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 that 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 total organic carbon (TOC), hydrogen index (HI), thermal maturity (T_max), 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 exhibit 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 free hydrocarbon (S₁) core data ∼55% higher and remaining generative potential (S₂) core data ∼90% higher. Although 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 T_max. 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.