The Spectrum of Fine-Grained Reservoirs from 'Shale Gas' to 'Shale Oil'/ Tight Liquids: Essential Attributes, Key Controls, Practical Characterization

A spectrum of combinations of rock and hydrocarbon properties in fine-grained rocks can result in significant production, effectively spanning ‘conventional’ tight oil to fractured ‘shale’ gas reservoirs, in four main families based on dominant porosity-permeability system and stratal relations (i.e., ‘Conventional’ tight, Hybrid/Interbedded, Porous ‘shale’, Fractured ‘shale’). Fine-grained reservoir types comprise ‘shale-oil’ reservoirs at lower thermal maturities and pressure-temperature (P-T) conditions to ‘shale-gas’ reservoirs at higher maturities and P-T conditions. These fine-grained reservoirs can contain a variety of pore types: inter-granular, intra-granular, fracture, intra-kerogen, and intra-pyrobitumen/char -- the last two ‘organic-hosted’ types are more obvious at higher maturities. ‘Shale-oil’ reservoirs share many attributes with ‘shale-gas’ reservoirs, but have some distinct differences. The key additional dimension is the properties of the hydrocarbon fluids: Over geological time, fluid density and phase influence fluid saturation in the matrix, and in the short term, viscosity and phase affect flow and production rates. Hence, two main classes of attributes affect ultimate ‘shale’ reservoir performance: rock properties (mainly permeability) and fluid properties (mainly viscosity) that are influenced by the full geological history of the reservoir. Overall reservoir permeability includes both matrix and fracture characteristics: Matrix permeability is a function of original depositional composition, texture, bedding, and stratal stacking plus burial history (thermal stress, diagenesis). Fracture permeability is a function of the same controls as matrix permeability along with structural history (mechanical stress). Fluid properties (viscosity, density) are also controlled by original depositional properties (which determine kerogen type) and burial/uplift history, along with present-day reservoir pressure and temperature. The higher thermal maturities of ‘shale-gas’ reservoirs result in contrasts with ‘shale-oil’ reservoirs: they tend to have less smectite due to illitization, but more obvious porosity associated with kerogen and bitumen. These factors modify the porosity-permeability system and well-log responses. Appreciation of the similarities and differences between ‘shale-gas’ and ‘shale-oil’ enables more efficient and effective exploitation of the full range of resource types.