Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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RESERVOIR FRAMEWORK ARCHITECTURE IN THE CLAMGULCHIAN TYPE SECTION (PLIOCENE) OF THE STERLING FORMATION, KENAI PENINSULA, ALASKA ABSTRACT The coal-bearing Kenai Group in the Cook Inlet and Kenai Peninsula is a significant gas-producing unit. A large percentage of the nonassociated gas, particularly in the upper part of the Kenai Group, is generated from mudstones and coalbeds. Study of sandstones and associated coaly sequences of the Sterling Formation in the upper part of the Kenai Group has revealed reservoir and source rock characteristics. Lithostratigraphic sequences and facies associations of the Clamgulchian Sterling Formation were formed in fluvial systems consisting of framework braided channels and overbank-flood-plain matrix onlapped by mires. Braided channel sandstones exhibit different levels of reservoir het- erogeneity based on the hierarchy, nature, continuity, and arrangement of bounding surfaces. Fining-upward sandstones formed in bedload, low-sinuosity braided channels provide homogeneous and less compartmentalized reservoir systems. Fining-upward sandstones formed in mixed bedload and suspended load, moderately high-sinuosity braided channels yield heterogeneous and highly compartmentalized reservoir systems. Coaly sequences related to the low-sinuosity braided channel sandstones are thicker than those related to the mixed bedload and suspended load channel sandstones. Mixed load fluvial systems readily supplied suspended sediments that suffocated organic accumulation in low-lying mires. INTRODUCTION The upper Miocene and Pliocene Sterling Formation is a coal-bearing unit within the Kenai Group in the Cook Inlet area, Alaska (fig. 1). Thin to thick coalbeds are com- mon in the lower and middle parts, and thin and sparse in the upper part of the Sterling Formation on the Kenai Pen- insula (Barnes and Cobb, 1959; Calderwood and Fackler, 1972; Memtt and others, 1987). This formation is best exposed along the northern coast of Kachemak Bay and east- ern coast of Cook Inlet (figs. 1 and 2). The Sterling Formation is as thick as 2,100 m and consists of inter- By Romeo M. Flores and Gary D. Stricker bedded sandstone, siltstone, mudstone, carbonaceous shale, and coal (fig. 3). Although Sterling coalbeds have not been mined for commercial use, methane gas associated with these coals is produced commercially (Kelly, 1963, 1968). Most nonassociated gas in the late Tertiary sandstone reservoirs is produced from methane gas generated from the coalbeds (Kelly, 1968). Multiple sandstone beds in the upper part of the Kenai Group at the Kenai and Swanson River fields, 25 and 65 km north of the study area, respectively (fig. 4), are the most consistent reservoir of methane gas (Kelly, 1968). These sandstone reservoirs, which are found at depths of 1,060-1,520 m, are clean, fine to medium grained, locally conglomeratic, poorly con- solidated units and are separated commonly by gray shale and rarely by lignite. A stratigraphic correlation by Church and others (1969) through the Kenai and Swanson River fields and nearby fields along the western part of Kenai Peninsula indicates that these sandstone reservoirs are within the Sterling Formation. The Kenai and Swanson River fields combined produced a total of 78,559,687 million cubic feet of gas from 1963 to 1967 in the upper part of the Kenai Group (Kelly, 1968). We made assessments of Tertiary coal and associated unconventional gas resources in Alaska in general, and of the coal-bearing Kenai Group in particular, during the summers of 1991 and 1992. We targeted the Kenai Group because of its abundant subbituminous coalbeds in the Tyonek and Beluga Formations, where these beds are as thick as 15 m. In addition, these coalbeds and associated shales and mudstones were the source rocks for con- densate and microbial (biogenic) gas for late Tertiary sandstone reservoirs in the Cook Inlet basin (Kelly, 1968; Magoon and Anders, 1990). Thus, knowledge of the paleoenvironments of these coalbeds and associated sandstones, particularly in the upper part of the Tertiary Kenai Group, is necessary to understand the accumulation and distribution of these organic deposits and related biogenic gas as well as the character of reservoir systems. The study focuses on the stratigraphic interval in the upper part of the Sterling Formation from 3.1 km south to 2.2 krn north of Clam Gulch (fig. 2), where potential sandstone reservoirs are well developed and exposed in two
RESERVOIR FRAMEWORK ARCHITECTURE, CLAMGULCHIAN TYPE SECTION, STERLING FORMATION 119 dimensions. In addition, occurrences of these sandstone STRATIGRAPHIC AND reservoirs are directly related to the presence of thin SEDIMENTOLOGIC SETTING coalbeds in the Clamgulchian type section. This relationship is best demonstrated by a genetic stratigraphic frame- Merritt (1986) indicated that coal accumulation in the work based on 30 measured sections described during the Kenai Group was related to a low-energy sedimentation summer of 1992. during a Miocene sea-level stillstand, which was succeeded
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Geologic Studies in Alaska by the U
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CONTENTS Introduction Cynthia Dusel
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CONTENTS CONTRIBUTORS TO THIS BULLE
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GEOLOGIC STUDIES IN ALASKA BY THE U
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LATE HOLOCENE LONGITUDINAL AND PARA
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DEEP-WATER LITHOFACIES AND CONODONT
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Table 1. Locality register for key
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Table 1. Locality register for key
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LITHOFACIES AND CONODONTS OF CARBON
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LITHOFACES AND CONODONTS OF CARBONI
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Table 1. Conodont faunules and lith
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DEPOSITIONAL SEQUENCES IN ATIGUN SY
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U-Pb AGES OF ZIRCON, MONAZITE, AND
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Page 73 and 74: U-Pb AGES OF ZIRCON, MONAZITE, AND
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Page 77 and 78: APPEAL FOR NONPROLIFERATION OF ESCA
Page 85 and 86: FAVORABLE AREAS FOR METALLIC MINERA
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Table 1. Isotopic ages of intrusive
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Table 1. Isotopic ages of intrusive
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GEOCHEMICAL EVALUATION OF STREAM-SE
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Table 3. Summary of geochemical sig
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GEOCHEMICAL CHARACTER OF UPPER PALE
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RECONNAISSANCE GEOCHEMISTRY OF BASA
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OSTRACODE ASSEMBLAGES FROM MODERN B
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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US. GEOLOGICAL SURVEY REPORTS ON AL
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U.S. GEOLOGICAL SURVEY REPORTS ON A
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U.S. GEOLOGICAL SURVEY REPOR TS ON
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REPORTS ABOUT ALASKA IN NON-USGS PU
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REPORTS ABOUT ALASKA IN NON-USGS PU
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Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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