Geological Evaluation For Co-mining Feasibility Of Unconventional Natural Gases In Coal Series From Linxing Block

Unconventional natural gases resources in coal series(coal series gases for short or CSG) are abundant in the Permo-Carboniferous strata of Ordos Basin. In recent years, there is a constant breakthrough in the exploration test of coal measure gases form the basin, but the co-mining potential and its heterogeneity are not clear and the coming development of CSG needs to be supported by the geological evaluation method of co-mining feasibility. Therefore, the author in this paper analyzed the occurrence and regional distribution of CSG reservoirs, studied the gas-bearing identification methods of the reservoirs, discussed the geological controls of CSG reservoir formation, then, set up the geological model and comprehensive evaluation method system of signal mining and co-mining CSG. The results show that the organic matter of source rocks in coal series is dominant of type III kerogen, with higher maturity and sufficient gas source. Several main coal seams are widely developed, with larger thickness. Owing to the larger thickness and good gas-bearing, the sandstones in the Taiyuan Formation are favorable gas reservoir, with developed dissolved pore in the whole study area. According to the samples tests, well logging data and interpretation, and geological controls such as sedimentary, diagenesis, hydrology and structure, the regional distribution of different reservoirs and gas-bearing in coal series are comprehensively analyzed, the geological factors of single mining or co-mining CSG are preferred, and four type of the source-reservoir-caprock assemblages for the main coal and sandstone reservoirs are refined. Combined with the numerical simulation of co-mining productivity from coalbed methane and tight sandstone gas, the cumulative contribution values of 20% and 80% from single type of reservoir were identified as the co-mining feasibility limits using gray correlation method. Co-mining comprehensive evaluation index was calculated based on the productivity formula. Referenced the co-mining feasibility limits, three critical limits(4.44, 8E-7 and 9E-4) of composite evaluation index were obtained. Co-mining feasibility was divided into four categories according to the critical limits. Through the evaluation of co-mining in various production layer combinations, the distribution of favorable co-mining areas was predicted and evaluated.