Abstract

A series of dip-oriented and strike-oriented sequence stratigraphic cross sections accentuating distribution of lithofacies in the Middle Permian (Guadalupian) Grayburg Formation have been built. The Grayburg Formation is a mixed carbonate (dominant)-siliciclastic (subordinate) composite sequence.

Grayburg Formation strata, and proposed reference sections, were studied in the: 1) Northwest Shelf type section; 2) Guadalupe Mountains surface reference section and proposed principle surface reference section; 3) Apache Mountains; 4) Central Basin Platform (northwest corner) Eunice Monument complex of unitized oil fields and a proposed subsurface reference section; and 5) Central Basin Platform (southeast side) McElroy field in another proposed subsurface reference section.

Six members of the Grayburg Formation are proposed, which in stratigraphic order from base to top are: 1) Premier Sandstone Member; 2) Lower Dolostone Member; 3) Metex Dolostone and Sandstone Member; 4) Loco Hills Sandstone Member; 5) Upper Dolostone Member; and 6) Stone Canyon Dolostone and Sandstone Member.

Hierarchy of Grayburg Formation sequence stratigraphy is proposed to contain the following: 1) one composite sequence (large third-order); 2) two simple sequences (small third-order); 3) 10 high frequency sequences (large fourth-order); 4) 21+ cycle sets (small fourth-order); 5) 82+ cycles (large fifth-order); 6) numerous beds/lamina (small fifth-order). Bedding, cyclicity, and rock types change systematically down-dip to up-dip via changes in accommodation space, wave/storm energy, and tectonics.

Grayburg Formation deposition was upon a tectonically-modified, storm-dominated, distally-steepened ramp that from up-dip to down-dip contained: 1) non-marine continental red bed succession (Bernal Formation); 2) inner ramp (non-reservoir – lateral and vertical seal); 3) ramp crest shoal (reservoir); 4) middle ramp (reservoir and non-reservoir); and 5) outer ramp (non-reservoir) depositional setting. Down-dip basin strata, if deposited, were eroded due to down-dip erosion of the ramp margin.

Up-dip, non-marine continental red bed strata of the Bernal Formation (Grayburg Formation time equivalent) are composed of red sandstone, siltstone and shale. Down-dip Grayburg Formation strata are composed of: 1) inner ramp – evaporites (anhydrite and halite) and interbedded non-porous dolomitic sandstone; 2) distal inner ramp – non-porous, anhydrite cemented, mud-rich dolostone and interbedded dolomitic sandstone; 3) ramp crest shoal – porous, peloid-ooid dolopackstone-dolograinstone and interbedded porous, dolomitic sandstone; 4) middle ramp – porous to non-porous fusulinid-poor to fusulinid-rich, peloid grain-dominated dolopackstone-dolograinstone strata and interbedded sandy dolostone strata; and 5) outer ramp – non-porous, dolomitized, solitary sponges (proximal), dolomitized small sponge clusters (1 m, 3 ft) (middle), and massive sponge dolo-bafflestone (distal). Most of the outer ramp and part of the middle ramp strata was eroded during post-Grayburg Formation subaerial exposure around the ramp margin in the Delaware Basin.

The composite sequence boundary at the top of the Grayburg Formation forms the most important surface in Middle Permian stratigraphy in the Permian Basin. Beneath the Grayburg Formation composite sequence boundary the Lower San Andres, Upper San Andres and Grayburg formations form ramp geometries. Above the Grayburg Formation composite sequence boundary the overlying Queen, Seven Rivers, Yates and Tansill formations form rimmed margins.

Grayburg Formation sequence stratigraphy and associated lithofacies distribution have added value by correctly correlating reservoir architecture, identifying miscorrelations, and helping solve day-to-day production-related problems in Grayburg oil fields. Examples include: 1) identification of down-dip intervals of strata once thought to be Upper San Andres that in actuality are basal, porous, productive Grayburg strata that onlapped the Upper San Andres composite sequence boundary; 2) identifying and avoiding highly porous strata charged with edge water during secondary recovery waterflood operations; and 3) conformance work identification and isolation of high-perm ooid dolograinstone strata, responsible for water cycling during waterflood operations.