Study On Quantitative Identification Of Biogenic Coalbed Methane And Its Reservoir Formation

Coal bed methane （CBM） mixed by thermogenic and biogenic gases has beensubsequently discovered and reported in many coal basins all over the word. It is importantto quantitatively investigate the relative amount of biogenic gas in CBM reservoir and theprocess of reservoiring. To understand thses, the Liulin and Luling CBM exploitation sitesare taken as the study areas. Quantitative method and reservoiring characteristic ofbiogenic CBM are emphatically discussed by using laboratory experiments and numericalsimulation technology. Major innovations are as follows.Firstly, based on hydrocarbon thermal simulation experiments in an autoclave closedsystem, the model for identifing the evolution stage of thermogenic gas generated byorganic matter is proposed. The evolutionary characteristics and ralationships betweencarbon and hydrogen isotopic compositions of gas with the evolution stage of samples arereviewed. The carbon isotopic compositions of methane, ethane, and propane changedfrom heavy to light, then back to heavy, whereas the hydrogen isotopic composition ofmethane and the carbon isotopic composition of carbon dioxide become increasinglyheavier with increasing evolution of organic matters （Rovalues）.Secondly, following mathematical statistics method, the distribution characteristics ofdifferent genetic types of coal type gas are systematically analyzed, and several charts foridentifying the genetic types of coal type gas are also raised. Results show that the δ¹³C₁value of biogenic gas, thermogenic gas, and a mixture of both types are less than-60‰,more than-30‰, and occur in between, respectively. From biogenic gas to thermogenicgas, the δ¹³C₁, δ¹³C_CO2-CH₄, δ¹³C_2-1, and CH₄/（C₂H₆+C₃H₈） values gradually become heavierwith the increase of Rovalues, and an obvious positive correlation exists among theaforementioned parameters. The charts of δ¹³C₁-δ¹³C_CO2-CH₄, δ¹³C₁-δ¹³C_2-1, andδ¹³C₁-CH₄/（C₂H₆+C₃H₈） can be used to divide the genetic types of coal type gas.Thirdly, by conducting secondary biogenic gas simulation experiments and combiningthe analysis of biogenic gas cumulative yields, hydrocarbon geologic and mathematicalmodels of biogenic gas are perfected. From the perspective of molecular structure, thechangements of samples from before and after during pyrolysis and microbial degradationare studied by using infrared spectrum analysis. The intermolecular forces of samples areweakening during the microbial degradation. Sulfocompound possibly has an inhibitoryaction on microbial degradation. By geochemical testing of methane carbon isotopiccomposition, a new method for quantitatively calculating the relative content of biogenicgas is presented. And the fractionation effect on methane carbon isotopic composition is also quantitatively inverstigated. By comparing these results, it is found that there is asmall deviation.Fourthly, using numerical simulation technology, the reservoiring characteristics ofbiogenic gas and the reservoiring affect of biogenic and thermogenic gases are discussed.Biogenic gas contributed to the methane content of coal seam presently is alsoquantitatively studied. Biogenic gas has obviously contribution on cumulative gasproduction and gas content in coalbed. An obvious periodicity exists in the process ofCBM reservoiring. CBM containing biogenic gas migrate in different way at differentevolution stage, mainly in the way of diffusion, percolation, and cap outburst dissipation.Primary biogenic gas has relatively small contribution on the gas content of coal seampresently, whereas secondary biogenic gas has much.