Biogenic methane and carbon dioxide generation in organic-rich shales from southeastern Brazil

Brazilian shale formations are potential biogenic gas systems that are still poorly understood. Organic-rich shales from Ponta Grossa (Devonian, Paraná Basin), Irati (Permian, Paraná Basin) and Tremembé (Paleogene, Taubaté Basin) Formations were analyzed in order to evaluate their biogenic methane (CH4) generation potential and to constrain environmental and geological conditions that may affect methanogenic metabolism. Additionally, the generation potential of carbon dioxide (CO2) was evaluated in combination with CH4 to constrain Brazilian organic-rich shales as sources of greenhouse gas to the atmosphere. Twenty-four shale samples were used for batch incubations in dry and wet conditions and after headspace refilling and acetic acid addition during one year. Physico-chemical parameters such as Total Organic Carbon (TOC), specific surface area and pore volume were characterized in order to understand general geologic controls on CH4 and CO2 production. Biogenic gas generation rates in dry tests reached up to 3.17 ml/t·d (milliliter per ton of shale per day) for CH4 and 2.45 × 103 ml/t·d for CO2 during the first 30 days, with wet samples producing about 54% more CH4 and 151% more CO2 than dry samples. Headspace refilling did not demonstrate significant changes on production rates, while acetic acid addition restarted or increased CH4 and CO2 production rates in several microcosms. Shales from the Irati Formation showed the highest methane production rates for both dry and wet tests, with unusual ranges of δ13CCH4 and δ13CCO2 that may be linked to hydrocarbon biodegradation. The studied shale formations have different pore volume and specific surface area, but without significant influence over biogenic methane generation.