Stratigraphic architecture of an Early–Middle Jurassic tidally influenced deltaic system (Plover Formation), Browse Basin, Australian North West Shelf

Abstract

Understanding the internal stratigraphic architecture of sand-dominated deltas is critical to assessing the extent and distribution of petroleum reservoirs. The stratigraphic architecture and evolution of a major Early–Middle Jurassic fluvio-deltaic system (Plover Formation) on the Australian North West Shelf has been established through integrated analysis of core, borehole image logs and wireline logs for the Calliance field in the Browse Basin. Six facies associations identified in cored intervals are interpreted as tidally influenced channel- and tidal channel-fill complexes (FA1–FA2), crevasse-splay deposits and interchannel marshes (FA3), heterolithic mouthbars and sandflats (FA4), sandy mouthbars (FA5) and offshore transition to offshore (FA6). Therefore, the overall depositional system in the study area is proposed to be a tidally influenced deltaic system, in which FA6 represents prodelta deposits, FA5 and FA4 constitute distributary mouthbar deposits of the delta front and tidally influenced channel and interchannel deposits represent the lower delta plain (FA2, FA1, FA3). Analysis of image lithology and fabric are used to extend interpretation to uncored intervals and to identify intrusive and extrusive igneous units and associated volcaniclastic facies within the formation. Five third-order stratigraphic sequences (S1–S5) record progradational (S1, S2 and S4) and retrogradational (S3 and S5) phases of delta evolution. Paleocurrent indicators derived from borehole image logs indicate common southerly directed sediment dispersal in S2 and S3 and increasingly complex with westerly directions in S4 and S5. Two rift-related depositional phases are recognised separated by a phase of uplift between S3 and S4. The stratigraphic succession of S2 is consistent with the depositional pattern expected in a synrift setting and deposition of the over-thickened sandy succession (FA5 in S4), which is also the major reservoir interval, was most likely controlled by syn-tectonic faulting.