Study On Shale Gas Enrichment Mechanism Of The Lower Cambrian Niutitang Formation In The Northern Guizhou

This study focuses on the organic-rich shale of the lower Cambrian Niutitang formation.Types of rock mineral assemblages,spatial distribution characteristics of sedimentary facies and the sequence stratigraphic framework are systematically sorted out based on petrology,mineralogy,sedimentary geochemistry,organic chemistry method and other theories and research methods and combined with lithologic profile,drilling data and a variety of testing analysis.On the basis of analyzing and summarizing the reservoir quality,a model of organic matter enrichment is established,the evolution process of organic matter is retrieved,preservation conditions of shale gas are revealed,the enrichment mechanism of shale gas are elucidated,and the favorable zone of shale gas distribution is proposed.The paper suggests that the shale of the Niutitang formation in the northern Guizhou region was established as a sequence framework of transition from transgression systems tract(TST)to condensation stage(CS)to highstand systems tract(HST)from the floor to top under the background of rapid transgression and slow regressive.The sedimentary facies sequence transitions from deep-water shelf facies to shallow-water shelf facies,and the petro-mineral assemblage shows a trend of increasing mineral clastics and decreasing clayey matrix.The proportion of transgressive systems tract(TST),the deep-water shelf facies and clay content in the shale of the Niutitang formation increases to the eastern facies region,while the proportion of the highstand systems tract(HST),the shallow-water shelf facies and detrital minerals increases to the western facies region bounded by the line of Longli-Huangpinglangdong-Shiqianbenzhuang-Yanheganxi,based on the sedimentary facies characteristics of the research area.This characteristic controls of the organic shale enrichment of the above two facies regions.The intergranular pores and intercrystal pores are main micropores structure in the western facies region,while the main development of dissolution pores and intergranular pores in the eastern facies region.The sedimentary environment of early Cambrian in the northern Guizhou region was transformed to the southeast deep water shelf,and the REDOX conditions were transformed from oxygen-poor environment to oxygen-deficient environment with some sulfurization characteristics.The continuous supply with the rising ocean current and submarine hydrothermal fluid led to the flourishing of a large number of phytoplankton,improved the paleoproductivity,and enriched a large amount of organic matter in the eastern facies region under the undercompensated low energy stable static water environment for a long time,which lays the foundation for the formation of shale with high organic carbon content.The western facies region was mainly dependent on the water stratification caused by the rising ocean current,which provides conditions for the enrichment of organic matter.The organic matter enrichment pattern of the Niutitang formation in the northern Guizhou region was formed by the control of multiple factors.The buried depth of the Niutitang formation in the study region is deep in the west and shallow in the east,while the structural style is simple in the west and complex in the east.The Niutitang formation experienced three stages of oil and gas enrichment,and finally produced mainly natural gas.The maturity of organic matter is high in the east and low in the west.The evolution of high maturity organic matter is the combined effect of buried role,tectonism and deep underground heat flow or magmatic activity.For the Niutitang formation shale gas,the whole area has certain sealing ability,deep and large faults have strong damage to gas reservoir,while shallow faults have little influence on gas reservoir.The thermal cycle is more open near the deep and large faults in the eastern facies region,which has a great influence on the preservation of shale gas.The differentiation preservation pattern of shale gas in Niutitang formation in the northern Guizhou region is established by summarizing the above characteristics.The hydrocarbon gases,nitrogen and carbon dioxide are caused by pyrolysis of organic matter,the hydrogen has both inorganic and biogenic characteristics in the shale gas of the Niutitang formation in the study region.The adsorbable gas content of shale is as high as 4.4m~3/t,most of the field analysismost of the gas content is less than 0.1m~3/t,the residual gas content in the laboratory test can be 2～17 times higher than that in the field test(methane content is as high as 94.16%).The results show that the shale of the Niutitang formation in the northern Guizhou region has strong heterogeneity,high predicted total gas content and is dominated by adsorbed gas,which can be accumulated to form large shale gas reservoirs in the place where the preservation conditions are suitable.Main controlling factors of shale gas enrichment,such as the abundance of organic matter,mineral composition and preservation conditions.The favorable selection of the Niutitang formation shale gas in the northern Guizhou region is Suiyangpuchang,Zhenganxieba,Wuchuanmaotian,Dejiangshaxi,Fenggangfengyan,Cengongtianma and Zhenyuanyangchang according to the favorable sedimentary facies belts,thick shale sections,high quality organic geochemical parameters,favorable preservation conditions and good reservoir physical properties and other results.