CORAL REEF FORMATION THEORY MAY APPLY TO OIL, GAS EXPLORATION

A Norwegian geophysicists has proposed a coral reef formation theory that has implications for hydrocarbon exploration.

The theory states that many coral reefs and carbonate buildups form at and are dependent upon nutrient rich fluids seeping through the seabed.

Martin Hovland, senior geophysicists with Den norske stats oljeselskap AS (Statoil), Stavanger, conceived the idea while grappling with the problem of explaining why large, living coral reefs were found in deep and cold (hostile) waters on the continental shelf off mid-Norway.

These cold water coral reefs, mainly consisting of the species Lophelia pertusa, have been known for more than 80 years but are still as enigmatic as when they were discovered.

They also occur in deep and cold waters off northwestern Europe and off the U.S. East Coast. They build cone shaped structures as large as 20 m high and 70 m wide at the base.

Hovland, who has been with Statoil since 1976, provides more lengthy and detailed description and partial documentation of the theory in the January 1990 issue of "Terra Nova," the official journal of the European Union of Geosciences.

SIZING UP CORAL REEFS

Hovland, main author of the book "Seabed Pockmarks and Seepages," had previously studied how and where gases and pore-water seep out through the seabed.

It became clear that the cold water coral reefs off mid-Norway always tend to be located on top of submarine clay ridges where seepage of methane and other minerals is likely to occur.

The cold water, ahermatypic corals are devoid of the algae that are normally found in tropic and shallow water corals.

They are therefore independent of sunlight.

From recent research at deep ocean warm vents it is known that numerous species of bacteria flourish in both the sediments and in the seawater column at seepage sites.

Either the seeping fluids originate in compacting sediments by hydrocarbon generation or by buried volcanic (hydrothermal) activity the associated bacteria-flora represents a readily available organic source that is directly used as food by plankton and other organisms.

The cold water coral reefs are thus situated in the middle of a local "food bowl" and are wholly dependent on the seeping minerals. If this nutrient source for any reason should be shut off, then the whole coral community would perish and become buried as a fossil reef below new sediments.

Based on the fact that seepage of methane and saline water has also been observed through numerous coral reefs and carbonate buildups in among other areas the Gulf of Mexico, the author has expanded his theory not only to explain the location of the Mexican gulf reefs and buildups but also certain coral reefs in the tropics, the problematic and paradoxal organic buildups such as stromatolites (structures that abounded in archaean times and held responsible for providing oxygen to the atmosphere), and carbonate caps, reefs, and atolls on seamounts.

Also, seepage of groundwater off the coastline may explain some of the coastal coral reefs, including barrier reefs.

The new coral reef formation theory could also explain why so many deeply buried (fossil) coral reefs and carbonate banks contain oil and hydrocarbon gases; i.e., why they are such interesting prospecting targets for the oil industry.

According to the new theory the reason is that the buried reef traps and accumulates the very minerals on which they at first were dependent while living on the seabed surface.

When the reefs become buried below layers of sand, silt, and clay the seepage becomes sealed and the hydrocarbons leaking up from below fill the pores in the buried reef body, thus rendering a confined subsurface hydrocarbon accumulation.

CONCLUSIONS

Hovland's article said the following seepage associated reef formation model emerged after combination of geological, geophysical, geochemical, and biological information from well documented reefs and known seepage sites:

Wherever minerals seep out of the seabed there will be a higher than normal density of chemosynthetically based micro-organisms suspended in the water column immediately above the seabed.
Filter feeders, of which most frame- and shell-forming (reef- or bioherm-forming) organisms belong, utilize this local supply of organic particles ("food") and gradually accumulate at the seepage site.
Depending on the nature of the seeping compounds, mainly CH4, CO2, and-or H2S, early digenetic mineral precipitation may also occur, whereby sediment grains and skeletal remains are cemented together to produce a firm and solid substratum.
Over time the reef may have grown extensively, depending on other environmental factors and the intensity of the seep, i.e., the volume of organic particle formation.
If the reef is buried before seep termination, then the seeping fluids will undoubtedly fill up all pore spaces within the buried reef and continue migrating through the capping sediments until a tight seal or cap is produced.
As long as seepage persists, series of new stacked reefs will form on top of and above the old, buried reefs.

The implications of this model are, among others, that buried hydrocarbon bearing carbonate reefs were originally and directly formed by seepage of the very fluids they contain, Hovland said.