Palynofacies Analysis and Submicron Pore Modeling of Shale-Gas Plays

The present study combines palynological applications with advanced microscopic techniques to characterize the Utica, Haynesville and Fayetteville shale-gas source rocks. This unprecedented approach could offer an alternative way to measure the total organic carbon (TOC) content using the 2D subsurface Scanning Electron Microscope (SEM) images. This approach is considered to be a faster and inexpensive method compared to conventional geochemical analyses. Palynofacies analysis provided valuable information about kerogen type and its degree of thermal maturation, which are key parameters in shalegas exploration. Moreover, it qualitatively allowed the estimation of important organic geochemical parameters such as vitrinite reflectance (Ro %) and numerical thermal alteration index (TAI). New high resolution microscopic solutions have successfully been exploited for source rock characterization at both microand nano-meter scales. In-situ Focused Ion Beam (FIB) and Scanning Electron Microscope (SEM) technologies provided new insights into rock fabrics such as porosity, permeability, tortuosity, anisotropy and kerogen content. Serial sectioning and sequential imaging using dual beam SEM/FIB instrument were implemented successfully to characterize the 2D kerogen content and 3D submicron-pore structures. Moreover, pores were found in organic matters with the size of nano level and occupy 40−50% of the kerogen body. A successful example of reconstructed 3D pore model from Fayetteville Shale is presented. Introduction Qualitative and quantitative characteristics of shale-gas source rock are used by petrophysicists, geochemists and reservoir engineers. These include submicron pore structure and organic matter analyses, which are important in the derivation of rock capillarity, wettability and storativity. Shale-gas source rocks hold large quantities of hydrocarbon reserves that have made significant impact on North American oil and gas market since early 2000s. The ambiguity behind gas storativity and deliverability can be demystified through the understanding of the relationship between organic matter content and porosity. Conventional standalone analyses are inadequate and not suited for unconventional gas rock characterization. The proposed combined approach provides important information for evaluating and appraising shale-gas plays. Palynofacies analysis identifies intervals of exploratory interest in terms of hydrocarbon content. It can also be used as a proxy to estimate organic geochemical values that are very expensive to obtain and thus, significantly reduces exploration and production costs. Pore imaging and modeling allows the evaluation of gas storage quantity and deliverability in shale-gas plays, which enables the development of optimized processes for hydrocarbon recovery. Palynofacies analysis as defined by Tyson (1995) is: “the palynological study of depositional environments and hydrocarbon source rock potential based upon the total assemblage of particulate organic matter.” In the present study, palynofacies analysis was carried out on five samples recovered from the Haynesville, Utica and Fayetteville shale-gas source rocks in order to evaluate their kerogen type and degree of thermal maturation. Three of the five studied samples are from the Utica Shale (two samples from Dolgeville member and one sample from the Indian Castle Member), while the fourth and fifth samples are from Haynesville and Fayetteville shales. The data obtained were used to qualitatively estimate some key organic geochemical parameters such as vitrinite reflectance (Ro %) and numerical thermal alteration index (TAI). Samples were also analyzed for TOC content in order to fully understand their source potential.