Biostratigraphic re-evaluation of the lower to middle Miocene succession in the Eastern Carpathians: a case study related to the oil fields of the Diapir Fold Zone, Romania

Romania has a long history of hydrocarbon production and tens of thousands of boreholes have penetrated Miocene strata. Many well cores or cuttings have been either lost or damaged, but lab reports containing valuable petrographic, paleontological and structural data are still available. Most of the knowledge of the subsurface relies on old descriptions and interpretations used by the oil industry. These data have not been recently updated, while research results from the last decade suggest potential changes in stratigraphy, especially for the lower to middle Miocene succession. In order to update, calibrate, and reduce uncertainties regarding the subsurface stratigraphic record, we have reviewed the lab reports and used equivalent field samples for an updated interpretation of the lower to middle Miocene succession. Core and cutting descriptions from boreholes covering an area of similar to 10,000 km(2) in the Diapir Fold Zone of the Eastern Carpathians have been selected and biostratigraphically re-evaluated based on microfossils and calcareous nannofossils. In many cases, highly uncertain ages were previously interpreted as Oligocene and early Miocene. Our recent data suggest that most of the lower Miocene is either difficult to determine or has been reinterpreted as middle Miocene (e.g., Cornu and Doftana formations). This significant change in ages requires an updated model for the timing of regional structural evolution and may open new exploration opportunities in this highly mature hydrocarbon area. This study demonstrates the need for a new complete and reliable stratigraphic framework for the whole Miocene stratigraphic record of the Eastern Carpathians.