鄂尔多斯盆地东南部山西组页岩气形成条件及富集主控因素

Formation conditions and the main controlling factors for the enrichment of shale gas of Shanxi Formation in the southeast of Ordos Basin

基于钻、录、测、试资料和泥页岩样品分析化验资料,在解剖页岩气典型井地质特征基础上,深入分析各成藏要素与页岩气富集之间的关系,总结页岩气富集控制因素。结果表明:鄂尔多斯盆地东南部山西组主要发育海陆过渡相泥页岩,具有较强的非均质性,主要体现出泥页岩有机质、物性等参数空间变化快的特点;山西组广覆式分布的泥页岩有机碳(TOC)含量较高,平均值为2.24%,干酪根类型主要为Ⅲ型,R_(O)平均值为2.48%,处于高成熟—过成熟阶段,为优质烃源岩;岩石组成具有富黏土特征(40.5%~88.5%),主要由伊/蒙混层和高岭石组成,具有相对较高的比表面积有利于提高页岩对气体的吸附能力;小于3 m薄砂岩夹层在山西组分布广泛,物性条件较好(平均孔隙度为3.37%,平均渗透率为0.1×10^(-3)μm^(2)),泥页岩生成的天然气可以短距离运移至这些砂岩夹层(纹层)中,有利于提高页岩含气性;研究区山西组含气性受不同岩性组合的影响具有较大的区别,其中山_(2)段厚层泥岩—煤组合含气性最好,其80%以上的样品含气量超过1 m^(3)/t。综合分析认为,泥页岩的高热演化程度、特殊的海陆过渡相储集条件以及良好的保存条件是影响研究区页岩气富集的主要因素,其使得鄂尔多斯盆地东南部山西组页岩气具备一定的勘探开发潜力,并在子长—延安—富县、宜川等地区圈定页岩气富集目标。

Based on the data of drilling,logging,testing and shale sample analysis,and on the basis of dissecting the geological characteristics of typical shale gas wells,the relationship between reservoir-forming factors and shale gas enrichment is deeply analyzed,and the controlling factors of shale gas enrichment are summarized.The results show that the shale of marine-continental transitional facies is mainly developed in Shanxi Formation in the southeast of Ordos Basin,which has strong heterogeneity,mainly reflected in the rapid spatial change of shale organic matter,physical properties and other parameters. The shale widely distributed in Shanxi Formation has a high content of organic carbon(TOC),with an average of 2.24%.The kerogen type is mainly type Ⅲ,and the average value of R_(O)is 2.48%,which is in the high mature-overmature stage and is a high quality source rock.The rock composition is rich in clay(40.5%-88.5%),mainly composed of illite-mongolian mixed layer and kaolinite,and its relatively high specific surface area is beneficial to improve the gas adsorption capacity of shale.Thin sandstone with thickness less than 3 m interbeds are widely distributed in Shanxi formation with good physical conditions(average porosity 3.37%,average permeability 0.1×10^(-3)μm^(2)). The natural gas generated by shale can be transported to these sandstone interlayers(laminae)in a short distance,which is beneficial to improve the gas-bearing property of shale. The gas-bearing property of Shanxi Formation in the study area is affected by different lithologic assemblages. The gas-bearing property of thick mudstone-coal assemblage of Shan2 is the best,and the gas content of more than 80% of the samples exceeds 1 m^(3)/t. It is considered that the high thermal evolution degree of shale,special marine-continental transitional facies reservoir conditions and good preservation conditions are the main factors affecting the enrichment of shale gas in the study area. It makes the shale gas of Shanxi Formation in the southeast of Ordos Basin have certain potential for exploration and development,and delineates the enrichment targets of shale gas in Zichang-Yan’an-Fuxian and Yichuan areas.