Micro-Structural Studies of Gas Shales

Abstract

Abstract To understand the deliverability of gas shales, one needs to understand the controls on porosity and permeability. The microstructure of shale is defined in part by the grain size which is typically less than 5 μm and composition. Gas shales unlike other lithologies contain significant quantities of organic matter in various stages of maturation. The geometry and nature of the mineralogical components and the organics would be easy to describe if the objects could be identified optically. Most investigations into shale microstructure have relied on technologies such as Scanning Electron Microscopy (SEM), X-ray imaging, Transmission Electron Microscopy (TEM) or Scanning Acoustic Microscopy (SAM). Each has advantages an limitations. Our focus here is limited to SEM studies we performed on gas shales. Our journey began with generic imaging of broken surfaces and has progress to imaging of ion-milled surfaces in dual beam SEM. The latest technology has provide three dimension images with maximum resolution of 4-5nm pores. Porosity is found on the microscale in organics, between grains, in pyrite framboids, fossils, within minerals and in the form of microcracks. The majority of pores in some shales an located in the organics. Other shales show the porosity to be largely associated with minerals. SEM resolved pore dimension agree well with "as received" NMR measurement. However, high pressure mercury injection measurements suggest that the paths connecting these pores are even smaller. Three dimensional reconstruction of sequentially ion-milled surfaces using a dual beam SEM provides controls on the volumetric distribution of pores and organics and their connectivity. Initial and limited analyses indicate that the shales investigated are dominated by smaller pores. Keep in mind that any such study only samples an extremely small portion of any reservoir and that while generalizations are tempting, statistical studies are required to establish the universality of such observations.