Interpretation of Methanic Diagenesis in Ancient Sediments by Analogy with Processes in Modern Diagenetic Environments

Abstract 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 (δ 34S ≃ −25 o/oo) iron sulfides accumulated. Organic matter oxidized during sulfate reduction gave rise to isotopically light calcite (δ 13C ≃ −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 CO 2reduction (methanogenesis) became the dominant form of anaerobic respiration. Carbonate precipitation accelerated as pH increased because of CO 2removal, while continued anaerobic oxidation of organic matter maintained bicarbonate activity at high levels. In the absence of dissolved sulfide, increased Fe +2activity favored siderite over calcite as the principal authigenic carbonate. During the early stages of methanogenesis, kinetic fractionation caused δ 13C of CH 4to change from-90 to-70 per mil and δ 13C 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.