1. Tight Sandstones of the Williams Fork Formation,
Mesaverde Group in Southern Piceance Basin,
Colorado
By : Yulini Arediningsih
GLGY 699 – Unconventional Gas
Reservoir Characterization and Evaluation
University of Calgary
April 4, 2011

2. Objectives
 To overview key features of tight sandstone
reservoirs of the Williams Fork formation,
Mesaverde Group in Southern Piceance Basin
including :
• Reservoir characteristics
• Fluid distribution in the reservoir and its controls
 To outline detection techniques of the natural
fractures related to basement fault features

3. Piceance
Basin
After Koepsell et al., 2003
(EIA, 2010 http://www.eia.gov/oil_gas/rpd/tight_gas.pdf

4. Current Status of Piceance Basin
Current Status of Piceance Basin
 Properties :
Porosity : 8.2% (avg)
Matrix Permeability : 250 nanodardy
Natural fracture permeability : 36-600 microdarcy
Water Saturation : 55%
Total OGIP : 311 TCF (106 TCF is from Southern Piceance
Basin i.e. Grand Valley, Parachute, Rulison and
Mamm Creek fields)
 Itsproved reserves is ranked 5th in the top 50 US gas fields (2009)
 Produced from a deep basin centered gas accumulation, in
particular from thick discontinuous lenticular fluvial sandstones of the
Williams Fork Formation, Mesaverde Group.
 Intensive resource development has increased total daily gas
production from < 200 MMCFD (2000) to > 1 BCFD
 EURs : 1 to 2 BCF/well  about 60-120 BCF/section
Reference : Kuuskraa and Prestridge (1996)  Cumella and Scheevel (2005)  EIA (2009)

5. Geology
Structures
 Common fold
structures,
 Closely associated with
the occurrence of
enhanced natural
fracture network
 NW – SE trending
 Have significantly
affected and
contributed to natural
fracture system in the
reservoir
Tremain and Tyler, 1997

6. Stratigraphy
Two Phase
Zone
Continuously
gas saturated
Interval
(1700’-2400’)
Gas source
850’
(Yurewicz, 2005)

7. Key Features of the Reservoirs
 GAS STORAGE : Continuously gas saturated tight sandstones
of the William Fork formation
 CONDUITS : Naturally occurring fractures
Abundant natural fractures in sandstones of the Iles formation and
upper part of the Williams Fork formation.
The fractures largely control fluid migration within the formation.
 Characterized by lower spacing and lower vertical and lateral extent
 GAS SOURCE : Cameo Coal zone at >6000ft depth, achieved
high thermal maturity
 PROCESSES AND CONDITIONS :
Pervasive natural fracturing results from extensive over pressuring
conditions in Cameo Coals due to huge volume gas
 A transition zone of mixed gas – water saturated sandstones above
the continuously gas-saturated interval.
 Restricted low fluid mobility because of low permeability sandstones

8. Water and gas distribution within the William Fork formation in Piceance Basin (After Yurewicz, 2005)

9. Detection of the Natural Fractures
Methods :
Integration of geologic, geophysical, and numerical basin modeling that can
cost effectively locate fractured areas where advanced seismic methods
can subsequently be applied.
Hi-Res aeromagnetic data + 2-D seismic
remote sensing imagery
to delineate the geometry of the to provide consistency
basement structure. to the interpretation
Linked with a numerical basin model
A prognostic fracture mapping model

10. Detection of the Natural Fractures
A prognostic fracture mapping model
Calibrated with local and
regional integrated fracture data
Selected sites for the 3-D multi-azimuth
P-wave reflection survey
The 3D seismic survey :
 To target subsurface fracture sets especially in southern part of the basin
 Target depth is 4000 to 7000 ft.
 To evaluate the P-wave azimuthal anisotropy (which affects AVO, velocity,
frequencies) and to determine relative fracture density and orientation.
 Production and well tests will verify the presence of open fracture sets and
thereby validate the integrated exploration approach.

11. Results and Conclusions
 Key reservoir features in the Piceance basin : thick, matured
Cameo coals, naturally occurring fractures due to over pressured
condition and gas-saturated reservoir with little movable water.
 The naturally occurring fractures control the mobility and
distribution of the fluid within the reservoir
 Fractured production trends : NW-SE, parallel to the faults of
overlying deep basement
 Numerous basement faults have been located indicating many
undrilled fracture prone areas are present in the basin.
 A 3-D survey provides a powerful technology for identifying
structural features that provide the essential fractured
permeability pathways necessary for commercial production
from low permeability reservoirs.

12. Selected References
Kuuskraa, V.A., and Prestridge, A.L. : Advanced Technologies for Producing Massively
Stacked Lenticular Sands, SPE 35630. This paper wav prepared for presentation at the
Gas Technology Conference held in Calgary, Alberta, Canada 28 April – 1 May 1996.
Cumella, S. and Scheevel, J. : Geology and Mechanics of the Basin-Centered Gas
Accumulation, Piceance Basin, Colorado, An extended abstract, adapted from AAPG
Hedberg Conference, April 24-29, 2005, Vail, Colorado.
EIA, 2009, Top 100 U.S. Oil & Gas Fields By 2009 Proved Reserves, US Energy Information
Administration.
Tremain, Carol M. and Tyler, R. 1997. Cleat, fracture, and stress patterns in the Piceance
Basin, Colorado: Controls on coalbed methane producibility. Rocky Mountain
Association of Geologists, Fractured Reservoirs: Characterizations and Modeling
Guidebook.
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)
Cumella, S., and Ostby, D., 2003, Geology of the Basin-Centered Gas Accumulation, Piceance
Basin, Colorado: Rocky Mountain Association of Geologists, Chapter 10, 171-193.