Ordos Basin, Eastern Gansu Region Triassic Yanchang Reservoir Characteristics And Its Controlling Factors

On the base of the studied results of former geologists, author suggests that the facies of the sandstones reservoir of Chang-8 in Longdong area is braided fluvial delta deposited on the background of regional lacustrine environment through comprehensive research of sedimentary framework, structure, sedimentary rhythm and logging response characteristics in this paper. The favorable facies belts for developing reservoir sand-bodies are the submerged distributary channels of delta front, mouth bar, and the delta plain distributary channels. After a large number of core description and observing, and experimental analysis, it is indicated the low mineral and textural maturity for the reservoir sand-bodies near the source area. The first type of pores is mainly the remained intergranular pores, second is feldspar dissolved pores. It belongs to the types of low porosity and extremely lower permeability reservoir, and extra lower porosity and permeability reservoir. The main diagenesis in the studied area included the mechanical compaction, cementation, infilling, dissolution, and authigenic mineral precipitation. According to the analysis of Ro of mudstone, compaction of sandstone, the contacts of clastic grains, pore types, the development and the type of authigenic minerals, the diagenesis of the reservoir sandstone of Chang-8 is during the period of late diagenesis A with four sequences of diagenesis development, and two diagenetic environments. By means of microscope observing, EMS analysing and the research of sedimentary and diagenesis, it brings forward that the sedimentation is the important factor of influencing reservoir property, while the hydrocarbon emplacement, the precipitation and dissolution of clay film are the key factors to control the preservation of original pores and the development of secondary pores in sandstone reservoir. There are three types of reservoir formation in this area and the types of Ⅰ and Ⅱ are the favorable ones by the comprehensive research and evaluation. Two favorable reservoirzones are suggested to be targets for the future exploration of Mesozoic period in studied area according to the evaluation of sand body characteristics, the contents of carbonates and chlorite, as well as diagenesis.