Kilometre-scale sandstone intrusions have recently been documented from the Palaeogene deep-water sequences of the northern North Sea. The distribution of Eocene sandstones within a 126 km23D4C seismic survey located in the Outer Moray Firth was mapped using surface and volume based interpretation of converted wave seismic data calibrated to a number of boreholes. About 35 conical sandstone intrusions, a few tens of metres thick and 1–1.5 km wide, were mapped within the lower Eocene. The intrusions are inclined about 15–20° and crosscut some 100–200 m of lower Eocene mudstones, terminating upward at the Middle Eocene unconformity, below the oil-charged, upper Eocene sandstones of the Alba and Chestnut reservoirs. Individual sandstone intrusions may contain millions of cubic metres of sandstone and often have excellent reservoir properties. Previous studies failed to define a viable migration path for the oil in the Alba/Chestnut reservoirs because they appeared to be completely encased in poorly permeable Eocene mudstones. However, detailed mapping and volume visualization of the converted wave seismic data demonstrates that the upper Eocene reservoirs have suffered post-depositional remobilization and that the conical intrusions provide a viable migration path between the Alba/Chestnut reservoirs and the underlying Paleocene aquifers. Despite more than two decades of exploration activity in the Outer Moray Firth, the widespread occurrence of conical intrusions in the lower Eocene is still not widely recognized, and it is likely that the sandstones may pose a hazard when drilling for deeper targets. However, it is also possible that large conical intrusions that do not connect with overlying sandstones may contain economical quantities of hydrocarbons.Supplementary material:The movie referred to in the article is available athttps://doi.org/10.6084/m9.figshare.c.4812948
The Sarir Sandstone is the principal reservoir for oil accumulations in the eastern Sirt Basin in Libya. The main phase of the rifting in this area took place in the Late Jurassic-Early Cretaceous, during which time the Sarir Sandstone was deposited as a non-marine, intra-continental clastic syn-rift sequence. Although successfully explored from 1959 onwards, the prolific eastern Sirt Basin is in a relatively immature stage of exploration regarding wildcat drilling and 3D seismic data acquisition. The most recent phase of exploration, utilizing 3D seismic techniques, revealed a complex structural development. The trap geometries are often related to E-W trending, basement-controlled fault systems, oblique to the NNW-SSE Sirt Basin trend. The fault systems were active during the Sarir Sandstone deposition, giving rise to structural as well as well as combined structural-stratigraphic traps. An increased understanding of trap architecture has led to both re-evaluation of older fields and new discoveries. Complex structural traps exhibiting four-way dip or fault closures, and combined structural-stratigraphic traps, have been successfully explored in recent years, and will continue to provide exploration opportunities. The prospective areas comprise the faulted basin margins and the Sarir Sandstone depositional and erosional edges. Modern geophysical techniques including high-resolution 3D seismic data acquisition are critical in discovering and developing the remaining potential.
The Messiah field, located in Arabian Gulf Oil Concessions 65 and 80 in the SE Sirte Basin, Libya, is a large combined structural‐stratigraphic trap on the flank of the Messiah High. The basement rocks of the Messiah High are onlapped by the Lower Cretaceous Sarir Sandstone, where the pay is trapped. The thickness of the Sarir Sandstone reservoir has been analysed using a statistical approach. Regression of the thickness of the Sarir Sandstone indicates that the unit wedges out against basement. The reservoir geometry developed as a result of syn‐rift tilting of the Basement and truncation at the top of the Sarir Sandstone. Post‐rift deformation of the Messiah field is restricted to subsidence with minor tilting, without significant faulting. The results of the linear regression are relevant to the exploration for similar traps in the area of the Messiah High.
Natural gas sourced from Carboniferous coal-bearing strata is produced from Carboniferous, Permian and Triassic reservoirs in the Southern Permian Basin in northwest Europe. The composition of this gas has been reviewed. In all reservoirs a distinct area rich in non-hydrocarbons, principally nitrogen, coincides with the basin depocentre in northern Germany. Furthermore, the average nitrogen concentration increases upwards in the stratigraphy. Carbon dioxide is a minor component which is geographically restricted and occurs mainly in Carboniferous and Zechstein reservoirs. It is generated indigenously in these reservoirs and does not migrate into other reservoirs. Hydrogen sulphide occurs only significantly in the Zechstein and is areally restricted to the basin margin facies of this reservoir. It is formed by thermochemical sulphate reduction. The maturity of the source rock, timing of the trap formation relative to the gas source maturation and reservoir lithology appear to be the dominant controls on the non-hydrocarbon gas composition in the Southern Permian Basin. For very high nitrogen gases (50-100 vol. %), an admixture of inorganic nitrogen to the thermogenic gas from organic sources cannot be excluded.
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