Abstract

The discovery of Parshall Field in Mountrail County, North Dakota, is an ongoing success story and is a credit to the efforts of petroleum geologists and independent oil companies. Results and data gathered from a geologic study of Montana’s Elm Coulee field was the model for the Parshall prospect that ultimately led to a multi-billion barrel, major oil field discovered by EOG Resources, Inc. This was in spite of the fact that the main reservoir objective of the prospect was absent, the resistivity of the new pay zone was extremely low (20-40 ohms) and the thermal maturity of the source rock bordered on immaturity (<0.65 percent Roe). Development progressed rapidly including newly-established reserves from the underlying Three Forks Formation. This huge oil resource pool adds greatly to the much-needed oil reserves for the U.S.

The discovery at Parshall was preceded by detailed geological analysis and some basic geochemical assessments. Geochemical analysis showed organic-rich, but low thermal maturity source rocks that were highly saturated with oil.

Integration of new, available data allows the presentation of a more accurate assessment of this major discovery in the Williston Basin. Geochemical data shows the oil crossover effect in the middle member of the Bakken Formation, the overlying Lodgepole Formation including the Scallion Member, and underlying Three Forks Formation in Parshall and Sanish fields. Such crossover is indicative of producible oil but does not quantify the recoverable amounts of oil. Although the upper and lower Bakken shales in Parshall Field are of low thermal maturity, oil was generated locally from these and migrated into the middle Bakken member, supported by oil fingerprinting and biomarker results. Furthermore, there is geochemical evidence from light hydrocarbons that some produced oil is from the immediately overlying and underlying Bakken shales, which is interpreted to be a function of stimulation energy reaching these highly oil saturated shales and other juxtaposed horizons. Light hydrocarbon derived GOR values for Bakken shales show good correlation to middle member produced petroleum GOR values.

The onset of petroleum generation is typically underestimated by most kinetic models. It is shown that the Friedman model provides results that demonstrate earlier generation consistent with compositional kinetic models. Pressure driven expulsion from hydrocarbon generation is thought to be further aided by the conversion of highly organic-rich Bakken shales that compact under lithostatic load, physically driving oil from the source rocks. Additional pressure and conduits for migration of early petroleum generation products are also hypothesized to be controlled in late diagenesis in part by release of kerogen-derived carbon dioxide and possibly by siderite decarboxylation, both combined with water and other organic acids.