Possible origin of inert gases in hydrocarbon reservoir pools of the Zindapir Anticlinorium and its surroundings in the Middle Indus Basin, Pakistan

Abstract

The natural gas in several gas fields in Pakistan is associated with varying percentages of inert gases (e.g., CO2, N2, and H2S). The heating capacity of such natural gas is measured in British thermal units (BTU) in Pakistan and is based on the combustion of the natural gases. The study area of this work belongs to the Middle Indus Basin, which also covers some parts of the Sulaiman Foldbelt and the Punjab Platform. Petroleum wells drilled in the study area contain inert gases (mostly CO2 and N2) in varying percentages, which decreases the BTU value of sweet gases and the economic value of the gas reserves.Attempts were made to analyze the varying compositions of inert gases (mostly CO2 and N2) along the deep-seated basement faults in the Zindapir Anticlinorium, eastern Sulaiman Foldbelt, but no specific relation could be established. Similarly, geothermal gradient zones were identified and the distribution of inert gases in these zones was studied, but even so, no well-established relation could be tracked. However, variations in the amounts of inert gases in the Chiltan Limestone of the Rodho Structure and the Afiband Structure point to the generation of in situ inert gases because both wells were drilled on the same anticlinorium and share the same geology, and possibly, the same source rock. Post-accumulation changes probably play an important role in the generation of in situ inert gases in varying concentrations. H2S is also present in some parts of the Indus Basin. Therefore, a brief discussion about the possible origin of the H2S is also included in this paper. Keywords: Inert gases, Zindapir anticlinorium and hydrocarbon reservoir pool, Middle Indus basin, Pakistan