Depositional environment and hydrocarbon potential of Oligo-/Miocene rocks in the Vrancea Nappe (Eastern Carpathians, Romania)

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Abstract

The isolation of the Paratethys at the Eocene/Oligocene boundary favored the formation of organic-rich sediments that have been deposited within the restricted basin. These sediments act as hydrocarbon source rocks in many parts of the Paratethys area including the Carpathian Foredeep. Oligo/Miocene rocks in the Eastern Carpathians have been studied previously using sedimentological techniques and some RockEval data. However a detailed study of the organic matter is still missing. For this reason, the present study aims (1) to determine the main factors controlling the deposition of organic matter, (2) to detect vertical variations in the source rock potential, (3) to test the possibility to distinguish oil generated from different units within Oligo/Miocene succession, and (4) to test, whether the source richness can be related to the gamma ray (GR) response in boreholes. To achieve the goals, more than 150 samples have been taken along the Tazlau river. The profile includes Eocene Bisericani Formation, Oligo-/Miocene Menilite Formation and lower Miocene Gura Soimului Formation. The Menilite Formation is composed from base to top by Lower Menilite Member (LMM), Bituminous Marl Member (BMM), Lower Dysodilic Shale Member (LDSM), Kliwa Sandstone Member (KSM), and Upper Dysodilic Shale Member (UDSM). The Bisericani Formation includes deposits of a slope apron system. A major change towards anoxic conditions caused deposition of the LMM with high TOC contents and a kerogen type II. Calcareous nannoplankton dates the overlying BMM into nannoplankton zones NP21-22. The organic matter mainly derived from autochthonous marine organisms including bacterial biomass. TOC contents are moderate because of dilution by carbonate minerals. Salinity and redox conditions varied from reduced to slightly enhanced and from strictly anoxic to dysoxic, respectively. The LDSM contains black shale and sandstone beds deposited in a deep marine depositional lobe. Anoxic environments caused accumulation of abundant organic matter with kerogen type II. Despite of high HI values, landplants form a significant part of the organic matter. A decrease in HI in the uppermost part of the LDSM is due to increasing contributions of landplants and increased oxygen contents. Salinity varied significantly. The channel fill sediments of the KSM separate LDSM and UDSM. The lower part of the UDSM accumulated in a lobe, whereas its upper part represents a basin plain setting. MTTC ratios increase upwards and reflect a trend from slightly enhanced to slightly decreased normal marine salinities. Whereas oxygen-depleted, but not strictly anoxic conditions, are indicated by Pr/Ph ratios, the presence of aryl-isoprenoids suggests temporary photic zone anoxia controlled by salinity variations. Organic and inorganic redox proxies suggest a subtly upward increase in the availability of oxygen within the UDSM. The organic matter is a mixture of autochthonous organic matter and landplants and is classified as kerogen type II. A major change towards oxic conditions occurred at the boundary between the UDSM and the Gura Soimului Formation. The GSF in the Tazlau profile is characterized by thick mega-olistolithes.

Details

Translated title of the contributionAblagerungsbedingungen und Kohlenwasserstoffpotential oligoäner/miozäner Gesteine der Vrancea Decke (Ostkarpaten, Rumänien)
Original languageEnglish
QualificationDipl.-Ing.
Awarding Institution
Supervisors/Advisors
Award date27 Mar 2015
Publication statusPublished - 2015