Abstract

The fluvio-deltaic Moogooloo Sandstone is an attractive petroleum exploration target because it is favorably positioned with respect to source and seal units. To assess and predict reservoir quality distribution in the Moogooloo Sandstone and laterally equivalent Keogh Formation, a petrological and petrophysical calibration data set was established by acquiring extensive core petrology, porosity, permeability, and thermal maturity data.

Significant differential rates of burial and subsequent uplift and erosion of the Moogooloo Sand stone led to a varying thermal regime and, consequently, major differences in diagenetic history across the basin. Quartz overgrowth abundance increases with increasing thermal maturity from less than 1% where burial temperature never exceeded 58°C to greater than 10% where maximum burial temperatures were around 100°C or higher. Other cements include calcite and ankerite that precipitated from depositional pore water of meteoric origin and that incorporated organic carbon. Early diagenetic chlorite grain rims are pervasively developed within some of the marine-influenced deltaic facies. Porosity loss is mainly due to quartz overgrowth cementation, grain contact dissolution, and, locally, carbonate cementation.

The strong correlation between thermal maturity, quartz overgrowth cementation, and reservoir quality results in a predictive relationship between log porosity (ϕ) and vitrinite reflectance (Ro), according to ϕ = 12.4 Ro-0.4540. Likewise, mean permeability can be modeled to order-of-magnitude accuracy, in most cases, using a power function regression equation containing porosity, grain size, clay volume, and thermal maturity terms.

Regional mapping of thermal maturity and porosity shows that the Moogooloo Sandstone generally lies within the oil preservation window and has a mean formation porosity ranging from 22% near outcrop to 12% in deep basinal areas. In borehole Kennedy Range 1, however, thermal history modeling, a coincident aeromagnetic anomaly, and the K-Ar age of secondary illite suggest unpenetrated igneous intrusives as the cause of anomalously high thermal maturity and low porosity. Similar aeromagnetic anomalies in the region, although small, pose significant risks for exploration.

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