Study On Controlling Factors Of CBM Reservoir In Fanzhuang-Zhengzhuang Block In Southern Qinshui Basin

The most typical characteristic of coalbed methane is that the coal seam is hydrocarbon source rock as well as reservoir rock. Coalbed methane mainly exists in the form of adsorption state. Therefore, there are a lot of differences between coalbed methane reservoir and conventional natural gas reservoir. In this paper, many disciplines theory and methods such as geology of coalbed methane and coal geology are used to study the research area of Fanzhuang-Zhengzhuang block in Qinshui Basin. Block is studied, including development of coal seam, physical characteristics of coal reservoir, gas-bearing characteristics and preservation conditions, to search controlling factors of reservoir of coalbed methane.In the block, the main coal seams in the research block under coalbed methane exploration are 3# coal of Shanxi Formation and 15# coal of Taiyuan Formation. They both have stable distribution and large thickness. Such a large thickness of coal seam is the material basis for the coalbed methane enrichment. The buried depth of coal seam is under "methane weathered zone". So coalbed methane is barely destroyed by the outside world. Shanxi Formation and Taiyuan Formation have been formed in the swamp environment. The coal and mudstone and silty mudstone have formed favorable strata combination for coalbed methane. Direct caprock is stable and more than 5m. The roof and floor have formed effective seal.Coal seams have experienced more than 4 km depth of burial. Abnormal thermal events happened during Yanshan epoch cause a high degree of metamorphism of coal seam with a large amount of coalbed methane. The coal rank mainly is anthracite III. The holes in coal of 3# and 15# focus on micropore. Micropores have large specific surface area so that the coal has high adsorption capacity. Cleats and fractures are rich in 3# and 15# coal. By the way, the coal seams have good pore-permeability characteristics. High gas content and saturation are the direct determinant of the coalbed methane reservoir.Groundwater system of Shanxi Formation and Taiyuan Formation is relatively independent. Water power is not strong and water content also is not large. They are barely disturbed by Ordovician water in limestone. The stratum in block is on the synclinal slope area as a whole. The exposed area of coal seam acts as water recharge area from atmosphere. To the basin, coal seams gradually transform to confined area. It is easy to form structure-water pressure sealing coalbed methane reservoir at such confined area. There is not large fault within block. The integrity and closure of the coal seam are not obviously affected. The only larger fault—Sitou Fault is proved to be not an open one. It is favorable to form the fault closed reservoir.Good development of coal seam is the basic foundation of coalbed methane enrichment-reservoir. Relatively good pore-permeability characteristics are the foundation to providing enough space for coalbed methane. High methane content is the direct material guarantee to reservoir. Good preservation condition is the final determinant to formation of coalbed methane reservoir. The enrichment- reservoir in the block is controlled by the four factors together. The study has certain practical guidance to coalbed methane exploration and development in this area.