Spatial prediction of caves in San Andres Dolomite, Yates field, West Texas
Persistent speculations that caves played a key role in the high flow rates of many early wells in the Yates field (203 wells potentialed for more than 10,000 BOPD each, 26 wells for more than 80,000 BOPD each) has raised questions of why the caves exist, how many there are, and how to incorporate them into reservoir management practice. This paper describes the use of probability theory to answer these questions. Among the geologic factors that contributed to the remarkable early productivity of Yates are zones of karst in the upper San Andres Dolomite, the principle reservoir unit. Hundreds of infill wells drilled after unitization of the field in 1976 have provided ample data on cave numbers and patterns. These data indicate that karstification was produced by dynamic lenses of fresh water beneath a cluster of islands formed when lowering of Late Permian sea level exposed San Andres limestone to rainfall and dissolution. The seemingly random occurrences of caves can be fitted into a geologic framework of systematic karst processes to produce mappable petrophysical parameters. The most important of these predicts, in probabilistic terms, where the caves are located. The contribution of cave porosity to total reservoir porosity can also be estimated. This cave component of porosity can be displayed as a petrophysical log and manipulated in the same way as matrix porosity.
- OSTI ID:
- 6131464
- Report Number(s):
- CONF-880301-
- Resource Relation:
- Conference: Annual meeting of the American Association of Petroleum Geologists, Houston, TX, USA, 20-23 Mar 1988
- Country of Publication:
- United States
- Language:
- English
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