Multiple Karst Events Related to Stratigraphic Cyclicity: San Andres Formation, Yates Field, West Texas
S. W. Tinker, J. R. Ehrets, M. D. Brondos, 1995. "Multiple Karst Events Related to Stratigraphic Cyclicity: San Andres Formation, Yates Field, West Texas", Unconformities and Porosity in Carbonate Strata, David A. Budd, Arthur H. Saller, Paul M. Harris
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Open caves and solution-enhanced joints influence porosity distribution and fluid flow in Yates field. Therefore, applying an accurate model for cave formation, describing the distribution of cave and karst features, and quantifying the contribution of caves to total pore volume is important in order to characterize the reservoir. Prior work showed that karst and caves in Yates field were formed by meteoric processes acting on subaerially exposed islands following San Andres deposition, and predicted that the number of open caves should decrease with depth below the top of the San Andres Formation. The current work addresses three related issues: the possibility of multiple karst events in the San Andres of Yates field and the relationship of these events to stratigraphic cyclicity, the 3-D distribution of caves in the reservoir and the areas where future well deepenings might encounter caves in the field, and the effect of subaerial exposure on porosity and permeability.
There is a relationship between cave distribution and sequence stratigraphy in Yates field. Sequence-stratigraphic interpretation indicates that four major cycles aggraded and prograded from west to east. Each major cycle has a clinoformal geometry, shoals upward overall, and was capped by a subaerially exposed island complex. On these exposed islands separate cave lenses formed as a result of meteoric processes. Because the clinoforms and their associated island complexes aggraded and prograded from west to east, deeper drilling in Yates field should encounter additional caves to the west of known shallower caves in the east.
An important economic issue in carbonate reservoirs is the effect of subaerial exposure on porosity and permeability. Cavernous porosity formed beneath major unconformities in Yates field, and when examined by stratigraphic cycle, the data show that cycles with abundant caves have lower average matrix porosity and lower total porosity than the cycles with few caves. This results from a combination of matrix porosity reduction due to the cave-forming process and a lower pre-cave matrix porosity. Because Yates field is relatively shallow, most caves have not undergone mechanical collapse. Therefore, in contrast to the reduction in porosity observed in zones with many caves, the extensive network of open vertical caves and solution-enhanced fractures increases total vertical permeability of the system.