Paper: The physics of gas chimney and pockmark formation, with implications for assessment of seafloor hazards and gas sequestration, Mar. Pet. Geol. 27, 82–91, 2010.

            Authors: L.M. Cathles, S. Zheng and D. Chen.

 

Summary of paper (By Brandon): This paper models the formation of gas chimneys in order to constrain the origin of pockmarks (with an overall industry point of view). In the model, gas pressure builds up at the base of several sediment layers, and at some P, the overpressure is enough to break the capillary resistance. This allows gas to rise through the sediment, building a chimney structure. Through a mathematical assessment it was found that once this chimney hits the halfway mark between the source of itŐs gas and the surface of the sediment, pockmark development can begin. The propagation of the chimney to the surface is then found to increase. The gas rises through cracks that form (similar to hydrofracture), allowing for quicker gas expulsion, where when the gas approaches the surface, itŐs expulsion lifts sediments and causes pockmark formation. 

 

Main questions that arose during discussion:

            (1) How do people feel about the model?

á            It was a simplistic 2-d model that ignored certain criteria like why the chimney remained open for the entire propagation (instead of collapsing in on itself at the base after the pressure drop), or including differential permeability with increasing sediment layers. DidnŐt have the timing of formation well constrained, but perhaps that allows for application to a variety of systems once specific variables were measured.

á            A long discussion developed around their permeability constraints, and how the two main parameters in their model were the worst constrained. However, it was concluded that most models in earth science are commonly oversimplified, and that the extreme lack of knowledge on the seafloor allows them some slack. It was also decided that if permeability wasnŐt constant in their model, i.e. if it was allowed to increase as you reached the surface (a realistic possibility because of pressure changes), the gas chimney would grow on even quicker timescales.

 

Briefly discussed how you could have a gas and a solid phase of the same composition in the same system, without any conclusions developing. Mentioned how having all the parameters in a table was extremely helpful.

 

(2) How does the capillary system allow for straight upward propagation of this gas rather than a plumose structure? Does this have to do with saturation conditions in the sediment or just an assumption they made in their model?

á            In their model, they didnŐt go into detail as to what controlled the width of the gas symmetry, or really why the structure remained vertical. They lumped it into Ôgeologic controlsŐ, and called it a day. May be a good approximation though, as seismically the gas chimney does look rather vertical.

á            This opened a brief discussion into what controls the spacing and localization of features like gas chimneys. In heterogeneous material, this phenomena seems to exist, so what other variable could be at play? Perhaps something to do with stress fields, evenly spaced cracks (contraction in some cases, perhaps structural in others).

 

Ended with a question focusing on the differences between gas chimneys and hydrothermal vents, where the idea is that not only is it a difference between gas extrusion vs. fluids, but also a matter of flux rate. These gas chimneys appear to be mostly a gas slug moving upward.