Abstract
The distribution of hopanoids and alkanes in bitumen extracts of coals, coaly shales, and shales from northwestern Taiwan has been measured to assess the maturity of potential petroleum source rocks in the studied area. The C31 homohopane isomers of the studied coals and coaly shales attain equilibrium at maturity corresponding to 0.6% Ro measured in coals or 0.65% Ro in adjacent shales. A proposed parameter using hopanoid ratios reveals a more consistent trend and approaches a steady value at maturity similar to the C31 homohopane isomer ratio. The results show that the onset of the main phase of petroleum generation in northwestern Taiwan takes place at maturity corresponding to VR of 0.6% Ro measured in coals or coaly shales and probably higher VR in shales. The distribution of pristane, phytane n-C17 and n-C18 confirms the validity of the conventional maturity indicators if the samples are from the same source, but shows depositional-or organic-source controls. The source influence on the phytane/n-C18 ratio appears to be larger than that on the pristane/n-C17 ratio. The phytane/n-C18 ratio is also less sensitive to maturity than that of the pristane/n-C17 ratio. The pristane/phytane ratios in the studied coals and coaly shales, in contrast to the pristane/n-C17 ratio, continue to decrease to maturities beyond the oil window up to 1.5% Ro and therefore may be applicable to high maturity sources.
Highlights
The sources and maturity of gas, condensate or light oil occurring in northwestern Taiwan have been of major research as well as exploration interest
This study focuses on analyzing biomarker molecules in bitumen extracts from shales, coaly shale, and coals in order to provide independent maturity indicators other than Vitrinite reflectance (VR)
Several observations and conclusions can be derived according to the distribution of biomakers measured from bitumen extracts of coals, coaly shales, and shales: (1) The change in C31 homohopane isomer ratios with vitrinite maturity of the studied coals and coaly shales is approaching a steady-state at maturity around or slightly higher than 0. 6% Ro measured in coals
Summary
The sources and maturity of gas, condensate or light oil occurring in northwestern Taiwan have been of major research as well as exploration interest. A previous study, reveals a discrepancy between the VR measured in organic matter from a variety of shales and adjacent coal seams (Bostick and Foster 1975). It has been found in our study area (Kuo et al 1987; Shen and Kuo 1991) that the average VR measured from the dispersed type-III organic matter in shales is slightly higher (0.04%) than those from adjacent humic coals (Fig. 1). The thermal maturities of source rocks may be evaluated using independent parameters such as biomarkers.
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