Organic geochemistry and petroleum potential of Permian outcrop and core samples from the southern Sydney Basin

Abstract

Permian sediments occur throughout the southern Sydney Basin, exposed on the coastline south of Wollongong, and penetrated by various boreholes. This study uses outcrop samples and samples from three boreholes held by NSW Resources and Energy at the core library at Londonderry (Department of Mines Callala DDH1 [Callala-1] from near Callala Bay, Elecom Clyde River DDH07 [ECR-7] from near Nowra, and Elecom Clyde River DDH01 [ECR-1] from near Wingello). Formations analysed include the Berry Siltstone, Nowra Sandstone, Wandrawandian Siltstone, the Snapper Point Formation, the Pebbley Beach Formation and the Yarrunga Coal Measures. The objectives are to determine the depositional environment, organic matter inputs, thermal maturity and petroleum generation potential of these formations, which were deposited when Australia was close to the South Pole. The rocks at outcrop and in ECR-1 are in the early oil window, while rocks from near Jervis Bay (Callala-1, ECR-7) are in the gas window. Total organic carbon content is heterogeneous and varies from 0.2–6.7%. The rocks were deposited in oxic to suboxic depositional environments. The Wandrawandian Siltstone contains biomarkers dominated by very high amounts of diahopanes and diasteranes, whereas these biomarkers are of lower relative abundance in the other formations. This is suggestive of a clay-rich sediment in an oxic, acid-catalysed depositional environment, with enhanced diagenetic alteration of the biomarkers, or alternatively an unusual organic input. The Pebbley Beach and Snapper Point formations are characterised by biomarker distributions dominated by terrigenously sourced terpanes (e.g. C24 tetracyclic terpane and C19 tricyclic terpane), corroborating their deltaic and shallow marine depositional environments, respectively. In contrast, the Wandrawandian Siltstone contains dominantly C21, C23, and C24 tricyclic terpanes. The Pebbley Beach Formation contains high amounts of C29 relative to C28 and C27 steranes, also consistent with a dominant terrigenous input.