Unconventional resources of hydrocarbons within "mature regions" of Northern Bulgaria.

The thick Pz-Mz sedimentary cover of Northern Bulgaria is under exploration for oil and gas since 1949. During past time one dozen of mainly small to medium pools in conventional reservoirs have been found. Now, these pools are depleted or in status of declination. Cumulative gas and oil extraction from these old plays till nowadays ranges some 15 million toe. In the meantime, some data collected from well tests and rock geochemistry has- revealed the existence of positive premises for extractable unconventional hydrocarbons, such as: a) shale gas, b) gas and oil from tight reservoirs and c) coal bed methane (CBM). Nevertheless this information is still not well known or its existence is totally neglected. Depicted here sedimentary bodies/units have a lot common with the models of unconventional natural gas and oil famous sources from Northern America. Their major descriptors are firstly its organic carbon content, then organic matter typology, grade of katagenetic evolution, subsurface depth, geo-thermal regime and geo-pressure ambiance. Accordingly these criteria the Mogilishtenska Formation (from the base of Upper Carboniferous series in Dobrudzha Coal Basin) is in fact promising source of coal bed methane (CBM) and gas from tight clastic reservoirs. More tight reservoirs of oil and gas are proven within the deposits of Babinska and Mitrovska Formations (Ladinian-Carnian) as in the Bachiyshtenska, Kostinska and Ozirovska Formations (Early Jurassic tomarine facieses). Most convenient reservoir for shale gas and oil is the lower portion of Etropolska Formation (Middle Jurassic), spread over area of 12 000 km². Some authors (e.g. geologists from Chevron) are strongly attracted by a segment of typical black shale (O?-S) from NE Bulgaria. This rock association is known from Vetrino-2 well only and the XRD probing indicate anchizonal grade of metamorphism (the fact is very unusual for now known producing shale gas formations). Obviously, the farther investigation, converting potential unconventional resources of hydrocarbons to assessed reserves in place, will need a totally new technological approach to the reservoir exploration and stimulation (these modern methods are under prohibition in Bulgaria by now). [ABSTRACT FROM AUTHOR]