Intraformational migration of petroleum: Insights into the development of sweet spot in the Cretaceous Niobrara shale-oil system, Denver Basin

摘要

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 (R-2 = 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.