Ordos Basin, Yanchang 6 Reservoir Characteristics And Its Controlling Factors

On the basis of formers' study, the paper suggests that the sandstone reservoir of Yanchang formation was deposited on the background of regional lacustrine environment according to sedimentary framework, structure, sedimentary rhythm and logging response characteristics. The meandering river delta system was established in the northeast Ordos basin and the favorable facies belts for developing reservoir of Chang-6 member were submerged distributary channels of delta front, mouth bar and distal bar. The braided river delta system was established in the southwest Ordos basin and the favorable facies belts for developing reservoir was turbidite formed in deep to semi-deep lake. Abundant core description and observing, and experimental analysis indicate that the reservoir sand-bodies mainly is feldspar sandstone, and the main type of pore is residual intergranular porosity, the second is feldspar dissolution porosity and laumontite dissolution porosity. It belongs to low porosity with extremely low permeability and extremely low porosity with extremely permeability reservoir. The pore texture of Chang-6 member is small sized porosity with minute throats. The main diagenesis in the studied area included the mechanical compaction, cementation, infilling, dissolution, and authigenic mineral precipitation. According to the analysis of R_o of mudstone, the contacts of clastic grains, pore types, the development and the type of authigenic minerals, the diagenesis of the reservoir sandstone of Chang-6 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 main influencing factor of reservoir. 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 four types of reservoir formation in this area and the types of Ⅰ and Ⅱ are the favorable ones and Ⅲ is comparatively favorable by the comprehensive research and evaluation. Seven favorable reservoir zones are suggested to be targets for the future exploration of Mesozoic period in researchful area according to the evaluation of sand body characteristics, the rang of rocks with hydrocarbon, diagenesis as well as combining with the conditions of oilness in the researchful area.