Fundamental Reservoir Units of Heterozoan Carbonates: Sedimentologic and Reservoir Properties of Shoaling- and Fining-Upward Cycles

Abstract

Hydrocarbon-bearing reservoirs in heterozoan carbonate deposits currently lack necessary predictive reservoir-analog models. Neogene complexes of the Cabo de Gata region of SE Spain provide ideal outcrops of heterozoan carbonates preserved as fining-upward cycles and fining-up-depositional-dip progradational clinothems. This pattern differs significantly from the coarsening-upward shoaling cycles of photozoan carbonates, suggesting the fundamental differences of heterozoan and photozoan carbonate reservoir architecture. Study of multiple outcrops of Miocene and Pliocene carbonates shows there are four fundamental cycle types. The cycles are documented using vertical and time-equivalent, lateral stratigraphic sections to provide a 3-D representation of facies. Each cycle contains coarse rhodolith and bivalve-rich gravel facies at the base, representing in situ production in deeper water. These facies systematically fine upward into sand-sized bioclastic facies that result from wave abrasion in shallower water, indicating a shoaling-upward origin for each cycle. Petrographic and petrophysical analyses allow for the quantification of sedimentologic variables and their control on reservoir character, as well as evaluation of geologic controls on reservoir-analog properties. Analyses include grain size and sorting, origin of sediment supply, grain constituents, abrasion parameters, mineralogy, and diagenesis as they relate to and are controlled by the shoaling- and fining-upward cycle. Preliminary porosity analyses demonstrate that the basal, and cycle-capping facies contain the highest porosity at 26–29%, whereas medially deposited coarse-grained facies contain 13–19% porosity, and with burial, may act as potential baffles to fluid flow. The main controls on porosity distribution appear to be grain sorting and the abundance of coralline algae. Analysis of the extent of diagenetic alteration correlates to grain size, and therefore enhances variation in reservoir quality throughout cycles. We propose that the shoaling- and fining-upward cycle could be used as a fundamental object-based unit for construction of subsurface geomodels in heterozoan reservoirs. Property modeling in Petrel will be used to develop 3-D reservoir-analog models for application. These results provide a vital reservoir unit and data applicable to recent discoveries in offshore Vietnam and Perla Field, Gulf of Venezuela, as well as application to future discoveries of heterozoan reservoirs.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015