Abstract: Sequence Stratigraphy and Petrophysical Variability of Backreef Carbonates, Permian Seven Rivers and Yates Formations, Guadalupe Mountains, New Mexico

David A. Osleger

Continuous exposures of outer shelf to pisolite shoal facies of the Permian Seven Rivers and Yates Formations were logged at a decimeter scale in Slaughter Canyon in the Guadalupe Mountains of southern New Mexico to distinguish stratal geometries, parasequence stacking patterns, and petrophysical variability. A cross-platform transect of eight closely-spaced sections spanning 3 km indicates that individual beds become progressively coarser and thicker seaward, culminating in steeply-dipping, massively-bedded coarse grainstones of the immediate backreef. Major and minor sequence boundaries were recognized on the basis of three criteria: (1) juxtaposition of quartzose siltstones and fine sandstones on top of pisolitic shelf crest facies, (2) laterally-correlative breccias developed on t p of a stack of upward-shallowing parasequences, and (3) deep vertical fissures developed within coarse backreef facies that are filled with large breccia clasts derived from adjoining fissure walls, siltstone that can be traced updip into bedded siltstone, and vertically-oriented fusulinids.

Petrography suggests that the four major facies associations recognized in backreef strata are characterized by distinctly different porosity types. Conventional porosity and permeability measurements determined from core plugs indicate that the highest permeabilities occur in outer shelf grainstones (avg. = 10.8 md) with intraparticle, interparticle and moldic porosity that show extensive connectivity. The lowest permeabilities occur in pisolite grainstones (avg. = 0.36 md) that have poorly-connected, fabric-selective intra- and interparticle porosity, and sandstones (avg. = 0.07 md) that exhibit dolomite and anhydrite cements and poorly-connected vuggy porosity. Vertical and lateral trends in petrophysical attributes identified within these outcrop exposures in the Guadalupes may pr vide a high-resolution model for subsurface cyclic carbonate reservoirs worldwide.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994