Analysis Of Formation Mechanism And Control Factors Of Tight Sandstone Reservoirs Of The He 8 Members Of Lower Shihezi Formation In The Northern Ordos Basin

Ordos Basin is the second largest sedimentary basins in China. There are the rich oil and gas resources. The tight sandstone gas reservoirs are mainly from the Carboniferous-Permian strata, these strata are important to the exploration and production of the natural gas in recent years. With the deepening and development of exploration work, the exploration of the low permeability tight sandstone reservoir become increasingly difficult and the prospecting situation is facing the severe challenges. To further optimize the development plan of gas fields, the formation mechanism and controlling factors of the tight sandstone reservoir need to be comprehensively analyzed. Based on a large number of field works and laboratory works, this paper have studied the deposition-sequence stratigraphy, reservoir rock composition, pore volume, porosity and permeability, diagenetic evolution of the tight sandstone, and them the formation mechanism and controlling factors of the tight sandstone was also concluded. These results will provide the useful guidance to the exploration and development of gas reservoir when they are used to the comprehensive evaluation of the gas reservoir.Based on the principles and methods of sedimentology, paleontology and stratigraphy and high-resolution sequence stratigraphy of continental deposit, this paper has studied the sedimentary facies and the genetic types of sand body by using the facies indicators. There are three major facies types:braided river, meandering river, plain of braided river delta, plain of meandering river delta. Two long-term sedimentary sequences, four mid-term sequences, twelve short-term sequences were established into the sequence stratigraphic framework of the He-8 Member. And then, the distribution of tight sandstone reservoirs in the different-level of sequence stratigraphic framework was analyzed, and the base level cycle which control to macroscopic reservoir heterogeneity was also discussed. Meanwhile, this paper has studied the reservoir characteristics based on the multidisciplinary research which include the sedimentary petrology, oil and gas geology, reservoir physics. The rock types of the reservoir sandstone are relatively simple. The particle size of the sands was mainly from medium to coarse, and sorting of the sands was mainly from moderate to well. The roundness of the sands was mainly the sub-angular. The contact interfaces between the sands were mainly the line. The types of the cementations were mainly pore cements and contact-pore cements. The secondary porosity are the main reservoir space, and the mainly combine-type was the remaining intergranular porosity and secondary dissolved porosity. The gas reservoir of the upper sub-member of He-8 Member is belong to low porosity and low permeability reservoirs, and the gas reservoir of the lower sub-member of He-8 Member is belong to the ultra-low porosity and low permeability reservoir. The diagenetic stage of the reservoir sandstone from the He-8 Member is mainly in the B section of the early diagenetic stage or early section of the late diagenetic stage. The densification of reservoir sandstones is due to the experienced strong mechanical compaction, and then the subsequent cementation makes the further porosity reduction. The formation time of the extensively developed quartz overgrowth may represent the end time of the densification of the reservoir sandstones, about 150Ma, namely the late Late Jurassic. The different sedimentary facies was a common controlling factor, and the diagenesis was also an important controlling factor to the reservoir quality of the tight gas reservoirs from the He-8 Member.