SEIMENTARY CHARACETERISTICS AND ENVIRONMENTAL ANALYSIS OF SINIAN DENGYING GROUP. HUBEI PROVINCE

The late Sinian strata in Hubei Province are situated in the eastern part of the shallow-sea area of the I'angtse shelf.The widspread Dengying group consists main-ly of dolomite generally 200-500m in thickness, and even up to more than 800m.Abundant small shell fossils are rich in the upper part of the Dengving group, and many blue-green algae in the middle so various types of rocks are formed. Accord-ing to the texture and genesis of the rocks, the dolomites can be grouped into six types:(1)Algae grain dolomite This type of rock includes oncolite-dolomite, lamina-ted dolomite, algae-ring dolomite, thrombolite-dolomite and algae pellet-dolomite.There are many depositional structures in algae grain-dolomite such as cross-bed-ding, scouring face and vados sand.This represents the lower part of the intertidal enironment.(2)Algae-bind dolomite This type includes stromatite dolomite, laminated do-lomite, grape-shaped dolomite sheet fissure, desiccation fissure and bird's-eye can often be found in algae-bind dolomite.This represents the part of intertidal environment. (3)Algae-mark dolomite This type of rock consists mainly of micrete. And desiccation fissure, desiccation break breccia and bird's-eye can be found in it.This reflects the upper part of the intertidal environment.(4)OÖlitic dolomite The cements mainly- are sparry calcites of two genera-tions .This is formed at the edge of carbonate platform in high energy environment.(5)Intraclastic dolomite The cements are sparry calcites.The depositional environment is as the oÖlitic dolomite.(6)Micritic dolomite Micrite-fine grain structure presents light color.The average content of Bolo mite is more than 90% and CaO/MgO=1.34.The laminated structure can often be seen, but desiccation fissure and break breccia can rarely be observed.This represents the clcsed and quiet restricted sea of intertidal and sub-tidal zones.The relationship between the above types of rocks and sedim entary structure indicates that at the end of late Sinian the environ went w as a carbonate platform for the shallow-water quiet and the energy was low.This is distributed in Jianghan plain, Hubei province, being a trumpet一like restricted sea platform.In this restri-cted sea, there are Huitingshan and Yanchihe.Algae flat is distributed in the northern part and the algae-dune of Hai-9 in the southern. The offshore lagoon facies is located at the north side of the restricted sea.The Sichuan-Guizhou platform is separated by a submarine groove in the west.At Cili there is an oolitic beach of platform edge.Yuanling-Xianning region belonges to the basin facies.Because the sedimentation at the end of late Sinian mainlw developed in the intertidal belt, dolomite predominated pores were well developed.The authigenic pores are bird's-eye, sheet fissure and intergranular pore.Secondary- pores are mainly solution-pore and intercry-stalline pore. Based on the above analysis. it is suggested that all these pores are well-developed and favourable for the oil and gas accumulation.