2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 175-10
Presentation Time: 10:30 AM



, farooqui@ciitlahore.edu.pk

Lower Cretaceous Sembar Formation of Western Sulaiman Foldbelt, Pakistan, is a well known source rock for most of the hydrocarbon reservoirs in the Upper Indus Basin. Recently presence of shale gas in the Sembar Formation has also been reported. We have studied the shales of the Sembar Formation for its geochemical characterization and interpreted the depositional environment and the diagenetic changes.

The geochemical and mineralogical studies of the Sembar Formation from four locations (two from the Loralai-Mekhtar areas and two from Quetta area), revealed that on average these shales are composed of 42% SiO2, 16% Al2O3, 0.67% TiO2, 10% CaO, 2% MgO, 3% K2O, 0.46% Na2O, 1.39% FeO, 2% Fe2O3 and 0.13% P2O5. The shale also contain 194±106 ppm Mn, 65±19 ppm Ni, 39±17 ppm Cu, 84±43 ppm Zn, 343±97 ppm Sr, 275±65 ppm Ba, 23±6 ppm Pb, 100±34 ppm Cr, 155±33 ppm Rb, 25±6 ppm Y, 130±45 ppm Zr, 15±3 ppm Nb, and 147±49 ppm V as average amounts of trace elements. The chemical analyses revealed that the average calcite content is 20%, the average quartz content is 16%, average pyrite content is 2.6%, average organic carbon is 3.3%, average organic sulphur 0.15%, and average inorganic sulphur is 1.4%.

The geochemical data indicate that the Sembar Shale is comparatively less siliceous in the Mekhtar and Loralai areas (Eastern part) as compared to Murree Brewery and Gawani Nala areas (Western part), indicating possibly different provenance. The quartz and clay minerals are positively correlated indicating conversion of feldspar and other less stable minerals into clays during moderate to intense paleoweathering conditions. Higher calcite/(Qtz+Clay) ratio in the Murree Brewery and Gawani Nala indicates significant biogenic activities in western part of the depositional basin. The positive and generally linear correlation between CaO and CO2, and between CO2 and CaO+MgO, also indicate biologically active depositional environment in the western part. Based on the values of the degree of pyritization it appears that most of the shales of the Sembar Formation were deposited in an aerobic water conditions in normal marine environment with temporal and special fluctuations in the amount of dissolved H2S in water.