Research On Reservoir Characteristics And Diagenesis Of Upper And Lower Shaximiao Formation In Xiaoquan-Xinchang-Hexingchang Area, Western Sichuan

On the basis of the predecessors'studies, the paper focuses on the reservoir characteristics and diagenesis of upper an lower Shaximiao formation of Middle Jurassic in Xiaoquan-Xinchang-Hexingchang area in western Sichuan basin. The sedimentary and diagenetic controls on porosity development and reservoir properties have been discussed in the paper.Feldspathic litharenite and lithic arkose constitute the main reservoir sandstones. The sandstones in Xingchang and Hexingchang have similar detrital grain composition. By contrast, the sandstones in Xiaoquan are quartz rich but have less detrital feldspar. Calcite occurs as cement. Matrix consists of clays and calcite. Authigenic chlorite is abundant. Based on the pore types and structure features, the reservoir is characterized by mesoporosity and thin-micro pore-throat. The value of porosity and permeability suggest that the reservoir quality is relatively bad; it can be classed as low to very low porosity and low permeability sandstone reservoir.The sandstones in the study area has a much more complex diagenetic and porosity evolution history and is now in the middle diagenetic phase B to late diagenetic phase. The main diagenesis includes compaction, cementation and dissolution which are closely related to reservoir porosity development. The main effective porosities are primary porosity surviving compaction and/or cementation and the secondary porosity produced by dissolution.This paper summarizes the geological factors that control sandstone reservoir qualities. The reservoir development is controlled jointly by sedimentary processes and diagenesis. The sedimentary facies constrained the distribution of reservoir. Good reservoir sandstones developed in delta-plain distributary sandbodies and delta front sandbar in Xinchang and Hexingchang area, and in delta-plain distributary pointbar and lake-shoreline beach bar in Xiaoquan area. In general, medium and fine grained, fair to good sorted with little clay matrix and feldspar abundant sandstones tend to form good reservoirs. Diagenesis plays a very important role in the development of reservoir. Compaction is the main cause of the primary porosity decreasing. Calcite cementation occurs as interstitial filling and destroys the pore space. Abundant pore lining authigenic chlorite is significant in porosity preservation because it inhibits quartz cementation and thus prevents reservoir deterioration. Dissolution is the main process to improve the reservoir properties, but different degree dissolution would produce different amount of secondary porosity and cause irregular distribution, which will result in heterogeneity of reservoir petrophysical parameters.