Biogenic Gas System in the Upper Miocene from the Moldavian Platform

Visnevschi, Eugeniu ¹; Tarigradschi, Victor ¹; Slusari, Boris ¹; Grigorescu, Stefan ¹; Pene, Constantin ¹
(1) Faculty of Geology & Geophysics, University of Bucharest, Bucharest, Romania.

In the Moldavian Platform has been discovered gas and gas-condensate reservoired in Albian, Badenian and Sarmatian (dry gas) sandstones. The Moldavian Platform represents the western part of the East European Platform. During Alpine orogeny, the western part of the platform was gradually underthrusted by the Eastern Carpathian Orogene. This structural evolution imprinted a monoclinal character of the deposits and they dip westward beneath the Carpathian Foredeep (Molasse) and Eastern Carpathian Flysch.

The older deposits than the Upper Sarmatian ones plunge step by step beneath Eastern Carpathians along major faults. The intense compressional regime and the high subsidence rate during Upper Miocene created a huge pile of Sarmatian deposits in front of Eastern Carpathians, with more than 3000 m in thickness. This geological evolution favored the formation of the lithostratigrafic and structural traps in the Sarmatian deposits. This study focuses on the modeling of the geological processes in relation with the generation and accumulation of the biogenic gas in the Sarmatian reservoirs from the Moldavian Platform. These reservoirs have been discovered with dry gas in the V.Seaca, the Bacau and the most important, the Roman-Secuieni field. The reservoir is a fluvial deltaic section of interbedded sands and shales having economic hydrocarbon intervals identified over a subsea depth range from 3000 to 3500 m in the Roman-Secuieni field (the deeper reservoirs). Individual sand beds are 1-15m thick separated by shale beds of a similar thickness scale. To evaluation of the reservoir potential we created a composite log and corresponding lithological section from well-log suites and we assigned each interval along the composite log a reservoir quality value as a percentage based on the gamma-ray (GR) and spontaneous potential (SP) curves normalized to the value of the best reservoir sand. Thus, the best sand is assigned a value of 100 and the best shales a value at or close to zero. The burial curves and the vitrinite reflectance values show that the Sarmatian deposits are thermal immature. The chemical composition of the hydrocarbon (content of methane is more than 98%) suggests a biogenic origin of them. These hydrocarbon gas could be generated by bacterial alteration of the organic matter from the Sarmatian pelites.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.