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Petroleum System of Early Cretaceous Forearc Basin: Sinop Basin, Central Black Sea Onshore – Northern Turkey

Abstract

Sinop Basin is an Early Cretaceous Forearc Basin that was initiated by extension during the Early Cretaceous in Central Pontides (north Turkey). Rifting began in the middle Barremian and Aptian to Albian syn-rift clastic dominated sediments including the shallow marine and turbiditic sandstones. Syn-rift rock stratigraphy units in the Central Black Sea onshore basins of Aptian to Albian age are exposed along the southern sector of Sinop Basin. Aptian to Albian aged black shales have good hydrocarbon source rock potential based on the geochemical properties. Rock-Eval pyrolysis analyses results of Early Cretaceous shale-rich intervals indicate that good to excellent oil-gas prone source rocks are present in paper and laminated shales. Source rocks facies of Early Cretaceous black shales was deposited within depression areas with anoxic environmental conditions. Total organic carbon (TOC) contents are between 0,8 to 2,5 % and hydrogen indexes (HI) values of reach up to 350 mg HC/g TOC are present in low maturity shale samples in Sinop Basin. Ekinveren oil seep is being observed southern part of Sinop Basin. Carbon isotope analysis, biomarker ratio and oil-source correlation studies indicate that, Ekinveren oil seep is being well correlated with Lower Cretaceous black shales in the basin. Albian aged sediments are well known in the Romanian offshore that have been one of the main reservoir targets for hydrocarbon exploration. Albian sandstones are composed of thick sand-rich proximal turbidite successions interbedded with siltstones and mudstones in Sinop Basin. Channelized stratified sandstones and non-channelized massive sandstone layers of proximal channel turbidite and sheet-sands have poor to good quality reservoir properties. Sand to shale ratio is increasing within lower part and decreasing to the upper part of Early Cretaceous succession in Sinop Basin. Potential reservoirs within the lower part of Early Cretaceous sedimentary package are represented by 20–30 m thick quartzite aranite facies overlying the Late Juriassic shallow marine to platform carbonate units. The reservoir quality of the Early Cretaceous sand-rich facies is controlled by mainly depositional facies and diagenetic alteration in Sinop Basin. The mineral composition of the turbiditc sandstones also has a major effect on porosity and permeability.