二连盆地吉尔嘎朗图凹陷低煤阶煤层气富集模式

二连盆地为我国典型低煤阶褐煤分布区,煤层气资源丰富,但煤层气富集成藏机制认识不足制约了该区低煤阶煤层气的勘探开发。为此以二连盆地吉尔嘎朗图凹陷低煤阶煤层气为研究对象,从煤层分布、含气性、煤层气成因、生物成因气模拟实验、保存条件等方面研究了该区煤层气富集的主控因素,并指出了下一步的勘探方向。研究结果表明:(1)浅水湖盆聚煤环境下,凹陷中部—缓坡带厚煤层发育,厚煤层弥补了含气量的不足;(2)含煤段堆积过程中,浅水湖泊周期性出现使得煤层上覆泥岩周期性发育,盖层条件有利;(3)凹陷中部—缓坡带位于地下水承压区,水动力侧向封堵有利于煤层气富集;(4)研究区煤层气为生物成因,原位条件下煤样产气0.25 m L/g,现今仍有生物气生成。结论认为:(1)厚煤层发育区、具备生物气生成以及良好的封盖条件并处于水动力承压区为吉尔嘎朗图凹陷煤层气富集成藏的关键;(2)吉尔嘎朗图凹陷煤层气富集模式为生物气+承压水封堵煤层气富集,中部—缓坡带L12—S88井区为下一步煤层气建产的有利区。

Accumulation patterns of low-rank coalbed methane gas in the Jiergalangtu Sag of the Erlian Basin

The Erlian Basin is a typical low-rank lignite distribution area in China. Its coalbed methane (CBM) resources are abundant, but the CBM enrichment and accumulation mechanisms are not understood sufficiently, which restricts the exploration and development of low-rank CBM gas. In this paper, with low-rank CBM gas reservoirs in the Jiergalangtu Sag of the Erlian Basin as an example, the main factors controlling CBM accumulation were analyzed from the aspects of coal bed distribution, gas content, CBM origin, biogenic gas simulation experiment and preservation conditions, and then the future exploration direction was pointed out. Some results were obtained. First, a thick coal bed is developed at the central belt–gentle slope of the Jiergalangtu Sag in the coal forming environment of shallow lake basins, and it compensates for the deficiency of gas content. Second, mudstones overlying coal beds are developed periodically due to the periodic occurrence of shallow lake during the accumulation of coal bearing sections, so the caprocks are favorable. Third, the central belt–gentle slope of the Jiergalangtu Sag is located in a confined groundwater area, where the hydraulic lateral sealing is favorable for CBM accumulation. Fourth, CBM in the study area is biogenic with an in-situ gas production ratio of 0.25 mL/g, and currently the generation of biogenic gas still continues. Fifth, the CBM enrichment pattern in the fault basin is that biogas and CBM gas are both sealed by confined water. It is concluded that the key factors for CBM accumulation in the Jiergalangtu Sag are thick coal beds, biogas generation, favorable seals/caprocks and hydraulic confined areas. Besides, the Well L12-S88 area in the central belt–gentle slope of the Jiergalangtu Sag is the favorable area for future CBM productivity construction.