Research On Shale Gas Accumulation Characteristics Of Key Strata In Well Enye 1 In Western Hubei Area

In recent years,great breakthroughs have been made in the exploration of shale gas in Western Hubei.The study on the characteristics of shale gas accumulateion in Western Hubei has an important significance for shale gas exploration.In this paper,the methods Rock-Eval,TOC,SEM and X-Ray diffraction were used for analyzing the qualities of source rocks and reservoir of Lower Paleozoic Niutitang and Longmaxi shales in Enye 1 well.Moreover,based on fluid inclusions microscopic petrographic observation,microthermometry,microscopic raman spectroscopy,composition and carbon isotope analysis,the Paleozoic shale gas forming conditions,forming stages,forming times and forming pressure in Enye 1 well were also studied.The main achievements are as follows:1)Lower Paleozoic mudstones are widely distributed in Western Hubei.The thickness of the Cambrian Niutitang Formation is about 249.50m in Enye 1 well.The lithology of Niutitang is mainly mudstone,limestone and mudy limestone,the TOC is about 4.67-6.32%,and the Tmax is higher than 500?which reaches the over maturity stage.Mudstone,siltstone and silty shale are the mainly lithology of Silurian Longmaxi,of which the thickness of organic-rich shale is nearly 70m.The TOC of Longmaxi is about 1.43-1.82%,and the Tmax value is 480-490?reaching the high maturity evolution stage.2)Microscopic and scanning electron microscopic observation results showed that the pore development types of Niutitang in Enye 1 well include organic matter pore,intragranular dissolution pore,intergranular pore and intergranular pore of clay minerals,the size of which is mainly in the level of nano-um.Organic pore in Longmaxi is relatively high and the inorganic pore is mainly composed by intergranular pore of clay mineral.The illite and illite-mont mixed layer are main composition of clay minerals in Niutitang and Longmaxi formation.3)Niutitang Formation has 3 process of hydrocarbon filling:the I stage hydrocarbon accumulation temperature is 140¹45?,and the time is about 324 Ma;the II stage hydrocarbon accumulation temperature is 160¹65?,and the time is about 265Ma;the last stage hydrocarbon accumulation temperature is 180-190?,and the time is nearly 230 Ma.Longmaxi has 2 process of hydrocarbon filling:the I stage hydrocarbon accumulation temperature is 90¹00?,and the time is about 258Ma;the II stage hydrocarbon accumulation temperature is 130¹40?,and the time is about 218 Ma.4)Microscopic laser Raman analysis of fluid inclusions showed that the natural gas components of organic inclusions in Niutitang are mainly CH₄,CO₂ and a small amount of H₂S.The gas components of organic inclusions in the Longmaxi are CH₄,no CO₂ and other non-hydrocarbon gases.In addition,the spectra peak of bitumen in organic inclusions of Niutitang and Longamxi are all very high,which represent the maturity of these 2 formations are in a high stage.5)Based on the laser Raman spectra peak of methane-rich inclusions and the homogenization temperature of associated brine inclusions,the accumulation pressure of Paleozoic shale gas in Enye 1 well were calculated.The shale gas accumulation pressure of Niutitang on pyrolysis gas stage is 57.64⁶6.5 MPa,and on crack gas stage is about 124.7 MPa.The shale gas accumulation pressure of Longmaxi is about 39.80MPa.6)Natural gas of Niutitang and Longmaxi in Enye 1 well are all belong to thermal origin,of which the hydrocarbon gas components are mainly CH₄,and non-hydrocarbon gas are mainly CO₂.Affected by thermal maturity,CO₂ content in Niutitang is obviously higher than Longmaxi Formation,and the?¹³C₁ is more heavier.The?¹³C₁ of Niutitang is-36.1^-20.8‰and Longmaxi Formation is-49.0^-40.6‰.Carbon isotope analysis of CO₂ showed that?¹³C_CO2O2 of 2 formations are all less than-10‰,which implys CO₂ are all belong to organic origin.