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ABSTRACT

During the late 1950s hydrocarbons were discovered in the Miocene Lagunillas Formation in Blocks III and IV of the Lake Maracaibo Basin, Venezuela. In ascending order, the Lagunillas is divided into three members: the Lower Lagunillas, Laguna, and Bachaquero. This study focuses primarily on the Lower Lagunillas and Laguna members, which are estimated to contain approximately 2550 MMB OOIP. Overall thickness of the Lower Lagunillas and Laguna members decreases from 630 feet in the southern part of the field to 435 feet along the northern margin. Prior to this study, the Lower Lagunillas and Laguna members were subdivided into six reservoir units for the purpose of reservoir management. However, production and engineering studies showed that this simplistic model, which assumed the six units were homogeneous and in pressure communication, did not adequately address the heterogeneities of the reservoir. A sedimentologic and sequence stratigraphic analysis that integrated eight cores, wireline log suites from 210 wells, and production and pressure data, was undertaken by a team of geoscientists and engineers from GeoQuest and Maraven. The result was a new geological model that contains 29 distinctive, correlatable reservoir layers consisting of 22 sandstones (reservoirs) and 9 shales (barriers). This revised model better explains the historical production performance of the field. Additionally, integration of the new reservoir correlation scheme with production and pressure data, gas chromatography of produced oil samples, and the interpretation of the 3D seismic survey identified several areas where sealing faults have further compartmentalized the reservoir.

The Lower Lagunillas and Laguna members were deposited in a variety of depositional environments ranging from fluvial and fluvial-estuarine to strandplain and fluvial-deltaic. The two members comprise part of a third-order sequence that includes at least part of the overlying Bachaquero Member. The contact of the Lower Lagunillas Member with the underlying La Rosa marine shale (Unit I) is interpreted as a type 1 sequence boundary.

The revised geological model was then utilized in a reservoir simulation of Blocks III and IV. Simulation results indicate that the shales associated with the nine mappable marine flooding surfaces and sealing faults have acted as barriers to vertical and horizontal communication and define reservoir compartments that contain bypassed oil. Recent drilling has supported the revised model and confirmed the importance of the shales and faults in controlling the distribution and movement of fluids. The revised reservoir model has also identified new opportunities to add reserves and significantly improve recovery from the field through infill drilling, horizontal wells, redesign of waterflood patterns, and recompletions.

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