Sedimentology and depositional model for the Early Miocene Oficina Formation in the Petrocedeño Field (Orinoco heavy-oil belt, Venezuela)

Abstract

The Early Miocene Oficina Formation in the Petrocedeño Field (Venezuela) produces extra-heavy oil with an initial gravity of 8.5° API. Reservoir zonation is successfully based on a 3rd-order sequence stratigraphic framework for which a detailed sedimentological understanding is crucial. This paper presents a, primarily core-based, detailed account of the sedimentology and sequence stratigraphy of the Oficina Formation in the Junín (formerly Zuata) Region of the Orinoco heavy-oil belt. In Petrocedeño, the Oficina Formation is typified by the long-term (approximately 7 Ma; 2nd-order sequence) change of a fluvial dominated deltaic system to a tide-dominated lower delta plain and subsequently a tide-dominated, subtidal, upper delta platform. A fluvial braidplain environment with mostly sand-dominated braided and sinuous rivers (Sequences 1 and 2) changed into a fluvially-dominated but notably tidally-influenced delta plain with straight and sinuous channel belts (Sequences 3–6). Subsequently, a mixed-energy (fluvial and tidal) delta front environment with numerous distributaries formed. This delta front drowned and changed into an estuary system with bay-head delta mouth bars (Sequence 7). Maximum transgression was reached in Sequence 8 with the development of fully marine shale. Subsequently, a low-gradient tide-dominated lower delta plain with distributaries and meandering channel belts established (Sequences 9 and 10). These were gradually transgressed to develop into proximal delta platform environments with a limited number of distal distributaries and large protected subtidal areas (Sequence 11). Together, the lower delta plain and upper delta platform facies form one of the few currently known subsurface examples of a tide-dominated delta. The change in depositional style was controlled by a long-term increase of accommodation space in the developing Eastern Venezuelan Basin. The lower part of the formation was primarily controlled by compressional tectonic activity (third-order Sequences 1 through 6). The upper half, however, was primarily controlled by Early Miocene eustatic changes (Sequences 7 through 11).