Steve Cumella 1. Search and Discovery Article # (2009) Posted July 30, Abstract

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1 AV Geology of the Piceance Basin Mesaverde Gas Accumulation* Steve Cumella 1 Search and Discovery Article # (2009) Posted July 30, 2009 *Adapted from oral presentation at AAPG Annual Convention, Denver, Colorado, June 7-10, Bill Barrett Corp, Evergreen, CO. Abstract A regionally extensive tight-gas accumulation in the Mesaverde Group of the Piceance Basin is currently being actively developed. Daily production has increased from under 200 MMCFD in the year 2000 to over 1 BCFD currently. Most gas production in the Piceance Basin is from discontinuous fluvial sandstones of the Williams Fork Formation of the Mesaverde Group. In some areas of the southern Piceance Basin, 10-acre well density has proven successful. EURs of typical wells in these areas range from 1 to 2 BCF per well, resulting in reserves of about BCF per section. The depth limits to the commercial gas accumulation are poorly defined, but it is possible that much of the deeper part of the basin may have commercial gas reserves. Within the area of commercial gas production, most gas is produced from a continuously gas-saturated interval in the Williams Fork. Productive intervals can attain gross thicknesses of over 3000 ft (900 m). The gas-saturated interval thins toward the basin margins where the Williams Fork gas reserves become sub-economic. This tremendous gas resource exists because of several important geologic circumstances. Large volumes of gas were generated from thick Mesaverde coals as they achieved high thermal maturity. Migration of this gas was inhibited by the very low permeability and discontinuous nature of the Mesaverde sandstone reservoirs. The rate at which gas was generated and accumulated in the reservoirs outpaced the rate at which gas could escape, resulting in overpressure. Eventually, the pressure of the gas phase in the pore system exceeded the capillary pressure of the water-wet pores, and water was expelled from the pore system, resulting in the development of an overpressured, gas-saturated reservoir with little movable water. References Johnson, R.C., 1989, Geologic history and hydrocarbon potential of Late Cretaceous Geological Survey Bulletin, v E, 51 p. age, low-permeability reservoirs, Piceance basin, western Colorado: U.S. Yurewicz, D.A., 2005, Controls on gas and water distribution, Mesaverde basin center gas play, Piceance Basin, Colorado (extended abstract): Search and Discovery Article #90042 (2005) ( Copyright AAPG. Serial rights given by author. For all other rights contact author directly.

2 Mesaverde Cameo Rollins Mancos Geology of the Piceance Basin Mesaverde Gas Accumulation Steve Cumella Bill Barrett Corporation Presenter s Notes: Piceance is fortunate to have, surrounding the basin, MV outcrop that can be correlated to the subsurface. This photo is of the Grand Hogback which forms the east side of the basin. The canyon is Rifle Gap north of Rifle. The MV is near vertical and the entire MV section is exposed along Rifle Gap. The WF is about 4000 thick here. The well log superimposed is only 3 miles to the SW. The Mancos forms the valley and the Rollins is the thick ss here. The coals in the lower WF commonly burn in outcrop and that s what causes the reddish color. The top of the MV is shown. The thick resistant intervals are amalgamated fluvial ss that appear to have some lateral extent.

3 Outline for Talk Stratigraphic overview of Mesaverde Group Geology of Williams Fork gas accumulation Permeability distribution in Williams Fork Shallower sands have better reservoir quality Open-hole logs identify wet versus gas sands much better than cased-hole logs Low porosity sands not traditionally completed can contribute significant gas volumes

4 T1N-R1E T1N-R1W T1N-R2W T1N-R3W T1S-R1E T1S-R1W T1S-R2E T2N-R2W T2N-R3W T2S-R1E T2S-R1W T2S-R2E T3S-R2E T10S-R100W T10S-R101W T10S-R103W W T10S-R89W T10S-R90W T10S-R93W T10S-R94W T10S-R95W T10S-R96W T10S-R97W T10S-R98W T10S-R99W T11S-R101W T11S-R102W T11S-R103W T11S-R88W T11S-R89W T11S-R90W T11S-R93W T11S-R95W T11S-R96W T11S-R97W T11S-R98W T11S-R99W T12S-R100W T12S-R101W T12S-R102W T12S-R103W T12S-R88W T12S-R89W T12S-R90W T12S-R91W T12S-R93W T12S-R94W T12S-R95W T12S-R96W T12S-R97W T12S-R98W T12S-R99W T13S-R100W T13S-R101W T13S-R102W T13S-R103W T13S-R89W T13S-R90W T13S-R91W T13S-R92W T13S-R93W T13S-R94W T13S-R95W T13S-R96W T13S-R97W T13S-R98W T13S-R99W T1N-R100W T1N-R101W T1N-R102W T1N-R103W 104W T1N-R90W T1N-R91W T1N-R92W T1N-R93W T1N-R94W T1N-R95W T1N-R96W T1N-R97W T1N-R98W T1N-R99W T1S-R100W T1S-R101W T1S-R102W T1S-R103W 04W T1S-R T1S-R89W T1S-R90W T1S-R91W T1S-R92W T1S-R93W T1S-R94W T1S-R95W T1S-R96W T1S-R97W T1S-R98W T1S-R99W T2N-R100W T2N-R101W T2N-R102W T2N-R103W R104W T2N-R92W T2N-R93W T2N-R94W T2N-R95W T2N-R96W T2N-R97W T2N-R98W T2N-R99W T2S-R100W T2S-R101W T2S-R102W T2S-R103W W T2S-R8 T2S-R89W T2S-R90W T2S-R91W T2S-R92W T2S-R93W T2S-R94W T2S-R95W T2S-R96W T2S-R97W T2S-R98W T2S-R99W T3N-R100W T3N-R101W T3N-R102W T3N-R103W N-R104W T3N-R T3N-R89W T3N-R90W T3N-R91W T3N-R92W T3N-R93W T3N-R94W T3N-R95W T3N-R96W T3N-R97W T3N-R98W T3N-R99W T3S-R100W T3S-R101W T3S-R102W T3S-R103W W T3S-R88 T3S-R89W T3S-R90W T3S-R91W T3S-R92W T3S-R93W T3S-R94W T3S-R95W T3S-R96W T3S-R97W T3S-R98W T3S-R99W T4N-R T4N-R89W T4N-R90W T4N R91W T4S-R100W T4S-R101W T4S-R102W T4S-R103W T4S-R88W T4S-R89W T4S-R90W T4S-R91W T4S-R92W T4S-R93W T4S-R94W T4S-R95W T4S-R96W T4S-R97W T4S-R98W T4S-R99W T5S-R100W T5S-R101W T5S-R102W T5S-R103W T5S-R88W T5S-R89.5W T5S-R89W T5S-R90W T5S-R91W T5S-R92W T5S-R93W T5S-R94W T5S-R95W T5S-R96W T5S-R97W T5S-R98W T5S-R99W T6S-R100W T6S-R101W T6S-R102W T6S-R103W S-R104W T6S-R89W T6S-R90W T6S-R91W T6S-R92W T6S-R93W T6S-R94W T6S-R95W T6S-R96W T6S-R97W T6S-R98W T6S-R99W T7S-R100W T7S-R101W T7S-R102W T7S-R103W S-R104W T7S-R89W T7S-R90W T7S-R91W T7S-R92W T7S-R93W T7S-R94W T7S-R95W T7S-R96W T7S-R97W T7S-R98W T7S-R99W T8.5S-R93W T8.5S-R94W T8S-R100W T8S-R101W T8S-R102W T8S-R103W R104W T8S-R89W T8S-R91W T8S-R92W T8S-R93W T8S-R94W T8S-R95W T8S-R96W T8S-R97W T8S-R98W T8S-R99W T9S-R100W T9S-R101W T9S-R102W T9S-R103W W T9S-R T9S-R89W T9S-R91W T9S-R92W T9S-R93W T9S-R94W T9S-R95W T9S-R96W T9S-R97W T9S-R98W T9S-R99W Structure Contours Top Rollins (Johnson, 1989) Isopach Williams Fork Continuous Gas Shows (Yurewicz, 2005) Current Daily Production ~1.3BCF/D Piceance Basin Colorado

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6 Amalgamated Channel Sandstones Discontinuous Williams Fork Sandstone Lenses Rollins Presenter s Notes: This photo is from the southwest part of the basin near Grand Junction. Rollins is the continuous white ss near bottom of photo. Lower part of WF contains low sand content with discontinuous ss. Higher in WF, amalgamated ss are thicker and more laterally continuous. Correlations indicate that the amalgamated ss are the strat equiv of the Upper marine ss.

7 100 ft 300 ft

8 Williams Fork Reservoir Factors Discontinuous Sands and Very Low Permeability Restricted fluid movement Gas Generation and Overpressuring Abundant gas generation from coals creates overpressuring because gas can t migrate out of basin center as fast as it is generated Natural Fracturing Extensive natural fracturing results from widespread overpressured conditions

9 T1N-R1E T1N-R1W T1N-R2W T1N-R3W T1S-R1E T1S-R1W T1S-R2E T2N-R2W T2N-R3W T2S-R1E T2S-R1W T2S-R2E T3S-R2E T10S-R100W T10S-R101W T10S-R103W W T10S-R89W T10S-R90W T10S-R93W T10S-R94W T10S-R95W T10S-R96W T10S-R97W T10S-R98W T10S-R99W T11S-R101W T11S-R102W T11S-R103W T11S-R88W T11S-R89W T11S-R90W T11S-R93W T11S-R95W T11S-R96W T11S-R97W T11S-R98W T11S-R99W T12S-R100W T12S-R101W T12S-R102W T12S-R103W T12S-R88W T12S-R89W T12S-R90W T12S-R91W T12S-R93W T12S-R94W T12S-R95W T12S-R96W T12S-R97W T12S-R98W T12S-R99W T13S-R100W T13S-R101W T13S-R102W T13S-R103W T13S-R89W T13S-R90W T13S-R91W T13S-R92W T13S-R93W T13S-R94W T13S-R95W T13S-R96W T13S-R97W T13S-R98W T13S-R99W T1N-R100W T1N-R101W T1N-R102W T1N-R103W 104W T1N-R90W T1N-R91W T1N-R92W T1N-R93W T1N-R94W T1N-R95W T1N-R96W T1N-R97W T1N-R98W T1N-R99W T1S-R100W T1S-R101W T1S-R102W T1S-R103W 4W T1S-R T1S-R89W T1S-R90W T1S-R91W T1S-R92W T1S-R93W T1S-R94W T1S-R95W T1S-R96W T1S-R97W T1S-R98W T1S-R99W T2N-R100W T2N-R101W T2N-R102W T2N-R103W R104W T2N-R92W T2N-R93W T2N-R94W T2N-R95W T2N-R96W T2N-R97W T2N-R98W T2N-R99W T2S-R100W T2S-R101W T2S-R102W T2S-R103W W T2S-R8 T2S-R89W T2S-R90W T2S-R91W T2S-R92W T2S-R93W T2S-R94W T2S-R95W T2S-R96W T2S-R97W T2S-R98W T2S-R99W T3N-R100W T3N-R101W T3N-R102W T3N-R103W N-R104W T3N-R T3N-R89W T3N-R90W T3N-R91W T3N-R92W T3N-R93W T3N-R94W T3N-R95W T3N-R96W T3N-R97W T3N-R98W T3N-R99W T3S-R100W T3S-R101W T3S-R102W T3S-R103W W T3S-R88 T3S-R89W T3S-R90W T3S-R91W T3S-R92W T3S-R93W T3S-R94W T3S-R95W T3S-R96W T3S-R97W T3S-R98W T3S-R99W T4N-R T4N-R89W T4N-R90W T4N R91W T4S-R100W T4S-R101W T4S-R102W T4S-R103W T4S-R88W T4S-R89W T4S-R90W T4S-R91W T4S-R92W T4S-R93W T4S-R94W T4S-R95W T4S-R96W T4S-R97W T4S-R98W T4S-R99W T5S-R100W T5S-R101W T5S-R102W T5S-R103W T5S-R88W T5S-R89.5W T5S-R89W T5S-R90W T5S-R91W T5S-R92W T5S-R93W T5S-R94W T5S-R95W T5S-R96W T5S-R97W T5S-R98W T5S-R99W T6S-R100W T6S-R101W T6S-R102W T6S-R103W S-R104W T6S-R89W T6S-R90W T6S-R91W T6S-R92W T6S-R93W T6S-R94W T6S-R95W T6S-R96W T6S-R97W T6S-R98W T6S-R99W T7S-R100W T7S-R101W T7S-R102W T7S-R103W S-R104W T7S-R89W T7S-R90W T7S-R91W T7S-R92W T7S-R93W T7S-R94W T7S-R95W T7S-R96W T7S-R97W T7S-R98W T7S-R99W T8.5S-R93W T8.5S-R94W T8S-R100W T8S-R101W T8S-R102W T8S-R103W R104W T8S-R89W T8S-R91W T8S-R92W T8S-R93W T8S-R94W T8S-R95W T8S-R96W T8S-R97W T8S-R98W T8S-R99W T9S-R100W T9S-R101W T9S-R102W T9S-R103W W T9S-R T9S-R89W T9S-R91W T9S-R92W T9S-R93W T9S-R94W T9S-R95W T9S-R96W T9S-R97W T9S-R98W T9S-R99W Structure Contours Top Rollins (Johnson, 1989) Isopach Williams Fork Continuous Gas Shows (Yurewicz, 2005) Piceance Basin Colorado

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11 Vertical Variation of Reservoir Quality in Williams Fork Sands in the lower Williams Fork have microdarcy perm and very little relative permeability to water, resulting in very low water production rates Sands in the upper part of the gas producing interval can have much better reservoir quality; they can be prolific gas producers, but sands with high water saturation can produce very high water rates

12 Shallower Sands Commonly Have Better Permeability GR ANIS FRAC Mesaverde 500 ud ud 4500 Top Gas ud Rollins

13 1,000 ud 0 Neutron-Density Above Top Gas Just Below Top Gas Lower Williams Fork

14 717 MCFD of 2,277 MCFD total, 1.28 BCF proportioned to 4.0 BCF EUR Top Gas Price Coal Top Seal Rollins

15 Emulated Open-Hole Curves from Pulsed Neutron Logs Don t Identify Pay As Well As Open-Hole Logs Numerous examples of CH logs from all logging companies show different log responses compared to OH logs These differences can cause completion of wet zones or skipping of highly productive zones Williams Fork fluvial sands are discontinuous; therefore it may not be possible to correlate a sand in a cased-hole well to an open-hole well, even at 10-acre density

16 CH OH OH CH Presenter s Notes: 32-11C,32-14,32-14D,32-15C

17 OH CH Wet? 346 Presenter s Notes: GM ,

18 Rollins Pay Example Mamm Creek Field OH CH OH

19 Low Porosity Sands Can Contribute Significant Gas Volumes Gas seeps on image logs are commonly seen in very low porosity sands These tight, well cemented sands are highly fracture prone Production logs show good contribution from tight sands that weren t usually completed in the past

20 Image Logs Show Gas Seeping from Thin, Tight Zones That Are Not Usually Considered Pay 25-0% Gas Seeps

21 Production Logs Show Good Production from Thin, Tight Pay

22 Conclusions The Mesaverde interval in the Piceance Basin is a world class gas resource with very little dry-hole risk, currently producing about 1.3 BCF/D Reservoir quality is best in sands in the upper part of the gas-bearing interval of the Williams Fork, but risk of water production is higher Wet versus gas sands can be differentiated much better with open-hole versus cased-hole logs Very low porosity sands not completed in the past can contribute significant gas