ABSTRACT Calcite twinning analysis across the central, unbuttressed portion of the Sevier thin-skin thrust belt, using Cambrian–Cretaceous limestones ( n = 121) and synorogenic calcite veins ( n = 31), records a complex strain history for the Sevier belt, Idaho and Wyoming, USA. Plots of fabric types (layer-parallel shortening, layer-normal shortening, etc.), shortening and extension axes for the Paris thrust (west, oldest, n = 11), Meade thrust ( n = 46), Crawford thrust ( n = 15), Absaroka thrust ( n = 55), Darby thrust ( n = 13), Lander Peak klippe ( n = 5), eastern Prospect thrust ( n = 6), and distal Cretaceous foreland ( n = 3) reveal a W-E layer-parallel shortening strain only in the Prospect thrust and distal foreland. Calcite twinning strains in all western, internal thrust sheets are complex mixes of layer-parallel (LPS), layer-normal (LNS), and non-plane strains in limestones and synorogenic calcite veins. This complex strain fabric is best interpreted as the result of oblique convergence to the west and repeated eastward overthrusting by the Paris thrust.

ABSTRACT The results of new detrital zircon analyses of 15 ( n = 1334) Sevier belt synorogenic (Jurassic–Eocene) conglomerates combined with U-Pb zircon ages from the literature ( n = 2638) support the structurally dynamic role of the western Paris thrust sheet as the dominant high-standing, out-of-sequence portion of the Sevier belt. This result requires modification of the traditional structural view of the thin-skinned Sevier fold-and-thrust belt having formed by west-to-east shortening over an ~100-m.y. period (ca. 150–50 Ma) with episodic thrust motions that become younger toward the craton (east), as constrained by numerous synorogenic deposits shed to the east from each thrust hanging wall. Sevier thrusting was preceded by deposition of the Jurassic Stump Formation, which has a maximum depositional age of 149 Ma and a unique detrital zircon and heavy mineral (garnet, magnetite) provenance. The oldest thrust, the Paris (Willard) thrust, eroded and deposited the Jurassic–Cretaceous Ephraim Conglomerate as a synorogenic fan devoid of quartzite clasts and with a detrital zircon provenance consistent with reworked sediment from the fold belt, but not from the hinterland or the Sierra Nevada arc of the orogenic system. All subsequent synorogenic deposits from the mid-Cretaceous Echo Conglomerate (Meade-Crawford thrust) to a variety of more easterly Eocene deposits (Sevier belt, Green River, Absaroka, and Bighorn basins) are rich in quartzite clasts. All the quartzite clasts were eroded from the Paris thrust hanging wall, which reached its peak orogenic height at ca. 95 Ma, 50 m.y. after first motion, and the Proterozoic Brigham Group remained a quartzite clast source for ~40 m.y. The detrital zircon signatures of these samples require additional sources of sediment, reworked from the hinterland and the Sierra Nevada and Idaho Batholith arcs, thus implying that long-distance sediment fairway(s) were active during the Mesozoic–early Cenozoic. Based on the same detrital zircon data, variable sources of sediment are inferred between each of the thrust sheets; however, within each thrust system, the source of sediment remained the same. The Teton Range was thrust up at ca. 50 Ma, long after the Sevier belt formed, and it was not a buttress to thin-skinned Sevier deformation. Rather, Teton–Gros Ventre–Wind River Laramide uplifts deformed the older Sevier belt with numerous back and out-of-sequence thrusts and synorogenic deposits, including the Darby thrust, which records the youngest displacement.

Abstract Improved geologic insights combined with advances in technology and innovative thinking, mainly since the laste 1990s, have driven Pinedale field’s development and unlocked a giant natural gas resource in stacked low-permeability fluvial sandstones. Understanding this field can provide a model for developing similar tight sandstone reservoirs around the world. This memoir contains 15 well-illustrated, peer reviewed chapters that describe the history of field development, the deposition and diagenesis of the reservoir rocks, geophysical characteristics of the field, special core analysis techniques used to better quantify the reservoir, petrophysical characteristics and interpretations of the reservoir, the types and abundance of natural fractures, and fluid production characteristics in the field. Finally, static and dynamic models for the field are presented in an attempt to integrate all the pieces of this giant geologic puzzle.

Abstract A synthesis of low-temperature thermochronologic results throughout the Laramide foreland illustrates that samples from wellbores in Laramide basins record either (1) detrital Laramide or older cooling ages in the upper ~1 km (0.62 mi) of the wellbore, with younger ages at greater depths as temperatures increase; or (2) Neogene cooling ages. Surface samples from Laramide ranges typically record either Laramide or older cooling ages. It is apparent that for any particular area the complexity of the cooling history, and hence the tectonic history interpreted from the cooling history, increases as the number of studies or the area covered by a study increases. Most Laramide ranges probably experienced a complex tectono-thermal evolution. Deriving a regional timing sequence for the evolution of the Laramide basins and ranges is still elusive, although a compilation of low-temperature thermochronology data from ranges in the Laramide foreland suggests a younging of the ranges to the south and southwest. Studies of subsurface samples from Laramide basins have, in some cases, been integrated with and used to constrain results from basin burial-history modeling. Current exploration for unconventional shale-oil or shale-gas plays in the Rocky Mountains has renewed interest in thermal and burial history modeling as an aid in evaluating thermal maturity and understanding petroleum systems.This paper suggests that low-temperature thermochronometers are underutilized tools that can provide additional constraints to burial-history modeling and source rock evaluation in the Rocky Mountain region.

Abstract Detachment folds basinward of Laramide Rocky Mountain arches are relatively poorly known, partially due to coverage by synorogenic strata that may conceal undiscovered anticlinal fields. This study documents the geometry and kinematics of the Beaver Creek Detachment system (BCD), which is located west of a series of NW-trending thrust faults and folds defining the Beaver Creek reentrant on the western edge of the Bighorn Arch. Possible origins for this proposed detachment include syn-Laramide detachment rooted in mountain-front faulting, syn-Laramide gravity slinding during mountain-front folding, and post-Laramide gravity sliding.

Abstract The following information was gathered from various sources and released for publication. Additional information exists among the many operators in Jonah field, but much of that data is considered proprietary. Data on the drilling and completion of individual wells can be found in Appendix A on the CD-ROM included with this volume. Dean DuBois of EnCana Oil and Gas (U.S.A.) Inc. reviewed and revised some of the data.

Abstract The discovery of a giant natural gas field within a mature petroleum province is a significant event. Understanding the factors that control such an accumulation is important if the oil and gas industry is to continue to develop natural gas resources. Jonah field, in the Greater Green River basin of southwest Wyoming, is the largest natural gas discovery in the onshore United States in the last 10-15 years with recoverable reserves ranging from 8 to 15 tcf natural gas. Since beginning widespread field development in August 1992, Jonah has produced approximately 1 tcf gas, 10.3 million barrels of oil, and 3.7 million barrels of water. Field production is still increasing with daily production presently at 666 MMCFGPD, 5800 BOPD, and 4000 BWPD from approximately 600 wells. Active drilling continues within the field as operators consider widespread downspacing. By virtue of being a tight-gas field, Jonah is, in many respects, nontraditional. Recent assessments of natural gas potential, for both the U.S. and the world, strongly suggest that most future gas resources will come from low-permeability sandstones in the deeper portions of sedimentary basins, and from fields that will undoubtedly share characteristics with Jonah. The subtle structure, the low-permeability nature of the reservoir, the challenging petrophysics, and the environmental sensitivity surrounding Jonah may foreshadow what explorationists have to look forward to as the demand for natural gas increases, not only in the United States, but throughout the world. This volume brings together previously unpublished material on Jonah field and attempts to integrate all aspects including geology, geophysics, reservoir engineering, drilling and completion, and regulatory affairs. As such, this is a definitive collection that provides a truly integrated perspective of this giant field.