The Distribution of Organic-Rich Mudstones in a Carbonate Setting: A Model Based on Pennsylvanian Source Rocks Deposited on an Isolated Carbonate Platform of the Paradox Basin, Southeast Utah, U.S.A.

John M. Guthrie¹, Kevin M. Bohacs¹, and Ruben D. Uribe^2
1 ExxonMobil Upstream Research Company, Houston, TX
² ExxonMobil Development Company, Houston, TX

A 3D continuum earth model of variations in organic-matter content was constructed for the platform carbonate setting of the Paradox basin based on outcrop, core, geochemical, and well-log data from the Gothic and Chimney Rock members of the Paradox Formation. Geostatistical modeling was used to fill in a high-resolution sequence-stratigraphic framework (meter-scale resolution) with continuous geobodies of organic-matter distribution throughout the basin. This revealed a distinctive, previously unreported, distribution of potential source rocks in this setting: they progressively lap landward onto underlying sequence boundaries, extending into relatively shallow water; their organic richness decreases basinward and contain their highest organic enrichment in intermediate water depths.

Major controls on organic matter accumulation are intra-platform bathymetry relative to the critical depth below the pycnocline where oxygen-deficient conditions can develop. During initial transgression, the entire water column is within the critical depth for maximum preservation of organic matter and organic-rich strata accumulate across the basin. The resulting maximum enrichment occurs within the basin center (a "bullseye" pattern), associated with isopach thicks. During the ensuing transgression, organic-matter-rich rocks are best preserved in intermediate water depths and form a "bathtub ring" around the basin. Local bathymetric lows on the carbonate platform tend to preserve the "bullseye" pattern and more open, sloping ramp areas preserve "bathtub ring" organic-rich rocks. The combination of these two sub-settings results in greater lateral variability in organic richness and more widespread distribution of organic-rich rocks in marginal carbonate platform settings thereby minimizing source risk.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005