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鄂尔多斯盆地深部煤层气差异富集的地质控制

Geological Control of Differential Enrichment of Deep Coalbed Methane in the Ordos Basin

  • 摘要: 鄂尔多斯盆地深部煤层气资源丰富,开发潜力大,是中国深部煤层气勘探开发的主战场。明确深部煤层气富集规律、圈定富集区段是煤层气勘探开发的重要基础。以中国石油长庆油田公司在鄂尔多斯盆地最新实施的多口深部煤层气评价井数据为分析基础,采用数学统计、模型计算和地质分析等方法,重点研究了盆地内深部煤层气差异富集的地质控制因素和富集规律。提出了物质组分效应、深度效应和保存效应是影响深部煤层气差异富集的“3种控制效应”。物质组分是控制煤层气富集的基础因素,其中灰分和煤变质程度是主要参数,低灰分、高变质煤储层含气量高。深度效应则是叠加孔隙度、温度和压力变化的综合效应,孔隙度随埋深增加而衰减,导致游离气随埋深增加会趋缓,吸附气和总含气量存在明显的临界转换深度,后者临界转换深度深于前者,一般在20003000 m,在此深度范围内,总含气量高。3000 m以深,深度效应加剧,总含气量缓慢降低,往往低于20 m3/t。保存效应是深部吸附气和游离气,尤其是游离气富集的重要控制因素,其中盖层类型是保存效应中的关键因素,而地下水矿化度和压力系数则是保存效应的重要外在显现。研究区煤岩组合类型中煤泥、煤灰、煤泥-煤灰组合具有良好封盖性,相应其地下水矿化度高,压力系数较大。叠加深度效应,20003000 m埋深范围内,总含气量往往高于20 m3/t,煤砂组合封盖性相对较弱,相应其地下水矿化度相对较低,含气量往往低于20 m3/t。耦合深度效应和保存效应,提出了鄂尔多斯盆地深部煤层气富集模式。

     

    Abstract: The deep coalbed methane (CBM) resources in the Ordos Basin are abundant and have great development potential, making it the main battlefield for exploration and development of deep CBM in China. Among them, clarifying the enrichment law of deep CBM and delineating enrichment sections are important foundation for CBM exploration and development. Based on the analysis of data from a number of deep evaluation CBM wells recentlyimplemented by China petroleum Changqing Oilfield company in Ordos Basin, the geological control factors and enrichment rules of deep coalbed methane differential enrichment in the basin are studied by means of mathematical statistics, model calculation and geological analysis. It has been proposed that the effects of material composition, depth, and preservation are the "three controlling effects" that affect the differential enrichment of deep CBM.The material component is the main factor controlling the enrichment of CBM, among which ash yield and coal metamorphism degree are the main parameters, low ash content, high metamorphic coal with high gas content. The depth effect is a comprehensive effect of superimposed porosity, temperature, and pressure. The porosity decays with the increase of burial depth, resulting in the free gas will tend to slow down with the increase of burial depth, and the adsorbed gas exists obvious critical conversion depth, and the critical conversion depth of total CBM content is deeper than the critical conversion depth of adsorbed CBM, which is generally in the range of 20003000 m, and in this depth range, the total CBM content is higher. For depths deeper than 3000 m, the depth effect intensifies, and the total CBM content gradually decreases, often below 20 m3/t. The preservation effect is an important controlling factor for the enrichment of deep adsorbed CBM and free CBM, especially free CBM, among which the type of cap rock is a key factor in the preservation effect, while groundwater total dissolved solids and pressure coefficients are important indications of the preservation effect.The coal-rock combination of coal-mud、coal-ash and coal-ash mud in the study area has good capping property, corresponding to its high groundwater salinity. Within the buried depth of 20003000 m, the total gas content is often higher than 20 m3/t, and the coal-sand combination capping property is relatively weak, corresponding to its relatively low groundwater salinity, gas content is often lower than 20 m3/t. At last, a deep CBM enrichment model in the Ordos Basin was proposed by coupling depth effect and preservation effect.

     

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