Geochemical studies of gases from oil and gas fields in the southern part of the caspian basin and their correlation with the results of oil geochemistry

Abstract

This article presents the results and interpretation of the compositional and isotopic analysis of carbon in 14 gas samples derived from oil and gas fields and structures (Liman, East Makat, North Kotyrtas, Zholamanov, S. Nurzhanov and West Prorva) of the southern part of Pre-Caspian basin. According to the results of this study, all gases have a thermogenic source and their organic matters were deposited in the marine environment, which is also consistent with the results of biomarker analysis of oils from these studied fields. A sharp enrichment of propane with heavy carbon isotope in gas №55 of East Makat field suggests its biodegradation, which is also confirmed by the results of gas chromatographic analysis of oil from this well. A star diagram of gases was plotted based on normalized values of carbon isotopic composition of C1-C5, according to the results of which the studied samples can be divided into 5 genetically different groups. The identified groups based on results of this gas analysis are also consistent with the results of «fingerprinting» of oil from these fields. Keywords: gas composition; gas isotopic composition; Rayleigh fractionation; Bernard diagram; Clayton diagram; Laurent diagram; Chung diagram; enrichment in heavy isotope.