Reservoir Forming Condition And Favorable Prediction In The Area Of East Junggar Basin

Coalbed methane(CBM) resource is rich in the eastern Junggar Basin, where the main coal lithotypes are mainly made up of brown coal and long flame coal, and is one of the most prospective low-rank CBM development districts in China. However, it is difficult to understand CBM accumulation condition and enrichment mechanism, calculate CBM resources and ascertain CBM favorable area, due to multi structural movement and complex coal-bearing environment. This paper tries to take the Jurassic coals in the eastern Junggar Basin as research object, and a systematic research on basic geology, coal-forming environments and reservoir characteristics was carried out firstly, and then three CBM accumulation models were proposed on basis of the research of geological controls. Finally, the volume method was used to calculate low-rank CBM resources in the eastern Junggar Basin, and analytical hierarchy(AHP) fuzzy prediction method also was used to analyze CBM exploration potential. Main achievements are as follows:(1) As a result of multistage tectonic activities, the nearly north-south striking thrust fold was developed between Kelameili Mountain and Bogoda Mountain, and “checkerboard type” structural framework(i.e., alternating uplifts and depressions) was formed before the development of coal seam. As a whole, both coal basin tectonic evolution and the current tectonic framework have an obvious controlling effect to coal seam growth and CBM accumulation. Since the Yanshan movement, the research area has been lifting continuously under the stress field of the crushing and compressive shearing, and shallow buried depth resulted in the low coalification and poor hydrocarbon-generating condition. In addition, overlying strata of coal measures was bad for CBM preservation due to its strong denudation.(2) There was a major difference between Xishanyao coal and Badaowan coal about macrolithotype of coal and coal rank, and the former was mainly made up of macro-pores, while the latter was micro-pores and transition pores. Reservoir physical properties of Xishanyao coal are better than Badaowan coal, because of the larger pore diameter and shallower burial depth. Moreover, Langmuir volumes of Badaowan coal was obvious larger than Xishanyao coal, but the opposite was Langmuir pressure.(3) Based on a systematic research on geological controls of CBM accumulation in the eastern Junggar Basin, three CBM accumulation models was proposed with the corresponding examples of CBM accumulation, which are “the southern mountain front” accumulation model, “the deep sag” accumulation model and “open slope” accumulation model.(4) The volume method was used to evaluate CBM resources of Xishanyao coal, i.e., in-place resources is 1596.82×108 m3, recoverable resources is 1223.43×108 m3, in-place resource abundance is 0.45×108 m3/km2, and recoverable resources abundance is 0.34×108 m3/km2. In addition, analytical hierarchy(AHP) fuzzy prediction method was used to build the standards of evaluation on CBM resources and selected areas, and a west part of the study area was forecasted as the prospective target areas for low-rank CBM exploration.