Sand Depositional Styles in Lower Lagunillas Member (Lower Miocene) of Central-Western Area of Maracaibo Lake, Venezuela, and Their Influence in Reservoir Modeling

I. Azpiritxaga, M. A. Lorente

Detailed sedimentologic and palynologic studies of two continuously cored wells in the lower oil-bearing member of the lower Miocene Lagunillas Formation, at Lake Maracaibo, and evaluation of 200 well logs, allowed definition of two main sedimentary units. A lower unit with four different sand facies (A₀, A₁, A₂, and A_g) and an upper unit with only one sand facies (A₃).

From base to top, facies A₀, A₁, and A₂ have specific organic assemblage and rocks texture, have sand development restricted to the north and eastern area, and a succession of northeast-southwest-trending sand bodies of lobate morphology. They represent a succession of distributary channel systems. Toward the west, a lateral change in facies (A_g) is shown by the development of sand bodies with southwest-northeast trend and elongate geometry, probably representing coastal sand bars.

The lower unit grades vertically into a widespread facies (A₃), characterized by sand bodies with a general northwest-southeast trend and elongate geometry (similar to A_g). Their organic assemblage and sedimentological characteristic indicate deposition in a coastal sand-bar complex.

Each facies has its own specific sedimentologic and petrophysical characteristics that make them behave as a "flow unit." As a consequence, the lateral and vertical changes in facies control physical continuity of reservoirs, directly influencing secondary-recovery processes. Recognition of these factors allows better reservoir modeling and eventually higher oil recoveries.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.