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William B. Hanson,1 Victor Vega,2 Dennis Cox2
1Consulatant and Independent Geologist, Vernonia, Oregon, U.S.A.
2BP America Production Company, Houston, Texas, U.S.A.
The work discussed in this chapter was completed while the senior author was employed by BP America Production Company in the capacity of senior geological associate. We thank BP America for permission to publish this work and for the privilege to work the Jonah asset for the past 3.5 yr (1999–2002). Ira Pasternack, geologist, friend, and colleague, introduced the senior author to the Bois marker and Lance stratigraphy at Jonah field and supported a solid handoff of the Jonah asset after the merger of Amoco and BP in early 1999. His dedication to the project is greatly appreciated. Paul Boissevain, after whom the Bois marker is named, is acknowledged for the early 3-D seismic work at Jonah and for preparing data showing that the Jonah faults may originate from the basement. Greg Partyka applied immensely skillful use of geophysical technology to the Jonah asset during 1999–2000. We thank Chris Christiansen for prolific petrophysical work, keen intellect and computing skills, and for many fervent conversations. Terry Young, BP geologist, is thanked for reading the manuscript draft and for being a talented, cordial, and dedicated colleague on the Jonah–South Pinedale anticline team. Appreciation is extended to Mike Mellor and Mai Nguyen of BP, who worked continuously and skillfully to organize records and enter data into the workstation environment during the fast pace of Jonah development. S. M. Hansen of Schlumberger is thanked for interpreting sedimentary structures with the FMI logs. We thank Dean DuBois and John Robinson for helpful and cordial conversations over the past few years. Keith Shanley is thanked for his astute interpretation of Jonah cores, probing conversations, and for reviewing this chapter. This manuscript benefited from a rigorous and challenging review by Donald S. Stone, and we appreciate his considerable effort to improve it.
Jonah field, Wyoming's second largest gas producer, is a structurally controlled trap located in the northwestern part of the Green River basin. Gas and condensate are produced from innumerable latest Cretaceous and early Tertiary overpressured tight-gas sandstones at depths of 7300–12,800 ft (2200–3900 m). Jonah field is remarkable for many reasons, including the large per-well reserves (relative to other tight-gas reservoirs), hundreds of feet of net pay, and a gross producing interval as great as 4000 ft (1220 m) thick. These superlative production characteristics exist, despite the fact that the structural trap is subtle and locally cryptic.
Advanced seismic techniques define the Jonah trap boundaries and add value when they are used to position wells in proximity to the faults and on subtle structures. One of the main seismic techniques is a three-dimensional, broadband, amplitude-based coherency algorithm that has edge-detection capabilities. This algorithm analyzes the reflector amplitude gradient and records the lateral change in amplitude with azimuthal angle, which allows the interpreter to illuminate a particular feature from the optimal angle to reveal the maximum detail in the data.
Gas entrapment at Jonah field is enabled by two bounding faults. Fault throw is variable but commonly less than 200 ft (60 m), and the major faults are nearly vertical. Two field-bounding fault zones, the west fault and south Jonah fault, intersect updip toward the southwest to create the overall wedge-shaped trap. The updip edge of a tilted fault block underlies the prolific Stud Horse Butte anticline and the Cabrito nose trends. The principal in-field faults terminate at the south Jonah fault to form four compartments, each comprised of a northeast-plunging, faulted nose or homocline bounded on the west and south by faults. The Jonah faults juxtapose high- and low-reserve wells; high-reserve wells are concentrated on the east side of the north- and northeast-trending faults, regardless of their sense of displacement.
The south Jonah fault is probably a left-lateral, wrench-fault zone. The south Jonah fault was active concurrently with Lance Formation deposition, resulting in thicker Lance north of the fault in the center of the field and thin along the updip edge of the tilted block. Subsequent post-Paleocene motion on the fault caused formation of the Stud Horse Butte anticline, which is evident in the basal Tertiary strata.
Jonah is a multipay field with numerous productive lenticular, fluvial sandstones. Whereas local structural features control the Jonah trap, regional structural elements shaped the fluvial system that deposited the reservoir sandstones. Regional structural features also affected the burial history that resulted in petroleum generation. Gas isotope composition and the thermal maturity of the producing strata indicate that the field produces hydrocarbons formed in deeper strata. The gas field shows evidence of paleostructural growth, which, in combination with the production pattern along the faults and the bottoms-up origin of the gas, may explain the enigmatic charging of these low-permeability sandstone bodies according to structural position, i.e., segregation by buoyancy. In this scenario, gas emplacement occurred before the reduction of sandstone permeability to the present condition.
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