Abstract

The Upper Permian Yates Formation produces gas from muddy very-fine-grained sandstone facies in the northeast part of the Central Basin Platform, West Texas. Lower Yates reservoir-interval siliciclastics and evaporites represent repetitive inner- and mid-shelf successions that record relative sea-level cyclicity. Lowstand-transgressive phases are marked by siliciclastics that were deposited in coastal mudflats, wadi outwashes, and aeolian dunes or sandsheets that were partly re-worked during transgressions. During highstands, marine flooding was followed by net evaporation in regionally extensive brine pans, resulting in widespread precipitation of swallow-tail gypsum (some of which was later re-worked as detritus into current laminae and altered to anhydrite) and halite. Brine incursion into pan-margin siliciclastics caused displacive halite precipitation (halite-mudrock facies or sabkhas) and poikilotropic halite precipitation in well-sorted sandstone. Over time, pans filled with evaporites. During falling relative sea level, coastal aeolian and alluvial siliciclastic environments migrated basinward.

Muddy sandstones, the primary Yates gas reservoirs, occur in the facies tract between halite-mudrock and aeolian sandstone facies and are compositionally transitional between those two facies. Muddy sandstones are marked by turbation produced during episodes of minor halite crystallization and dissolution. Halite precipitation was inhibited by the clay content of the facies and concentrated in coarser-grained sandy lenses. Stratigraphic seals are provided by regionally extensive anhydrite and halite strata. Although very well-sorted fine-grained aeolian sandstones in the upper Yates originally possessed high primary porosity, permeability, and reservoir-potential, pore occlusion by halite has destroyed their reservoir potential but enhanced their potential as seals.

Gamma ray-neutron porosity-density well logs commonly recorded through the Yates interval provide diagnostic response associations for five generalized facies that are most important for gas-reservoir characterization: 1) anhydrite (low GR, very high density, very low neutron porosity), 2) halite (low GR, very low density, very low neutron porosity), 3) halite-mudrock (intermediate GR, low density, low neutron porosity), 4) muddy sandstone (high GR, intermediate density, intermediate neutron porosity), and 5) halite-occluded sandstone (intermediate GR, low density, very low neutron porosity). Particularly significant is neutron-density-log crossover that occurs in the halite-occluded sandstone and can be misinterpreted as indicating gas content where cores are not available for log calibration.