Geology of the Lance Pool, Pinedale Field
Thomas Meyer, Stephen R. Kneller, Robert McDermott, Mark Longman, 2014. "Geology of the Lance Pool, Pinedale Field", Pinedale Field: Case Study of a Giant Tight Gas Sandstone Reservoir, Mark W. Longman, Stephen R. Kneller, Thomas S. Meyer, Mark A. Chapin
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The Pinedale field, a giant gas and gas condensate field, is located in the northwest part of the Green River Basin in southwest Wyoming. This field has an estimated ultimate recovery (EUR) of 39 trillion cubic feet of gas equivalent (tcfe), making it the third largest gas field in the United States based on 2009 statistics from the U.S. Energy Information Administration. Additionally, the natural gas condensate production makes it the 49th largest oil field in the same study. Production is principally from overpressured, gas-saturated, tight gas sandstones and siltstones of the Lance Pool comprised of the Upper Cretaceous Upper Mesaverde interval, the Upper Cretaceous Lance Formation and the lowermost part of the lower Tertiary Wagon Wheel Formation.
The presence of natural gas in the Pinedale anticline area has been known for decades. The California Company drilled the first well within the current Pinedale field production limits in 1939 and encountered gas but completion attempts failed. Despite numerous wells drilled over the ensuing years, it was not until the late 1990s that the first commercial production was achieved in Pinedale field. This success was largely the result of improved multistage fracture stimulation techniques, which had been successful in nearby Jonah field’s tight gas, low-permeability Lance Pool sandstones and siltstones.
Pinedale field is a complex accumulation and still has some disagreements as to its true nature. The trapping mechanism within the field has both stratigraphic and structural components and the reservoir has further been shaped in areas by natural fracturing. The field is coincident with the thrust-bounded Pinedale anticline, which is situated approximately 5 mi (8 km) west of the west edge of the Wind River Mountains, another thrust-fault-bounded feature. The Wyoming thrust belt is located approximately 30 mi (48 km) to the west. The town of Pinedale, Wyoming, is at the northeast end of the field. Jonah field, also a giant gas field producing from the interval equivalent to the Lance Pool in Pinedale, is adjacent to and just west of the southern limit of Pinedale field. The absolute field limits of Pinedale are still being defined, but the current field is about 30 mi long (48 km) and up to 5 mi wide (8 km) in places.
Producing rocks in the Lance Pool mainly consist of alluvial plain deposits. The source sediments were Paleozoic and Mesozoic sedimentary rocks eroded from surrounding uplifts and deposited in a rapidly subsiding basin. The Lance Pool hydrocarbon-bearing sandstone, siltstone, and shale column locally exceeds 6000 ft (1829 m) in total thickness. The primary reservoirs in the interval are fluvial channel sandstones deposited by migrating rivers, which flowed generally from northwest to southeast across the depositional area. The resulting reservoir bodies are complex laterally and vertically stacked discontinuous multistory and single-story channel-fill and overbank deposits with as much as 1600 ft (488 m) of net productive sandstone and siltstone in the Lance Pool. Total drill depths range up to 15,000 ft (4572 m) subsurface. Average porosity in these gas-bearing sandstones is about 7% with just 0.005 mD (5 microdarcies) of permeability. The reservoir is overpressured from about 0.57 pounds per square inch per foot (psi/ft) at the top of the Lance Pool gradually increasing to 0.85 psi/ft near the base of the productive interval.
The source of the abundant gas in the Lance Pool at Pinedale field has been the subject of much discussion. The Lance Formation itself contains relatively little organic matter and has never been hotter than the oil-generation window. The top of the dry gas window occurs at a depth of about 15,000 ft (4572 m) in the Rock Springs Formation. The Upper Mesaverde interval has some carbonaceous shale layers and thin coals that were in the wet-gas window and could have generated a relatively small amount of gas. Isotopic analyses of the produced gas in the Lance Pool indicates higher thermal maturity (Ro >2%), which imply a source deeper than the deepest depth penetrated to date by drilling to the upper Hilliard Shale in the field. These potential deep source beds include the lower portions of the upper Cretaceous Hilliard Shale, and the lower Cretaceous Mowry Shale, possibly with a relatively minor contribution from coals in the lower part of the Upper Cretaceous Rock Springs Formation. Hydrocarbon generation and migration into the reservoir rocks of the Lance Pool at rates in excess of the rate of leak-off charged the section and resulted in the overpressured, gas-saturated reservoir.
To date more than 2000 wells have been drilled in the field with cumulative production through the end of 2011 at 3.3 trillion cubic feet (tcf) of natural gas and 25.4 million barrels (MMBLS) of gas condensate.
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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.