Abstract

The post Early Cretaceous burial history of the Wind River Basin is well known because of a nearly continuous deposition in the basin trough from the Late Cretaceous to the end of the Laramide orogeny in the early Eocene. In the basin trough, Upper Cretaceous Mesaverde strate reach a depth of up to 18,500ft deep, a temperature of at least 200 °C, and over 2.0% mean random vitrinite reflectance (Rv-r). In contrast, along the southwestern margin of the basin, the Mesaverde Formation records several cycles of erosion and deposition during Late Cretaceous to early Eocene time and, as a result, has only reached 0.4 to 0.5% Rv-r.

Coals and coaly organic matter (OM) in the Mesaverde Formation and overlying uppermost Cretaceous to Paleocene strata have commonly generated thermogenic gas in the Wind River Basin. Peak temperatures were apparently produced during peak burial in the late Oligocene to Miocene by an increase in heat flow related to intermittent periods of volcanism during this time. Generally temperatures have now declined due to erosion of about 3,000 feet of rock from the basin area. Rock-Eval and vitrinite reflectance analysis of the Upper Cretaceous to Paleogene coaly source rocks from the Wind River Basin show that intense gas generation commences at 0.4-0.6% Rv-r, peaks near 1.2% Rv-r, and rapidly declines near 2.0% Rv-r. Present depth versus Rv-r data suggest peak gas generation occurs at about 10,000 to 16,000 ft depending on the local thermal maturation gradient.

In the Upper Cretaceous to Paleogene rocks penetrated by wells across the Wind River Basin, the initiation of gas generation becomes successively older from the southwest basin margin northeast to the deep basin trough. Along the southwest basin margin, thermogenic gas was generated mostly in the Miocene. In the deep basin trough, gas generation started mostly in the Eocene.