SIMPLE MIXING THEORY FOR KEROGEN CHARACTERIZATION IN HYDROCARBON BEARING SHALE

The complex configuration of shale reservoirs reflects on their being source, trap and cap rocks the same time; their complicated compositions; uncertain lithologies; overlapping oil and gas windows; strong dependency on thermal maturity; enigmatic pore systems and permeability. These complexities have rendered most well-established concepts and models of geology, geophysics, and petrophysics inapplicable.

As the well-known models do not directly apply to shale reservoirs, there is need to modify them or derive new ones. I derive here a simple mixing theory by reducing the complex configuration to two distinguishable phases such as organic and inorganic and assuming that the overlap between them is negligible. This perspective is another way to distinguish shale reservoirs from conventional ones (sandstone and carbonates) whose properties are mainly controlled by their inorganic constituents. Then I use the idea that a hydrocarbon bearing shale of negligible pore space is a mixture of kerogen (organic) and minerals (inorganic) with significant hydrocarbon in the kerogen. Also, I introduce two dimensionless variables and defined as weight fraction and volume fraction of kerogen with respect to the host shale. I adopt two kinds of averages: harmonic and arithmetic to arrive at the main result expressed by three equations. With these equations the true kerogen density, its volume and weight fractions can be computed what reduces the uncertainties in characterizing these reservoirs. These results are then applied to characterize the kerogen of the Barnett, Eagle Ford, Marcellus and Mancos shale.