Source of hydrocarbons in the supergiant Romashkino oilfield (Tatarstan): Recharge from the crystalline basement or source sediments?

Romashkino is a supergiant oilfield in Tatarstan with reserves of more than 5 billion tons of oil. The relatively small thickness of the sedimentary cover on the South Tatar arch and the location of Devonian producing beds directly above the crystalline basement were used as arguments for speculations about a deep source of hydrocarbons and a role of the basement as a conduit for fluids. It was found that several wells at Romashkino exhibit “anomalous” behavior characterized by a steady and, sometimes, increasing production, which was interpreted as an indication of recharge from below along basement faults. We present the results of isotope and geochemical studies of crude oils from the sedimentary units of Tatarstan, oils from “anomalous” wells at the Romashkino field, and organic matter from both sedimentary and basement rocks. The results of isotope analysis (comparison of carbon isotopic compositions in the fractions) revealed a genetic link between oils from the sedimentary rocks and organic matter from the Late Devonian Domanik-type facies beds. Because the organic matter within the South Tatar arch does not reach sufficient maturity for hydrocarbon generation, we supposed that sediments of similar type and age (Domanik facies) in the deeper parts of the Ural foredeep and Buzuluk depression could be a likely source of hydrocarbons in this area. The study of oils from “anomalous” wells revealed their close similarity to the other oils from the sedimentary cover. The analysis of the composition of hydrocarbons, which are present in trace amounts in basement rocks, provided no evidence to support the role of deep abiogenic hydrocarbons in recharging oil pools at Romashkino and other fields of Tatarstan.