TITLE
Geochemical Insights into Oil Sources, Maturity, and Migration Pathways of the Pennsylvanian (Desmoinesian) Strawn Group, North-Central Texas
Lucy Tingwei Ko (Speaker), Xun Sun, Eric Radjef, Kelly Hattori, Bureau of Economic Geology, Jackson School of Geosciences, the University of Texas at Austin
ABSTRACT
The Strawn Group, deposited during the Pennsylvanian icehouse period, represents significant sandstone, carbonate, and mixed carbonate-sand reservoirs on the Eastern Shelf of the Permian Basin in Texas. These reservoirs have long been of interest due to their complex depositional histories, heterogeneous reservoir properties, and substantial hydrocarbon production potential. Sandstone reservoirs within the Strawn Group primarily consist of fluvial-deltaic and nearshore marine deposits, characterized by varying degrees of porosity and permeability influenced by depositional environment, diagenesis, and tectonic history. In contrast, the carbonate reservoirs, composed predominantly of limestone and dolostone, reflect shallow marine depositional settings with localized reefal and shoal complexes. Diagenetic processes such as dolomitization, cementation, and dissolution significantly influence reservoir quality, enhancing or reducing reservoir performance.
This ongoing study integrates geochemical analyses–bulk oil compositions, biomarkers, and diamondoids of produced oils alongside HAWK pyrolysis of cuttings–to investigate sandstone and carbonate reservoirs of the Strawn Group. Our objectives are to better constrain oil sources, maturity, and the timing of hydrocarbon generation, migration, and entrapment. We also evaluated source-rock distribution, richness, quality, thermal maturity, and link these to oil fingerprinting and typing.
Geochemical results show that oils from carbonate reservoirs in southeastern Scurry County are primarily sourced from a late oil window origin, with additional evidence of minor mixing with a secondary, higher-maturity source likely a gas condensate or wet gas. In contrast, oils from the Goen Limstone reservoirs (Runnels Co.) share biomarker affinities with the Barnett Shale of the Fort Worth Basin, yet diamondoid signatures indicate mixing with a secondary, more mature source. Similarly, mixed sourcing is evident in oils from the Truby Sand Field (Jones Co.) and Gray Sand reservoir (Coleman Co.). Ongoing oil sampling, reservoir characterization, and data acquisition in the Strawn Group will continue to refine our understanding of petroleum system elements, including carrier beds, sealing and migration roles of faults, and trap efficiency. Migration losses via carrier beds and leakage along faults remain important factors to assess.
BIO
Lucy Ko is a Research Assistant Professor at the Bureau of Economic Geology, UT-Austin, working with the MSRL (Mudrock System Research Laboratory) and STARR programs. Trained as an organic and isotope geochemist, she earned her M.S. in Geology from the Colorado School of Mines in 2011, worked as a basin modeler in Platt River associates, and completed her Ph.D. at UT-Austin in 2017. Since 2007, her research has focused on the unconventional reservoirs, integrating sedimentology, stratigraphy, petrography, and geochemistry (XRF, XRD, Rock-Eval/HAWK) to evaluate depositional processes, diagenesis, and reservoir quality. She also develops quantitative petrography workflows, applying machine learning and image segmentation in mudstones to advance understanding of the redox conditions and elemental partitioning in sedimentary rocks.
