Geologic Framework of Oil and Gas Genesis in Main Sedimentary Basins from Romania Oprea Dicea

IONESCU, N., and C. D. MORARIU, IPGG, Bucharest, Romania

The main hydrocarbon-bearing structural units belong to the orogenic domain of margin-plate type: the flysch and molasse organic zone of the Eastern Carpathians, the Paleogene and Neogene molasse zone of the Southern Carpathians (Getic depression), the post-tectonic basins with molasse formations (Transylvanian and Pannonian depressions), and intraplate-type basins (Moesian, Moldavian, and Scythian platforms and the Black Sea Plateau).

Oil and gas fields located in Moldavic nappes are encompassed in Oligocene and lower Miocene formations, mostly in the marginal folds nappe, where Kliwa Sandstone sequences have high porosity, and in the Black Sea Plateau.

Hydrocarbon accumulations are frequently present in areas extending the marginal folds nappe. They are present from the half-window beneath the Tarcau nappe and from the half-pools hosted in faulted anticlines or scale-folds, and they occur at different levels in superimposed subunits. There are some relations between the depth of foreland units beneath the nappes, the window, and the half-window and the possibility of finding new prospective stages. The big longitudinal and transversal faults of the platform margins have a determinant role in the thickness of the underthrusted nappe package.

In the external zone of the Carpathian foredeep, the hydrocarbon accumulations are encountered in the median and western parts of the bending zone, or its asymmetrical flanks. The accumulations are confined in Brach anticline folds, growth anticlines, monoclines and faulted monoclines, and stratigraphic and lithologic traps.

In the internal zone of the foredeep, the most varied structures containing hydrocarbons are in the so-called diapir folds. There are all types of salt structures, from exaggerated to cryptodiapir folds. Some accumulations are common to sub-Carpathian nappes, not the folded deposits of the internal foredeep.

The origin of the hydrocarbon accumulations from the Carpathian foredeep seems to be connected to the Oligocene-lower Miocene bituminous formations of the marginal folds and sub-Carpathian nappes.

In the Gethic depression, the hydrocarbon accumulations originate in Oligocene and Miocene source rocks and host in structural, stratigraphical, and lithological traps. The accumulations connected with tectonic lines that outline the areal extension of the Oligocene, Miocene, and Pliocene formations are in the underthrusted Moesian platform.

The hydrocarbon accumulations related to the Carpathian foreland represent about 40% of all known accumulations in Romania. Most of them are located in the Moesian platform. In this unit, the oil and gas fields present a vertical distribution at different stratigraphic levels, from Paleozoic to Neogene, and in all types of reservoirs, suggesting multicycles of oleogenesis, migration, accumulation, and sealing conditions. The structures are moderately to strongly fractured, tabular or slightly undulated; some of them are controlled by sedimentogenesis factors (unconformities, facial variations), forming typical paleovalley, pinch-out, and reef traps.

Oil and gas fields of the Moldavian and Scythian platforms relate both to the evolution of sedimentary sequences of the external flank of the Carpathian foredeep and to the depositional and the thermal history of their Paleozoic and Mesozoic formations.

The hydrocarbon deposits known so far on the Black Sea continental plateau are confined in the Albian, Cenomanian, Turonian-Senonian, and Eocene formations. The traps are of complex type structural, lithologic, and stratigraphic. The reservoirs are sandstones, calcareous sandstones, limestones, and sands. The hydrocarbon source rocks are the pelitic and siltic Oligocene formations. Other older source rocks are probably Cretaceous.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)