Study On Petroleum Geological Characteristics Of Xujiahe Formation In Guang'an Area, Sichuan Basin

In the Guang’ an area, Xujiahe Formation has a wide gas-bearing area but with "spot-like" distribution pattern. It follows the distribution of "upper-gas and lower-water" on the whole. For the whole central Sichuan, a unique pure gas cap exists in the well Guang’an2area, while other areas have simultaneous production of gas and water. Based on the theory of the petroleum geology and the geochemical characteristics of hydrocarbon, combining with previous studies and analysis of well logging and testing data etc.. the characteristics of structure, sedimentary and gas-water distribution of Xujiahe Formation are summarized in the Guang’an area, then the detailed study of the petroleum geological features of Xujiahe Formation and its relationship with the gas-water distribution was made.In the study area, the source rocks were mainly distributed in T3X1, T3X3and T3X5, the thicknesses of the source rocks have little change and stable distribution in the plane. There were differences of source rocks thickness and its hydrocarbon generation between different layers. The thickness of T3X5source rocks is largest, the T3x3’takes the second place, the T3X1’is worst. The organic matter abundance of T3X5is also highest. The average organic carbon content are1.66%,1.13%and1.21%respectively, the chloroform bitumen "A" content are0.631%,0.239%and0.109%respectively, which indicate that all source rocks are good hydrocarbon source rocks, whose organic matter types are III type, and all are in the period from the later mature stage to the early high mature stage, so that the hydrocarbon generation intensity of source rocks is not high.In the study area, the main reservoirs are T3X4and T3X6. The main rock types of T3X6are litharenite rock and feldspar litharenite rock, the rock types of T3X4are mainly feldspar Iitharenite and lithic quartz sandstone. The reservoir space types of T3X6are mainly intergranular dissolved pore, primary intergranular pore and intragranular dissolved pore, the T3X4have two kinds of reservoir space types mainly, which are intergranular dissolved pore and intragranular dissolved pore. In addition, T3X4also develops some low dip-angle fractures. The reservoir porosity of T3X6and T3X4are minute, at the same time, the porosity and permeability are also poor, so both are low porosity and low to extra-low permeability reservoir beds, and the heterogeneity of reservoir beds is serious.The differences of methane, the heavy hydrocarbons content and dry coefficient of T3X6and T3X4are significant:the average methane content of T3X6is89.36%, while the T3X4’ is92.66%. The average heavy hydrocarbons content of T3X6is9.94%, while the T3X4’is6.61%. The average dry coefficient (Cl/Cl+) of T3X6is0.91, while the T3X4’ is0.94. In addition, the methane and ethane carbon isotope of T3X4is obviously heavier than T3X6’, which indicates that the gas of T3X6and T3X4are from T3X5and T3X3source rocks respectively.Source rocks, reservoir beds and tectonic of the study area have certain effects of gas-water distribution. In macro sense, tectonic amplitude controls the distribution of "upper-gas and lower-water". But inside the gas reservoir, the joint effects of these three factors make the complex gas-water distribution pattern.