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 CalgaryApril 4, 2011

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

(EIA, 2010 http://www.eia.gov/oil_gas/rpd/tight_gas.pdf Retrieved 8/3/2011)

After Koepsell et al., 2003

Piceance Basin

Current Status of Piceance Basin Properties :

Porosity : 8.2% (avg)Matrix Permeability : 250 nanodardyNatural fracture permeability : 36-600 microdarcyWater Saturation : 55%Total OGIP : 311 TCF (106 TCF is from Southern Piceance

Basin i.e. Grand Valley, Parachute, Rulison and Mamm Creek fields)

Its proved 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)

Current Status of Piceance Basin

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

(Yurewicz, 2005)

Gas source850’

Continuously gas saturatedInterval (1700’-2400’)

Two Phase Zone

Stratigraphy

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

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

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

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.

Detection of the Natural Fractures

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.

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.