Features of Deepwater Lithofacies, Yanchang Formation in Ordos Basin and Its Petroleum Significance

Key words: 
• deepwater lithofacies /
• Ordos Basin /
• Yanchang Formation /
• Ch7 member /
• conventional oil and gas resources

Abstract: Deep-water deposit of Ch7 member, in Trssic Yanchang Formation, is widely distributed in the center of Ordos basin. Study on particle size, sedimentary textures and structures and organic geochemistry shows that sedimentary rock in deep water can be summed up as four types, i.e. oil-shale lithofacies, dark-mudstone lithofacies, siltstone-mudstone lithofacies and sandstone lithofacies. The oil-shale lithofacies is characterized by high organic carbon content and good type of organic matter, with higher value of resistivity and natural gamma, clear negative anomalies of SP curves, and the curves of acoustic interval transit times varies sharply; dark-mudstone lithofacies characterized by being rich with organic carbon, mid-low value of resistivity, midhigh value of natural gamma and high value of density and acoustic interval transit times; siltstone-mudstone lithofacies by mid value of resistivity, low natural gamma, high density and sharply varied curves of acoustic interval transit time, with mud content ranges about 20~70%；and sandstone lithofacies by low value of resistivity and acoustic interval transit time, and natural gamma, with mud content lower than 20%. The shape of different lithofacies are defined and configuration difference of lithofacies at deepwater slope and lake bottom plain is illustrated in detail. It is pointed out that deep-water rock has great significance for unconventional oil and gas resources. For example oil shale lithofacies and dark-mudstone lithofacies, as the main source rock of Mesozoic, contribute greatly for the formation of large scale reserviors in Ordos basin; the deepwater sandstone lithofacies provides extensive spaces for large-scale tight oil; and deepwater siltstone-mudstone and oil shale lithofacies is the important accommodation for shale oil and has the potential of economic value.