LAWRENCE — Knowing the thermal history of rock layers can help geologists locate faults, oil reservoirs and other subsurface features. To test and advance a dating technique used to gauge when and how fast sedimentary rocks were heated during and after deposition, the Kansas Geological Survey at the University of Kansas has been awarded a $110,000 grant from the American Chemical Society’s Petroleum Research Fund.
Sedimentary rocks, including limestone and shale, are ubiquitous in Kansas, and the presence of oil and other hydrocarbons in sedimentary rocks is influenced, in part, by their thermal history. Structural and physical changes to the rocks also can be tied to the degree and rate at which temperatures within the earth rise and fall over time.
“Establishing the thermal history of an area is very important for determining the timing and magnitude of a host of geologic events, including faulting and the alteration of geologic formations and reservoirs,” said Tandis Bidgoli, KGS geologist and lead investigator on the project.
Being able to pinpoint when and how geologic events occurred, in turn, leads to a better understanding of such processes as the migration of hydrocarbons and the formation of impermeable rock layers that trap fluids and gases underground.
Scientists obtain information about the thermal history of some types of rock using the dating technique called (U-Th)/He thermochronometry. As part of the process, the amount of helium (He), uranium (U) and thorium (Th) in certain minerals within a rock is analyzed to determine the rock’s age. Because helium accumulates steadily over time due to the radioactive decay of uranium and thorium, the more helium found, the older the rock.
Although the dating technique has worked well in other places, in Kansas it has been of limited use because the state’s common marine limestones and shales generally lack the relevant minerals. Recently, however, a new approach has been proposed that would rely on a different source of helium, uranium and thorium. That source — a microscopic tooth-like fossil known as a conodont — is abundant in Kansas. As part of their project, the KGS researchers will be testing the viability of using conodonts to date rocks and help establish thermal histories.
The microfossils for the KGS study are from rock cores recovered from two boreholes drilled in western Kansas and from rocks in the Beaver Dam and Mormon mountains of eastern Nevada and western Utah.
“The geology and thermal histories of these sites are well-studied and well-characterized, making them ideally suited for the evaluation of this dating technique,” Bidgoli said.
The American Chemical Society, which has 157,000 members, is a nonprofit organization that supports scientific inquiry in the field of chemistry.