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Abstract

The Cretaceous Niobrara Formation is one of the latest unconventional exploration targets in North America. In this paper, an overall Type-II marine organofacies with some variability is present within a core which consists of alternating marls and chalks, covering a maturity range of 0.8–1.0 %Rc (calculated vitrinite reflectance). Enhanced oil saturation index (OSI) values have previously been reported for the chalk, and here a positive trend (R2 = 0.92) between the OSI and the percentage of saturates in gross fractions is present. Thus, the saturate fraction gradually increases from 40% to above 60% as the OSI increases from 40 to 550 mg HC/g TOC (total organic carbon). Extremely high OSI values (>300 mg HC/g TOC) are characteristic of Niobrara chalks. Therefore, the chalk intervals represent primary sweet spots. Using a combined analytical approach, petroleum has been shown to migrate from the organic-rich marls into juxtaposed chalks, and that this has fractionated oils into higher quality liquids. Extractable organic matter in the chalks is clearly enriched in saturate hydrocarbons, while the NSO-containing polar compounds are preferentially retained in the shale and marl units. Compared with the significant fractionation effects between these gross fractions, fractionation within a given compound class is nevertheless indistinct.