Characteristics of Tight Oil Reservoir Chang8 Member of Yanchang Formation in Wuqi-Zidan Area of Ordos Basin

Laiyi Ren, Yonghong He, Chao Yang, Xinting Duan, and Hongjin Li
Shanxi Yanchang Petroleum (Group) Co., Ltd. Research Institute, Xi'an, Shaanxi Province, China

By comprehensive study of reservoir rock characteristics, pore characteristics, pore throat structure, properties and other factors, the tight oil of reservoir basic characteristics was analyzed in the Triassic Yanchang Formation, Ordos Basin. The results showed that: Oil accumulated in oil shale or interbedding tight sand which was adjacent to source rocks was referred as tight oil. Normally, the in-situ permeability of reservoir is less than 0.1md and oil has not yet experienced large-scale and long-distance migration. The tight oil sand of Yanchang Formation was mainly in the core area of petroleum system, which is located in the central basin and the sedimentary center of the middle-late Triassic, i.e. the scope among Dingbian- Jingbian - Ansai - Qingyang – Fuxian and in the middle layer of Yanchang Formation in profile, mainly in Chang6-Chang10 reservoir group. Most of oil zones are closed to the source rock and symbiosis with it. Characteristics of tight oil sand in Chang8 member are multiple causes of large-scale composite overlay, tight sandstone, complex pore structure, abundant oil content, low-pressure low-productivity. Fine-grained feldspar sandstone and lithic feldspar sandstone dominated in Chang8 reservoir and debris grain content is 79.27% ~ 84.15%, with overall good sorting of grain, poor roundness, mainly angularity grain, line contact. Cement contents are mainly authigenic clay minerals, carbonate, zeoliteand silica. The pore types include intergranular pore, intragranular dissolved pore, micropore and microfissure in filler hole, and the average porosity is 2.37%. The main pore type is small - middle pore micro throat or small- pore micro throat. The porosity of sandstone is 3.8 ~ 15.6%, and the average is 8.4%, the permeability is 0.004 ~ 85.81 × 10^-3 μ m², the average is 0.77 × 10^-3 μ m², these characteristics are typical properties of the tight oil reservoir. The tight oil reservoir is mainly controlled by sedimentary facies and diagenesis. The northeast, northwest, southwest of the three direction of delta distributary channel controlled the distribution of tight reservoir. The laumontite solution and chlorite growth rim improved storage property of the tight reservoir, and carbonate cementation, compaction and pressolution significantly reduce the petrophysical properties of reservoir storage.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013