Deformation band control on hydrocarbon migration

The hydrocarbon potential of sandstone reservoirs may be significantly affected by postdepositional compartmentation by grain-scale deformation mechanisms such as pressure solution, dislocation creep, microfracturing, and small-scale faulting. Deformation bands, sub 1-mm-thick elongate layers characterized by intense localized brittle fracturing, are important small-scale structural features controlling fluid flow in reservoir sands. They can act as fluid pathways or as barriers to fluid flow, depending on the timing relationship between hydrocarbon charge and deformation. Characterization of deformation-band scale, distribution, and timing of deformation relative to hydrocarbon charge is therefore critical in a full assessment of reservoir history. Integrated CL and UV studies of fluid inclusions from the Jeanne d'Arc Basin, Newfoundland, demonstrate that hydrocarbon charge predated deformation-band formation because oil-trapping quartz overgrowths are clearly cut by deformation bands. In contrast, porous, bitumen-rich sandstones from the Upper Old Red Sandstone of Caithness, NE Scotland, contain hydrocarbon inclusions trapped in quartz overgrowths that formed after deformation. Detailed petrographic work and CL imaging of these Old Red Sandstone samples demonstrates that hydrocarbons are absent in cemented deformation bands and occur only in undeformed sandstone pores and late quartz overgrowths. Post-deformation hydrocarbon migration pathways were thus constrained by effectively sealed deformation bands. In a third example, from the Jurassic of Skye, there is evidence for hydrocarbons in early (pre-band) cements, entrainment of hydrocarbons during band development, and a later pore-filling oil charge, indicating a complex relationship between the migration history and the bands.