COMPARING AND CONTRASTING NATURAL CARBON DIOXIDE ACCUMULATIONS IN SOUTHERN CALIFORNIA

The USGS is investigating domestic natural accumulations of carbon dioxide (CO₂). This work addresses natural reservoirs as analogs for long-term storage of anthropogenic CO₂ and natural CO₂ supply for enhanced oil recovery that may displace nearby anthropogenic supplies. This study focuses on two differing examples of accumulations in southern California. Four gas samples were collected from mud volcanos at Davis-Schrimpf Field (DSF), which is part of the Salton Sea geothermal system. In this system quaternary magmatic intrusions induce both contact metamorphism with sediments and hydrothermal reactions. Another gas sample was collected in Los Angeles at La Brea Tar Pits (Seep 61/67), which overlie Salt Lake Field in the northern Los Angeles Basin. The field, discovered in 1902, produces oil from folds and fault blocks in the late Miocene and Pliocene Repetto and Puente Formations. Both CO₂ accumulations are leaking gas that is likely flowing through a fracture network or faults. While leakage of CO₂ is a risk in anthropogenic storage, gas escaping to the surface may provide geochemistry results along with insight into its migration pathway and timing.

Gases sampled for this study were analyzed for both CO₂ concentration and the carbon isotope ratio δ13C (CO₂), as possible source indicators. CO₂ gas concentrations at DSF ranged from 93.4 to 98.4 percent and concentration at La Brea reached 20.8 percent with the remaining gas being mostly methane. The ratio for δ13C (CO₂) at DSF ranged from -3.8 to -3.3 per mil and the ratio at La Brea reached 26.3 per mil. Based on these results, the rift system may host geothermal or volcanic sources of CO₂, whereas the petroliferous sedimentary basin may host a source of CO₂ enriched through biodegradation or methanogenesis. The volcanic origin of CO₂ occurs through degassing of magmatic bodies, though this process may be complicated by decarbonation reactions in the geothermal system. The origin of CO₂ due to biodegradation typically occurs in mature oil and gas fields, and is not necessarily due to exploration practices. In contrast to preliminary results from the Santa Maria and Ventura Basins presented last year, less overlap of δ13C (CO₂) ratios exits in this study, making for less complicated identification of the CO₂ source in these accumulations.