2014 Rocky Mountain Section AAPG Annual Meeting

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Diagenesis of the Upper Three Forks Formation – The North Dakota Perspective


The upper part of the Three Forks Formation in the Williston basin of North Dakota and Montana is one of the prime targets for oil exploration in the onshore part of the US today. The unit is mainly composed of dolomite, yet the details of dolomite formation and its relative timing are unknown. This study is the first that combines cement generations and porosity into a diagenetic scheme based on detailed microscopical observations. The upper Three Forks Formation shows a total of seven dolomite generations along with some anhydrite and pyrite. Most of the rock consists of an inclusion-rich dolomite, likely dolomite II, that forms mm- to sub-mm- size rhombic crystals showing overgrowth of five more alternating clear and inclusion-rich dolomite generations, and in places a core of iron-rich dolomite I. Porosity types in the Three Forks Formation are intercrystalline, intracrystaline, and "moldic" which here stands for the dissolution of entire dolomite crystals. Detrital components are quartz, feldspars, mica, and clay particles.

The Three Forks Formation was most likely deposited on a mixed carbonate-siliciclastic ramp as a limestone unit with varying amounts of detrital input. Initial replacement of limestone into dolomite probably occurred early entirely changing the texture of this unit. Several dolomite phases occurred during burial post-dating early dolomitization. The effective porosity, characterized by intercrystalline and "moldic" pores, is linked to the dolomitization, most likely originally to an early event as no late dolomite is seen filing these pores. Up to centimeter-size voids, though, representing mostly non-effective porosity, are generally partly filled with several generations of dolomite and leaving some part of the vugs open. This indicates that most likely the voids were formed before the last few generations of dolomite cement, and also that not all open space was easily occluded by these dolomitizations but left some of the porosity untouched.

Based on a limited data set, porosity distribution in the upper Three Forks Formation does not show a clear link to the distribution of dolomite. However, it does show a trend to overall increased values from the east (less than 1%) to the west (around 5%) with a north-south extending zone of maximum porosities (about 10-12%) around 103.50. It is therefore likely that potential hot spots in this basin are rather located in western North Dakota while towards the east porosities are less promising.