Study On The Characteristics And Formation Mechanism Of Sinian Reservoirs In Sichuan Basin

This paper focuses on the Denying formation reservoirs in Sinian system.Based on a detailed observation of its characters of lithofacies, paleaogeography, sedimentary facies, petrography, fluid pack-injection in cavity, the densification process of the formation etc, the present paper summed up its spatial distribution feature and made sure there were multistages fluid pack-injection within the cavity system. In addition, the paper also constructed the sequence of diagenetic evolution of the Denying formation, thus found out the mechanism that made the reservoirs of fine quality. Through these processes, the paper finally gets some significant results below:Restricted platform is the main sedimentary environment of the Denying formation, the south-west part of which contains minor sabkha deposit of evaporative platform. The microfacies of the Denying formation includes algal-bearing muddy flat, algal flat, lagoon, algal grain bank, algal reef, shallow platform and semi-deep platform. It is proved that the terrestrial materials contained were mainly from Songpan ancient block mass of western Sichuan and central Yunnan.The lithological types of reservoir in Dengying formation are 15, and it is more complex in Leshan-longnvshi palaeohigh than in southeast Sichuan basin. The main type of reservoir in Leshan-longnvshi palaeohigh appears to be algal structure with some crystalline granular structure and grain structure, but in southeast Sichuan basin, it becomes mainly with crystalline granular structure partly with algal structure and grain structure. The second and forth members of the Denying formation are the most potential reservoirs, especially those close to weathering surface are more better. It is attractive that more or less bitumen, which presents at the reservoir displayed by significant wells and field sections, indicates the hydrocarbon infilling taken place in Dengying formation of Sichuan basin.Generally, they are characterized to be tight with low matrix porosity, usually between l%-2%. The types of reservoir are not much correlative with porosity and permeability in the southeast Sichuan. The whole characters of pore and throat structure indicate that the quality of micrite dolomite and fine crystal siliceous dolomite are comparatively better, argal aggregate dolomite or residual dolarenite follows, and the worst is the fine-medium crystal dolomite. But the algal boundstones and algal grainstones are most suitable for the development of reservoir. Matrix porosity and whole core analysis bulk porosity characters has obvious difference in different districts. Matrix porosity test indicates that the quality of reservoir of central Sichuan is the best, and the inferior distributes at the southeast and southwest part of Sichuan basin. Bulk method suggests that Ziyang-Weiyuan district is best, and the Central Sichuan region follows.The main reservoir spaces of Dengying formation are cracks, solution pores and cavities. This is regional and correlate with Leshan-longnvshi palaeohigh . In the plane, corrosion holes most developed in ZiYang, followed by Weiyuan, and relatively poor in the central and southeast part of Sichuan basin. Vertically, except some developed in the fourth member, most of them distribute from the member III to the top of member II, 70 meters near to the surface weathering. But in the remaining area of the Dengying formation member IV, the reservoir of Dengying formation member IV is slightly better than that of member II and member III. Most of the cracks in Dengying formation are structural cracks formed by multi-stage,and maybe underwent multi-phase transformation(early broken—filling—re-broken—re-filling). Part of the cracks seen on the sore experienced at least two or three active stages. Cracks in Weiyuan developed best, followed by Ziyang, and then Gaoshiti - Anping stores and the southeast of Sichuan basin is the worst. The thickness of effective reservoir (porosity is bigger than 3%) becomes thin from Leshan-Longnusi paleohigh to the slope. It developed better at the top of the Dengying formation member II and bottom of the formation member III vertically.The characters of infillings in pores and cavities in the formation are similar with different region of Sichuan Basin. The multistage salt fluids are characterized by cross formation migration. The typically relatively complete mineral filling sequences are as following: wall rock (micrite dolomite)→1^st generation fine crystalline granular dolomite→2^nd generation coarse crystal granular dolomite→3^rd generation coarse crystal or megacryst dolomite→4^th bitumen→5^th calcite and quartz/calcite/quartz→6^th bitumen. The 1^st generation fine crystalline granular dolomite often lack of cracks. The geochemical characteristics of host rocks and filling minerals reveal that the salt fluid is not from host rocks, but most likely from source rock.The diagenesis of Dengying Formation is complicated and contains more than 35 kinds. The main diagenesis includes algae-micrite, compaction, pressure solution, cementation, dissolution, recrystallization, dolomitization, hydrothermal alteration, silicatization,etc. The main dolomitization in the formation is penecontemporaneous and burial dolomitization. Further more there are conditions for hypergene dolomitization and hydrothermal dolomitization in the formation. The Dengying Formation can divide as five kinds of silicatization and the five relative sequences are initially established. The Dengying Formation contains hypergene karstification (includes interbedding karstification), burial dissolution and (folding) structure karstification.In the Dengying Formation, diagenesis and multistage fluid complexs its densification. The densification processes of the crystalline dolomite are following: the compaction during early diagenesis and the sparry cementation in fenestra (reducing about 40%～45% of the porosity)→early silicatization / recrystallization (little or no change of the porosity)→the uplifted dissolution / infilling (or standing burial)→-Kreburial) presolution / recrystallization (hydrocarbon infilling)→the dissolution during deep burial-Mnfilling and recrystallization→the fracturing of tectonic uplift/brecciation ( or standing burial )→the leaching of the freshwater / silicatization / dedolomitization ( or standing burial)→infilling. The densification processes of the grain dolomite are following: the cementation (minor dissolution) of the freshwater, seawater and the mixing water of freshwater and seawater (reducing the 10%～20% of the porosity )→compaction→ypergene karstification→silicatization / recrystallization→presolution / tectonic karstification / recrystallization / dissolution during deep burial/infilling (hydrocarbon infilling)→uplift/tectonic crack (or standing burial) dissolution / silicatization / infilling.The information mechanism of the Sinian system Dengying formation reservoirs are is mainly controlled by diagenesis. Its characteristics include the crust karstification, dissolution-recrystallization of chemically corrosion-fluids, cracking, the low oil saturation and the proper structural situation.