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

Abstract

Using log, core, and outcrop data, the authors 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, 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 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 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 suggest that all the sea level fluctuations had relatively low magnitudes rangingmore » 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.« less