Investigation On Charging And Losing Mixed-gases In Reservoirs From Different Source Kitchens

Mixed-gases reservoirs in which natural gas came from different source kitchens are prevalent in gas-bearing basins. It is necessary to identify mixed-gases and calculate mixed ratio of its end-member gases for forecasting resources and distribution of natural gas. Losing is inherent in natural gas accumulated in reservoirs. Gas components and isotopic compositions would change when natural gas loses from reservoirs. Investigating geochemistrical characteristics of losing gas is of importance in natural gas exploration.An investigation method of charging mixed-gases in reservoirs was brought forward, which is made up of following steps: 1) analyzing geological conditions and natural gas characteristics, to identify the source-mixed gas and its end-members, 2) adopting geochemistrical data of natural gas produced from laboratories and sampled from gas fields, to find quantitative relationships of identifying parameters of end-member gases and their thermal maturities, 3) establishing quantitatively identification mode of mixed-gases, based on physical change laws of mixing gases, and 4) using the results gained from the above 3 steps, to analyze formation process of mixed-gases reservoir. Some typical mixed-gases reservoirs were investigated by the method. The result shows that more than 90% natural gas in Kela-2 gas field charged in the Kuche period, 16%~78% of natural gas in Jingbian gas field is the Lower Paleozoic oil-type gas, 70% natural gas in Wucan-1 well came from type III organic material, the percentage of natural originated from the Jurassic is 60% in Jialingjiang gas reservoir of Wolonghe gas field, and 54% natural gas in Fengshen-1 well came from type I organic material.Accumulated gas would lose ceaselessly by diffusing through cap rock and (or) dissoluting in formation water. The difference of losing velocities among various gas components/molecules with different isotopes results in fractionations of component and isotopic compositions in natural gas. Based on the Fick law and the Herry law, diffusion and dissolution fractionation modes were established respectively. According to the diffusion fractionation mode, CH4 in Sulige gas field has decreased 1.08 percent, whileδ13C1 increased 0.112 percent because of natural gas losing by diffusing from reservoir. According to the dissolution fractionation mode, CO2 in Weiyuan gas field has decreased 14.79%, while CH4 increased 13.97% because of natural gas losing by dissolving in formation water.With mass of gas losing from and reserved in reservoir now, mass of gas charged into reservoir could be known, and then charge-losing history of natural gas reservoirs could be rebuilt. Biogas in eastern Chaidamu Basin has been losing from reservoirs mainly by diffusing. Distribution and enrichment of biogas in the area are controlled by the charge-losing balance of biogas. After investigating on charge-losing history of biogas reservoirs in eastern Chaidamu, a parameter of gas losing ratio was put forward for evaluating the charge-losing process of natural gas reservoir. According to the biogas losing ratio from reservoirs in eastern Chaidamu, charge-losting process is advantageous in accumulating natural gas when gas losing ratio is less than 0.6, while the process is disadvantageous when the ratio is more than 0.6. According to the geological factors that control charge-losing biogas in reservoirs eastern Chaidamu, geological conditions of Sebei and Tainan sections are most advantageous in accumulating biogas, geological conditions of Yitai, Tuoya, Senie and Huoda less advantageous than that of Sebai and Tainan, and Nanya section disadvantageous in accumulating biogas.