ICE 2022


Modern low-latitude shallow-water carbonate systems are typically dominated by reef-building photosynthetic organisms (photozoans) when photic zone conditions are ideal. However, under adverse conditions (e.g., increased nutrients, cooler water), photozoans can be greatly reduced, and heterozoans (e.g., mollusks, echinoids, bryozoans) can dominate (termed transitional carbonate systems; TCS). Distinguishing between the two systems is important because facies types, stratal architecture, dominant mineralogy, diagenetic potential, and resultant reservoir character can be different. Despite their importance as reservoirs, TCS are not well understood or recognized. During the Miocene, adverse conditions created by well-documented upwelling in the Caribbean resulted in widespread development of shallow-water TCS. This project focuses on the early Miocene Cicuco Field, NW Colombia to better understand controls on deposition and reservoir character of a possible TCS. Our initial results from core, well log, and seismic data indicate shallow-water carbonates, and associated siliciclastics developed on basement paleo-highs. Core study indicates grainy carbonate (packstones and grainstones) and mixed carbonate-siliciclastic (fossiliferous siltstones and sandstones) facies are dominant. Carbonate facies include heterozoan and photozoan components. Dominant heterozoans include gastropods and bivalves. Photozoans are limited to corals (mostly Porites), large benthic foraminifera, and red algae, all known to tolerate elevated nutrients, turbidity, and cooler water conditions. The facies characteristics are similar to those found in nearby areas in Colombia (e.g., El Dificil Field) and TCS systems documented around the Caribbean, which formed under adverse photic zone conditions related to upwelling. Initial core data also indicate a general vertical facies trend. Mixed carbonate-siliciclastic facies are dominant in basal portions (nearest underlying basement) and pass upwards to grainy heterozoan and photozoan carbonates, including abundant in-place and reworked coral-rich facies in upper portions. Similar vertical facies patterns have been documented in time-equivalent rocks elsewhere in the Caribbean, and interpreted to reflect changing upwelling conditions tied to relative sea-level fluctuations. Our ongoing studies are continuing to evaluate the significance of regional and local controls on deposition and reservoir character.