Abstract Large quantities of natural gas have been produced from underpressured Cretaceous reservoirs of the San Juan Basin since 1951, yet the reasons for the under-pressuring and the containment mechanisms remain a subject of inquiry. In this investigation, compilations of reservoir pressures from the 1950s and early 1960s are used to minimize the perturbations caused by later gas production. The pressures are projected to two basin-scale cross sections showing the structural configuration and stratigraphy of Cretaceous and younger rock units. Gas pressures in the Dakota Sandstone vary according to location, with pressure/depth ratios of 0.36 psi/ft (8.16 kPa/m) in the west and 0.41 psi/ft (9.27 kPa/m) in the east, where pressures approach hydrostatic values. Gas pressures in the sandstones of the Mesaverde Group are remarkably consistent, with pressure/depth ratios of 0.24 psi/ft (5.42 kPa/m), except in the southeast corner of the gas accumulation where the pressure/depth ratio is 0.35 psi/ft (7.91 kPa/m). Pressure-elevation plots, in conjunction with cross sections and measurements of hydraulic head in water wells, show that the gas system is not buoyant in the way that a conventional gas accumulation is buoyant. Underpressuring in this basin reflects the absence of bottom water and the presence of top water. The pressure reference for the gas is at the edge of the gas accumulation instead of at the bottom, and the preproduction gas pressure is determined by the elevation of the lateral transition from downdip gas to updip water on the southwestern limb and other margins of this asymmetric basin. No pressure discontinuity between gas and water exists at the updip edge of the gas accumulation; hence, no seal in the usual sense exists, and there is no need for one. The hard seal of a shale or an evaporite formation is replaced by a capillary soft seal caused by a transition from low-permeability downdip rocks to high-permeability updip rocks. Hydrodynamic trapping, an explanation that has been cited for many years, is not required. Instead, the gas is just sitting in a pancake-shaped volume bounded by a low-permeability base, a gentle stratigraphic rise on one side, and more steeply dipping monoclines on the other three sides. The gas does not escape from the edges of the basin because no excess gas pressure can exist in the absence of an underlying aquifer.

Abstract This five-day field trip examines Pennsylvanian to Jurassic strata in the Paradox Basin on the Colorado Plateau in southeastern Utah. The trip will emphasize four major themes: 1) Permian-Triassic stratigraphy, 2) characteristics of non-marine depositional systems, 3) paleogeography, and 4) paleoclimate. The trip follows depositional facies in the Permian and Triassic section from proximal continental settings near the Ancestral Rocky Mountains in the Uncompahgre Highlands to distal marine settings within the Paradox Basin. Evolution of these depositional systems along the west coast of Pangea from the late Paleozoic to the early Mesozoic is a key indicator of the paleogeographic and paleoclimatic history of the region. Late Paleozoic red beds encountered during the trip provide additional, high-resolution paleoclimate information. The trip will highlight unconformities, and their origin as a result of sea-level change, regional tectonics, or salt diapirism. The Permian-Triassic unconformity, which is expressed worldwide, will be examined during the trip. In southeastern Utah, the Permian-Triassic unconformity influenced the diagenetic history of the White Rim Sandstone, resulting in formation of the Tar Sand Triangle—the largest tar sand deposit in the U.S. Rocks examined in detail during the trip include the Permian Cutler Group (informal lower Cutler beds, Halgaito Formation, Cedar Mesa Sandstone, Organ Rock Formation, White Rim Sandstone, and De Chelly Sandstone), the overlying Lower Triassic Moenkopi Formation, and locally the Upper Triassic Chinle Formation. The trip also passes through the underlying Pennsylvanian section and the overlying Jurassic section, including eolianites exposed in and around Canyonlands National Park. This field guide includes several sections. The introduction and following text sections present short discussions of the major rock units and topics to be presented on the field trip. The field trip section of the field guide begins with a brief list of significant geologic features encountered during the drive across the Rocky Mountains from Denver to Grand Junction. We will not stop at any of the sites in the list. The remainder of the field trip section provides brief descriptions of the individual field stops that will be visited.

Abstract The Middle Jurassic San Rafael Group and Upper Jurassic Morrison Formation of the San Juan basin consist of complexly interrelated conglomerate, sandstone, silt- stone, mudstone, limestone, and gypsum. The San Rafael Group, consisting of the Entrada Sandstone, Wanakah Formation, Cow Springs Sandstone, and sandstone at Mesita, was deposited in eolian, sabkha, minor fluvial, and marine or possibly lacustrine environments. The Morrison Formation, consisting of the Salt Wash, Recapture, Westwater Canyon, and Brushy Basin Members, was deposited in widespread fluvial, lacustrine, and eolian environments. The presence of eolian rocks in both the San Rafael Group and Morrison Formation led to mapping and correlation problems in the southern half of the basin. The chief problem was that a widespread eolian facies of the Recapture Member of the Morrison was considered a part of the eolian Cow Springs Sandstone by earlier workers. Recent work in the southern and western parts of the basin shows that eolian beds of the two units can be distinguished by lateral relationships and sedimentologic features including sorting, sedimentary structures, and crossbed dip-vector resultants. Separation of the two units yields an improved understanding of depositional processes and paleoenvironmental distributions. Problems in this interval still exist in the southeastern part of the basin where the relationship of the San Rafael Group to the Recapture Member of the Morrison remains unclear.

Abstract Approximately 1,800 geophysical logs and 100 measured sections provided data for several types of isopleth maps of the Westwater Canyon and Brushy Basin Members of the Upper Jurassic Morrison Formation in the southern San Juan basin, New Mexico. These types of maps include: isopach, sandstone:mudstone ratio, percent sandstone, net sandstone, and average number of mudstone interbeds per 100 ft (30 m) of section. We also constructed a paleotopographic map on the base of the Westwater Canyon and a structure contour map on the base of the Upper Cretaceous Dakota Sandstone. These maps illustrate depositional unit geometry, sandstone depocenter distribution, and large-scale lithofacies variations within the units. The Westwater Canyon is thinner and less sandy over paleotopographic highs and is thicker and sandier along paleotopographic lows, which suggests active structural control of facies distribution during deposition. Sedimentation of the Brushy Basin Member was also affected by some of the same active structural elements. Detailed reflection seismic studies have defined basement faults that were periodically reactivated since the Precambrian. These faults exerted a significant influence on depositional patterns in the Morrison Formation. Primary uranium ore in the Westwater Canyon +Member is restricted to sandstone depocenters associated with east-southeast-trending isopach thicks. Remnant ore deposits are relict primary deposits that lie in oxidized ground updip from a regional oxidation-reduction (redox) interface. Sedimentologic controls seem to be similar to those for primary ore, and in general these deposits have been preserved from oxidation by stratigraphic variations and by structures. Redistributed ore deposits are also concentrated in the vicinity of isopach thicks, but in rocks with relatively low sand-stoneimudstone ratios. However, the location of redistributed ore is much more closely related to the position of the regional redox interface. The geographic form of this interface was influenced regionally and locally by Laramide structures.