Record Of Ancient Climates Can Be A Map To Riches

Virginia Tech researchers are looking at the rock record from the last 600 million years to study the past history of climate change and to determine where oil and gas may be hidden. J. Fred Read, professor of geological sciences, and Ph.D. student Thomas C. Wynn of Mebane, North Carolina, will present their research at the 36th annual meeting of the Northeastern Section of the Geological Society of America (GSA) in Burlington, Vermont, March 12-14.

Read is looking at the signatures of the ice house and the greenhouse worlds in the stratigraphic record since the Cambrian period. "The earth has gone through three major glacial stages in 600 million years -- about 300 million years apart," he says. The most recent ice age started some 35 million years ago and peaked in the last 700,000 years (the later part of the Pleistocene). "We are actually in a little greenhouse phase of an ice house world now," he says.

He is looking particularly carefully at the greenhouse earth from 540 to 450 million years ago, at the end of the Cambrian through the early Ordovician periods. Because there were no large ice caps at the poles, sea level changes were small; so, millions of years of change can be seen in relatively thin layers of accumulated sediments (cycles) on the continental shelves. Just a few meters of sediment (about 3 to 20 feet) represent about 15 to 20 thousand years. "The sediments built up to sea level, then the sea dropped and left the sediment exposed, such as seen today in the cyanobacteria-covered tidal flats of the Bahamas," Read says. (Cyanobacteria were formerly called '"blue-green algae," but were renamed when it was determined that it is not a plant). These ancient sedimentary cycles are well exposed along Interstates 81 and 77 in Virginia. "We are fairly confident the sea level changes are driven by the earth's wobble on its axis (orbital precession)," he says. "The sea level rising and falling as the continental shelf subsides gives us a continuous record of greenhouse times."

While Read studies the greenhouse period, Wynn is studying the transition to ice house times by looking at chips brought up during drilling of the wells in the 1930s to the 1980s. Compared to the subtle accumulation of deposits during the earth's greenhouse phase, "a big glaciation in the Carboniferous age resulted in 50- to 100-meter changes in sea level, which left sediment layers (cycles) that are five times thicker than during the greenhouse period, and are spaced about 100,000 to 400,000 years apart," Read explains.

The researchers are studying the global climate record left in the ancient soils throughout Virginia, West Virginia, and Kentucky. Wynn is looking at 200 oil and gas wells throughout West Virginia to construct detailed cross sections of the sediment types (some of which are now oil and gas reservoirs) laid down during each rise and fall in sea level. Where as oil companies may distinguish three types of materials -- limestone, sandstone, and shale, Wynn is dividing 2,000 to 4,000 feet of sedimentary succession into 10-foot layers (the interval covered by each bag of the well cuttings), and characterizing the detailed rock types present, he explains. Read adds, "We find potential petroleum reservoirs in the ooid sands layer, of which there is a huge band around the United States. The petroleum migrated into these rocks during the large mountain building event that formed the present Appalachians. The immense oil fields in the Caspian basin are in Carboniferous rock that formed under similar climate and sea level conditions as those are finding in Kentucky and West Virginia, except that the Caspian carbonates are huge isolated platforms like the Bahamas."

In addition, Read’s group is looking at the relatively recent transition from greenhouse conditions to ice house conditions (about 35 million years ago) in sediments beneath the North Carolina coastal plain. "We hope to get a 1,500-foot-deep look at several localities using continuous cores that will be drilled by the U.S. Geological Survey. Read says. "We are part of a mutlidisciplinary team from federal and state surveys and universities that will be looking at fossils, isotopes, and the sedimentary rocks to determine the sedimentary succession and evidence for climate change."

"The sea level changes cause the porous beds to be interlayered with finer, less porous beds, which separate the individual reservoirs with low-permeability rock. These sea level changes during ice house times appear to relate to long- and short-term eccentricity reflecting the cyclical change of the earth's orbit from circular to elliptical every 100,000 and 400,000 years," says Read. "The changes in the shape of the orbit interacting with the ice sheets amplifies the climate change." Thus the rock record tells the story of climate and offers the prize of how oil and gas reservoirs are layered.

Read's paper, "Sequence stratigraphy of Cambro-Ordovician passive margin carbonates, U.S. Appalachians," will be presented at 2:10 p.m. Monday March 12 in the Sheraton Conference Center Diamond Salon I. He will discuss the sea levels, climate controls, and subsidence history that formed the continental shelf of the Appalachians during a time of major greenhouse conditions "It is very different from the modern continental shelf that formed under icehouse conditions. There were beautiful carbonate beaches throughout the Appalachians, just like in the Bahamas." Wynn's paper, "Subsurface high resolution sequence stratigraphy via well-cuttings, Mississippi Carbonates, Appalachian basin," will be presented at 4:30 p.m. Monday, in the Sheraton Conference Center Diamond Salon I. He will discuss a time in the earth's history (the Carboniferous) when there was a change from greenhouse to ice house and the stratigraphic signature of that transition.

Read's group's research has been supported by the National Science Foundation for the last 26 years, and is also sponsored by the American Chemical Society and the petroleum industry.