Late Jurassic Reefs of the Western Caucasus and Crimea: Hydrocarbon Implications for the Eastern Black Sea

Late Jurassic reefs are potentially important reservoir facies in the Eastern Black Sea Basin. Seismic reflection data from the northern Shatskiy Ridge of the Eastern Black Sea show apparent reef complexes up to 1-2 km thick and 10-20 km wide. Reefs may also be present in the area of the Mid Black Sea High. Widespread onshore exposures of the Late Jurassic reefs in the Russian Western Caucasus and Crimea provide excellent reservoir analogues for offshore targets.

The Russian Western Caucasus and Crimea formed part of the northern margin of Neotethys during the Late Jurassic, and carbonate production began in Mid-Late Callovian time. Outcrops of the Late Jurassic reefs can be grouped into coral-dominated, siliceous sponge-microbial and microbial types. Patchy and massive coral-dominated reefs formed at shallow-water platform margins or in slightly restricted deeper-water mid shelf settings. Siliceous sponge-microbial and microbial reefs occur as lenses and mounds and are restricted to deeper-water mid-outer shelf environments. The development of these reefs was controlled mainly by local variations in water depth, light, and the availability of nutrients.

The reefs exhibit a complex pattern of porosity development reflecting independent diagenetic histories involving near-surface and deep-burial dissolution, dolomitization and dedolomitization. Porosity is particularly common in coral-dominated reef facies and consists of both primary and secondary types. The amount of visual porosity estimated at outcrop is up to 5%.

Coral-dominated reefs analogous to onshore outcrops in the Russian Western Caucasus are likely to occur along the northwestern margin of the Yuzhnyi-Adler carbonate platform in the Eastern Black Sea. Possible isolated deeper-water reefs imaged on the northern Shatskiy Ridge could be largely composed of siliceous sponge-microbialite and microbialite facies. Similar reef facies may be present on the Mid Black Sea High.