About This Item
Share This Item
Most of the Paleozoic section of Williston basin is a thick accumulation of numerous carbonate to evaporite, shallowing-upward, cyclic sequences. These sediments were deposited in broad epeiric seas, and the deposition of the evaporite facies marked the final stage of each cycle. Many of the sequences display a pervasive replacement dolomite in the uppermost portion of the carbonate units. This secondary dolomitization is, at least in part, an early diagenetic event, synchronous with evaporite deposition.
The inception of evaporite precipitation resulted in the seepage of a dense, magnesium-rich brine into the underlying sediment. The heavy brine moved down the gentle regional dip, displacing the more normal marine interstitial pore fluid and dolomitized the primary calcitic sediments en route. The supply of magnesium-ions decreased away from the source, and correspondingly, the degree of dolomitization decreased and larger crystals formed because of slower nucleation at fewer sites.
The seepage refluxion of dolomitizing brines gave rise to the frequently observed textural variation of cryptocrystalline, impervious dolomite grading into a finely sucrosic, permeable dolomite downdip. This diagenetic facies change provides the critical updip barrier for potential hydrocarbon reservoirs. The lateral pool boundaries are controlled by either structural relief or a similar diagenetic facies change. The reservoirs are capped by the tight evaporites.
Hydrocarbon production is attained from diagenetic traps in the Mississippian Oungre Zone and the Ordovician Red River "C" Zone. Reservoir creation and the pooling mechanism are the result of dolomitization by seepage refluxion of dense, magnesium-rich brines beneath restricted, hypersaline lagoons.
End_of_Article - Last_Page 1341------------