Sandstone formation begins with sediment that turns to rock during diagenesis. Diagenesis defines the chemical and physical changes the sediment undergoes during burial, including cementation ("sediment glue") and compaction. "We want to understand what controls sandstone diagenesis," Reed said, "how it turns into sandstone."
Reed, along with professors Kenneth Eriksson and Michal Kowalewski, is evaluating three potential controls on sandstone diagenesis, including initial grain composition, past climate, and the environment of sediment deposition. They sampled Pennsylvanian (time period approximately 300 million years ago) sandstones from the Appalachian basin with known framework grain compositions, climatic histories, and depositional environments to compare and contrast diagenesis from the controlled sample set.
"Sandstone is made predominantly of quartz grains," Reed said. "We are looking at how quartz and an additional grain type—the lithic grain or rock fragment—influence how the sediment eventually turned to sandstone." They also are looking at how the climate, whether humid or arid, affects diagenesis. Finally, they are studying how depositional environments such as marine or stream settings compare in their influence on diagenesis.
After collecting and quantifying data from 70 rock samples under the microscope, Reed analyzed the results using statistical techniques. He and his colleagues found that upper Pennsylvanian samples are predominantly lithic arenites and lower Pennsylvanian samples are mostly quartz rich. Quartz was the most significant burial cement in terms of volume.
Considering the effects of climate, various cements, and environment, they found that quartz cement, most abundant in the lower Pennsylvanian quartz-rich sandstones, had less compaction-related loss of void space that could harbor fluids than the more lithic, upper-Pennsylvanian sandstone. "In view of the volume of quartz cement, the most likely source of silica-rich fluids is deeply buried brines mobilized by thrust loading associated with the building of the Appalachian mountains 250 million years ago," Reed said.
The reason for the work is that sandstone is one of the most important reservoir rocks exploited, Reed said. "It holds fluids—hydrocarbons such as oil and gas or water," he said. "If we can understand what happened during sandstone diagenesis, we can understand how the reservoir is built. If we can understand this, can we say something about the water or petroleum systems? Can we exploit this for gas or oil or water?"
For example, sediment with space between it is an excellent place for finding oil or gas and is a good reservoir, he said. If the space between the sediment is filled with cement during diagenesis, then the sandstone has no place to store oil or gas.
Reed’s work will help scientists understand how climate, environment, and grain control sandstone diagenesis in order to determine whether a particular sandstone could be a possible source of petroleum, natural gas, or water.
Reed, from Parkersburg, W.Va., will present his findings in "Aspects of Pennsylvanian Sandstone Diagenesis, Central Appalachian Basin: Qualitative and Quantitative Analysis" at 1:40 p.m. April 3 during the meetings of the North-Central and Southeastern Sections of the Geological Society of America in Lexington, Ky. The meeting will be at the Hyatt Regency Hotel.
Jason Reed can be reached at 540/231-8162 or jareed@vt.edu.
PR CONTACT: Sally Harris (540) 231-6759 slharris@vt.edu