ALBERT

Description: Throughout exploration Block 2 of the Orange Basin offshore the South African continental margin, different natural gas leakage features and the relationship between natural gas leakage with structural and stratigraphic elements were studied. This study also quantifies liquid/gas hydrocarbon generation, migration and seepage dynamics through the post-rift history of the basin.The interpretation of seismic data reveals two mega-sequences: Cretaceous and Cenozoic that are subdivided by major stratigraphic unconformities into 5 and 2 sub-units, respectively. The basin is also divided into 2 structural domains: 1. an extensional domain characterized by basinward dipping listric normal faults rooted at the Cenomanian/Turonian level identified between 500 to 1500 m of present-day depth, 2. a compressional domain that accommodates the up-dip extension on the lower slope, and which is characterized by landward dipping thrust faults. One hundred and thirteen observed gas chimneys are identified and classified into stratigraphically-controlled (sa-c) and structurally-controlled (s-c) chimneys. The ratio of s-c versus s-ac chimneys is estimated as 2:5, which suggest a strong stratigraphic control on natural gas leakage. The chimneys either terminate at the seafloor where active leaking gas is manifested by pockmarks, or are sealed within the Miocene (14 Ma) sequence as paleo-pockmarks. The s-c chimneys are located along the normal faults in the extensional domain, and terminate as seafloor mounds up to 1500 m in diameter and with heights between 10 to 50 m. The sa-c pockmarks range between 100 to 400 m in diameter, and are linked to stratigraphic onlaps and pinch-outs within the Aptian sequence. Several giant chimneys, with diameters of more than 7 km, are also identified. At least one of these displays apparent internal gravitational collapse structures. Bright spots indicative of gas presence within these large chimneys were identified, but there is no evidence of acoustic turbidity or seismic pull-downs within these large structures. This suggests the giant chimneys are inactive paleo-gas-escape structures.Modelling suggests that gas from the lower Aptian and the Barremian source rocks migrates laterally-updip to the proximal parts of the basin where it accumulates beneath the Cenomanian/Turonian sequence that acts as a regional seal. Across the shelf-break and the upper slope, chimneys and pockmarks are fed from younger Cenomanian/Turonian source rocks. The migration model also indicates that fluids are about 24 times more likely to flow out of the study area than to be preserved within it.Since methane gas escaping across the sea floor into the exosphere (combined hydrosphere and atmosphere) may contribute to Earth’s climate fluctuations, and because escaping gas must have been cut off when at least half of identified s-c chimneys were sealed within the Miocene sequence, decrease of gas escape along the southern African continental margin may have to be factored into global Neogene cooling models.

Description: This study uses a 3D petroleum system modelling approach to investigate the links between hydrocarbon generation and migration with observed gas chimneys and gas leakage features along the western margin of offshore South Africa. In addition we have investigated the impact and timing of mass failures and related sediment mass movement on the petroleum system and hydrocarbon maturation history within the southern Orange Basin.Our 3D-model covers this passive margin’s evolution from Early Cretaceous drift initiation to the establishment of a present day continental margin, extending from the shelf to the deep marine domain. The model is based on interpreted 2D seismic profiles and borehole data including sedimentological and geochemical analyses as well as heat flow data. The model includes a proven source rock of Aptian/Albian age, and a second assumed Cenomanian-Turonian source. Several heat flow and uplift-erosion scenarios have been tested with the model to assess consistency with calibration data and present day surface heat flow values. After calibration against known well temperatures and vitrinite reflectance hydrocarbon generation and migration have been modelled to investigate the initiation, duration and spatial distribution of petroleum accumulation and leakage within and throughout the sedimentary column.The main sedimentary depocentres of the Orange Basin developed from east to west, with a siliciclastic basin infill and aggradation during the Cretaceous and westward progradation during the Late Cenozoic. A Late Cretaceous episode of margin instability occurred in the north western part of the study area followed by a second phase of Late Cenozoic mass movement in the south-western part of the study area. At the location of the Cretaceous mass failure the increase in sediment load profoundly affected the underlying source rocks seen in modelled maturation and petroleum generation potential. Albian source rocks started hydrocarbon transformation 100 to 85 Ma ago (phases I to III) in the centre of the basin and between 75 to 65 Ma ago (phase IV and VI) towards the distal part of the basin. The highest generation rates occurred at 75 Ma, followed by a rapid decrease until 15 Ma and a slight increase in generation potential until present day (phase V) caused by enhanced Cenozoic sediment load west of the Cretaceous shelf break. This recent increase accounts for the location of present day gas generation, though it does not substantially affect the overall regional maturation history.Today’s gas leakage features that have been observed in the shelf area (

Description: Basin-scale stratigraphic correlation is the fundamental base for successful reservoir exploration, and especially when dealing with cross-border areas. Differences in lithostratigraphic and chronostratigraphic nomenclature between sub-basins and countries often result in problematic estimations of reservoir geometries and potential. This study combines available biostratigraphic, biofaunal and lithofacies data, together with sequence-stratigraphical correlations of the Lower Jurassic from the Central European Basin (CEB), to propose a genetic-based framework of transgressive and regressive depositional units. The determination of four major biofacies environments, composed of (I) polyhaline open-marine/offshore environments, (II) upper mesohaline marine–brackish environments, (III) lower mesohaline brackish environments and (IV) low oligohaline to freshwater continental environments comprising very rare marine phytoplankton and terrestrial spores and pollens, were translated into 12 biofacies reconstructions of ammonite (sub-) chronozone levels. Variations of biofacies reconstructions in time and space were supplemented by biostratigraphically constrained large-scale progradational and retrogradational sedimentary architecture. Retrogradation is accompanied by increasing polyhaline environments and pinpoint basinwide third-order flooding events, whereas progradation is accompanied by decreasing polyhaline environments pointing to third-order regressions. The outcomes of this study support exploration of Lower Jurassic deep geothermal reservoirs or CO 2 storage sites in the eastern CEB (especially Germany and Poland). Supplementary material: A list of all documented Liassic ammonites known from the eastern European shelf area (Denmark, The Netherlands, Sweden, Germany, Poland; wells and outcrops) is available at https://doi.org/10.6084/m9.figshare.c.3923467