Diagenesis and densification process of the Chang 8 Interval of Triassic Yanchang Formation, western Jiyuan area, Ordos Basin
Wang Yang1, 2, Liu Luofu1, 2, Li Shutong3, Ji Haitao1, 2, Li Linze1, 2, Luo Zehua1, 2, Xu Tong1, 2
1 State Key Laboratory of Petroleum Resources and Prospecting,China University of Petroleum(Beijing),Beijing 102249; 2 Basin and Reservoir Research Center,China University of Petroleum(Beijing),Beijing 102249; 3 Key Laboratory of Petroleum Resources,Gansu Province/Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics,Chinese Academy of Science,Lanzhou 730000,Gansu
Abstract：The mechanics of diagenesis and densification of the reservoir rocks from Chang 8 Interval,Triassic Yanchang Formation,western Jiyuan area,Ordos Basin were investigated through analyses of cores,cast thin sections,scanning electron microscope images,carbon and oxygen isotopes,and fluid inclusion. The reservoir rocks are mainly composed of lithic arkose and feldspathic litharenite. The reservoir rocks went through diagenesis processes include compaction,pressure solution,cementation,alteration,dissolution and structural fracturing so that the reservoirs are considered to be at late middle-diagenetic stage A. The typical diagenesis sequence includes mechanical compaction,cementation of cholorite rim,cementation of early micritic calcite,cementation of quartz overgrowth,dissolution of feldspar and rock fragments,cementation of authigenic kaolinite,cementation of authigenic quartz and cementation of medium term(ferrous)calcite in choronological order. The results show that: (1)The close proximity towards provenance and rapid fluctuation of lake level contributed to sediment supply. Facies distribution is the main control of reservoir properties where distributary channel facies and interdistributary bay facies show favorable and low reservoir qualities,respectively. (2)The original porosity of Chang 8 Interval during early burial stage is 41.35% and decreased rapidly due to compaction,resulting in a 50.67 percent drop of original porosity. Cementation resulted in an additional 37.48 percent decrease of original porosity. Dissolution increase the porosity by 3.26%,howeverthe reservoir quality is still limited with this slight increase of porosity. (3)The sedimentary and diagenetic factors above lead to the densification of the Chang 8 Interval reservoirs in western Jiyuan area,and the densification process of Chang 8 Interval can be divided into four periods: Rapid porosity reduction through compaction,porosity reduction through early cementation,porosity increase through dissolution and densification through cementation. (4)The porosity of the reservoir is far below 10% during hydrocarbon accumulation,therefore,the reservoirs of Chang 8 Interval are interpreted to be densified through compaction prior to hydrocarbon accumulation.
Wang Yang,Liu Luofu,Li Shutong et al. Diagenesis and densification process of the Chang 8 Interval of Triassic Yanchang Formation, western Jiyuan area, Ordos Basin[J]. JOPC, 2017, 19(5): 892-906.
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