GARDNER, MICHAEL H., Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

ABSTRACT: The Compound Effect of Multiple Scales of Cyclicity on Hydrocarbon Seals in Sand-Rich Deep-water Systems: Delaware Mountain Group (Permian, Guadalupian), West Texas

Cyclic patterns of deep-water sedimentation within a coupled eolian-turbidite system are a primary control on development of seals in Delaware Mountain sandstone reservoirs. Because sandstone is volumetrically dominant, the most important controls on hydrocarbon accumulation are seals that impede updip hydrocarbon migration and influence hydrodynamic charge relations. At the top of each low-order cycle, the additive effects of marine deepening at all scales of cyclicity constructively combine to form vertical and lateral seals.

Three orders of cyclicity are identified in the Delaware Mountain Group. Low-order cycles approximate the Brushy Canyon. Cherry Canyon, and Bell Canyon Formations and form asymmetric siliciclastic successions that are up to 500 m thick and have minor carbonate interbeds. The proportion of sandstone relative to limestone and siltstone increases vertically. Low-order cycles are bounded by regionally correlative carbonates and siltstones. Large-scale cycles consist of 5 to 8 intermediate-scale cycles characterized by 10-to 150-m-thick eolian-derived turbidite successions. Intermediate-scale cycles contain 2 to 7 high-frequency symmetric cycles that are 2 to 30 m thick and are bounded by siltstones. High-frequency cycles form an upward-bed-thickening hemicycle of thinly interbedded sandst ne and siltstone, ripple-laminated sandstone, and erosive-based, structureless sandstone overlain by an upward-bed-thinning hemicycle of similar facies in reverse order.

Hydrodynamic trapping within base-level-rise portions of long-term cycles controls hydrocarbon accumulations in Delaware Mountain reservoirs. Relative sea-level fall produced a basinward shift in platform accommodation, base-level fall, dune progradation, and downslope transport of eolian sands as high-density sediment gravity flows. Subsequent relative sea-level rise generated a landward shift in accommodation, base-level rise, and transgression over platform dunes. This was concurrent with basinal deposition of progressively thinner, lower energy sandstone beds and increased bioturbation. Clastic sediment starvation and highstand shedding of basinal carbonates produced laterally extensive low-permeability and low-porosity strata that form vertical seals, whereas compartmentalization of lower energy, laterally discontinuous reservoir sandstone forms lateral seals.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.