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Petrographic Evidence of the Origin of Differential Compaction in the Chert and Siliceous Shale Beds in the Woodford Shale of Oklahoma

Abstract

The Devonian Woodford Shale in the Arkoma Basin in Oklahoma is dominantly composed of organic-rich, black, siliceous mudrock. In specific locations and intervals, the Woodford also contains sharply interbedded, 2- to 8-cm-thick black chert. Regional analysis suggests that the Woodford Shale was deposited on a restricted deep shelf overlain by a stratified water column, and was bordered on the south by a deep, open marine basin. The chert beds are interpreted to have originated as siliceous oozes of opal-A derived from radiolarian blooms that were episodically transported up out of the basin by upwelling currents onto the outer shelf. Post-Devonian tectonism resulted in burial into the dry gas window. Thin section petrographic analysis has revealed that the chert beds contain common, round, silica-filled algal cysts, in contrast to the interstratified siliceous mudrock beds, which contain compacted algal cysts. Why are the algal cysts in the chert beds relatively uncompacted while other cysts in the adjacent siliceous mudrock beds intensely compacted? Various hypotheses have been offered regarding this difference in the degree of compaction, but most presume that the diagenetic processes, and their timing, were the same in the chert and siliceous mudrocks. The answer may lie in several decades of research on the Monterey Formation of California, which is a biogenic quartz-rich formation much like the Woodford. Applying the work of Behl and Garrison (1994) and Issacs (1981) on the Monterey to the Woodford it is suggested here that the original opal-A deposits of the Woodford may have been converted to “proto-chert” beds of porcelaneous opal-CT by 1500m burial depth. The author has a handsample of the Aguajito Shale Member of the Monterey Formation collected in the Carmel Valley area of California. It is composed almost entirely of porcelaneous opal-CT by XRD analysis, and it is slightly harder than common school chalk. The sample contains abundant, round, sub-millimeter-scale molds of probable microfossils that show no evidence of compaction. The interbedded siliceous mudrocks however, were derived from more clay-rich sediment, and as a result took a different route on their diagenetic journey. The original sediment may have possessed up to 75% porosity at the time of deposition based on work by Baldwin (1971), and increasing burial depth and overburden stress resulted in compaction (with dewatering) as the dominant, initial, diagenetic process. Based on Baldwin’s (1971) curve mechanical compaction in the siliceous mudrock may have reduced porosity to about 20% at 1500m, which represents a loss of over 70% of the original sediment volume. Plastic algal cysts within the siliceous mudrock facies would have been crushed as a result, whereas their cohorts within the opal-CT beds were “sheltered” from compaction by the relative competency of the porcelanite that had developed early in the burial history.