ABSTRACT: Formation Resistivity as an Indicator of Oil Generation--Bakken Formation of North Dakota and Woodford Shale of Oklahoma

James W. Schmoker, Timothy C. Hester

With the onset of oil generation in organic-rich, low-porosity shales, nonconductive hydrocarbons begin to displace conductive pore water. As this process continues, formation resistivity increases from the low levels typical of water-saturated shales and can reach hundreds of ohm-m if sufficient oil is generated. In this study, formation resistivity of selected organic-rich shales is compared with geochemical indicators of hydrocarbon generation and thermal maturity in order to quantify relationships between resistivity and oil generation.

The upper and lower shale members of the Bakken Formation (Upper Devonian and Lower Mississippian) of the Williston basin, North Dakota, and the Woodford Shale (Upper Devonian and Lower Mississippian) of the Anadarko basin, Oklahoma, are used here as illustrative examples. An increase of volatile hydrocarbons (S₁) in core samples indicates that a resistivity of about 35 ohm-m marks the onset of observable oil generation in these three organic-rich shales. This resistivity value is used to map regions of the study areas where the Woodford Shale and the Bakken Formation have generated oil and where free oil might possibly be produced from fracture systems.

Crossplots of formation resistivity versus vitrinite reflectance (R_o) indicate that the level of thermal maturation required for oil generation is about R_o = 0.44% in the upper member of the Bakken Formation, R_o = 0.50% in the lower member of the Bakken Formation, and R_o = 0.57% in the Woodford Shale of the study area.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990