Organic matter maturity and hydrocarbon potential of the Lower Oligocene Menilite facies in the Eastern Flysch Carpathians (Tarcău and Vrancea Nappes), Romania

Abstract

Bulk organic geochemical and molecular composition data have been used to analyse the hydrocarbon potential and organic matter maturity of the Lower Oligocene Menilite facies from two adjacent tectonic units of the Eastern Flysch Carpathians (Tarcău and Vrancea Nappes), Romania due to the importance of these source rocks in hydrocarbon exploration in entire Paratethys realm. The data show strong variability in organic matter quantity and quality. Organic carbon content reaches peak values in the siliceous facies of the Lower Menilite Member (up to 8.6 wt% TOC), which contains type II kerogen. With increasing contribution of flysch sedimentation mixed type II/III kerogen gains importance. The biomarker distribution reveals strong variation in the supplied organic matter common for flysch-influenced sedimentary environments. Terrigenous input is marked by epicuticular wax imprint in n-alkane distribution and occurrence of conifer biomarkers, while marine organic matter origin is expressed by the occurrence of short-chain n-alkanes and hopanes especially in the siliceous facies. Thus, these source rocks can be classified as oil-prone and subordinately mixed oil/gas-prone. The maturity in the outer tectonic unit (Vrancea) is low (Tmax ∼425 °C, Ro ∼0.4%) but increases towards the inner Tarcău Nappe (Tmax ∼430 °C, Ro ∼0.5%) reaching onset of hydrocarbon generation. The studied rocks have good petroleum potential, but hydrocarbons were generated only in more mature Tarcău Nappe, where solid bitumen veins were observed. Bitumen impregnation of numerous vitrinite grains possibly suppressed vitrinite reflectance, thus leading to more accurate maturity assessment based on molecular proxies (biomarker maturity indices). The observed difference in maturity levels between the nappes results from the more inner position of the sampled Tarcău Nappe succession within the orogen relative to the Vrancea unit. This is related to different burial histories, as well as variation in subsequent erosion and exhumation levels. The actual hydrocarbon potential in the studied area varies due to local interplay of these critical factors.