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The Maracangalha injectite complex: An overlooked hydrocarbon play in the Lower Cretaceous Recôncavo Basin, NE Brazil
... Paleocurrent readings indicate transport towards the southwest, with slight deviations towards the southeast. The studied interval shows Soft Sediment Deformation Structures (SSDS), such as penecontemporaneous faults, balls and pillows, small-scale sand injections and various degrees of sand and mud mixing, which in turn affect primary porosity and permeability (Silveira et al., 2023). This occurrence causes heterogeneities within the deposits, impacting reservoir predictions. ...
... Paleocurrent readings indicate transport towards the southwest, with slight deviations towards the southeast. The studied interval shows Soft Sediment Deformation Structures (SSDS), such as penecontemporaneous faults, balls and pillows, small-scale sand injections and various degrees of sand and mud mixing, which in turn affect primary porosity and permeability (Silveira et al., 2023). This occurrence causes heterogeneities within the deposits, impacting reservoir predictions. ...
... The prodelta deposit of the Morro do Camaragibe outcrop was significantly affected by postdepositional processes such as upward migration of interstitial fluids, contemporary seismic activity, among others, "resulting in a mixture of mud and sand that affects facies distribution" (Silveira et al. 2023), leading to heterogeneities in its deposits and so impacting reservoir predictions. The outcrop shows both deformed and undeformed facies associated with different types of Soft Sediment Deformations (SSDS), including sand injections. ...
The late Aptian Maceió Formation is often used as an outcrop analogue for offshore reservoirs in the Brazilian marginal basins. Although its deposits are well-known for a long-time, processes involved in their sedimentation and facies distribution are still poorly explored. Its deposits are associated with fan deltas that migrated and transported sediments along the basin axis forming channels and lobes basinwards (Morro do Camaragibe outcrop, focus of this study), favoring the generation and deposition of sediment gravity flows in various ways. In the succession exposed in this area, the following architectural elements were identified: proximal turbidite lobes, intermediate lobes, and distal lobes. These deposits were significantly affected by post-depositional processes, resulting in a mixture of mud and sand that impacts the quality of the reservoirs. The main objective of this work is to produce a high-resolution sedimentological description at a detailed scale (1:20), as well as individual facies tract along key depositional events within the succession. Sedimentary logs combined with petrographic data and gamma ray profiles acquired from CPS (counts per second) helped analysis assisted heterogeneity analysis. Moreover, a drone was used for a three-dimensional visualization of the outcrop and its depositional architecture. Outcomes of this work can be used to predict geometries, facies trends and distributions in lobe complexes as an analogue to support seismic and borehole data for hydrocarbon exploration.
... The Apraius Miranga-Norte diapir has a northwest-southeast trend, which would be coincident with that of the Itanagra-Araças transfer fault (Destro et al., 2003), which is 10 km away from the diapir. However, 3D seismic images do not show a direct and clear relationship between the basement faults and this diapir, despite previous works (Destro et al., 2003;Silveira et al., 2023). The basement faults probably did not act on the Pedra do Salgado diapir, whereas the Carijó diapir is broadly influenced by a large listric fault confined to the rift section. ...
Mud diapirs are present in different tectonic environments around the globe and are particularly prevalent in large deltas. The onshore, Jurassic-Cretaceous Recvo Basin NE Brazil, represents an atypical example of mud diapir occurrence in a rift basin. The intensity of mud diapirism is different in the three sectors of the basin, with the central sector exhibiting the best development In the southern sector, diapirism is less significant, and it is practically non-existent in the northeast sector. 3D seismic, outcrop and well data were used to geomorphologically characterize the onshore mud diapirs in the central sector of the Recvo Basin and correlate them with the structural and morphological aspects of the basin, in addition to elucidating the possible origins of their movement. After seismic interpretation, four mud diapirs were identified in the study area: Apraius-Miranga Norte, Pedra do Salgado, Biriba and Carijwo internal seismic facies were identified as F1 and F2, with F1 presenting more disruptive features and F2 being more reflection-free. The eastern diapirs, close to border fault conglomerates, are dominated by disrupted seismic facies (F1). The diapiric movement is associated with paleo-seismicity and the rupture of an ancient delta front together with rapid basinward advancement of conglomerates during an active rifting phase. The Biriba diapir was possibly the first to arise, followed by Carijpraius-Miranga Norte and lastly Pedra do Salgado; this variable timing influenced the location and migration of depocenters in the area. #xD;Keywords: Mud diapir; Recvo Basin; 3D seismic interpretation. #xD;
This paper details and describes a suite of 143 sub-seismic-scale clastic injectites encountered within the early Cretaceous, early post-rift of the deep-lacustrine North Falkland Basin. The injectites, referred to here as the Sea Lion Injectite System (SLIS), are encountered below, above and in-between the hydrocarbon-bearing, deep-lacustrine turbidite sandstones of the Bleaker 30, Sea Lion North, Sea Lion, Casper and Beverley fans. Sedimentary structures are documented within the injectites including: planar laminations, mud-clast imbrication and clast alignment. Clasts align along cm-scale foresets formed through ripple-scale bedform migration in a hydraulically-open fracture. The style of flow within the injectite system is interpreted as initially through fluid turbulence during an open fracture phase, which was followed by a later stage where laminar flow dominated, most likely during the closing phase of the fracture system. The host rocks display evidence for ductile deformation, which along with ptygmatic folding of dykes and internally injected mud-clasts, suggests a period of injection into relatively uncompacted sediments. Evidence for brittle fracturing, in the form of stepped margins may be indicative of a separate phase of emplacement into more-compacted sediments. This variability in deformation styles is related to multi-phased injection episodes into host strata at different stages of consolidation and lithification at shallow burial depths. Injectites have been identified in four stratigraphic groupings: above the Bleaker 30 Fan and within/above the Sea Lion North Fan; within the hydrocarbon-bearing Sea Lion Fan; overlying the Sea Lion Fan; and above/below the hydrocarbon-bearing Casper and Beverley fans. This spatial association with the hydrocarbon-bearing fans of the North Falkland Basin is important, considering the ability of injectite networks to form effective fluid-flow conduits in the subsurface. Consequently, the findings of this study will improve the characterization of sub-seismic scale injectites (and therefore fluid conduits) within otherwise impermeable strata.
The objective of the present study was to interpret the direction of gravitational sedimentary flows in the turbidite deposits of the Maracangalha Formation (Recôncavo Basin), indicating their source areas. Thus, the indicative deformational structures existing in the formation were analyzed. The structures observed ranged from brittle to highly ductile and viscous states, with a diversity of deformational styles. A total of 284 planar and linear measurements was collected in three distinct subareas, separated according to their degree of deformation. The structures found were separated into four groups according to their formation process: (i) pre-deformation structures; (ii) plastic deformation structures; (iii) intrusion structures (fluidization), related to either early or syn-sedimentation events; and finally (iv) brittle deformation structures, with late sedimentation. Some structures were observed to be good indicators of apparent mass movement direction, which yielded predominantly SSW directions, suggesting a partially confined flow, parallel to the main axes of Recôncavo Basin, towards its depocenter.
Empty elliptical vesicles are observed in outcrops of Barremian very fine clayey sandstone to siltstone lacustrine slurry deposits of the Pitanga Member (Maracangalha Formation), exposed in the Maré Island, Southern Recôncavo Basin, Brazil. These sedimentary features have been traditionally interpreted as water escape structures triggered by the diapirism of the underlying shales of the Candeias Formation. This work proposes that vesicles were generated during massive gas hydrate dissociation as a result of tectonic activity in a paleolake system. Tectonic uplift would have triggered both the reduction of the confining pressure as well as an increase in lake bottom temperature, resulting in the instability of gas hydrate and causing intense release of both methane - or carbon dioxide (CO2) - and water. On one hand, this proposal has a strong impact on paleoenvironmental interpretations, giving support to the current hypothesis that rocks related to the Pitanga Member would have been deposited under water columns deep enough for gas hydrate formation and subsequent dissociation. On the other hand, it provides new insights on the genesis of fluid escape structures in sedimentary rocks, both lacustrine and marine, providing a paleobathymetric indicator.
L arge-scale vertical to subvertical clastic intrusions (as much as 67 m [219 ft] wide and >100 m [>330 ft] high) are present in Cretaceous strata (Cerro Toro Forma-tion) of the Ultima Esperanza district, southern Chile. The injectites emanate from the margins of submarine-channel deposits that accumulated at water depths of 1000–2000 m (3300–6600 ft) in the Magallanes foreland basin. The remobilized sediment is very coarse, consisting of sandy matrix conglomerate, muddy matrix con-glomerate, and poorly sorted sandstone. The injectite bodies sometimes bifurcate upward and are circular in plan view and, thus, are geometrically analogous in many respects to numerous injection features mapped seismically in the North Sea Basin. The remobilization of coarse sediment was likely induced after the burial of the parent deposit to at least a few hundred meters. The controlling factors on injection are difficult to discern; however, it is probable that the highly energetic process in-volved gas charging of the source body and, potentially, a seismic event trigger asso-ciated with the uplift of the Patagonian Andes. 17 Hubbard, S. M., B. W. Romans, and S. A. Graham, 2007, An outcrop example of large-scale conglomeratic intrusions sourced from deep-water channel deposits, Cerro Toro Formation, Magallanes basin, southern Chile, in A. Hurst and J. Cartwright, eds., Sand injectites: Implications for hydrocarbon exploration and production: AAPG Memoir 87, p. 199 – 207.
Consolidation laminae and dish structures form high-density zones that are interpreted to have formed by disruption of primary structures during gravitational collapse of the grain framework during water escape and consolidation. They are not associated with higher contents of clay-size material than adjacent units, but the chloritic clay minerals associated with consolidation laminae and dish structures have a different microtexture than observed elsewhere. Clay mineral texture has a direct effect an petrophysical characteristics, in particular water saturation and conductivity in hydrocarbon-saturated intervals. CT-scans identify consolidation laminae and dish structures as zones of high density that correspond to tighter packing of sand grains and are unrelated to clay distribution. Dish structures may form independently of consolidation laminae or by further modification of fragmented consolidation laminae.
Release faults are rift cross faults, which develop to accommodate the variable displacements of the hanging-wall block along the strike of normal faults. Release faults are nearly perpendicular or obliquely oriented to the strike of the normal fault they are related to. They have maximum throws adjacent to the parent normal fault. and die out in the hanging wall away from it. They form to release the bending stresses in the hanging wall and do not reflect the orientation of the regional stress field in a basin. Commonly, they show normal-oblique displacements and are preferentially located along the strike ramps. Release faults may also act at the scale of an entire basin, reaching displacements of thousands of meters. Joints, shale, and salt diapirs may develop in association with release faults. Because all these structures represent domains of stress release, they may work as conduits for oil migration and oil traps in extensional basins. This is the case of the Reconcavo basin in northeastern Brazil, a Cretaceous failed rift, connected to the eastern Brazilian continental margin basins. In the Reconcavo basin, two large-scale release faults, with displacements in the order of 3 km, developed in the hanging wall of the rift border faults and control the location of the main oil fields.
Sandstone intrusions are found in all sedimentary environments but have been reported most commonly from deep-water settings. They also appear to be more frequently developed in tectonically active settings where applied tectonic stresses facilitate development of high fluid pressures within the sediments. A variety of mechanisms have been cited as triggers for clastic intrusions. These include seismicity induced liquefaction, application of tectonic stresses, excess pore fluid pressures generated by deposition-related processes and the influx of an overpressured fluid from deeper within the basin into a shallow sand body. The formation of sandstone dykes and sills is investigated here by considering them as natural hydraulic fractures. When the seal on an unconsolidated, overpressured sand body fails the resulting steep hydraulic gradient may cause the sand to fluidize. The fluidized slurry can then inject along pre-existing or new fractures to form elastic intrusions. The scale and the geometry of an intrusive complex is governed by the stress state, depth and pre-existing joints or faults within the sedimentary succession, as well as the nature of the host sediments. For the simplest tectonic setting, where the maximum stress in a basin is vertical (gravitational loading), small irregular intrusions commonly result in the formation of sills at shallow depths within a few metres of the surface, whereas at greater depth dykes and sills forming elastic intrusion networks are more typical. A simple relationship is derived to calculate the maximum burial depth at which a dyke-sill complex forms as a function of the source-bed to sill height, the bulk density of the surrounding sediments, and the ratio of the vertical to horizontal effective stresses, K-0. When applied to three examples of large-scale dyke-sill complexes developed within Paleocene and Eocene deep-water reservoir sand bodies of the North Sea, maximum burial depths in a range of 375 to c. 500 m, 450-700 m and 550-850 m are estimated for intrusion of each of the three complexes.
The Nauchlan Member of the Late Eocene Alba Formation (UK North Sea) consists of a deep-water channel fill that was extensively modified by post-depositional sand remobilization and injection. Sandstone textures, facies associations and the geometry of the channel fill were affected. A suite of sand-rich facies was produced by large-scale fluidization and injection within the channel fill and above it. These facies, termed here unstratified facies, are characterized by the absence of stratification surfaces and by discordant relationships with bedding in the adjacent succession. They reflect variable degrees of disruption of the primary sedimentary structures caused by escaping pore fluid, the velocity of which is estimated at least in the order of 0·1 ms−1. Adjacent mudstones were severely disrupted by hydraulic fracturing, and fragments of fractured mudstone were incorporated into the fluidized sand. Average porosity was decreased in the sandstones affected by fluidization. Two main phases of sand injection are inferred to occur at different burial depths. A shallow burial phase (below 100 m) produced thin dykes with ptygmatic folds. The second phase occurred at the boundary between Eocene and Oligocene (≈ 300 m burial depth) and resulted in large-scale tabular wing-like dykes that project from the edges of the channel fill. The significant pore-fluid overpressure, which was required to hydraulically fracture the thick mudstone seal and to fluidize the large volume of sand, was likely to be built up by static liquefaction of the source sand and was possibly enhanced by hydrocarbon gas influx.
Three processes of water escape characterize the consolidation of silt‐, sand‐and gravel‐sized sediments. Seepage involves the slow upward movement of pore fluids within existing voids or rapid flow within compact and confined sediments. Liquefaction is marked by the sudden breakdown of a metastable, loosely packed grain framework, the grains becoming temporarily suspended in the pore fluid and settling rapidly through the fluid until a grain‐supported structure is re‐established. Fluidization occurs when the drag exerted by moving pore fluids exceeds the effective weight of the grains; the particles are lifted, the grain framework destroyed, and the sediment strength reduced to nearly zero. Diagenetic sedimentary structures formed in direct response to processes of fluid escape are here termed water escape structures.
Four main types of water escape structures form during the fluidization and liquefaction of sands: (1) soft‐sediment mixing bodies, (2) soft‐sedimsnt intrusions, (3) consolidation laminations, and (4) soft‐sediment folds. These structures represent both the direct rearrangement of sediment grains by escaping fluids and the deformation of hydroplastic, liquefied, or fluidized sediment in response to external stresses.
Fundamental controls on sediment consolidation are exerted by the bulk sediment properties of grain size, packing, permeability, and strength, which together determine whether consolidation will occur and, if so the course it follows, and by external disturbances which act to trigger liquefaction and fluidization. The liquefaction and fluidization of natural sands usually accompanies the collapse of loosely packed cross‐bedded deposits. This collapse is commonly initiated by water forced into the units as underlying beds, especially muds and clays, consolidate. The consolidation of subjacent units is often triggered by the rapid deposition of the sand itself, although earthquakes or other disturbances are probably influential in some instances.
Water escape structures most commonly form in fine‐ to medium‐grained sands deposited at high instantaneous and mean sedimentation rates; they are particularly abundant in cross‐laminated deposits but rare in units deposited under upper flow regime plane bed conditions. Their development is favoured by upward decreasing permeability within sedimentation units such as normally graded turbidites. They are especially common in sequences made up of alternating fine‐(clay and mud) and coarse‐grained (sand) units such as deep‐sea flysch prodelta, and, to a lesser extent, fluvial point bar, levee, and proximal overbank deposits.
The Recôncavo-Tucano-Jatobá rift consists of a series of asymmetric grabens which are separated by basement highs and transfer faults. Opening of the rift took place in a NW direction oblique to the N-S rift trend. Well defined transfer faults parallel the opening direction. They were responsible both for offsetting en-echelon depocenters in the Tucano and Recôncavo basins and for switching of the asymmetry of half-grabens across the Vaza-Barris fault zone. The transfer faults show characteristic features such as change of movement sense along strike and with time, and “cactus-shaped” fault structures as well as flower structures in cross section.The sigmoidal plan-form of the rift is probably due to faulting following pre-existing weaknesses through a complex mosaic of basement blocks. Basement anisotropy is thought to have controlled the asymmetry of the half-grabens and the localisation of the Vaza-Barris transfer.A 2° anticlockwise rigid rotation of the East Brazilian Microplate relative to the São Francisco Craton around a pole located near the eastern termination of the Jatobá Graben describes the calculated 20% extension in the South Tucano and Recôncavo grabens, the oblique northwestward opening, and the eastward shallowing of the Jatobá Graben.Gravity modelling suggests important crustal thinning, locally up to 45%, below the rift. Mantle upwarping is localized near the faulted margin of the rift. With localized thinning in a 100 km wide basin during a 20 Ma rifting event, lateral, as well as vertical heat conduction would be important and may account for the absence of a post-rift thermal subsidence phase.
Almost two hundred years of research is reviewed that focuses on the physical characteristics of sandstone intrusions. It is concerned with mechanisms of sand injection, particularly with fluid-grain transport and sedimentation processes during the remobilization, injection and extrusion of sand. Outcrop and subsurface studies in combination with laboratory experimental data are drawn on to present the state-of-the-art of sand injection. The text covers 1) geometry, internal structure, and microtexture of deformed parent units, injected and extruded sandstones, 2) host-strata and their seal characteristics that contribute to basin-wide overpressure generation, 3) common trigger mechanisms for sand injection such as high magnitude seismicity and the rapid injection of large volumes of fluids, 4) fluid types that drive sand into fractures, 5) hydrofracture mechanisms that induce regional-scale seal failure, 6) liquefaction and fluidization processes that transport sand into fractures, 7) sedimentation processes in fractures, 8) the flow regime of fluidized sand during injection, 9) post-sand-injection fluid flow and diagenesis, 10) porosity and permeability characteristics of injected sandstones and 11) post-sand-injection fluid-flow over geological timescales. Processes of sand remobilization, injection, and extrusion are complex and depend on many interrelated factors including: fluid(s) properties (e.g. pressure, volume, composition), parent unit and host-strata characteristics (e.g. depositional architecture, grain size and distribution, clay-size fraction, thickness, permeability) and burial depth at the time of injection. Many studies report erosional contacts between host strata and injected sands and these record high-velocity, erosive flow during injection. The flow regime is poorly constrained and similar features are interpreted as records of laminar and turbulent flow, or both, during injection. Internal structures are common in sandstone intrusions and can be accounted for by a variety of processes. The interpretational limits largely result from a lack of laboratory experiments that focus on developing analogues for sand injection. The relationship between grain fabric developed during injection and its control on permeability in sandstone intrusions is poorly understood and failure to advance this field of research will hinder the quantitative characterization of sandstone intrusions as fluid-flow conduits during basin evolution. We conclude that future research should focus on: 1) quantification of sediment transport modes under different flow conditions in different fracture dimensions with laboratory data relevant to sand injection; 2) estimation of the effect of injection on the bulk permeability of otherwise low-permeability seals (host strata) so that their effect on fluid flow can be assessed at all scales; and 3) incorporation of sand injection into quantitative basin models. Although an enormous amount of data have arisen from existing studies there remains a need to advance many fields of research related to sand injection so that the significance of these important structures can be fully appreciated in the geological record.
A major intrusive sandstone complex of Late Jurassic age is spectacularly exposed in Jameson land, East Greenland. It is probably the largest in the World, and covers an area of 55x70 km with a thickness of 200–400 m, and forms the Upper Oxfordian–Volgian Hareelv Formation. The complex consists of black basinal mudstones and highly irregular sandstone bodies, dykes and sills. The sand was derived from collapse of the front of sandy shelf-margin wedges, which triggered hyperconcentrated to concentrated density flows, and deposited massive sands further down the slope, at the base-of-slope and in the basin. The sand of some flows was loaded into the slope muds while elsewhere it flowed in steep-sided gullies formed by retrogressive slumping of the slope muds. All sand bodies were liquefied subsequent to burial and the sand was intruded into the surrounding black compacted muds and mudstones. Intrusion took place repeatedly over a long time interval, in environments ranging from very shallow to relatively deep burial, and the primary sediment structures of the sands were generally lost during these processes. It is rarely possible to determine the degree of post-burial remobilization but it ranges from rather small-scale modifications to wholesale liquefaction and out-of-place intrusion of the sand over many tens of metres. Sandstone dykes and sills occur ubiquitously and were emplaced by all combinations of stoping and dilation. The intrusive sand bodies range in dimensions from centimetres to many hundreds of metres. Deposition took place during the most important Mesozoic rift event in East Greenland and the pervasive remobilization and liquefaction of all sand bodies in the Hareelv Formation is interpreted as having been caused mainly by cyclic earthquake shocks. Additional important factors were slope shear stress, build up of pore pressure due to loading, slumping, upwards movement of pore waters expelled from the compacting muds, and also possibly of biogenic and thermogenic gas. The Hareelv Formation is an excellent field analogue for deeply buried hydrocarbon reservoirs, which have been modified by remobilization and injection of the sands.
Giant sand injection complexes form, intricate, basin-scale fluid plumbing systems and document the remobilisation and intrusion of several tens of cubic kilometres of sand within the shallow crust in stratigraphic units 100's metres thick. This is the first detailed and extensive account of the Panoche Giant Injection Complex (PGIG), a regionally significant outcrop (>300 km2) and part of a larger subsurface development (>4000 km2) identified in boreholes and on seismic reflection data. Magnificent exposure of the PGIC occurs along the north western margin of the San Joaquin Valley and presents the opportunity to examine the regional geological significance of a giant sand injection complex and its origin in the context of a late Cretaceous – early Paleocene forearc basin. Between 25 and 49 km3 of sand were remobilised and injected, at least 0.35 km3 of which extruded onto the paleo-seafloor. Large sandstone intrusions often >10 m thick and laterally extensive on a kilometer scale formed saucer-shaped intrusions, wing-like intrusions and a variety of sill geometries along with volumetrically smaller randomly oriented dikes in a 200–300 m thick interval. Dikes prevail below and above this interval, some reaching the paleo seafloor and extruding sand. Networks of propagating hydrofractures form intensely brecciated host strata, some of which were intruded by sand. All intrusions formed in a single pulsed event in which the most intense hydrofracturing caused by supra-lithostatic fluid pressure occurred approximately 600 to 800 m below the paleo seafloor. A crudely orthogonal arrangement of dikes is preserved with most oriented normal, and less commonly oriented parallel to the oceanic trench associated with the late Mesozoic to early Tertiary North Pacific subduction. Dikes orthogonal to the trench opened against the minimum horizontal stress, which was parallel to the trench. Dikes parallel to the trench opened against the regional maximum horizontal stress along minor faults formed in extension caused by shallow crustal deformation. There is no evidence that compressional tectonics influenced the onset of elevated pore fluid pressure necessary to promote sand injection. However, tectonic compression was responsible for creating the basin physiography that locally increased subsidence and accelerated chemical diagenesis in the basin centre. PGIC outcrop, located along the basin margins, was unlikely to have experienced heating above 70 °C, equivalent about 2 km burial, so the effects of chemical diagenesis in the host strata of the injection complex had negligible potential to evolve significant pore water volume. In a deeper part of the basin approximately 150 km to the south, lateral equivalents of the host strata were subjected to heating >100 °C and would expel significant volumes of water displaced by quartz cementation and clay dehydration that caused lateral pressure transfer to the north and western margin of the basin where the PGIC formed. Estimates of the total volume of water expelled from the deep basin suggest that a fluid volume equivalent to a gross rock volume reduction <1% would have provided a fluid budget sufficient to fluidise and inject the sand that forms the PGIC. In terms of areal and vertical extent, volume and architecture the PGIC shares strong similarity with the regionally developed giant injectite systems of Tertiary age in the North Sea basin. In both cases regional sand injection is genetically linked to pressure transfer toward the basin margin from more rapidly subsiding basin centres. Aqueous fluid is derived from thermally driven chemical diagenesis of thick deep water clastic sandstone and smectitic mudstone or from deeper, stratigraphically older, aquifers.
The Early Cretaceous chronostratigraphic positioning of the northeast Brazilian basins is mainly based on ostracod distribution, including a biozoning proposal of local stages. The ostracod species were recovered from shales and siltstones samples along Gameleira, Manguinhos and Praia da Falha outcrops of the Maracangalha Formation (Recôncavo Basin). In the analyzed samples, six genera were recognized, out of a total of 1036 specimens. The non-marine ostracod fauna consists of the species Cypridea lunula Krömmelbein1962, Cypridea ellipsoidea Krömmelbein and Weber1971, Cypridea aff. C. vulgaris Krömmelbein1962, Cypridea sp. 1, Cypridea sp. 2, Cypridea sp. 3, Cypridea spp., Reconcavona? polita Viana, 1966, Reconcavona sp., Ilhasina amphotera Krömmelbein1963, Ilhasina sp. aff. I. remanei cuneiformis Krömmelbein1963, Ilhasina sp., Candona? condensa Krömmelbein1962, Brasacypris sp. and Alicenula sp. The biochronostratigraphic position of the Manguinhos outcrop in the Rio da Serra Stage (Berriasian–Valanginian) was possible through the identification of the guide species Reconcavona? polita, index in subzone RT-004.2. In the Praia da Falha outcrop the faunal association suggests an age range between the late Rio da Serra to Aratu Stage (Valanginian-Hauterivian), through the recording of the species Ilhasina amphotera and Cypridea lunula.
Recent exploration activity in the Austral-Magallanes Basin has revealed the presence of sand injection complexes of Upper-Cretaceous-Palaeocene age. The discovery of these large-scale sandstone intrusions documents the first onshore example of sand injection complexes as hydrocarbon exploration targets and confirms a more widespread phenomenon than previously known. Integration of regional understanding, three-dimensional seismic reflection data, exploration wells, and whole core have allowed the interpretation of large sand injectites associated with a deep-water depositional system in the Austral-Magallanes Basin. These injectites are characterized seismically by circular to elongate amplitude anomalies with cross-cutting and discordant stratigraphic relationships, additionally, detailed sedimentological analysis from exploration wells and core have confirmed the presence of facies consistent with injected sand associated with a deep-water depositional system. The presence of giant sand injection complexes in the Austral-Magallanes Basin records a previously undocumented period of pore-fluid overpressure that led to large-scale hydraulic fracturing of the overburden and subsequent fluidisation of sand derived from a deep-water depositional system. The discovery of intrusive traps such as sand injection complexes, defines a new play with significant exploration potential, however, additional evaluation is required to understand the nature and degree of primary sand body geometrical modification and the subsequent impact on reservoir distribution, trap geometry, migration pathways, and seal.
The integration of rock and well logs data is fundamental for the interpretation of depositional systems and sequences, a fundamental support for E&P activities. This approach was applied on the low permeability turbidite sandstones and slurries of the Maracangalha Formation, in the Massapê Oil/Gas Field, Recôncavo Basin. Well-log interpretation allowed the recognition of 23 turbidite stages, each of which records a specific period of growth of a fan system. These stages are composed of four main facies, all observed in each one of these stages: (a) fine to medium sandstones, with porosities greater than 9%, being the best reservoirs; (b) silty to very fine sandstone slurry facies, that corresponds to very muddy reservoirs with porosities less than 9%, forming important permeability barriers; (c) siltstones and (d) shales. Turbidite stages appear in the Gamma Rays logs (GR) with funnel, block and serrated log motif patterns, corresponding to dominantly thinning and fining upward signatures, recognized in several wells in the Massapê Field. The detailed correlation of these well-logs showed that these stages are present in the whole field. The mapping of high resolution turbiditic stages, considering the correlation of their four facies, will allow a better location of wells, resulting in a more efficient exploration and production development, capable of increasing the recovery of important oil reserves that still exist in the area, optimizing costs and giving greater robustness to E&P processes.
The Tumey Giant Injection Complex (TGIC) is a regionally developed sandstone intrusion complex emplaced into the deep-water Kreyenhagen Shale (Eocene) in the San Joaquin Basin, Central California. Detailed geological mapping, stratigraphic reconstruction and outcrop description, supported by structural analysis, allowed the architectural characterization of the TGIC. The complex is described as two main stratigraphically constrained intervals: (1) a lower interval (250 m thick) emplaced into clay-rich mudrock, consisting dominantly of sills with stepped and multilayered geometry; and (2) an upper interval (200 m thick) characterized by injection breccia and large wing-like intrusions (c. 600 m width × 100 m high) emplaced within predominantly biosiliceous mudrock strata. The intrusions in both intervals were derived from turbiditic channel fills intensely modified by sand fluidization. Sandstone intrusions and fractures affecting host
strata are dominantly oriented sub-parallel to the basin axis striking between NW–SE and N–S, mainly dipping to NE and forming asymmetric saucer-shaped intrusions, suggesting structurally driven hydraulic fracturing and sand emplacement. The absence of a deep aquifer and potential sand sources underlying the complex suggests a lateral contribution of fluid flow. The TGIC occurs at a scale similar to injection complexes recognized in the subsurface and is a valuable reservoir analogue for hydrocarbon accumulations associated with sand injectites.
Despite the potential of sandstone-filled normal faults to significantly influence fluid transmissivity within reservoirs and the shallow crust, they have to date been largely overlooked. Fluidized sand, forcefully intruded along normal fault zones, markedly enhances the transmissivity of faults and, in general, the connectivity between otherwise unconnected reservoirs. Here, we provide a detailed outcrop description and interpretation of sandstone-filled normal faults from different stratigraphic units in central California. Such faults commonly show limited fault throw, cm to dm wide apertures, poorly-developed fault zones and full or partial sand infill. Based on these features and inferences regarding their origin, we propose a general classification that defines two main types of sandstone-filled normal faults. Type 1 form as a consequence of the hydraulic failure of the host strata above a poorly-consolidated sandstone following a significant, rapid increase of pore fluid over-pressure. Type 2 sandstone-filled normal faults form as a result of regional tectonic deformation. These structures may play a significant role in the connectivity of siliciclastic reservoirs, and may therefore be crucial not just for investigation of basin evolution but also in hydrocarbon exploration.
This work discusses the significance of particular types of soft-sediment deformations very common within turbidite deposits, namely convolute laminations and load structures. Detailed facies analyses of the foredeep turbidites in the Marnoso-arenacea Formation (northern Italy) and Annot Sandstones (south eastern France) show that these deformational structures tend to increase near morphological obstacles, concomitantly with contained-reflected beds. The lateral and vertical distribution of convolute laminae and load structures, as well as their geometry, has a well-defined depositional logic related to flow decelerations and reflections against bounding slopes. This evidence suggests an interaction between fine-grained sediment and the presence of morphologic relief, and impulsive and cyclic-wave loadings, which are produced by flow impacts or reflected bores and internal waves related to impinging bipartite turbidity currents.
Several productive Paleogene deepwater sandstone reservoirs in the North Sea show evidence of having undergone post-depositional remobilization and clastic injection, which can result in major disruption of the primary reservoir distribution (e.g., Alba, Forth/Harding, Balder, and Gryphon fields). Case studies of deepwater sandstones from UK Quadrants 9, 15, 16 and 21 are presented to illustrate the wide spectrum of remobilization features, which range from centimeters (e.g., core-scale) to hundreds of meters (e.g., seismic-scale). Most common are clastic injection structures such as dikes and sills. Sills of massive sand, over 20 m thick, have been identified. Intrusions associated with the propagation of syn- to early post-depositional, dewatering-related polygonal fault systems in adjacent deepwater mudrocks are also common. The scale of the clastic intrusion and remobilization has significant impact on reservoir architecture and production performance, including changes in (a) original depositional geometries; (b) reservoir properties; (c) connectivity, (d) top reservoir surface structure, (e) reservoir volumetrics, and (f) recovery/performance predictions.
There are several prerequisites for sandstone intrusions to form: the source sediment must be uncemented, and the ‘parent’ sand body must be sealed such that an overpressure with a steep hydraulic gradient can be generated. The seal on the overpressured sand body must then be breached for the sand to fluidize and inject. The stress state within the basin, burial depth, fluid pressure and the nature of the sedimentary host rock all contribute to the final style, geometry and scale of intrusion. At shallow depths, within a few meters of the surface, small irregular intrusions are generated, more commonly forming sills, whereas at greater depth larger and more continuous dikes and sills form clastic intrusion networks. Field examples from the Ordovician in Ireland, and Panoche Hills in California are used to illustrate the control of burial depth/stress on intrusion scale.
Earthquake induced liquefaction, tectonics stresses and build-up of excess in-situ pore pressure are the most commonly cited explanations for the occurrence of clastic intrusions. However, our work suggests that the large-scale, ‘catastrophic’ sandstone intrusions within the North Sea Paleogene, which remobilized hundreds of cubic meters of sediment, probably require the presence of fluids migrating from deeper within the basin (e.g., gas charge) to drive the injection. Deepwater sand bodies within the North Sea that appear most susceptible to remobilization occur in mud-dominated successions and include (1) narrow, elongate channel or gully-filled sands (i.e., non-leveed channel systems), and (2) isolated sand-rich mounds (e.g., ‘ponded’ sand bodies and terminal fan lobes). Sand bodies located above rift-related basin-forming faults, which periodically appear to have acted as vertical fluid escape pathways, were especially susceptible to remobilization. Sand remobilization may influence reservoir distribution in other mud-dominated, deepwater depositional systems.
Depositional and remobilized sandstone units are identified in core from the Eocene sand-rich deep-water Nauchlan Member and termed stratified and unstratified facies, respectively. The unstratified facies association records an increased intensity of sand remobilization, and inferred fluidization, upward. Unstratified facies have lower average porosity and permeability than stratified facies. Bulk density and acoustic velocity are higher in unstratified facies than in stratified facies. The general geometric relations of the reservoir can be inferred from a correct identification of the facies. Correlation of borehole data with (3D PS) seismic data enables the seismic to be used as a lithology indicator. A modified interpretation of sandbody geometry is made that incorporates sand injection features and provides a more accurate reservoir model.
Caruaçu Member (Maracangalha Formation) is an oil and gas producer in Recôncavo Basin, Brazil. This unit crops out in the coast of Todos os Santos Bay, near the Aratu Navy Base, in the Great Salvador, and in the Bom Despacho Ferry Station in the Itaparica Island. Caruaçu shows channel-fill and proximal levee facies in the first locality and sheet sands in the latter. In the southwest of the Cassarongongo and Taquipe fields area, these sandstones can be recognized in logs and have characteristic reflections in seismic lines. The channel cut is represented by white (negative polarity) reflections and channel filling is marked by black (positive polarity) reflections, which onlap the white ones. Levees are marked by elevations in the channel borders, which are channels flowed down the slope of the Maracangalha deltaic complex and sporadically formed crevasse-splay deposits. In the southeast Recôncavo, channels flowed into a large splay area, whose sheet sands coalesced laterally and vertically into thick sand bodies. Caruaçu sandstones form unconventional oil and gas traps as they frequently do not present oil or gas/water contact. In the deepest part of the basin, they behave as tight-gas sands.
The geological expression of hydraulic fracturing is varied and is controlled primarily by the magnitude of the differential stress and the intrinsic properties of the rock. The orientation and type of fractures that develop within a basin are determined by the state of stress, which in turn is controlled by the geological boundary conditions. During the early stages of burial and diagenesis the formation of hydraulic fractures is thought to be an important factor in the movement of fluids through and out of low-permeability, semilithified sediments. Unfortunately, these fractures are not generally preserved and are presumed to heal once the fluid pressure is relieved. The low-permeability Mercia Mudstones of the Bristol Channel Basin, southwest England, however, contain bodies of sand that, during the opening of the basin, were injected along some of the hydraulic fractures in the mudstones, preserving them as sedimentary dikes and sills. Field observations indicate that fluid pressures within the Mercia Mudstones were also very high during basin inversion and that hydraulic fracturing provided a transient permeability that relieved this excess pressure. The fractures are not visible in most of the mudstones but have been preserved within evaporite-rich horizons as a network of satin spar veins. Thus, the chance preservation of the sedimentary dikes and satin spar veins shows that at different times during the evolution of the basin, fluids migrated through low-permeability units along transient networks of hydraulic fractures. In addition, the orientation and spatial organization of these fractures reflect the boundary conditions operating at various stages in the basin history.
I. Stratigphy. [J. M.]
At the end of the year 1859, Mr. Samuel Allport communicated to the Geological Society his discovery of a series of fossiliferous rocks, apparently of Mesozoic age, in the neighbourhood of Bahia (Brazil). In 1870 the same formation was more adequately described by Prof. C. F. Hartt, who regarded it as probably equivalent to the Neocomian of Europe; and, in 1878, the whole of the Cretaceous basin of Bahia formed the subject of a memoir by Dr. Orville A. Derby. The work of all these authors emphasized the importance of systematic collecting from the highly-fossiliferous deposits of the series in question; and it has been my pleasure and privilege, at intervals during the past thirty years, to devote considerable time to this task. Most of the fossil mollusca thus obtained have been monographed by Dr. C. A. White, while the fossil vertebrata have been described by the late Prof. E. D. Cope and Dr. A. Smith Woodward. No summary of results, however, has hitherto appeared; and, as my opportunities tor continuing the work are now almost at an end, I venture to offer to the Geological Society some general observations, to precede a discussion of my collection of vertebrate fossils, which has been prepared by Dr. Smith Woodward
As already remarked by Allport, the Cretaceous rocks of Bahia have a general north-westerly dip, but the series is so much disturbed and contorted that the actual dip differs in almost every section. It was, in fact
Load structures are a type of soft-sediment deformation structure comprising synforms (load casts and pseudonodules) and antiforms (flame structures and diapirs) at an interface. They form in response to unstable density contrasts (density loading) or lateral variations in load (uneven loading) when sediment becomes liquidized or otherwise loses strength. They are here classified into five varieties: simple and pendulous load casts, in which the upper (denser) layer is laterally continuous; and attached pseudonodules, detached pseudonodules and ball-and-pillow structure, in which discrete masses of the upper layer are separated by matrix. Conceptual models demonstrate that there are several possible modes of formation for each type of load structure. One interpretation of the variation of load structure morphology is as a deformation series representing varying degrees of deformation, controlled by the magnitude of the driving force and/or the duration of its effective action. An interpretation of the commonly observed pattern of wide load casts and narrow flame structures is presented in terms of their differential growth. Fluidization has an important influence on the development of load structures and their relationship to other products of sediment mobilization.
Submarine fans of Late Palaeocene and Early Eocene age form important hydrocarbon reservoirs in the Bruce-Beryl Embayment, northern North Sea. The Early Eocene fans are the main reservoirs in the Forth-Gryphon oilfields and in the giant Frigg gasfield. Significant oil discoveries have also been made in Late Palaeocene fans. Forth and Gryphon lie on the flanks of the Crawford Anticline, a drape structure that developed during the Palaeocene above the crest of a Mesozoic tilted fault block. The Early Eocene fans pinchout against the flanks of the anticline implying continued growth of the structure throughout the Eocene. Growth was accompanied by the development of major gravity slides that detached in a sequence of altered, basaltic tephras at the base of the Eocene sequence. Seismic-scale, post-depositional deformation (sandstone diapirism and the intrusion of clastic sills and dykes) connected with this sliding dramatically modified the original depositional geometries of the fans. A detailed account of the deformation features, illustrated with core, wireline log and three-dimensional seismic data is presented together with a discussion of their exploration/appraisal significance.
A number of large, unusually shaped sandbodies have been interpreted from three-dimensional seismic data of hydrocarbon-bearing Tertiary submarine fan deposits of the North Sea. The unusual sandbody shape is considered to have resulted from post-depositional liquification of turbidite deposits. Three processes can cause liquifaction: 1) fluidization, which results from pore fluid movement; 2) liquefaction, caused by the agitation of grains during cyclic shear stress; and 3) shear liquification which results from the movement of grains during the application of a shear stress across the sandbody. The large-scale deformation of sandbodies and hence their geometry may vary according to which liquification process or processes are active. The rheology of the surrounding material and the nature of the stress field active at the time of deformation will play an important part in controlling the behaviour of the sandbody during remobilization. -from Author
Observations on outcrop of a regionally developed sand injectite are used to infer and estimate the pore-pressure conditions in the shallow crust that caused the fluidization and injection of tens of cubic kilometres of sand. The estimated pore-fluid pressures at the base of the injection complex (at 1500 m burial depth, below a regionally developed shale-dominated seal) are from 22.26 to 25.08 MPa, which respectively correspond to 0.81 and 0.95 lithostatic pressure. A theoretical basis for prediction of sand injection is defined and applied to the prediction of pore pressure at the time of sand injection, the depth at which seal failure occurred, and the density and granular content of the fluidized flow. Lateral variations in the style and abundance of sandstone intrusions are described and these all fit into a remarkably uniform tripartite division of parent units, an intrusive complex and an extrusive complex. A sill zone (intrusions are dominated by sills) occurs in a restricted stratigraphic interval 200-270 m thick. Location of the base of the sill zone is directly related to the thickness of the overburden, and an isobaric surface at the time of sand injection, the lithostatic equilibrium surface, is defined at the base of the sill zone. When the sills formed an extended period of supra-lithostatic pressure occurred within the sill zone.
An architectural hierarchy of elastic sills is recognized in the Panoche Giant Injection Complex in which staggered, stepped with erosive top surfaces, and multi-layered geometries occur in that stratigraphic order upward. Genetic relationships between parent depositional sand bodies, the sand injections, a zone of hydraulic fracture and a palaeo sea floor are seen at a scale previously observed only by using seismic data. Sills and randomly oriented dykes intrude into a hydraulically fractured shale unit above and below which dykes predominate. Erosive surfaces (scallops) are identified on sills that, along with smaller erosional features, record low-viscosity turbulent flow during sand injection. Sand extrusions occur where dykes reach the palaeo sea floor.
Subsurface and outcrop data are used to describe sand injectites, a group of genetically related features that includes sandstone dykes and sills, but also structures within depositional sand bodies. Fluidization is identified as the process by which sand is injected but we draw attention to the lack of constraints regarding fluidization velocity and fluid viscosity. Injectites are shown to develop between <10 m and 500 m below the seafloor. No relationship between depth of generation and injection geometry is found. Liquefaction of sand may produce sufficient excess pore fluid to create small sand injections during shallow burial. Large injectite bodies are identified on seismic data that may exceed 4 x 10(7) m(3) are unlikely to be related to sand liquefaction. The general validity of hydraulic fracture as the mechanism for seal failure and propagation of injections is questioned. The association between the formation of polygonal faults and sand injection provides one of several alternative mechanisms for seal failure. Multi-phase intrusion is proposed as a likely mechanism for the formation of large sand intrusions, both because of the cyclical nature of most of the process invoked in their formation, and the author's own observations. Many of the processes of sand injection remain poorly constrained.
Delta-front sedimentation dominated deposition of the Cretaceous Pitanga Member of the Candeias Formation and the Caruaçu Beds of the Marfim Formation, Recôncavo Basin, Brazil. The Pitanga Member is subdivided into three sedimentary facies according to differences in lithology and sedimentary structures, whereas the Caruaçu Beds are subdivided into ten such facies. The three sedimentary facies of the Pitanga Member are (1) Disrupted Sandstone-Mudstone Facies, containing slump fragments, pull-aparts, and small-scale faults and folds deposited by slumping and sliding as a cohesive sediment; (2) Massive Sandstone Facies, poorly sorted sandstones containing dispersed clasts of coarse sand, granules, and mudstone blocks deposited by mass flow and slurrying during cohesion loss; and (3) Dish-structured Sandstone Facies; poorly sorted sandstones with dish structures and steplike load structures deposited by grain flow. The ten facies of the Caruaçu Beds were deposited in distributary channels (Quartzose Sandstone Facies; Micro-cross-laminated Facies), interdistributary bays (Parallel-laminated Mudstone Facies), delta fronts and delta-front troughs by subaqueous slumping as cohesive sediment (Disturbed Sandstone-Mudstone Facies, Pebbly Mudstone Facies), mass flowand slurrying due to cohesion loss (Massive Sandstone Facies), and grain flow (Dish-structured Sandstone Facies, Conglomerate Facies). These facies can be grouped into a delta-plain association, a high delta-front association (slump dominated) and a low delta-front trough association (massflow, slurry, and grain-flow dominated). Their areal distribution indicates both southward progradation of the deltas and repeated slumping. Triggering mechanisms of subaqueous slumping, mass flow and grain flow in this example include growth faulting, slope instability in response to local diapiric upwarp, and loading of water-saturated muddy sediments with overlying sand and gravel transported over short distances by high-gradient streams. Deposition occurred predominantly in delta-front troughs.
A large number of km-scale, saucer-shaped sandstone bodies of enigmatic origin have recently been documented in the North Sea and the Faroe Shetland Basin. This study utilises three-dimensional seismic data, calibrated by well data, to examine two such bodies that exhibit very similar saucer-shaped geometries in cross-section. The Volund and Danica structures, located 250 km apart are interpreted as end members of a spectrum of large-scale remobilised and injected sandstones present in the North Sea Palaeogene. Both are characterised by a central 1–2 km-wide low area surrounded by a discordant, 2–300 m tall inclined dyke complex, that tips out into a bedding concordant body interpreted as a shallow-level sill and/or partly extruded sandstone. The origin of the central concordant sandstone body as either injected (laccolith) or depositional is of key importance to a complete understanding of the origin and prospectivity of these structures. The key criteria for recognising an injected vs. depositional origin for the central concordant sandbody are: (1) a flat, nonerosional base; (2) ‘jack-up’ of the overburden equal to the underlying sand thickness; (3) equally thick layers of encasing mudstones; and (4) paleogeographic context. This study suggests that the Danica structure was deposited as a channel sandstone and remobilised in situ; this led to the formation of wing-like intrusions along the channel margins. In contrast, the Volund structure overburden displays a forced-fold geometry, arguably a diagnostic feature of an intrusive origin. The ability to recognise and differentiate completely injected vs. in situ remobilised sandbodies is important both from a basin analysis, hydrocarbon exploration and rock mechanics points of view. An improved understanding of these aspects will lead to a reduction of risks associated with the exploration and development of such a sandbody and an enhanced understanding of sediment remobilisation and fluid flow on a basin scale.
Experiments demonstrate that fluid escape structures can be produced as a result of unstable fluidization behaviour where a lower base layer of granular material is inhibited from fluidizing by the presence of an overlying non-fluidizing top layer. Before the base layer can fluidize the weight of the overlying material must be balanced, and this is accomplished by base layer material pressing against the bottom surface of the confining top layer forming a static layer. This static layer allows the top layer to lift away from the base layer which is then free to fluidize. -from Authors
Deformations formed in unconsolidated sediments are known as soft-sediment deformation structures. Their nature, the time of their genesis, and the state in which the sediments occured during the formation of soft-sediment deforma-tion structures are responsible for controversies regarding the character of these deformations. A defi nition for soft-sediment deformation structures in siliciclastic sediments is therefore proposed. A wide variety of soft-sediment deformations in sediments, with emphasis on deformations in siliciclastic sediments studied by the present author, are described. Their genesis can be understood only if their sedimentary context is con-sidered, so that attention is also paid to the various deformational processes, which are subdivided here into (1) endo-genic processes resulting in endoturbations; (2) gravity-dominated processes resulting in graviturbations, which can be subdivided further into (2a) astroturbations, (2b) praecipiturbations, (2c) instabiloturbations, (2d) compagoturbations and (2e) inclinaturbations; and (3) exogenic processes resulting in exoturbations, which can be further subdivided into (3a) bioturbations – with subcategories (3a') phytoturbations, (3a'') zooturbations and (3a''') anthropoturbations – (3b) glaciturbations, (3c) thermoturbations, (3d) hydroturbations, (3e) chemoturbations, and (3f) eoloturbations. This sub-division forms the basis for a new approach towards their classifi cation. It is found that detailed analysis of soft-sediment deformations can increase the insight into aspects that are of im-portance for applied earth-scientifi c research, and that many more underlying data of purely scientifi c interest can, in specifi c cases, be derived from them than previously assumed. A fi rst assessment of aspects that make soft-sediment deformation structures in clastic sediments relevant for the earth sciences, is therefore provided.
Bacia do Recôncavo: sumário geológico e setores em oferta
- Bastos
On the deformational structures in systems with reversed density gradients
- Anketell
Arquitetura das seqüências estratigráficas desenvolvidas na fase de lago profundo no Rifte do Recôncavo
- Cupertino
Cupertino, J.A., Bueno, G.V., 2005. Arquitetura das seqüências estratigráficas
desenvolvidas na fase de lago profundo no Rifte do Recôncavo. Boletim de
Geociências. Petrobras, Rio de Janeiro 13 (2), 245-267 maio/nov.