Guthrie, John M.¹, Ruben Uribe¹, Kevin Bohacs¹
(1) ExxonMobil Upstream Research Company, Houston, TX

ABSTRACT: Spatial Variability of Organic-Rich Rocks: A Critical Element for Defining the Petroleum System of Pennsylvanian Carbonate Reservoirs of the Paradox Basin, SE Utah

Lateral variations in source rock thickness, richness, and quality are critical to accurately estimating hydrocarbon potential and expelled hydrocarbon volumes within a basin. Shallow-shelf carbonate reservoirs of the Pennsylvanian Paradox Formation in the Paradox basin of southeastern Utah contain about 1.3 BBO original oil-in-place (OOIP). The hydrocarbons in these reservoirs were generated from organic-rich rocks of the Gothic and Chimney Rock formations.
In this study, a high-resolution sequence-stratigraphic framework for these source rocks was constructed for interpreting variations in organic richness using outcrop, core, and well-log data from approximately 350 wells. The source rocks were subdivided into parasequences and sequences using detailed total organic carbon (TOC) profiles derived from well-log analysis and correlated throughout the basin. TOC was restored to original TOC (OTOC) and an integrated 3D model in goCad© was constructed to help visualize source rock variations in detail.
The Gothic and Chimney Rock formations contain mostly Type II marine, oil-prone, black, laminated mudstones with OTOC up to 9% and original hydrogen index (OHI) up to 500 mg HC/g TOC. These source rocks were deposited in bathymetric lows surrounding carbonate algal mounds in the platform interior as well as lapping onto the mainland. Maps of thickness and OTOC show that the relatively intricate bathymetry of this carbonate-platform setting resulted in the richest source rocks being deposited as a rim around the margins of the bathymetric lows. This work provides new models for the deposition of source rocks in carbonate-platform settings.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.