The Characteristics Of The High-Quality Reservoir Of Dengying Formation In Southwestern Of Sichuan

Based on the sedimentary facies, reservoir petrology and its physical property etc,we unraveled the lithogeneous sequence, formation-and conservation-mechanism ofthe high-quality reservoir in the upper Sinian Dengying formation at southwestern ofSichuan basin.It is characterized with the restricted platform eulittoral zone-tidal flat facies indeposit of the Dengying formation, with such microfacies: lime mud flat, algal flat,intraclast beach and oolitic beach etc. In particular, the high-quality reservoirs arewidespread in the facies of intraclast beach and lime mud flat with deposit ofhigh-porosity dolarenite and crystallized dolostone.The rock types of Dengying formation are ophyton dolarenite, algal-laminateddolomite, algal lumpy dolomite and oncolite dolomite etc. Due to lack of insolubleresidue, e.g. argillaceous fraction and algae, dolarenite and crystalline grain dolomiteare usually with high-porosity under the process of recrystallization and dissolution.The types of reservoir space are emposieu and tectoclase, as well as minorintercrystal-and intergranular-pore. The porosity of Dengying formation in Weiyuanand Ziyang areas is similar with a range of1~2%. However, there are differentfeatures in reservoir of Weiyuan and Ziyang areas, e.g. tectoclase. The reservoir inZiyang area is characterized with major emposieu and minor tectoclase, thus alow-permeability and high-homogeneity. On the contrary, the reservoir ischaracterized with major tectoclase and minor emposieu, thus a good connectivity inthe Weiyuan field.The diagenesis in Dengying includes cementation, compaction and pressuresolution, recrystallization, fracturing, silication and pyritization etc. In particular, thepressure solution, recrystallization and fracturing are very helpful to the formation of high-porosity reservoir.Due to different environment of deposition, diagenesis and reconstruction in later,the pore in Dengying formation has different evolutionary processes, e.g., both of thegrain dolomite and algal-laminated dolomite deposited in subtidal high-energyshallow and intertidal-supratidal zone, are similar in the evolution of pore, in contrastto the micrite dolomite deposited in subtidal low-energy environment. As a typicallithogeneous sequence of the grain dolomite and algal-laminated dolomite, it was asfollow: cementation, depositional karstification, filling compaction cataclasis,epidiagenetic stage karst, filling recrystallization, buried corrosion,(oil and gas)filling, hydrothermal reconstruction cataclasis, silication, folding phasekarstification filling.Thus, the formation-mechanism of the high-quality reservoir in Dengyingformation includes such factors as:(a) the depositional environment has significantlycontrolled the development of favorable belts with high-porosity deposits, inparticular, the grain dolomite and algal dolomite deposited in banks;(b) therecrystallization and dissolution are very helpful for the formation of the intercrystalpore and corroded cave, as the most important pore in reservoir; and (c) fractureprovides important passageway for late fluid, oil and/or gas, to significantly improvethe storage ability of reservoir.Several factors favor the conservation of the high-quality reservoir in Dengyingformation, such as:(a) oil and/or gas filling in time can prevent from the formation ofauthigenic mineral, cementation and metasomasis etc;(b) a suitable structure position,e.g., the paleohigh position, which is the migrating area for oil and/or gas as well ashigh-porosity.