Sequence Stratigraphic Patterns and Reservoir Implications in the Oligocene and Cretaceous, Northern Monagas, Eastern Venezuela

Abstract

A sedimentological study based on 876 ft of core, electric logs, and cuttings from the well MUC-20 in Mulata field, Northern Monagas, has shown that the Cretaceous to Oligocene section exhibits a variety of facies, ranging from coarse-grained cross-stratified sands, fine-grained bioturbated sands, marine mudstones with varying proportions of silt and thin coal seams. These sediments accumulated in environments varying from fluvial to deltaic to marine shelf. The study of stacking patterns and facies successions indicates that six type I sequence boundaries are visible in the Cretaceous to Oligocene section. The lowest sequence boundary is Maestrichtian and is an incised valley complex filled with lowstand fluvial sandstones. The overlying sequence boundary is interpreted to be at the Cretaceous-Paleocene boundary (68 m.y. ) and is overlain by estuarine transgressive tract deposits. The third sequence boundary is thought to represent the 30 my eustatic sea level fall and incorporates a major stratigraphic hiatus from the lower Paleocene to Upper Oligocene. This sequence boundary is overlain by estuarine transgressive tract deposits. The other three sequence boundaries were recognized within the Upper Oligocene and are capped by nearshore and coastal sediments comprising coastal barrier and tidal inlet sandstones, and marsh and lagoonal mudstones. Themore » sediments overlying the fourth and fifth sequence boundaries are interpreted as lowstand shoreline deposits. The uppermost sequence boundary is represented by barrier sandstone transgressive tract deposits. The identification of these sequence boundaries and the parasequences comprising the systems tracts formed the basis for fieldwide reservoir correlations which enabled detailed mapping of the individual reservoir units. The Maestrichtian fluvial incised valley complex forms a widespread sand sheet, whereas the transgressive barrier sandstones are more restricted, and tend to shale out laterally.« less