Comparative Analysis On Development Characteristics Of Organic-Rich Shale In The Extensional And Compressional Settings

The Lower Paleozoic in the west of Middle Yangtze have favorable geological conditions for shale gas accumulation,and have great potential for resource exploration and development.The shale of the Qiongzhusi Formation was deposited in the Craton basin under extensional background,and the shale of Wufeng-Longmaxi Formation was deposited in the foreland basin under compressive background.The two formations are the focus of exploration work in southern China,but the two sets of organic-rich shales have obvious differences in sedimentary facies distribution,developmental characteristics of organic-rich shales,gas-bearing properties and controlling factors for the development of organic-rich shales due to their different basin properties.Therefore,this paper combines drilling and field profile data,focus on the sedimentary characteristics and evolution of Qiongzhusi and Wufeng-Longmaxi formations in the study area,and the development models of organic-rich shale in different basins were established to provide guidance for the exploration and development of Lower Paleozoic shale gas in western Middle Yangtze.Through relevant research,this paper mainly obtains the following understandings:(1)Qiongzhusi Formation and Wufeng-Longmaxi Formation mainly developed deep-water shelf facies,semi deep-water shelf facies and shallow water shelf facies in the western Middle Yangtze.Organic rich shale is forme in deep-water shelf environment.As the tectonic process(extension)weakened,Qiong Zhusi Formation transferred from the first to the third member from deep shelf to shallow shelf.With the intensification of tectonism(extrusion),from Wufeng Formation to 4 layer of the first submember of the first Member of the Longmaxi Formation transited from deep water shelf facies to shallow water shelf facies,and the development characteristics of deep water shelf facies under different tectonic backgrounds were significantly different.The continental shelf of Qiong Zhusi Formation was thick and developed for a long time,mainly in black siliceous and carbonaceous shale.The hydrodynamic conditions were weak and the lamina was not developed.Compared with Qiongzhusi Formation,the Wufeng-Longmaxi Formation has a thinner depth and shorter development time,mainly deposited siliceous shale,clay siliceous shale,horizontal bedding and massive bedding,with high graptolite abundance and differentiation.On the plane,the distribution of sedimentary facies was significantly controlled by paleogeographic pattern.Qiongzhusi Formation developed in passive continental margin for a long time,but in the middle Yangtze platform for a short time under the control of first-order structural differentiation--"platform-basin differentiation" paleogeographic pattern.Under the control of the paleogeographic pattern of second-orderstructural differentiation--"trough-platform differentiation",deep water shelf facies developed thickly in the western Hubei trough and thickly in the Middle Yangtze platform.The Wufeng-Longmaxi Formation,under the control of the "uplift-depression" paleogeographic pattern,presents a zonal sedimentary pattern along the western Hunan-Hubei underwater palaeouplift,and the deepwater shelf facies are mainly developed in the post-uplift depression.(2)Mineral and petrological characteristics of Qiongzhusi Formation and Wufeng-Longmaxi Formation were identified.The mineral compositions of Qiongzhusi and Wufeng-Longmaxi formations in the western part of the Middle Yangtze were both composed of quartz,feldspar,clay,carbonate and pyrite,but there were significant differences in the proportion of quartz,clay and carbonate minerals.The average content of Qiongzhusi(36.01%)was lower than that of Wufeng-Longmaxi(54.64%),and the average content of clay minerals was lower than that of Wufeng-Longmaxi,while the average content of carbonate minerals was higher than that of Wufeng-Longmaxi.In the longitudinal direction,the water gradually became shallower,and the sediments of Qiongzhusi Formation changed from deep shelf to shallow shelf.Qiongzhusi Formation was located in the passive continental margin,and the sediments were mainly siliceous shale.In the western Hubei trough and the Middle Yangtze platform,the central Hubei ancient land expanded further,the terrigenous input increased,and the lithofacies gradually changed from siliceous shale to calcrete shale and limestone.The Wufeng-Longmaxi Formation,on the other hand,receives clastic rock deposits,and the content of clay minerals increases obviously.The lithofacies of the Wufeng-Longmaxi Formation changes from siliceous shale to siliceous mixed shale.(3)The TOC content of Qiongzhusi and Wufeng-Longmaxi formations in the western Middle Yangtze was higher than that of the latter,and the TOC content changed significantly.The high TOC content appeared at the bottom of the first and second stage,and the TOC content decreased at the third stage,while the TOC content of the Wufeng-Longmaxi stage decreased at 3 layer of the first submember of the first Member of the Longmaxi Formation.After the third layes,the TOC content decreased significantly.The TOC of Qiongzhusi Formation increased gradually from northwest to southeast in the passive continental margin,west Hubei trough and platform,respectively.The TOC of Wufeng-Longmaxi Formation was high in DaxianWulong-Lichuan-Padong and Zhangjiajie-Cili Guangming.TOC content increased gradually from southeast to northwest in the study area as a whole.(4)The gas content of Qiongzhusi and Wufeng-Longmaxi formations was lower than that of Qiongzhusi,which was mainly due to the large burial depth and high thermal evolution.The distribution of maturity Ro of organic matter in Qiongzhusi Formation was 1.8% to 4.4%,reached the late stage of over-maturity;The maturity Ro of organic matter in Wufeng-Longmaxi Formation ranges from 2.05% to 3.13%,indicating the early stage of overmaturity.