沁水盆地武乡南煤层气赋存主控地质因素及富集区预测

Controlling geological factors and coalbed methane enrichment areas in Southern Wuxiang Block, Qinshui Basin

通过对沁水盆地武乡南区块石炭—二叠系太原组15号煤和山西组3号煤的沉积环境、地质构造以及含气性和煤层赋存特征开展研究,探讨了煤层气富集主控地质因素,并采用模糊数学评价和层次分析法,综合评价了研究区煤层气的有利富集区。结果表明:研究区太原组潮坪灰岩-砂泥岩相、山西组三角洲分流间湾泥岩-粉砂岩相和泥炭沼泽-黑色泥岩相为煤层主要形成环境,其中潮坪和三角洲分流间湾中泥炭沼泽区为煤层发育的最佳地带;两层煤赋存形态一致,均为走向北北东向、倾向向西的单斜构造,埋深自东向西逐渐加深;煤层厚度太原组15号煤由西南向东北逐渐变厚,山西组3号煤则整体从北向南逐渐变厚。煤层含气量主要受控于煤层厚度、埋深、顶板岩性特征以及地质构造。含气量与煤层厚度、埋深呈正相关性;砂质泥岩、泥岩封盖性能好,有利于煤层气的赋存;构造因素中封闭性较差的正断层导致煤层气的逸散,逆断层则相反,往往具有较好的封闭性。因此,研究区煤层气富集区为构造简单或逆断层发育、煤层厚度大,埋深大以及顶底板岩性为厚层砂质泥岩和泥岩区域,同时多层次模糊数学综合评价表明,研究区煤层气富集区15号煤、3号煤分别位于勘探井W26-W31-W41和W1-W20-W2一带。

Using the coal samples from the Carboniferous-Permian Taiyuan Formation(No.15) and Shanxi Formation(No.3) in southern Wuxiang block, Qinshui Basin, Shanxi Province, the authors studied the sedimentary facies, geological structures, gas-bearing property, and coal seam occurrence characteristics, investigated the main geological controlling factors of coalbed methane(CBM) enrichment, and comprehensively evaluated the CBM favorable enrichment areas based on the fuzzy mathematics evaluation and analytic hierarchy process method. The results show that(1) The peat flat in the Taiyuan Formation, and the peat swamp in the Shanxi Formation are the most favorable facies belts for the development of coal seams.(2) No.15 coal seam of the Taiyuan Formation and No.3 coal seam of the Shanxi Formation have the same occurrence pattern. They are monoclinic structures with a north-northeast strike and a westward dipping trend. The buried depth deepens gradually from east to west. The thickness of No.15 coal seam increases gradually from southwest to northeast, and No.3 coal seam gradually thickens from north to south as a whole.(3) The larger coal thickness and buried depth, the better sealing ability with sandy mudstone/mudstone, and simple geological structures or with developed reverse faults will benefit the increase of gas content in coal reservoirs.(4) Finally, the comprehensive evaluated results using multi-level fuzzy mathematics indicate that the CBM favorable enrichment areas of No.15 and No.3 coal seams are the areas surrounding the exploration wells of W26-W31-W41 and W1-W20-W2 respectively.