Origin and migration of oil and natural gas in the western part of the Ukrainian Outer Carpathians: Geochemical and geological approach

doi:10.1016/j.marpetgeo.2019.02.018

Marine and Petroleum Geology

Volume 103, May 2019, Pages 596-619

https://doi.org/10.1016/j.marpetgeo.2019.02.018 Get rights and content

Highlights

•
Oil and natural gas were generated from Type II and II/III kerogen.
•
Oil underwent biodegradation and slight water washing.
•
Gaseous hydrocarbons and CO₂ are both of thermogenic and microbial origin.
•
Oligocene Menilite and Cretaceous Spas beds are main source rocks for oil and gas.

Abstract

We established the generation and migration processes as well as the influence of secondary processes of oil and natural gas accumulated in Lower Cretaceous-Oligocene strata of the western part of the Ukrainian Outer Carpathians based on the results of organic geochemical analyses of oil and natural gas related to the geological setting and source organic matter. The Oligocene Menilite beds from the Boryslav-Pokuttya Nappe are the main source rock for oil and natural gas accumulated in the fields of the Boryslav-Pokuttya and Skiba nappes. Co-sourcing of oils and natural gas accumulated in the Skiba Nappe in the study area by both Menilite and Spas beds of this nappe cannot be excluded. In some cases (e.g. Novoshidnytsya field) oil and natural gas originated within the Oligocene Menilite beds of the Boryslav-Pokuttya Nappe, migrated through thrusts and fissure zones and accumulated in the Lower Cretaceous reservoirs of the Skiba Nappe. Oil from small Vovche field and Pohar seep of the Krosno Nappe was generated by Menilite source rocks of this tectonic unit and migrated to the traps. Sporadically oils underwent secondary processes, mainly biodegradation and occasionally water washing. Hydrocarbon gases and carbon dioxide were generated mainly during low-temperature thermogenic processes from Type II kerogen, and partly during the microbial CO₂ reduction process. Only natural gas from the Mala Volosyanka oil and gas seep of the Krosno Nappe was documented to have undergone biodegradation.

Introduction

The Outer Carpathians are one of the largest petroleum provinces of Central Europe and constitute one of the oldest petroleum-producing regions in the World. The records of oil seeps near the town of Boryslav date back to the second half of the 18th century (Fedyshyn ed., 1998). The early attempts of its commercial recovery were made in 1810–1817. In 1853, after Jan Zeh and Ignacy Łukasiewicz developed the process of distillate extraction during oil refining, Boryslav became established for commercial oil production. Oil was extracted with the help of draw-wells. Exploitation of oil began in 1854 and natural gas production in 1921 (Fedyshyn ed., 1998; Kotarba and Koltun, 2006). Since 1861 shallow wells have been drilled using the manual percussive method. In 1865 there were more than 5000 draw-wells in and around Boryslav, eventually there were about 20000. In 1908 the borehole Oil City was put onto production with 3000 tons of oil and 900 thousand m³ of gas per day from Boryslav Sandstone. This and other boreholes with oil daily production exceeding 1000 t brought Boryslav worldwide fame.

Presently 41 petroleum fields exist in the Ukrainian Outer Carpathians (Fedyshyn ed., 1998). The main petroleum producing tectonic unit is the Boryslav-Pokuttya Nappe, where of 39 existing fields, 37 are oil and 2 gas-condensate. The fields occur in a wide depth range at all levels of folds of this nappe. The deepest oil has been tested at a depth of 5800 m. Two shallow oil fields have been discovered in Skiba Nappe. Oil accumulations occur in sandstone reservoirs of Upper Cretaceous Stryi beds, Paleocene Yamna beds, Eocene Vyhoda beds and Oligocene Menilite beds. Accumulations are hosted by tectonically and lithologically sealed anticline structures. As of 1st January, 2011 (Vishnyakov et al., 2014) the residual recoverable reserves in Boryslav-Pokuttya Nappe are: oil 49 million tons, condensate 2.5 million tons, and gas 40 billion m³. In the study area, from the Polish-Ukrainian border in the west and Stryi town in the east (Fig. 1), fifteen oil and eight gas samples were collected from Boryslav, Novoshidnytsya, Pivdenna Stynava, Strilbichi, Staryi Sambor and Vovche fields, and Kozieva, Mala Volosyanka, Pohar and Strilbichi seeps (Table 1). Moreover, the results of the previous published geochemical studies from nine oil and gas fields (Blazhiv, Novoshidnytsya, Oriv, Pivdenna Stynava, Pivdenny Monastyrets, Staryi Sambir, Ulychno, Verkhne Maslove, Vovche) (Kotarba et al., 2007; Więcław et al., 2012) of the study area were also used for genetic interpretation. A set of analytical methods used in previous works (Kotarba et al., 2007; Więcław et al., 2012), especially for isotopic analyses of natural gases and for biomarker analyses of oils, was considerably smaller than the currently used one. In Appendix A Supplementary data, all analysed petroleum fields and seeps were described based on Fedyshyn ed. (1998), Vishnyakov et al. (2014) and unpublished Ukrainian and Polish archival data.

The aim of this paper is determining the origin and migration of oil and natural gas and influence of secondary processes (biodegradation, water washing and evaporative fractionation), occurring in the study area of the western part of the Ukrainian Outer Carpathians (Fig. 1) based on the results of bulk, biomarkers, stable carbon isotope analyses of oil samples, and molecular and stable carbon and hydrogen isotope analyses of crude oil samples and natural gas samples. Interpretation of these results is performed in relation to geological setting (Section 2) and genetic type and maturity of source rocks (Section 3).

Section snippets

Geological setting

The Outer Carpathians are a constituent of the Alpine belt and extend from the Vienna Basin in the west to the Moesian Platform in the east, including the territory of Czech Republic, Slovakia, southern Poland, western Ukraine and northeastern Romania (Fig. 1). The Ukrainian Carpathians represent the central segment of the Carpathian Arc extending between Poland and Romania. Their geological structure is described in numerous publications (e.g., Danysh et al., 1974; Burchfiel and Royden, 1982;

Source rocks in lithostratigraphic profiles of the Ukrainian Outer Carpathians of the study area – a review

Two main organic-rich episodes are recorded in the Cretaceous to Neogene sequences: (i) the Lower Cretaceous Spas beds in the Skiba Nappe at the base, and (ii) the Oligocene-Lower Miocene Menilite and Krosno beds in the Boryslav Pokuttya, Skiba and Krosno nappes at the top (Fig. 2).

Lower Cretaceous Spas beds in the Skiba Nappe are as much as 200 m thick, predominantly black shales with minor sandstone and siltstone intercalations. They have commonly higher than 2.0 and sometimes over 7 wt% TOC

Samples and sampling procedure

Fifteen crude oil and eight natural gas samples were collected from the Lower Cretaceous–Oligocene sandstone reservoirs of the study area of the western part of the Ukrainian Outer Carpathians (Fig. 1).

Associated gases (dissolved in oil) were taken from a separator into a glass container (∼500 cm³) at pressures of at least 0.2 MPa. Natural gases from seeps were collected in glass containers filled with NaCl saturated solution. The sample containers were labelled with the date and well name, and

Oil

Oils were analysed for API gravity using an Anton Paar DMA5300M density meter and for sulphur content with a Leco SR-12 analyzer. Before fractionation by column chromatography, the oils were topped under nitrogen (5 h) at a temperature of 60 °C and the asphaltene fraction was precipitated from oil with petroleum ether. The maltenes were separated into fractions of saturated hydrocarbons, aromatic hydrocarbons and resins by column chromatography, using alumina/silica gel (2:1 v/v) columns

Oil

The analysed crude oils accumulated in the Upper Cretaceous-Oligocene sandstone reservoirs of the Boryslav-Pokuttya, Skiba and Krosno nappes of the Outer Carpathians in the study area (Table 1) range in their physical and geochemical properties. Density of oils varies from −0.8 to 37.5 API (0.833–1.078 g/cm³) and sulphur content ranges from 0.11 to 0.60 wt% (Table 2). In almost all oils concentration of saturated hydrocarbons exceed 50 wt% with the exception of sample S-Va collected from a seep

Origin

Origin of analysed oils was established based on the results of sulphur content, density, stable carbon isotope composition of oil and its fractions (Table 2), whole-oil GC analysis (Table 3) and GC-MS analysis of saturates (Table 4) and aromatics (Table 5). Distributions of n-alkanes are variable, usually monomodal, with a distinct maximum in the short-chain range (C₆ - C₁₀, Fig. 7A), characteristic of hydrocarbons generated from marine organic matter (e.g. Peters et al., 2005). Visible

Comparison of origin and migration of oil and gas

Parameters and indices of oils as well as geochemical characteristics of natural gas accumulated in fields and flowing out in seeps in the study area indicate one predominant type of source organic matter. Some observable small differences are result of facies changes and/or secondary processes. The most prolific/productive oil-prone source rock are the Oligocene Menilite beds occurring in all nappes (Fig. 3, Fig. 4, Fig. 5), but only in Boryslav-Pokuttya and Krosno nappes are mature enough (in

Conclusions

Results of organic geochemical study of oil and associated and non-associated natural gas accumulated in the Upper Cretaceous-Oligocene sandstone reservoirs of the Boryslav-Pokuttya, Skiba and Krosno nappes of the western part of the Ukrainian Outer Carpathians reveal that:

1.
All oils and natural gas accumulated in the Boryslav-Pokuttya Nappe and in folds of the Skiba Nappe thrusted over the former were generated from the Menilite beds of the Boryslav-Pokuttya Nappe. Co-sourcing of oils and

Acknowledgements

The research has been financially supported by the National Science Centre (Poland) grant No. UMO-2014/15/B/ST10/00131. Comments and suggestions by three anonymous reviewers were very helpful. Analytical and editorial works by Joanna Gawęda-Skrok, Hieronim Zych and Tomasz Kowalski from the AGH University of Science and Technology is kindly acknowledged.

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Research paperOrigin and migration of oil and natural gas in the western part of the Ukrainian Outer Carpathians: Geochemical and geological approach

Highlights

Abstract

Introduction

Section snippets

Geological setting

Source rocks in lithostratigraphic profiles of the Ukrainian Outer Carpathians of the study area – a review

Samples and sampling procedure

Oil

Oil

Origin

Comparison of origin and migration of oil and gas

Conclusions

Acknowledgements

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Geochim. Cosmochim. Acta

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Biostratigraphical characteristics of the type sequence of the menilite formation on chechva river (Ukrainian Carpathians)

Paleontol. Sb. (L'vov)

Early generation characteristics of a sulfur-rich Monterey kerogen

AAPG (Am. Assoc. Pet. Geol.) Bull.

Understanding source, distribution and preservation of Australian natural gas: a geochemical perspective

Aust. Prod. Pet. Explor. Assoc. J.

Carpathian foreland fold and thrust belt and its relation to other basins

AAPG (Am. Assoc. Pet. Geol.) Bull.

Origin of gaseous hydrocarbons in subsurface environments: theoretical considerations of carbon isotope distribution

Chem. Geol.

Guidelines and recommended terms for expression of stable-isotope-ratio and gas-ratio measurement results

Rapid Commun. Mass Spectrom.

Geochemistry and accumulation of carbon dioxide gases in China

AAPG (Am. Assoc. Pet. Geol.) Bull.

Flysch Carpathians

Research paper
Origin and migration of oil and natural gas in the western part of the Ukrainian Outer Carpathians: Geochemical and geological approach