下扬子地区中上二叠统页岩有机孔发育特征

Characteristics of organic pores in Middle and Upper Permian shale in the Lower Yangtze region

下扬子地区中上二叠统发育一套海陆过渡相页岩,其显微组成与海相页岩存在明显的差异,为了研究该套页岩有机孔的发育特征及影响因素,开展了有机岩石学、扫描电镜、氩离子抛光扫描电镜、气测孔隙度和压汞等相关分析。研究表明,中上二叠统页岩中有机孔整体发育较好,但不同有机质组分中孔隙的发育存在明显的差别,具体表现为镜质体内孔隙发育差,固体沥青内发育少量孤立的孔隙,腐泥质内具有丰富的孔隙。黄铁矿或黏土矿物常与有机质形成有机/矿物复合体,复合体内有机孔普遍发育较好,可能与黄铁矿或黏土矿物促进有机质生烃和分解有关。中上二叠统页岩TOC含量与比表面积之间存在明显的线性正相关性,但与孔隙度之间具有复杂的关系,当w(TOC)<6.16%时,孔隙度随TOC含量的增加而增加,而当w(TOC)>6.16%时,孔隙度普遍较低且与TOC含量之间存在微弱的负相关性。孔径分布特征也揭示高TOC页岩的中—大孔体积明显低于低TOC页岩。页岩孔隙结构发育特征表明,TOC含量越高、固体沥青组分以及贫氢组分的含量越高,这些组分占据的矿物孔隙越多,降低了页岩总的孔隙空间,且TOC含量越高,页岩越易被压实,造成中大孔塌陷,进一步降低了页岩的孔隙度。

A set of transitional shale reservoirs developed in the Middle and Upper Permian strata in the LowerYangtze region, with a maceral composition significantly different from that of marine shale. Organic petrology,scanning electron microscopy, field emission scanning electron microscopy in combination with argon ionpolishing, gas？filled porosity and mercury intrusion experiments were conducted in order to reveal the characteris？tics of organic pores and their influencing factors. The results showed that organic pores were well developedoverall in the Middle and Upper Permian shale, but have significantly different characteristics in differentmaceral grains. Vitrinite generally has no or rare pores developed, solid bitumen could produce a small quantityof large？scale isolated pores, and sapropelinite usually contains a large number of small pores, which is the maincontributor to organic porosity. Pyrite and / or clay minerals could be mixed with organic matter and then enhancethe development of pores in organic matter grains, which may be related to the effect of pyrite and clay mineralson organic matter generation and decomposition. Although there is a positive relationship between specific surfacearea and TOC content, the relationship of porosity with TOC content is rather complex. The TOC content has apositive relationship with porosity for the Middle and Upper Permian shale when w（ TOC） 〈 6. 16%, whereasporosityis generally low and has a slight decreasing trend with increasing TOC content when w（TOC） 〉6.16%.The pore size distribution also revealed that the high TOC content shale has lower meso-macropore volume thanthe shale with a low TOC content. The characteristics of pore development in the Middle and Upper Permian shale illustrated that solid bitumen and hydrogen-depleted components increased with increasing TOC content,and these macerals would fill in the mineral intergranular pores and reduce the overall space of shale. Mean-while, with higher TOC content, the shale is more likely to be compacted, resulting in the collapse of mesoporesand macropores, further reducing the porosity of shale.