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Examination of the Silurian-Ordovician Stonewall Formation's Stratigraphy, Hydrocarbon Production and Potential — Williston Basin, North Dakota


The Stonewall Formation consists of three carbonate-evaporitic cycles deposited during the Late Ordovician-Early Silurian. Each Stonewall cycle consists of three general lithofacies: 1) tan to very dark grey/black bioturbated to laminated carbonate mudstone to wackestone, 2) thin greyish green dolomitic and/or argillaceous, silty to sandy mudstone, and 3) nodular to laminated anhydrite that is sometimes interlaminated to interbedded with dolomite. The anhydrite intervals are basin centered and pinch out moving towards the Stonewall's peripheral extent. Similar anhydrite intervals are present in the underlying Red River Formation in which previous interpretation has proposed subtidal deposition within a hypersaline silled basin. The silty-sandy argillaceous mudstone beds thicken towards the peripheral extents of the Stonewall Formation and may be marginal marine to terrestrial in origin. Commercial hydrocarbon production from the Stonewall Formation within North Dakota's portion of the Williston Basin began in 1979. To date there have been 64 productive vertical wells with perforations within the Stonewall Formation that have cumulatively produced over 7.5 million barrels of oil, 17 BCF gas, and 3.9 million barrels of water. Stonewall wells produce from the carbonate facies within the middle and/or upper two cycles. Productive areas for the Stonewall Formation display increased porosity within the carbonate facies. Natural facture systems may also play an important role in vertical Stonewall production. The Stonewall Formation appears to be a self-sourced petroleum system. Analyzed Stonewall core samples from western North Dakota (north McKenzie and southern Williams Counties) yielded TOC values up to 2.5% but with low S2 (<3 mg/g) and Hydrogen index values (<140). Measured Tmax values (455–460°) from this samples set, however, suggests most of the original kerogen has been converted to hydrocarbons, which has significantly degraded the original organic-richness of the Stonewall Formation. Portions of the carbonate facies may have originally contained 1–6% TOC with adequate S2/hydrogen index values to classify as good to excellent quality source rock. The Stonewall Formation may contain a continuous petroleum accumulation and has potential to be an unconventional resource play.