Estimation of Formation Pressures from Log-Derived Shale Properties(*)

ABSTRACT Sedimentary rocks during burial maintain hydrostatic fluid pressure within their pore space in the fluids within the sediment are allowed to escape as the sediment compacts. If the fluid is not permitted to escape, compaction is retarded, and the fluid pressure rises (the sediment becomes overpressured) and ultimately approaches the pressure exerted by the overlying rocks and contained fluids. The actual fluid pressure existing in a given permeable formation can be determined by standard pressure bomb measurements. The determination of the fluid pressure in shales, with their low permeability, has previously been difficult or impossible. The fluid pressure within the pore space of shales can be determined by using data obtained from both acoustic and resistivity logs. The method involves establishing relationships between the common logarithm of shale transit time or shale resistivity and depth for hydrostatic-pressured formations. On a plot of transitime versus depth, a linear relationship is generally observed, whereas on a plot of resistivity versus depth, a nonlinear trend exists. Divergence of observed transit time or resistivity values from those obtained from established normal compaction trends under hydrostatic pressure conditions is a measure of the pore fluid pressure in the shale and thus in adjacent isolated permeable formations. This relationship has been empirically established with actual pressure measurements in adjacent permeable formations. The use of these data and this method permits the interpretation of fluid pressure from acoustic and resistivity measurements with an accuracy of approximately 0.04 psi per foot, or about 400 psi at 10,000 feet. The standard deviation for the resistivity method is 0.022 psi per foot and for the acoustic method 0.020 psi per foot. Knowledge of the first occurrence of overpressures, and indeed of the precise pressure-depth relationship in a geologic province, enables improvements in drilling techniques, casing programs, completion methods, and reservoir evaluations.