Geochemical Characterization Of Warchha Sandstone And Sardhai Formation, Potwar-Sub Basin, Pakistan (T-1442) (MFN 7085)

Abstract

Pakistan is having a tremendous hydrocarbon potential in its different basins, among all these basins the most petroliferous basin is Potwar sub-basin, where exploration activities began more than a century ago. Although, the origin of petroleum has been under study from the last few decades. Focus on Hydrocarbon geochemistry of source rock and crude oils start extensively during recent times, to explore new hydrocarbon plays organic geochemistry plays an important role. Several studies were carried out on source rocks, and it is generally believed that the Patala Formation (Paleocene) plays as a source rock in most of the Potwar sub-basin but the correlation between source rock and crude oil analysis has not yet been confirmed by any reliable geochemical study. Total organic carbon content of Permian Warchha and Sardhai formations in 10 outcrop samples from Pidh village, Karuli Village and Zaluch Nala, and 51 well cuttings from Chak Naurang South-1 was evaluated to classify the source rock characteristic. In case of outcrop as well as cuttings samples, TOC values < 0.5% indicate the poor source rock quality of present day Permian formations. Similar source rock quality is reflected by S1 and S2 hydrocarbons. Well cuttings fall in category of Type IV kerogen type and therefore low present day hydrocarbon potential. Thermal maturity of Permian source based on Tmax values of studied samples show marginal maturity except for few samples. However, the vitrinite reflectance measurements of studied samples have range 0.84 to 0.87 %, close to peak oil window. Organic matter of both the formations appears oxidized, inertinite-Type IV organic matter. The source rock extracts and oils show presence of unresolved complex mixture (UCM) that indicates biodegradation. The distribution of n-alkanes of source rock extracts with even-odd predominance EOP in alkanes < nC20 represents marine source input. In oils, carbon range upto n-C35 is observed with little OEP that indicate oil samples have high organic matter input from land plants. Pr/Ph values of crude oils indicator of slight sub-oxic environment of deposition of the source rock of Chak Naurang oils while Permian strata was deposited under anoxic condition. The ratios of saturates/aromatics for source rocks, Sardhai and Warchha is > 1 while Pr/Ph <1, C29/C30 hopanes > 1 shows source is carbonate v deposited in anoxic environment. While saturates/aromatics ratio 1.17 (≥1), Pr/Ph 1.06 (≥1), C29/C30 hopanes 0.71 (Low ≤1) and DBT/P 0.61 (≤1) suggest that source of analyzed crude oil is predominantly marine shale. C27 steranes are predominately derived from the marine source and C29 steranes are sourced by terrestrial organic matter, the source horizon of Sardhai and Warchha show >55 % C27 steranes whilst oil samples have very high abundance of C29 steranes (>55%). C32 homohopane isomerization index and sterane isomerization index show thermal maturity of the sedimentary organic material (source rock of oils) in oil-window. Sardhai and Warchha have calculated vitrinite reflectance values of 0.78% and 0.80% while values for oil samples have range 0.97-0.98%, these values correspond to mature source rock that is in oil window. The n-alkanes in the range C6-C12 were not detected in extracts, probably some of light hydrocarbons were lost during extraction process and some due to biodegradation. In the analyzed oils, light hydrocarbons are present along with UCM and most probably resulted from multiple source rocks.