Nano-scale texture and porosity of organic matter and clay minerals in organic-rich mudrocks

Abstract

Speculation exists in the oil and gas shale community about the form and distribution of the organic matter within the nanometer scale texture of mudrocks. In addition, the presence of micropore and mesopore networks either exclusively within the organic matter or as pore systems in the mineral components of these formations are not well understood. There is little published quantitative data with respect to the development of porosity in organic matter (OM) with thermal maturity in a burial diagenetic sequence. This paper presents a comparative study of pore-size distribution (PSD) in a burial sequence from the Baltic Basin, along with a selection of samples from other unconventional hydrocarbon reservoirs of various age and origin. Methods include quantitative mineral analysis by X-ray diffraction (XRD), RockEval pyrolysis, and subcritical gas-adsorption (SGA) analysis with N2 at 77.3K. SGA with N2 is effective in quantifying the volume of small pores that are below the detection limit of imaging techniques. Analyses were performed on aliquot samples in the natural state and after OM removal by treatment with buffered sodium hypochlorite (NaOCl).The results indicate that the clay hosted micro- and mesoporous network forms the textural structure of mudrocks at the nanometer scale. The distribution of OM with respect to the clay microstructure is heterogeneous. OM exists as separate particles or laminations where clay porosity may be open to adsorption, or OM can partially or completely occupy the space between clay aggregates within dimensions <5nm. The presence of micropores and fine mesopores within the OM itself are only observed in thermally mature samples where the RockEval Hydrogen Index (HI) is <100. The relative abundance of micro- and fine mesopores in the thermally mature mudrocks is controlled by both the clay and the OM content. Comparison of high-pressure methane adsorption measurements from OM-removed and natural aliquots demonstrate that thermally mature OM with well-developed microporosity has significantly greater adsorbing potential than clay-hosted porosity with comparable volume.