Abstract

Siliciclastic sediments are a critical component of mid-upper Guadalupian platform and basin facies both in the Guadalupe Mountain outcrops and in Grayburg, Queen, and Seven-Rivers/Yates reservoirs of the Permian Basin. Siliciclastic facies can serve as reservoir sweet spots, thief zones, and barriers to flow depending on the setting within the basin. Mixed siliciclastic-carbonate strata ranging from San Andres through Tansill shelf units are considered here at a range of stratigraphic scales from supersequence bounding events through cycle-scale patterns to re-evaluate architecture and timing of input and bypass of these sandstones. A combination of detailed mapping on photomosaics, closely spaced measured sections, and 3D mapping on high-resolution airborne and ground-based lidar data are used to better understand the genetic and geometric character of these sandstones.

The “lowstand bypass/transgressive preservation” model for Guadalupian siliciclastics is considered solid, but there are additions, modifications, and retractions to consider. There is a broad consensus that the arid eolian origin of the wind-blown clay-free siliciclastics does not impact the carbonate factory with which they are intercalated because the lack of clays in this system does not effect light penetration. Regardless, there is strong evidence that siliciclastics provide a first-order displacive effect on the mud factory, considerably reducing the carbonate mud production on the shelf and thereby impacting the accumulation of fine-grained carbonate mudstones that are potential source rocks. The change in sediment supply associated with the input of siliciclastics and concomitant loss of carbonate mudstones is a primary influence on the transition from carbonate ramp to reef-rimmed architecture of the upper Guadalupian.

Although the Guadalupe Mtn outcrops may not be the ideal analog of the northwest shelf as a whole, these outcrops can aid in the characterization of sheet-like vs. channel-form geometries of major sandstone units that define the bases of 2^nd-order supersequences. The basal “Premier sand” of the Grayburg (base CS10), the Shattuck member of the basal Seven Rivers (base CS11), and the Triplet sand-carbonate-sand of the uppermost Yates (base CS12) are confirmed to be sheet-like at the regional scale of the Guadalupe Mtns. One modification to the sheet-like sandstone geometry is the documentation of local thinning of the Triplet sands proximal to the reef margin. The decreased sandstone thickness is associated with syndepositional tectonism driven by compaction-related faulting. A second modification of the “lowstand bypass/ transgressive preservation” model for reciprocal sedimentation is the overall progradational pattern associated with carbonate interbeds within regional sandstone intervals including:Grayburg, Shattuck, and basal Triplet sands. At all scales, from 2^nd order supersequence to cycle-scale reservoir thief zones and compartments, the siliciclastics within the Guadalupian section are critical to our understanding of shelf stratigraphic patterns.