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Understanding the Effects of Base-Level Rise on the Stratigraphic Distribution of Shale Plays


Conceptual modeling of the numerous allogenic controls on deposition of potential self-sourcing shale hydrocarbon reservoirs suggests that they commonly share similar paleotectonic, eustatic, paleoclimatic, and paleogeographic histories. For example, both the Jurassic Haynesville Formation and the Cretaceous Eagle Ford Shale were deposited in semi-restricted basins in the Gulf of Mexico during rising sea level, warm or transitional climates, and at relatively low latitudes. That deposition of organic-carbon-rich shale was not “simple” has gained support from the petroleum community, but not all facets of its stratigraphic distribution are fully understood. New to this model, however, is the following question: Is the stratigraphic distribution of organic-carbon-rich sediment related to processes that are not accommodated by standard sequence-stratigraphic models, which generally assume symmetrical changes in relative sea level? Self-sourced shale was commonly deposited during rapid, high-frequency relative sea-level rise that accompanied low-frequency eustatic rise to highstand. Conceptually, even if the sedimentation rate is presumed to have been constant proximal to sediment sources, a positively skewed rate of base-level rise requires that the sedimentation rate in distal locations be negative with respect to the former as sediment is trapped in coastal settings by flooding. The relative rates of base-level rise (i.e. creation of accommodation) and distal sedimentation were more strongly divergent than in proximal locations, so the early part of a transgressive systems tract in distal settings was characterized by erosive ravinement, not just hiatus. As the rate of base-level rise slowed after maximum starvation, the recovery of sediment supply was delayed so that the maximum flooding surface is younger distally. The development of a condensed section, therefore, was prolonged. This may help explain the deposition of organic-rich-carbon sediment at relatively shallow depths.