ABSTRACT

Haas Field produces from the "Glenburn" zone of the Mission Canyon Formation. Oil is trapped at or near the facies change from porous carbonates to updip evaporites, a relationship which typifies many Mississippian fields in the northeastern Williston Basin.

Carbonate rocks cored in Haas Field represent two main depositional facies. A cyclic, coated-grain facies, located on the downdip (basinward) margin of the field, represents a carbonate shoal deposited under conditions of fluctuating, relative sea level. In this facies, upward-shallowing cycles are recorded by the repeated, vertical transition from fossiliferous wackestone (deeper conditions) to non-fossiliferous oolite and pisolite (shallower conditions). Some upward-shallowing cycles culminated in subaerial exposure as shown by the occurrence of vadose (microstalactitic) cements and laminated, micritic crusts in the capping grainstone and packstone.

A peloidal, intraclastic facies, located in the updip (landward) part of the field and constituting most of reservoir, represents restricted lagoonal or very low energy, tidal-flat environments. In this facies, intervals of sparsely fossiliferous lime wackestone, packstone and porous dolomite are punctuated by thin, laterally extensive beds of tight dolomite, which form local "marker beds".

Depositional cycles show the relationship of the shoal environment to the lower-energy environments which were sheltered in its lee: periodic emergence of the carbonate shoal formed a barrier which restricted circulation into the adjacent lagoon. The thin "marker beds" of evaporitic dolomite formed during these periods of maximum restriction. Depositional cycles are also reflected by the present distribution of porosity. In the shoal facies, "clean" oolite and pisolite were plugged by calcite cements, whereas "muddier" lime wackestone and packstones developed secondary porosity.

The Haas Field reservoir is largely a product of late, burial diagenesis rather than of near-surface processes. Early diagenesis was limited to the formation of fibrous marine cements in grainstones, to the formation of sheet cracks and tepee structures, and to a vadose overprint including microstalactites and laminated, micritic crusts. Most of the dolomitization accompanied pressure-solution and stylolitization. Leaching of secondary porosity, formation of coarse calcite and anhydrite cements, and oil migration postdated the formation of stylolites. Both the development of pressure-solution features and of secondary porosity are related to the original depositional facies; consequently these phenomena may be predictable on a local or regional scale.