Study On Shale Gas Accumulation Conditions Of The Lower Cambrian Niutitang Formation In The Micangshan Area

The mud shale of the Niutitang Formation of the Lower Cambrian in the Micangshan Area has good sedimentary thickness,high abundance of organic matter,high maturity of organic matter,good reservoir property and good preservation conditions,which meet the favorable conditions for shale gas accumulation.Therefore,it is of great significance to study the conditions of shale gas accumulation in the Niutitang Formation in the Micangshan Area for the exploration and development of shale gas resources.Guided by the theories of sedimentology,oil and gas geochemistry and oil and gas geology,this paper summarizes the main controlling factors of shale gas enrichment in the study area through the specific research on the lithologic combination and distribution characteristics,sedimentary facies and lithofacies paleogeographic characteristics,organic geochemistry characteristics,reservoir characteristics and preservation conditions of the shale gas in the Niutitang Formation,and formulates the favorable prediction map of the shale gas in the Niutitang Formation in the study area.The results show that a deep-water shelf sedimentary system is widely developed in the Niutitang Formation in the study area.According to the sedimentary facies markers,three subfacies,namely,shore-shore facies,shallow-water shelf facies and deep-water shelf facies,are identified.In the study area,the sedimentary facies show the characteristics of shore-shallow-deep-water shelf successively from north to south,in which black organic mud shale mostly exists in the sedimentary environment of deep-water shelf.According to the results of organic geochemical analysis,the total organic carbon content of the organic rich mud shale in the Niutitang Formation in the study area ranged from 0.19%to 10.66%,and the single index was evaluated as a good source rock.Rich organic matter organic matter type is given priority to with?type kerogen of the shale;The maturity of organic matter is generally high,generally between 0.91%and 3.56%.On the whole,the thermal evolution degree of source rocks has entered into the stage of high-over-maturity.Through X-ray diffraction analysis of whole rock,it is concluded that the constituent minerals of the Niutitang Formation are mainly brittle minerals,clay minerals,and a small amount of pyrite and siderite,without carbonate minerals.Among them,the content of brittle minerals accounted for 62%?91%of the constituent minerals,and the average content was75%.Clay mineral content accounts for 9%?38%of the constituent minerals,with an average content of 25%.Clay mineral mainly contains illite,eamon mixed layer,chlorite and smectite,among which illite content is the highest.The effective porosity of the shale of the Niutitang Formation in the study area is 2.10%?7.14%,and the permeability is 0.0015×10^-3m²?0.009×10^-3m².The reservoir space of shale gas mainly contains two types of pores and fractures,among which the pore types can be divided into intrgranular pores,intergranular pores,dissolution pores and organic matter pores.A large number of developed pores and fractures in the study area provide a good basis for shale gas enrichment.Based on the study of organic geochemical characteristics,reservoir physical characteristics,preservation conditions and main control factors of mud shale enrichment in the Niutitang Formation in the Micangshan Area,the prediction map of shale gas distribution in the Niutitang Formation in the Micangshan Area was compiled according to the classification criteria of favorable and abundant shale gas areas and a variety of factors.This paper holds that the favorable shale gas enrichment areas of the Niutitang Formation of the Lower Cambrian in the Micangshan Area are mainly distributed in the fucheng-jiandongzi-guanyin'an area in the east of the study area and the xiaoba-yingshuiba area in the north of the study area.