Diagenesis of Deeply Buried Eagle Mills Sandstones: Implications for Paleo-Fluid Migration and Porosity Development: ABSTRACT

Abstract

ABSTRACT Eagle Mills strata (Triassic-Jurassic) unconformably overlie the Paleozoic basement complex and thus, form the basal sedimentary unit within the Gulf of Mexico Basin. Jurassic-aged basaltic dikes and sills have intruded Eagle Mills strata. Werner-Louann evaporates disconformably overlie the Eagle Mills Formation. These sedimentary and igneous rocks preserve the earliest record of Gulf of Mexico rifting. Deeply buried (15,000 to 18,000 ft/4,573 to 5,488 m) Eagle Mills sandstones have subarkosic and sublithic modal compositions. These sandstones exhibit evidence of a complex and prolonged diagenetic history, including: early chlorite cementation; mechanical compaction; quartz and feldspar overgrowths; nonferroan calcite and dolomite cementation; dissolution of framework grains; kaolinite precipitation; ferroan calcite cementation; albitization; saddle dolomite and anhydrite cementation; and pyritization. Pyrobitumens coat early cements indicating that most diagenesis post-dated hydrocarbon migration. Marked shifts in paleo-water chemistry are recorded by diagenetic phases in Eagle Mills sandstones. The Eagle Mills paragenetic sequence is indicative of progressive burial into a high-temperature (>150° C) regime where thermochemical sulfate reduction was the dominant diagenetic process. Pervasive late burial dissolution of detrital feldspars in some Eagle Mills sandstones provides direct evidence for deep-seated sourcing of metal cations which promoted diagenesis of overlying Mesozoic strata. These petrographic data support interpretations of the Mesozoic Gulf Coast Basin which imply vertical mass-transfer of diagenetic fluids along faults.