Compositional fractionation of petroleum from reservoir to wellhead in the Niobrara shale oil play

Abstract

Hydraulic fracturing is routinely used to stimulate unconventional reservoirs. In the Niobrara shale oil play there are several fracking targets, but the actual stimulated rock volume is not well constrained. This means that allocating produced oil to specific reservoirs, in essence oil-reservoir correlation, is not straightforward. The data presented here support that the chalk units are the principal production zones in the Niobrara shale oil play. Several lines of evidence, such as the presence of heavy-end n-alkanes (>n-C31), consistence in composition of gross fractions, the distribution of C27-28-29 diasteranes, as well as the pristane to phytane ratio, all suggest that oils were produced from the A- & B- chalks, particularly from the A-Chalk. During production, a selective partitioning of lighter hydrocarbons into the migrating fluids could nevertheless occur, in particular as regards the n-alkanes domain and their carbon isotopic compositions. Regarding the latter, a depletion of 13C in the produced oil's n-alkanes by more than three per mil relative to all of those core extracts was observed. This finding suggests that compound specific isotope analysis in both oil-reservoir and oil-source rock correlations cannot be taken at face value, i.e. by simple matching of values. Besides that, fractionation appears to result in the enrichment of rather aliphatic nitrogen-containing compounds in crude oils.