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The Importance of the Malm-Neocomian Carbonate Buildups in the Petroleum System of the Moesian Platform

Abstract

The heterogeneous basement of the Moesian Platform is overlain by a very thick sequence of sedimentary deposits. The marine Lias-Lower Cretaceous sedimentary cycle started with a lower clastic (Lias-Dogger) sequence followed by the carbonate Malm-Neocomian cycle. The carbonate sedimentation started in relative deep waters and was uniform. The Upper Tithonic sedimentation was differentiated in a basinal pelagic facies in Central Moesia, a neritic littoral facies with reefal episodes in East and West and a lagoonar-evaporite facies in eastern extremity of the Platform. The dynamics of marine waters has contributed to the spatial development orientation of carbonate buildings, determining reefal buildings of barrier, pinnacle and fringe typ. The hydrocarbon system of Moesian Platform is characterized by alternatively deposits of source, reservoir and seal rocks. The factors controlling the hydrocarbon distribution are the source rocks presence, West-East normal faults trend and the presence of areas of high geothermal gradient. The highest values of geothermal gradient (60 C/100 m) were recorded west of Arges River, the eastern part of the Platform outlines a geothermal minimum anomaly (1.5–20 C/100 m). Source rocks are found in the Silurian, Lower Devonian, Middle Triassic, Upper Lias-Dogger and Badenian-Sarmatian formations (0.5–2.5% TOC). West of Arges River liquid hydrocarbons could be generated starting from 1000 m depth, at the East, liquid hydrocarbons could be generated starting from 2500–3000 m depth. To the East, the Devonian bituminous limestone and Silurian shists with Graptolites can compensate the small thickness or lack of the Lias-Dogger bituminous shists. The reservoirs show good primary inter-granular porosity in detritic organogenous and oolitic limestone and secondary fissured and cavernous porosity. Resevoirs from carbonate buildings were sealed at the top by draping of covering formations, laterally by compacted inter-reefal limestone. There are 126 oil and gas fields in the Moesian Platform, located from Devonian up to Neogene, from 350 m to 4900 m in depth. The majority of the fields are oil bearing, two third in the Mesozoic formations. Seismic data interpretation has found the existence of more than 50 carbonate buildings, more than 20 in the eastern sector of the Moesian Platform.