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Regional Assessment of the Toolebuc Formation: A Shale Oil Play in South-West Queensland, Australia



A regional assessment of the Toolebuc Formation has been conducted to evaluate its potential as a shale oil target. It shares many characteristics with successful shale oil plays in the United States of America. This study has focussed on mapping key parameters for the formation, including depth, thickness, lithology, mineralogy, maturity (Rv-max and Tmax), total organic carbon (TOC) and mud gas compositions and has identified a possible play fairway covering approximately 20 000 km2 where the formation is deepest. The Early Cretaceous Toolebuc Formation is laterally extensive over approximately 630 000 km2 in the Eromanga and Carpentaria basins in western Queensland, Australia. It is relatively shallow, extending from outcrop to approximately -1500 mASL. It is commonly between 10-20 m thick, though it thickens to over 30 m in a belt through the central Eromanga Basin. The formation exhibits a distinct gamma-ray anomaly with a serrated appearance that may contain multiple peaks. It contains three facies: a calcareous mudstone, a coquinitic limestone and a kerogenous calcareous mudstone, all of which can be detected lithologically, mineralogically and on wireline logs. The mineralogy varies in each facies, though overall, the formation contains approximately 40% quartz and feldspar, 40% clay minerals (smectite-dominated), 20% calcite and 4% pyrite. TOC concentrations for the Toolebuc Formation are commonly over 10 wt% and in some areas of the formation may exceed 30 wt%. Thermal maturity of the formation has been modelled based on reflectance profiles (Rv-max) established from petroleum wells across the Eromanga Basin. These values range from 0.31% to 0.78%, with the area of highest maturity where it is buried deepest in the central Eromanga Basin. Modern day temperatures for the formation have been calculated for six petroleum wells in the deepest area, with temperatures of up to 102.6°C at the base of the formation. The Toolebuc Formation has a consistent gas peak in mudlogs from wells across the Eromanga and Carpentaria basins, with butane and pentane detected. Isotopic analysis of desorbed gas from GSQ Julia Creek 1, where the formation is approximately 180 m deep, suggests an early thermogenic origin. Wetness and balance ratios have been calculated for wells in the Eromanga Basin to determine the character of these hydrocarbons. These ratios suggest that the Toolebuc Formation may have generated oil in the deeper, more mature areas.