Early Permian Stratal Architecture and Depositional History, Northern Perth Basin, Western Australia

Abstract

The Perth Basin is a proven petroleum province with considerable gas and oil production from conventional reservoirs since the 1960s, and large reserve estimates associated with organic-rich shale-bearing Permian and Triassic formations. These unconventional resources are current exploration targets. The Carboniferous-Permian interval in the northern part of the basin forms an important Late Paleozoic cold- to cool-climate succession and a long-term record of shallow to marginal marine deposition in glacial to post-glacial conditions. The succession is well exposed on the Irwin Terrace on the north-east of the basin, correlative of hydrocarbon-bearing strata to the west, and thus provides an opportunity to develop a sequence-stratigraphic framework. Although major lithostratigraphic units are broadly correlatable in outcrop, marked lateral facies changes and locally limited biostratigraphic data further compound basin-scale interpretations. The lower part of the succession (Carboniferous to Early Artinskian Nangetty Formation and overlying Holmwood Shale, ~1700 m thick) is characterized by coarse siliciclastic facies, diamictites, sandy mudstones, and minor carbonate facies. The overall dominance of muddy facies locally with dropstones suggest a proglacial shelf setting. Fossiliferous and bioturbated siltstones appearing in the upper part indicate gradual shallowing. The overlying Early Artinskian High Cliff Sandstone records a significant change in deposition with higher energy sandstone facies associated with progradation of a deltaic system. A distinct erosion surface overlain by conglomeratic facies and small dropstones in its upper part may be correlated over tens of kilometers based on gamma-ray logs in surrounding wells. Prospective shale gas formations (mid Artinskian to mid Kungarian, ~600 m thick) form the upper part of the overall succession. Continued progradation is recorded by channel-filling cross-bedded and ripple cross-laminated sandstone, heterolithic and mudstone facies with coal lenses (Irwin River Coal Measures). Sedimentary structures such as mud drapes, flaser bedding, syneresis cracks, plant fossils and roots, and locally well-developed bioturbation reflect the development of a tide-influenced to marshy setting associated with climatic amelioration. The succession is capped by transgressive mudstones with some large dropstones and locally thicker sandstones of the Carynginia Formation deposited on a storm-affected lower shoreface.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015