Factors controlling source and reservoir characteristics in the Niobrara shale oil system, Denver Basin

Abstract

This paper clarifies the controls of oil retention in the Niobrara Formation, Denver Basin, in the western United States. Sweet spots have been recognized using a total of 98 core samples from 5 wells with maturities covering the oil window. Oil retention in the source rock samples (carbonate content 30 wt. %) is controlled by carbonate content, which is positively related to the amount of retained oil. Petrographic features indicate that oil or bitumen is stored in porous calcite fossils (i.e., coccolith and foraminifera), which provide additional space for petroleum storage. Chalk samples (carbonate >85 wt. %) are characterized by anomalously low Tmax values caused by the influence of heavy petroleum or bitumen. The amount of this bitumen is higher than the initial petroleum potential of kerogen in A and B chalks and thus must have been emplaced here. The most likely sources are juxtaposed organic-rich marl layers. Thus, sweet spots occur where carbonate content is either low (high TOC) or high (low TOC), whereas production of petroleum from the pore space of presumably brittle chalk seems more attractive than production from organic- and clay-rich rocks.