2014 Rocky Mountain Section AAPG Annual Meeting

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Differentiating Green River Formation Oils Sourced from Freshwater and Saline Lacustrine Facies in the Uinta Basin, Utah


Recognized petroleum source rocks in the Eocene Green River Formation, Uinta Basin include the Black shale facies and Mahogany zone shale. Recently, the informal Uteland Butte member of the Green River Formation has been targeted for development using horizontal drilling and hydraulic fracturing. Organic-rich Uteland Butte shales represent a freshwater lacustrine facies containing interbedded dolomite reflecting intermittent saline conditions. Several wells are producing oil out of dolomite-rich beds in the Uteland Butte in the western Uinta Basin. Oils from six wells in Duchesne and Uintah Counties were provided by the Bill Barrett Corporation and examined using bulk characterization methods to determine sulfur content, stable carbon isotopic signature, API gravity, and C15+ saturate, aromatic, resin, and asphaltene content. Distributions of total, saturated, and aromatic hydrocarbons were measured by gas chromatography (GC) and relative concentrations of biomarkers were determined by GC-mass spectrometry. The results indicate that oil from one well resembles Green River type A, which is derived from the Black shale facies. Oil from another well is consistent with Green River type B, which is sourced from Mahogany zone shale. The remaining four wells produce similar waxy oils that have distinctly different properties from those attributed to the established source rocks. Differences include lower carbon preference index values, higher pristane-phytane ratios, and other parameters consistent with a freshwater lacustrine origin. We will present results of geochemical analyses to ascertain if these oils originate from Uteland Butte shale or if another source is indicated.