Evolution of Barnett Shale organic carbon structure and nanostructure with increasing maturation.
Résumé
The organic nanopore network of shales is believed to be the location of hydrocarbon trapping in source rocks. Better understanding of organic carbon structure and nanostructure constitutes a key point for estimating the oil and gas in-place at the local and basinal scale. Here we characterize, a maturation series from Mississippian Barnett Shale at multiple scales down to the nanometer scale. Organic petrography, Rock-Eval pyrolysis and Raman microspectroscopy are used to determine the organic matter properties (e.g. maturity, type, TOC content). Organic pore network attributes (e.g. pore size distribution, surface area, polyaromatic layers characteristics) are estimated using low-pressure gas adsorption analyses and direct imaging by high-resolution transmission electron microscopy (HRTEM). Based on Raman data we show that Barnett Shale organic matter has only reached the level of carbonization, even within overmature samples; graphitization has not yet been triggered. Interestingly, the investigated samples exhibit an additional broad Raman band centered at about 1480 cm-1, likely due to the presence of retained hydrocarbons within the organic matter nanoporosity. Low pressure gas adsorption analyses evidence a positive correlation between surface areas, mesopores (pores of about 2-50 nm of diameter) and maturity level. HRTEM images confirm Raman data: even the overmature organic matter remains disordered. Such disorientation of nanometer-sized polyaromatic layers creates a nanoporosity network in organic matter that appears mostly developed in samples of gas window maturity. Furthermore, carbon nanoparticles of concentric nanostructures, known to result from the thermal cracking of hydrocarbons, are detected in overmature samples.