晋东南沁水盆地的煤层气储层主要是石炭-二叠系煤层,其厚度变化明显受控于当时的沉积环境及层序地层格架。本文对该盆地含煤岩系的太原组和山西组进行了高分辨率层序地层分析,并探讨了主采煤层15号和3号煤层在层序地层格架下的分布模式。以区域性分布的与下切谷砂岩共生的间断面、不整合面、海侵方向转换面、下切谷砂岩底面、由深变浅—再由浅变深的沉积相转换面以及共生的古土壤层为界,将含煤岩系划分为3个三级复合层序和9个四级层序。15号厚煤层和3号厚煤层位于三级海侵(泛)面附近,前者形成于障壁—潟湖及滨外陆棚沉积环境,较低的泥岩堆积速率与较慢的可容空间增长速率相平衡;后者形成于三角洲平原分流间湾环境,较高的泥炭堆积速率与较高的可容空间的增长速率相平衡。太原组的以"根土岩—煤层—海相石灰岩"旋回为代表的四级层序中的煤层可能形成于"海相灰岩层滞后时段",即从海平面抬升到陆棚之上到碳酸盐岩真正沉积下来之前的时段,因为缓慢的海平面抬升速率与泥炭堆积速率保持较长时间的平衡,从而聚集了厚层的泥炭/煤层。
The Qinshui Basin in southeastern Shanxi Province is an important base for coalbed methane exploration and production in China. The methane reservoirs in this area are Carboniferous and Permian coals and their thickness are strongly controlled by the depositional environments and the sequence stratigraphic framework. This paper analyzed the high-resolution sequence stratigraphy of the Permo-Carbonifersous Taiyuan and Shanxi Formations. The accumulating models of the No. 15 and No. 3 coals, the major coals in the Qinshui Basin, in the context of sequence stratigraphy were discussed. Bounded by regional disconformity, transgression direction switching surface, the base of incised valley fill sandstone, facies-reversal surface and associated palaeosols, the coal measures were subdivided into 3 third-order composite sequences and 9 fourth-order sequences. The No. 15 coal of the Taiyuan Formation was formed in the barrier-lagoon and offshore shelf settings and was developed at the transgressive surface of the composite sequence Ⅱ, where the lower peat accumulation rate was balanced by the slower accommodation creation rate. In contrast, the No. 3 coal of the Shanxi Formation was formed in a fluvialdelta setting, and was developed at the maximum surface of the composite sequence Ⅲ, where the higher peat accumulation rate was balanced by the higher accommodation creation rate. The coal in the fourth-order sequence represented by "seat earth-coal-marine limestone" could be formed during the "lag time" of marine deposits, when development of balanced accommodation creation rates and peat accumulation rates might favor the accumulation of thick coals.