Interpretation of vitrinite reflectance–depth profiles in the Northern Denison Trough, Bowen Basin, Australia

Abstract

The Denison Trough, Bowen Basin, Australia, is a mature hydrocarbon province that contains Permian to Triassic coal and gas bearing sediments deposited in deltaic to shallow marine shelf conditions. The sandstone reservoirs of the Permian Aldebaran Sandstone, Freitag Formation and Catherine Sandstone of the Northern Denison Trough (NDT) were evaluated for their CO2 storage potential as part of the Queensland Government’s ZeroGen CO2 Capture and Storage Project, which found that diagenetic processes acting on the sandstones have resulted in poor permeability and reduced porosity, making them unsuitable for large scale injection and storage in this area.To better understand the factors acting to degrade these reservoirs, the purpose of the current study was to establish a thermal maturity framework for the Permian sandstone reservoirs of the Northern Denison Trough based on vitrinite reflectance measurements of coals and Rock-Eval analysis of coals and shales from the Aldebaran Sandstone through to the Catherine Sandstone.Mean maximum vitrinite reflectance of coals (Rmax) ranges from 0.55% to 0.93%, and estimated maximum burial temperatures calculated from vitrinite reflectance fall between 83 oC – 125 oC. Calculation of palaeotemperatures from vitrinite reflectance data has been hampered by inversions of depth-reflectance profiles at the top of the Aldebaran Sandstone and scatter in vitrinite reflectance data. This has been attributed to depositional effects (including the major marine flooding surfaces at the top of the Aldebaran Sandstone) that have affected vitrinite reflectance evolution in the NDT, producing low reflecting, fluorescing, perhydrous vitrinite near marine flooding surfaces, and subhydrous vitrinite when oxidising conditions prevailed. A vitrinite reflectance difference of up to 0.2% Rmax is detected in an isometamorphic coal, an equivalent palaeotemperature difference of approximately 19 oC. The abundance of perhydrous vitrinite has resulted in largely suppressed vitrinite reflectance throughout the examined Permian interval in the Northern Denison Trough, and the presence of both perhydrous and subhydrous vitrinite introduces an error in palaeotemperature calculation in high volatile bituminous coals.Rock-Eval analysis provides supporting and calibrating maturity data. Tmax over the studied interval ranges from 421 oC to 447 oC, generating a positive, broadly linear correlation between Rmax and Tmax that is applicable specifically to the NDT. Tmax suppression was also detected in perhydrous coals. Hydrogen index serves to evaluate the level of vitrinite reflectance suppression through an inverse relationship between the two parameters. The Rock-Eval data confirms the oil generation potential of coals in the NDT while shales associated with the same stratigraphic intervals have lower hydrocarbon generation potential.This study brings new insights into the thermal maturity framework in the NDT and highlights the importance of reliable vitrinite reflectance data for calculation of maximum burial temperatures, particularly in underexplored or frontier areas.