ABSTRACT: Recognition of Hydrocarbon Expulsion Using Well Logs: Bakken Formation, Williston Basin

R. Cunningham, F. B. Zelt, P. D. Snavely, III, S. R. Morgan, Q. R. Passey, R. L. Webster

The Upper Mississippian-Lower Devonian Bakken Formation forms a source/carrier/reservoir system in the Williston basin. Hydrocarbon expulsion within the Bakken has been identified by overlaying sonic and resistivity logs. Typically, these curves track in organically lean, water-saturated mudrocks because both respond mainly to porosity; however, in thermally mature organic-rich rocks and hydrocarbon reservoirs or carrier beds, the curves separate due to the anomalously high resistivity associated with replacement of pore water by hydrocarbons. Sonic/resistivity-log overlays for wells throughout the Montana and North Dakota parts of the Williston basin reveal significant increases and maximum in-curve separation within the middle siltstone member of the Bakken at subsurfac temperatures of about 170 and 200°F, respectively.

Sequence-stratigraphic characteristics of the Bakken define the framework within which the expulsion process operates. The organic-rich upper and lower shale members represent the transgressive and early highstand systems tracts of two adjacent depositional sequences. A sequence boundary within the intervening middle siltstone member separates nearshore siltstone and sandstone of the late highstand systems tract in the lower sequence from cross-bedded subtidal to intertidal sandstones of the lowstand systems tract in the upper sequence. Reservoir properties vary across this sequence boundary. We attribute the log separation in the siltstone member to hydrocarbons expelled from the adjacent shales. Abrupt shifts in several geochemical properties of the shale members, indicative of hydr carbon generation, occur over the same subsurface temperature range as the rapid increase in log separation in the middle siltstone, thus indicating the contemporaneity of generation and expulsion.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990