Anadarko Woodford Shale: Improving Production by Understanding Lithologies/Mechanical Stratigraphy and Optimizing Completion Design

Craig Caldwell
Cimarex

Since late 2007 over five hundred horizontal Woodford wells have been completed in the Anadarko Woodford play, west-central Okla-homa, along a northwest-southeast trend approximately 100 miles (161 km) in length and 25 miles (40 km) wide. Shallowest produc-tion to date occurs at approximately 8,500 ft. (2,590 m) along the east side of play, and deepest production occurs at 16,100 ft. (4,900 m) along the southwest margin of the play.

Seven mudrock lithologies, defined on the basis of percent quartz, clay, dolomite, and TOC, make up the 15 lithostratigraphic units that compose the Basal, Lower, Middle, and Upper Woodford in the cen-tral part of the Anadarko Woodford play (Canadian Co.). The Lower, Middle, and Upper Woodford in this area are of approximately equal thickness (70 to 100 ft./21 to 30 m). This area of thick total Wood-ford (175 to 330 ft./53 to 100 m) overlies an erosional Hunton thin. By contrast, the Basal Woodford, 10 to 50 ft. (3 to 15 m) thick in the central part of the play, thickens to over 100 ft. (30 m) to the southwest.

The Basal Woodford in the central part of the Anadarko Woodford play is composed of organic-poor clayey mudrock (OPCM) character-ized by 41 to 52% clay, core analysis gas-filled porosities of 1 to 4.5%, and TOC values of generally 0.5 to 4.0%. The Lower and Mid-dle Woodford in the central part of the play are made up of 10 to 30 ft. (3 to 9 m) intervals dominated by one of three lithologies: clayey mudrock (CM) (38% clay and 41% quartz), clayey siliceous mudrock (CSM) (27% clay and 55% quartz), and less common dolomitic clayey mudrock (DCM) (33% clay, 32% quartz, and 15% dolomite). These mudrock lithologies generally have core analysis gas-filled porosities averaging 5.6 to 6.8% and TOC values averaging 5 to 6.5%. The Upper Woodford is predominantly CSM and siliceous mudrock. Sili-ceous mudrocks (SM) average 14.5% clay and 75% quartz. Quartz in these mudrock lithologies is predominantly biogenic but includes silt-size detrital grains. Clay is predominantly illite, and dolomite is generally ferroan. Sedimentary structures, while dominated by paral-lel lamination, include burrows, millimeter-scale scour surfaces, and rare soft-sediment deformation features. SM and CSM display bed-limited dolomite-healed fractures.

In the central part of the play the density/neutron log response has a strong relationship with clay and silica content as determined by XRD. Silica-rich mudrock intervals (SM and CSM) display density/neutron cross-over and are readily distinguishable from clay-rich intervals (CM, DCM and OPCM). Thus, mudrock lithologies and lithostrati-graphic units can be mapped using density/neutron logs.

Marked stratigraphic changes occur southwest of the central part of the play. In this area the Woodford is 175 to 250 ft. (53 to 76 m) thick and composed largely of OPCM of the Basal Woodford and CSM and SM of the Upper Woodford. The intervening Lower and Middle Woodford in this area have thinned to a combined thickness of ap-proximately 20 ft. (6 m). North and east of the central part of the play, where the Woodford thins to between 50 and 125 ft. (15 and 38 m), the Basal Woodford is absent. The Lower and Middle Wood-ford have markedly thinned in these areas, and the Upper Woodford makes up 50 to over 75% of the total Woodford thickness.

"Brittle" silica-rich mudrocks (SM and CSM) display mechanical prop-erties significantly different than those of the more ductile clay-rich mudrocks (CM, DCM, and OPCM). Frac‘ stages in the clay-rich litholo-gies frequently treat at higher pressures and are less likely to place the designed amount of proppant than those stages in the more sil-ica-rich lithologies. Thus, mudrock lithologies and the mechanical/lithostratigraphy model developed here assist in completion design and provide information valuable to the understanding and prediction of regional variations in Woodford production.

AAPG Search and Discovery Article #90184 © AAPG Woodford Shale Forum, Oklahoma City, Oklahoma, April 11, 2013