Depositional Analysis of the Brigadier Formation in the Rankin Trend, North West Shelf, Australia
Scardigno, Marco F
Honours Degree 2005
University of Adelaide
The Rankin Trend of the Dampier Sub-Basin is a major provider of gas in the North West Shelf. The Triassic to Jurassic fluvio-deltaic reservoirs have received much attention, including the Brigadier Formation (also know as the D Unit), which is currently producing in the North Rankin and Goodwyn Fields. However, a detailed investigation into the depositional processes that formed the Brigadier Formation in the Rankin Trend had not previously been undertaken.
The Brigadier Formation is a siliciclastic, marginal marine unit. Its facies indicate tidal-dominated, estuarine, restricted marine and offshore marine environments, with continued fluvial sediment supply. Coarsening-up successions, prominent throughout the Brigadier Formation, are interpreted to represent the progradation from shelf mud to tide-dominated delta sand, topped with clean distributary channel, mouthbar or shoreface sand.
The Brigadier Formation was deposited as the transgressive systems tract of a 3rd order sequence, overlain by marine shales of the succeeding highstand systems tract, and overlying the lowstand systems tract of the fluvial-based Mungaroo Formation. The sequence is backstepping and represents the gradual transgression and filling of a sandy, coastal-plain incised-valley system over many higher-order sea level cycles. Times of high relative sea level resulted in a restricted marine embayment, while a coastal plain incised-valley system developed at times of falling relative sea level. Image logs suggest a prominent southwest fluvial palaeocurrent, while net to gross maps show a very clear trend from high net to gross in the North Rankin and Perseus Fields, to low net to gross to the southwest in the Western Flank.
An understanding of subsurface geology and depositional models is fundamental to the exploration, development and production of reservoirs, especially in those whose reservoir quality is based primarily on grainsize and clay content, such as the Brigadier Formation.
The application of chronostratigraphy was essential for the creation of high resolution correlations and palaeogeographic maps. The use of biostratigraphy and chronostratigraphy led to major changes in some of the well correlations, such as a 130 metre vertical change in the Keast-2 to Dockrell-2 correlation.
This study provides reservoir facies analysis, high resolution well-to-well correlations, palaeogeographic interpretations, depositional environments and analogues, and reservoir dimensions for the Brigadier Formation. The development of genetic reservoir facies from core samples, together with an integration of much data from image logs, palynology, well logs, seismic and sequence stratigraphic and petrological studies, led to an understanding of probable depositional environments and models for the Brigadier Formation.