4-D Stratigraphic Architecture and 3-D Reservoir Zonation of the Mirador Formation, Cusiana Field, Colombia

Andres A. Fajardo and Timothy A. Cross

A high-resolution sequence stratigraphic study using 2300 feet of core calibrated with geophysical logs from 14 wells and 1800 measurements of porosity and permeability established the 4-D stratigraphy and 3-D reservoir zonation of the Mirador. Virtually all reservoir-quality facies are trough cross-stratified sandstones which occur in channel facies successions in the lower Mirador, but in bay-head delta and estuarine channel facies successions in the upper Mirador. Petrophysical properties and the geometry, continuity and volume of reservoir-quality sandstones change regularly as a unction of their stratigraphic position.

Two long-term asymmetrical base-level rise cycles were defined. The lower long-term cycle comprises three intermediate-term landward-stepping asymmetrical base-level rise cycles. Each consists of a basal amalgamated channel sandstone overlain by crevasse splay successions which are succeeded by lake facies successions. These vertical facies successions reflect increasing accommodation-to-sediment supply (A/S) ratio through each intermediate-term cycle.

The upper long-term cycle comprises four intermediate-term, landward-stepping symmetrical base-level cycles. These cycles consist of estuarine channel, bay-head delta and bay-fill facies successions. The transition from channel to bay-head to bay-fill facies successions represents an increase in A/S ratio, and the reverse transition indicates a decrease in A/S ratio.

Sixteen reservoir zones were defined within the Cusiana field. Reservoirs within the upper and lower long-term cycles are separated by a continuous middle Mirador mudstone which creates two large reservoir divisions. At the second level of zonation, the reservoir compartments and fluid-flow retardants coincide with the intermediate-term stratigraphic cycles. A third level of reservoir compartmentalization follows the distribution of facies successions within the intermediate-term cycles. A strong stratigraphic control on reservoir properties occurs at the three scales of stratigraphic cyclicity. In all cases as A/S ratio increases, porosity and permeability decrease.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California