Hydrocarbon occurrences in the western Anatolian (Aegean) grabens, Turkey: Is there a working petroleum system?

Abstract

Western Anatolian graben basins are filled with continental sediments and minor carbonates deposited in lacustrine, alluvial, and fluvial environments. Individual grabens are composite basins and comprise multiple depocenters or subbasins, with sediment thickness rarely exceeding 3000 m (9843 ft). A number of hydrocarbon occurrences are reported from the basins. Regional geologic framework justifies these occurrences by providing a generous heat budget, good quality source rock, fair reservoir seal sequences, and suitable traps, although no economically producible reserves have been discovered to date. The rich source rock with an average total organic carbon of 2.34% and hydrogen index up to 800 mg/g is oil and gas generative at maturation. However, the composite nature of the grabens controlled the stratigraphic variability and led to sporadic distribution of the lacustrine deposits hosting the source rock facies in the basins. The depth of burial is generally insufficient (2000 m [6562 ft]) for the source rock to effectively generate and expel hydrocarbons. This limitation was locally overcome by additional heat input into the system through deep-reaching faults, volcanism, and/or after volcanic hydrothermal circulation. Thermal maturity of organic matter effectively increases in the vicinity of the major border structures, volcanic rocks, and zones of polymetallic mineralization, yet this effect is typically constrained within a narrow zone adjacent to heat source and diminishes within short distances. Thus, the hydrocarbon system remains regionally inefficient. Exploration success in the region depends on the discovery of deeper Neogene depocenters that could provide sufficient source rock burial depths.