The Corrib Gas Field contains in excess of 10 12 SCF (28.3×10 9 Sm 3 ) of gas in place in a Triassic fluviatile sandstone reservoir, sealed by late Triassic evaporites and trapped in a relatively simple, but underfilled, NE–SW-trending anticlinal structure. In contrast, the post-Triassic overburden manifests a more complex evolution with indications of flipped polarity, extensional faulting on a crestal detachment structure during the Jurassic, succeeded by alternate episodes of burial and exhumation from the Early Cretaceous onwards. Compactional, thermal and stratigraphic frames of reference have been used to assess the magnitude and timing of the principal exhumation episode, which occurred during the early Cretaceous. Large uncertainty is associated with exhumation estimates derived from individual techniques, but stratigraphic analysis and seismic interpretation support the conclusion that kilometre-scale (800–1700 m) exhumation and erosion of the Jurassic overburden has occurred in the Corrib area. A number of post-Aptian re-burial and exhumation events also occurred but are unlikely to have exceeded the earlier maximum burial of source, reservoir and seal rocks achieved during the Late Jurassic. Continued local extensional reactivation of the listric detachment structure, post-Aptian, has resulted in a pattern of heterogeneous exhumation within the field area. However, the severely attenuated post-rift stratigraphic record throughout the Slyne Basin suggests that long-wavelength processes, such as mantle hotspot activity and shoulder uplift linked to the rifting and thermal subsidence of the Rockall Basin, were the primary drivers of exhumation in the Corrib area during the Cretaceous-Tertiary period.
The Erris Trough is a narrow, elongate, Mesozoic basin lying adjacent to the eastern margin of the Rockall Trough to the northwest of Ireland. Limited drilling in the area has proven Carboniferous, Permo-Triassic, Lower–Middle Jurassic and Cretaceous sediments beneath a thin Tertiary cover. Considerable variation in structural style and preserved stratigraphy is observed along the basin. Based on interpreted fault polarity and observed pre-Cretaceous stratal dip, the Erris Trough can be subdivided into three structural sub-basins separated by diffuse, poorly defined, overlapping or divergent transfer zones.Carboniferous basin development in the area of the Erris Trough was significant but is poorly constrained. Permo-Triassic rifting produced a series of half-graben, some of which were controlled by down-to-the-southeast faults. This extensional phase was followed by post-rift subsidence during the Early and Middle Jurassic. In addition, N–S and NW–SE faults locally influenced depositional patterns. In the southern part of the Erris Trough a Middle to Late Jurassic rifting event produced a reversal of the earlier basin geometry with the generation of northwest- and west-downthrowing normal faults. Along the western margin of the Erris Trough, footwall uplift associated with this event induced massive, kilometre scale, uplift and erosion implying Late Jurassic rifting within the Rockall Trough. Restricted basins may have been developed in the area during the Cretaceous as a result of ‘ponding’ of deposition to the east of this zone of footwall uplift. A minor extensional phase occurred during the Aptian–Albian resulting in local reactivation of the Late Jurassic faults concurrent with rifting in the Rockall Trough. A westerly tilt of the basin was established during the Late Cretaceous–Early Tertiary caused by downflexing associated with thermal subsidence of the Rockall Trough. Regional uplift and erosion occurred during the Oligocene–Miocene.Basin modelling indicates that Lower Jurassic source rocks may have generated oil during the Early Eocene to Early Miocene, dependent on an elevated geothermal gradient during the Early Tertiary. Late Carboniferous sediments may have generated hydrocarbons at a number of times during the basin’s history.
The Central Irish Sea area, from Kish Bank to St. George's Channel and Cardigan Bay, consists of a series of Late Palaeozoic to Cenozoic extensional and transtensional basins which have experienced a multiphase inversion history. Potential hydrocarbon source rocks of Carboniferous and Jurassic ages have been recognised in this area. In the Kish Bank and Central Irish Sea Basins, maturation modelling of these source rocks is hampered by the severely truncated rock record and by the relative paucity of vitrinite throughout much of the preserved post‐Palaeozoic (Triassic) section. Vitrinite reflectance data from six exploration wells have been used to quantify the peak palaeotemperatures attained by the rocks in this area and to estimate the magnitude of net exhumation at these locations. An apparent palaeogeothermal gradient of ˜26°C/km is recorded by the Jurassic sediments in well 42/21‐1, whereas significantly higher palaeogeothermal gradients of 74–78°C/km are interpreted for the Westphalian/Stephanian sediments in the area. At least two periods of rock exhumation have occurred; during the Late Carboniferous‐Late Permian, and again sometime between the latest Jurassic and early Tertiary. Estimates of net exhumation vary from ˜350 m at well 42/12‐1 to ˜1,900 m at well 42/17–1. Our interpretation suggests that the higher palaeogeothermal gradients recorded by the Westphalian/Stephanian sections reflect elevated heat flows during Stephanian to Early Permian times.
Uplift, erosion and removal of overburden have profound effects on sedimentary basins and the hydrocarbon systems they contain. These effects are predictable from theory and from observation of explored exhumed basins. Exhumed basins are frequently evaluated in the same way as ‘normal’ subsiding basins, leading to errors and unrealistic expectations. In this paper we discuss the consequences of exhumation in terms of prospect risk analysis, resource estimation, and overall basin characteristics.Exhumation should be taken into account when assigning risk factors used to estimate the probability of discovery for a prospect. In general, exhumation reduces the probability of trapping or sealing hydrocarbons, except where highly ductile seals such as evaporites are present. Exhumation modifies the probability of reservoir in extreme cases; for example, where a unit may have been buried so deeply before uplift that it is no longer an effective reservoir, or where fracturing on uplift may have created an entirely new reservoir. The probability of sourcing or charging is affected by multiple factors, but primarily by the magnitude of the post-exhumation hydrocarbon budget and the efficiency of remigration. Generally gas will predominate as a result of methane liberation from oil, formation water and coal, and because of expansion of gas trapped before uplift. These factors in combination tend to result in gas flushing of exhumed hydrocarbon basins.Compared with a similar prospect in a non-exhumed basin, resource levels of a prospect in an exhumed basin are generally lower. Higher levels of reservoir diagenesis influence the standard parameters used to calculate prospect resources. Porosity, water saturation and net-to-gross ratio are adversely affected, and (as a consequence of all three) lower recovery factors are likely. Hydrostatic or near-hydrostatic fluid pressure gradients (as observed in exhumed NE Atlantic margin basins) will also reduce the recovery factor and, in the case of gas, will adversely affect the formation volume factor.Hydrocarbon systems in exhumed settings show a common set of characteristics. They can include: (1) large, basin-centred gas fields; (2) smaller, peripheral, remigrated oil accumulations; (3) two-phase accumulations; (4) residual oil columns; (5) biodegraded oils; (6) underfilled traps. Many basins on the NE Atlantic seaboard underwent kilometre-scale uplift during Cenozoic time and contain hydrocarbon systems showing the effects of exhumation. This knowledge can constrain risk and resource expectation in further evaluation of these basins, and in unexplored exhumed basins.
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