SEDIMENTOLOGY, ICHNOLOGY, BIOSTRATIGRAPHY, AND SEQUENCE STRATIGRAPHY OF THE MIDDLE MIOCENE OFICINA FORMATION, ORINOCO OIL BELT, VENEZUELA
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. Nine facies (FA-FI), grouped in five facies assemblages (FA1-5), have been identified in the Oficina Formation in the Orinoco Oil Belt. FA1 occurs in the lower member, encompassing fluvial braided channels (FB), floodplains (FG2), swamps (FH1) and paleosols (FG3). FA2 occurs in the middle member and consist of meandering estuarine-channel deposits (FA, FC, FD, FE, and FI). FA3 occurs in the middle member, including tidal flats and tidal creeks (FC, FD, FE, FF, and FG2), swamps (FH1 and FH2), and paleosols (FG3) formed in tide-dominated estuarine systems. FA4 is present in the uppermost part of the middle members, including sandbars (FC, FD, FG1), paleosols (FG3), and swamps (FH2) formed in the outer part of estuaries. FA5 occurs in the upper member and consists of deltaic distributary channel (FC and FD), floodplain and interdistributary bay (FG2) and swamp (FH1) deposits of the lower delta plain of tide-dominated deltas. The sedimentary succession in the Oritupano Field represents the upper member of the Oficina Formation, therefore correlating with the deltaic deposits identified in the Orinoco Oil Belt. Eleven facies (FJ-FS), grouped in four facies assemblages (FA6-9), have been recognized in the Oritupano Field. FA6 is present in the lower part and consists of deltaic distributary-channel (FJ) and interdistributary-bay (FK) deposits of the delta plain of a wave-dominated delta. FA7 is present in the middle part and consists of sandy mouth-bar (FL), proximal delta-front (FM1), storm-dominated distal delta-front (FM2), and prodelta (FN) deposits formed in an area encompassing the delta front and the prodelta of a wave-dominated delta. FA8 is present in the upper part, including upper- to middle-shoreface (FO) and lower-shoreface (FP) deposits formed in a wave-dominated shoreface. FA9 is present in the upper part, including deposits of the upper offshore (FQ), lower offshore (FR) and shelf (FS) formed in an offshore-shelf complex. The Oficina Formation contains four softground ichnofacies (Scoyenia, depauperate Cruziana, Skolithos, and archetypal Cruziana) and two substrate-controlled ichnofacies (Teredolites and Glossifungites). The Oficina Formation in the Orinoco Oil Belt and Oritupano areas provides an ideal opportunity to study faunal distribution and ichnofacies because it comprises a wide range in depositional environments formed under variable salinity conditions within a single stratigraphic unit. Freshwater conditions in the fluvial deposits, as well as in the inner portions of the estuary and delta plain, are further supported by the presence of Scoyenia Ichnofacies, whereas brackish-water segments of the estuarine and delta-plain deposits are characterized by the Skolithos and depauperate Cruziana Ichnofacies. Rapid ichnofaunal changes are distinctive of delta-front and prodelta deposits, where archetypal marine ichnofacies (i.e. Skolithos and Cruziana) alternate with stressed expressions of marine suites (i.e. depauperate Cruziana Ichnofacies), indicating rapid changes in salinity conditions due to times of freshwater discharge and return to fully marine conditions. Shoreface, offshore and shelf are characterized by the Skolithos and archetypal Cruziana Ichnofacies, indicating persistence of normal-marine salinity conditions. Salinity is a crucial factor in the development of benthic organisms and is independent of physical sedimentological processes. Therefore, understanding ichnofaunal distribution is very important for paleoenvironmental characterization of marginal-marine settings. In addition, the Glossifungites and Teredolites Ichnofacies indicate erosional exhumation of coastal-plain deposits, providing insights into sequence-stratigraphic interpretations. The Oficina Formation (15.97-12.7 Ma) in the Orinoco Oil Belt comprises a single 2nd-order sequence, which is divided into two third-order depositional sequences (DS1-2). Third-order sequences provide a better understanding of reservoir distribution and are associated with sea-level changes. DS1 is bounded by sequence boundaries U-1 (15.97 Ma) and U-2 (13.82 Ma) and includes maximum flooding surface MFS-1 (14.91 Ma). It consists of thick lowstand systems tract (LST) and transgressive systems tract (TST) strata, and a thin highstand systems tract (HST) package. DS1 is associated with incised-valley systems formed during a relative sea-level fall. Fluvial valley-fill is recorded by FA1. The fluvial valleys were replaced by estuarine valleys during the Langhian relative sea-level transgressive episode. The estuarine valley-fill displays a retrogradational stacking pattern, comprising FA2, FA3 and FA4. Thin deltaic deposits also occur in the uppermost interval of DS1, forming a thin HST. DS2 is bounded by U-2 (13.82 Ma) and U-3 (12.7 Ma) and includes MFS-2 (13.53 Ma). It consists of a thin TST and a thick HST formed during the Serravallian sea-level highstand. In DS2, transgressive deposits in the lower part form a thin TST interval reflecting delta abandonment, which rests directly on top of the underlying highstand systems tract (HST) deposits of DS1, therefore mantling a flooding surface/sequence boundary. The bulk of DS2 is represented by FA5, displaying a progradational stacking pattern. The Oficina Formation of the Orinoco Oil Belt shows similarities in sedimentologic, ichnologic and sequence-stratigraphic aspects to other marginal-marine units worldwide (most notably the Cretaceous McMurray Formation of Alberta), representing a broad spectrum of latitudinal contexts. These similarities therefore stress the importance of tidal dominance and relative sea-level changes as main controls on deposition, regardless of latitudinal controls. However, latitude may have played some role in controlling the establishment of extensive coastal wetland systems, the abundance of tidal channels and the types of burrowing organisms.
Orinoco Oil Belt, Delta, Estuary, Ichnofacies, Salinity, Sequence stratigraphy, Biostratigraphy, Sedimentary facies, Tide-dominated environments
Doctor of Philosophy (Ph.D.)