Late Jurassic Reefs in the Russian Western Caucasus, Crimea and Pontides: Implications for the Eastern Black Sea

Seismic data have revealed the likely occurrence of Late Jurassic reef complexes up to 1-2 km thick and 10 20 km wide on the northern Shatskiy Ridge in the Eastern Black Sea (Figure 1). Reefs may also be present on the Mid Black Sea High. Widespread onshore exposures of Late Jurassic reefs in the Russian western Caucasus, Crimea and Pontides (Zonguldak Zone, Central Pontides and Eastern Sakarya Zone) provide excellent reservoir analogues for offshore targets (Figure 1).

The Russian western Caucasus, Crimea and northern Pontides formed part of the northern margin of Tethys during the Late Jurassic. Reef development was widespread throughout Oxfordian to Tithonian time. These reefs, similar to the Late Jurassic reefs in other part of northern Tethys, can be grouped into coral-dominated, siliceous sponge-microbial and microbial types (Figure 2). Coral-dominated reefs occur as patchy and massive forms; they formed mostly on shallow-water platforms, although some (platy) coral communities also developed in slightly restricted deeper-water 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, the availability of nutrients, and sedimentation rate.

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%.