Structural constraints on Lower Carboniferous shale gas exploration in the Craven Basin, NW England

摘要

Detailed interpretation of a 3D seismic data volume reveals the detrimental effect that post-depositional tectonic deformation has had on buried Lower Carboniferous (Dinantian-Namurian) shales and its consequences for shale gas exploration in the SW part (Fylde area) of the Craven Basin in NW England. The structural styles primarily result from Devono-Carboniferous (syn-sedimentary) extension, post-rift subsidence and Variscan inversion, a renewed phase of Permo-Triassic extension, and Cenozoic uplift and basin exhumation. In contrast to the shallow dips and bedding continuity that characterizes productive shale gas plays in other basins (e.g. in the USA and Argentina), our mapping shows that the area is affected by deformation that results in the Bowland Shale Formation targets being folded and dissected into fault-bound compartments defined by SW-NE striking (Lower Carboniferous and Variscan) reverse faults and SSW-NNE to N-S striking (Permo-Triassic) normal faults. The fault networks and the misalignment between the elongate compartments they contain and the present-day minimum horizontal stress orientation limit the length over which long lateral boreholes can remain in a productive horizon, placing an important constraint on optimal well positioning, reducing the size of the shale gas resource and affecting well productivity. Our subsurface mapping using this high-fidelity dataset provides an accurate picture of the Upper Palaeozoic structure and demonstrates that faulting is denser and more complex than apparent from geological mapping of the surface outcrop. That structural complexity has direct and significant consequences for: the location of well pads; the lateral continuity of target shale gas horizons; the evaluation of the risk of inducing seismicity on seismically resolvable (large displacement) fault planes prior to drilling; and the likelihood of faults with small throws (below seismic resolution) being present.