The Study Of Densification Genetic Mechanism Of Sandstone Reservoir In Xujiahe Formation Southern Sichuan Basin

with the advancement of oil-gas exploration theory and production technology, the tight sandstone gas reservoirs has a highly proportion in oil-gas resources and attracts great attention. Looking for commercially valuable oil-gas enrichment zone in tight sandstone reservoir is the key issue, and this kind of low porosity and low permeability reservoir developed fine physical properties in effective reservoirs so called “sweet spot”. To accurately understand the formation mechanism and distribution of the effective reservoir in tight sandstone reservoir is the foundation of “sweet spot” reservoir prediction. As the important gas produced reservoir in China and typical tight gas reservoir developed area, sandstone in Xujiahe Formation southern Sichuan basin(Shunan) buried deeply and experienced complex diagenesis, caused tight-super tight reservoir characteristics; however it still developing the relatively quality reservoir area to gas reservoir. Densification leading to sand bodies developed in study area, but the characteristics of the oil-gas enrichment zone is hard to find which have brought difficulty to gas exploration. Due to heterogeneity of the Xujiahe group in this area is strong, oil and gas secondary migration, lack of motivation, therefore, focus of Xujiahe formation of oil and gas exploration is still looking for "dessert" reservoir, which is mainly look for areas with high porosity and layers.Based on results of previous researches, directed by basin analysis, reservoir sedimentology, petroleum geology, etc., through core information, logging data, numbers of measured physical property information and thin section data, studying characteristics of reservoir petrology and physical property, analyzed the connection between the physical property and sand body of reservoir included debris clast components, clast structure, interstitial material and lithological characteristics; Analysis and recognizing the pore throat structure and reservoir classification evaluation by mercury injection data and cast thin section image of reservoir sand bodies; Through the thin section analysis, scanning electron microscope(SEM) awareness the main reservoir space type, cement, interstitial material characteristics and diagenesis of reservoir; combined with X-ray diffraction analysis of clay minerals and based on the studies of primary pore evolution and secondary pores genesis, studies the reservoir diagenetic sequence and porosity evolutional relationship; Through the observation of homogenization temperature of fluid inclusions, organic matter maturity, fluorescent thin section microscope, determines the formation time and hydrocarbon charge stages of silicious cementing material, summary the control effect of deposition and diagenesis to reservoir quality, classification of tight reservoir, and analyze the genetic mechanism of different types of tight reservoir; summary the development control factor and distribution of “sweet spot” and achieve the following results:(1) The tight sandstone in study area of Xujiahe Formation experience compaction(pressure solution), cementation, dissolution, replacement; possess various diagenesis types of precipitation and cataclasis of authigenic mineral such as authigenic quartz, authigenic calcites, authigenic chlorite, etc.; compaction(pressure solution) and cementation is the main factor of formation of Xujiahe Formation, and the dissolution is the key factor of the formed effective reservoir.(2) The very first densification genetic mechanism study of tight sandstone reservoir in Xujiahe Formation Shunan area, and determines the key factor of densification, especially detailed study the material sources and formed mechanism of cement, the results show that the compaction is the main cause of reservoir densification; secondary quartz overgrowth and calcareous cementation is the key factor of reservoir densification within diagenetic process; the forming of high-quality reservoir is the result of time-space optimal combination with tectonic setting, deposition environment and diagenesis, etc..(3) Identified the relationship between time and oil-and gas charging of densification in study areas, determines the reservoir densification occurs before the oil-and gas charging. And the densification process is divided into three stages: Contemporaneous Period- Early Diagenetic Period A: the stages with porosity reduced rapidly; Early Diagenesis Period A- Early Diagenetic Period B: the stages with strong mechanical compaction. Early Diagenesis Period B- Middle Diagenesis Period A: the stage with strong cementation. The densification within the three stages caused the present-day reservoir densification levels.(4) Proposed the key factor of “sweet spot” genetic mechanism of tight sandstone reservoir is the fine preservation condition of primary pore and the fine corrosion level of secondary pore in Xujiahe Formation Shunan area. The primary pore preservation condition mainly include rigid particle content and chlorite coat resist silicious cementation; The secondary pore formation condition mainly include reservoir have eutectic feldspar content and have channel the dissolution fluid can free access. Both of them develop in the sand body of distributary channel with high rigid particle content and chlorite coat could resist the growth of authigenic quartz which preserves additional primary intergranular pore to become the favorable growth area of “sweet spot”.The study result not only guide the high-efficiency exploration deployment of clastic reservoir in Xujiahe Formation southern Sichuan basin, promoting the exploration development process and have important guiding significance in tight sandstone field, also have practical significance to achieve the breakthrough of oil-gas exploration in Sichuan Basin; moreover, The result have theoretical significance to low-penetration tight sandstone reservoir experienced deep buried and the “sweet spot” prediction in tight sandstone reservoir especially, provide studies value to enrich theories of petroleum geology and reservoir sedimentology.