Study Of Shale Gas Geologic Conditions Of The Middle Permian Tongziyan Formation In Southwestern Fujian

The Tongziyan Formation of Middle Permian in Southwest Fujian Provinces contains abundant coal resources and is one of the most economical coal-bearing sediments in Fujian Province.It can be divided into three members form base to up,including P₂t¹member,P₂t²member and P₂t³member in ascending order.The P₂t¹member and P₂t³member are dominated by detrital coastal deposition facies,but the P₂t²member is the shallow marine deposition facies.The organic-enriched shale depositions are?relative?moderate developed in these three members are provide material sources for shale gas formation.Recently,the investigations and evaluations of shale gas resources have been carried out in the in other provinces through all the whole country,but the shale gas in Fujian has not been studied systematically.In this thesis,we have focused the Tongziyan Formation of Middle Permian in Southwest Fujian Province,in order to explore the geological conditions of shale gas formation in this area.Firstly,we identified the depositional environments as well as spatial-temporal distributions of shale by the characteristics of regional strata and sedimentary environment of Tongziyan Formation.The results show that Tongziyan Formation of Middle Permian in Southwest Fujian is a coal-bearing deposit of marine-continental transitional facies,which could be the source rock of the shale gas.In addition to the temporal distributions,the Tongziyan Formation can be divided into three members in spatial distributions,which can be divided into three regions?western,central and eastern region?.The P₂t¹member of the Tongziyan Formation is thicker in the eastern region and thinner in the western region;the sedimentary range and thickness of the P₂t²member,which mainly deposited in the middle and eastern regions,and the thickness of decreases gradually toward the east,is smaller than that of the P₂t¹and P₂t³members;the P₂t³member has larger thicknesses of shale in Yongding,Longyan,Yong'an and the middle of eastern region.These results from comprehensive comparisons show that the shales in the middle region of the study area are the most abundance developed,which can provide the most favorable conditions for the formation of shale gas.Secondly,we explored the geochemical and reservoir physical characteristics of the Tongziyan Formation in the study area,by using a large number of organic geochemical and reservoir physical property analyses.The results show that the type of organic matter of Tongziyan Formation in Southwest Fujian is III,the total organic carbon?TOC?contents are moderate.There are higher hydrocarbon-generation potential in some members and regions.However,there is an uncertain factor for the formation of shale gas of the Middle Permian Tongziyan Formation in Southwest Fujian for it is in the stage of over-maturation for the average vitrinite reflectance?Ro?of Tongziyan Formation in the study area is greater than 3.5%.The mineral fractions are mainly composed of quartz and clay minerals,yielding few potash feldspar,plagioclase,calcite,dolomite,pyrite,siderite,and barite.The average brittleness index is almost greater than40%,indicating that the whole area has a certain degree of fracturing.The shale of the Tongziyan Formation has lower porosity,but the permeability is better,yielding porosity and permeability ranges from 0.27%to 3.80%,and 0.00001×10^-3?m²?0.04553×10^-3?m²respectively.The nitrogen adsorption experiment was used to explore the pore morphology of the shale reservoir,in order to understand the pore characteristics of the shale in the Tongziyan Formation,The result show the pore is open,and mainly include slit holes with open parallel walls,and a small amount of 3 types of ink bottle holes and 2 types of airtight holes with closed at one end.Micropores?diameters are less than 2 nm?and mesopores?diameters range from 2 nm to 50 nm?contribute most of the pore specific surface area and are the main spaces for gas adsorption in shale.The sorption isotherm shows that the Langmuir volumes are 1.71 m³/t and 1.47 m³/t in the Yongding and Datian respectively in the middle region.The gas-bearing properties of the Tongziyan Formation are similar to the typical shale gas basins in China and the United States.Finally,we analyzed the preservation conditions of shale gas in detail in Tongziyan Formation by integrating of tectonic movement,magmatism and caprock conditions.We provided the direction for the next shale gas works in the study area,by comparing the prospect areas of each member,which was evaluated by the shale gas accumulation factor superposition analysis method.The tectonic movements of the Variscan-Indosinian period formed regional folds in southwest of Fujian.It was strongly reformed by the Yanshan movement later,which was not conducive to the preservation of shale gas.The study found that the Tongziyan Formation in Fushi syncline and Yong'an Datian Syncline in Yongding and Datian of the middle region have relatively complete development and burial depths?more than 500 m?,which are beneficial to the preservation of shale gas.There are many faults in the study area,which not only have control effects on sedimentary strata,but also become magmatic hydrothermal intrusion channels,and thus have unfavorable effects on the preservation of shale gas.The influence of tectonic movement on shale preservation is also manifested in the thrust nappe structure.The basal thrust nappe structure mainly causes the coal-bearing strata of the Tongziyan Formation to be covered by the overthrust nappe in a large area,as well as the coal-bearing strata of Tongziyan Formation under the overthrust nappe is relatively intact,and only the upper strata is replaced,which has little effect on gas preservation;interlayer thrusting nappe structure mainly causes large-scale repetition of Tongziyan Formation coal measures strata in space,the development of coal-bearing strata folds in the upper and lower walls of faults,and the increase of strata thickness of Tongziyan formation in unit area,which is also conducive to the generation and preservation of gas;the caprock thrust nappe structure causes the strata structure of the Tongziyan Formation to be fragmented,making gas storage more difficult and having lower exploration value.The research shows that the Cuipingshan Formation?P₃cp?as the caprock has relatively stable lithological characteristics and sedimentary thickness,with a thickness of 180 m?640 m,and then has better physical condition for conservation.Meanwhile,assuming the Cuipingshan and Tongziyan formations successively deposited in large areas without disturbing,they are benefit for gas preservation.There are five prognosis prospective blocks in the study area.Block I is located 7 km northeast of Jiangle country,block II is in Jiafu coal mine area of Yong'an,block III is in the middle part between Yong'an city and Datian country?near the Datian coal mine area?,block IV and block V are located in Longyong coal field,but the block IV is near Zhangping city,the block V is in Yongding area of Longyan.Based on the conditions of resource,reservoir and comprehensive supporting conditions,the comprehensive evaluation and analysis of the five prognosis prospective blocks of the Tongziyan Formation were carried out.Results show that,the comprehensive evaluation of block II is the best one,which has the highest evaluation value?0.533?.The evaluation values of block IV and block V are relatively similar,which are 0.519 and 0.526 for blocks IV and V respectively.The block III has the lowest evaluation value of 0.507,showing the exploration value is relatively poor.The blocks II and V are more conducive to the next step of work.Finally,we using analogy method to calculate middle Permian shale gas resources from the Tongziyan formation in Southwestern Fujian,the inferred shale gas resourceis 121.70?183.07×10⁸m³,yielding the median is 152.38×10⁸m³.