Controls on permeability heterogeneity in the Tocito Sandstone (Upper Cretaceous), northwest New Mexico
— Mark R. Lambert, Rex D. Cole, and Peter S. Mozley

The Tocito Sandstone Lentil of the Mancos Shale is an Upper Cretaceous (Coniacian) shallowmarine sandstone and mudrock complex that is a major hydrocarbon producer in the San Juan Basin. Most Tocito pay intervals show significant variations in reservoir heterogeneity at a variety of scales. The results of this study strongly suggest that permeability heterogeneity at the macroscopic scale (i.e., well-to-well scale) is largely controlled by lithofacies variations. This conclusion is based on a systematic analysis of permeability variations in the Tocito. Minipermeameter measurements were conducted on outcrops (21 windows, 2 horizontal transects, and 2 vertical transects) and two shallow cores at Hogback oil field (anticline), and four conventional cores from the deep subsurface. A total of 2649 permeability measurements were made; values ranged from 0.0005 to 17.9 darcies. Overall, the permeability values from outcrop and the shallow subsurface cores are dramatically higher than those from the four deep subsurface cores. Average (geometric) permeability values for the outcrop population and the combined shallow-core populations are 2.0 and 0.7 darcies, respectively, whereas the average for the deep-subsurface cores is only 0.001 darcies. In the outcrop and subsurface, the large- and medium-scale cross-stratified sandstone lithofacies have the highest permeability values, followed by the interbedded sandstone and shale, ripple cross-stratified sandstone, and the muddy bioturbated sandstone lithofacies. Siltstone and mudstone interbeds, laminations, and mud drapes are also common in the Tocito. These mudrocks, which have greatly reduced permeabilities, create significant flow barriers and baffles and compartmentalize the Tocito both vertically and laterally. Petrographic examination indicates that permeability enhancement in outcrop and the shallow subsurface is due to dissolution of calcite cement and framework grains by meteoric water, plus microfracturing produced by weathering and decompaction.