Small-Scale Sandstone Injectites Surrounding Oil-Filled Reservoirs – Examples From the North Falkland Basin, South Atlantic and West of Shetland, UKCS

Abstract

Despite recent hydrocarbon discoveries within large-scale sandstone injectites, smaller features remain poorly understood. Smaller-scale (1-10 cm wide) sandstone injectites are regularly observed in many hydrocarbon-filled reservoirs, but typically overlooked. However, the fact that they are commonly intersected by a 6-8 inch core barrel, some of them being 2 cm in width and completely vertical, suggests that they form laterally pervasive zones of injection directly within and above hydrocarbon reservoirs; a heterogeneity that is very rarely modelled.

These features are observed in close proximity to the oil-filled reservoir rocks of the Sea Lion Fan in the North Falkland Basin and the Solan Field, west of Shetland. Sandstone injectites, which occur around the Sea Lion Fan, contain intra-formational clasts, comprising fragments of the host shale into which the sandstones intruded. Both these clasts and the margins of the injectites demonstrate evidence of ductile deformation associated with the intrusion. The observation of ductile deformation of the clasts and the host material suggests that the intrusion occurred prior to the lithification of the host sediments, with some sediments showing a complete lack of cohesion. Furthermore, ptygmatic folding of some of the injectites provides additional support that these features formed prior to advanced compaction of the succession.

Intrusion of injectites requires a two-stage process: a primer and a trigger. If the priming mechanism for intrusion can be attributed to a particular process or event, that event has the potential to provide important information regarding the early stages of a developing sedimentary basin. One mechanism that may have acted as a primer for injection may have been increasing hydrostatic pressure. An increase in hydrostatic pressure can be related to a range of processes, including: compaction, regional uplift, diagenesis and fluid migration. However, given the proximity of the injectites to oil-filled reservoirs, it is possible that hydrocarbon generation and migration is responsible.

In the case of Sea Lion, this implies that oil charge occurred relatively early. Given that main oil generation is modelled to have initiated in the late Cretaceous in the North Falkland Basin, these findings suggest an earlier phase of oil generation and charge, possibly from a second deeper source rock as the main source interval would not have reached thermal maturity at the time of injection.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018