Sequence stratigraphic analysis of fluvial deposits using facies characterization and wireline log correlation: case of the late Early-early Middle Devonian Snake Cave Interval, Darling Basin, Australia

Abstract

In this article, we present the application of wireline log correlation for sequence stratigraphic analysis to the late Early to early Middle Devonian Snake Cave Interval in the Neckarboo Sub-basin (DM Kewell East DDH-1 well) and Hillston Trough (DM Mossgiel DDH-1 well), Darling Basin. The results of an integrated study using characterising wireline log signatures (gamma ray, sonic, resistivity shallow, long spaced neutron and limest neutron porosity) and core description data supplemented by limited palaeontological reports indicate that three major upper Lower to lower Middle Devonian cycles have controlled sedimentation of this stratigraphic succession. We distinguished seven core sedimentary facies, which were grouped into six electrofacies defined by wireline log signatures. The electrofacies were defined by describing and analysing the graphic gamma ray logs using basic geometrical shapes, such as bell, serrated bell, funnel, serrated funnel, cylinder (block) and linear. Six electrofacies informally named electrofacies SA, SB, SC, SD, SE and SF correspond to seven core facies (MS1-MS7), which have progressively greater sandstone/shale ratios and better electrofacies quality. These electrofacies are characterised as follows: electrofacies SA corresponds to the conglomeratic braided channel-fill, electrofacies SB corresponds to channel lag and point bar, electrofacies SC corresponds to crevasse splay sandstones and flood-plain fines, electrofacies SD corresponds to multistorey fluvial channel-fills, electrofacies SE corresponds to sandy braided channel-fills and electrofacies SF corresponds to distributary channel sands and local small delta-plain. The upper Lower to lower Middle Devonian sequence (Snake Cave Interval) of the Neckarboo Sub-basin and Hillston Trough is characterised by two major third-order sequences (Snake Cave sequence 1 and Snake Cave sequence 2) that consist of ten systems tracts and ten sequence surfaces. Snake Cave sequence 1 (SCS1) consists of two cycles with progradational to retrogradational trends (LST1 to TST1 and LST2 to TST2 separated by a marine flooding surface) divided into five sequence surfaces (SBSC1, TS1 MFS1, TS2 and MFS2, in ascending order) as defined by wireline log signatures. One cycle has progradational, retrogradational to aggradational trends (LST3, TST3 to HST1) and has seven sequence surfaces (e.g. MFS2, LST3, TS3, MFS3 and SBSC2, in ascending order). Snake Cave sequence 2 (SCS2) consists of one unit with progradational, retrogradational to aggradational trends (LST4, TST4 and HST2, in ascending order) with three sequence surfaces (e.g. SBSC2, TS4 and MFS4). No TST4 and HST packages were observed, and the MFS4 is poorly developed. The recognition of sedimentological sequence stratigraphic changes above and below marine flooding surfaces (MFS1-4), where deposition sequences have been defined without adequate biostratigraphic control, is easy since depositional sequence and bounding surface correlation can always be clearly demonstrated. A sequence stratigraphic architecture that can be used to accentuate and strengthen Darling basin exploration also documents significant changes in the local and regional tectonostratigraphic setting.