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.

The New Mexico Bureau of Mines and Mineral Resources (NMBM&MR), with the participation of other individuals, has been involved in a long-term coal quality study. This project was funded by the New Mexico Research and Development Institute (NMRDI) with contributions from several companies. The NMBM&MR has developed a large data set for the San Juan basin coal fields using quality data from this project and data collected from public and private sources through an 11-yr cooperative project with the U.S. Geological Survey (USGS) for entry into the National Coal Resource Data System (NCRDS). The most complete set of quality and thickness data exists for the economically important Fruitland Formation coals. Evaluation of these data suggest that some trends in the attributes of the Fruitland coals exist. The trends appear to support the premise that the characteristics of these coals are a consequence of their depositional environments. These environments were influenced by the relative position of the shoreline and the rate of shoreline movement. The thickness and quantity of the coals and the ash and sulfur content appear to be significantly influenced by their position relative to the shoreline and the rate of shoreline shift. The moisture content and Btu value appear to have been influenced by these same controls, but the degree of coalification of the northern Fruitland Formation coals has also been influenced by the heat from the massive intrusive complexes of the La Plata and San Juan Mountains in southern Colorado.

A series of 33 Late Cretaceous (earliest Campanian through Maastrichtian) paleoshoreline maps was developed to document the migrational evolution of the western edge of the North American Western Interior Seaway. The maps represent a geologic span of roughly 18 million years, and portray the estimated positions of the strandline for each standard Western Interior ammonite zone, beginning with the Clioscaphites choteauensis zone and continuing to the end of the Mesozoic. We attempted correlation of all significant mammal-bearing localities known from the Western Interior with the ammonite-based marine zonation. First approximations of correspondence between ammonite zones and North American Land-Mammal “Ages” (NALMAs) include: Lancian ( Sphenodiscus through “Triceratops” zones); “Edmontonian” (a name not yet faunally defined; Didymoceras cheyennense through Baculites clinolobatus zones); Judithian (the smooth, late form of Baculites sp. through Exiteloceras jenneyi zones); and Aquilan ( Scaphites hippocrepis through Baculites asperiformis zones). Correlations emphasize use of provincial biostratigraphic terminology designed specifically for use in the Western Interior. On the basis of temporal constraints suggested herein, known mammalian fossils from the upper Fruitland and/or lower Kirtland Formations of New Mexico probably are of “Edmontonian,” not Judithian age. Although considerable latitudinally based taxonomic diversification of Judithian mammals is now recognizable across the Western Interior, comparative data are inadequate to defend a similar statement for the remaining Late Cretaceous NALMAs. Quantitative evaluation of geographic patterns of shoreline change suggests occurrence of a general, regional regression of the sea during the entire geologic interval represented in the study. We favor explanation by a slow sea-level depression resulting from topographic evolution of the world’s mid-oceanic system of ridges and volcanic plateaus. Local and subregional asynchronous episodes of shoreline transgressions, stillstands, and regressions are superimposed upon the general regressive trend, and probably represent influences of local tectonism, not eustatic changes in sea level. Strandline evolution of the epeiric sea during the last 20 million years of the Cretaceous in the North American Western Interior is inconsistent with: (1) existence of geologically brief (1 to 10 m.y.) global fluctuations in sea level; and (2) the concept that the late Campanian was represented by an unusually high global sea level.