Geological characteristics of unconventional hydrocarbon resources

This paper reviews the geological characteristics of unconventional hydrocarbon plays from shales, tight formations, and coal bed, which are gaining much attention and are being commercially produced in significant amounts recently due to the modern development of directional drilling and hydraulic fracturing technologies. In fact, the hydraulic fracturing techniques are being increasingly adopted by the petroleum industry, and thus the distinction between conventional and unconventional hydrocarbons becomes less meaningful than before, leading to unconventional hydrocarbons becoming a new type of conventional hydrocarbons. The unconventional hydrocarbons trapped in tight reservoirs have much different characteristics than the conventional hydrocarbons stored in structural and stratigraphic traps. The unconventional hydrocarbons have the following geological features: 1) the hydrocarbon source and reservoir rocks coexist; 2) porosity and permeability are very low; 3) nanometer-sized pore-throat are widely distributed; 4) hydrocarbon-bearing reservoirs are continuously distributed; 5) no obvious trap boundary occurs; 6) there is little effect of buoyancy and hydrodynamics; 7) Darcy's law does not apply; 8) there is poor separation of water; oil, and gas; 9) there occurs no clear interface among oil, gas, and water as well as pressure system; and 10) oil and gas saturation varies. In summary, continuous accumulation of hydrocarbons over a large area is the most distinguishing characteristic of unconventional petroleum.