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BookmarkScienceDaily (Apr. 6, 2001) — (Blacksburg, Va., April 6, 2001) -- Jason Reed, a doctoral student in geological sciences at Virginia Tech, has determined that impervious cementing minerals deposited millions of years ago surround porous compartments in thick sandstone now more than 1,000 feet underground. Water, gas, or petroleum can be trapped within the porous compartments.
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Reed will present his research at the 50th annual meeting of the Southeastern Section of the Geological Society of America (GSA) on April 5-6 in Raleigh, N.C.
Reed, who is from Parkersburg, W.Va., is studying core drilled by the U.S. Geological Survey and the West Virginia Geological and Economic Survey collected from coal fields near Beckley, WV He explains that, approximately 300 million years ago -- during the Carboniferous period, loose sediment built up in ancient rivers followed by tidal environments during sea level rise, until the deposit was many feet thick. The material compacted into sandstone -- a porous rock. In the process of being buried thousands of feet beneath the surface of the earth, the sandstone was cemented by various agents.
"We are looking at a particular kind of cementing agent -- carbonate minerals, which we found isolated within distinctive zones that may extend laterally for many kilometers," says Reed. He reports finding iron-rich and iron-magnesium-calcium carbonates. "The iron-rich carbonates appear to have formed early in the burial history. Later, as the sandstone was buried more deeply, higher concentrations of iron as well as magnesium and calcium were introduced into the system." The element abundance and distribution in these carbonate cements reflect different formation conditions. The composition of the early iron-rich samples indicate that marine fluids influenced their formation during sea level rise. The cements that formed during deeper burial were likely affected by chemical reactions involving clay minerals and organic matter located in surrounding shale.
The sandstones involved in this study have up to 10 percent open space. The cementing minerals fill in the openings to form horizontal barriers to the flow of liquid or gas, leaving 20 to 50 meters (60 to 150 feet) of uncemented sandstone that may contain liquids or gas. "Such sub-surface compartments can persist for many square miles," says Reed.
"Knowledge of compartmentalized sandstone is important to petroleum, natural gas, or ground water exploration," says Reed. "If we understand the geometry of the compartments, we have an opportunity to predict fluid migration into and out of reservoirs. If we can discover something about the process, perhaps we can apply it on a global scale."
The sandstone layer Reed is studying is now 1,100 feet deep. He is also trying to determine the original depth before erosion. "It would be deeper in northwest West Virginia, for instance," he says.
