Tens Of Billions Of Additional Barrels Of Oil Remain To Be Tapped Miles Below Gulf Of Mexico, Cornell Geologist Says

These untapped oil and gas reserves can be found by matching hydrocarbon chemical signatures with geologic models for stratigraphic layers under the sea floor, says Lawrence M. Cathles, a professor of chemical geology at Cornell in Ithaca, N.Y.

"The undiscovered gas and oil potential of the Gulf of Mexico is very large," says Cathles. "We have produced only a small fraction, and the deep-water potential for finding more there is big. In terms of potential, it is bigger than the North Sea. It's about a big a deal as there is."

Cathles will present his findings in a talk, "Massive Hydrocarbon Venting with Minor, Constantly Replenished (Flow-Through) Retention in a 100 x 200 km Area Offshore Louisiana Gulf of Mexico," at the 225th national meeting of the American Chemical Society in New Orleans at 1:30 p.m. CST on March 27.

The northern Gulf of Mexico basin is one of the world's most active areas of hydrocarbon exploration. A study of an area of about 9,500 square miles, found that hydrocarbons currently are being naturally generated from strata deposited during the Tertiary and Jurassic periods, miles below the sea floor. Hydrocarbons are leaking through natural vents at hundreds of locations, and these vent sites have been visited and studied by Cathles and other researchers using small submarines. What makes this area offshore of Louisiana important is the presence of two types of hydrocarbon deep below the gulf floor: the deeper, early-maturing Jurassic and the later-maturing Tertiary. Each has a distinctive chemistry. As these sources mature, the hydrocarbons migrate upward toward the surface through what can be thought of as a myriad of small streams and ponds, much like a natural water system. Just how much liquid hydrocarbon is retained within this subsurface network is a matter of crucial interest, Cathles says.

More than 70 percent of the hydrocarbons that have been naturally generated have made their way upward through the vast network of streams and ponds and vented into the ocean. The hydrocarbons are digested by bacteria, which then become food for the gulf's marine life. The earlier-generated, sulfur-rich, carbonate-sourced Jurassic hydrocarbons are replaced by the shallower, later-generated, shale-sourced Tertiary hydrocarbons which fill the producing reservoirs in the northern part of the study area. This displacement of Jurassic by Tertiary oil provides geologists with a measure of the remaining untapped oil and gas below the gulf's floor.

The hydrocarbons hidden within the subsurface ponds and streams are about 8 to 10 percent of the Gulf of Mexico's total hydrocarbons. In the study area this represents about 60 billion barrels of oil and 370 trillion cubic feet of gas and is the hydrocarbon that could be extracted, Cathles says. (The remaining hydrocarbons, about 20 percent, stay stored in the source strata.)

Cathles says that the telltale chemistry of the hydrocarbons reflects the streams and ponds through which they migrated, and thus could point to the ponds that remain to be discovered and produced. Ultimately he hopes that looking at the hydrocarbon chemistry in this new way could provide geologists with accurate information on the presence and size of the deeper reservoirs. He says: "By combining chemical data from currently producing reservoirs with seismic images of the subsurface using computer migration models, drilling for new deep reservoirs can be facilitated."

Funding for the research was provided by the Gas Research Institute in a joint project with the Woods Hole Oceanographic Institution.