ABSTRACT The Navarro Formation produces oil from thin sandstones encased in thick marine shales. Nearby outcrop faunal analysis places these mudstones as being deposited in a middle-shelf depositional environment at 100 km or greater from time-equivalent shorelines based on regional correlation. This study integrates core, drill cuttings, and well logs to identify ten lithofacies grouped into four facies associations (FA-1 to FA-4), representing a dynamic mud-dominated depocenter, with characteristics of both subaqueous deltas and shallow-water contourites. High fluvial inputs from the northwest fed an energetic shelf with storm waves and strong geostrophic currents, transporting fine sediment far across the shelf before deposition as elongate, muddy clinoforms that prograded perpendicular to the currents. Additional accommodation along the Mexia–Talco fault promoted more aggradational parasequence stacking and thicker clinoforms locally. Toesets comprise muddy laminated deposits (FA-1) that grade upward across topsets into bioturbated, silty contourite facies (FA-2, FA-3), forming coarsening-upward parasequences. The clinoform rollover lay near storm-wave base, where storm-ebb flows transitioned into along-shelf geostrophic currents, depositing sandy tempestites (FA-4) thick enough to outpace bioturbation and preserve primary structures, yielding sandstone fairways aligned with rollover zones. This study presents a predictive model for isolated sandstone bodies on mud-dominated highstand shelves, informing unconventional reservoir characterization and hydrocarbon migration pathways.
Burton et al. (2026) studied this question.