海陆过渡相页岩气储层非均质性及其主控因素——以鄂尔多斯盆地东缘大宁—吉县区块山西组为例

Heterogeneity of shale gas reservoirs in marine-continental transitional facies and its controlling factors:an example of Shanxi Formation in Daning-Jixian Block on eastern margin of Ordos Basin

海陆过渡相页岩沉积受海相与陆相环境的双重控制,页岩气储层具有较强的非均质性。在岩心观察的基础上,采用X线衍射全岩矿物分析、干酪根显微组分鉴定、TOC质量分数测试、场发射扫描电镜、低温气体吸附及高压压汞等方法,分析鄂尔多斯盆地东缘大宁—吉县区块山西组山_(2)^(3)亚段(Ps^(3))页岩气储层非均质性特征。结果表明:山西组页岩气储层非均质性包括矿物组分、有机质质量分数及类型、微观孔隙类型、微观孔隙结构。Ps^(3)上段为潮控三角洲相页岩,脆性矿物质量分数低(平均为39.2%),TOC质量分数较低(平均为2.3%),有机质类型为Ⅲ型,有机质孔发育不佳,孔隙结构参数较差,为中等页岩气储层。Ps^(3)下段为河口海湾相页岩,其中封闭潟湖、海湾微相页岩脆性矿物质量分数高(平均分别为74.1%、67.2%),TOC质量分数高(平均分别为7.3%、11.2%),有机质类型为Ⅱ型,储集空间类型多样,且发育大量复杂有机质孔,孔隙结构参数优,为优质页岩气储层。页岩气储层非均质性受沉积环境和成岩作用的共同控制。封闭潟湖—海湾微相有利于Ⅱ型干酪根形成,易产生大量有机质气泡孔,富生物成因硅也有利于有机质孔的形成与保持。三角洲相页岩发育Ⅲ型干酪根,有机质孔发育不佳,且陆源硅质对储集空间贡献不大。该结果为海陆过渡相页岩气“甜点”段优选提供指导。

Shale gas reservoirs are strongly heterogeneous because the deposition of marine-continental transitional shale is under the dual control of marine and terrestrial environments.Systematic analyses were performed to investigate shale reservoir heterogeneity in the Shansubmember(Ps^(3))of the Shanxi Formation in the Daning-Jixian Block on the eastern margin of the Ordos Basin,including core observation,mineral analysis of bulk rock through X-ray diffraction,organic maceral analysis of kerogen,total organic carbon(TOC)content measurement,field emission scanning electron microscopy,low-temperature gas adsorption,and high-pressure mercury injection.The results showed that macroscopic shale reservoir heterogeneity in the Shanxi Formation included mineral composition,organic matter content and kerogen types,whereas microscopic heterogeneity included reservoir space type and pore structure parameters.The upper part of Psmainly contained tidal-dominated deltaic shale with a low content of brittle minerals(average value is 39.2%),a low TOC content(average value is 2.3%),typeⅢkerogen,poor development of organic pores,and poor pore structure parameters.It is a nonpremium shale gas reservoir.The lower part of Psis dominantly estuarine-bay shale in which the shales of the closed lagoon and bay have a high content of brittle minerals(average values are 74.1%and 67.2%,respectively),a high TOC content(average values are 7.3%and 11.2%,respectively),typeⅡkerogen,diverse types of reservoir space,and numerous complex organic matter pores that developed with excellent pore structure parameters.It is a high-quality shale gas reservoir.The shale gas reservoir heterogeneity is controlled by depositional environment and diagenesis.Formation of typeⅡkerogen and numerous organic bubble pores are favorable in the closed lagoon and bay,with biogenic silicate contributing to the formation and maintenance of organic pores.Deltaic shales are characterized by typeⅢkerogen and poorly developed organic pores,with limited contribution from terrigenous siliceous materials to the reservoir space.The study of reservoir heterogeneity lays the foundation for the selection of the"sweet spot"section of marine-continental transitional shale gas.