Yates Formation (Late Guadalupian) Facies Tracts, and Depositional and Diagenetic Models Permian Basin

H. S. Nance
John A. and Katherine G. Jackson School of Geosciences The University of Texas at Austin

The Yates Formation in the study area is composed of carbonates, evaporites, and siliciclastics deposited in middle- and inner-shelf environments under conditions of regional aridity. Middle shelf carbonate facies comprise grainstone to wackestone deposited in subtidal, lagoonal, and tidal flat settings during sea-level highstands. Siliciclastic facies comprise clayey siltstone to finegrained sandstone deposited in lagoons, on tidal flats, and on arid alluvial plains during sea level regressions and lowstands. Siliciclastics deposited during lowstands were reworked during transgressions. Sandstone is the primary hydrocarbon (oil) reservoir facies in middle-shelf successions.

Inner-shelf carbonate facies are mainly algal-laminated dolomite wackestone to packstone. Evaporite facies comprise anhydrite (originally gypsum) and halite that was precipitated in regionally extensive brine pans during marine transgressions. Siliciclastic facies comprise halite-mudrock, muddy sandstone, and fine- to very fine sandstone deposited during regressions and partly reworked during transgressions. The coarser sandstones are very well sorted with halite-occluded porosity. Other cements in sandstones include dolomite and anhydrite. Muddy sandstone is the primary hydrocarbon (gas) reservoir facies in inner-shelf successions.

Geophysical well-logs can be interpreted to distinguish carbonates from siliciclastics in middle-shelf successions. Relatively siliciclastic-free carbonates typically show conspicuously low radioactivity, whereas siliciclastics are more radioactive. Geophysical well-logs can be interpreted to distinguish evaporites from siliciclastics in inner-shelf successions. Relatively siliciclasticfree evaporites typically show conspicuously very low radioactivity, whereas siliciclastics are increasingly more radioactive with increasing clay and feldspar content. Bulk-density and neutron porositylogs can be used to discriminate anhydrite from halite whereby anhydrite shows very high density and negative porosity values while halite shows very low density (approximately 2.16) and zero porosity. Mixtures of halite and siliciclastics show increasing radioactivity and density as abundances of siliciclastics increase. Halite-occluded sandstone produces a density-neutron log crossover pattern that may be mistaken as an indication of gas in the facies.

AAPG Search and Discovery Article #90089©2009 AAPG Southwest Section Meeting, Midland, Texas, April 26-29, 2009