The Study Of Pores Characteristics And Shale Gas Occurrence Of Longmaxi And Niutitang Formation In East Of Sichuan And West Of Hunan Area

Based on core description,thin section analysis and core testing data,we studied the Longmaxi Formation in the east of Sichuan and the Niutitang Formation in the west of Hunan province by multiple techniques in order to characterize the pore structures.The field emission scanning electron microscopy?FE-SEM?was performed to investigate organic matter?OM?pores,interparticle?interP?pores and intraparticle?intraP?pores.The size distribution of micropores,mesopores and macropores were characterized by utilizing CO₂ adsorption,N₂ adsorption and mercury injection analytical methods.The fractal dimensions of the micropores,mesopores and macropores were calculated.We build the supercritical DA model by considering the adsorption potential theory and the characteristic equation by incorporating temperature and pressure parameters.The shale gas accumulation mechanism and evolution model were analyzed based on the characteristic equation.The major conclusions can be drawn as following:1.The Longmaxi Formation can be divide into two intervals:the upper interval is shallow-water shelf deposits and the lower interval is deep-water shelf deposits.This formation primarily comprises grey to dark shales and mudstones and the lower interval is more rich in silica.This formation is dominated by quartz,clay,and a less content of feldspar,carbonate and pyrite.The clay minerals are dominated by illite/smectite mixed layer and illite with a minor amount of chlorite.The kerogen is dominated by type I organic matter and currently over-mature.The Longmaxi Formation has an average TOC of 2.66 wt.%with the lower interval more organic-rich.The Niutitang Formation is interpreted to be deep-water shelf deposits.It is dominated by quartz,clay and carbonate,and a less content of feldspar pyrite.The clay minerals are dominated by illite with a minor content of illite/smectite mixed layer and chlorite.The kerogen is dominated by type I organic matter and currently over-mature with an average TOC content of 2.46wt.%.2.FE-SEM observations show that OM pores display elliptical and spongy-like shape.InterP pores display silt-like and angular shape.IntraP pores display circular,elliptical,and angular shape.OM pores and interP pores are better connected than intraP pores.Among different lithofacies,siliceous shales have less OM pores but with wider pore sizes.Mixed shale lithofacies have more OM pores with narrower pore sizes,whereas,argillaceous shale has least OM pores with narrowest pore sizes.Base on CO₂ adsorption,N₂ adsorption and mercury injection analysis,the pore size distributions of Longmaxi Shales show that the average pore volume and specific surface area of Longmaxi shale is higher than Niutitang shale.The pore volume and specific surface area of siliceous shale is higher than the mixed shale and argillaceous shale lithofacies,which is beneficial for gas accumulation.Organic matter is more beneficial for pore development,and rigid grains can improve porosity by sheltering primary pores from compaction,where clay minerals have limited contribution to total porosity.3.The combination of SEM images of shale reservoirs and“box-counting”method can be effectively used to to characterize the fractal dimensions of pore cross sections.Our results show that OM pores have a stable fractal structure with relatively high fractal dimension.InterP pores have a simple structure,whereas IntraP pores have complicated structures with a wide range of fractal dimensions.Porous medium fractal theory was applied to calculate the fractal dimensions of micropores and mesopores.The results show that the Longmaxi Shale has a more complicated pore structure than the Niutitang Shale.Micropores and mesopores of Longmaxi siliceous shales have higher fractal dimensions,indicative of a more complex pore network.Several parameters include pore volume,specific surface area and fractal dimension can be used to quantitatively characterize pore structure of shale reservoirs.The Longmaxi Shale has a better porosity development and a more complex pore structure than Niutitang Shale.4.By considering the adsorption potential theory,we derived the supercritical DA model.It displayed an excellent imitative effect for methane adsorption data,and it can be comparative with three-parameter Langmuir model and SDR model.The results show that the lower interval of the Longmaxi Formation has a greater gas sorption capacity?GSP?than the upper interval,and the GSP of Longmaxi Shale is higher than Niutitang Shale.The GSP of reservoirs is control by several factors.First of all,TOC is the key factor.Clay components have a little contribution because of none smectite.The specific surface area of pore is usually benefit for adsorption,and the simple pores surface may support the homogeneous adsorption.Moisture is a negative factor,the hydrophobicity of Longmaxi shale is greater than Niutitang shale.Temperature cut down the GSP,while pressure can promote it.Longmaxi Shale and Niutitang Shale have different sensitivity for temperature and pressure.Base on the adsorption potential theory,characteristic function can be derived to indicate GSP.This function incorporates temperature and pressure parameters,and reveals the temperature-pressure coupling effect.5.The change of reservoir gas content in situ and different depth reveals that the occurrence of adsorbed gas and free gas is control by geothermal and pressure coupling effect,and it also be affected by lithology,moisture,and gas components.Form the upper of the formation to the bottom,the distinction of temperature and pressure can be ignore,so the occurrence of adsorbed gas is controlled by TOC,moisture,and gas components.The occurrence of free gas is controlled by pore volumes and water saturation and gas components.When the change of temperature and pressure is remarkable,both of them must influence occurrence of adsorbed gas deeply,and moisture,gas components also have an affection.And free gas is also controlled by temperature,pressure and water saturation and gas components.The evolvement rule of gas content in the Longmaxi Formation reveals that methane generated through thermal cracking was free phase mainly,then it transformed into adsorbed gas in the uplift stage because of relative closed system,so reservoir could contain more gas.While the porosity development of the Nititang Formation is worse than the Longmaxi Formation,reservoir must be lack of spaces for free gas,but it could contain adsorbed gas with a high TOC.