Sedimentological and ichnological characterization of delta front mouth bars in a river-dominated delta (Upper Cretaceous) from the La Anita Formation, Austral Basin, Argentina

Abstract

Integrated sedimentological and ichnological studies in deltaic systems are key to determine whether a delta is river-dominated, tide-dominated, wave-dominated or hybrid, and thus predict the distribution of their geometries and main reservoirs. The Austral Basin is one of the most important oil and gas basins from Argentina, and the Upper Cretaceous La Anita Formation has shown excellent attributes as a hydrocarbon reservoir. Analysis of eleven cores, supplemented by a large volume of well logging data and two-dimensional and three-dimensional seismic, allowed to recognize a river-dominated deltaic system affected by hyperpycnal discharges and storm events. Four main facies associations were recognized (offshore, prodelta, delta front and interdistributary bay), being the delta front facies associations, the most recurrent facies and constituting the major reservoir of the La Anita Formation. Three types of deltaic mouth bars have been characterized in the delta front. Type I corresponds to active mouth bars with preservation of sedimentary structures and bioturbated by suspension and deposit feeding organisms. Type II represents the stopped progradation and the abandonment in the construction of mouth bars, which are completely reworked by suspension and deposit feeding organisms and, in some cases, affected by fair-weather or storm-wave processes at the top. Type III comprises hyperpycnal mouth bars characterized by transitional and recurrent intervals of sedimentary structures, high concentration of phytodetritus, and paucity of bioturbation. The amalgamation and coalescence of these three types of sandy mouth bars produce lobe-like geometries and are the main processes of delta front construction and progradation. This proposed model allows to explain the principal mechanism of construction of the delta front and can be applied to the analysis of other reservoirs where mouth bar architectural elements are dominant.