Study On Oil & Gas Geochemistry Characteristic And Origin In Turpan Depression

Turpan Depression is regarded as a typical coal-formed rich oil accumulating area with over 2000 meters thick Middle-Lower Jurassic coal measure formation in northwestern China. And up to now there remain some viewpoints about oil & gas formation mechanism because of the complexity and diversity of the oil & gas geochemistry and distribution characteristics in this area; albeit the theory of oil generated from coa1 has been one of the most important frontier research fields in petroleum geology since the 1990’s, the effectiveness of which in hydrocarbon exploration needs further verification.In order to investigate the oil & gas origin, this paper made some in-depth analyses of the different types source rocks distribution scale, hydrocarbon-generating potential, evolution and generated hydrocarbon history, biomarker characteristic. Further to the above, and taking into account of the exploration activities and petroleum geology and organic geochemistry research results in recent years, the paper analyzed the oil & gas geochemical characteristics and origination discovered in different sags and various formations, and reviewed the theory of coal-formed oil and its application results.There exist four different types of oils discovered in Taibei sag. The first type is discovered dominantly in mid-Jurrassic reservoir, characterized by having relatively high abundance of stigmastane , Tetra-cycle terpane and high Pr/Ph ratio and heavy carbon isotope etc, and the geochemical characteristic is similar to that from the Coal measures source rocks; the second type is discovered in such shallow reservoirs as Mid-Upper Jurrassic, Cretaceous and Paleogene in Shenbei area, west of Taibei sag with low Pr/Ph ratio and high cholestane content and low maturity, and the biomarker characteristic is similar to the Jurrassic Qiketai Formation lacustrine source rock; the third type is only discovered in Huoyanshan-Qiketai structure belt and Pubei-Shenquan structure belt, having mixed oil characteristic originated from coal measures and Qiketai lacustrine source rocks; the fourth type is discovered in Trassic reservoir and a few wells such as Hongtai-2, Shen-401 and Shennan-202, having the similar biomarkers with the Permian source rock.The oils discovered in Tainan sag are primaryly heavy oils and oil-source correlation indicates that the heavy oils originate mainly from the Taibei sag Permian Taodonggou group lacustrine source rock; and the biodegraded degree is controlled by palo-structure background, fault activity and pore connectivity among oil pool; from west to east and from lower oil zone to upper zone of Triassic reservoir the heavy oils have more severe biodegradation characteristics; low oil maturity, the dissipation of light components and biodegradation are the major factors to make the oils heaviver.The oil discovered in Tuokequn sag is mainly originated from Triassic lacustrine source rock.The comparison results between coal-formed oil theory and its application in exploration activity indicate that: (a) the coal in Turpan Depression is not much richer in containing hydrogen maceral composition than in the adjacent basins; (b) the hydrocarbon generating and expelling potentials of mudstone in coal measures formation are better than those of coalbed; (c) coal does not demonstrate substantial generation and primary migration of hydrocarbon in the early mature stage; (d) there exists a distinct difference between the inferred petroleum reserves and the proved petroleum reserves in different areas in Taibei sag; (e) the coal-formed oil viewpoint is not coincident with the oil & gas geochemistry and distribution characteristics; (f) the natural gases discovered in Taibei Sag are primarily hybrid-sourced gases. Integrated the above mentioned research results with other geological and geochemical evidence, the author considers that although coal may generate oil, but the oil is difficult to be expelled out and to form commercial oilfields, and oils currently found in Taibei depression middle Jurassic formation can be classified as“the hybrid-sourced oil”coming from the Permian lacustrine hydrocarbon source rocks and Jurassic coal measure rather than classified as“coal-formed oil”in a strict sense.