Abstract
Mud volcanoes act as prospecting indices in the exploration of oil and gas deposits with gases and muds excreted. A methodology and an analytical protocol have been developed in order to classify the petroleum provinces according to the geochemical information available from the mud volcanoes. Such a study allows us to evaluate the potential of the petroleum system beneath. The Crimean-Caucasus region is renowned for mud volcano activity with well-known areas from west to east: Kerch Peninsula, Kuban and Azerbaijan. A methodology concerning the geochemical sampling and analysis of the mud volcanoes located onshore was applied to the Kerch Peninsula. Three field trips were organised by IFP in 2000, 2001 and 2002 in order to collect geochemical data, in agreement with the Museum of National History in Kiev. During these geological and geochemical surveys particular attention was given to the Boulganack mud volcano, which is a favourable site for the study of mud and gas samples due to the variety of the types of the vents. Specific tools were built in order to take samples of mud, up to 20 m deep in the vents, and gas more than 25 m away from the bubbling point in safe conditions in the deep mud lakes. Moreover, monitoring of the sampling of the gas was undertaken on the Boulganack mud volcano for one year in order to study the variation in the composition of the gas in time and space in the various vents. The proportions of nitrogen, carbon dioxide and hydrocarbons (methane to ethane) were measured. The vents of the Boulganack mud volcano present about 90% of methane and 10% CO2 , with a few percent of heavier hydrocarbons in one particular vent: Andrusov. Pavlov expelled more CO2 (25% to 40%), with the greatest temporal variations. Only Obrudchev expels as much carbon dioxide as methane (50/50). Sometimes, nitrogen was also measured (Pavlov, Obrudchev and Central Lake). The high dryness of the gas implies a loss of heavy hydrocarbon compounds relative to a typical thermogenic gas. Chemical gas compounds are highly dependent from one vent to the other. The comparison of the quantity of hydrocarbon included in the mud released from the mud volcanoes of the Kerch Peninsula and those of Azerbaijan (used as a reference) shows very little hydrocarbon in the mud from the mud volcano in the Crimea, whereas accumulations of hydrocarbons exist in the fluids expelled from mud volcanoes in Azerbaijan. In fact, the ranking of the petroleum provinces is linked to the occurrence of the free hydrocarbons in the mud excreted by mud volcanoes. Analysis by chromatography of the saturated hydrocarbons presents a high degree of biodegradation when the samples are taken at the surface. When they are taken with the core barrel at greater depth in the vents, the hydrocarbons are preserved with a typical continental origin which can be compared with the organic matter of the Maykop formation. The modelling of the hydrocarbon window geohistory confirms that the hydrocarbons are mainly produced within the lower Maykop. The generation of gas is favoured, in agreement with the quality of the type III source rocks in this western part of the Caucasus.
Highlights
Mud volcanoes bring to the surface sediments, fluids and gas that may come from deep horizons, and so the sampling and studying of these products may help to understand better the petroleum system in frontier petroleum exploration areas.The Crimean-Caucasus region is renowned for mud volcano activity with well-known areas from west to east: Kerch Peninsula, Kuban and Azerbaijan.The Taman and Kerch Peninsulas (Fig. 1) are located between the Caucasus in the east (Russia) — which is an incratonic range with a tertiary evolution integrated within the alpine compression — and the Crimea (Ukraine) in the west, with a complicated and active structuration since the end of the Jurassic.The Indolo-Kuban basin corresponds to a fore deep in front of the Great Caucasus
The concentration of kerogen being diluted within the whole volume of the outflow of mud, parameters such as the percentage of Total Organic Carbon (TOC) or the petroleum potential (S2 peak) issued from the Rock-Eval analysis [24] must not be interpreted in the same manner as the classical samples taken on the outcrops
The source rocks related to the Maykop formation (Oligo-Miocene), from samples taken from Azerbaijan and from the Kerch Peninsula, have different petroleum potential (Fig. 21)
Summary
Mud volcanoes bring to the surface sediments, fluids and gas that may come from deep horizons, and so the sampling and studying of these products may help to understand better the petroleum system in frontier petroleum exploration areas.The Crimean-Caucasus region is renowned for mud volcano activity with well-known areas from west to east: Kerch Peninsula, Kuban and Azerbaijan.The Taman and Kerch Peninsulas (Fig. 1) are located between the Caucasus in the east (Russia) — which is an incratonic range with a tertiary evolution integrated within the alpine compression — and the Crimea (Ukraine) in the west, with a complicated and active structuration since the end of the Jurassic.The Indolo-Kuban basin corresponds to a fore deep in front of the Great Caucasus. During the one-year monitoring, the weekly sampling consisted of taking gas samples in sealed glass tubes from every vent of the Boulganack mud volcano, i.e. Obrudchev, Oldenburg, Great Lake, Pavlov, Abikha and Tishenko, and several secondary vents characterised by temporary activity on Andrusov. The various vents of the Boulganack mud volcano contain generally about 90% methane and 10% CO2: Oldenburg, Abihka, Tishenko 1 & 4, Andrusov 2 & 6, and Central Lake (Fig. 15).
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