Study Of Coal Measure Strata Free Gas Accumulation Mechanism And Accumulation Model

Coal strata in Qinshui Basin are rich in natural gas resources, and free gas mainly exists in the roof and floor of coal-bed and in the conventional reservoirs of coal seam formation in the. The study of free gas accumulation mechanism and accumulation model has great meaning in exploring free gas in Qinshui Basin.This paper make a definite concept of four process stages of the revolution of the coal strata in Qinshui Basin by comprehensive study which including Yanshanian-major natural gas accumulation stages of development. In Yanshanian, there are five sets of main source-reservoir-cap assemblage, and the self-generating and self-preserving reservoirs of Shanxi formation and Taiyuan formation are tow main sets.Through the analysis of the accumulation elements and the main controlling factors of free gas, the paper summarizes the rule of the distribution of free gas:(1) structures of high position are favorable to the accumulation of free gas; (2) the coal bed methane content of the roof and floor of coal seam is higher as the gas content of the coal seam is higher; (3) the closeness of the roof of coal seam and covering layer decides the horizon of the gathering of free gas, the good sealing condition is good for accumulation.The conclusion is the accumulation mechanism, including multi-source and multi-phase state, dynamic transformation, unidirectionally accumulate and escape. Natural gas of coal measure strata in this area comes from many kinds of source rocks, such as coal, dark mudstone and dark carbonate rock, and exists in many states. So we call that"multi-source and multi-phase state". Since Indo-Chinese period, different phasestates of natural gas have different processes of transformation, and they're interconvertible. That is so-called"dynamic transformation", while the "unidirectionally accumulate and escape" emphasizes the irreversibility of the process of accumulation.In this paper, I set up the accumulation model according to the accumulation process and secondary evolution of free gas, and free gas reservoir of Qinshui Basin could be classified into three types:native, adjustment and transformation. And the adjusted gas reservoir could be further divided into native-transformation type and adjusted-transformation type.By evaluating free gas prospect synthetically based on the overall consideration of the content of coal seam gas, properties of reservoir and closeness of seal, the most favorable gas-bearing zones located in the central belt of the basin (I zone) and the relatively favorable gas-bearing zones located in the edge of the basin (II zone) are recognized.