POSSIBLE CONTROLS ON POROSITY PRESERVATION IN THE ANDAMAN FOREARC GAS HYDRATE SYSTEM: OSR, AOM, AND/OR MARINE SILICATE WEATHERING

In the Andaman forearc gas hydrate system, anomalous porosity (~60-65%) is preserved over a 400 m thick stratigraphic interval composed of biosiliceous carbonate oozes and volcanic ashes deposited between 3.8-9.6 Ma. Gas hydrate exists solely within this interval of preserved porosity. SEM and EDS imaging of the sediments documents the extensive presence of microcrystalline authigenic carbonates. These carbonates were precipitated prior to compaction and consistently through time during early diagenesis to a degree that preserved their porosity without completely cementing them. There are three possible origins for these early-formed authigenic carbonates: (1) formation from excess alkalinity produced during organoclastic sulfate reduction (OSR) (2) formation near the sulfate-methane transition zone (SMTZ) via the anaerobic oxidation of methane (AOM) and/or (3) formation below the SMTZ via marine silicate weathering coupled to methanogenesis. All three of these authigenic carbonate formation pathways result in unique C isotopic signatures and bulk carbonate mineralogy that are distinct from each other. In this presentation, we will discuss the three formation pathways for these authigenic carbonates. The implications of their presence in this stratigraphy and their consistent effect on porosity preservation suggest a possible long-lived, potentially steady state, early diagenetic mechanism that may help preserve porosity in other marine gas and gas hydrate reservoirs globally.