Upper Jurassic carbonate build-ups along the northern margin of the Tethys: a broad underexplored reservoir play fairway between the Vienna and the Amu Darya basins

In this very large scale overview of Upper Jurassic carbonate build-ups along the northern margin of the Tethys we compiled case studies where these features were targeted for oil and gas. The Upper Jurassic, Oxfordian to Tithonian, carbonate play fairway is well known regionally, spreading from the Lusitanian Basin to the Central Asian Salt Basin (CASB). The size of the commercial reserves associated with this reservoir play fairway strongly varies along strike. For example, the exploration success targeting these build-ups in Portugal is negligible, in Poland it is considerable and in Turkmenistan it is outstanding.

The patchy nature of discoveries and failures associated with this play fairway is, of course, due to several other factors rather than just the reservoir quality of the build-ups. In the CASB of Turkmenistan and Uzbekistan the uppermost Jurassic salt provides the super-seal above numerous reefs charged by gas. Besides the critical presence or lack of efficient hydrocarbon charge, the biggest challenge appears to be the proper subsurface identification of these sometimes subtle features. Whereas in some areas, like in the Russian Eastern Black Sea region, the sheer size of the reefal build-ups makes them easy to image on seismic reflection data, in some other basins, like in Poland or Bavaria, their seismic expression is very subtle, at best. The seismic detectability is strongly dependent on the particular carbonate facies of the build-ups, i.e. reflecting the geometric differences and lithologic contrasts between large coral dominated high-relief reefs versus much smaller siliceous sponge and microbialite reefs.

Our compilation suggests that a good understanding of the paleogeography in any given basin is also critical in predicting the presence of reservoir-quality Upper Jurassic carbonate build-ups. In the Vienna Basin, for example, isolated reefs can be found only on the Austrian side of the basin as opposed to the Moravian side, reflecting the geometry of the paleo-shelf in relation to the adjacent deep-water basin.

Regionally, where Upper Jurassic build-ups are coral-dominated reefs, we have found reasonable porosity and permeability values for at least gas reservoir targets. Examples of oil production associated with these reservoirs seem to be less widespread. We consider the Oxfordian to Tithonian carbonate play fairway as regionally underexplored. There are basins along this vast fairway, for example in Ukraine and Georgia, where the play may deserve more systematic exploration efforts than in the past.