Abstract

Karst features, including open caves, bit drops, collapse breccias, enlarged fracture systems, and allothunous void fillings are well known in the San Andres formation (Guadalupian, Permian) in Yates field, Pecos County, Texas. Resulting porosity in caves and fractures is a significant contributor to both storage and production of hydrocarbons. Minor karst features such as pits, infiltration of overlying sandstone, and red oxidation, have been reported from San Andres outcrops on the Algerita Escarpment, Guadalupe Mountains, Texas and New Mexico. However, karst features in San Andres fields other than Yates have rarely been reported.

Examination of several 475’ cores cut in the San Andres formation in Vacuum field, Lea County, New Mexico, reveals numerous cored intervals with karst overprints. These features include 3 to 25 foot high, anhydrite filled caves, brecciated zones containing infiltrated quartz sandstone and collapsed carbonate blocks, laminated cave floor deposits, and sand filled fractures. Unlike Yates field, these karst features are relatively tight and constitute local barriers to vertical fluid flow. Karst features appear to be most abundant in ooid/peloid grain-rich packstones deposited close to the shelf margin, updip locations display numerous tidal flat capped cycles with less frequent karst development.

Karst-related dissolution, cave development, collapse structures, and sandstone infiltration can be impressive in individual cores, but the lateral extent of such phenomena is limited. It is important to distinguish between widespread, correlative depositional features and localized karst overprints when developing depositional models in San Andres reservoirs.