Discovered in 1955, the Lower Lagunillas member reservoir of the Miocene Lagunillas formation of Bloques III and IV of the Bachaquero field was originally estimated to contain 2 billion barrels of oil. This reservoir interval traditionally has 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. Field performance, however, has indicated that the reservoir is more complex; the present study was initiated to provide a revised development strategy.

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 (FT-IR) measurement of clay concentrations and prompt neutron capture boron measurements that are indicative of a brackish water-depositional setting. We used this geological model 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 tidal 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 increase recovery further from this mature field.

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