Research On Diagenesis Of Shan2Sandstone Reservoir In Northern Yulin Area In Ordos Basin

Ordos Basin is the second largest petroleum-bearing basin in china, and also one of the largest and earliest discovered and developed basins. The second member of Shanxi Formation (shan2reservoir) in Lower Permian in East Ordos Basin is the most important gas-bearing strata in Upper Paleozoic and has indicated a promising prospect for exploration. The study area is located in the east of the basin. And the Shan2reservoir has characteristics of thin bed, low-porosity and low-permeability.On the basis of earlier studies, the paper studied the reservoir characteristics and diagenetic types of Shan2reservoir combined with Regional geological and tectonic backgrounds of the Ordos Basin by using various analysis and measurement methods, including description and observation of drilling cores, identification and quantitative statistics of thin sections under microscope, grain size analysis, analysis of scanning electron microscope, capillary pressure and X-ray diffraction. With these studies, the paper discussed the effect of diagenesis to reservoir property, thus established a model of diagenetic evolution and porosity evolution, and classified the diagenetic fades. With middle-term base-level cycle of Shan2reservoir as a unit, a detailed description of reservoir distribution and physical properties of Shan2reservoir was made and finally the favorable exploration areas were predicted.Quartz sandstone and lithic quartz sandstone are the main reservoir rock types in the study area which has a very high compositional maturity. With a lower matrix content, the sandstones are extremely deficient in feldspar. Sandstones are often medium to coarse grained, and poorly to moderately sorted. The composition of authigenic minerals in sandstone is characterized by quite rich in quartz and kaolinite. Reservior space always occur as the combination form of several kinds of pores, and are mainly of secondary dissolution pores and kaolinite intercrystalline micropore.The primary intergranular pore takes a second place. Besides, microfractures developed. The pores are positively correlated with the permeability and the changes of the permeability are controlled by the development of the pores.Diagenesis that the reservoir had experienced mainly were compaction, cementation and dissolution, including organic acid dissolution and thermal dissolution. Mechanical compaction and cementation were the main factors that caused the great loss of primary pores. While the dissolution of the upper Paleozoic of Ordos Basin was the key to form high quality reservoirs, and the dissolution fluid was mainly from the acid fluid of coal formation and hydrothermal fluids from the deep, which dissolved the vast majority of feldspars, part of quartz and its secondary enlargement, matrix and rock debris. Microfractures connected the pores, and transported dissolution fluid, thus improving the reservoir quality. The diagenetic stage of the reservoir in the second member of Shanxi formation in Yulin area had entered into the B stage of middle diagenesis generally, while Part of them are in the A stage of late diagenesis.3types of diagenetic facies are developed in the study area, and feldspar dissolution-authigenic kaolinite diagenetic facies and moderate dissolution illite-kaolinite diagenetic facies are the most profitable diagenetic zones for developing high quality reservoirs. Favorable exploration area are mainly in the central and southern study area.