Characteristics of the First Occurrence of Jurassic Petroleum in the Zagros Basin, Iran

After two well tests in the Asmari well#A, located in the North Dezful zone, it was concluded that in the Jurassic Mus/Alan/Neyriz and Upper Sargelu reservoirs, highly mature colorless oil and gas were trapped, including 4%-6% H2S. The alternation of Garau shale and the Gotnia anhydrite seal was so efficient that it did not allow the upward migration of petroleum from Jurassic reservoirs to higher levels. Descriptive ratios, chromatograms, pick correlation and cross plots demonstrated that the oil and gas have been derived from a TOC-enriched sequence, consisting of the base of the Garau and the top of the Sargelu Formations. This highly organic matter-rich sequence is traceable as an oil shale in other parts of the North Dezful zone, such as the Gashun section. The petroleum accumulations in both reservoirs are identical, have the same maturity and the same source. Diagrams of delta C-13(2) versus delta C-13(3), delta C-13(1) versus wetness of gas (C-1/C-2+C-3) and delta C-13(1) versus delta DC1 suggest that the gas is derived from a highly mature source. There are indications of TSR effects on the original petroleum that could have changed the volumetric and isotopic composition of the oil and gas. This result requires more careful study of the petroleum components to be undertaken.