Abstract

Analysis of lithologic and electric log data from the Ordovician Red River Formation of eastern Montana and western North Dakota has been utilized to propose a diagenetic model of the nature, extent, and position of porosity development in this stratigraphic unit. Abundant evidence suggests that porosity of reservoir quality developed around the perimeter of small structures exhibiting slight topographic expression during Ordovician time. Porosity of less than reservoir quality developed on the crests of these Ordovician highs. Secondary End_Page 804------------------------------ dolomite porosity has been destroyed by the precipitation of secondary minerals. Porosity development and reduction by cementation can be explained by near-surface geochemical conditions which are thought to have existed in and around these topographic highs. Dolomitization occurred as meteoric waters were introduced into the subsurface from partly exposed tidal flats. Microcrystalline dolomite was precipitated as waters with high Mg/Ca ratios percolated down from the tidal flats. Rapid nucleation precluded the formation of sucrosic dolomite and porosity. As the Mg/Ca ratio dropped owing to dolomite crystallization, nucleation rates dropped and sucrosic dolomite and porosity developed downdip from the crests of the topographic highs. In this manner, a ring-shaped zone, of reservoir quality porosity formed surrounding paleotopographic highs. Application of this proposed model to petroleum exploration in the Williston basin provides a significant guide for the drilling and accurate development of Ordovician Red River prospects. End_of_Article - Last_Page 805------------

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