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BASIN SUBSIDENCE AND THERMAL MATURITY OF TERTIARY SEDIMENTS IN BULGARIAN EXCLUSIVE ECONOMIC ZONE - PRECONDITIONS FOR GAS HYDRATE RESOURCES
Abstract
The discovered industrial and semi-industrial hydrocarbon reservoirs, seeps and offshore gas hydrate fields in the Bulgarian Exclusive Economic Zone (BEEZ) are evidence of the generation and migration of oil and gas in the Western Black Sea basin. Marine gas hydrates from the deep water palaeodelta of Danube are seismically studied recently, but the issues about their origin and thermal maturity of sediments still need significant clarification. Therefore, the aim of the paper is to study the basin subsidence and reconstruct thermal maturity of Paleogene and Neogene deposits, which are the most perspective stratigraphic intervals for source rocks in the Black sea area. The geological, geochemical and geophysical methods and previous results are applied in order to define and model the perspective sediments for gas generation and possible accumulation of marine gas hydrates in the deep water environment. The most important for the evolution of the Western Black Sea basin during the Tertiary period were post-rift thermal cooling and subsidence, as well as widespread post-Middle Eocene compression. After the Paleocene, 4 major periods of basin subsidence (Eoc2; Eoc3-Oli; Mio; Plio-Q) can be separated, distinguished by three erosional events. The most expressive basin subsidence occurs during the Middle Eocene and with respect to the zonal distribution a clear tendency is for increasing of the burial towards the abyssal plane. The Tertiary sediments in the Bulgarian waters differ with a relatively low degree of thermal maturity, related to their depth of deposition. This is also due to the lower values of paleo-heat flow in the Western Black Sea basin. The highest generation potential in the BEEZ, in terms of organic matter and their maturity, is possessed by the popular in Black Sea ? "Maikop Series" (Ologocene Ruslar Formation in Bulgarian zone). However, everywhere in the area there are favorable conditions for biogenic methanization from the Oligocene and especially from the Miocene deposits, and promising localities for the prospecting and exploration of gas hydrates have been identified in the palaeodeltas of the Danube and Dnieper Rivers.
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