Deep-seated hydrocarbons in the seep “Brecciated Limestones” of the Maiella area (Adriatic foreland basin): Evaporitic sealing and oil re-mobilization effects linked to the drawdown of the Messinian Salinity Crisis

Abstract

The 13C-depleted “Brecciated Limestones” record a hydrocarbon seep event that occurred between 5.56 and 5.532 Ma in the Maiella area. They unconformably overlie the “Primary Lower Gypsum” and crop out as carbonate buildups and authigenic patches fed by fluid migration pathways filled with flow-mobilized pelites. Tar occurs as solid bitumen shows, corresponding to pore-filling tar in the microfacies, and as a distinctive brown facies, resulting in finely impregnating patterns in thin section. Geochemical rock characterization confirmed that the measured Total Organic Carbon, with values up to 11.07%, mostly consists of migrated hydrocarbons in the carbonate buildups and the authigenic patches, while the flow-mobilized pelites are devoid of oil traces. Bitumen characterization showed an overall homogeneity among the samples, established by their similarities in the tricyclic fraction and their uniform bitumen δ13C signals (−26.3/−28.66‰ PDB-1). Gas chromatography–mass spectrometry revealed five distinctive markers indicative of an oil that originated and migrated from a carbonate source rock: high C29/C30 hopane ratio; low Ts/Tm ratio; abundant C24 tetracyclic terpane; absence of diasteranes; and occurrence of methylsteranes. Considering the regional source rock setting, the carbonate source rock might presumably be Upper Triassic/Lower Liassic in age. We favor a conceptual model that envisages a step-wise migration of hydrocarbons, which originated from a deep-seated source and migrated through a hydrofractured reservoir (Bolognano Fm.). The Primary Lower Gypsum of the Messinian Salinity Crisis provided an efficient seal until the major evaporative drawdown of the Mediterranean Sea at ∼5.55 Ma triggered a renewed hydrocarbons migration, which occurred first with a vigorous gaseous release and then with a seeping oil flow. Oil impregnated the early cemented “Brecciated Limestones” and partly contributed to their precipitation, whereas a self-clogging effect supposedly prevented oil impregnation in correspondence with the feeder channels formed during the first gaseous migration phase.