Summary Discovered in 1955, the Lower Lagunillas Member reservoir of the Miocene Lagunillas Formation of Bloque IV of the Bachaquero field was and was originally estimated to contain 2 billion barrels of oil. This reservoir interval has traditionally been interpreted to have been deposited in a delta plain setting and to comprise three reservoir subdivisions that were developed as a single drainage unit. Sedimentological interpretation of four cored wells has led to the development of a new model of deposition in tidally influenced lower delta plain and delta front settings. This model is supported by Fourier Transform Infrared (FTIR) measurement of illite concentrations and prompt neutron capture boron measurements that are indicative of a brackish water depositional setting. The conceptual geological model has been used to guide correlation of wireline logs from 46 wells in the central part of Bloque IV and to provide a high-resolution sequence stratigraphic model of the Lower Lagunillas reservoir. Eleven genetic layers are identified that are separated by locally developed intraformational seals into up to eight drainage units. High permeability, tidally influenced channel-fill sands have acted as preferential conduits for gas influx, leaving bypassed oil in lower quality sands that were deposited as lagoonal deltas and bars. This reservoir model has been supported by re-examination of production data and by openhole measurements in a recent infill well and cased-hole logging of two other wells in the study area. The new model will form the basis for redevelopment of the Lower Lagunillas reservoir to further increase recovery from this mature field. Introduction This sedimentological investigation formed part of a wider study aimed at generating a detailed characterization of the Lower Lagunillas reservoir in a pilot study area located in the central part of Bloque IV of the Bachaquero field, Lake Maracaibo, Venezuela. A prerequisite to providing reliable estimates of log-derived reservoir properties in the interwell areas and ultimately predicting the distribution and movement of fluids within the pilot study area was to establish a robust geological framework. This was achieved by interpretation of core and wireline log data within a high-resolution sequence stratigraphic framework. In this account, we will illustrate how evaluation of dynamic reservoir data within this high-resolution sequence stratigraphic framework has been used to validate the model and to successfully identify targets for horizontal infill wells within this mature reservoir. Geological Setting The Maracaibo basin covers an area of some 50,000 km2 in the northwestern part of Venezuela (Fig. 1). The Cretaceous to Middle Eocene sedimentary fill of the basin was deposited in an extensional tectonic regime created by the opening of the proto-Caribbean, as the North American and South American plates began to drift apart. In the Late Eocene, the tectonic regime became transpressional (strike-slip) as a result of the development of a convergent plate margin in northwest South America. Late Eocene, Oligocene and Early Miocene sediments are not present in the area of Bloque IV and a prominent unconformity represents substantial erosion of the underlying Eocene deltaic sediments of the Misoa Formation, during the initial, Late Eocene, phase of basin inversion (Fig. 2). Subsequent deposition of Miocene sediments took place in a foreland basin setting under a transpressive tectonic regime. The oldest Miocene rocks of the area, the La Rosa Formation, overlie the base Miocene unconformity and were deposited during an Early Miocene (Burdigalian) transgressive-regressive cycle of sedimentation (Fig. 2). This marine transgression moved progressively from the northeast to the southwest of the basin.