Geological Conditions And Exploration Potential Of Shale Gas In The Niutitang Formation Of The Lower Cambrian In The Yuanma Basin And Its Periphery Area

The Lower Cambrian Niutitang Formation,with large thickness and abundant shale-gas resources widely distributed in the Yuanma basin and its peripheral area,is an important target bed for shale gas exploration in South China.However,there is no breakthrough has been made due to two key factors.One is the complicated preservation condition by the reconstructions of the multi-phase tectonic movements.The characteristics,styles and burial process of key tectonic period on shale-gas preservation condition is not clear.Another one is the lack of systematic study on the parameters variation of Niutitang Formation,especially the sedimentary environment,geochemical characteristics,reservoir physical properties.Therefore,in this paper,the author focus on the study of the black shale of the Lower Cambrian Niutitang Formation in the Yuanma basin and its peripheral area.Based on the previous researches,combining with geological data,drilling data and seismic data obtained by the author in recent years,the geological condition and exploration potential of the shale gas of the Niutitang Formation is studied according to the theories on reservoir sedimentology,petroleum geology and geochemistry,etc.Firstly,the lithologic characteristic,sedimentary environment and distribution of organic-rich shale in the Niutitang Formation is analyzed with plenty data including outcrops,drilling cores and geochemical testing data.Meanwhile,the hydrocarbon potential of the Niutitang Formation is studied by analysing the organic geochemical data such as TOC,types and maturity of organic matter,etc.Secondly,the reservoir characteristics such as mineral composition,types of reservoir space,pore structure and gas content ability is investigated with testing data,and the reservoir is evaluated integrated with logging data.Thirdly,the tectonic types and styles are summarized by seismic data and tectonic profiles,and the thermal evolution of hydrocarbon generation and burial process of organic-rich shale is studied in detail,as well as the coevolution of tectonics,burial process and hydrocarbon generation history on the shale-gas preservation condition,then the shale-gas preservation condition is evaluated combining with tectonic styles,top and bottom conditions,and fault-fracture system.Finally,baesd the above researches,the prospect and favourable areas of shale gas of the Niutitang Formation in the Yuanma basin and its peripheral area are identified,and their shale-gas resource is estimated by using probabilistic volume method.The main research results are listed as followings:1.The shale types and its distribution of the Niutitang Formation in the Yuanma basin and its peripheral area are controlled by sedimentary environment.By the detailed observation and description of the outcrop sections and drill cores,several types of rocks such as clayey mudstone/shale,silty mudstone/shale,siliceous mudstone/shale,silicastone and muddy limestone are recognized.The sedimentary environment of the Niutitang Formation in the study area is dominated by slope faices and basin facies.The data of geochemistry of element show that the shale of the Niutitang Formation formed in oxygen-deficient and oxygen-poor lentic environment.The average total organic content(TOC)of black shale formed in slope facies is more than 2%in average,mainly ranges from 3.0%to 6.0%.But in contrast,the TOC of shale formed in basin facies is usually more than 6.0%.The types of organic matter is mianly consist of type?,minor is type?1.The equivalent organic maturity(Ro)ranges from 2.06~3.55%,average of2.79%,and the Ro in the Yuanma basin is lower than that in its peripheral area,which shows the shale in the Niutitang Formation is under overmature stage and the shale has a certain extent of potential for producing dry gas.2.Reservoir features indicates that the shale in the Niutitang Formation has good gas content ability and fracturing ability.The reservoir features has been studied by the analysis of the mineral composition,reservoir physical properties,reservoir space,pore features and gas content ability,as well as the controlling factors of the development of organic-rich shale and reservoir evaluation.According to the whole-rock X-ray diffraction,brittle mineral(including quartz,potash feldspar and anorthose,etc.)content in the shale ranges from 5.2%to 91.61%,average of 59.14%.Clay minerals(mainly composed of illite with an average of 89%)content is from 5.0%to 50.96%,average of23.98%.The content of carbonate minerals mainly consist of calcite,dolomite ranges from 0%to 62%,average of 9.37%.Micropores(mainly consist of organic pore,intergranular pore and mold pore)and micro fractures(mainly consist of interbedded fracture and bed-parallel fracture)are widely developed in shale.Five types of lithological facies such as clay-rich siliceous shale,silica stone,siliceous shale,mixed shale and mud shale has been identified,and the Niutitang Formation has high content of siliceous mineral,which is similar to that in North America and Jiaoshiba area of east of Sichuan,but different from Shuijingtuo Formation which is characterized by high calcareous content in Yichang area.These features indicate that different sedimentary environment has obviously controlling effect on minerals composition of shale.3.Structural preservation conditions is closely related to tectonic styles,which is the key factor for shale gas accumulation.On the basis of the analysis of the geological profiles and 2D seismic data,the structure styles including faulted-bend anticline,back thrust and face thrust structure,growth structure,thrust-imbricate structure and thrust duplex structure are summarized,as well as the gas content ability of shale under different tectonic styles.The results shows,the face thrust structure and the growth structure are the most favorable structures for shale gas preservation,the burial depth of shale within the faulted-bend anticline and the footwall of thrust structure areas is usually too deep for economical exploration,eventhough the shale gas accumulation is also favorable in these areas.Back thrust,thrust-imbricate structure and thrust duplex structure aren't the favorable structures for shale gas preservation.4.Integrated with the shale-gas evaluation parameters such as lithological facies,geochemical features,sedimentary environment,reservoir physical properties and tectonic preservation conditions of the Niutitang Formation,Jishou-Changde area,Qiangongpin syncline,Xupu depression,south and north part of Chenxi depression are identified as prospective area for shale gas exploration of the Niutitang Formation,with an area of 4719.03 km~2.Caotang depression and Yuanguping-Changde areas are identified as favorable areas with an area of 2979.86 km~2.5.By using probabilistic volume method and the resource assessment software for shale gas(SGRE2.0),the geological resources of shale gas of Niutitang Formation in the prospective area is about 12701.57×10~8m~3(P50),and that in the favorable area is about 2289.76×10~8m~3(P50).Therefore,it can be concluded that the Niutitang Formation of the Lower Cambrian in the Yuanma basin and its peripheral area has good potential for shale gas exploration.