The Study Of Geological Formation Features Of CBM Reservoir In Heshituoluogai Basin

Through analyzing the geological setting of coal formation, basin evolution, sedimentary environment, distribution of coal, the research studies several controllable geological accumulation factors of low-rank CBM, such as tectonic enviornment, sealing capability, and fluid system, and evaluates the potential of CBM resources, and then predicts the promising targets.Heshituoluogai Basin comes into being and has a great developmen during the Hercynian and Indosinian respectively, which laid the foundation for the present structural pattern. Tow sets of coal-bearing strata form in early Yanshan, and then accept the late Yanshan and Himalayan movement transformation, resulting in that the development and preservation of coal are significantly affected by the forward thrust faults. Because of the buriment depth, the Mohetai depression, Bailishannan depression and Kangteneng depression in Hoxtolgay Basin affected by the faults are relatively small.The Badaowan coal in Hoxtolgay Basin is distributed from Bailishan to Hoxtolgay in the form of layered and quasi-layered sturcture, and mainly in the upper part of Badaowan Formtion. The Xishanyao Formation, which is the main coal-bearing group of basin, is characterized by stable coal distribution and relatively simple coal structure. Coal is mainly concentrated in upper of lower subformation of Xishanyao and partly in lower of upper subformation of Xishanyao.The rank of coal in the study area is from lignite to long-flame coal, so the reservoir has the characteristics of low-rank coal. First of all, the porosity of reservoir is relatively high, from 6.30%～24.33%. Secondly, the pore system developes two types of pore. The first type is characterized by the content of holes reducing from the large hole to porous; the other type is mainly porous-small holes and large holes, especially micro-small hole. The third feature is that the coal reservoir has bad capacity of storage gas.The Badaowan Formation in Hoxtolgay Basin contains much mudstone which has better sealing capability in part area. The deep shale rock deposits of lake transgression of Xishanyao Formation becomes a regional cap. Thick dark mudstone which forms in deep lake environment in Sangonghe Formation and Toutunhe Formation becomes the regional barrier layers.The main part of basin is part of runoff and the shallow strata have shallow groundwater supply. To some extent, the CBM seal capcity is affected, however, there may be local stagnation region in the deep of depression. Shallow runoff causes the dispersion of coal bed methane and the damage of gas reservoir, whereas water retention environment is conducive to the accumulation and preservation of coalbed methaneAccording to coal development, the role of structural controllment, coal rank, coal equality and coalbed methane enrichment, there are three CBM favorable blocks in study area, which are Bailishannan depression, Mohetai depression and kengteneng depression. The favorable blocks in Hoxtolgay Basin are mainly affected by subsiding extent of secondary depression and preservation condition of secondary depression.