Origin and exploration prospect of Upper Paleozoic crude oil from Hangjinqi area, Ordos Basin

Abstract

A set of favorable source-reservoir-caprock assemblage was found in the Upper Paleozoic transitional facies in the Hangjinqi area of the Ordos Basin, and the large-scale Dongsheng gas field has been discovered. A small amount of crude oil has been produced from the Lower Shihezi Formation reservoir in the area; however, the origin and exploration prospect of the oil are unclear. Based on the studies including physical property, n-alkanes distributions, biomarker compositions, and carbon isotopic ratios, the origin and exploration prospect of the crude oil has been discussed. Results indicate that the Upper Paleozoic crude oil from the Hangjinqi area is mature oil with a low sulfur content and a high wax content, and the steranes are dominated by terrestrial plants and assisted by a small amount of aquatic organism. The Pr/Ph ratio is 2.16-2.26, and the diasteranes and rearranged hopanes are in high contents with a relative low content of gammacerane. The carbon isotopic ratios of the oil range from -27.7‰ to -26.8‰, [JP+1]and the carbon isotopic[JP]compositions of individual n-alkanes are heavy at first and then become light with the increasing of carbon numbers. These characteristics suggest that the oil was generated by the dark mudstone in terrigenous coal measures under weak oxidation environment. The comprehensive analysis combining organic petrography, hydrocarbon generation simulation, and pyrolysis of source rocks in the Shanxi Formation indicates that, the dark mudstone in the Permian Shanxi Formation in the Hangjinqi area displays a certain amount of oil generation potential. In combination with the location of oil producing wells and the spatial distribution of source rocks, the dark mudstone in the Shanxi Formation might be the potential source rocks for the produced oil. The oil exploration of the Upper Paleozoic strata in the Hangjinqi area deserves attention as suggested by the favorable conditions of source-reservoir-caprock assemblage, and the adjacent areas of the Borjianghaizi fault are potential favorable areas for oil accumulation.

Keywords

• origin of crude oil
• dark mudstone
• hydrocarbon exploration potential
• upper paleozoic
• hangjinqi area
• ordos basin