Marine Gas Hydrate: response to change of seafloor temperature, ocean sulfate concentration, and compositional effect
Hirasaki, George J.; Chapman, Walter G.
Doctor of Philosophy
The global inventory of carbon in gas hydrate at present day is comparable to that in oil & coal reserve, therefore, gas hydrate could have played an important role in earth carbon cycle, e.g., during the Paleocene Eocene Thermal Maximum (PETM) event. However, ocean floor temperatures were ~6°C higher than today, so the hydrate abundance under warmer conditions was a question to be clarified. By using numeric simulations, this work showed that gas hydrate abundance is not only affected by ocean floor temperature, but, more essentially, greatly dominated by the organic carbon buried into sediment. During PETM, higher organic carbon contents due to less dissolved oxygen at seafloor and increased methanogenesis rates, both resulted from higher ocean temperatures, enhanced hydrate accumulation. Therefore, though hydrate stability zone would be thinner and shallower than present-day, depending on water depth and sedimentation rate, gas hydrate abundance could be still higher in some marine sediment columns than present-day value. The quantity of carbon stored in marine gas hydrates during PETM may have been similar to that of present-day. The ocean sulfate concentration is as another factor affecting hydrate abundance. From seafloor to sulfate-methane transition (SMT) zone, sulfate consumes a certain portion of organic carbon. Via numerical models, this work proposed and demonstrated that the organic carbon remaining at SMT, should be regarded as the real organic carbon content available for methanogenesis, which contributes to gas hydrate inventory. This work also revealed that lower ocean sulfate is favorable for higher gas hydrate inventory because it consumes less organic carbon in a shallow zone of sediment from seafloor to SMT. By using an example mixed gas system, this work showed that a transition zone which contains both solid hydrates and free gas can span over a thick zone (~300m). The gradual change of seismic impedance across the transition zone diminishes the strength of the Bottom Simulating Reflector (BSR). The results provide a possible mechanism for enigmatic weak-to-absent BSR in prolific hydrocarbon basins across the world.