Study On Shale Gas Accumulation Rules And Exploration Area Of Dawuba Formation Of Carboniferous In Guizhou Province

In recent years,China has achieved exploration breakthroughs in shale gas exploration in the Cambrian and Silurian systems.In some blocks,China has realized the shale gas capacity construction,but the scale of resource reserves and production capacity is still in a small size,and it is necessary to further expand the exploration horizon and exploration range.Guizhou Province has abundant shale gas resources,and there are many sets of black shales,the Carboniferous Dawuba Formation is one of them.In this paper,the regional lithology,lithological combination and deposition of the Dawuba Formation,organic geochemistry,reservoir physical properties,and gas—bearing properties and hydrocarbon accumulation characteristics are studied.The preliminary results are as follows:(1)This paper analyzes the sedimentary pattern of the Shuicheng—Ziyun rift trough and divides the sedimentary facies for the Carboniferous Dawuba Formation.The sedimentary and distribution of the Carboniferous—Dawuba Formation in Guizhou Province is controlled by the Shuicheng—Ziyun rift trough.The rift trough has a low asymmetric distribution pattern in a high northeastern part and a low southwestern part,it has two sedimentary depression centers naming Ziyun and Weining.The bulge area is formed between the Shuicheng and Liuzhi.The complex sedimentary geomorphic environment caused rapid changes in the stratum thickness and rock combination for the Carboniferous Dawuba Formation.According to the changed sedimentary environment,the sedimentary facies of the Dawuba Formation are divided into four phases including rift edge slope phase,transition shallow sea—semi—deep sea phase,trough basin phase,and shallow sea phase in the uplift area,and it is in transition with the old division of the Weining Formation lagoon.(2)This paper establishes the sedimentary model for the Carboniferous Dawuba Formation and studies the control law of its shale organic geochemical conditions.The Carboniferous striking dam group has a rift—edge—slot phase retention sedimentary model with shallow seawater on the edge slope,strong evaporation,high proportion of carbonate interbeds,abundant land margins and low organic matter abundance,its kerogen type is II₂-III;In the shallow sea—semi—deep sea,the retention and reduction environment is obviously enhanced,and it is almost unaffected by the continental margin source,and the carbonate rock content is reduced.The pyrite begins to enrich and develop bone fossils,and the kerogen type is II₁-II₂.In the stagnant basin phase,the sedimentary water body is further deepened,the reducing environment is strong,the plankton begins to develop,and the bone needles and algae fossils develop.It is almost unaffected by terrestrial debris and organic matter,carbonate rocks develop,siliceous rocks appear in large quantities,with abundant pyrite,with high organic matter abundance,and its kerogen type is I-II₁.(3)This paper studies the shale reservoir characteristics of the Carboniferous Dawuba Formation and analyzes the influence of sedimentary environment on shale reservoir conditions.The black shale of the Dawuba Formation is mainly composed of quartz and clay minerals,and it also contains calcite and a small amount of feldspar,dolomite and pyrite.The clay minerals are mainly composed of illite and yimon,and the content of brittle minerals is from 30%to 60%.It develops microscopic pore types,including:fracture pores,interparticle pores,intercrystalline pores,clay inter—plate pores,rigid particle edge cracks,pyrite nodule intercrystalline pores,particle edge cracks and organic matter pores.The pore size is between 3.37 and 6.84 nm,with an average of 7.09 nm.There are plenty of micropores and mesopores,and the contrast surface area provides the most amount of contributions.The change of sedimentary environment has an important influence on microscopic pores,which is characterized by the fact that the specific surface area in the edge phase of carbonate mineral enrichment is generally less than 12m²/g,while the specific surface area in the semi—deep sea—sink phase of the rift trough with lower carbonate mineral is generally more than 20 m²/g.(4)This paper studies the shale gas accumulation law of the Carboniferous Dawuba Formation and summarizes its accumulation model.The black shale of the Carboniferous in the study area is more than 1m³/t,which is positively correlated with organic matter abundance,quartz content,pyrite content and specific surface area.Different material groups affect the accumulation of shale gas.In addition,the Yanshanian tectonic movement has a great influence on the shale gas accumulation of the Dawuba Formation,and the stable area with less tectonic deformation is beneficial to shale gas accumulation.If there is a good cap preservation condition,the strong squeezing deformation zone can form a fractured shale gas reservoir;When the local layer is shallow,the shale gas enrichment is greatly affected by the well depth,and it is necessary to provide a page of sufficient thickness rock as a capping condition.When the buried depth is increased,the thickness of the cap shale will be gradually reduced.According to the characteristics of shale gas enrichment above,this paper summarizes the shale gas accumulation model of the shallow sea—slot basin weak deformation zone in the Dawuba in the Carboniferous System in Guizhou Province.(5)This paper compares the prospects of shale gas exploration in the Carboniferous Dawuba Formation,and optimizes the shale gas favorable area.By comparing the shale geological conditions of the Longmaxi Formation in Jiaoshiba,Antrim,Ohio,New Albany,Lewis and Barnett in the United States,the results show that the black shale of the Carboniferous Dawuba Formation is thick,its gas content,quartz content,TOC and other indicators are similar to that of shale layer.Although the kerogen type is complex,the maturity changes greatly,and the porosity is low,it has a good material basis for the formation of shale gas,and it has shale gas exploration prospect.In view of the current low level of shale gas exploration in the Dawuba Formation,according to the shale gas accumulation law,the sedimentary environment,organic matter abundance,shale thickness,tectonic deformation strength and fracture development are used as conditions,and refer to mineral composition,the specific surface area and the degree of exploration,six zones including Changshun,Wangjing,Qinglong,Guanling,Shuicheng and Luodian are optimized.According to the probabilistic volume method,the shale gas resources under P50 are about 515.7 billion m³,624.5 billion m³,566 billion m³,746 billion m³,261.9 billion m³,and 128.82 billion m³,the total geological resources are 284.3billion m³,which provides guidance for the next step in shale gas exploration.