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 been known as a proven source of gas with minor oil accumulations across the Thrace Basin of northwest Turkey. However, based on well data 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 informally subdivided 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 be deposited during the earliest Oligocene (?Pshekian) under the prevailing marine conditions. The UMF is characterized by a very rich and 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 and Eastern Paratethys. The UMF shows the geochemical characteristics of a typical transgressive sequence such as higher TOC, hydrogen index (HI), and relative hydrocarbon potential (RHP) values than those for the regressive LMF. On the RHP basis, three short-term transgressive to regressive cycles are recognized in the entire MF in the wells studied. The early mature UMF samples showed a fair to good 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 apparently low TOC, HI, and S2 values. Better understanding of the MF will eventually aid a better understanding of the paleoenvironment of the Eastern Paratethys.

Source rock potential and depositional environment of the Lower Oligocene İhsaniye Formation in NW Turkey (Thrace, Karaburun)

“…This is also the case for the Ropotamo well, located relatively near the Karaburun area (Mayer et al, 2018). Gürgey and Batı (2018) Figure 9). Remarkable are partly very high HI values (up to 744 mg HC/g TOC) in the Upper Mezardere Formation, which have not been observed in any other Oligocene succession in the Black Sea area (Sachsenhofer et al, 2018a(Sachsenhofer et al, , 2018b.…”

“…However, as the Pınarhisar Formation accumulated at the shallow margin of the Çatalca gap, it is likely that it was restricted. Gürgey and Batı (2018) studied the Lower Oligocene succession, up to 2500-m thick, in the center of the Thrace Basin. They distinguished a Pshekhian(?)…”

“…Based on the presence of marine dinocysts (e.g., Wetzeliella gochtii) and high amounts of green algae (e.g., Pediastrum spp. ), Gürgey and Batı (2018) argued for a brackish-marine environment. Hence, it is not clear if a marine connection through the Thrace Basin existed in Solenovian time and more information on the Solenovian succession in the Thrace Basin is needed.…”

“…The petroleum potential of the İhsaniye Formation has not previously been studied in detail, but regionalscale analogue data are available from age-equivalent Oligocene rocks in the West Black Sea offshore Bulgaria, where Sachsenhofer et al (2009) and Mayer et al (2018) showed that the Lower and Upper Oligocene successions hold a fair to good petroleum potential and, respectively, poor to fair from the Thrace Basin (Burkan, 1992;Sedat and Yalçın, 1997;Gürgey, 1999;Gürgey et al, 2003Gürgey et al, , 2005Hoșgörmez et al, 2005;Huvaz et al, 2007;Gürgey and Batı, 2018).…”

Biostratigraphy and paleoenvironments of the Oligocene succession(İhsaniye Formation) at Karaburun (NW Turkey)

“…In the central part of the Thrace Basin, the succession begins with turbidites (Keşan and Ceylan formations) and passes up into pro-delta shales with sandstones (Mezardere Formation) (e.g., Siyako and Huvaz, 2007). Recent work (Gürgey and Batı, 2018) demonstrates that the Mezardere Formation is at least partly Early Oligocene in age and thus age-equivalent to the İhsaniye Formation. These are in turn overlain by continental sandstones and mudstones with lignite horizons (Osmancik and Danişmen formations).…”