Results of Deep Drilling on the Pinedale Anticline
Mark Longman, Denis E. Foley, Joel Scoville, 2014. "Results of Deep Drilling on the Pinedale Anticline", 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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Pinedale field produces gas from the nearly 6000-ft (1800 m) thick “Lance Pool” composed mainly of fluvial sandstones in the Lance Formation and in the uppermost Mesaverde Group above the Ericson Sandstone. Below this thick gas-productive interval is another 10,000 ft (3000 m) of sedimentary section that has so far proved noncommercial but which has been penetrated by only three deep wells on the 35-mi-long (56 km), 2- to 3-mi-wide (3–5 km) anticline. All three of these deep wells yielded abundant gas shows from intervals in the Ericson, Rock Springs, and Hilliard Formations in the hanging wall of the thrust-fault that formed the Pinedale anticline. Considering the size of the anticline, the abundance of gas shows, and the major hydrocarbon source potential of the Cretaceous Mowry and Hilliard Shales now at depths below 18,000 ft (5500 m) on the anticline, it seems likely that it is only a matter of time before the proper confluence of improved understanding of the potential reservoir zones, improved completion techniques, and higher gas prices leads to commercial development of one or more of these deeply buried reservoirs.
The three wells that are the focus of this study are the Wagon Wheel #1 drilled in 1969 and 1970 by El Paso Natural Gas, the Stewart Point 15-29 drilled by Questar (now QEP) in 2004 and 2005, and the Mesa 10D-33 drilled by Ultra Petroleum from 2006 to 2008. In the Wagon Wheel #1, the liner below intermediate casing at 12,086 ft (3684 m) was accidently filled with cement and no completion of the deeper intervals below the Lance Pool could be attempted. In the Stewart Point 15-29 well, reservoir pressures were so high following fracture stimulation of three stages in the uppermost Hilliard and lower Blair from 18,541 to 19,435 ft (5651–5924 m) that rock fragments were produced from the well bore, plugging it. When this plug of debris was eventually cleaned out months later, both reservoir pressure and production declined rapidly over a period of days. Additional fracture stages were then added up hole in the Rock Springs Formation, but these also had limited reservoir capacity, causing production to decline quite rapidly. This well was then completed in the Lance Pool and produced nearly 7 BCF of gas and 63,000 barrels of condensate between October 2006 and September 2013.
Ultra’s Mesa 10D-33 well has the only current production from the Rock Springs, Blair, and Hilliard Formations. This well produced about 264 MMCFG and 57,500 barrels of water from June 2008 to December 2013 from a gross interval of 15,410 to 19,400 ft (4697–5913 m) and was still producing at a rate of 14 to 130 MCFGPD. The reservoir rocks include shaly and silty intervals in the Hilliard and very fine-grained marine and fluvial sandstones in the Blair and Rock Springs Formations.
Extensive source rock analyses of cuttings from the SP 15-29 well including vitrinite reflectance work shows that the top of the oil generation window (Ro = 0.6%) occurs in the Eocene Wasatch Formation at a depth of about 2600 ft (790 m), that the top of the wet gas window (Ro = 1.0%) occurs near the top of the upper Mesaverde interval at about 12,500 ft (3810 m), and that the top of the dry gas window (Ro = 1.4%) occurs in the Upper Rock Springs interval at a depth of about 15,000 ft (4570 m). In contrast, gas isotope analyses of both mud gas samples and head gas from cuttings isojars shows that the gas in the Lance Pool is dominantly dry gas with an equivalent vitrinite reflectance thermal maturity of 2 to 2.5%. This is even higher than the maximum vitrinite reflectance recorded in the deepest sample of the Hilliard Shale from 19,520 ft (5950 m) where the measured Ro was 1.8%. This suggests that the gas in the Lance Pool migrated upward through more than 10,000 ft (3000 m) of sedimentary section from source beds in the Hilliard and possibly Mowry Shales at depths below 20,000 ft (6000 m). Significant amounts of isotopically distinct dry gas were also generated in the coal beds in the lower Rock Springs interval from 16,200 to 18,477 ft (4938–5632 m), but this gas mostly stayed within that interval rather than migrating into the overlying sandstone reservoirs in the upper Rock Springs interval and Lance Pool.