Palynofacies and geochemical analysis of Oligo-Miocene bituminous rocks from the Moldavidian Domain (Eastern Carpathians, Romania): Implications for petroleum exploration

Abstract

A palynological, palynofacies and geochemical investigation conducted on Oligo-Miocene bituminous rocks of the Lower Dysodilic Shale Formation and the Upper Dysodilic Shale Formation (Eastern Carpathians) has allowed recovery of pollen and spore assemblages associated with marine palynofossils (dinoflagellates and prasinophyte algae) and phytoclasts. The general composition of the assemblages suggests an anoxic depositional environment from a distal basin to a highly proximal shelf. The palynological assemblage identified in the Lower Dysodilic Shale Formation exhibits an abundance of dinoflagellate cysts and prasinophyte algae, with some taxa, such as Wetzeliella gochtii, Rhombodinium draco and Cordosphaeridium gracile, indicating a Rupelian–early Chattian age for these deposits. In contrast, the Upper Dysodilic Shale Formation displays diverse assemblages of palynomorphs (more pollen and spores) and its age is older than middle Aquitanian. Geochemical (Total Organic Carbon content and Rock-Eval pyrolysis) and palynofacies (optical and scanning electron microscopy) analyses performed on samples from the Lower Dysodilic Shale Formation suggest that it contains type II kerogen (oil prone), consisting of abundant amorphous organic matter (AOM), extracellular polymeric substances (EPS) and coccoid bodies (bacteria or algae). This kerogen comes from a marine source (derived from phytoplankton and bacteria), and likely accumulated in a distal suboxic-anoxic basin. The Total Organic Carbon (TOC) content suggests good to excellent petroleum potential, especially for generating mixed oil and gas. The level of kerogen maturation (inferred from the Thermal Alteration Index, Tmax and prasinophyte algae fluorescence) lies at the boundary between immature and mature phases. The Upper Dysodilic Shale Formation is abundant in translucent and opaque phytoclasts, suggesting a continental organic matter source and type III kerogen, and thus would yield mainly gas. This organic matter was principally deposited in a highly proximal shelf setting.