Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
An Exceptionally Well-Preserved Calcisponge-Dominated Reef Facies in the Upper San Andres Formation (Permian), East Vacuum Grayburg San Andres Unit, Lea County, New Mexico
(1) ConocoPhillips, Houston, TX.
(2) Wahlman Geological Services, LLC, Houston, TX.
An exceptionally well-preserved calcisponge-dominated doloboundstone reef in the Guadalupian (Permian) high frequency sequence G9 (upper San Andres) is present in core from the distal margin of the East Vacuum Grayburg San Andres Unit (EVGSAU 0524-007, W/2SENW, Sec 5, T17S R35E), Lea County, New Mexico. The reef grew on the shelf-margin to upper slope immediately seaward of crestal shelf-margin shoals.
The basal 108 feet of the core is massive doloboundstone, and three overlying thin doloboundstone intervals are interbedded with skeletal dolowackestone, dolopackstone, fusulinid dolograinstone, and dolorudstone (reef talus). A karst fracture extending through 30 feet of the interbedded sequence is filled with anhydrite, breccia clasts, geopetal internal sediment, and fusulinid packstone.
The biota of the boundstone comprises abundant calcareous sponges, common thrombolitic microbial encrustations and masses, moderately common Tubiphytes, sparse fistuliporid bryozoans, rare Archeolithoporella red algae, and a sparse associated reef fauna of bryozoans, corals, brachiopods, and crinoids. Guadalupia and Lemonea are the most abundant calcisponge genera. Smaller calcisponges include common Amblysiphonella and Discosiphonella (Cystauletes), sparse Cystothalamia, and possible Sollasia. Massive to tabular forms of Guadalupia and Lemonea occur throughout the reef, and branching forms increase downward.
Rapid dolomitization occurred very early during diagenesis and preserved the depositional fabric of the reef. Framework cavities are typically lined by isopachous rims of bladed to radiaxial layered cements. Geopetal laminated dolomitic mud, silt, and very fine-grained peloidal packstone directly follow these cement rims in many cavities. Finally, the cavities were filled by medium-crystalline anhydrite, which was probably precipitated relatively early during paragenesis. Coarse anhydrite, sparse void-filling medium- to coarse-crystalline dolomite cements, rare calcite cements, and minor pore-lining bitumen are later diagenetic features.
The boundstone has six percent average porosity, and average permeability is 26 millidarcies. The distribution of calcisponges and other skeletal grains and the patchy distribution of pore-filling anhydrite control the pore system. The reef facies is a minor component of the dominantly dolograinstone/dolopackstone San Andres reservoir.