Detailed Sequence Stratigraphic Framework of the Cretaceous Mowry Shale, Big Horn Basin, Wyoming: Implications for Development and Distribution of Hydrocarbon Play Elements

The Late Cretaceous Mowry interval of the present day Big Horn Basin records deposition in the proximal to medial portion of the Western Interior Seaway under systematically variable conditions of organic-matter production, preservation, and dilution. These variations control the quality and distribution of key play elements throughout the basin. A fully integrated sequence stratigraphic framework was constructed over a broad area (>18,000 km²) at the bedset to parasequence scale that incorporates surface and subsurface data.

Correlation of eight parasequences with associated chronostratigraphic timelines (ash beds) and construction of isopach maps at the parasequence scale reveals thickness trends and lateral variations. In the proximal setting, parasequences are thicker, more blocky in character, and show overall lower concentrations of total gamma. This probably records deposition closer to sediment source and shoreline, where dilution and consumption inhibit organic-matter preservation. In the distal setting, parasequences are thinner, overall total gamma concentrations are higher, and packages consist of higher contrast facies, making them easier to identify. This probably reflects a depositional environment farther from sediment source (decreased dilution) as well as higher organic-matter preservation, and lower energy conditions to better preserve fluctuations in facies belts during shoreline progradation.

Depositional environment and sequence stratigraphy appears to be controlling factors of lithofacies play elements in the Mowry Shale. Key play elements quality and distribution appears to be best developed around the maximum flooding downlap surface. Hydrocarbon source potential increases towards the south-eastern part of the basin, where finer-grained facies have the highest total organic content (up to 2.2%) in the late TST to early HST. Seal potential also increases towards the southeast, mainly in the late TST where landward migration of the shoreline pushes coarser-grained facies belts farther on-shore. Reservoir quality increases farther from sediment source and shoreline, within the late TST to early HST where TOC and silica content are the highest. Developing a sequence stratigraphic framework within the Mowry interval reveals genetically related packages that can be correlated throughout the basin, and enables prediction away from sample control and targeting of key play elements that are essential to economic success.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California