Are seep carbonates quantitative proxies of CH4 leakage? Modeling the influence of sulfate reduction and anaerobic oxidation of methane on pH and carbonate precipitation

Abstract

Seep carbonates tell us where and when CH4-charged fluids escaped from the subsurface, thus providing qualitative information to reconstruct the activity of petroleum systems. The potential of seep carbonates as quantitative proxies for the amount of CH4 leaked, however, remains largely unexplored, which limit their applicability as exploration tools. This paper tackles the quantification of the CH4 flux - seep carbonate relationship by simulating the coupled sedimentary carbon (C) – sulfur (S) cycles in a reaction-transport modeling (RTM) framework. We first establish a theoretical basis demonstrating that the stoichiometry of diagenetic reactions and the ambient pH of pore waters are the main drivers of the rate of change in the saturation state of carbonate minerals (ΩCal), while the concentrations of total dissolved inorganic carbon and sulfide are only of secondary importance. It results that anaerobic oxidation of methane (AOM) is the main driver of carbonate precipitation, while organoclastic sulfate reduction (SR) has a minor impact. We further show that SR mostly drives carbonate dissolution, but can also contribute to precipitation when pH is low (<7–7.1). The RTM simulations reveal that an increase in upward fluid flow triggers an intensification of peak AOM rates, associated to a shallowing and thinning of the zone of carbonate precipitation. Such behavior leads to an almost linear relationship between the amount of carbonate precipitated and flux of CH4 (nCH4 = 3.3–5.2 * nCaCO3), until, eventually, full cementation occurs. We thus define a “quantitative domain” at moderate fluid flow and a “threshold domain” at high fluid velocities, where full cementation solely provides a lower bound estimate of the amount of CH4 leaked. We also show that in contrast to a traditional view of seep carbonate formation mainly controlled by venting activity, sedimentation rate and water depth also play major roles, via their control on residence time and saturation concentration of CH4, respectively. The interpretation of vertical seep carbonate stacks should thus not solely focus on changes in fluid flow, but also consider changes in sedimentation rate and/or water depth.