The structural evolution of the Erris Trough, offshore northwest Ireland, and implications for hydrocarbon generation

Abstract

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.