泥岩微观组构对有机质赋存形式和孔隙发育的影响研究:以白垩系Eagle Ford页岩为例

Influence of microscopic fabric on organic matter occurrence and pore development in mudrock:A case study of the Cretaceous Eagle Ford Shale

以美国Texas州白垩系Eagle Ford页岩为研究对象,采用氩离子抛光场发射扫描电镜技术,并结合X射线能谱分析,对位于生油窗的原始岩心样品和位于生气窗的柱状模拟样品开展镜下有机质赋存形式和孔隙发育、演化特征研究。结果表明,Eagle Ford页岩样品在镜下的不同微区可以划分出硅质黏土质条带、钙质有孔虫化石和富颗石藻透镜体3种典型组构类型,并且不同类型组构中有机质赋存形式和孔隙发育、演化特征存在显著差异。其中,硅质黏土质条带中富集大量无定形干酪根和少量结构型干酪根,无定形干酪根在生油窗由于结构内部滞留了大量吸附油基本不发育孔隙,到了生气窗伴随吸附油裂解开始形成丰富的纳米级海绵状孔隙,而结构型干酪根由于异常低的生烃潜力,基本不发育孔隙;有孔虫化石腔室内部赋存的有机质代表早期充注的运移沥青,在生油窗发育大量气泡状孔隙和丰富的纳米级海绵状孔隙,但在生气窗纳米级孔隙发生相互融合,则开始以发育微米级孔隙为主;富颗石藻透镜体中赋存的有机质可能代表相对后期充注的运移沥青,在生油窗发育丰富的纳米级海绵状孔隙,并未见气泡状孔,且在生气窗由于异常高的有机质转化率,原矿物粒间孔重新暴露,有机质几乎被消耗殆尽。该研究结果不仅证实了泥岩中有机质孔隙的发育、演化特征受有机质类型和成熟度的双重控制,同时也为从泥岩微观组构、有机质赋存形式和有机孔发育非均质性三者之间的内在联系反演地下富有机质泥岩的生—排—滞留烃过程并揭示高过成熟阶段有机质孔隙的发育载体和保存机理提供了重要研究思路。

The characteristics of organic matter(OM)occurrence and pore development in the Cretaceous Eagle Ford Shale of South Texas,USA were studied at the micrometer scale through analyses of the original oil-window core and artificial gas-window cylinder samples by Ar ion milling/scanning electron micrography(SEM)method,combined with energy-dispersive spectroscopy mapping.Three typical rock fabric types—siliceous-argillaceous seams,foraminifera fossils,and coccolith-rich lenses—were observed under microscope which revealed significantly varying OM occurrences and pore development characteristics.Specifically,in siliceous-argillaceous seams,enriched amorphous kerogen developed few pores within the oil window due to oil sorption until nano spongy pores began to form in large numbers within the gas window as a result of sorbed-oil cracking;whereas structured kerogen,of small quantity,had little or no pore development through the entire petroleum-formation period due to its abnormally low hydrocarbon generation potential.In foraminifera fossil chamber,filling OM represented migrated bitumen that charged during the early stage and developed an abundance of bubbly pores and spongy nanopores within the oil window,and then,with coalesce of nanopores,micron scale large pores began to dominate within the gas window.And in coccolith-rich lenses,filling OM of migrated bitumen charged in the late stage was characterized by abundant spongy nanopores within the oil window,whereas within the gas window abundant mineral intergranular pores re-exposed due to high OM conversion rate.This study demonstrated that the formation and development of OM-hosted pores in marine mudrock were controlled by both OM type and thermal maturity.It provided an important approach to studying the process of hydrocarbon generation,expulsion,and retention as well as revealing the carrier medium of OM-hosted pores and pore preservation mechanisms in mature marine shales in the subsurface by investigating the spatial relationship between microscopic fabric,OM occurrence,and pore heterogeneity in organic-rich mudrock.