Geochemical characteristics of natural gases and source rocks in Obayied sub-basin, north Western Desert, Egypt: implications for gas-source correlation

Western Desert represents a major oil and gas province in Egypt producing more than 50% of the country's oil production. Oil and gas blend occurs in most producing fields, however, the genetic link between gas and liquid hydrocarbon phases are not well-constrained. Obayied sub-basin in the Western Desert where oil and gas phases coexist in the Middle Jurassic sandstones of the Khatatba Formation provides an ideal place to investigate the link between oil and gas generation. Geochemical analyses on rock samples (Rock-Eval pyrolysis, vitrinite reflectance, R-o) and gases (molecular and isotopic composition) were conducted in order to identify the genetic characteristics of the hydrocarbon phases produced. Maturity-relevant parameters (Rock-Eval T-max, vitrinite reflectance R-o) elucidate that only Middle Jurassic Khatatba organofacies capable of generating wet and dry hydrocarbon gases. Additionally, the enrichment of C-7 normal alkanes, mono-branched alkanes relative to poly-branched components in the Obayied gases reflect their generation via cracking of oil. Basin modelling results confirm gas generation through both primary and secondary cracking. However, secondary cracking of liquid hydrocarbon phases is volumetrically more significant. Primary cracking of the Khatatba organofacies likely predate and catalyze the secondary cracking of the liquid phases and therefore the volume of generated gas increases incrementally eastward where both processes coexist. The present study highlights the significant role of secondary cracking in the generation and accumulation of huge gas accumulations in the basins containing oil-prone source intervals.

Automated summary

The Western Desert in Egypt is a significant oil and gas province, accounting for more than 50% of the country's oil production. This study investigates the genetic link between oil and gas phases in the Obayied sub-basin, where both phases coexist. The results show that Middle Jurassic Khatatba organofacies is capable of generating wet and dry hydrocarbon gases. Secondary cracking of liquid hydrocarbon phases is found to be more significant in gas generation and accumulation. This study highlights the importance of secondary cracking in the formation of huge gas accumulations in basins with oil-prone source intervals.