Researsh On Shale Reservoir Characteristic Of Wufeng-Lower Longmaxi Formation In Dianqianbei Depression

In this paper,shale reservoir of the Wufeng Formation of Upper Ordovician and lower section of the Longmaxi Formation of Lower Silurian in the Dianqianbei area is taken as the research object.Based on the previous work,two exploration wells in the research area are systematically sampled,and measurements including X-ray diffraction,SEM,organic geochemical testing,low-temperature nitrogen adsorption and other testing techniques were applied to find out the characters of minerals composition,pores,organic geochemistry in the study area.The results show that:(1)the Wufeng and lower member of Longmaxi Formation in the study area is divided into two main lithofacies member from top to bottom,namely clayey siliceous mixed shale lithofacies member and calcareous siliceous mixed shale lithofacies member.As a whole,those two formed in the environment of detention,low energy and anoxia,with high paleo marine productivity and weak input intensity of terrigenous clasts.The inorganic minerals are mainly quartz(4%?45%),clay(10%?50%),and carbonate(7.2%?40.7%),with an average content of 36.06%,31.06%,and 23.89%,respectively.Clay minerals are mainly consisted of illite(I),chlorite(C)and illite montmorillonite mixed layer(I/S).Among them,the content of I/S is the greatest(26%?58%),with an average of 45.52%;followed by illite(23%?56%),with an average of 38.43%,and chlorite(2%?30%),with an average of 15.94%.(2)The total organic carbon content(TOC)of shale in the study area ranges from 0.23%to 7.40%,with an average of 2.94%.Samples with TOC greater than 2%occupied 88.50%of the total,and the organic matter abundance is high.TOC values of shales increase from the top to the botton in the Wufeng and Longmaxi Formation.In organic macerals,the content of exinite is the greatest,ranging from 86%to 92%with an average of 89.67%,followed by sapropel,ranging from 4%to 7%with an average of 4.67%.The type of organic matter is mainly type?₁ kerogen with high degree of evolution.The vitirnite reflectance is ranged between 2.47%and 2.71%,with an average of 2.57%.It has entered the stage of over mature thermal evolution and is in the stage of thermal cracking dry gas generation,indicating the shale reservoir is high quality hydrocarbon source rocks.(3)Pores of shale reservoir in the study area mainly include organic pores,intergranular pores,intragranular pores,and crack.The average pore diameter of the shale is 8.7-10.4 nm,with an average of 9.51 nm.The BJH total pore volumeis ranged from 0.0247 ml/g to 0.037ml/g,with an average of 0.032 ml/g The specific surface area is ranged 19.30-28.20 m²/g,with an average of 22.59 m²/g.There are good positive correlations between pore volume and specific surface area.Shale porosity distribution ranges from 3.30%to 9.87%,with an average of 6.02%.Vertically,the porosity from Wufeng to Longmaxi formation gradually increases.(4)TOC,brittle mineral content,and clay mineral content control the physical properties of the reservoir.TOC and brittle mineral content are directly proportional to specific surface area and pore volume as well as porosity.Clay minerals have negative correlation with pore specific surface area and pore volume as well as porosity.Because of the differences of TOC,brittle mineral content and clay mineral content between Clayey siliceous mixed shale and Calcareous siliceous mixed shale,the porosity of Calcareous siliceous mixed shale shale is greater than that of Clayey siliceous mixed shale.