Abstract

In order to understand the characteristics and origin of the tight gas sands in the Mesaverde Group, a stratigraphic and sedimentologic framework must first be constructed. Because the Mesaverde Group consists of over 5,000 feet of multiple, stacked wedges of siliciclastic sediments deposited in a variety of nonmarine to marine environments, it was decided to focus our initial studies on one formation, the Almond.

For the most part, the Almond was deposited during the early stages of the T4 transgression of Weimer. In the area of the Rock Springs Uplift, it has been informally divided into upper and lower members. Whereas the lower Almond is dominantly nonmarine to brackish, the upper Almond is dominantly estuarine to open marine. This upper and lower member terminology has been carried into the subsurface, but is based more on lithofacies than chronostratigraphic equivalency. What is generally termied "upper" Almond consists of a series of landward-stepping strandline sandstones separated by transgressive disconformities, which stratigraphically rise and become younger from east to west. Historically, the best production is from these "upper" Almond reservoirs. On the Rock Springs Uplift and in the western Washakie Basin, the "upper" Almond sandstones appear to have been deposited within an inlet-dominated barrierisland system. The "upper" Almond sands in the Dripping Rock field, however, were deposited within a storm-dominated environment. Both regional tectonics and localized movement along basement block faults appear to have influenced Almond deposition.