川西震旦系台内坳槽西缘地层划分与沉积演化

The stratigraphic division and sedimentary evolution of the western Sinian intraplatform trough in the western Sichuan Basin,South China

近年来四川盆地的天然气勘探揭示,震旦系发育一条贯穿盆地的近北西—南东向的台内坳槽,其形成演化控制坳槽周缘灯影组的储层发育与气田形成。选取紧邻震旦系台内坳槽西缘的绵竹清平大水闸、燕子岩剖面,系统开展地层划分、沉积演化及储层发育特征研究。研究表明:(1)碳同位素变化曲线可有效厘定震旦系陡山沱组与灯影组的界线,坳槽西缘陡山沱组与灯影组均发育,且灯影组发育完整;(2)陡山沱组以深水斜坡相沉积为主,灯影组主要为浅水碳酸盐台地相沉积,陡山沱期至灯影期经历了由深水斜坡向浅水碳酸盐台地的沉积演化;(3)坳槽西缘灯二段发育厚层叠层石白云岩规模优质储层,与坳槽东侧台缘带可以类比,揭示坳槽西缘同样具备形成大气田的储层条件。

The Sinian is extensively distributed in the Yangtze Block,South China,comprising the Doushantuo and Dengying Formations from base to top.In the recent years,hydrocarbon exploration in the Sichuan Basin has revealed that an approximately NW-SE trending intraplatform trough was formed during the Sinian,and its formation and evolution controlled the reservoir development and gas field formation of the Dengying Formation around the trough.Now the Anyue Field of the Dengying Formation with much research has been found in the eastern intraplatform trough.However,the Sinian sequence,sedimentary evolution and reservoir characteristics in the western intraplatform trough still need to be further studied.In this paper,two representative outcrops close to the western edge of the Sinian intraplatform trough,i.e.the Dashuizha and Yanziyan sections in Mianzhu County,are selected to systematically analyze the stratigraphic division,sedimentary evolution,and reservoir characteristics.The results of this paper show that:(1)The carbon isotope curves can effectively distinguish the Sinian Doushantuo and Dengying Formations.The carbon isotope at the top of the Doushantuo Formation is characterized by negative shift(up to-8‰),which is considerably compared with that at the base of the Dengying Formation with positive shift.Both the Doushantuo and Dengying Formations were developed in the western intraplatform trough.The Doushantuo Formation at least includes Members2to4,and the typical cap dolomite of Member1is not occurred.And the Dengying Formation is completely developed,from base to top,including Members1to4.(2)The Doushantuo Formation is composed of a mixed succession including the lower dominant siliciclastic rocks and the upper dominant carbonate rocks.Furthermore,the Doushantuo Formation is dominated by deep-water slope facies deposition with the features of syn-depositional slump folds,rhythmites of interbedded carbonate and mudstone,and olistostrome breccias.The Dengying Formation comprises dolomites with minor chert and mudstone,and it is mainly shallow water carbonate platform facies deposition with the development of microbial laminated dolomites and botryoidal dolomites.Thus,during the Sinian the study area experienced a sedimentary evolution from deep-water slope to shallow water carbonate platform.(3)Thick microbial laminated dolomite reservoir with large scale and high quality was developed in the Dengying Member2in the western intraplatform trough,which could be compared with eastern intraplatform trough,revealing that the western intraplatform trough has also the excellent reservoir conditions of large gas field formation.