GEOCHEMICAL CHARACTERIZATION AND SOURCE EVALUATION OF PATALA SHALE, UPPER INDUS BASIN, PAKISTAN

Abstract

Upper Indus Basin is considered to be one of the prolific petroleum production Basins found in Pakistan. The initial discovery for the hydrocarbons in the Potwar subbasin was made in 1915, while in the Kohat Sub-Basin the discovery of hydrocarbons started with a discovery from Chanda-01 by Oil and Gas Development Company Limited in 1999 (OGDCL). Although there has been no such discovery of hydrocarbons in Hazara Basin. However, some of the low-quality coal is found in the carbonaceous shale of Patala formation (Late- Paleocene to Early- Eocene age). Late- Paleocene to Early- Eocene Patala Formation is considered as important potential source rock present in the Upper Indus Basin. The potential to generate hydrocarbons is dependent on the type of organic matter, quantity of preserved organic matter, and its thermal maturity. Outcrop samples from Potwar sub-basin and Hazara Basin and well cuttings of Chanda-1 (Kohat Sub-Basin), Sadkal Center Deep-1, and Sadkal-1 (Potwar sub-basin) were analyzed for geochemical characteristics to understand the organic matter source, environment of deposition, thermal maturity, and probable type of hydrocarbon generation and the outcrop samples of Patala Formation from Potwar sub-basin are correalated with the outcrop sample from Hazara basin to establish well-defined correlation based on biomarkers and screening analysis. The quantification of organic content of Patala Formation demonstrates different trend of organic richness. The well cuttings of Patala Formation fall in the rank of fair to very good source rock, and outcrop samples appeared poor to excellent. The variation in organic richness of outcrop samples of Potwar sub-basin and Hazara Basin reflect facies variation. The qualitative assessment of Patala Formation was carried out by Rock Eval pyrolysis that indicates thermal maturity, and hydrocarbon potential, with reference to generated hydrocarbons (S1) and remaining potential (S2), Patala Formation ranked as fair to good source rock. Patala Formation mainly comprises of terrestrial organic matter, Type-III and in few cases Type-II and Type-II/ III (mix). Type-II and III kerogen is of both oil and gas prone, while type-III is gas prone. In case of outcrop samples from Hazara Basin, very low values of hydrogen index and accordingly T-max resulted from weathering, oxidation of organic matter. RE shows immature source rock and Type-VI kerogen type.vi The outcrop samples show higher abundance of aromatic hydrocarbons, being > 50%. There is odd to even predominance or bimodal distribution for outcrop samples in saturated hydrocarbon fingerprinting, OEP values are > 1.5 and CPI values of outcrop samples are greater than one, indicating comparatively low maturity of source rock and terrestrial input. However, Patala Formation shows higher concentration of higher nalkanes in all the outcrop samples. Isoprenoid/n-alkane ratio indicates the terrigenous organic matter input for all the outcrop samples. Outcrop samples show higher terrestrial source contribution than algal or bacterial as terpane biomarkers fingerprint is dominated by hopanes, tricyclic terpanes and tetracyclic terpane are not appeared in any of the terpane chromatogram (m/z 191). Higher relative abundance of C29 sterane indicates higher terrestrial influence with very little marine input. Sterane maturity parameter, S/S+R of outcrop samples reflect source rock has not attained equilibrium for isomers and values of these parameters are comparable to the accepted value for onset of the oil window. ββ/ αα+ββ values indicate the Patala Formation from Hazara Basin and Potwar sub-basin did not attain peak phase of maturity for oil generation. Oleanane biomarker is known as source and age indicator, it has origin in land plants. Angiosperms evolved from Late Cretaceous to early Tertiary time period and present in rocks of late Cretaceous or younger. All the outcrop samples have demonstrated Oleanane peak in ion chromatograms, indicating terrestrial source input and showing an age of Late- Paleocene to Early- Eocene to Patala Formation.