Authigenic fluid inclusions in lithic sandstone: A case study from the Permo‐Triassic Gunnedah Basin, New South Wales

Abstract

The Gunnedah Basin is a structural subdivision of the Permo‐Triassic Sydney‐Bowen Basin, a major Permian coal province in eastern Australia. Coarse‐grained sediments of the Permian Maules Creek, Porcupine and Watermark Formations within the basin are predominantly volcanolithic to quartz‐lithic conglomerate and sandstone that were deposited in alluvial, marine fan delta and marine shelf environments. Triassic strata of the Digby and Napperby Formations were deposited in alluvial and lacustrine environments respectively. Fluid inclusions in authigenic quartz overgrowths and healed grain fractures identified in sandstone from these rocks comprise four distinct types (Types 1–4) generated during three discrete diagenetic episodes (GI [oldest]‐GIII [youngest]). Two of the inclusion types (Types 3 and 4) are hydrocarbon‐bearing and their distribution indicates that migration of hydrocarbons occurred late in the diagenetic history of the rocks, although homogenisation temperatures imply that emplacement of these inclusions pre‐dated maximum burial temperature. Microthermometric analysis of 57 aqueous inclusions gave homogenisation temperatures ranging from 62°C to 140°C. The majority of samples are now located at depths shallower than those at which the fluids were entrapped and data are used to constrain the palaeothermal and tectonic history of the basin. Freezing runs yielded first melts at between −65°C and −31°C suggesting the presence of a Ca‐Na‐Cl system. Ice melts ranged from −8.5°C to −0.2°C and are interpreted to represent much lower salinities than would have been present in the original marine depositional environment of the Porcupine and Watermark Formations.