Mineralogical Controls on Rock Fluid Properties--The Delaware Mountain Sandstones, Waha Field, West Texas

HAYS, PHILLIP D., and THOMAS T. TIEH, Texas A&M University, College Station, TX

Reservoir rocks of the Guadalupian Delaware Mountain Group in Waha field, Reeves Co., Texas are very fine-grained (mean grain size 0.10 mm), subarkosic sandstones interbedded with organic-rich siltstones and thin limestones. Shales and detrital clays are notably rare. The Delaware Mountain Group, including in descending order the Bell Canyon, Cherry Canyon, and Brushy Canyon formations, are currently buried at depths of 4500-8000 ft (1370-2440 m) in this area. Production is primarily from the Bell Canyon and Cherry Canyon formations.

Diagenesis of the Delaware Moutain Sandstones is unique in several aspects: The lack of shale in the sequence precludes significant changes associated with shale diagenesis. Abundant organic matter within the formations may have played an important role in diagenetic processes. Pore fluids migrating from the overlying Castile evaporite sequence are a likely source of early halite cement in the Delaware Mountain Group. The hydrodynamic nature of the basin also contributed to the diagenetic pathway of this sandstone. The suite of authigenic minerals present in the Delaware Mountain Group reflects these conditions. Early in their burial history the sandstones were extensively cemented with calcite, halite, and anhydrite. These cements partially arrested burial compaction and preserved a igh percentage of minus cement porosity. Large scale dissolution subsequently removed cement and detrital material, creating abundant secondary porosity. Widespread chlorite authigenesis then occluded porosity, blocking pore throats and seriously reducing permeability. Dolomite, ferroan/mangenous dolomite, potassium feldspar overgrowths, quartz overgrowths, brookite, and anatase constitute the late authigenic phases. These minerals precipitated as pore waters approached saturation due to continued detrital dissolution and increased cation activities in solution. Solubilities of these cations may have been augmented by the formation of organic complexes generated by degradation of organic material. This is corroborated by the greater abundance of authigenic minerals, such as brookite, obs rved in close proximity to organic-rich zones.

In the study area, reservoir porosities generally range from 20 to 25%. However, permeabilities are low (usually less than 50 md) and water saturations are high due to the presence of pervasive pore-lining chlorite. Reservoir quality of these sandstones has been controlled to a large extent by diagenetic processes.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)