Who Hexed Hoodoo Hills? A Case History of a Subthrust Mountain-front Test, Wind River Basin, Wyoming
Charles A. Hinson
El Paso Production, Houston, Texas
Hoodoo Hills was an 18,600’ test of a large structural prospect located beneath the Owl Creek Thrust, which forms the northern structural boundary of the Wind River Basin located in central Wyoming. The Hoodoo Hills structure is analogous to Madden Field, a structural culmination located six miles east of Hoodoo Hills. Both features are comprised of large folds formed by rollover into deep seated thrust faults. Prospective reservoirs were the tight gas sands of the Upper Cretaceous section, particularly the Shannon sands, which have produced over 250 BCF in Madden Field.
The prospect concept was based on the idea of testing the Shannon Formation in a more structurally favorable location than the Moncrief Lysite Unit 12-1. The 12-1 well was drilled on the flank of the Hoodoo Hills structure, several thousand feet down dip from the Hoodoo Hills location, and it had a stabilized flow of 2 MMCFPD @ 1,000 psi from the 1st Shannon sand without benefit of hydraulic fracturing. Log porosity within the reservoir section ranged from 8–12%, similar to the equivalent section which is productive in Madden Field.
A structural interpretation integrating both geology and 2D seismic data was performed in order to determine whether the Hoodoo Hills structure formed a four-way closure similar to Madden Field or if the crest of the structure had been removed by the overriding Owl Creek Thrust. Conventional 2D seismic data were unable to image the structure beneath the Owl Creek Thrust so PSDM (Pre-Stack Depth Migration) processing was performed. The PSDM data not only imaged the deep structure beneath the Owl Creek Thrust, but the data also confirmed that the Hoodoo Hills structure had indeed been truncated by the emplacement of the Owl Creek Thrust.
The additional structural information from the PSDM data was incorporated into the geologic model of the prospect. The excellent gas show in the Moncrief well was subsequently interpreted to be the down dip extension of a cross fault trapped gas accumulation. The trapping mechanism was interpreted to be cross fault “smear” of Cretaceous shale beneath the Owl Creek thrust plane. The balanced structural cross section, shown in Figure 1, was constructed along the PSDM seismic line traverse and it illustrates the concept and interpreted trapping configuration. On the basis of this model the Hoodoo Hills well was drilled.
As predicted, the well encountered the Shannon sands beneath a small thrust fault in the footwall of the Owl Creek Thrust. The geologic section above the small thrust was barren of shows and devoid of pore pressure changes, but numerous good gas shows and a 0.2 lb/gal mud cut were encountered within the Shannon sands immediately below the thrust (Figure 2). Porosity analysis of log data (neutron porosity) and sidewall core data within the prospective reservoir section was not encouraging (5-6% porosity). It was decided to deepen the well in order to evaluate the sands of the Frontier Formation.
Good shows were encountered in the Frontier sands and indicated as much as 9% neutron porosity within one sand. However, analysis of three sidewall cores within the same sand indicated poor porosity development (1-4%). Both the Shannon and Frontier sands were hydraulically fractured, and extensively flow tested, but only noncommercial quantities of gas were produced.
The balanced cross section of Figure 2 demonstrates that the basic structural concept of the prospect was valid. Gas charged Shannon sands were encountered in a structurally favorable location beneath the Owl Creek Thrust four thousand feet high to the Moncrief 12-1 show well. In addition, the thrusted footwall section between the overriding Owl Creek Thrust and the underlying Shannon sands appears to have provided an effective seal. However, the structure and the lithology encountered in the hanging wall of the Owl Creek Thrust was a surprise.
The mystery surrounding the prospect remains unresolved. Why did the Shannon sands, completed with modern fracture technology in the Hoodoo Hills well, test so much poorer than the same sands in a well 4,000’ structurally lower having no fracture treatment? What caused the reservoir properties to degrade so rapidly between two wells barely a mile apart? Who hexed Hoodoo Hills remains an unanswered question.