| Tight sandstone gas is one of the main type of unconventional natural gas, and it is also the biggest unconventional natural gas resources in the development in the world currently, which makes the most important contributions to the rapid improvement of the natural gas production and reserves of our country. Howerver, tight sandstone gas reservoir usually has the characteristics of strong reservoir heterogeneity, low permeability, complex relationship between gas and water and unclear gas accumulation period, which leads to much difficulty in the development of tight sandstone gas and prediction of productivity.The Xujiahe Formation gas reservoir of Anyue gas field is a typical tight sandstone great gas reservoir which has the hydrocarbon generation intensity less than 2 billion cubic meter per square km. Currently, tight sandstone reservoir distribution and controlling factors of Anyue gas field are complex, and its heterogeneity is strong. What is more, this area has the low production, and it exists that water produce phenomenon in the process of gas production because of the disputed gas gathering methods and complex gas-water relationship and unclear controlling factor of gas and water, which finally leads to the production of gas well in this gas field decrease. And therefore, conducting the research of Xujiahe Formation tight sandstone gas reserves of Anyue gas field has a very important practical significance for reasonable gas production.On the basis of regional geological background, we researching the natural gas geochemistry patterns of tight sandstone gas of Xujiahe Formation, analyzing the distribution rule of gas and water, finding out the main controlling factors of affecting the tight sandstone gas reservoir forming mechanism of objective layer by drilling data, logging data, geochemical analysis, tests data analysis and conducting accumulation simulation experiment under different displacement pressure, which provides a geological basis for follow-up evaluation of oil and gas exploration for study area, and also provides reference for the evaluation and development of abnormal natural gas accumulation including tight sandstone gas of other domestic area.Xujiahe Formation could be divided into 6 sections in the longitudinal. Xujiahe Formation has three sets of reservoir and cap rock:the lower combination making the lower and upper of Xul section as the hydrocarbon rocks, Xu2 section as reservoir layer, Xu3 section as cap rock; the middle combination making the Xu3 section as the hydrocarbon rocks, Xu4 section as reservoir layer, Xu5 section as cap rock; the upper combination making the Xu5 section as the hydrocarbon rocks, the lower part of Xu6 section as reservoir layer, the upper part of Xu6 section and the Jurassic system as cap rock.The natural gas of Xujiahe Formation of Anyue gasfield mainly is hydrocarbon gas, and its methane content is 80.16%-98.58%, and its dry coefficient is 0.83-0.998, there are both dry gas and moisture. The composition of non-hydrocarbon gases mainly are N2、 CO2, in which the content of N2 is 0.27%-7.33%, the content of CO2 is 0.03%-1.55%. The methane content of natural gas of Xujiahe Formation of Anyue gasfield is 82.57%-87.28%, and its dry coefficient is 0.86～0.89, which is a typical moisture gas reservoir. The methane carbon isotope value distributes from -43.8‰ to -30.5‰.The Ethane carbon isotope value distributes from -33.0‰ to -21.7‰, which infers that natural gas reservoirs of Xujiahe Formation has the characteristics of both coal-formed gas and oil type gas. The δ13C1 value of Anyue gasfield is from -44‰ to -40.5‰, which is less than the value of adjacent gas reservoir of middle-Sichuan area, and its δ13C2 value all are heavier than -29.0‰, which is a typical coal-formed gas reservoir. The methane hydrogen isotope value of natural gas of every gas field of Xujiahe Formation distributes from-191.0‰ to -156.0‰, which shows it belongs to continental origin natural gas. The Ethane hydrogen isotope value distributes from -154.0‰ to -119.0‰, in which, the Ethane hydrogen isotope value of Anyue and Hechuan gas field are from -132.0‰ to-120.0‰,which are obviously heavier than other gas reservoir(those hydrogen isotope value is from -191.0‰ to -139.0‰). The ancient salinity of Hechuan and Anyue area is higher, which may leads to the Ethane hydrogen isotope value of this area to be much heavier.The analysis of relative content of C7 light hydrocarbon series compound show that the n-heptane content of natural gas of Xujiahe Formation is from 10% to 40%; and the methyl cyclohexane content is from 40% to 80%, and the content of every kind of dimethyl cyclopentane is from 10% to 30%, from which we can find out that the Methyl cyclohexane predominates and the natural gas of Xujiahe Formation is terrigenous origin natural gas combined with the carbon isotope characteristics of natural gas of Xujiahe Formation. What is more, the analysis of relative content of C5-C7 series compound shows that the proportion of shall naphthene of Xujiahe Formation minly distributes from 10% to 30%, and the most part of relative content of alkane is higher than 60%. On the whole, it shows that characteristics of high paraffin、low aromatics and terrestrial facies origin gases. But it could be influenced by the water washing effect in the process of accumulation, which caused the natural gas reservoir has the characteristics of low content of aromatics on the whole, and can not reflect the original light hydrocarbon composition and structural characteristics.The distribution of Xu2 section natural gas of Anyue gasfield infers that gas reservoir belongs to the lithologic gas reservoir, its difference of gas and water is not obvious, and its form is complex. By the analysis of process of the simulation experiments of natural gas, natural gas accumulation area size、gas saturation are affected by the filling dynamics and fixed condition of the reservoir physical property, and the dynamics trap is the main way of gathering for the tight sandstone gas reservoir accumulation. According to the analysis of natural gas geochemical patterns and hydrocarbon rocks maturity of whole the Xujiahe Formation gas reservoir of Anyue gas field, we conclude that the distribution and change rules of upper two are roughly anastomosis. Combined with the higher transport coefficient (4.6%-5.2%) of gas reservoir of Xujiahe Formation, we think that tight sandstone gas reservoir of Xujiahe Formation has the characteristics of the proximal efficient gathering.The accumulation patterns of tight sandstone gas of low intensity of hydrocarbon generation:the Indosinian movement is the gathering oil and gas phases of high pore and high permeability; during the period of Yanshan movement, the hydrocarbon source rocks came into the stage of large hydrocarbon expulsion, and the oil and gas furturely gathered in the reservoir of better physical property, on the same time, the low degree of gas and water differentiation, complex gas and water distribution are caused by flat tectonic and not big dip angle; during the late period of Yanshan movement, with the increasing buried depth and reducing physical property of reservoir, it led to the lateral migration of oil and gas being limited; during the tectonic period of Himalayan movement, the oil and gas reservoir redistribute by the effect of early and late period of tectonic activity, at this time, the gas and water redistributed by the influence of gravity differentiation, and then oil and gas gathered in the structural high position, water layer was located at the relatively low structural positions.The Anyue gasfield of main controlling reservoir factors of tight sandstone gas have three factors:The effective hydrocarbon source rocks control the filling degree of the gas reservoir; distribution of effective reservoir controlling gas reservoir; the growth of fracture provide the good channel for the migration of oil and gas, which further control the accumulation of oil and gas and the distribution area. |
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