AAPG/GSTT HEDBERG CONFERENCE

“Mobile Shale Basins – Genesis, Evolution and Hydrocarbon Systems “

June 4-7, 2006 – Port of Spain, Trinidad & Tobago

 

 

Mud volcanoes – a result of supercritical water formation at depth?

 

Martin Hovland¹, Håkon Rueslåtten², Helge Løseth³, Christine Fichler³, Hans Konrad Johnsen³

 

¹Statoil ASA, Stavanger, Norway

²Numerical Rocks ASA, Trondheim, Norway

³Statoil ASA, Trondheim, Norway

 

 

Geological setting of mud volcanoes

Mud volcanoes occur in deep sedimentary basins located at relict or active plate boundaries. Some pertinent examples are:

-         Chandragup, located on the Makran Accretionary Wedge, Pakistan.

-         Abundant mud volcanoes in Azerbaijan, located on a very deep (>20 km) backarc-related sedimentary basin.

-         Marine mud volcanoes, such as the Campeche Knolls (shale/salt diapirs), in the Gulf of Mexico, located on the flank of a previous spreading system.

-          Large mud volcanoes on the Mid Mediterranean Ridge, located on a subduction/accretionary system.

-          Mud volcanoes on and off Trinidad, located on a transform plate boundary.

 

Mud volcanoes have been studied for more than 100 years. Hedberg (1974) concluded that they were consequences of over-pressure caused by oil and gas generation at depth. A problem with this hypothesis are the difficulties of explaining the formation of some of the products that well up together with hydrocarbons in most mud volcanoes.

 

Products of mud volcanoes

In general, the terrestrial and ocean-bottom mud volcanoes produce three main components: very fine-grained clayey material (‘mud gel’), water of varying chlorinity, and hydrocarbons: both liquids and gases (Brown, 1990; Hovland et al., 1997; Milkov, 2000; Planke et al., 2003).

Planke et al. (2003), found that terrestrial mud volcanoes in Azerbaijan emitted brines with net additions of B, Na, Al, Cr, Fe, Mn, Ni, Cu, Zn, As, Cd, Ba, U, Cl, and Br and a net removal of Ca, Mg, K, and SO₄, compared to seawater. This is despite the Caspian Sea being an intra-continental drainage basin (draining the Volga river), without marine contact, and despite there being any underlying salt deposits in the Azeri/South Caspian sedimentary basin.

Recently, a completely new type of fluid-releasing piercement structure was found in the Campeche Basin, off Yucatan, Mexico: an ‘Asphalt volcano’ (MacDonald et al., 2004). At least two such features (one named ‘Chapopote’) were found at 3,000 m water depth. They occur at the apex of large, deep-rooted vertical salt piercement structures (salt diapirs). The Campeche volcanoes also produce light hydrocarbons, which are detectable on the sea-surface with satellite technology (MacDonald et al., 2004). There is a possible link between inferred hydrothermal processes at the root of the Chapopote salt stock and the venting asphalt on the seafloor (Hovland et al., 2005). It is suggested that the process that produces asphalt material in Chapopote is similar to the process that causes the petroleum venting from terrestrial and marine mud volcanoes. Consequently, these volcanoes can be reckoned in the same family as mud volcanoes.

 

A new formation hypothesis

Due to the lack of a convincing and unifying model for the formation of the World’s numerous terrestrial and marine mud volcanoes, we are currently examining the possible role of supercritical water formed at depth in sedimentary basins. From observations of deep-sea hydrothermal vents, it is known that ‘phase separation’ occurs (Bischoff and Rosenbauer, 1989). ‘Phase separation’ is just another term for ‘supercritical water’, which forms at elevated temperatures and pressures. For seawater, the supercritical point is around Tc=405^oC, and Pc=300 bars (equivalent to a seawater hydrostatic pressure of 2,800 m water depth).

In deep sedimentary basins at depths beyond 10 km, it is conceivable that porewaters can locally (and temporarily) achieve temperatures of 400^oC or more. At pressures above 300 bars, the water will become supercritical, with all the ramifications involved. When the pressure is too great for water to boil (> 221 bars for pure water, and > 300 bars for normal seawater), it attains the supercritical state, which is neither vapor nor liquid, but something in between. The density of water changes rapidly with both temperature and pressure, and is intermediate between that of liquid water (1 g/cm³) and low-pressure water vapor (<0.001 g/cm³). At supercritical conditions, the density is approximately 0.3 g/cm³ (Tester et al., 1993; Bellissent-Funel, 2001). Similarly, the ionic dissociation constant falls from 10^-14 at ambient conditoins to 10^-23 in the supercritical state.  Furthermore, Raman spectra of deuterated water in the supercritical region show only a small residual amount of hydrogen bonding. As a result, the supercritical water acts essentially as a non-polar, dense fluid, and its solvation properties resemble those of low-polarity organic fluids (Tester et al., 1993). Supercritical water can therefore be regarded as a non-polar fluid which is able to dissolve organic liquids (oils), but unable to dissolve common sea salts (Armellini and Tester, 1991). There are also strong indications that supercritical water is corrosive to silicate rocks (e.g., as seen from the precipitation of silica at venting sites on the sea floor), a property which is of particular importance when studying deep hydrothermal systems and alteration of rocks and sediments.

From scientific drilling performed by the Ocean Drilling Program (ODP), it is also known that hydrothermal systems continue to be active even long after they have been covered by sediments. This has been demonstrated in the Guaymas Basin, off Mexico/California, and at Middle Valley at the sediment-covered Juan de Fuca Spreading Ridge, NE Pacific Ocean. By virtue of these main properties, we suggest that supercritical water actually causes mud volcanoes to form, and as such, represents the ‘motor’ of creating these features, including asphalt volcanoes.

Besides the high pressure, a temperature of at least 405^oC is needed to form supercritical water. We believe this is achievable locally in deep sedimentary basins (at depths beyond 10 km), e.g., by local high heat-flows close to the basement due to the migration of hot fluids from active hydrothermal systems. It is suggested that such supercritical point-sources at depth would migrate upwards due to buoyancy.

We envisage that all the products: gases, liquids (oil and water/brines), and a slurry of mud (with some clasts from the sidewall-rocks of the conduit) start their long transit to surface in a slurry, driven by supercritical water. However, upon cooling and decompression inside the conduit, the water will enter into a two-phase state (with boiling). In this situation we will also have a vapor-driven process, until it condenses upon further adiabatic and conductive cooling (Fig. 1).

Other compounds, e.g., some hydrocarbons, will also condense to liquid states, upon the upward migration. It is also expected that inorganic components dissolved in the water and amorphous material in the slurry will precipitate and/or crystallize during the ascent to surface. Such processes can explain the migration of all the products associated with mud volcanoes, and thus represent a unifying model for this type of natural phenomenon. Although very few terrestrial mud volcanoes are known to have an elevated temperature signature, some of the marine ones have. Thus, recent extreme heat-flow and temperature values were measured on the Vodyanitskiy mud volcanoes in the Black Sea supports our new formation model (Poort et al., 2005).

 

References

Armellini, F.J., Tester, J.W., 1991. Experimental methods for studying salt nucleation and growth from supercritical water. Journal of supercritical fluids 4, 254-264.

 

Bellissent-Funel, M.-C., 2001. Structure of supercritical water. Journal Molecular Liquids 90, 313-322. 

 

Bischoff, J.L. and Rosenbauer, R.J., 1989. Salinity variations in submarine hydrothermal systems by layered double-diffusive convection. Journal of Geology 97, p. 613-623.

 

Brown, K.M., 1990. The nature and hydrogeologic significance of mud diapirs and diatremes for accretionary systems. Journal of Geophys. Res. 95, 8969–8982.

 

Hedberg, H. D., 1974. Relation of methane generation to undercompacted shales, shale diapirs and mud volcanoes. AAPG Bull. 58, 661-673.

 

Hovland, M., Hill, A., Stokes, D., 1997. The structure and geomorphology of the Dashgil mud volcano, Azerbaijan. Geomorphology 21, 1-15.

 

Hovland, M., MacDonald, I.R., Rueslåtten, H., Johnsen, H.K., Naehr, T., Bohrmann, G., 2005. Was the Chapopote asphalt-volcano, Gulf of Mexico, generated by supercritical water? Accepted for publication in EOS, American Geophysical Union.

 

MacDonald, I.R., Bohrmann, G., Escobar, E., Abegg, F., Blanchon, P., Blinova, V., Brückmann, W., Drews, M., Eisenhauer, A., Han, X., Heeschen, K., Meier, F., Mortera, C., Naehr, T., Orcutt, B., Bernard, B., Brooks, J., de Faragó, M., 2004. Asphalt volcanism and chemosynthetic life in the Campeche Knolls, Gulf of Mexico. Science 304, 999-1002.

 

Milkov, A.V., 2000. Worldwide distribution of submarine mud volcanoes and associated gas hydrates. Marine Geol. 167, 29-42.

 

Planke, S., Svensen, H., Hovland, M., Banks, D.A., Jamtveit, B., 2003. Mud and fluid migration in active mud volcanoes in Azerbaijan. Geo-Marine Letters 23, 258-268.

 

Poort, J.,Kutas, R., Vassilev, A., Klerkx, J., 2005. Heat flow variability in the seep dominated nothern margin of the Black Sea. (Abstract), Proceedings of the VIII International Conference Gas in Marine Sediments, Univeristy of Vigo, Spain, Sept. 5-10, 2005.

 

Tester, J., Holgate, H.R., Armellini, F.J., Webley, P.A., Killilea, W.R., Hong, G.T., Berner.H.E., 1993. Supercritical water oxidation technology. In: Emerging technologies in hazardous waste management III, American Chemical Society, 35-76.

 

 

 

 

 

 

 

 

Fig. 1

A conceptual diagram, showing the most important aspects of our new mud volcano formation model.