Clastic injectites, internal structures and flow regime during injection: The Sea Lion Injectite System, North Falkland Basin

Abstract

This paper details and describes a suite of 143 sub-seismic-scale clastic injectites encountered within the early Cretaceous, early post-rift of the deep-lacustrine North Falkland Basin. The injectites, referred to here as the Sea Lion Injectite System, are encountered below, above and in between the hydrocarbon-bearing, deep-lacustrine turbidite sandstones of the Bleaker 30, Sea Lion North, Sea Lion, Casper and Beverley fans. Sedimentary structures are documented within the injectites including: planar laminations, mud-clast imbrication and clast alignment. Clasts align along centimetre-scale foresets formed through ripple-scale bedform migration in a hydraulically-open fracture. The style of flow within the injectite system is interpreted as initially through fluid turbulence during an open fracture phase, which was followed by a later stage where laminar flow dominated, most likely during the closing phase of the fracture system. The host rocks display evidence for ductile deformation, which along with ptygmatic folding of dykes and internally injected mudclasts, suggests a period of injection into relatively uncompacted sediments. Evidence for brittle fracturing, in the form of stepped margins may be indicative of a separate phase of emplacement into more-compacted sediments. This variability in deformation styles is related to multi-phased injection episodes into host strata at different stages of consolidation and lithification at shallow burial depths. Injectites have been identified in four stratigraphic groupings: above the Bleaker 30 Fan and within/above the Sea Lion North Fan; within the hydrocarbon-bearing Sea Lion Fan; overlying the Sea Lion Fan; and above/below the hydrocarbon-bearing Casper and Beverley fans. This spatial association with the hydrocarbon-bearing fans of the North Falkland Basin is important, considering the ability of injectite networks to form effective fluidflow conduits in the subsurface. Consequently, the findings of this study will improve the characterization of sub-seismic scale injectites (and therefore fluid conduits) within otherwise impermeable strata.

Hybrid event bed character and distribution in the context of ancient deep‐lacustrine fan models

“…The centimetre‐scale injectites are interpreted as post‐depositional, and related to over‐pressuring within the fan sandstones, as was discussed in detail by Dodd et al . (2020). Hybrid event beds encountered in the Hemi‐Limnic Mudstone FA are interpreted as the distal expression of flows in neighbouring active systems.…”

Evolution of a sand-rich submarine channel-lobe system and impact of mass-transport and transitional flow deposits on reservoir heterogeneity: Magnus Field, northern North Sea

“…However, inclusion of sand injectite fabrics in the northern MSB, may indicate the presence of larger injectites which can act as efficient fluid flow conduits between disconnected reservoir units, casting uncertainty on seal integrity in the area (e.g. Hurst & Cartwright 2007;Cobain et al 2017;Dodd et al 2019).…”

The Influence of Complex Palaeobathymetry on Development of Deep-Lacustrine Fan Systems

“…1). Several of these wells cored the Sea Lion Fan, providing significant geological insight into post-rift basin stratigraphy and the geology of these potentially analogous deep-lacustrine turbidite fan deposits (Williams, 2015;Dodd et al, 2019;Dodd et al, 2020;Dodd et al, 2022).…”