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AAPG Bulletin, Preliminary version published online Ahead of Print 1 April 2024.

Copyright © 2024. The American Association of Petroleum Geologists. All rights reserved.


Organic geochemical characterization of Deltaic Paleogene Rock units in Coos Bay, Oregon: Kerogen type and richness in response to depositional environments

Allison K. Barbato12 , John M. Armentrout3 , Leslie B. Magoon4 , Thomas Demchuk5 , Craig Barrie6* , and Sophie Warny12

1 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA, USA
2 Museum of Natural Science, Louisiana State University, Baton Rouge, LA, USA
3 University of Oregon, Department of Earth Sciences, Eugene, OR, USA
4 Stanford University, Department of Geological Sciences, Stanford, CA, USA
5 PetroStrat Inc., The Woodlands, TX, USA
6 Applied Petroleum Technology, Sven Oftedalsvei 6, 0950 Oslo, Norway
* Previous Affiliation: GeoMark Research, Ltd., 9748 Whithorn Drive, Houston, Texas 77095

Ahead of Print Abstract

Coastal depositional environments are known to host source, reservoir, and seal rocks. The middle Eocene Coaledo Delta in Coos Bay, Oregon, with its well-preserved coastal units, has been identified for its potential to generate and accumulate petroleum. Previously encountered gas implies a viable, gas-prone source rock exists in the Coos Bay area. This study assesses 84 outcrop and 12 core samples from Coos Bay using parameters such as TOC, Hydrogen Index (HI), Tmax, and organic petrography to elucidate which formation likely produced the encountered gas. Results were integrated with lithologic observations to relate geochemical changes to depositional environments over time.

Results indicate that the Lower and Upper Coaledo contain organic-rich coal seams and siltstone that are the most prospective gas-prone source rock units, with kerogen sources ranging from terrigenous to marine. The samples are generally immature, potentially suggesting gas migration from depth. Laminated mudstones exhibited higher average TOC and HI values compared to bioturbated mudstones, but HI appears to be more affected. Substantial discrepancies between outcrop and core geochemistry are evident, with S1 core data ~55% higher, and S2 core data ~90% higher. While part of this variance is attributed to weathering, lithology and depositional environments may also play a role. Furthermore, this study discusses the importance of a hierarchical approach to filtering S2 data for the determination of Tmax. This involves evaluating S2 peak morphology using statistical descriptors such as bimodality, skewness, and kurtosis in combination with an S2 > 0.15 mg HC/g rock.

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Please cite this AAPG Bulletin Ahead of Print article as:

Allison K. Barbato , John M. Armentrout , Leslie B. Magoon , Thomas Demchuk , Craig Barrie , Sophie Warny: Organic geochemical characterization of Deltaic Paleogene Rock units in Coos Bay, Oregon: Kerogen type and richness in response to depositional environments, (in press; preliminary version published online Ahead of Print 01 April 2024: AAPG Bulletin, DOI:10.1306/01252423011.