Early dolomitization: its significance in creating subtle diagenetic hydrocarbon traps in the Williston Basin

CIM Bulletin, Vol. 77, No. 864, 1984

ALF HARTLING, Area Geologist, Saskatchewan Oil and Gas Corporation Regina, Saskatchewan

Most of the Paleozoic section of the 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 fades 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, Mg-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 Mg-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 ofdolomitizing brines gave rise to the frequently observed textural variation ofcryptocrystalline, impervious dolomite grading into a finely sucrosic, permeable dolomite down-dip. This diagenetic fades change provides the critical up-dip barrier for potential hydrocarbon reservoirs. The lateral pool boundaries are controlled by either structural relief or a similar diagenetic fades change. The reservoirs are capped by the tight evaporites. Hydrocarbon production is attained from diagenetic traps in the Mississippian Oungre Zone and the Ordovidan Red River 'C' Zone. Reservoir creation and the pooling mechanism are the result of dolomitization by the seepage refluxion of dense, Mg-rich brines beneath restricted, hypersaline lagoons. When formulating an exploration/exploitation program for these oil-prone horizons, it is essential to understand the complete geological history in order to establish where the reservoirs will be found and to define the area/ extent of each accumulation.

Mots Clés: Mineral exploration, Williston Basin, Hydrocarbon reservoirs, Dolomitization, Sedimentation, Oungre Zone, Red River 'C' Zone.