Origin and migration of oil from the Ukrainian Outer Carpathians to their Mesozoic basement: a case of Lopushna traps

Lopushna oil field has been discovered in Mesozoic-Paleogene strata of the platform basement under the Ukrainian Outer (Flysch) Carpathians. Oils from two main accumulations, occurring in the Upper Cretaceous sandstone and Upper Jurassic limestone reservoirs, have been analysed in order to identify their origin and relation to the known petroleum systems in the region of the Outer Carpathians and the basement of the Carpathian Foredeep. Results of geochemical investigations of oils, including biomarker and isotopic data, show a good correlation with oils, accumulated in the Carpathian flysch strata. Oils are low-sulphur and did not undergo significant degradation. They were generated from Type II or II/III kerogen, deposited in anoxic environments in clastic sediments. Generation occurred at the peak of “oil window”, probably in the temperature range of 122 to 126oC. The presence of oleanane indicates the Cretaceous or younger source rocks. Good correlation on biomarker and isotopic data with the organic matter dispersed in Oligocene Menilite Beds, occurring at the top of the Carpathian flysch succession, allowed authors to identify them as source rocks for oils of the Lopushna field, which thus can be regarded as an oil family of the Outer Carpathians. The migration distance for both Lopushna oils and oils accumulated in the Carpathian flysch strata was similar. A number of sub-vertical dislocations, on which the subsided flysch strata occur at the same depth level with the Jurassic and Cretaceous rocks of the platform, can be assumed as possible migration pathways from the mature source rocks of the Oligocene Menilite Beds of Boryslav-Pokuttya, Skyba or Krosno units to the Lopushna type traps, which suggests the possibility of existence of other hydrocarbon accumulations of this type. The Lopushna field so far is unique containing this type of oil found in the basement of the Ukrainian Carpathian Foredeep.