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Influence of Salt Diapirism on the Basin Architecture and Hydrocarbon Prospects of the Western Iberian Margin

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Abstract

The continental edge of the Western Iberian Margin is marked by two contrasting basins, the mostly onshore Lusitanian Basin and the mostly deepwater Peniche Basin. These two rift basins were originated during the opening of the North Atlantic Ocean and have similar characteristics and are geographically separated by an elongated basement high that strikes quasiparallel to the Portuguese coastline. The basins evolved during the Mesozoic as part of the Iberia Peninsula. The petroleum geology history of the relatively unexplored Lusitanian Basin is reconstructed in this chapter through the detailed stratigraphic mapping of outcrop and legacy exploratory drilling. The lesser-known Peniche Basin, to the west of the bounding basement high, is part of the deepwater continental margin of Iberia; knowledge of this basin is derived from seismic stratigraphic comparisons with Lusitanian Basin.
... A key observation here is that diapir O is very close to one of the oil seeps in the study area (Fig. 2) proposing the likelihood of salt tectonics as the main factor influencing oil accumulation and seepage. Salt tectonics has influenced petroleum prospectivity and seepage locations in several similar sedimentary basins worldwide (dos Reis et al., 2017;Macgregor, 1993). It has also been noted that intrusion of some of the diapirs (e.g. ...
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This work reports remote sensing aided identification of salt deposition and remobilization within a sub-rounded structure with approximately equant dimensions in the Mandawa Basin. A calculated surface flow direction has revealed the sub-rounded structure from Shuttle Radar Topography Mission—Digital Elevation Model (SRTM DEM) data. Internally, the body contains several structural highs and lows reflected by multiple peaks and troughs displayed by profiles drawn across the body. Seismic interpretation results confirmed these structural highs and lows, and they conform to salt build-ups (salt pillows and domes) and sedimentary minibasins, respectively. Seismic work, a follow-up investigation of the remote sensing findings, involved qualitative interpretation of 24 seismic profiles, 7 of which were chosen to represent the findings. East Lika 1 well stratigraphy was used to assign the age of the studied interval, and confirm that features of interest are associated with salt deposition and remobilization. Moreover, the East Lika 1 well tie corroborated that the interpreted salt deposition and the associated build-ups are within the Nondwa Formation of the Pindiro Group. Remote sensing observation, coupled with seismic interpretation, has revealed that the Mandawa salt basin is more extensive than reported before. Oil seeps, which are potential indicators of a working petroleum system, have been reported in the Mandawa Basin. One of these seeps is adjacent to a mapped salt dome. This observation probably indicates the influence of salt tectonics in the Mandawa petroleum system. Further work on a better quality seismic dataset may help resolve the problem of hydrocarbon whereabouts as far as the influence of salt tectonics is considered.
... This leads to significant differences in the structural styles and evolution of salt-influenced basins relative to those unaffected by salt. Thick layers of salt have been encountered in several basins across the European Atlantic margin, including offshore Iberia (Alves et al., 2006;Pena dos Reis et al., 2017;Ramos et al., 2017;Wilson et al., 1989;Zamora et al., 2017), offshore France (Chapman, 1989;Ferrer et al., 2012), on the United Kingdom Continental Shelf (UKCS) (Jackson & Stewart, 2017;Stewart et al., 1996) and offshore Norway (Jackson et al., 2019;Rojo et al., 2019). Salt is also present in basins on the conjugate margin of Atlantic Canada (Deptuck & Kendell, 2017;Jansa et al., 1980). ...
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In contrast to much of the European Atlantic margin, the influence of salt on basin evolution has received little attention to date in the basins west of Ireland, despite salt being proven in several boreholes. Using an extensive seismic reflection database coupled with data from exploration wells and shallow boreholes, this study maps the distribution and composition of salt layers and investigates their role in the structural evolution of the Slyne and Erris basins offshore west of Ireland. Two salt‐prone intervals have been proven. The Upper Permian Zechstein Group is present throughout the Slyne and Erris basins, while the Upper Triassic Uilleann Halite Member is only developed in the Northern Slyne and Southern Erris sub‐basins. Both sedimentary units mechanically detach pre‐, post‐, and intra‐salt stratigraphy. Both salt layers underwent halokinesis during basin development, creating a variety of salt‐related structures. Salt pillows and salt rollers formed in the Zechstein Group, causing folding and rafting in the overlying Mesozoic section, driven by active faulting within the pre‐salt basement. Halokinesis in the Uilleann Halite Member caused thin‐skinned crestal collapse of the overlying Jurassic section above anticlines cored by Zechstein salt. Where both Triassic and Permian salt are present, unique structural geometries are formed when two stratigraphically discrete but kinematically linked halokinetic structures develop. The most common structural configuration consists of a Zechstein salt pillow and an Uilleann Halite salt wall separated by Lower Triassic sandstones. The fold‐axis of the salt pillow trends parallel to the strike of the salt wall. The results of this study provide a framework for the evolution of halokinetic structures in other basins on the Irish Atlantic margin, give a greater insight into the Permian and Triassic paleogeography of the region, and have more general implications for the evolution of salt‐related structures in rift basins with multiple stratigraphically discrete salt layers.
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The onshore Lusitanian Basin is dominated by two large diapiric salt walls which extend for up to 100 km. Seismic sections indicate that stratal onlap onto an intervening salt pillow initiated in Early Jurassic times. Well-exposed diapir flanks reveal three types of halokinetic-related unconformities (hook, low-angle wedge and high-angle flap-onlap – a new term) of Kimmeridgian to Turonian age, indicating that diapirism was active throughout this period. Stratal dip-fanning and wedge-thinning is predominantly caused by original sedimentary depositional processes with multiple low-angle unconformities (1°–5° pinchout angles) in both carbonate- and clastic-dominated sequences. No significant sedimentary slumping was observed in the clastic-dominated strata but important slumping of flank material is present in the carbonate-dominated sequences. Two tar accumulations are derived from oil trapped in sandstones on the flanks of the São Pedro de Moel and Santa Cruz salt diapirs. These are interpreted to be exhumed, but now biodegraded, oilfields. To our knowledge, these are the only exposed examples of salt-flank hydrocarbon accumulations that have been documented in the literature.
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Integration of extensive fieldwork, remote sensing mapping and 3D models from high quality drone photographs relates tectonics and sedimentation to define the Jurassic–early Albian diapiric evolution of the N‐S Miravete anticline, the NW‐SE Castel de Cabra anticline and the NW‐SE Cañada Vellida ridge in the Maestrat basin (Iberian Ranges, Spain). The pre‐shortening diapiric structures are defined by well‐exposed and unambiguous halokinetic geometries such as hooks and flaps, salt walls and collapse normal faults. These were developed on Triassic salt‐bearing deposits, previously misinterpreted because they were hidden and overprinted by the Alpine shortening. The Miravete anticline grew during the Jurassic and Early Cretaceous and were rejuvenated during Cenozoic shortening. Its evolution is separated into 4 halokinetic stages, including the latest Alpine compression. Regionally, the well‐exposed Castel de Cabra salt anticline and Cañada Vellida salt wall confirm the widespread Jurassic and Early Cretaceous diapiric evolution of the Maestrat basin. The NE flank of the Cañada Vellida salt wall is characterized by hook patterns and by a 500‐m long thin Upper Jurassic carbonates defining an upturned flap, inferred as the roof of the salt wall before NE‐directed salt extrusion. A regional E‐W cross‐section through the Ababuj, Miravete and Cañada‐Benatanduz anticlines shows typical geometries of salt‐related rift basins, partly decoupled from basement faults. These structures could form a broader diapiric region still to be investigated. In this section, the Camarillas and Fortanete minibasins displayed well‐developed bowl geometries at the onset of shortening. The most active period of diapiric growth in the Maestrat basin occurred during the Early Cretaceous, which is also recorded in the Eastern Betics, Asturias and Basque‐Cantabrian basins. This period coincides with the peak of eastward drift of the Iberian microplate, with speeds of 20 mm/yr. The transtensional regime is interpreted to have played a role in diapiric development.
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The relatively well-studied Lusitanian Basin in coastal west-central Portugal can be used as an analogue for the less well-known Peniche Basin in the deep offshore. In this paper the Lusitanian Basin is reviewed in terms of stratigraphy, sedimentology, evolution and petroleum systems. Data comes from published papers and technical reports as well as original research and field observations. The integration and interpretation of these data is used to build up an updated petroleum systems analysis of the basin. Petroleum systems elements include Palaeozoic and Mesozoic source rocks, siliciclastic and carbonate reservoir rocks, and Mesozoic and Tertiary seals. Traps are in general controlled by diapiric movement of Hettangian clays and evaporites during the Late Jurassic, Late Cretaceous and Late Miocene. Organic matter maturation, mainly due to Late Jurassic rift-related subsidence and burial, is described together with hydrocarbon migration and trapping. Three main petroleum systems may be defined, sourced respectively by Palaeozoic shales, Early Jurassic marly shales and Late Jurassic marls. These elements and systems can tentatively be extrapolated offshore into the deep-water Peniche Basin, where no exploration wells have so far been drilled. There are both similarities and differences between the Lusitanian and Peniche Basins, the differences being mainly related to the more distal position of the Peniche Basin and the later onset of the main rift phase which was accompanied by Early Cretaceous subsidence and burial. The main exploration risks are related to overburden and maturation timing versus trap formation associated both with diapiric movement of Hettangian salt and Cenozoic inversion. ON-LINE FULL VERSION http://onlinelibrary.wiley.com/doi/10.1111/jpg.12648/epdf
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The objective of the present work is to analyze the sub-surface geometry of the Matacaes (Torres Vedras, Portugal) salt diapir, taking into consideration the geodynamic context that led to its formation. With this objective, a campaign took place in the surrounding area of the Matacaes salt diapir in order to gather gravimetric data, as well as the acquisition of global positioning (GPS) and altimetry data, with a total of 233 measurement points with special relevance to the flanks of the diapir. After the correction and processing of the gathered data, different anomaly maps were constructed (Bouguer, Regional and Residual Anomaly), which enabled the identification and delimitation with more precision of the Matacaes salt diapir, and also the recognition of the depocenters in Bombarral, Turcifal and Arruda sub-basins. A NW-SE profile was drawn over the Bouguer anomaly map, in order to cross perpendicularly the main structures that exist in the area under study (Structural Lineament Torres Vedras - Montejunto) in order to intercept the Matacaes salt diapir (outcropping). The gravimetric modeling of the selected profile enabled the construction of two hypothetical models of the sub-surface geometry of the Matacaes salt diapir. The most appropriate model, whichexplains the geometry of the Matacaes Diapir, is the one that takes into account the different tectonic periods (distensive-compressive) that were registered in the Lusitanian Basin, with special relevance to the Miocene compressive period. In this context, the Matacaes Diapir has intermediary characteristics between a namakier and a "salt tongue".
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Most lakes are well flushed and the chemical constituents of their waters do not accumulate beyond the potable range. In unusual circumstances, however, the solute load may be increased and the lake then becomes saline. This is caused either by evaporation exceeding inflow or by the inflow being saline or both. Saline lakes are quite common in certain parts of the earth, but they have neither the size nor abundance of normal lakes. For this reason, few extensive studies have been made of such lakes and consequently their hydrologic, geochemical, sedimentological, and biological environments remain little explored. This is a pity, for saline lakes have much to teach us about processes under extreme conditions and, once we understand them, we will have greatly improved our understanding also of normal lakes. This paucity of studies is also surprising because saline lakes not only are of economic significance but are important in the geologic record as sensitive indicators of past tectonic and climatic events.
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The Lusitanian Basin (Portugal) is located on the western margin of the Iberian Plate with maximum sedimentary thickness around 5 km. In spite of containing promising petroleum systems and more than a hundred wells, its hydrocarbon generation potential has not been fully evaluated yet. These sediments range from upper Triassic to upper Cretaceous, mainly from the Jurassic age. The Lower Jurassic is particularly well represented in the Lusitanian Basin, specifically in the Peniche region, where there are more than 450 m thick outcrops of carbonate rocks aged between Sinemurian and Toarcian. This study presents the evaluation of the hydrocarbon generation potential of the Pliensbachian - lower Toarcian of the Peniche section; which includes the Vale das Fontes Formation, Lemede Formation and Cabo Carvoeiro Formation (CC 1 and CC 2 members) based on 233 rock samples analyzed for total organic carbon (TOC), Rock-Eval pyrolysis and biomarkers. Available data of oxygen and carbon stable isotopes were also used in the paleoenvironmental interpretation. Marls and limestones with the bituminous fades (Mlbf) of the Vale das Fontes Formation, present the highest TOC values, up to 14.95%, corresponding to the ibex (upper part), davoei and margaritatus ammonites zones and part of the NJ4a and NJ4b nannofossils biozones (Pliensbachian). In thermically immature sections (Tmax about 430°C), this member also shows high S2 values (10 to 50 mgHC/g rock) and Hydrogen index (HI: 200 to 555 mgHC/g TOC). The correlation between HI × OI shows that type II kerogen is pre-dominant in the portion of the section where TOC values are higher. The good positive correlation between TOC values and the biomarkers ratio P/nC17 and steranes (C27/C29) throughout the Mlbf Member suggests a gradual increase of anoxia and higher contribution of algae in relation to terrestrial organic matter. Usually δ18O variations observed in the basins of the Iberian Peninsula have been explained by temperature changes. This work proposes an alternative interpretation, by correlating δ18O alterations with data on biomarkers to indicate salinity variations. Thus, the presence of gammacerane in the marls and limestones with Uptonia and Pentacrinus (Mlup) and lumpy marls and limestones (Lml), associated to the increase of δ18O values would indicate an environment with salinity above normal levels. In addition to this, the increase of moretanes rates towards the maximum flooding (MF) interval - which is located in the δ13C positive excursion in the upper part of the Mlbf Member - margaritatus ammonites zone and NJ4b nannofossil biozone - associated with the absence of gammacerane, suggests gradual salinity reduction. The presence of gammaceranes at the uppermost part of the Mlbf Member, with the more-fanes rate reduction and the increase of Ts/Tm values seems to indicate a new cycle of increasing salinity.