Source Rock Evaluation of Datta Shale, Kohat Sub-Basin, Pakistan (T-1423) (MFN 4990)

Abstract

The Kohat sub-basin, a northwest extension of the Indus basin in KPK, is the main hydrocarbon producing basin of the province. Despite the fact that the first hydrocarbon discovery was made in the adjacent Potwar sub-basin in 1915, the discovery of hydrocarbon in the Kohat sub-basin is relatively recent and started with a breakthrough discovery from Chanda-01 by Oil and Gas Development Company Limited in 1999. Following this discovery, around 10 hydrocarbon fields have been discovered in the Kohat sub-basin since then. Jurassic Datta Formation is producing liquid hydrocarbons in the Kohat area. Datta sandstone is acting as a reservoir. However, little is known about the source potential of Datta shale. The potential to generate hydrocarbons is function of the organic matter type, quantity of preserved matter and its thermal maturity. Outcrop samples from Chichali gorge and well cuttings of Chanda field, and oil samples from Chanda wells were analyzed for geochemical characteristics to understand the organic matter source, environment of deposition, thermal maturity, and probable type of hydrocarbon generation and correlation of Datta source characteristics with crude oil to establish oil-source correlation. The quantification of organic content of Datta shale indicates variation in organic richness. The well cuttings of Datta shale falls in the category of good to very good source rock. The variation in organic richness in comparison to outcrop samples of Chichali gorge reflects facies variation. The qualitative assessment of Datta shale by Rock Eval pyrolysis indicates thermal maturity, hydrocarbon potential. With reference to generated hydrocarbons (S1) and remaining potential (S2), Datta shale ranked as fair to good source rock. Datta shale mainly comprises marine organic matter, Type-II with some terrestrial input in few cases. Type-II organic matter is oil/gas prone whilst type-III is gas prone. In case of outcrop samples very low values of hydrogen index result from weathering, oxidation of organic matter. RE shows maturity of the samples in peak oil generation phase. It is supplemented by VR values. The crude oil samples and Datta source rock extract show higher abundance of saturated hydrocarbons, being > 40%. The relative ratio of fractions; saturate HCs: aromatic HCs: NSO compounds show good correlation of Chanda oil to Datta shale. There is no odd to even predominance or bimodal distribution for oil and extracts saturated hydrocarbon fingerprinting. OEP and CPI values of rock extract and oil are close to one, indicating comparable maturity. However, Datta shale extracts show higher concentration of higher n-alkanes compared to Chanda oil. Isoprenoid/n-alkane ratio indicates the mixed organic matter input for Chanda oil while Datta shale has predominant marine organic matter. Both, the oil and extract show higher bacterial source contribution than algal as terpane biomarkers fingerprint is dominated by hopanes with low relative abundance of tricyclic terpanes reflected. Higher relative abundance of C29 sterane indicates terrestrial influence with marine input. Sterane maturity parameter, S/S+R of crude oils from Chanda field reflects its source rock has attained equilibrium for isomers and values of these parameters are comparable to those from Datta shale. ββ/ββ+αα values indicate the source of Chanda oil and Datta shale has attained peak oil generation phase. Oleanane biomarker is source and age indicator, it has origin in land plants, angiosperms evolving from Late Cretaceous and early Tertiary period. Oleanane index values for oil and source rock extract demonstrate Jurassic-Cretaceous age. The stable carbon isotope of Datta shale and Chanda oils show positive oil-source correlation.