Study of Stratigraphy and Structural Styles in the Subsurface of Southern Sindh Monocline, Pakistan: Using Seismic and Well Data.

Abstract

The study area is the part of Southern Sindh Monocline. Sindh Monocline is an important hydrocarbon producing area of Pakistan from where a large number of hydrocarbon fields have been discovered in the tilted fault blocks. The area is highly deformed and is characterized by extensional tectonic activities which are related to rifting phases experienced by the Indo-Pakistan plate. Thus the study of structural styles formed due to these tectonic activities is much important as the cretaceous rifting episodes had a profound effect on the formation and occurrence of hydrocarbons in Sindh Monocline. To solve structural problem twenty five seismic lines and well information has been used for the interpretation of stratigraphy and structural styles within the subsurface of study area. Total Eight reflectors have been marked and were named as R1 of Upper Ranikot Group (Lakhra Formation), R2 of Lower Ranikot Group (Khadro Formation), R3 of Parh Limestone, R4 of Upper Goru, R5 of Top Lower Goru Formation, R6 of Sand Below Badin Shale (C- Sand unit) of Lower Goru Formation, R7 of Sembar Formation and R8 of Chiltan Limestone respectively from top to bottom. Ultimately the average depths were calculated as of Upper Ranikot Group at 355.05 meters, Lower Ranikot Group at 646.00 meters, Parh Limestone 690.21 meters, Upper Goru at 1434.26 meters, Top Lower Goru at1937.66 meters, Sand Below Badin Shale ( C- Sand unit)at 2075.45 meters, Sembar Formation at 2475.41 meters and Chiltan Limestone at 2932.00 meters. Through this study faults have been also marked on seismic lines which are normal faults by nature; collectively form horsts and grabens which is the evidence of effect of extensional tectonics in the area. Faults showing the north-west to south-east trend in the studies area. Interpretation of seismic data reveals that as a result of major and minor tectonic events, the study area has gone through extensive structural as well as depositional changes forming horst and graben structures up to Paleocene (Khadro Fm) horizon while the top most Upper Ranikot Formation is unaffected. Structures in the area provide basic elements of petroleum system. Grabens play main role for producing of hydrocarbons while faults are providing migration pathway for hydrocarbons from source to reservoir rocks while the faults also providing the trapping mechanism for hydrocarbons. Other main constituents are also present in the studied area.