Characterization of organic-rich shales for petroleum exploration & exploitation: A review-Part 1: Bulk properties, multi-scale geometry and gas adsorption

Abstract

Shales, the most abundant of sedimentary rocks, are valued as the source-rocks and seals to porous petroleum reservoirs. Over the past-twenty years, organic-rich shales have also emerged as valuable petroleum systems (reservoir, seal, and source rocks contained in the same formation). As such they have become primary targets for petroleum exploration and exploitation. This Part 1 of a three-part review addresses the bulk properties, multi-scale geometry and gas adsorption characteristics of these diverse and complex rocks. Shales display extremely low permeability, and their porosity is also low, but multi-scale. Characterizing the geometry and interconnectivity of the pore-structure frameworks with the natural-fracture networks within shales is essential for establishing their petroleum exploitation potential. Organic-rich shales typically contain two distinct types of porosity: matrix porosity and fracture porosity. In addition to inter-granular porosity, the matrix porosity includes two types of mineral-hosted porosity: inorganic-mineral-hosted porosity (IP); and, organic-matter-hosted (within the kerogen) porosity (OP). Whereas, the fracture porosity and permeability is crucial for petroleum production from shales, it is within the OP where, typically, much of the in-situ oil and gas resources resides, and from where it needs to be mobilized. OP increases significantly as shales become more thermally mature (i.e., within the gas generation zones), and plays a key role in the ultimate recovery from shale-gas systems. Shales’ methane sorption capacities (MSC) tends to be positively correlated with their total organic carbon content (TOC), thermal maturation, and micropore volume. Clay minerals also significantly influence key physical properties of shale related to fluid flow (permeability) and response to stress (fracability) that determine their prospectivity for petroleum exploitation. Clay minerals can also adsorb gas, some much better than others. The surface area of the pore structure of shales can be positively or negatively correlated with TOC content, depending upon mineralogy and thermal maturity, and can influence its gas adsorption capacity. Part 2 of this three-part review considers, in a separate article, the geochemistry and thermal maturity characteristics of shale; whereas Part 3, addresses the geomechanical attributes of shales, including their complex wettability, adsorption, water imbibition and “fracability” characteristics. The objectives of this Part 1 of the review is to identify important distinguishing characteristics related to the bulk properties of the most-prospective, petroleum-rich shales.

Keywords

shale gas / shale lithofacies / shale porosity / shale methane adsorption / shale fractal dimensions / shale surface area

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