Interpretation of Methanic Diagenesis in Ancient Sediments by Analogy with Processes in Modern Diagenetic Environments
Donald L. Gautier, George E. Claypool, 1984. "Interpretation of Methanic Diagenesis in Ancient Sediments by Analogy with Processes in Modern Diagenetic Environments", Clastic Diagenesis, David A. McDonald, Ronald C. Surdam
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Methanic diagenesis commonly dominates pore-water chemistry in organic carbon-rich sediments from depths of tens of centimeters to 1000 m or more, and is coincident with the depths of principal sediment dewatering. As a result, methanic diagenesis in organic carbon-rich mudstones may control early diagenesis in adjacent sand and sandstone and can result in the accumulation of economic quantities of biogenic methane. Chemical aspects of methanic diagenesis in ancient marine sediments can be reconstructed by analogy with processes in modern diagenetic environments, on the basis of the mineralogy, texture, and isotopic composition of concretionary carbonate cements and other related authigenic minerals.
This sort of diagenetic reconstruction is well illustrated by means of examples from the Upper Cretaceous Gammon Shale from southeastern Montana. Bioturbated mudstones of the Gammon accumulated in oxic, open marine waters, but dissolved oxygen was probably depleted from pore waters a few tens of centimeters beneath the sediment/water interface. Sulfate reduction took place beneath this depth and was most important in a zone of mixing at the base of bio-turbation, where isotopically light (δ34S ≃ −25 o/oo) iron sulfides accumulated. Organic matter oxidized during sulfate reduction gave rise to isotopically light calcite (δ13C ≃ −21 o/oo) that formed discrete concretions and that formed the interior portions of zoned calcite-siderite concretions. Sulfate was exhausted at depths of about 5–10 m, and CO2 reduction (methanogenesis) became the dominant form of anaerobic respiration. Carbonate precipitation accelerated as pH increased because of CO2 removal, while continued anaerobic oxidation of organic matter maintained bicarbonate activity at high levels.
In the absence of dissolved sulfide, increased Fe+2 activity favored siderite over calcite as the principal authigenic carbonate. During the early stages of methanogenesis, kinetic fractionation caused δ13C of CH4 to change from-90 to-70 per mil and δ13C of bicarbonate to change from −22 to approximately zero per mil over a depth interval of a few meters in the sediment column.
Interpretation of methanic diagenesis in the Gammon Shale illustrates only part of a single diagenetic pathway for one type of organic carbon-rich mudrock. And yet, the implications are clear: Early diagenesis of muds is dominated by processes involving organic matter and by the products of organic matter decomposition. Because of the economic significance of organic carbon-rich mudrocks as source beds for hydrocarbons and because their diagenesis probably controls mineral precipitation and dissolution in many reservoir rocks, it is of the utmost importance that diagenesis in ancient mudstones be understood.
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Clastic diagenesis has evolved from a very descriptive science to a much more process-oriented study. This evolution has been driven by the realization that many hydrocarbon reservoirs have significant diagenetic compotents directly affecting porosity and permeability characteristics. The prediction in time and space of reservoir characteristics affected by diagenesis can greatly reduce the risk in the search for hydrocarbon accumulations, particularly in subtle targets lacking pronounced structural expression. This publication contains three sections designed to increase understanding in the processes controlling clastic diagenesis: Conepts and Principles; Aspects of Porosity Modification; and Applications of Clastic Diagenesis in Exploration and Production. The first two sections deal with processes controlling various aspects of clastic diagenesis, and the third section applies these principles and observations to specific examples. Altogether, the three sections contain 22 chapters.