EXAMINING THE EFFECTS OF SUBSURFACE HYDROGEOCHEMISTRY ON METHANE PRODUCTION IN THE PALEOCENE-EOCENE WILCOX GROUP OF NORTH-CENTRAL LOUISIANA

Determining geochemical trends that indicate methanogenesis is invaluable as the world increases its reliance on natural gas relative to other fossil fuels such as coal. Recent research has identified the Olla oil field, which produces from the Paleocene-Eocene Wilcox Group of north-central Louisiana, USA, as a “hot spot” for microbial methanogenesis due to low-salinity porewaters, large microbial populations, high alkalinities, and δ¹³C-enriched dissolved organic carbon (DIC) and CO₂ (McIntosh et al., 2010, GSA Bull., v.122, p.1248-1264). The “2800 ft sand”, an oil- and gas-producing interval of the Wilcox Group in the Olla field, was flooded in the 1980s with 9.0x10⁷ m³ of CO₂. This research tests the hypothesis that the CO₂ injection is connected to the increased microbial activity by examining the geochemical characteristics of formation waters and gases in sandstone intervals in the Wilcox Group (including the “2800 ft sand”) inside and outside of the Olla field. The goals of this research are to determine if there are factors inhibiting methanogenesis in the formations outside of the Olla field, and/or if there are other hydrogeochemical factors associated with increased microbial activity in the Olla field. The field site spans a 90-km northwest to southeast transect in north-central Louisiana. Twenty-one formation water and gas samples from the Wilcox Group were collected from 6 oil fields in July 2012 to complement previous samples collected in the area in 2006 and 2011. Initial results show that Cl^- concentrations increase downdip from NW (averaging 567 mmol/L in the Crossroads field) to SE (averaging 1896 mmol/L in the Cypress Bayou field). δ¹³C DIC values reach a maximum of +26.0‰ in the Olla field. No apparent trend was observed in δ¹³C DIC moving radially from the Olla field. Alkalinity (up to 57.7 meq/kg, averaging 35.1 meq/kg) is higher within the Olla “2800 ft sand” compared to outside the Olla Field (up to 13.8 meq/kg in the Colgrade Field, averaging 9.2 meq/kg). Major and trace element concentrations and water isotopes (δ¹⁸O H₂O, δ D H₂O, and δ¹³C DIC) of produced fluids, together with gas isotopes (δ¹³C CO₂) will be used to further constrain hydrologic controls on methanogenic conditions.