Basic characteristics of Lower Silurian Longmaxi Formation in SE margin of Sichuan Basin
Ma Wenxin1, Liu Shugen1, Huang Wenming2, Zeng Xiangliang1, Wang Jia1
1. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059; 2. Geological Exploration & Development Research Institute, CNPC Chuanqing Drilling Engineering Company Limited, Chengdu 610051
Lower Silurian Longmaxi Formation is one of the most important source rocks and shale gas exploration beds.Using the rock eval,chromatography-mass spectrometry,and other analytical techniques,the geochemical characteristics of Longmaxi Formation has been systematically analyzed based on the studies of petrographic features.The thickness of Longmaxi Formation ranges from 60 m to 420 m,and is comprised by eight lithofacies,including laminated and nonlaminated mudstone/shale,dolomitic siltstone,laminated lime mudstone/shale,argillaceous siltstone,laminated and nonlaminated silty mudstone/shale,fine grained silty sandstone,calcareous concretions and organic matter enriched noncalaceous shale.The depositional environment gradually changes from cutoff basin,deep shelf,and to shallow shelf bottom-up.Porosity measured on core samples of the shale from the Longmaxi Formation in exploratory wells ranges from 0.58% to 0.67%.The microporosity observed in the thin sections of the shale is dominated by the intercrystal and intragranular pores.TOC of Longmaxi Formation ranges from 0.55% to 4.41% in SE Sichuan Basin,but only the lower part is high quality source rocks,the mid-upper with general source rocks.TOC is very high in Shizhu profile,east Chongqing-west Hubei Province,with range of 0.55%～5.88%,but which is lower than 0.3% in other regions.Organic matter range from type Ⅰ-kerogen to type Ⅱ1-2-kerogen in Longmaxi Formation,but is dominated by the latter.The maturity of organic matter in Longmaxi Formation is very high,with late stage of post-maturation,and Ro is just lower than 2.0% in high point of local structure or upper part of Longmaxi Formation.The lower part of Longmaxi Formation(mainly aquatic algae)is deposited in restricted deoxidation conditions,which is revealed by chromatography-mass spectrometry features of saturated hydrocarbon,But becoming more and oxidation up.The heterogeneity of Longmaxi Formation shale is very strong,and siltstone bedded in lower part shale is more denser in Jianshen #1 well.Shale reservoir in Jianshen #1 well is more developed in Dingshan #1 well.