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The AAPG/Datapages Combined Publications Database

Rocky Mountain Association of Geologists

Abstract


The Paradox Basin Revisited – New Developments in Petroleum Systems and Basin Analysis, 2009
Pages 95-130

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

John M. Guthrie, Kevin M. Bohacs

Abstract

Accurate maps of source rock thickness and richness variations are critical for estimating hydrocarbon potential and expelled hydrocarbon volumes within a basin. This study investigated the spatial variability of organic richness in the Gothic and Chimney Rock shales of the Middle Pennsylvanian Paradox Formation of the Hermosa Group in the Paradox Basin of southeastern Utah. These “shales” are predominantly composed of black, parallel-bedded (“laminated”), carbonate-dominated mudstone that underlie shallow-shelf carbonate reservoirs in the Aneth Field having estimated original oil-in-place (OOIP) of 1.3 billion barrels of oil (BBO).

A high-resolution sequence-stratigraphic framework was constructed to interpret the spatial variability of organic richness using outcrop, core, and well-log data from approximately three hundred and fifty locations. The source rocks were subdivided into parasequences and sequences using detailed lithological variations, sedimentary structures, and stacking patterns from core and outcrop descriptions, total organic carbon (TOC wt. %) profiles from well-log analysis, and well-log correlation and stratal geometry. TOC was restored to original TOC (OTOC wt. %) and a three-dimensional model was constructed to portray the variability of organic richness throughout the basin.

The Gothic and Chimney Rock contain mostly Type II marine, oil-prone, organic matter with OTOC contents up to 10 wt. % and hydrogen index values up to 480 mg HC/g TOC. These source rocks were deposited in bathymetric lows surrounding carbonate algal mounds on the shelf platform as well as lapping onto the basin margin. Maps of thickness and OTOC show that the relatively intricate bathymetry of the carbonate-platform setting resulted in the richest source rocks being deposited as a rim around the margins of bathymetric lows. This work provides new insights for the deposition of source rocks in carbonate-platform settings and has important implications for exploration.


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