Hydrocarbons, hydrogen, and organic acids generation by ball milling and batch incubation of sedimentary rocks

Abstract

Pulverized rock samples are widely used in laboratory experiments, e.g., to assess microbial or abiotic processes in batch incubation tests. However, it is unclear if ball-milled samples accurately reflect in-situ conditions and if observed processes are affected by by-products artificially generated during the sample preparation procedure. As such, this study examined the effects of dry ball milling on the release of gases, which include C₁–C₄ hydrocarbons, carbon dioxide (CO₂), hydrogen (H₂), and low molecular weight organic acids (LMWOAs) from different sedimentary rocks. The experiments involved pulverization using a gas-tight zirconium oxide planetary ball mill followed by wet and dry batch incubation and thermal desorption up to 200 °C. During milling, all sedimentary rocks, except a low organic carbon sandstone, yielded methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), H₂, CO₂, and unsaturated hydrocarbons, e.g., ethene (C₂H₄). Sandstones only yielded H₂, CH_4, and CO₂. Stable carbon isotope signatures of these products are similar to thermogenic gases. The gases were also detected in subsequent wet incubation experiments (after gases from milling had been removed). Additionally, formate, acetate, and citrate, were detected in all samples except for sandstone. Pyruvate, malate, and succinate were also detected in some samples. Thermal desorption products of powdered limestone, shale, and pyrite concretion samples included organic acids, such as acetate and formate, which were found at higher levels in milled samples than in crushed particles (1 mm). The original geological thermal maturity of the studied sedimentary rock samples was low (below “oil window”) and, thus, considerably below “gas window,” suggesting that most of the detected gases were generated during ball milling. H₂ generated during ball milling may be derived from the reduction of water from fluid inclusion or phyllosilicates, involving the formation of reactive mineral surfaces or radicals. Notably, the concentrations of gaseous hydrocarbons derived from milling are relatively high and comparable to “wet” gases. At the same time, these data indicate that substantial amounts of gases and LWMOAs might be artificially generated during laboratory batch experiments with milled samples. Hence, ball milled rock samples must be used with caution if assessing (bio-)geochemical processes for natural environments.