Formation Resistivity as an Indicator of Oil Generation in Black Shales

Timothy C. Hester, James W. Schmoker

Black, organic-rich shales of Late Devonian-Early Mississippian age are present in many basins of the North American craton and, where mature, have significant economic importance as hydrocarbon source rocks. Examples drawn from the upper and lower shale members of the Bakken Formation, Williston basin, North Dakota, and the Woodford Shale, Anadarko basin, Oklahoma, demonstrate the utility of formation resistivity as a direct in-situ indicator of oil generation in black shales.

With the onset of oil generation, nonconductive hydrocarbons begin to replace conductive pore water, and the resistivity of a given blackshale interval increases from low levels associated with thermal immaturity to values approaching infinity. Crossplots of a thermal-maturity index (R_o or TTI) versus formation resistivity define two populations representing immature shales and shales that have generated oil. A resistivity of 35 ohm-m marks the boundary between immature and mature source rocks for each of the three shales studied.

Thermal maturity-resistivity crossplots make possible a straightforward determination of thermal maturity at the onset of oil generation, and are sufficiently precise to detect subtle differences in source-rock properties. For example, the threshold of oil generation in the upper Bakken shale occurs at R_o = 0.43-0.45% (TTI = 10-12). The threshold increases to R_o = 0.48-51% (TTI = 20-26) in the lower Bakken shale, and to R_o = 0.56-0.57% (TTI = 33-48) in the most resistive Woodford interval.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.