An improved understanding of the origin and mechanism of Permian-Triassic natural gas-condensate accumulations in the Gavbendi High, Southwest Iran: An integrated approach

Abstract

Numerous gas-condensate discoveries have been made in the Permian-Triassic reservoirs in the southwest of Iran. However, the origin and post-generation alterations of these gas-condensate accumulations remain unresolved. Thus, a multifaceted approach was utilized to investigate their origin and formation mechanism. For this study, molecular and isotopic compositions of 11 gases and their associated condensate samples together with source rock extracts were integrated to better address the source of the gases. The molecular data on associated condensates suggested that the gas-condensates were derived from two different source rocks. The reservoirs in central and northern parts of the study area with their low concentrations of tetracyclic terpanes and high extended tricyclic terpane ratios were generated from the Early Triassic Kangan source rocks, whereas the condensates at eastern and western parts with higher contents of tetracyclic terpanes and lower values of extended tricyclic terpane ratios showed good affinity with the Silurian Sarchahan Formation. The gas-condensates at central parts of the Gavbendi High with high amounts of saturate to aromatic ratios together with high fractionation index were accumulated through migration of gas from the Kangan source kitchen in northern part of the Gavbendi High. However, the gas accumulations in western side of the Gavbendi High were formed by migration of gas from the western Silurian source kitchen. Moreover, the Permian-Triassic gas reserves with their relatively low differences between carbon isotope values of ethane and propane and less negative carbon isotope values of methane together with less negative carbon isotope values of CO2 and negligible H2S contents were subjected to low extent of oil cracking and generated mainly by kerogen cracking through progressive maturation of source rock. In addition, multiple charging of the reservoirs in northern part of the study area plays an important role in the Permian-Triassic sequence which has not been previously identified.