Research On Relationship Between Graptolitic Shale Of Longmaxi Formation In Southern Chongqing And Shale Gas Reservoir Forming

The Longmaxi Formation graptolite shale facies developed in south region of Chongqing with good shale gas accumulation geological condition. This paper selects the Longmaxi Formation graptolite shale facies as the research object. A data collection, geological investigation, experimental test and theory research is used in the study to analy the fossil specimens developmental characteristics, shale gas accumulation conditions and graptolite biological contribution to shale gas accumulation of Longmaxi Formation shal. The results are as follow.The study area has a characteristic of simple structure and good strata outcropped. The research section of the Longmaxi Formation consists of two parts. The lower is argillaceous deep-water shelf deposit, which includes carbonaceous shale, clay shale, calcareous shale. The upper is plaster shallow shelf deposit which includes calcareous shale, siltstone and lenticular limestones. The research section of the Longmaxi Formation identifies mainly 20 genera and 71 species fossil stone with 5 genera and 8 species of the Ordovician Wufeng Formation and 17 genera and 63 species of the Siluric Longmaxi Formation. The Siluric Longmaxi Formation of the study area is divided into seven graptolite zone.Ice age regressive events of the late Ordovician and global climate warming events in the early Silurian changed the sedimentary environment and deposition rate which affected the growth of the graptolite and preservation conditions which led abundance of graptolite having changed. The abundance abundance of graptolite in the lower Longmaxi Formation is large while upper is small which corresponds with the characteristic of organic carbon content. Both have good correlation. The graptolite body carbon content is extremely high while carbon content of the surrounding rock is low in the study area. Therefore, the abundance of graptolite is the key factor for controlling the organic matter abundance of source rock, graptolite provides the material foundation for shale gas formation, and it is parent material for shale gas major hydrocarbon generation.Brachiopods fossils of Wufeng Formation in study area preserve mineralization stereo- half stereo which is formed in the horizon with low organic matter content. The graptolite of the Longmaxi Formation is saved in organic matter flattening film form which is formed in the horizon with high organic matter content. Pyritization petrochemical graptolite is found in middle-upper part of the Longmaxi Formation which is formed in the horizon with reducing environment and low organic matter content. Therefore, carbon film graptolite fossil can reflect the high organic carbon content of the study area.Due to the filling and metasomatism, brachiopods fossils of the Wufeng Formation and Pyritization petrochemical graptolite of the Longmaxi Formation have less effective pore which have limited contribution to reservoir pore. Effective porosity of carbon film graptolite fossil is relatively good. The openness of the pore is also good. Carbon film graptolite fossil can form a certain pore space and inner surface area, provide good clearance between the layers and connectivity which can provide a certain amount of storage space and seepage channel.Graptolite fossils exists residual organic carbon which can develop a large number of organic hole to provide a large number of adsorption point of shale gas. In the meantime, graptolite affects the pore development by controlling the content of total organic carbon, especially the development of microporous- transition hole which provides a large number of occurrence of space for the free gas and adsorbed gas. As a result, Graptolite fossil is one of the important factors for shale gas occurrence.Thesis research results have the reference value on the relationship between graptolite and shale gas accumulation and exploration of Chongqing south region graptolite shale facies shale gas.