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

Kadir Gürgey,Zühtü Bati

doi:10.3906/yer-1710-24

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.

Highlights

During the Eocene/Oligocene transition, the Paratethys Ocean extending from France in Europe to Mangyshlak in inner Asia began to separate from the Tethys Ocean (Figures 1a–1c) (e.g., Rögl, 1999; Linda et al, 2003; Popov, 2004, 2010)
Palynological sample processing and analyses Palynological preparations from the composite well cuttings were processed at the Turkish Petroleum Corporation (TPAO) Research and Development Center Laboratories in Ankara, following the standard laboratory techniques the details of which were given in Bati (1996)
The Mezardere Formation, for the first time, is informally subdivided into the ?Pshekian Lower Mezardere Formation based on the common occurrences of Glaphyrocysta cf. semitecta and absence of Wetzeliella gochtii corresponding to ?NP21/22 zones and the Solenovian Upper Mezardere Formation based on the very rich and diverse dinocyst assemblage having abundant occurrences of age-diagnostic Wetzeliella gochtii and corresponding to NP23/24 zones

Summary

Introduction

During the Eocene/Oligocene transition, the Paratethys Ocean extending from France in Europe to Mangyshlak in inner Asia began to separate from the Tethys Ocean (Figures 1a–1c) (e.g., Rögl, 1999; Linda et al, 2003; Popov, 2004, 2010). Similarity between the Lower Oligocene Mezardere oils of Western Turkey and Western Turkmenistan oils could be additional evidence that these oils are sourced from the Lower Oligocene source rocks deposited in the Eastern Paratethys (Figures 1a–1c) (Gürgey, 1999). Because of this and other reasons that will be discussed chronostratigraphic. The gradual isolation of the Paratethys during the Pshekhian (Nannoplankton zones NP21/22) to Solenovian (NP23) may have caused the basin-wide occurrence of organic-rich sediments, deposited in a dysoxic–anoxic environment (Popov et al, 1993; Rögl, 1998, 1999) that constituted the active hydrocarbon source rocks in most parts of the Paratethys. The Maikop Group all over the Eastern Paratethys, in the South Caspian Basin (Saint-Germes et al, 2000), the Menilite Formation in the Alpine Foreland Basin/Carpathians (Sachsenhofer et al, 2011), and the Ruslar Formation in the Kamchia Depression (Western Black Sea) onshore and offshore Bulgaria (Sachsenhofer et al, 2009; Bechtel et al, 2014) as well as the Tard Clay in the Pannonian Basin (Vetö, 1987; Bechtel et al, 2012) are good examples of organic-rich and active shale source rocks deposited in the Paratethys.

Objectives

Methods

Results

Discussion

Conclusion