Factors controlling the reservoir accumulation of Triassic Chang 6 Member in Jiyuan-Wuqi area,Ordos Basin,NW China

In the Triassic Yanchang Formation, Jiyuan-Wuqi area, Ordos Basin, the Chang 6 reservoir is contacted to the Chang 7 high-quality source rock, but the oil pools are unevenly distributed, and complex in oil and water distribution. Through cores observation and fracture statistics, combined with comprehensive analyses of physical property, mercury injection, logging and geochemical data, and comparisons of the sandbodies scales, reservoir physical properties, argillaceous laminae and fractures between source and reservoir in the eastern and western oil-bearing areas and in the central water producing area, it is found that the hydrocarbon accumulation patterns are different in the eastern, central and western areas, and the characteristics of hydrocarbon migration under the background of double-provenance were sorted out. The study results show that the crude oil in the eastern area has different Pr/Ph and sterane distribution from that in the western area. The oil and gas primarily migrated vertically. The high-quality source rocks and favorable source-reservoir-cap combinations lay the foundation for large-scale oil and gas accumulations. Vertically, the oil and gas enrichment is controlled by the scale of sandbody and the difference of physical properties, while on the plane, it is controlled by the connectivity of sandbodies, the argillaceous laminae between source rock and reservoir, the reservoir physical property and the fractures. The sandbodies of oil-rich zones in the eastern and western areas have large thickness, low shale content, good physical properties, weak heterogeneity, few argillaceous laminae and abundant fractures, all of which are favorable for the vertical migration and accumulation of oil and gas. In contrast, in the middle area with converging provenances, the reservoirs, composed of thin sandbodies, features rapid variation in lithology and physical properties, strong heterogeneity, poor continuity of sandbodies, abundant argillaceous laminae between source rock and reservoir, and few fractures, makes it difficult for the oil and gas to migrate vertically, and results in low oil enrichment degree ultimately. For the exploration of continental multiple-provenance tight reservoirs, not only the good-property source rocks and reservoirs, but more importantly the source-reservoir contact relationship and the effect of fractures on the hydrocarbon migration and accumulation should be considered.

Geochemical Characteristics and Origin of Crude Oil and Natural Gas in the Southern Slope Zone,Kuqa Foreland Basin,NW China

Continuous exploration has triggered a heated debate on hydrocarbon resource potential in the southern slope zone of the Kuqa foreland basin,and sources of the Mesozoic-Cenozoic oil and gas have become a key problem to be solved in this region.Composition and organic geochemical parameters of crude oil and natural gas from the southern slope zone of the Kuqa foreland basin were illustrated in order to reveal their origin by using a combination of gas chromatograph(GC),gas chromatogram-mass spectrum(GC-MS)and carbon isotope analyses.The characteristics of crude oil,such as low density,viscosity,solidification point and sulfur content,and high wax content,indicate that source of the crude oil is continental.The biomarker compositions of crude oil are characterized by low to medium molecular weight compounds(n-C_(12) to n-C_(20)),high Pr/Ph ratios(>1.0),low phytane/n-C18 ratios(0.06-0.54),and predominant regular sterane C_(29).All biomarker parameters clearly indicate that the crude oil was derived mainly from algae and aquatic plankton and deposited under weak reduction-oxidation environment,and has the characteristics of mixed kerogens.The Cretaceous crude oil was mainly derived from the Triassic lacustrine source rocks,which also contributed to the Paleogene crude oil together with Jurassic coal source rocks.Natural gas is characterized by moderate methane content,high heavy hydrocarbon and nitrogen content,and no hydrogen sulfide.The methane and ethane in Paleogene natural gas are relatively rich in ^(13)C withδ^(13)C_(1) andδ^(13)C_(2) values ranging from-37.3‰to-31.2‰(mean=-34.25‰)and from-25‰to-21.3‰(mean=-23.09‰),respectively,indicating the coal-derived gas from the Middle and Lower Jurassic strata.Hydrocarbon products in the southern slope zone of the Kuqa foreland basin are primarily generated from source rocks in the mature stage.The low-amplitude structural and lithologic traps with the updip pinch-out sand bodies or plugging secondary fault at relatively high tectonic positions are the most favorable areas for discovery and breakthrough in the study area.Results of this study will provide useful information for controlling factors of reservoirs and oil and gas exploration deployment in the southern slope zone of the Kuqa foreland basin.