SOURCE ROCK CHARACTERIZATION AND THERMAL MATURITY EVALUATION OF EOCENE-MIOCENE SHELF MARGIN DEPOSITIONAL UNITS IN COOS BAY, OREGON

Coastal depositional environments, and their downslope depositional facies, have been identified around the world as sites where effective hydrocarbon source, reservoir, and seal rocks are deposited. Due to its world-class, exceptionally preserved coastal-depositional units and thick organic-rich strata, the Middle Eocene Coaledo Delta has been identified in Coos Bay, Oregon as a potential site for the generation and accumulation of hydrocarbons. This basin lies adjacent to the Tyee Basin, which has produced natural gas from Eocene aged coal and carbonaceous mudstone, making Coos Bay Eocene strata excellent candidates for further exploration. Non-commercial quantities of oil and gas were previously identified in the Lower and pre-Coaledo units in 1980, and again in the early 2000’s. Viable source rocks in Coos Bay have already been recognized, yet the full extent of their hydrocarbon potential remains unknown. This research provides new insight into the source rock characteristics of 78 outcrop and 10 well-core mudstone samples from Eocene to early Miocene rock units in Coos Bay Basin for their total organic carbon content (LECO TOC wt.%), their thermal maturity characteristics (measured and calculated %R_o, and T_max), and places their hydrocarbon potential in context of regional exploration and production.

Organic geochemistry results indicate that the pre-Coaledo Beds of Sacchi Beach, Lower Coaledo, Middle Coaledo and Upper Coaledo formations all contain samples with TOC values between 1-5 wt.%, with some samples in the well-core reaching as high as 48 wt.%. However, to be considered a viable source rock, the rocks containing ample TOC wt.% must also exhibit adequate thermal exposure (%R_o ≥ 0.6). Samples from those four intervals have a thermal maturity value of %R_o ≥ 0.6 – 0.7, placing them just within the oil generation window. Results from directly measured and calculated %R_o values suggest the majority of samples are thermally immature, which supports previous thermal maturation studies completed in 1980. However, previous reports were unable to identify samples which exhibited a TOC >1%, paired with %R_o values > 0.6, making these findings a novel discovery for the area. This research is therefore a meaningful update on the source rock characterization and hydrocarbon potential of the Coos Bay Basin.