Lower and Middle Jurassic organic-rich rocks of W and SW Ukraine, NE Romania and S Moldova: occurrence, hydrocarbon generation zones and mineralogy

Abstract

The organic-rich strata of the Lower and Middle Jurassic occur within separate blocks, covering significant areas of the Carpathian Foredeep basement, the Bârlad Depression and the Dobrogean Foredeep. The deposition of these strata resulted from an anoxic event, leading to the formation of a globally important source rock level. The Rock-Eval pyrolysis results for the Middle Jurassic rocks in the Ukrainian part of the Carpathian Foredeep basement indicated a total organic carbon content of 1.61–14.98 wt%, with type II/III kerogen at an early mature to mature stage. Based on time-temperature index calculations, the top of the oil window occurs at the depth of 2200 m in the Ukrainian part of the Carpathian Foredeep basement, and at 1000 m in both the Bârlad Depression and the Dobrogean Foredeep. The former achieved thermal maturity during the Neogene period, while the latter two entered the oil window towards the end of the Cretaceous period. Across the entire study region, the Lower and Middle Jurassic strata are situated in the oil window over extensive areas, suggesting their potential inclusion in the petroleum system. Hydrocarbons generated by the Toarcian–Bathonian organic-rich rocks may contribute to the formation of Mesozoic–Miocene accumulations in the Carpathian Foredeep basement and some oil fields in the Carpathian flysch sequence. In the Bârlad Depression and the Dobrogean Foredeep, hydrocarbon occurrences may be expected in the Upper Jurassic reservoir rocks, potentially sourced by the Middle Jurassic black shales.

Keywords

• toarcian–bathonian
• black shales
• total organic carbon
• burial and maturation history
• oil window
• mineral composition