Sedimentary facies of a tide-dominated estuary and deltaic complex in a tropical setting: The middle Miocene Oficina Formation of the Orinoco Oil Belt, Venezuela

Abstract

Although the middle Miocene Oficina Formation of the Orinoco Oil Belt represents most of Venezuela's hydrocarbon resource, a comprehensive and detailed sedimentary facies model for the whole belt has never been put forward. Based on the analysis of cores and well logs, nine sedimentary facies (FA-I), forming five facies assemblages (FA1-5), have been characterized. Both sedimentologic and ichnologic datasets have been integrated in this study. The Oficina Formation records the typical succession of fluvial incision during relative sea-level fall, fluvial deposition during lowstand, transition from fluvial to estuary valleys during the subsequent transgression and deltaic progradation during highstand. FA1 consists of fluvial-braided channel (FB), floodplain (FG2), and swamp (FH1) deposits, as well as paleosols (FG3). This facies assemblage occurs in the lower member, representing the infill of lowstand fluvial valleys. FA2 consists of meandering estuary-channel deposits (FA, FC, FD, FE, and FI). This facies assemblage occurs in the middle member, representing the infill of tide-dominated estuary valleys during the early stages of the Langhian transgression. FA3 consists of tidal-flat and tidal-creek (FC, FD, FE, FF, and FG2), and swamp (FH1 and FH2) deposits, together with paleosols (FG3). This facies assemblage is present in the middle member, revealing backstepping and retrogradation within the estuary system during a continuing transgression. FA4 consists of outer-estuary sandbar (FC, FD, and FG1); swamp (FH2) deposits and paleosols are present at the margins of the estuary (FG3). This facies assemblage occurs in the uppermost part of the middle members, representing a late stage of the Langhian transgression, culminating in a maximum flooding surface. FA5 consists of deltaic distributary channel (FC and FD), floodplain and interdistributary-bay (FG2), and swamp (FH1) deposits. This facies assemblage occurs in the upper member, recording sedimentation in the delta plain of a tide-dominated delta during a highstand. Freshwater conditions in the fluvial system, as well as in the inner portions of the estuary and the delta plain are further supported by the presence of the Scoyenia Ichnofacies, whereas brackish-water segments of the estuary are characterized by the Skolithos and depauperate Cruziana Ichnofacies. The substrate-controlled Teredolites, and Glossifungites Ichnofacies occur in connection to erosional exhumation during ravinement. The absence of fully marine ichnofaunas is consistent with the embayed nature of the Orinoco Oil Belt. The tropical and humid character of the Oficina depositional systems is manifested in the development of widespread wetlands in marginal-marine settings, with formation of swamps and embayed areas, typically displaying evidence of waterlogged paleosols with pervasive root trace fossils. The abundance of crustacean burrows, such as Ophiomorpha and Thalassinoides, is also typical of low-latitude marginal- and shallow-marine settings. Although tidal forces tend to be weaker in lower latitudes because Coriolis effects are stronger there, both the estuary and delta systems are regarded as tide-dominated, most likely reflecting the embayed nature of the shoreline.