Organic matter characteristics and conventional oil potentials of shales from the Early Jurassic Datta Formation in the Upper Indus Basin, Northern Pakistan

Abstract This study deals with organic matter in the Early Jurassic Datta shale in the Upper Indus Basin, Pakistan. Organic geochemical and microscopic techniques were used to investigate organic matter input, environmental conditions of deposition as well as petroleum generation potential. The geochemical results suggest that the shales display mainly fair to good source rock generation potential, with poor petroleum generation potential in some samples. The HI varies between 32 and 519 mg HC/g TOC, with many of the samples appearing to be mainly Type II and mixed Type II-III kerogen, and thereby exhibiting the ability to generate both oil and gas. The dominance of such Type II kerogen in some samples is consistent with the high HI values (> 300 mg HC/ g TOC) and high fluorescence alginite macerals. A section of the shale within the Datta Formation appears to be gas-prone, with HI values of less than 200 mg HC/g TOC. Biomarkers also indicate the presence of mixed organic matter of planktonic/bacterial and land plant input, which were deposited in a marine environment under relatively oxic conditions. The biomarker characteristics of the extracted shales correlate well with those of the oil from the Chanda Oilfield, indicating that the Datta shales are likely to be the effective source rock. This is supported by the thermal maturity, which ranges from moderate to peak-maturity, as indicated by vitrinite reflectance values of 0.69-0.82%, and the biomarker maturity indicators.