The Pato-Onado area occupies 2000 km² south of the Eastern Venezuelan fold belt, north of the Oficina area, and east of the Anaco Fault.

The objective of the study is to establish new areas for exploration and locate new prospects. We have applied a sequence stratigraphy approach to 3000 km of seismic lines and well data in order to define zones of favorable sandstone reservoirs within each sequence. The analysis includes the Lower Miocene section from Merecure Formation through the lower part of the Oficina Formation.

Three third-order sequences are established, sequences C, D, and E, and examined in detail because they contain most of the volume of production and reserves in adjacent areas. Maps of thickness, sand percentage, and interpreted paleoenvironments and paleogeography are based upon integrated study of well logs, cores, petrophysical data, and biostratigraphy. Taking into account the relations between these properties and the patterns of sedimentation of transgressive and regressive parts of each sequence, we define the limits of maximum flooding surfaces and the related lateral variations of sandstone bodies.

Sequence C is the oldest and contains the largest percentage of sand. Its thickness increases toward the north and represents the submergence of a deltaic lobe, with environments that vary from continental to transitional marine in the central area, to middle neritic in the lateral flanks of the lobe.

Sequence D represents an increase in water depth and a slight northeastward migration of the delta, with the development of amalgamated channels at the base of the sequence oriented along the axis of the lobe. The highest percentage of sand is concentrated along two axes trending northeast parallel to the orientation of the lobe. The total thickness of the sequence increases to the northeast. The paleoenvironments vary from transitional to inner neritic, with some continental aspects in the central lobe.

Sequence E forms the most distal part of the deltaic lobe. In this sequence the percentage of shale increases and the major percentage of sand is located along the depositional axis oriented northeast-southwest, including peripheral distal bars. Paleoenvironments vary from transitional to inner neritic. The paleo-water depths increased to the west, and the maximum flooding surface produced widespread submersion with no change in the depositional axes.

These sequences were affected by an extensional tectonic regime that produced normal faults, resulting in the formation of combination stratigraphic-structural traps. Middle Miocene deformation is responsible for the entrapment of hydrocarbons which probably migrated during the last 15 million years as the foreland basin tilted northward. Light crude and condensate are thought to originate from Cretaceous and possibly Tertiary source rocks.

Integration of maps and sections of the sequences with structural maps permits the definition of areas with the best development of prospective sands and favorable fault geometry. Several new prospects and the improved evaluation of previous prospects have resulted.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas