We present new 3D seismic and well data from the Ebro Margin, NW Mediterranean Sea, to shed new light on the processes that formed the Messinian Erosion Surfaces (MES) of the Valencia Trough (Mediterranean Sea). We combine these data with backstripping techniques to provide a minimum estimate of the Messinian sea level fall in the EBRO Margin, as well as coupled isostasy and river incision and transport modeling to offer new constraints on the evolution of the adjacent subaerial Ebro Basin. Four major seismic units are identified on the Cenozoic Ebro Margin, based on the seismic data, including two major prograding megasequences that are separated by a major unconfirmity: the MES. The 3D seismic data provide an unprecedented view of the MES and display characteristic features of subaerial incision, including a drainage network with tributaries of at least five different orders, terraces and meandering rivers. The Messinian landscape presents a characteristic stepped‐like profile that allows the margin to be subdivided in three different regions roughly parallel to the coastline. No major tectonic control exists on the boundaries between these regions. The boundary between the two most distal regions marks the location of a relatively stable base level, and this is used in backstripping analysis to estimate the magnitude of sea level drop associated with the Messinian Salinity Crisis on the Ebro Margin. The MES on the Ebro Margin is dominated by a major fluvial system, that we identify here as the Messinian Ebro River. The 3D seismic data, onshore geology and modeling results indicate that the Ebro River drained the Ebro Basin well in advance of the Messinian.
The thick sequences of Upper Palaeozoic strata present in the Southern North Sea Basin were probably deposited on a basement of Caledonian low-grade metamorphic rocks intruded by numerous late-Caledonian granitoids which form part of the subsurface extension of the Mid-European Caledonides linking the English Lake District and Northern Pennines to the Ardennes. This basement, along with the better known onshore areas of the British Isles and Ireland was intensely fragmented during late Devonian to mid-Carboniferous times by a major phase of extensional tectonics.Active fault-bounded tilted blocks developed in the Dinantian with deposition of fluviatile redbeds, marine carbonate/deltaic clastic cycles and deltaic coastal plain facies. These are penetrated by a number of wells on the southern flanks of the Mid-North Sea High, but details of basin geometries and facies distributions remain unclear.Crustal stretching appears to have given way to thermal subsidence by early to midNamurian times. This was coincident with a major climatic change, probably forced by glacial expansion in Gondwanaland. Thc major drainage system that resulted dominated sedimentation in the SNSCB, punctuated by numerous marine transgrcssions of undoubted glacio-eustatic origins. Within this broad dcpositional framework, Namurian facies were closely controlled by basement tilt block topography, with good cvidcncc for a gradual southerly migration of the active fairways of fluviodeitaic sandbodies. Wcstphalian facies were dominated by the interplay between glacio-eustatic and sedimentary processes on a vast low-gradient alluvial plain with much less pronounced tectonic control.By latc-Wcstphalian B timcs a phase of basin inversion caused thc gradual clcvation of thc arca as a rctro-arc forcland basin. Vadosc diagencsis and pcdogcncsis occurred in the fluviatilc facies of thc so-called Barrcn Rcd Mcasurcs (BRM). With continucd comprcssivc dcformation a scrics of over 50 NW-SE oricntatcd folds, somc associated with thrusts, dcvcloped. Significant crosion and truncation of these invcrsion anticlincs was accompanicd by thc deposition of thc BRM facies in characteristic growth fold basins, with marked onlap around thc basin margins. The Carboniferous play in thc SNSCB has been greatly influcnccd by thc subsequent Mcsozoic/Cenozoic geological history of the basin, including periods of subsidcncc and uplift. Wc demonstrate that variations in original detrital mineralogy and various diagcnetic processes havc cxcrted significant controls upon Carbonifcrous reservoir quality.
The hydrocarbon prospectivity of the offshore Central Irish Sea Basin has been assessed with particular reference to Block 42/12, which was formerly operated by Hydrocarbons Ireland Ltd (British Gas Exploration & Production). Data from the two wells drilled in 42/12, and other wells and boreholes in the area, have been integrated with seismic, gravity and magnetic data. Onshore outcrop and sub-surface information were also utilized. Interpretation of these data shows that the proven basin-fill consists of several megasequences bounded by major unconformities. At least three separate phases of extension are recorded, in the Carboniferous, Triassic and Jurassic. These were followed by post-rift, regional thermal subsidence, but the stratigraphy is further punctuated with evidence of compression and transpression associated with the Variscan, Cimmerian and Alpine inversion events. An assessment of hydrocarbon prospectivity indicates that the primary reservoir interval occurs in the syn-rift Lower Triassic Sherwood Sandstone Group. This is sealed by the overlying Middle-Upper Triassic Mercia Mudstone Group consisting of shales with major halite intervals. Westphalian coal-bearing facies, proven in 42/12-1 and 42/17-1, are potential gas-prone source rocks, which are late to overmature for oil generation to early mature for gas generation. Unproven Namurian marine source rocks may occur in the main basin depocentre. Burial history modelling suggests that source rocks probably generated hydrocarbons by the Early Cretaceous, with possible further generation prior to the main Tertiary inversion. Tilted fault-block and rollover anticline trapping structures were probably formed during Permian to Jurassic rifting, pre-dating the main phases of hydrocarbon expulsion. Exploration to date has centred on the southern margins of the basin. Future exploration of the deeper and potentially prospective parts of the basin in the north should resolve uncertainties concerning the possible presence of Namurian source rocks and Permian reservoir and seal facies, and the nature of a post-Triassic section identified on seismic data. The Central Irish Sea Basin is an under-explored Late Palaeozoic to Mesozoic basin which is stratigraphically analogous to the prolific East Irish Sea hydrocarbon province to the northeast. This paper presents the results and interpretations of past exploration activity and reviews these in terms of their impact on the tectono-stratigraphic evolution and future hydrocarbon prospectivity of the basin.
A number of oil discoveries have been made in recent years in the East Irish Sea Basin (EISB). These have renewed industry interest in a basin which many thought to be essentially gas prone. British Gas, over a number of years, has developed and refined a basin model for the area based upon in excess of 23 000 km of seismic data and 40 wells. This model shows that the present-day distribution of oil and gas is controlled by complex interrelationships between the thermal and tectonostratigraphic history of the EISB. An integrated approach has been adopted in creating the model, which includes data generated from many sources including AFTA, detailed diagenetic and geochemical studies and more conventional geological, geophysical, and petrophysical analysis. The paper will address several aspects of the current basin model, including oil–source rock correlation, the origins of hydrogen sulphide within the EISB, and the problems and solutions in validating a basin model when so much of the rock record is missing.
A methodology for the correlation of offshore Silesian (Upper Carboniferous) sequences is developed using an integrative approach based upon biostratigraphy, downhole logging and geochemistry. We develop the combined use of miospore zonation, gamma ray spectrometry, formation velocities and carbon/sulphur ratios to determine the location of marine horizons and other lithofacies in a thick (>l km) largely uncored Silesian sequence in the Southern North Sea Gas Basin. At least nine major marine horizons are recognized within a basin-fill sequence of Marsdenian, Yeadonian and Langsettian ages (Namurian B/C and Westphalian A) in the well. These horizons help to provide a detailed subdivision of the offshore sequence, although the correlations must remain tentative until further deep wells are drilled in the area. The implications of the correlations for offshore palaeogeography are that the area of northern Quadrant 48 was a deep, periodically anoxic basin until infill by successive submarine fan lobes in late Marsdenian times. Yeadonian delta front and coastal plain deposits pass upwards into fluvio-deltaic/wetland facies of the Langsettian.
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