The wide continental shelf of the Gulf of Lion (up to 70 km) has been the object of numerous investigations since the early days of oceanography. Yet, the question of sequences duration, the mechanisms of deposition and factors implied remained unanswered. A study of a very dense grid of Very High Resolution (VHR) seismic reflection (Sparker) data associated with surficial cores both, collected by IFREMER between 1992 and 2001 on the outer shelf and upper slope offshore of Sète in the Languedoc region gives a new insight into these issues. Analysis of the 3D geometry of the sedimentary record reveals a basic depositional pattern consisting of a pair of horizontally juxtaposed and downlapping prisms. Prism PI with low angle clinoforms (<1°) lies on the upper part of the shelf and is interpreted to be prodeltaic-offshore deposits. Prism PII with steeply dipping clinoforms (∼4°) lies on the outer shelf between 40 and 70 km from the present day coastline and is interpreted to be littoral deposits.Results obtained from integrating lithology, palynology, micropaleontology, seismic stratigraphy, stratigraphic simulation, support the hypothesis that the basic depositional pattern records a 100 000-years glacioeustatic (interglacial/glacial) cycle. As previously suggested by Aloïsi [Aloïsi, J.C., 1986. Sur un modèle de sédimentation deltaïque: contribution à la connaissance des marges passive, Thèse de Doctorat d'Etat. Université de Perpignan, 162 pp], prisms PI corresponds to deposition at high sea level and prisms PII to deposition during low sea level at glacial maxima. Five sequences of paired prisms capped by five major erosion surfaces have been identified and modelled showing that the corresponding glacioeustatic cycles (the last five cycles at least) are recorded on the shelf of the western part of the Gulf of Lion.
We have conducted elemental, isotopic, and Rock-Eval analyses of Cenomanian–Santonian sediment samples from ODP Site 1138 in the southern Indian Ocean to assess the origin and thermal maturity of organic matter in mid-Cretaceous black shales found at this high-latitude location. Total organic carbon (TOC) concentrations range between 1 and 20 wt% in black to medium-gray sediments deposited around the Cenomanian–Turonian boundary. Results of Rock-Eval pyrolysis indicate that the organic matter is algal Type II material that has experienced modest alteration. Important contributions of nitrogen-fixing bacteria to the amplified production of organic matter implied by the high TOC concentrations is recorded in δ15N values between −5 and 1‰, and the existence of a near-surface intensified oxygen minimum zone that favored organic carbon preservation is implied by TOC/TN ratios between 20 and 40. In contrast to the marine nature of the organic matter in the Cenomanian–Turonian boundary section, deeper sediments at Site 1138 contain evidence of contributions land-derived organic matter that implies the former presence of forests on the Kerguelen Plateau until the earliest Cenomanian.
Jurassic shales and mudrocks from the Haltenbanken area offshore Norway and red claystones from Carboniferous and Permian intervals of Northern Germany were used in a study of the hydrocarbon sealing efficiency of clastic sediments. The investigations comprised geochemical and mineralogical analysis of the pelitic rocks, petrophysical characterisation by mercury porosimetry and specific surface area measurements, and laboratory experiments to assess the transport properties with respect to both molecular transport (diffusion) and volume flow (Darcy flow). Effective diffusion coefficients of methane in the water-saturated rock samples at 150°C lay between 1.4 × 10−11 and 4.5 × 10−10m2/s and showed a distinct correlation with TOC content. Permeability coefficients, measured by means of a steady-state method, ranged from <1 nDarcy (<10−21m2) for Permian (Rotliegend) and Carboniferous red claystones up to 4.3 μDarcy (4.3 × 10−18m2) for a bioturbated Jurassic siltstone.The experimental data were used to calculate maximum sustainable gas and petroleum column heights, hydrocarbon leakage rates by pressure-driven volume flow (Darcy flow), and diffusive gas losses for simple, hypothetical scenarios. Computed maximum gas column heights range from 20 m up to >2000 m. Hydrocarbon column heights calculated on the basis of a rich condensate lay between 3 and 340 m. Depending on temperature, pressure, reservoir geometry and seal thickness, diffusive losses can be expected to require tens of millions of years to significantly affect the contents of commercial size natural gas reservoirs.
The South African plateau is bordered by passive margin basins preserving the terrigeneous sediment produced during onshore erosion. As such, these basins potentially provide a record of the variation in onshore elevation and relief over time. Here we bring new constraints on the uplift and erosion of the Southern African plateau over the last 150 Ma from the perspective of the stratigraphic architecture of these basins. We review published data to quantify the terrigeneous supply eroded in the drainage area and preserved in the basins. The novel aspect of our approach is the integration of the evolution of the whole domain in sedimentation (i.e. not only the platform) as well as the onshore eroding region.
Laboratory experiments and drilling observations are used to estimate vertically upward fluid flow rates of approximately 4 mm/yr in Keathley Canyon, northern Gulf of Mexico. Based on uncertainty in pressure and permeability models, flow rates exceed 1.3 mm/yr but are less than 28 mm/yr. Consolidation experiments document that permeability decreases from 10−15 m2 at the seafloor to 10−18 m2 at 300 m below seafloor. I use these experimental data with logging-while-drilling data to constrain a permeability function for the basin. Sediment discharge from an open borehole filled with weighted mud is used to estimate a minimum overpressure gradient of 4.3 kPa m−1 in the Keathley Canyon mini-basin. The overpressure gradient and permeability model are input into Darcy's law to estimate an average flow rate for the basin. These flow rates are consistent with estimates of compaction-driven flow from existing regional-scale models of flow in the northern Gulf of Mexico. Hydrate stability calculations for the basin predict a 25 m deepening of the base of hydrate stability due to overpressure.
The earliest attempts to find oil in Britain, dating from 1918–1922, based on anticlines flanking the Pennines, were largely unsuccessful. Renewal of exploration in the 1930s, aimed at a broader range of prospects, met with much criticism but went ahead after nationalization of the unknown and undiscovered oil resources (1934). The south of England ranked as first priority but the initial drilling of the new campaign carried out in 1935–1937 yielded only minor quantities of oil and gas. Attention was transferred in 1938 to Upper Palaeozoic prospects in the Midlands and north, resulting in small gas and oil discoveries in Scotland and Yorkshire, and discovery of a series of commerical oilfields in the Upper Carboniferous of Nottinghamshire. In the 1950s the first commercial discovery in the Jurassic was made at Kimmeridge in Dorset. Further Carboniferous discoveries were made in Nottinghamshire and Lincolnshire, and a series of fresh objectives defined by seismic reflection were drilled in the Mesozoic basin of southern England. This phase of exploration was terminated at the end of 1964 by adverse fiscal changes. Continuation of effective exploration operations remained uneconomic until the oil price rise in 1973.
In April and May 2005, cores were acquired and sub-sampled for gases in lease blocks Atwater Valley 13 and 14 and Keathley Canyon 151 during deep subseafloor drilling conducted as part of the JIP study of gas hydrates in the northern Gulf of Mexico. Sample types included sediment headspace gas, free gas derived from sediment gas exsolution, and gas exsolution from controlled degassing of pressurized cores. The gases measured both onboard and in shore-based labs were nitrogen, oxygen, hydrogen sulfide, carbon dioxide, and the hydrocarbons methane through hexane. The presence of seafloor mounds, seismic anomalies, a shallow sulfate–methane interface, and similar gas compositions and isotopic compositions near the seafloor and at depth suggest an upward flux of methane at both sites. Sediment gases at the Atwater Valley sites, where seafloor mounds and adjacent sediments were cored, strongly suggest a microbial source of methane, with very little thermogenic gas input. Sediment gas from all cores contained from about 96 to 99.9% methane, with the balance composed primarily of carbon dioxide. Methane to ethane ratios were greater than 1000, and often over 10,000. Gases from cores at Keathley Canyon were similar to those at Atwater Valley, however, deeper cores from Keathley Canyon contained more ethane, propane, and butane suggesting mixing with minor concentrations thermogenic gas. The isotopic composition of methane, ethane, and carbon dioxide were measured, and δ13C values range from −84.3 to −71.5‰, −65.2 to −46.8‰, and −23.5 to −3.0‰, respectively, all consistent with microbial gas sources, early diagenesis of organic matter and perhaps biodegradation of petroleum. The presence of deep microbial gas at these sites here and elsewhere highlights a potentially significant, predominantly microbial gas source in the northern Gulf of Mexico.
One of the principal scientific, technical and environmental challenges for the next century is undoubtedly the exploration and understanding of the deep oceans. Close collaboration between the hydrocarbon industry and scientific community is allowing us to push back this frontier and so to develop new models for deep-water sedimentary systems. The turbidity current paradigm is under scrutiny and refinements proposed for massive sands, megabeds and immature turbidites. Source area and sediment type are key controls. Bottom currents play an important part in the shaping of margins, the generation of hiatuses and bounding surfaces, the winnowing of sands and ventilation of ocean basins. It is at the level of architectural elements and their three-dimensional geometry that much activity is currently focused. Most advance has so far been made in terms of channel types, dimensions, aspect ratios, stacking patterns and hierarchies; to a lesser extent this is true for lobes, levee complexes, contourite drifts and sheet sands. It is only after this phase of study that we will be able to significantly improve our models for the larger-scale systems—fans, ramps, slope-aprons, basin plains and drifts.
A formal method for describing data collected from field studies is used to generate stochastic geological models of sedimentary successions using a method based on syntactic pattern recognition. Using this method an analogue model developed from field data can be encoded as a grammar. The grammar is composed of symbols which represent geological entities. Valid patterns formed by the symbols are described by a set of production rules. In order to demonstrate the potential of the syntactic method, 2D simulations of interpreted cross-sections from Brushy Canyon outcrops are presented here, as well as 2D simulations of seismic facies from the Bengal Fan.
This 2D modelling study attempted to quantify the processes of hydrocarbon (HC) generation, expulsion and migration along a regional section (NW–SE) in the North Alpine Foreland Basin of Western Switzerland. Modelled excess pressures increase towards the Alpine front, and are mainly related to the lithology distribution; excess pressure compartments are centred around shaly or evaporitic intervals of low permeability. By late Jurassic times, a first major phase of HC generation is initiated in the deepest part of the Permo-Carboniferous grabens in the external part (i.e. NW) of the Molasse basin. A second more important generation phase starts in Oligocene–Miocene times in the internal parts close to the Alpine front. Migration of HC seems to be controlled predominantly by the layer-cake geometry of the Mesozoic passive margin sequence. In contrast to many studies where vertical buoyancy driven migration dominates, the main driving mechanism for the migration and accumulation of HC is the excess pressure evolution. Large overpressure zones in the frontal part of the orogen (i.e. Subalpine Molasse) can drive deep fluids far updip into the foreland. The build-up of overpressured zones depends strongly on the subsidence rates, lithology and the occurrence of heterogeneities such as faults. Modelling results suggest that the presence of vertical fault zones have a dramatic influence on the pore pressure evolution (pressure drain-off), and in consequence on the HC accumulation pattern.
The Yampi Shelf on Australia's North-West Shelf is highly prospective, with two discrete hydrocarbon sources producing dry gas and oil. To reduce exploration uncertainty relating to gas flushing and poor top seal capacity, a study was undertaken to characterise hydrocarbon migration in the area. It used a combination of seismic amplitude and structural data integrated with shipboard water column geochemical sniffer (WaSi) data, satellite Synthetic Aperture Radar or SAR data and aircraft-acquired Airborne Laser Fluorosensor (ALF) data. Data were acquired synchronously and in staged programs, to allow both direct comparison and time-series analysis of results. Massive natural dry gas and oil seepage was detected, though the relative abilities of WaSi, SAR and ALF to detect and characterise this seepage were markedly different. The spatial distribution, concentration, and relative composition of the detected seepage were controlled principally by the regional seal's thickness and capacity, rather than by the inherent composition and flux of the migrating hydrocarbons. WaSi preferentially identified gas seepage, often in basin-ward locations, because the high relative permeability of gas favoured its early leakage, even through thick seals. SAR preferentially identified oil seepage, which was episodic and largely restricted to the basin-margin at the regional zero-edge-of-seal, reflecting the low relative permeability of oil, even through thin seals (it leaked ‘late’). ALF principally detected low-level oil seepage from charged traps, and was hence most useful for trap ranking. The ability of these remote sensing tools, as well as that of seismic data itself, to detect hydrocarbons appears critically dependant upon interplays between the relative sensitivity of the assorted tools to detect various hydrocarbon phases and the capacity of the top seal itself. The study has demonstrated that the interactions between geology and hydrocarbon charge are predictable, and that understanding these interactions is crucial for the reliable interpretation of remote sensing data.
The Middle America Trench is the topographic expression of the subduction of the Cocos Plate beneath the Caribbean Plate. In this study, the subduction process was simulated in a numerical basin model to determine the fate of the organic matter. During subduction, temperature rises and the organic matter embedded in sediments generates hydrocarbons. However, equilibrium temperatures are not reached due to the fast subduction process, shifting the petroleum generation zone to depths exceeding 10,000 m. The average organic carbon content of the subducted sediments is low but the fast subduction leads to a high rate of hydrocarbon generation. For example, along a 1 km wide section along the subduction zone a methane generation rate of about 9×105 m3/a is calculated. The hydrocarbon generation process occurs at much greater depth than in conventional sedimentary basins. Due to the low migration velocity, oil is transported from the kitchen area to a greater depth and finally transformed to methane. Interestingly, calculated migration velocities of methane are mostly lower than the current subduction velocity. Thus, even methane is further subducted. This modelling result is confirmed by published data on gas composition at the trench, where (almost) no thermogenic gas has been detected. The subduction of large amounts of methane to very great depths could have an effect on volcanism in the back arc region or on metamorphic processes.
This paper documents a new method for describing channel-related sedimentary deposits based on formal language theory. Using this method an analogue model of a sedimentary deposit can be encoded as a grammar. A program, called a parser, has been developed which can generate stochastic maps of these sedimentary deposits based on information in a specified grammar. The maps of sedimentary deposits generated by the parser have the same type, spatial arrangement, shape and size distribution as the analogue model. The successful generation of depositional maps represents a crucial step in the ongoing development of a new technique designed to generate 3D static geological models of sedimentary successions. The maps can be conditioned to match sparse hard data in the form of channel segments interpreted from seismic horizon maps.
When working in complex areas, the explorationist needs to construct 3D models and to check their coherency. Coherence can be analyzed on the current geometry but could also be quantified through restoration. To achieve this restoration, i.e. the search for an initial geometry with a realistic deformation pathway between the initial and the final geometries, appropriate tools are necessary. These include classical cross-section balancing, surface unfolding and volumetric restoration. In this paper, we will describe a workflow to do quantitative structural geology in complex areas. This includes the construction and quality control of the surfacic model, the line balancing, the cross-section, surface and multi-surface restoration and the volumic restoration. In addition to the quality control of the geometries, the restoration allows us to compute strain tensors. This can be done in the three types of restoration and visualized through ellipses and ellipsoids (resp. 2D and 3D) that represent the main strain or stress vectors. We shall discuss the relationship between these strain tensors and the expected fracture network, in density as in direction.
An integrated evaluation of the tectonic and depositional history of the Upper Jurassic of the Fife and Angus area of UK Quadrants 31 and 39 has been carried out through 3D seismic interpretation and a range of geological studies. Sedimentological and petrological data indicate that the Upper Jurassic sands in the study area are the lateral equivalents of the Fulmar Formation. They can be divided into three groups of sandstones, deposited by different processes during an interval of almost 10 million years: (i) open shelf, Group 1 sandstones comprise sands dominated by storm/wave activity, which accumulated in fault-bounded mini-basins (‘embayments’); (ii) Group 2 sandstones, possibly deposited by storm-induced flows; and (iii) Group 3 sandstones, which may represent subaqueous dunes deposited in a tidally-influenced, shallow marine environment.Sedimentological and heavy mineral studies suggest that the sands in the study area were sourced from the adjacent Mid North Sea High and were transported along the shelf probably by wave, storm and tide-generated currents. Biostratigraphic dating indicates that the Upper Jurassic sands in the Fife/Angus area were deposited during Late Kimmeridgian to upper Middle Volgian. An earlier depositional period with a possible Callovian age is also inferred. Deposition of Jurassic sediments in the study area occurred during two transgression periods (Callovian and upper Late Jurassic), separated by an erosional/non-depositional phase (Oxfordian to Middle Kimmeridgian). The accumulation of Late Jurassic sands started in the Fife embayment and progressed northwards. It was confined, however, to the eastern side of the embayments, possibly due to the hydrodynamic conditions of the area.
The Eocene of the northern North Sea is characterised by fine-grained mudstones and isolated sandbodies. Three-dimensional (3D) seismic data reveal that the Eocene mudstones are intensely deformed by polygonal faulting and contain numerous discordant amplitude anomalies. The anomalies identified in the Tampen Spur area are similar to discordant anomalies previously described from the Eocene in the South Viking Graben and the Outer Moray Firth. Detailed mapping shows that the anomalies are often conical in three dimensions, with dimensions ranging from 100 to 200 m height and a few hundred metres to a few kilometres lateral extent. Well calibrations show that the anomalies arise from sandstones tens of metres thick encased in mudstone. The discordant amplitude anomalies are interpreted as large-scale conical sandstone intrusions fed from deeper Paleocene sandbodies. Individual conical intrusions may contain millions of cubic metres of sandstone and may provide fluid migration paths, potential reservoirs, and pose a drilling risk when exploring for deeper targets.
Optimum hydrocarbon recovery from clastic reservoirs depends on the depth of our knowledge of the detailed architecture of intercalated shaly baffles and barriers. In the case of deep marine turbidite fields, the usual investigation methods are often unsatisfactory due to a scale gap between seismic and well bore data. This gap can be reduced however, if we have more information concerning the distribution of finer-scale heterogeneities.This paper describes the detailed study and modelling of outcrop analogues from the Eocene Morillo turbidite system (south-central Pyrenees, Spain). The southern margin of the Morillo turbidite system provides evidence of the contact between the uppermost channel storey of the complex and associated sandy overbank wedge deposits. Lithofacies are firstly used to describe heterogeneity distribution. Petrophysical characteristics derived from reservoir analogues are then applied to these lithofacies. The model is flow simulated and two distinct well patterns are tested to evaluate the impact of the different heterogeneity levels encountered in the outcrop.These models significantly increase our understanding of the precise distribution of the inherent heterogeneities at a detailed level previously difficult to achieve.
In this paper a channel levee system and the associated depositional lobe are described. The proposed example derives from a recently acquired 3D survey in the West Africa deep-offshore. It is mainly based on a detailed 3D seismic reconstruction, attribute map interpretation and log data from a single well to calibrate seismic responses and sedimentary facies.The stratigraphic section under consideration, informally named the A 100 Sequence, is about 80 ms TWT thick, and of early Pliocene age. The attribute maps focused on this interval clearly show the presence of two narrow (up to 250 m wide) low-sinuosity slope channels that can be followed for more than 32 km in an E–W direction (down slope to the W) as far as the western border of the 3D acquisition. Over most of their length both channels are characterized by the low amplitude aspect of the axial belt and by brighter responses in the flanks (presence of thin-bedded sands in the levee areas). This character, associated with the common convex-down geometry of the reflections lying just above the channel axis, suggests a predominant fine-grained infilling of the thalwegs.One of these channels, termed the Southern Channel, shows a high amplitude lobe-shaped zone in the middle part of its course. The ‘anomalous’ development of this depositional element has been related to a local reduction of the slope gradient, probably induced by the synsedimentary growth of an adjacent mud-cored anticline.Because of hydrocarbon occurrence, the lobe area and the associated feeder channel have been investigated in detail through careful picking of all the mappable reflections inside the channel-lobe system. The resulting physical-stratigraphic framework and the related attribute maps suggest that channel development occurred through distinct growth stages. The lower stage (Stage 1) is expressed by symmetric levees flanking the main channel to the east and by a depositional lobe/lobe fringe area to the west. Between the levee belt and the lobe' a transitional zone occurs where the presence of isolated bypassing bars has been inferred. The upper stage (Stage 2) seems to record a phase of overall bypassing of flows within the channel conduit, producing the westward propagation of the channel and consequent dissection of the previous stage lobe. A contemporaneous lateral spillover from the channel axis of low-density turbidites constructed prominent gull-wing shaped levees that uniformly covered the stage 1 elements.
Using the new high-quality 3D seismic data, this paper addresses the salt structures in the KL11 area of the Laizhouwan depression in the southern offshore Bohai Bay basin. In the study area, the salt in the Sha-4 Member of the Paleogene Shahejie Formation thickened, and then formed an S–N trending salt wall, which changes shape regularly along its trend from salt diapir to salt pillow. The change in thickness of the suprasalt layers record five growth phases of the salt wall from the Eocene to the Quaternary: (1) early diapirism, (2) active diapirism, (3) passive diapirism, (4) relative structural quiescence, and (5) arching. The evolution of the salt structures was mostly governed by the multi-phase compression induced by the dextral strike-slip of the Tan–Lu fault, which formed a restraining bend in the study area. There was an original passive stock in the south, which was later tectonically squeezed by E–W compression and became a diapir. As the shortening propagated to the north from the original stock, the salt pillow was created in the north. Relative structural quiescence then followed until the next phase of compression, which arched the thick roof of the salt wall.
Three-dimensional (3D) seismic data from the continental margin offshore Israel (Eastern Mediterranean) have been used to analyse the compressional structures within the toe regions of two major buried submarine landslides: the ISC and the T20. Both landslides are developed within a Plio-Pleistocene slope succession composed predominately of claystones, limestones and siltstones. The high spatial resolution provided by the seismic data has allowed a detailed analysis of the geometries and deformational structures within the toe regions of the two landslides, and this has been used to develop a mechanical model for their development. Importantly, it has been recognised that submarine landslides may be divided into two main types according to their form of frontal emplacement: frontally confined and frontally emergent. In the former, the landslide undergoes a restricted downslope translation and does not overrun the undeformed downslope strata. In the latter, much larger downslope translation occurs because the landslide is able to ramp up from its original basal shear surface and translate in an unconfined manner over the seafloor. We propose that these two types of submarine landslides are end members of a continuum of gravity-driven slope failure processes, which extends from landslides where the headscarp is completely evacuated, to landslides where the material remains entirely within the headscarp. The differentiation of these two end members is of critical importance as their respective mechanisms of formation, downslope propagation and emplacement are significantly different, and hence need to be taken into consideration when analysing their respective kinematics.
Three-dimensional seismic and well data are integrated to investigate the geometry and controls on a series of sand-rich slope systems in the Kyrre Fm (Upper Cretaceous) on the Måløy Slope, offshore Norway. Slope systems were fed by sediments eroded from mainland Norway to the east and transported across a relatively narrow shelf into four canyons developed at the shelf edge. These canyons were not formed through erosional or mass-wasting processes during the Late Cretaceous, but represent a series of underfilled canyons developed during an earlier, Late Jurassic erosional phase. Channels, which are commonly arranged into laterally or vertically stacked channel complexes, were fed sediment through the shelf-edge canyons and may be associated downslope with small terminal fans. The canyons and their associated depositional systems were not active synchronously, with a clear southward migration of the active depositional systems. On the slope, syn-depositional topography was formed via: (i) differential compaction of mudstone-rich strata across underlying Late Jurassic canyons which resulted in the formation of a series of E–W-trending structural lows; and (ii) differential compaction of mudstone-rich strata across the underlying Late Jurassic fault blocks which resulted in N–W-trending structural highs. Both of these features had a variable influence on the incision, fill and overall spatial distribution of slope channels/channel complexes and associated fans. A large fan which overlies the shelf-edge canyons and associated downslope depositional systems represents the final depositional unit within the study area. The fan effectively ‘seals’ the underlying shelf-edge canyons, suggesting it was not supplied by sediment routed through the canyons. The results of this study support previous studies which indicate that shelf-edge canyons may be a first-order control on the location of sand-rich, turbidity current-fed depositional systems on submarine slopes. Furthermore, this study demonstrates that differential compaction may be a key control on slope morphology in submarine settings and the associated topography can markedly influence depositional patterns.
Three-dimensional (3D) seismic reflection data have recently been shown to be an excellent tool in the study of submarine mass-transport complexes (MTCs), from which kinematic indicators can be identified. Kinematic indicators are geological structures or features which may be analysed to allow the direction, magnitude and mode of transport to be constrained. The various indicator types have been classified according to where they may typically be found within the MTC body – the headwall domain, translational domain and toe domain. Aspects of their formation, identification using seismic data and their kinematic value are discussed, and illustrated using examples taken from 3D seismic data from the continental margin of Norway and the Levant Margin, both of which have been influenced by repetitive large-scale slope failure in the recent past. The imaging of kinematic indicators using seismic surveys which provide large areal coverage allows swift and confident evaluation of the direction of translation, and in many cases also allow the degree of translation of the displaced slide material to be constrained. Imaging of the basal shear surface, analysis of internal architectures and determination of transport direction are areas which are of particular benefit from the analysis of 3D seismic. The descriptions and applications of the various kinematic indicators detailed in this study should find broad applicability for seismic interpreters working on MTCs in many different settings and locations.
Gas and minor oil production from the northwestern Uralsk basin is associated with TTGF, Çaganskaya and Darinskaya fields located along a NE–SW aligning chain of barrier reefs. Most important reservoir rocks of the basin are associated with shelf, reef, pinnacle reef, atoll and bar facies of the carbonate complex, deltaic or tidal facies of the platform deposits and clastic basinal fan or slope facies. The basin comprises multiple petroleum systems, characterized by deep marine black-shales of Paleozoic which shows up to 10% TOC. The evaluation of these petroleum systems were investigated by utilizing 1-D and 3-D basin models constructed using the geological, geophysical and geochemical data. The sub-salt section of the northwestern Uralsk basin demonstrates higher hydrocarbon potential and lower exploration risk compared to the post-salt sequence despite the necessity for drilling extreme depths (e.g. over 5000 m) particularly in the Inner Zone. Various traps associated with salt tectonics favor hydrocarbon accumulations in the Artinskian/Filippovskiy, Frasnian, Bobrikovskiy/Tulskiy and Bashkirian proven reservoir rocks. Among these reservoirs, the Artinskian/Filippovskiy is the major oil bearing reservoir rock particularly in the Teplovskaya, Gremiachinskaya and Teplovskaya fields. It has good hydrocarbon charge access to multiple source rocks. Besides, Kungurian salt forms efficient seal on the top. Among the 15 investigated and modeled source rocks, the most expulsive ones are Afoninskiy, Givetian, Famennian, Tournaisian, Tulskiy, Serpukhovian and Vereiskiy, which intensively charge reservoirs of the northwestern Uralsk basin starting from Triassic. Source to trap timing risk is minimal, particularly within the Inner and Escarpment zones since traps are older. Gas and oil generation, expulsion and migration model results point out a total of 179 Mboe gas and 89 Mbbl oil potential (subsurface conditions) associated with Gremiachinskaya, W. Teplovskaya, Teplovskaya, Tokarevskaya, E. Gremiachinskaya, Ulianovskaya and Tsyganovskaya structures.
Using high quality 3D seismic data within the Lower Congo Basin (LCB), we have identified pockmarks that are aligned above the sinuous belt of a buried turbiditic palaeo-channel, 1000 m beneath the seafloor. Geochemical analyses on cores (GC traces), taken in the centre of four of these pockmarks along this channel, show no clear evidence for migrated oil. But, some features of the GC traces, including elevated baselines (UCM>34 μg/g) and a broad molecular weight range of n-alkanes with little odd–even preference, may be interpreted as indicating the presence of thermogenic hydrocarbons in the cores.Seismic profiles show that these pockmarks developed above two main features representative of pore fluid escape during early compaction: (1) closely spaced normal faults affecting the upper 0–800 ms TWT of the sedimentary column. This highly faulted interval (HFI) appears as a hexagonal network in plane view, which is characteristic of a volumetrical contraction of sediments in response to pore fluid escape. (2) Buried palaeo-pockmarks and their underlying chimneys seem to be rooted at the channel–levee interface. The chimneys developed during early stages of burial and are now connected to the HFI.This study shows that the buried turbiditic channel now concentrates thermogenic fluids that can migrate through early chimneys and polygonal faults to reach the seafloor within some pockmarks. Using a multidisciplinary approach within the Lower Congo Basin, combining 3D seismic data and geochemical analyses on cores, we trace the fluid history from early compaction expelling pore fluids to later migration of thermogenic hydrocarbons.
From the measurements of bore-hole breakout, hydrofracturing, and rock acoustic emission in the Zhangqiamg depression, Liaohe field, China, we have determined that the orientation of the maximum principal compressive stress is nearly east–west with a small angle of pitch (no more than 10°), and the regression equations of stress gradient with depth are σ1=8.359+0.0142H (r=0.96) and σ3=5.801+0.00437H (r=0.94). The present-day three-dimensional (3D) stress field of the Zhangqiang depression is approached using measured data and by 3D modeling. The approach shows that a full 3D crustal stress field analysis of an oil basin is consistent with the actual measurements. And then we use fluid potential or gradient to discuss the quantitative correlation between crustal stress and hydrocarbon migration and accumulation. The results show that low fluid potential areas surrounded by high regions are eligible for hydrocarbon accumulation and have been tested at the Keerkang oil field. The results also show that the distribution of favorable areas for hydrocarbon accumulation varies in different layers in space and is used in hydrocarbon exploration in the Zhangqiang depression. Therefore, the understanding of the 3D crustal stress field of a basin is essential for evaluating hydrocarbon migration and accumulation, and the technology and methodology developed here can be applied to other petroliferous basins.
3D basin modelling is now used by some oil companies for exploration purposes. The potentiality of this tool is not fully expressed, but it seems that it should become the core of the basin evaluation process in the near future.In this paper the methodology of 3D basin modelling in a relatively mature area is presented. This methodology is based on a four step process which includes: (1) 3D block building, (2) gridding, (3) backward modelling, and (4) forward modelling. These processes are illustrated through a case study performed on the Franklin structure (Central Graben, North Sea, UK).The main results of this study show that petroleum migration is a 3D process that cannot (in this specific case) be properly addressed by 2D basin modelling alone. Furthermore, the global hydrocarbon mass balance, only accessible by 3D modelling, shows that only 2% of the total hydrocarbons generated by the source rocks are present in the main reservoir.
Three periods of sustained gas seepage in geological time have been revealed in Danish block 5604/26 in the North Sea by the use of exploration 3D seismic data. The most recent period is indicated by a cluster of seismic chimneys which ties in to buried craters near the seabed, and possible present gas escape through the seabed, along with amplitude anomalies indicating a shallow gas sand charged by gas migrating from a deeper level. The cluster of seismic chimneys indicative of vertical gas migration is visible down to 1.5 s TWT (1500 ms), and therefore the gas is interpreted as migrating from a deeper stratigraphic horizon. Below this level it becomes difficult to see the chimneys due to complex faulting. The faults may work as gas migration pathways. The geometry of the cluster of seismic chimneys indicates that the gas has been migrating from one point. The nearest possible source of the gas is an underlying prospect where an oil and gas discovery has been made. Two earlier periods of gas seepage are indicated by mounds, possibly carbonate buildups over gas seepages in Pliocene time, and,similarly, buried craters formed by gas seepages in an earlier period in Pliocene time. The results of this study are that gas seepage is a periodical process in geological time and that its presence and associated features can be used as an indicator of deeper prospective reservoirs.
In the last decades gas hydrate occurrence along the Chilean continental margin has been well documented. In order to better define the seismic character of the hydrate-bearing sediments, we performed a detailed velocity analysis by using the pre-stack depth migration on part of multichannel reflection seismic line RC2901-734 located offshore Coyhaique.The velocity model shows a hydrate bearing layer above the BSR, with high velocity (1700–2200 m s−1) and maximum thickness of 250 m and a free gas bearing layer below the BSR, characterized by low velocity (1250–1400 m s−1). A weak reflector at about 70 m below the BSR marks the base of the second layer.By knowing the BSR depth, the seafloor depth, and the sea bottom temperature, the geothermal gradient was estimated. The resulting gradient varies from 35 to 95 °C km−1, with highest values at the structural high, and the lowest values located in the accretionary prism and in the fore arc basin.In order to quantify the amount of gas phase, the velocity model was converted into a gas-phase concentration model by using a theoretical approach. The results indicate that highest concentrations of gas hydrates, up to 23% of the total volume, are located in the fore-arc basin, and that highest concentration of free gas, up to 3% of the total volume, are located at the structural high, which may be considered as a natural trap for migrating fluids. Average concentrations are equal to 12% and 1% of total volume for gas hydrate and free gas, respectively.
Scaled sandbox models were used to investigate the 4D evolution of pull-apart basins formed above underlapping releasing stepovers in both pure strike-slip and transtensional basement fault systems. Serial sectioning and 3D volume reconstruction permitted analysis of the full 3D fault geometries. Results show that very different pull-apart basins are developed in transtension compared to pure strike-slip. Both types of models produced elongate, sigmoidal to rhomboidal pull-apart systems, but the transtensional pull-apart basins were significantly wider and uniquely developed a basin margin of en-echelon oblique-extensional faults. Dual, opposing depocentres formed in the transtensional model whereas a single, central depocentre formed in pure strike-slip. In transtension, a distinct narrow graben system formed above the principal displacement zones (PDZs). Cross-basin fault systems that linked the offset PDZs formed earlier in the transtensional models.Sequential model runs to higher PDZ displacements allowed the progressive evolution of the fault systems to be evaluated. In cross-section, transtensional pull-aparts initiated as asymmetric grabens bounded by planar oblique-extensional faults. With increasing displacement on the PDZs, basin subsidence caused these faults to become concave-upwards and lower in dip angle due to fault block collapse towards the interior of the basin. In addition, strain partitioning caused fault slip to become either predominantly extensional or strike-slip. The models compare closely with the geometries of natural pull-apart basins including the southern Dead Sea fault system and the Vienna Basin, Austria.
The Maastrichtian continental Whitemud Formation in the Cypress Hills of southwestern Saskatchewan, Canada, has been kaolinized by in situ leaching to a depth of at least 20 m, produced by a lowered water table. The first stage of leaching took place during accumulation of the coal zone in the middle of the 20 m-thick Whitemud. A second stage took place after deposition of the lower few meters of the overlying organic-rich Battle Formation.The U-Pb age on bentonite zircon in the lower Battle is 66.5 ± 0.2 Ma, indicating the last stage of sea fall. Using an average compacted sediment accumulation rate of 72 m/m.y., based upon the thicknesses of magnetozones 29r and 30r, the time spanning the two stages of lowered sea level is 200,000 years or less.The Colgate Member of the Fox Hills Formation in Montana is an exact correlative of the Whitemud, and kaolinization of the Colgate is due to the same sea level fall from the International Boundary to central North Dakota. The age correlates to the sea level cycle marking the base of supercycle TA-1, and the short duration indicates that this sea level cycle was glacioeustatic.
The Triassic reservoirs of the Judy Field, an overpressured petroleum accumulation in the Central North Sea, have been studied to determine their pressure and petroleum filling history. The magnitude of overpressure in the Pre-Cretaceous aquifer is similar across the field at about 24 MPa (3500 psi), but with some higher pressure laterally in areas closest to the major depocentres. Pressure modelling shows that the magnitude of the overpressure can be attributed almost entirely to disequilibrium compaction, and largely due to late rapid Tertiary burial, although modelling does require nanoDarcy (i.e. shale-like) permeability in the Cretaceous chalk section. The contributions from other mechanisms, which may be relevant in this setting, i.e. gas generation and lateral transfer, appear to be small. The existence of both aqueous and petroleum phases in secondary fluid inclusions in microfractures in quartz and feldspar grains allows estimation of palaeopressure. The data show fracture healing from 115 to 136°C during a time when reservoir palaeopressures were above hydrostatic. Reconstruction of the temperature history of the reservoir places the timing of the fluid inclusion formation during fracture healing at between 3 and 1 My ago, coincident with a period of rapid burial. The palaeopressure estimation using fluid inclusions validates the pressure modelling studies, which are derived independently from commercial basin modelling software. The fluid inclusions also revealed a low-GOR oil over the whole field, prior to late stage flooding by gas leading to the variable (low to high) GOR fluids plus gas condensates within the field today.
A unique set of high-quality downhole shallow subsurface well log data combined with industry standard 3D seismic data from the Alaminos Canyon area has enabled the first detailed description of a concentrated gas hydrate accumulation within sand in the Gulf of Mexico. The gas hydrate occurs within very fine grained, immature volcaniclastic sands of the Oligocene Frio sand. Analysis of well data acquired from the Alaminos Canyon Block 818 #1 (“Tigershark”) well shows a total gas hydrate occurrence 13 m thick, with inferred gas hydrate saturation as high as 80% of sediment pore space. Average porosity in the reservoir is estimated from log data at approximately 42%. Permeability in the absence of gas hydrates, as revealed from the analysis of core samples retrieved from the well, ranges from 600 to 1500 millidarcies. The 3-D seismic data reveals a strong reflector consistent with significant increase in acoustic velocities that correlates with the top of the gas-hydrate-bearing sand. This reflector extends across an area of approximately 0.8 km2 and delineates the minimal probable extent of the gas hydrate accumulation. The base of the inferred gas-hydrate zone also correlates well with a very strong seismic reflector that indicates transition into units of significantly reduced acoustic velocity. Seismic inversion analyses indicate uniformly high gas-hydrate saturations throughout the region where the Frio sand exists within the gas hydrate stability zone. Numerical modeling of the potential production of natural gas from the interpreted accumulation indicates serious challenges for depressurization-based production in settings with strong potential pressure support from extensive underlying aquifers.
The passive Eastern Continental Margin of India (ECMI) evolved during the break up of India and East Antarctica in the Early Cretaceous. The 85°E ridge is a prominent linear aseismic feature extending from the Afanasy Nikitin Seamounts northward to the Mahanadi basin along the ECMI. Earlier workers have interpreted the ridge to be a prominent hot spot trail. In the absence of conclusive data, the extension of the ridge towards its northern extremity below the thick Bengal Fan sediments was a matter of postulation. In the present study, interpretation of high resolution 2-D reflection data from the Mahanadi Offshore Basin, located in the northern part of the ridge, unequivocally indicates continuation of the ridge across the continent–ocean boundary into the slope and shelf tracts of the ECMI. Its morphology and internal architecture suggest a volcanic plume related origin that can be correlated with the activity of the Kerguelen hot spot in the nascent Indian Ocean. In the continental region, the plume related volcanic activity appears to have obliterated all seismic features typical of continental crust. The deeper oceanic crust, over which the hot spot plume erupted, shows the presence of linear NS aligned basement highs, corresponding with the ridge, underlain by a depressed Moho discontinuity. In the deep oceanic basin, the ridge influences the sediment dispersal pattern from the Early Cretaceous (?)/early part of Late Cretaceous times till the end of Oligocene, which is an important aspect for understanding the hydrocarbon potential of the basin.
The Triassic through to the Middle Jurassic (Bathonian) stratigraphy of the Kerr McGee 97/12-1 exploration well is described using petrophysical logs, in association with lithological and palaeontological analyses. Over 6210′ of sediment was penetrated, with sixteen Triassic and fourteen Lower–Middle Jurassic sedimentary units recognised, with a total measured depth (T.D) of 7310′ within the Sherwood Sandstone Group, Budleigh Salterton Pebble Beds of Early/Middle Triassic, Olenekian/Anisian age. As expected the sedimentary sequences are comparable to those described from the Wessex Basin, especially the Portland–Wight and Dorset (sub) basins. Of particular note, however, is the very thick halite sequence (1074′) within the Carnian, Dunscombe Mudstone Formation and also an expanded “Paper Shales” (27′) sequence within the earliest Toarcian. A normal fault is noted at 1260′ with the Late Bajocian, Inferior Oolite Group in fault contact with the latest Toarcian, Bridport Sand Formation, Down Cliff Clay Member. The Late Triassic (Rhaetian) to Middle Jurassic (Bathonian) yielded rich microfaunal and palynomorph assemblages, all of which have been previously described throughout northwest Europe. Four microfossils groups (foraminifera, ostracods, dinocysts and miospores) have been used to interpret stratigraphic ages and palaeoenvironments.This study provides new lithological and microfossil data for the Triassic to Middle Jurassic (Bathonian) sequences preserved in the offshore Portland–Wight Basin and provides a key tie-point/reference section for future offshore drilling.
Two-dimensional spectral analysis of aeromagnetic data has been used to determine the mean depths to buried magnetic basement rocks in the Abakaliki Anticlinorium of the Lower Benue Trough in Nigeria. The average thickness of the sedimentary cover overlying the basement which may contain petroleum reservoirs has thus been determined. The results indicate a two depth source model with the depth to the deeper sources (identified with the crystalline basement) varying between 1200 and 2500 m. Isolated linear magnetic features which are prominent on the aeromagnetic maps were also studied in detail. The anomalies over such features (which in many cases have spatial dimensions that are greater than 10 km), were modelled in terms of dyke-like bodies using non-linear optimization techniques. The results obtained from these studies compare favourably with those from gravity and ground magnetic investigations carried out in the are.. The sediment thickness in the area, the occurrence of numerous intrusive bodies and the deformational history of the sedimentary rock sequences in the Abakaliki Anticlinorium suggest that this part of the Benue Trough may not hold much promise in terms of hydrocarbon accumulation.
The Bandar Abbas syntaxis marks the transition between the Zagros collisional belt and the Makran subduction-related prism and constitutes a major structural feature within the Alpine–Himalayan mountain belt. Within the foreland part of the Zagros (the Zagros Fold Thrust Belt, ZFTB), the transition is marked by a sudden change in structural trends and style and a rapid decrease in seismicity and salt diapirism from west to east of the syntaxis. The depth to decollement and lithology affect the geometry and size of the folds on each side of the syntaxis, while different mechanical coupling along the basal decollement affects the degree of internal rock deformation. Thus the short and broad folds of the western side of the syntaxis are decoupled along the 8-km deep Hormuz evaporites, visible at surface in numerous salt diapirs. Strong seismicity in this area suggests that the basement is involved in the deformation. By contrast, the long and narrow folds of the eastern side of the syntaxis appear to be controlled by frictional slip along a 6-km deep detachment. The absence of seismicity suggests that the basement is not involved in the deformation in this area. We propose that the basal decollement for these structures corresponds to the eroded top of the NNE–SSW trending Oman Mesozoic nappes resulting from Late Cretaceous obduction of Tethys on the Arabian margin. These structures are visible onshore in the northern tip of the Oman peninsula and seen in offshore seismic profiles continuing northward beneath the Strait of Hormuz. Finally, map view observations show that the Main Zagros Thrust and the Zendan Fault—separating the ZFTB from the internal zones of the orogen—are shallow low angle thrust faults rather than steeply dipping faults.
The Navarin Basin, located in the Bering Sea, Alaska, is composed of three major units: (1) a thick Late Eocene and Oligocene overpressured shaly section; (2) a Miocene sandy section; and (3) a Late Miocene to Pliocene section, characterized by high porosity and a possible zone of abnormal pressure due to the presence of diatomaceous shales. The basin was affected by folding during the early stages of basin filling, whereas strike-slip faulting occurred until mainly Miocene time. To model the fluid flow and compactional history of the basin, a cross-section was used, controlled by four wells for which porosity and pressure data were available. A first approximation used a one-dimensional model to bracket the range of parameter values which govern the fluid flow/compaction equations, using data from the Arco COST No. 1 well. A sensitivity analysis then allowed: (1) determination of the best set of parameters of the model; (2) the behaviour of each parameter and its relative importance to be assessed; and (3) the role of an erosion event in influencing the evolutional history of the basin to be tested. With a slight readjustment of the parameters to allow for lateral fluid flow, a two-dimensional model was then constructed. The model reproduced correctly the variation of porosity and pressure data with depth for the four wells. The main overpressure in the deeper part of the basin arose during the earliest stages of basin filling by shale because of strong undercompaction. In the shallower part of the basin, high porosity values were obtained in agreement with the data, whereas some overpressure was developed in association with the undercompaction and low permeability of the diatomaceous shales. Different assumptions have been investigated, such as the permeable or impermeable nature of faults, the role of a rock fracturing coefficient and the anisotropy of permeability but, because of the location of the wells on structural highs, no major variations were observed relative to the well data, so that the impact of such different assumptions on the total dynamic history of the basin cannot be determined from presently available data.
Overpressure development in a sedimentary basin is directly related to the rate of fluid escape from the sediments. The model used here for fluid pressure evolution is a two-dimensional model (GEOPETII, developed at the University of South Carolina), which includes a dynamic indicator inversion method so that present day indicators of dynamic evolution with depth, such as porosity, permeability, formation thickness and fluid pressure, can be used to evaluate the parameters controlling the temporal behaviour of geological processes as well as those in intrinsic equations of state. In general, the dominant factors influencing fluid pressure evolution are the lithology, faulting/fracturing of rocks and sedimentation rate; other factors, such as fluid thermal expansion, dewatering of clays, as well as hydrocarbon generation, also contribute to the abnormal fluid pressure, but are relatively less important. A shale-dominated section can lead to overpressure as high as 300 atm at about 2000±500 m sub-mudline depth with a sedimentation rate of 50 m/Ma, whereas in a section dominated by sand and sandy shale, low to zero overpressure obtains.
We employ a discrete-element technique to investigate the influence of mechanical stratigraphy on the transition from trishear to kink-band fault-propagation fold forms above a blind basement thrust fault. We find that a ‘classic’ trishear fold, consisting of a broad, upwards-widening monocline with limb dips increasing towards the fault tip, occurs where the cover stratigraphy is weak and has no strong mechanical layering. In contrast, we find that marked, bedding-parallel mechanical layering in the cover promotes much narrower, kink-like fold geometries with more constant limb widths and dips. We discuss the modelling results in terms of their implications for structural interpretation and the application of kinematic models of fault-related folding in contractional settings.
Normal faulting and halokinesis have been important controls on the deposition and subsequent deformation of Mesozoic and Tertiary strata in the North Sea. In addition to the previously documented mechanisms of salt withdrawal, dissolution and differential sedimentary loading, it is recognized that gravity-driven thin-skinned extension above inclined salt layers has played an important part in North Sea basin development. Commercial section restoration software has been used to facilitate depth conversion, restoration and decompaction of seismic sections selected from an interpreted regional database in the western central North Sea, allowing validation of the interpretations, and a graphical and highly quantitative description of salt-assisted extension. Results of this work show that Zechstein Group evaporites were deposited in shallow sag basins during the Permian. Triassic sedimentary pods were generated by localized deposition in synclinal basins and grabens above the evaporites. Bedded salt became folded, while mobile salt flowed to fill anticline cores. Since the early Jurassic, regional tectonic tilting related to post-rift subsidence and increasing sedimentary overburden have caused allochthonous Mesozoic and Tertiary strata to extend by gravity spreading above the mobile salt layer, which detaches the allochthon from the underlying autochthonous Late Palaeozoic rocks. Concave-up listric normal faults sole out in the salt layer, propagate into the overlying cover sewuence, and have been active at different geological times causing stratal thickening and folding within the allochthon. Antithetic and synthetic normal faults have developed, producing complex upward branching fault systems. In map view, the listric faults form curvilinear en echelon arrays, the faults linked by relay ramps. Fault blocks are typically 3–7 km wide, 2–3 km thick and 7–10 km long. Salt movement during the Jurassic-Tertiary has been driven by active extension of the cover, causing salt to fill potential voids created by fault block rotation. Thus salt highs occur beneath sites of extension. The listric faults generally dip in the same direction as the sub-salt surface, although there are also some major counter-regional faults. During extension, regional dips have increased up to about 5, which is sufficient for gravity-driven extension above a salt layer. A total extension of about 6% has occurred. The gravity-driven thin-skinned extension documented in the western central North Sea is a phenomenon which can be recognized elsewhere in the North Sea basin, and can be readily compared with similar phenomena already documented in offshore Angola, Brazil, Nova Scotia and the Gulf of Mexico.
The relative importance of petroleum emplacement in inhibiting diagenetic processes and preserving porosity and permeability in Lower Cretaceous, Thamama Group (Kharaib Formation) carbonate reservoirs of Abu Dhabi, UAE, and in Callovian-Kimmeridgian carbonate reservoirs of the Amu Darya Basin in Uzbekistan and Turkmenistan, has been evaluated by combining geologic, petrophysical and geochemical data. When petroleum emplacement is synchronous with and prior to significant burial cementation in carbonates, primary petroleum inclusions are trapped in the cements. The process appears to be characterised by steep intra-field porosity-depth trends within a more gradual regional decline in porosity with depth. This has profound implications for the prediction of porosity in carbonate reservoirs.
The concentration of Ca in the formation waters of petroleum reservoirs can play a major role in influencing the outcome of a number of processes that are of great significance to the oil industry. For example, formation water Ca concentration affects the risk of carbonate scale formation during production. In order to better understand the concentrations of Ca in formation waters, we have investigated the chemistries of formation waters from a range of onshore and offshore basins worldwide, using published sources, as well as unpublished data held by BP. Although calcium and sodium are the principal cations in almost all formation waters they vary enormously in their relative proportions. We have identified three distinct trends on a plot of XCa (Ca/(Na + Ca)) against Cl. Most data lie on a high-Ca trend, here termed Trend 1, and show an increase in XCa with salinity. We interpret this as tracking equilibration with Ca and Na-bearing minerals, with the ratio (mol Ca/mol Na2) remaining approximately constant irrespective of salinity for chloride-dominated fluids. At very high salinities, Br-enriched bittern brines that have taken part in dolomitisation lie at the Cl-rich end of this trend. Some brines remain Na-dominated up to very high salinities and define a distinct low-Ca trend, Trend 2. These are associated with dissolution of halite beds and are interpreted to arise when the amount of Na in the pore fluid greatly exceeds the amount of Ca available in minerals. We refer to such brines as mass-limited; the sparsity of Ca in the rock-fluid system constrains XCa to a low value. Remarkably few brines lie between these trends. Finally, dilute formation waters show very large variations in XCa and may have bicarbonate as the dominant anion. They define a distinct low-Cl trend, Trend 3. We conclude that the behaviour of Na and Ca in most formation waters reflects equilibration with minerals, and concentrations of Ca in solution are sensitive to pH and PCO2 as well as to chloride concentration. For some brines however, the amount of salts in solution is sufficient to overwhelm the buffering capacity of the wallrocks.Highlights► There are 3 distinct trends for Ca and Na in oilfield waters globally. ► Many brines are rock-buffered, but some swamp the exchange capacity of the host. ► NaCl-dominated brines arise from dissolution of halite. ► The most CaCl2-rich brines are bittern brines that participated in dolomitisation. ► Variations in brine chemistry do not reflect changes in seawater through time.
The mol% CO2 of gas reservoired in sandstones tends to increase with increasing temperature along both the US Gulf Coast and the Norwegian continental shelf. Each of these provinces displays identical linear correlations between temperature and the log of the partial pressure of CO2 (i.e. PCO2) in the reservoir fluid (that is, the product of the mol% CO2 × the reservoir pressure). This correlation is the same both in Gulf Coast reservoirs younger than 17 Ma and in Norwegian reservoirs of Jurassic to Triassic age (144–245 Ma), indicating that it is not dependent on reaction kinetics. Norwegian shelf data indicate that the same correlation also applies for gas/condensate and oil reservoirs.We suggest that the log PCO2versus temperature correlation is the result of inorganic chemical equilibria between feldspar, clay, and carbonate minerals. A trend of decreasing pH with increasing temperature is fixed by equilibria between aluminosilicate minerals: feldspar-kaolinite at lower temperatures, followed by various reactions between kaolinite, illite, chlorite, and feldspar as temperatures exceed 110–140°C. Responding to the pH set by the aluminosilicate system, CO2 activity in the formation water and thereby CO2 partial pressure in any gas phase present is controlled by equilibrium with carbonate minerals. This model predicts that CO2 released by organic maturation will, in general, result in precipitation of carbonate minerals below 100–120°C and that there should be widespread carbonate dissolution at higher temperatures.
The French Guiana transform margin and Demerara abyssal plain have been recently surveyed in the framework of the EXTRAPLAC French Program of extension of the continental shelf (Guyaplac survey, Ifremer-IFP–SHOM-IPEV). Based on the interpretation of some of the data collected during the Guyaplac survey (Simrad-EM12 multibeam bathymetric data, backscatter imagery, and 3.5 kHz profiles), the area can be divided into three morphostructural domains.(1)The western Guiana margin, including a part of the Demerara plateau, an important bathymetric relief prolonging the continental platform off Guiana and Surinam. This domain is bounded by (1a) the NW–SE trending northern border of the Demerara plateau which appears quite steep and corresponds to a transform segment of the margin, (1b) the N–S eastern border of the Demerara plateau which corresponds to a divergent segment of the margin.The Demerara plateau shows a segmented morphology, low slope gradients, and a very rough surface (ripples perpendicular to the slope direction). NNW–SSE structural steps seem to correspond to collapses of 100 km long blocs towards the east. Slumps initiate along these directions. The observed rough bathymetry seems to be related to creeping processes. At a greater scale (seismic data), this part of the margin has been totally destabilized (numerous imbricate transparent masses rooted at about 0.5 s.t.w.t.t. below seafloor). The NW–SE trending northern border of the Demerara plateau corresponds to a cliff-like continental slope, probably slightly smoother than other transform margins (Ghana/côte d'Ivoire margin).The N–S eastern border of Demerara plateau is characterized by numerous small-scale imbricate slumps. Some of these failures seem to be emplaced in the prolongation of the NNW–SSE structural steps identified on the Demerara plateau.(2)The eastern Guiana margin corresponds to a NW–SE oriented gullied transform margin segment. The associated continental slope is very steep and characterized by numerous imbricate slumps and related debris flows. Some undulated masses, probably corresponding to creeping sediments or to older mass-wasting events are still imprinted on bathymetry. This transform margin segment is nearly entirely destabilized and eroded.(3)The Demerara abyssal plain. This domain is characterized eastwards by channels belonging to the Amazon turbidite system and westwards, at the foot of Demerara continental slope, by sediment waves probably created by contour currents.To conclude, it seems that there is a strong relationship between the structure (transform and divergent segments) and the emplacement of recurrent slope instabilities. These are probably related to the steepness of the slopes but also to subsidence histories generating in some cases huge deep-seated collapses of the whole margin. Fluid ascents are common everywhere in the area, probably enhancing slope instability. Their origin is not constrained but the black shales or Cretaceous organic-rich layers could be good candidates.
The Lower Miocene Green Channel Complex from the Dalia M9 Upper Field, Block 17, offshore Angola is an excellent example of a deepwater sinuous channel. This sinuous Channel Complex is located in the upper portion of a Confined Channel System, which is approximately 150 m deep, 2 km wide, and tens of kilometers long. The Green Channel Complex itself is approximately 40 m deep and 2 km wide and was formed by the lateral migration and local avulsion of a single channel that was approximately 300 m wide and 40 m deep.
In the Castle Creek study area, a vertically dipping, 2.5 km-thick succession of basin-floor to base-of-slope Neoproterozoic rocks are superbly exposed. In part of that outcrop, inner-bend (point-bar) deposits of sharp-based, laterally accreting sinuous channels are exposed, of which one is described in detail (Isaac Channel unit 2.2—IC2.2). IC2.2 is up to 13 m thick and extends laterally for at least 400 m. Lateral-accretion deposits, or simply lateral accretion deposits (LADs), are inclined at 7–12° toward the channel base and are about 120–140 m long. Grain size changes little obliquely upward along an individual LAD, or vertically upward through the channel-fill. LADs consist of two repeating and interstratified kinds: coarse-grained LADs consisting of strata up to granule conglomerate, and fine-grained LADs composed of thin- to medium-bedded finer-grained turbidites. In the lower part of the channel-fill, strata consist only of amalgamated coarse-grained LADs composed of decimetre-thick beds composed of very coarse sandstone/granule conglomerate that grade upward to medium sandstone. Tractional sedimentary structures are absent and fine-grained strata, specifically mudstone, occur only as isolated patches of intraclast breccia. In the upper part of the channel-fill, however, LADs consist of a rhythmic interfingering of coarse- and fine-grained LADs. Coarse-grained LADs consist of 2–3 bed-thick packages that are separated and then pinch-out rapidly into fine-grained LADs. Close to their up-dip pinch-out these coarse strata consist commonly of poorly sorted, ungraded very coarse sandstone/granule conglomerate overlain abruptly by planar-laminated or medium-scale (dune) cross-stratified, medium-grained sandstone. Fine-grained LADs are composed of mudstone interbedded with thin- and medium-bedded Tbcd and Tcd turbidites that obliquely downward and become truncated as the super- and subjacent coarse-grained LADs amalgamate.
Existence of gas-hydrate in the marine sediments elevates both the P- and S-wave seismic velocities, whereas even a small amount of underlying free-gas decreases the P-wave velocity considerably and the S-wave velocity remains almost unaffected. Study of both P- and S-wave seismic velocities or their ratio (VP/VS) for the hydrate-bearing sediment provides more information than that obtained by the P- or S-wave velocity alone for the quantitative assessment of gas-hydrate. We estimate the P- and S-wave seismic velocities across a BSR (interface between gas-hydrate and free-gas bearing sediments) using the travel time inversion followed by a constrained AVA modeling of multi channel seismic (MCS) data at two locations in the Makran accretionary prism. Using this VP/VS ratio, we then quantify the amount of gas-hydrate and free-gas based on two rock-physics models. The result shows an estimate of 12–14.5% gas-hydrate and 4.5–5.5% free-gas of the pore volume based on first model, and 13–20% gas-hydrate and 3–3.5% free-gas of the pore volume based on the second model, respectively.
Broad-range side-scan sonar (GLORIA) images and single- and multi-channel seismic reflection profiles demonstrate that the margin of north-western Hispaniola has experienced compression as a consequence of oblique North American-Caribbean plate convergence. Two principal morphological or structural types of accretionary wedges are observed along this margin. The first type is characterized by a gently sloping (≈4°) sea floor and generally margin-parallel linear sets of sea-floor ridges that gradually deepen towards the flat Hispaniola Basin floor to the north. The ridges are caused by an internal structure consisting of broad anticlines bounded by thrust faults that dip southwards beneath Hispaniola. Anticlines form at the base of the slope and are eventually sheared and underthrust beneath the slope. In contrast, the second type of accretionary wedge exhibits a steeper (≈6–16°) sea-floor slope characterized by local slumping and a more abrupt morphological transition to the adjacent basin. The internal structure appears chaotic on seismic reflection profiles and probably consists of tight folds and closely spaced faults. We suggest that changes in sea-floor declivity and internal structure may result from variations in the dip or frictional resistance of the décollement, or possibly from changes in the cohesive strength of the wedge sediments. The observed pattern of thickening of Hispaniola Basin turbidites towards the insular margin suggests differential southwards tilting of the Hispaniola Basin strata, probably in response to North America-Caribbean plate interactions since the Early Tertiary. Based upon indirect age control from adjacent parts of the northern caribbean plate boundary, we infer a Late Eocene to Early Miocene episode of transcurrent motion (i.e. little or no tilting), an Early Miocene to Late Pliocene period of oblique convergence (i.e. increased tilt) during which the accretionary wedge began to be constructed, and a Late Pliocene to Recent episode of increased convergence (i.e. twice the Miocene to Pliocene tilt), which has led to rapid uplift and erosion of sediment sources on the margin and on Hispaniola, generating a submarine fan at the base of the insular slope.
The Maury Channel is a deep-sea sediment transport system located in the Iceland Basin and extends from the Icelandic plateau southwards towards the Charlie-Gibbs Fracture Zone (CGFZ). This study has utilised multibeam bathymetry and multi-channel seismic reflection survey data along 480 km of its 1200 km pathway. In the northern reach of the channel it is predominantly broad (>20 km) and shallow (∼10 m). Further to the south the channel narrows (5–10 km) and locally deepens to 150 m prior to finally discharging onto the Eriador Plain to the north of the CGFZ. DSDP Site 115 in the Iceland Basin can provide insight into the evolution of the system as it sampled a suite of volcaniclastic turbidites of unequivocal Icelandic provenance. This sequence produces distinct amplitude anomalies on seismic reflection profiles allowing it to be mapped over an area of at least 26,000 km2. The southern edge of the high velocity unit is delimited by onlap onto the flanks of the Miocene (and younger) Gardar Drift. The drift appears to have initially acted as a barrier to southerly flows and promoted ponding of flows in the Maury Fan. Continued sediment supply from Iceland eventually filled the Maury Fan leading to the overspilling of the Gardar Drift dam. A result was the initiation of the Maury Channel. To the south of the drift, where the seabed is steep, flows are confined to the channel, whereas to the north of the drift, where the gradient is less, unconfined flow pathways dominate. The Maury Channel system highlights the interaction between turbidity currents and bottom currents on abyssal plains. The growth of sediment drifts not only mould the seafloor through their bathymetric development but also, through the building of seafloor topography, influence the passage and behaviour of gravity-driven sediment-laden flows along the seafloor.