Organic geochemistry, organic petrography and basin modelling of the Upper Cretaceous Second White Specks and Upper Belle Fourche alloformations in the Western Canada Foreland Basin
Abstract
The Second White Specks is an unconventional shale play in the Western Canada Foreland Basin, endowed with over 450 billion barrels of light oil in place. Inconsistent production performance, however, has hindered its economic viability as a resource play. One potential explanation for erratic production behavior, on local and regional scales, is heterogeneous distribution of retained oil charge in the source rock. This study focuses on the analysis of geochemical properties and thermal history of the organic matter to assess lateral variability in generated hydrocarbon in the Second White Specks.
The methodology comprised characterization of the organic matter integrating organic geochemistry and basin modeling. Rock-Eval and organic petrography determined thermal maturity, paleoenvironmental conditions, kerogen type and hydrocarbon generation. Because exhumation magnitude is a critical input in the basin models, and previous results are disparate and uncertain, exhumation magnitudes were estimated across the study area, using a robust and reproducible compaction method. Finally, 1-D and 2-D basin models integrated borehole temperature, thermal maturity, kerogen data and exhumation estimates.
Geochemical analysis reveals the Second White Specks is rich in organic matter with high generation potential. The kerogen is type II, dominated by liptinite macerals with secondary vitrinite and inertinite components. Altered bituminite indicates intense bacterial degradation. The organic matter richness decreases with thermal maturity because of hydrocarbon generation, although enrichment in the center of the basin suggests local enhanced preservation. The source interval in most wells in the regional “oil window” fairway is thermally mature, and modeling shows generation started in the Paleocene. Two wells, however, exhibit anomalous characteristics that provide insights into local charge variability. In one case, the organic matter is inexplicably immature despite close proximity to a thermally mature well. In the other case, variability in heat flow delayed the onset of hydrocarbon generation, resulting in anomalously low charge.
The estimated exhumation exhibits the expected general trend of increase toward the deformation front. The high lateral resolution provided by this study, however, revealed an unexpected local pattern suggesting possible control by basement structures.