ABSTRACT: Cyclostratigraphy and Duration of the Yates Formation (Permian, Late Guadalupian) of the Permian Basin

J. M. Borer, P. M. Harris

Using log, core, and outcrop data, we have identified patterns in cyclic sequences of the mixed siliciclastic-carbonate Yates Formation of

the Permian basin. A composite well log, representing the entire formation through a single sea level-sensitive facies tract (pisolite shoal crest), exhibits two orders of cyclicity with Milankovitch band periodicity. At the shelf margin, large-scale cycles consist of subequal dolomite- and siliciclastic-rich intervals. Small-scale cycles are carbonate and siliciclastic couplets that exhibit regular changes in lithology and thickness (bundling) that define the large-scale cycles. Cycle packaging changes across the shelf due to the updip (landward) pinch-out of high-frequency carbonate intervals.

Several lines of evidence (e.g., 4:1 cycle bundling, a good fit between depositional cycles and plots of astronomical parameters, and limited age constraints based on calculated subsidence/accumulation rates and regional stratigraphy) suggest that the depositional cycles are the product of orbitally forced sea level fluctuations with 100- and 400-k.y. periods (Milankovitch eccentricity cycles). This implies that the five large-scale cycles present in the Yates represent about 2 m.y. of deposition.

A lower frequency (third-order) sea level cycle was also active during Yates deposition. This 1.5- to 2-m.y. cycle is apparent from Fischer plots (cycle thickness corrected for regional linear subsidence plotted against time) and a lithofacies cross section. The third-order cycle may be related to a low-frequency Milankovitch signal or tectonics.

Fischer plots and stratigraphic analysis (using average slope/cycle data calculated from regional cross sections) suggest that all the sea level fluctuations had relatively low magnitudes ranging from approximately 2 m for the 100-k.y. cycles, 8-12 m for the 400-k.y. cycles, and 5 m for the 1.5- to 2-m.y. cycle.

The architecture of Yates Formation reservoirs is related to the stacking of depositional sequences during the three orders of sea-level fluctuations. In general, the low frequency (1.5-2 m.y.) cycle controlled the lateral position and reservoir quality of 400-k.y. depositional cycles (reservoir units); whereas, high-frequency (100 k.y.) sea level cycles controlled heterogeneity within reservoir units.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990