龙马溪组页岩黄铁矿微观赋孔特征及地质意义

Micropore Characteristics and Geological Significance of Pyrite in Shale Rocks of Longmaxi Formation

随着页岩气地质理论的不断完善,页岩气储层研究也更加精细、量化,黄铁矿作为页岩气储层普遍发育的物质成分,其矿物学特征、赋孔特征与地质意义引起了关注。为细化、量化对页岩储层黄铁矿的地质认识,通过氩离子抛光—场发射扫描电镜(FE-SEM)、能谱(EDS)、X射线衍射(XRD)等实验手段结合图像处理技术(Image Processing),以渝东南地区龙马溪组中下部优质页岩储层样品为例,探究页岩储层黄铁矿的发育类型和特征,量化表征评价页岩基质莓状黄铁矿在纳米尺度下的孔隙发育特征,并在此基础上讨论黄铁矿的页岩气地质意义,尤其是其储层意义。实验结果表明,莓状黄铁矿是龙马溪组页岩基质中最主要的黄铁矿类型,集合体直径介于3~10μm之间;莓状黄铁矿集合体内部晶体间有机质纳米孔发育,孔隙直径主要分布在100 nm以下,在页岩储层孔隙分类中莓状黄铁矿孔隙应归入有机成因孔隙类型;莓状黄铁矿集合体及其控制的有机质可以为页岩储层贡献0.7%~7%的孔隙比例,是对储集空间具有正贡献的、不应忽视的孔隙类型;基于图像处理技术(Image Processing)的孔隙分类表征与评价技术可以为储层孔隙研究提供新思路,是实现不同类型孔隙量化研究的可行方法。龙马溪组基质黄铁矿既可以贡献一定储集空间,也与有机质具有成因联系,可以为优质储层发育机理研究与优质储层勘探预测提供依据,在页岩气储层研究与勘探开发中具有重要地质意义。

The study on shale gas reservoirs has become much more meticulously and quantitatively along with the rapid improvement of shale gas geological theories.As a common material composition of shale gas reservoirs,the mineralogical and pore development characteristics as well as geological significance of pyrite has drawn a lot of attention,especially the nanoscale pores in framboidal pyrite.By using Argon Ion Polishing-Field Emission Scanning Electron Microscopy(FE-SEM),Energy Dispersive Spectroscopy(EDS),and X-ray Diffraction(XRD)combined with Image Processing technology to study on the mineralogical characteristics of different types of pyrite in reservoir rocks and to make quantitative characterization and evaluation of nanoscale pore characteristics as well as the reservoir significance of pyrite in high quality shale reservoir rocks of middle-lower Longmaxi Formation in Southeast Chongqing area,the results showed that the framboidal pyrite was the most developed type of pyrite in the shale matrix of Longmaxi Formation with aggregate diameter ranging between 3-10μm.The pores in the framboidal pyrite aggregate were mainly nanoscale pores developed in organic matter filling among the crystallites with diameters mainly concentrated below 100 nm.The pores in framboidal pyrite provided the reservoir with 0.7%-7%proportion of pore network in shale matrix and should be classified into Organic-Matter pores.Characterization and evaluation of classified pores based on Image Processing might provide a new approach to study on reservoir pores,being a feasible method to quantitatively study on different types of pores in different compositions.In addition to the reservoir space contributed by framboidal pyrite to the reservoir,the favorable conditions of organic matter preservation under reducing environments provided a basis for the study of high-quality reservoir formation mechanism and the prediction of high-quality reservoirs.Thus,pyrite,especially framboidal pyrite developed in shale matrix,has important geological significance for shale gas in both reservoir study and the exploration of high-quality reservoirs.