Researchers Use Tiny Bubbles To Determine Formation Temperatures For 300 Million Year Old Rocks

If Jason Reed, a geological sciences student from of Parkersburg, W.Va., can define the thermal history of Carboniferous (300-million-year-old) quartz sandstones from the Appalachian plateau, then he will be the first to have done so using fluid inclusions.

The research will be presented during the 113th annual meeting Geological Society of American meeting in Boston Nov. 1-10, 2001.

Reed's study focuses on quartz cement. He is using a technique called "fluid inclusion micro thermometry" to determine the formation temperature for quartz cement. This cement, which holds the sandstone together, formed under heat and pressure. However, it is not entirely solid. Tiny, rare pockets in the cement contain fluids trapped millions of years ago. "These fluid inclusions are being used to determine the temperature at which the quartz cement formed – temperatures greater than 140 degrees centigrade (284 *F)," says Reed.

Fluid inclusions record a snapshot of the sandstone’s history, analogous to a dragonfly in amber from a prehistoric time. The cement-bounded fluid may have begun as drops of ancient ocean or river water, or be leftover from an underground reservoir that formed as the sandstone was buried. As the rocks continued to sink, compress, and heat up, the fluid evolved. The sources of the fluids remain a major scientific problem, says Reed, but he has been able to estimate the salinity and chemical compositions of the fluids using fluid inclusion techniques.

"If I know the temperature, I can use microscopic techniques to help constrain when in the past 300 million years the cements formed," he says. "The timing is important. A number of things can happen to sediment as it turns to rock. I want to know how quartz fits into the history of the formation of the rock. It's scientifically interesting because we gain an understanding of how certain Carboniferous sandstones formed, and because I'm looking at these sandstones in a way that hasn't been done before."

The predominantly quartz sandstones being studied occur within the central Appalachian basin. Reed is studying samples from depths ranging from 170 to 400 meters (more than 1,000 feet) although they have been buried much deeper in the past (more than three miles). "The thermal history of the rocks also allows us to determine the maturity of organic material at maximum burial," he says for the sake of those who are wondering about sources of petroleum. "The deeper the material is, the higher the temperature. Because oil and natural gas can only be formed at certain temperatures, if you can determine the temperature, you can say something about oil and gas formation."

An understanding of quartz cement formation provided by fluid inclusions will also tell him about rock porosity. "With low porosity, there is no room for oil and natural gas, or for water reservoirs, for that matter."

Reed's poster at the GSA meeting explains the fluid inclusion micro thermometry process and his early findings. The poster, "Fluid inclusions in quartz overgrowths: Implications for paleothermometry and quartz authigenesis, Pennsylvanian sandstones, central Appalachian basin" (Dis 25519), will be on display in Hynes Convention Center Hall D from 1:30 to 5:30 p.m., Monday, Nov. 5, 2001. Authors are Reed and Virginia Tech geological sciences professors Kenneth Eriksson and Robert Bodnar.