Gas In Place And Its Controlling Factors Of The Shallow Longmaxi Shale In The Northern Guizhou Area

At present,the commercial development shale gas blocks in China are mainly located in the lower Silurian Longmaxi Formation shale in the Sichuan Basin,while the marine shale reservoirs outside the Sichuan Basin have a relatively shallower burial depth(<1500 m)since they have suffered strong structural transformation.It is generally believed that the shallow shale has poor shale gas preservation conditions and low gas-in-place(GIP)content.However,the latest exploration practice shows that the shallow Paleozoic marine shales still have good exploration potentials.In this study,the shallow Longmaxi shale samples were collected from two exploration wells(XK2 well and XK3 well)in northern Guizhou Province,which is located outside the of Sichuan Basin,and their organic geochemical and mineralogical characteristics,reservoir properties,connate water characteristics and methane high pressure adsorption capacities were systematically studied.The GIP content was tested and the gas content(free gas and adsorption gas content)of the shale was evaluated.On the above basis,the effects of TOC content,mineral composition,porosity,pore structure,water content and preservation conditions on the contents of different gas sates are discussed,and the main controlling factors of the gas content of shallow Longmaxi shale in the study area are determined.The following understandings and results are obtained:(1)The TOC contents of the shallow Longmaxi shale in northern Guizhou range from 0.94-8.04%,with an average of about 4.22%.The Longmaxi shale are in the over-mature stage,and their EqRo values are in the range of 2.95 and 3.19%,with an average of about 3.08%.The shale samples are mainly composed of quartz and clay minerals.The organic pores and inorganic pores are both well developed in the studied shales,and the pore types of shale samples mainly include internal pores of organic matter pores,internal pores of mineral grains,intergranular pores between mineral grains,and microscopic fractures.The total porosity of the shale is 3.03-5.50%,with an average value of about 4.33%.TOC content is the main controlling factor of the porosity of Longmaxi shale.(2)The connate water content of the Longmaxi shale ranges from 5.01 to 12.38mg/g,with an average of 7.59 mg/g;the water saturation of the shale ranges from27-84%,with an average of 46%.The connate water is mainly controlled by the clay minerals content,and the average proportion of connate water in inorganic pores is about 80%.The connate water mainly occurs in mespores less than 10 nm and micropores between 0.4 and 0.6 nm,which significantly reduces the non-microporous BET surface area.Therefore,clay minerals is the main controlling factor of connate water properties of shallow Longmaxi shale in the study area.(3)The gas-in-place content of the shallow Longmaxi shale in the study area is relatively high(between 0.83 and 3.81 m~3/t with an average of 2.08 m~3/t),and it is dominated by adsorbed gas(with an average proportion of 76%);the shale gas is typically dry gas.Compared with Longmaxi shale inside Sichuan Basin,a much greater TOC content(generally>4%)will be required for the shallow Longmaxi shales outside the Sichuan Basin with prospective shale gas resources.(4)Compared to the dry shale,connate water reduces about 40%of the adsorption capacity of the shale.The measured excess adsorption result of the water-bearing shale is more comparable to the actual adsorption gas content of the shale.However,the difference between the two is affected by the temperature of high-pressure adsorption experiment and the lost gas in the shale GIP content test.(5)The TOC content has a definite positive linear correlation with the total gas content,adsorbed gas content and free gas content of the shale samples.The porosity and pore structure(non-micropore BET specific surface area and micropore volume)directly affect shale adsorbed gas content and total gas content.TOC content controlls the total pore porosity,pore structure and adsorption capacity of the studied Longmaxi shale,and in turn affect the shale gas content;the organic pores store 76%of the adsorbed gas content and 90%of the free gas content of shale.Quartz is beneficial to the occurrence of shale free gas,but has little effect on the content of adsorbed gas.The connate water mainly reduces the adsorption capacity of the shale(mainly clay minerals),leading to the limited contribution of clay minerals to adsorbed gas content and total gas content,and consequently reducing the adsorption gas content and total gas content of shale.Therefore,TOC content is the main controlling factor of gas content of the shallow Longmaxi shale in the study area.(6)Based on the above observations,it is concluded that the exploration of shallow shale gas outside of sichuan basin should give priority to the areas with high TOC content(more than 4%),high siliceous mineral content,and low clay mineral content,which are far away from large faults and have good top and bottom plates.The above study results have an important reference significance for the further exploration and development of the shallow Longmaxi shale in southern China.