Late Jurassic-earliest Cretaceous reef analogues for the Eastern Black Sea; palaeogeographic setting, facies, strontium isotope stratigraphy and reservoir potential

Seismic data have revealed the likely occurrence of Late Jurassic earliest Cretaceous reef complexes up to 1 2 km thick and 10 20 km wide on the northern Shatskiy Ridge in the Eastern Black Sea. Reefs may also be present on the Mid Black Sea High. Widespread onshore exposures of Late Jurassic earliest Cretaceous reefs in the Pontides, Russian western Caucasus and Crimea are a similar size to those in the subsurface and form excellent analogues to these potential reservoirs.

The study region formed part of the northern margin of Tethys during the Late Jurassic earliest Cretaceous. In common with much of the north Tethyan margin, reefs in the study region can be grouped into coral-dominated, siliceous sponge-microbial and microbial types. Coral-dominated reefs occur as patchy and massive forms, and can be subdivided into higher-diversity and low-diversity (platy) types. The former developed at shallow-water platform margins and in platform interiors, whilst the latter occurred in 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, the availability of nutrients, and sedimentation rate. Thick reef occurrences are often associated with fault-bounded platform margins.

In many regions, karstic surfaces / unconformities punctuate the Late Jurassic earliest Cretaceous carbonate stratigraphy. These may be associated with zones of enhanced secondary porosity and form markers on seismic. The resolution of existing biostratigraphic data is insufficient to test if these surfaces are synchronous between regions. A pilot strontium isotope study of a well-exposed section in the eastern Pontides has been carried out as a result. The section contains a major truncation surface with a local relief of at least 45 m. Isotopic values supported by foraminiferal biostratigraphy indicate that it represents an intra-Tithonian to Berriasian hiatus. This technique has the potential to be applied to other outcrops in the region.

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%.