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Title: Spatial prediction of caves in San Andres Dolomite, Yates field, West Texas

Abstract

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 alsomore » be estimated. This cave component of porosity can be displayed as a petrophysical log and manipulated in the same way as matrix porosity.« less

Authors:
; ;
Publication Date:
OSTI Identifier:
6131464
Report Number(s):
CONF-880301-
Resource Type:
Conference
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
Subject:
02 PETROLEUM; 04 OIL SHALES AND TAR SANDS; CAVES; SPATIAL DISTRIBUTION; OIL FIELDS; PETROLEUM GEOLOGY; TEXAS; DISSOLUTION; FORECASTING; LIMESTONE; MAPPING; OIL WELLS; RAIN; RESOURCE MANAGEMENT; ATMOSPHERIC PRECIPITATIONS; CARBONATE ROCKS; CAVITIES; DISTRIBUTION; FEDERAL REGION VI; GEOLOGIC DEPOSITS; GEOLOGY; MANAGEMENT; MINERAL RESOURCES; NORTH AMERICA; PETROLEUM DEPOSITS; RESOURCES; ROCKS; SEDIMENTARY ROCKS; USA; WELLS 020200* -- Petroleum-- Reserves, Geology, & Exploration; 040201 -- Oil Shales & Tar Sands-- Site Geology-- (-1989); 040500 -- Oil Shales & Tar Sands-- Properties & Composition

Citation Formats

Nosal, E.A., Carlson, J.L., and Craig, D.H. Spatial prediction of caves in San Andres Dolomite, Yates field, West Texas. United States: N. p., 1988. Web.
Nosal, E.A., Carlson, J.L., & Craig, D.H. Spatial prediction of caves in San Andres Dolomite, Yates field, West Texas. United States.
Nosal, E.A., Carlson, J.L., and Craig, D.H. 1988. "Spatial prediction of caves in San Andres Dolomite, Yates field, West Texas". United States. doi:.
@article{osti_6131464,
title = {Spatial prediction of caves in San Andres Dolomite, Yates field, West Texas},
author = {Nosal, E.A. and Carlson, J.L. and Craig, D.H.},
abstractNote = {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.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 1988,
month = 1
}

Conference:
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  • Persistent speculation 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.
  • Production from the San Andres dolomite, principal reservoir unit in the Yates field, is controlled by structure and by stratigraphic variation in depositional diagenetic facies, resulting in subdivision of the field into eastside and westside areas. In the east, the upper San Andres is dominated by peloidal, fusulinid, and algal carbonate sand and gravel shoals that were deposited as an easterly prograding platform margin sequence. Carbonate lithologies in the west Yates are typically peloidal lime mudstones and wackestones with a restricted biota of dasyclad algae and gastropods, and thin interbeds of gray-green clay shale-all deposited in intertidal and lagoonal environments.more » Two general porosity groups comprise most of the matrix porosity in the San Andres Formation: (1) compact or dense vuggy and moldic dolomite (west Yates), and (2) sucrosic dolomite with fine molds and fine to coarse intergrain and intercrystal pores (east Yates). Most of the reservoir storage in the San Andres of the Yates field is in sucrosic dolomite. Fractures, vugs, and caves have greatly enhanced drainage from the highly porous reservoir rock matrix and have contributed to the remarkable high productivities of many wells drilled in the field during its early history.« less
  • Since its discovery in 1926, the Yates field has produced 1.04 billion bbl of oil from an estimated original oil in place of approximately 4 billion bbl. The reservoir produces from Upper Permian dolomites, siltstones, and sandstones at depths from 1000 to 1900 ft (305 to 580 m). Wells with extraordinary flow rates were common during the early life of the field. Of the 636 wells drilled before unitization of the field in 1976, 203 potentialed for more than 10,000 BOPD, 26 for more than 80,000 BOPD, and one for more than 200,000 BOPD. These remarkable early productivities were duemore » to several geologic factors: (1) reservoir pressures that were considerably in excess of those needed to support flowing wells; (2) an immense network of reservoir pores, ranging in size from micropores to small caves, tied together by open fractures; and (3) an oil column that, at discovery, filled at least 350 ft (107 m) of closure in the broad Yates dome, which covers an area of 35 mi/sup 2/ (91 km/sup 2/).« less
  • The Mallet Lease, Hockley County, western Texas, produces oil from the Permian San Andres Dolomite as part of the larger Slaughter Field. The Mallet Lease is being considered for infill drilling and for tertiary recovery. This paper discusses the engineering and geologic basis of the reservoir description that is used in history matching 40 years of primary and secondary performance. Emphasis is given to integration of reservoir description with necessary fluid flow properties needed to match performance and also the use of long term data to insure proper reservoir representation. There are no discontinuous, isolated portions of the reservoir thatmore » would be tapped by infill wells. Most of the remaining oil is located in the tighter portions of the producing intervals which bears on prospects for infill drilling and tertiary oil recovery.« less
  • Although nearly 100% (19 million bbl of oil) of the estimated recoverable oil in the Emma San Andres reservoir has been produced, indications are that as much as 15 million bbl of recoverable, movable oil remains. Detailed study of facies and diagenesis in the reservoir reveals considerable lateral and vertical variability, which has important implications for recovery and injection efficiencies in the field. The upper San Andres Formation in the Emma field comprises a shallowing-upward sequence of pervasively dolomitized, shallow-water subtidal to supratidal carbonates that accumulated on a gently east-sloping ramp. The lowermost part of the reservoir interval is composedmore » of laterally extensive fusulinid packstone deposited in an open-marine ramp setting. Abruptly overlying this facies is a sequence of interbedded fusulinid/algal grainstone, formed in a migrating sand-shoal complex, and burrowed, skeletal mudstone deposited on a restricted inner ramp. The grainstone is restricted to the western and central parts of the field. These deposits are overlain by pisolitic and cryptalgal mudstones and terrigenous siliciclastics of supratidal origin. Anydrite is abundant throughout the section. Porosity and permeability are facies controlled. Highest permeabilities are developed in shoal complex grainstone (100 md) and in fusulinid packstone (925-50 md). Lateral and vertical facies variations resulted in considerable reservoir heterogeneity. Trace-element and isotope data indicate that most of the San Andres dolomite and associated sulfate (anhydrite) precipitated from downward-moving, seawater-derived brines during shallow burial in the late Guadalupian. Subsequent anhydrite dissolution has locally enhanced original porosity.« less