Abstract: Petrofacies Evolution of Cretaceous and Paleogene Siliciclastites from the East Carpathians (Romania)--Related to Clastic Reservoir

Nicolae Anastasiu

The East Carpathians Belt--the first oil and gas province discovered in the Romanian territory--comprises more geotectonic units of which the external Dacides and Moldavides consist of carbonate and siliciclastic flysch.

The source rocks (bituminous facies) have been identified in the Cretaceous (Black Shale) and the Oligocene (Dysodilic shale) formations.

The oil and gas bearing reservoir rocks are mostly represented by arenitic and sandy-calcareous bodies from the Audia, Tarcau and Marginal Folds Nappe. The Siriu (K), Horgazu-Hangu (K), Tarcau (Eoc), Tazlau (Eoc-Ol), Lucacesti (Ol), Fierastrau (Ol), Kliwa (Ol) facies pointed out a large variety of siliciclastic petrofacies: grain-supported polimictic conglomerates, quartz sandstones, litharenites, feldarenites, and rare, graywackes.

The areal distribution of clastic petrofacies shows a sensitive change in petrographic framework of source area (from granitoides to metamorphic rocks, to volcanic rocks, to shelf sediments) and in space position (both extrabasinal and intrabasinal one). On the basis of clast mineralogy (especially feldspars and lithic fragments) it was distinguished three tectofacies provenance: recycled orogen, foreland uplift remanent basins and magmatic arc.

The temporal evolution of Dacides and Moldavides petrofacies from Cretaceous to Paleogene shows some trends which could be used for correlations in siliciclastic reservoir bodies. Therefore, it was distinguished:

-- two "quartz events"--at the level of the Albian (glauconite sandstone), and the Oligocene (Kliwa sandstone), too;

-- three volcanoclastic events: at the level of the Turonian (Bota facies), the Senonian (Hangu facies), the Eocene (Tarcau facies). The petrochemical evolution was from andesitic to rhyolitic and trachytic clasts.

From these points of view, the comparing of the reservoir will be possible by the clasts of quartz, feldspar and lithic fragments.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994