The Waltman Shale Member of the Fort Union Formation is a thick lacustrine shale deposited in the Wind River basin during Paleocene time. Available geologic and geophysical data suggest that the depositional conditions were relatively constant during deposition. These data also indicate that Lake Waltman was deep, with maximum water depths on the order of 500 ft. Sonic logs indicate that within deeper portions of the basin a velocity inversion is seen in the Waltman Shale Member, where the expected increase in acoustic velocity with depth due to compaction is not observed. An examination of available data reveals that this anomaly does not appear to be a result of either differences in organic matter content or mineralogy. The inversion appears to be most likely the result of overpressuring within the unit. The pyrolysis data indicate that the overpressuring within the unit. The pyrolysis data indicate that the overpressuring is associated with hydrocarbon generation and the inability of the system to effectively expel these hydrocarbons through a massive shale. The proposed relationship among the acoustic inversion, overpressuring, and active hydrocarbon generation is supported by the disappearance of the acoustic anomaly and the anomalous free hydrocarbon content in the vicinity of sandmore » layers in some of the wells examined. These sands allow for the expulsion or drainage of hydrocarbons from the shale and a return to normal pressures and a normal seismic velocity profile.« less