AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Alpine inversion, Neogene tectonics and East Carpathians Foreland hydrocarbon traps.

Abstract

The Alpine inversion and the Neogene evolution are proven to have a major impact on Eastern European hydrocarbon systems. The East Carpathians Foreland, part East European Plate has a Precambrian basement and Paleozoic, Mesozoic and Miocene sediments. The sedimentary basin within the platform was highly influenced by tectonic Orogen - Foreland relationship. The opening of Alpine Tethys, the Mid Cretaceous and Paleogene inversion and movements were accompanied by a partial removal of Mesozoic cover. Miocene tectogenesis determined the Outer Carpathian Flysch units to get over the Neogene Tertiary Molasse Zone. The presence of the hydrocarbons in this area is due to the association in a certain manner of a reservoirs, traps and migration pathways. High resolution seismic records helped us to clarify the relationship between sedimentary basin dynamics and hydrocarbon distribution. The complex structure of molasse deposits overthrusts on the foreland along of the NNW-SSE Pericarpathian Fault. The westward descent of the platform is achieved along of some fault systems with the same orientation, affecting the basement and sometimes, the sedimentary cover as well. The duplex Paltinoasa - West Paltinoasa System Faults is proven to play the most important role in structural traps alignments. More or less eastward advancing of Pericarpathian Unit surpassed or left behind Paltinoasa Faults system. The interlining zones prove to be the most favourable for hydrocarbon structural traps. The vertical and the strike-slip neotectonic movements have also played an important role, horst and graben structures were created on both sides of the longitudinal faults. Beginning with Alpine inversion and continuing with Neogene deformations the area manifested differences in stress and subsidence. So, different type of hydrocarbon traps has been formed: wide anticlines between normal faults (in the North), strong symmetric anticlines flanked by high step normal faults and roll-over on reverse faults (in Central part), and large monoclinal structures (in Southern part). Depending on the stress in the area, the faults that affect the reservoirs may be tight or not. The base Neogene tectonic style suggested by the TWT isochronous map represents an essential element in establishing the perspective structural alignments and the geometry of the main faults. Seismic data show another alignment of structures further west, under the Pericarpathian Unit, at about 3000 - 3500 m depth. The stratigraphic traps are present in the Sarmatian deposits. After a short break at the end of Badenian, when the Parathetys waters withdrew, the sedimentation restarted, the first deposits thickness proving a continue subsidence process but with discontinuous character. The next sediments become more complex, sand and sandstones (even oolotic limestones), lacustrine and deltaic deposits. The delta-front sandy bodies are the main objectives for biogenic gas accumulations. During Upper Sarmatian the sedimentation continued only in the southern part of the platform. Seismic images and wells correlation offered a new interpretation regarding the distribution of the productive levels. Even if the traps are litho-stratigraphic, the hydrocarbons distribution is tectonically controlled, being connected with the westwards descending, gravity collapse and pinch-outs within each step. Although Moldavian Platform is the oldest platform on Romanian territory, only Mid -Upper Miocene reservoirs are productive. For Paleozoic and Mesozoic formations, protection disruption has influenced the preservation. And that, because the major tectonic events that have controlled the structural evolution. Alpine inversion had a negative impact; the Neogene development had a positive one.