SEQUENCE STRATIGRAPHY OF NON‐MARINE AND MARINE DEPOSITS OF CAMBRIAN-PERMIAN SYSTEM OF SALT RANGE PAKISTAN

Abstract

Sequence stratigraphy study has been successfully implemented for better understanding of sedimentary basins, its depositional controls, age, distribution of facies and prediction of petroleum play fairways. Any evaluation of sedimentary basin without sequence stratigraphic study will be leading to unreliable correlation of rock strata across basin with respect to age and facies distributions particularly while understanding is required to predict reservoirs. The sequence stratigraphy methodology has been used in present M.S research project as a tool for the study area located in southern Potwar region and Salt Range. This research project is aimed to understand the sequence architecture of Precambrian, Cambrian-Permian rocks using surface and subsurface geological information from out crops, wells and seismic data by correlating and delineating a reliable frame work of depositional system. One of key aim is to evaluate petroleum play of Cambrian and Permian section containing clastics reservoirs. Beside geological information the present work incorporated subsurface seismic data acquired in the area by OGDCL, POL and PPL. This task was supported by field work at three locations where observations were made for sequence boundaries, stratigraphic positions, contacts, facies analysis, thickness, structural features and other rock properties. This work incorporated limited data of wells and seismic survey due to permissions. Litho and sequence stratigraphic correlation in Salt Range is challenging due to change in facies and thickness in Cambrian and Permian levels. Stratal truncations at various levels are associated with the change in environment of deposition. Lateral changes in thickness and facies are not yet clearly understood based on only lithological characteristics. The Cambrian-Permian rock strata exposed in Salt Range and drilled in southern Potwar Basin demonstrated presence of sequence boundaries marked by unconformities and correlative conformities. In the study area three major sequences of the 2nd to 3rd order can be referred after study which are broadly based on lithology, stratal surface geometries, age and presence of three regional unconformable surfaces. Each sequence also indicated number of transgressive and regressive surfaces and where field data and well logs supported evidences and helped to configure internal architecture. The stratal architecture of Cambrian is difficult to understand for proper correlation howevervii based on pronounced unconformaties and spatial change of stratal relationship using surface and well data a sequence framework has been established. The preserved Cambrian system in the study area demonstrated high energy marine to aeolian and to fluvial system of deposition and intercepted by unconformaties. The sequence boundaries and surfaces has been analyzed from base to top in this setting. The truncation of high energy marine cycle at base by aeolian phase has been marked by probable surface of sequence boundary while at top it is marked by regional boundary which is a base Permian unconformity. The understanding of overlying depositional cycles of Permian stratigraphy commencing from glacio fluvial system at base (marking a sequence boundary) and entering to fluvial marine above and subsequently followed by fluvial system and ultimately truncated at top after marine nonclastics (marking upper sequence boundary) sketches another major sequence in the study area.