ABSTRACT

Oungre strata form a small but productive portion of the Mississippian Ratcliffe Beds within the Williston Basin. Oungre sedimentation was characterized by lime mudstone deposition on a broad, shallow marine shelf interior. Numerous shoals developed in response to sea level fluctuations and prolific algal sedimentation. These subtle bathymetric features are overlain by the Oungre Evaporite which marks the final phase of Oungre basin filling.

Seepage refluxion of Mg-rich brines through the sediment beneath restricted hypersaline lagoons of the Oungre Evaporite, dolomitized the uppermost few meters of the Oungre Zone. A permeable dolomite reservoir was created downdip of the lagoonal area as the Mg-ion content of the brines waned and crystal size increased correspondingly. The up-dip textural change from finely- to micro-sucrosic dolomite forms a permeability barrier to hydrocarbon migration.

Two distinct trapping mechanisms are identified in the Oungre pools: 1) strati-structural and 2) structural. Dolomites form the reservoirs and the extent of dolomitization commonly determines the up-dip limits of the pools. Localized salt tectonics or gentle basin subsidence controlled the structural elements in all cases. A successful Oungre exploration model requires the integration of detailed geology and geophysics to predict depositional trends, diagenetic changes, and to highlight the often subtle structural events.