Abstract

A geochemical study was conducted with rock samples collected from outcrops over and around the actively producing (oil and gas) Little Buffalo Basin field in the southwest Bighorn Basin. Sample lithologies, consisting of calcite-cemented siltstones and sandstones from the Cretaceous Cody Formation, were analyzed for their carbon and oxygen compositions in an effort to detect evidence of vertical hydrocarbon microseepage. This is the first study of its type performed in the Bighorn Basin, and employs a significantly higher sampling density than has been used in previous studies elsewhere.

Although no visible surface geochemical alterations of any type are known to exist at Little Buffalo Basin, this field produces from several horizons. The shallowest production is from a gas zone capped with over 1200 feet of dense calcareous shale.

The results of this investigation show that anomalously low δ¹³C values of calcite are found in surface outcropping samples over the Little Buffalo Basin field. The systematic distribution of these low δ¹³C values is closely correlated with the subsurface production axis of the oil and gas field. In contrast, oxygen isotopic compositions are found generally to slightly increase as the production axis is neared. Isotopic evidence gathered in this investigation indicates that vertical hydrocarbon microseepage occurs and can be detected by judicious surface sampling techniques and the use of stable isotopes even in oil fields capped with thick impermeable sequences of clay.