MS

thesisFrom the time of its discovery in 1962 the San Arroyo the Field in Grand County, Utah, has been a source of oil and gas from productive intervals in the Cretaceous Cedar Mountain, Dakota, and Mancos Shale Formations, and in the Late Jurassic Entrada and Morrison Formations. At present, natural gas is the chief commodity produced from reservoirs along the San Arroyo anticline which is the dominant structure in the field. The productive sandstones are elongate along the regional structural grain and most of them occupy the crestal part of the anticline. Stratigraphic complexity due to eolian, fluvial, and transitional sedimen­tary deposition common in beds of the region, precludes full structural control of the accumulations. The reservoirs are controlled by the local stratigraphy, structure, and morpho­logy of the sandstone bodies within the section. The Arroyo fault, a new geologic feature disclosed by detailed structural mapping, traverses the northern part of the field from southeast to northwest running along the north flank of the San Arroyo anticline. This fault is important as a structural linkage between the San Arroyo Field and the adjacent Prairie Canyon Field in Garfield County, Colorado. A slight tectonic event, deduced from the particular distribution of the Buckhorn Conglomerate, is believed to have taken place regionally at the beginning of the Cretaceous this event influenced the orientation of the stream channels along a regional structural grain. It is concluded that: a) a preferential accumulation of gas exists along the length of the sandstone bodies; b) the Arroyo fault conforms to the regional structural pattern and is a potential controlling feature for probable reservoirs at the north border of the San Arroyo Field; c) the difference in quality between the gas produced from the Entrada Sandstone and that produced from the Dakota Sandstone indicates two different sources for the gas contained in these formations