The Influence of Depositional and Maturation Factors on the Three-Dimensional Distribution of Coal Rank Indicators and Hydrocarbon Source Potential in the Gunnedah Basin, New South Wales
Three-dimensional modelling of vitrinite reflectance has been used to enhance the understanding of lateral and vertical rank variations in the Permian coals of the Gunnedah Basin, New South Wales, Australia. The level of organic maturity of the coals has been investigated using both petrographic (vitrinite reflectance and fluorescence) and chemical methods (proximate and ultimate analyses, and electron microprobe data). The coal is of high-volatile bituminous rank, with a mean maximum vitrinite reflectance of between 0.56 and 1.1%. In addition to maturation-induced trends, a significant influence of depositional environment has been identified on vitrinite reflectance and other coal rank indicators in different parts of the sequence.
Lower than normal vitrinite reflectance is developed in several parts of the Permian sequence, where marine strata overlie the coal-bearing interval or where lower delta plain facies are present. The coals in these intervals have a perhydrous character, increased fluorescence intensity and contain framboidal pyrite, that combine to make them distinctive in petrographic studies. When plotted against depth all vitrinite reflectance values in these parts of the sequence are shifted to the lower side of the more “normal” depth/reflectance regression line. Such anomalies can be recognised at equivalent horizons over wide areas, suggesting basin-wide marine flooding events. If not allowed for in some sections rank, as expressed by vitrinite reflectance or volatile matter content, would appear to decrease instead of increase with depth.
Coals in other parts of the section have anomalously high vitrinite reflectance values, and contain hydrogen-poor material described elsewhere as ‘pseudovitrinite’. Data from such coals plot to the right of the regression line in vitrinite reflectance profiles.
Chemical and petrographic studies show that the different vitrinite types follow separate coalification tracks, and hence both high and low-value anomalies need to be taken into account when interpreting maturation patterns. The depositional controls and the rank trends both have implications to maturation studies, and to prospectivity mapping for coalbed methane and petroleum generation.
KeywordsCoal Seam Volatile Matter Vitrinite Reflectance Coal Rank Marine Influence
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