Reservoir Quality And Distribution Of The Late Jurassic Sands, Dampier Sub-Basin, Western Australia.
Dubsky, Matthew Karel
Honours Degree, 1999
University of Adelaide
AbstractInvestigations following drilling in the Dampier Sub-basin show the Late Jurassic Angel Formation has high porosity (up to 20 %) and high permeability (up to 1000 mD) at depths of 3000+ m sub sea. As overlying sediments do not reflect this trend, the aim of the study is to investigate the diagenetic events and subsequent reservoir quality within the Angel Formation together with determination of the seal capacity of intraformational shales.
Drill core was provided from 6 wells and sampled for detailed petrography, including compositional determination, X-ray diffraction, cathodoluminescence and scanning electron microscopy. In addition, 3 shales from Mutineer #1B were analysed by mercury injection capillary pressure. The petrography showed that all samples are quartz-arenites with cementation by quartz overgrowths, carbonate, pyrite, authigenic clays, alteration plus dissolution of quartz and carbonate.
Thin section analysis and cathodoluminescence indicate the quartz overgrowth cementation is the earliest diagenetic event and has been etched later by up to two stages of carbonate emplacement and subsequent dissolution that increases porosity. The carbonate mainly occurs as pore-filling, concretionary dolomite nodules that sporadically cement the rock. Compaction following dissolution does not significantly affect reservoir quality. Late stage cements, particularly pyrite and zeolite, only have local detrimental effects.
A series of thin sands in Mutineer #1B show incomplete oil saturation. Mercury injection capillary pressures show the thin shales separating the sands are able to hold a substantially greater hydrocarbon column than present in the well so another mechanism is needed to explain the variable saturation.
The study has shown that the anomalous reservoir quality of the Late Jurassic Angel Formation is associated with large increases in secondary porosity by dissolution of dolomite that re-opens the pore-network.