Diagenesis of Eagle Mills Sandstones: Implications for Paleo-Fluid Flux in Rift Basins

William C. Dawson

The Eagle Mills Formation (Triassic-Jurassic) occurs in deep (12,000 to 18,000 ft) wells in northeastern Texas and southwestern Arkansas. Eagle Mills "redbeds" are unconformably underlain by Paleozoic (?) basement and unconformably overlain by Werner-Louann evaporites. Eagle Mills strata are a heterogeneous sequence of siliciclastic lithofacies representing nonmarine paleoenvironments developed within grabens. Basaltic dikes and sills transect Eagle Mills lithofacies. The Eagle Mills sequence is the earliest record of Gulf of Mexico rifting. Eagle Mills sandstones exhibit petrographic and geochemical evidence of a complex diagenetic history during progressive burial. Diagenetic phases include: chlorite, nonferroan calcite, quartz overgrowths, kaolinite, anhydrite, albitized feldspars, ferroan calcite/dolomites, and late pyrite. Pyrobitumens coat early cements indicating that most diagenesis post-dated hydrocarbon migration.

Extensive secondary porosity (dissolution of detrital feldspars) developed in a deep burial setting where thermochemical sulfate reduction was the dominant diagenetic. Feldspar dissolution/alteration sourced metal cations required for diagenesis of overlying Mesozoic reservoirs. Eagle Mills-derived diagenetic fluids were modified by interactions with Louann-Werner evaporites. Fluid circulation was probably induced by high temperatures generated by emplacement of basaltic intrusives. Comparable high temperature diagenesis occurred in other rift basins (e. g., West Africa) suggesting that significant vertical migration of diagenetic fluids and hydrocarbons characteristizes rift basins.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995