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

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124 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 The middle part of the Clamgulchian type section of the Sterling Formation is composed of alternating fining- upward sandstone, and mudstone and siltstone facies associations (fig. 6). Fining-upward sandstones average 27 m in thickness and the mudstone and siltstone sequences average 3 m in thickness. The fining-upward sandstones are amalgamated into multiscoured (as many as five internal scour bases) bodies and are interbedded with thin to thick interbedded mudstone and siltstone sequences. Although the multiscoured sandstone bodies display multistory (six tiers) and multilateral architecture as in the lower interval, here the multistacked sandstone bodies are commonly separated by thin mudstone-siltstone lenses with basal scours. The mudstone and siltstone sequences, which .- - + V) I Mud I Isand l~ebble EXPLANATION Conglomerate Sandstone Siltstone Mudstone Coal and carbonaceous shale b--d - - Trough crossbeds , Planar crossbeds Megaforesets Convolute laminations - - Ripple laminations -A, TVN AA Root marks T - 30 Tonstein METERS Figure 7. Composite vertical facies association in upper part of Clamgulchian type section of the Sterling Formation. contain coarsening-upward sandstone, are also laterally gradational into the fining-upward sandstones particularly in the upper parts of the multistacked sandstone complexes. The upper part of the Clamgulchian type section of the Sterling Formation consists of interbedded fining-upward sandstone, mudstone and siltstone, and coal, carbonaceous shale, and tonstein facies associations (fig. 7). The fining-upward sandstones range from 18 to 38 m in thickness and like those in the lower and middle intervals also occur as amalgamated multiscoured bodies. The multiscoured sandstones are also multistory (three tiers) and interbedded with thin to thick (1.5 to 31 m) mudstone and siltstone sequences and thin (0.4 m) coal, carbonaceous shale, and tonstein sequences. The multiscoured sandstone bodies in the upper part of the interval consist of as many six internal scour bases and are locally interrupted by nu- merous scour-based mudstone and siltstone lenses. The amalgamated multiscoured sandstones also exhibit multilateral architecture, but have more pronounced offset or en echelon lateral arrangements than those in the middle interval. However, as in the middle interval, the fining-upward sandstones also locally grade laterally into mudstone and siltstone and coarsening-upward sandstone sequences. The three groups of facies associations include both the fining-upward sandstone and mudstone and siltstone sequences throughout the stratigraphic interval. The coal, carbonaceous shale, and tonstein sequence is not present throughout and generally thins upward in the stratigraphic interval. Although the coal, carbonaceous shale, and tonstein sequences occur in thick mudstone and siltstone sequences, thin and less coaly sequences are associated with the thinner mudstone and siltstone sequences. In addition, the thick coal, carbonaceous shale, and tonstein sequences are interbedded with thin fining-upward sandstones. RESERVOIR ARCHITECTURE Successions of the fining-upward sandstones throughout the stratigraphic interval show both upward thickening and changes in reservoir architecture. The fining-upward sandstones increase twofold in thickness upward from the lower to the upper part of the study interval. This is accompanied by increased multistory and internal heteroge- neity of the sandstone complexes. The number of multistacked, fining-upward sandstones increases threefold upward and is accompanied by an upward increase in the frequency of internal bounding surfaces (compare for ex- ample the multistory and internal basal scours in figs. 5- 7). Sedimentary structures within sandstone bodies bounded by scour surfaces range from homogeneous trough crossbeds in the lower part and ripple laminations in the upper part to heterogeneous, haphazardly arranged convolute laminations, megaforesets, trough and planar crossbeds in the lower part and ripple laminations in the
RESERVOIR FRAMEWORK ARCHITECTURE, CLAMGULCHIAN TYPE SECTION, STERLING FORMATION 125 upper part (see figs. 8-10). Framework architecture of the thick complexes of sandstones in the upper part of the stratigraphic interval also records inclusion of more basally scoured mudstone and siltstone lenses (see figs. 5-7). Thus, framework architecture of the fining-upward sandstone reservoir system indicates different levels of het- erogeneity, based on the hierarchy, nature, continuity, and arrangement of bounding surfaces. The thin fining-upward sandstone reservoir system with fewer multistacked sandstone bodies and accompanying bounding scour surfaces in the lower part of the Clamgulchian type section of the Sterling Formation is a more homogeneous and less compartmentalized reservoir system (fig. 8). That is, compart- mentalization by permeability barriers (such as bounding surfaces) subdivides a potential reservoir system into many smaller individual reservoirs of more homogeneous proper- ties. First-order bounding surfaces that compartmentalized the fining-upward sandstone reservoir system include a continuous sharp basal scour surface, and gradational mar- ginal and upper contact with surrounding fine-grained sedi- ments. Second-order bounding surfaces are the internal, discontinuous sharp basal scour surfaces of the multistacked sandstone bodies. Third-order bounding surfaces within the multistacked sandstone bodies are short, discontinuous bounding surfaces of single bedforms such as trough crossbeds, megaforesets, convolute laminations, and ripple laminations. Fourth-order bounding surfaces within these bedforms include very short, asynchronous laminae surfaces. The transition from the trough-crossbedded lower part to the ripple-laminated uppermost part of the sandstone bodies is locally disrupted by some megaforeset and large-scale convolute bedforms. In addition, the upper part of the sandstone reservoir system is locally interrupted by subordinate basally erosional lenses of mudstone and siltstone. Juxtaposition of the reservoir sandstone system with thick, potential source rocks for gas consisting of the coal, carbonaceous shale, and mudstone sequences and thick impermeable rocks consisting of mudstone and siltstone sequences make them ideal targets for exploration. In contrast, the thick fining-upward sandstones in the middle and upper parts of the Clamgulchian type section of the Sterling Formation, which consist of a larger number of stacked, single sandstone bodies with accompanying bounding scour surfaces and multiple, unorganized bedforms, represent very heterogeneous and highly compartmentalized reservoir systems (figs. 9, 10). Although Coal and I c~~~o".c.o"s shale I Mire Figure 8. Sandstone reservoir framework architecture in lower part of Clamgulchian type section of the Sterling Formation.
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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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APPEAL FOR NONPROLIFERATION OF ESCA
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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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