Sandstone Continuity in the Mesaverde Formation, Rulison Field Area, Colorado

Abstract

Analysis of data from an outcrop study of Mesaverde sandstone lenses showed that they could be characterized by length:thickness ratio (normal distribution, mean of 20, and standard deviation of 0.29 log cycle). A reservoir model based on these data yielded results consistent with well tests and production history. Introduction Rulison field produces gas from multiple sandstone reservoirs in the Mesaverde formation of Upper Cretaceous age. The field is believed to contain a large volume of gas but is not commercial because the wells are capable of producing only a relatively small volume of gas each day. The low rates of production are caused by the low permeability and the production are caused by the low permeability and the shape and discontinuity of the individual sandstone bodies. Rulison and other similar fields in the area are not fully developed because the wells will not repay the cost of drilling and completing. Conventional stimulation techniques have not been effective in increasing the productivity of the wells. The field is the site of a joint industry-government experiment to stimulate gas producing rates by fracturing the reservoirs with an underground nuclear explosion. This experiment, known as Project Rulison, is being conducted by Austral Oil Co., Inc., and the U. S. Atomic Energy Commission. The study reported in this paper was part of the preshot investigation of the feasibility of part of the preshot investigation of the feasibility of nuclear stimulation at Rulison field. General Geology The sequence of rocks present at Rulison field, which is located in the Piceance basin (Fig. 1), and their relation to the general stratigraphy of the area are shown on Fig. 2. Rocks ranging in age from Quaternary to Precambrian are thought to be present. The local structure at Rulison field -northeast regional dip - is not important to the gas accumulation. The entrapment of gas is stratigraphically controlled and is primarily a result of the discontinuity of the individual sandstones. The gas deposit is an aggregate of hundreds of separate small accumulations in individual discontinuous sandstone bodies. Mesaverde Formation Stratigraphy The Mesaverde formation as referred to here includes all sediments below the base of the Ohio Creek conglomerate (Paleocene) and above the top of the Mancos shale (Cretaceous). This definition is used by most geologists working in the subsurface of the area. The Mesaverde includes the stratigraphic equivalents of the Lance, Lewis, Williams Fork, and lies formations as they are recognized in outcrops around the Piceance basin. We have made no attempt to identify these formations in this study. The Mesaverde formation in Rulison field ranges in thickness from approximately 3,400 ft in the southwestern portion of the field to 3,900 ft in the northeast. It consists of a sequence of sandstones, shales and coals that were deposited near the western shoreline of a broad, shallow sea that extended north-south across the area. The varied and changing local conditions associated with the flood plain and shoreline during Mesaverde time deposited a complex sequence of rocks. These conditions also generated and preserved a large volume of gas. The gas was apparently formed and trapped at the time of deposition or shortly thereafter. JPT P. 911