Assessing thermal maturity of organic-rich Maykop facies in the Western Black Sea

The Black Sea developed as part of the Alpine-Himalayan folded belt overlying newly formed oceanic or thinned pre-Cretaceous crust. During its formation, a thick sedimentary sequence in the western part of the basin comprising of clearly defined units starting at the Upper Cretaceous was buried. Recent research has revealed that much of this buried organic matter is being transformed into petroleum components such as oil and gas. In fact, there is direct evidence of this active petroleum system, i.e. oil reservoirs being encountered in deep and shallow-water locations, mud volcanoes, gas seepage sites and gas hydrate deposits. Moreover, it has been recently suggested that large amounts of thermally reactive organic matter have been exposed to sufficiently high temperatures and consequently converted into mobile hydrocarbons. The free gas presence in marine sediment of the Black Sea is confirmed by the observed Bottom Simulating Reflectors (BSRs) often associated with gas hydrate deposits located directly above. BSRs are widely present in the western Black Sea basin; however, their presence does not provide any quantitative information on the free gas accumulations located directly beneath.

Present-day hydrocarbon budget of the Western Black Sea Based on the present-day steady-state conditions assuming a bottom water temperature of 9 °C and the pore water salinity of 3, the upper limit of the gas hydrate stability zone (GHSZ) coincides with a water depth of about 665m. However, due to transient temperature profiles in the sub-surface and the sediment deposition history, multiple BSR levels have been observed. Therefore, as methane gas can be formed in both biogenic as well as thermogenic processes, it is necessary to distinguish between these two sources. To address this, German researchers from the GEOMAR Helmholtz Centre for Ocean Research Kiel: Dr. Ewa Burwicz and Dr. Matthias Haeckel, designed a new application of 3-D numerical modelling to estimate the total amounts of both biogenic and thermogenic methane gas and other petroleum components. Their work is currently published in the research journal, Marine and Petroleum Geology.

The researchers used the commercial petroleum software PetroMod^®, to reconstruct the entire depositional history of the Western Black Sea basin to assess basin-scale estimates of organic matter maturation, petroleum component generation and expulsion from the source rock, as well as migration and accumulation. Through this spatio-temporal reconstruction, they investigated the main factors leading to the present-day methane gas budget and its distribution in the hosting marine sediments. In addition, the model was able to estimate the rates and total amounts of the in-situ biogenic methane generation and thermally-driven organic matter maturation in the source rocks. In summary, the Burwicz-Haeckel study presented a novel numerical model for reconstructing the depositional history (98–0 Ma) of the Western Black Sea sub-basin. Most important, the amount of biogenic methane generated on a basin scale almost doubles the amount of its thermogenic equivalent (~3,100 Gt and ~1,560 Gt, respectively) due to insufficient thermal maturation of the organic-rich Maykop facies at present.. Overall, their findings were in agreement with the isotopic composition of methane reported for the region and other modeling studies.