Palynological and petroleum geochemical assessment of the Lower Oligocene Mezardere Formation, Thrace Basin, NW Turkey

Abstract

: The Oligocene clastic sequence of the Mezardere Formation (MF) with laterally variable organic richness has long beenknown as a proven source of gas with minor oil accumulations across the Thrace Basin of northwest Turkey. However, based on welldata for the thick MF, neither detailed work in relation to age dating and stratigraphy nor a close linkage between the depositional facies/environments, organic richness/organic proxies, and cyclicity has been established yet. In the present study, the MF was informallysubdivided into Lower MF (LMF) and Upper MF (UMF) based on the distinct differences in palynological and geochemical data.Based on the common occurrences of Glaphyrocysta cf. semitecta and absence of Wetzeliella gochtii, the LMF is considered to bedeposited during the earliest Oligocene (?Pshekian) under the prevailing marine conditions. The UMF is characterized by a very richand diverse dinocyst assemblage having abundant occurrences of age-diagnostic Wetzeliella gochtii and a Solenovian age is assigned.Common Pediastrum occurrences in the UMF may suggest fresh water input as is the case for many source rocks of the Central andEastern Paratethys. The UMF shows the geochemical characteristics of a typical transgressive sequence such as higher TOC, hydrogenindex (HI), and relative hydrocarbon potential (RHP) values than those for the regressive LMF. On the RHP basis, three short-termtransgressive to regressive cycles are recognized in the entire MF in the wells studied. The early mature UMF samples showed a fair togood source rock potential (average TOC = 1.14 wt. %; HI = 283 mg oil/g TOC) and low to moderate genetic petroleum potential (GP= 3.65 mg oil/g rock) and source potential index (SPI = 1.44 t oil/$m^2$). The LMF samples were not evaluated due to their apparentlylow TOC, HI, and S2 values. Better understanding of the MF will eventually aid a better understanding of the paleoenvironment of theEastern Paratethys.