Geological characterization of unconventional shale-gas reservoirs

The growth of natural gas production in the United States has been boosted by shale-gas production in the Appalachian Basin in the Northeast, the Permian Basin in western Texas and New Mexico, and the Haynesville Shale in Texas and Louisiana. Shale gas is an unconventional gas resource. It differs from the conventional gas resource, which produces natural gas from granular, porous, and permeable formations. The term “shale gas” refers to thermogenic or biogenic gas produced from organic-rich, fine-grained, low-permeability sedimentary rocks (e.g., shale, mudstone, mudrock, and associated lithofacies) upon extensive fracturing. The unconventional shale gas resource is a self-contained system where the rock functions as the hydrocarbon source, migration pathway, reservoir, and seal. Additionally, the shale gas resource requires enhanced drilling/production technologies (e.g., hydraulic fracturing) in order to be produced at economic rates. The gas occurs as free and adsorbed states in the pore spaces. Much gas is adsorbed on the surface of minerals (especially clay minerals) and OM. Therefore the approach to the study of the geological characteristics of a shale gas play is distinctive. To accurately predict the shale gas resource potential, there must be a thorough assessment of the paleodepositional environment, organic geochemistry, thermal maturity, reservoir thickness, mineralogy, mechanical property, porosity, and permeability.