Fluid inclusion studies in search for hydrocarbons in marine and mountain regions Katarzyna Jarmołowicz-Szulc POLISH GEOLOGICAL INSTITUTE CENTRE OF EXCELLENCE Research on Abiotic Environment REA

Objectives The aims of the presentation are: - to show characteristic features of fluid inclusions in the quartz and in the quartz cements - to interpret the results of the isotope, microthermometry and fluorescence studies in the context of fluid evolution and formation of the rocks.

Quartz - one of the best known and studied minerals in the world - has been occurring in different geological environments, in that as the cement in the sedimentary rocks in the hydrocarbon reservoirs,

as well as the vein mineral of certain characteristics called the Marmarosh diamonds.

Localization and structures

The Marmarosh diamonds The second variety of the quartz mentioned, has been studied for about a century in the Carpathians in Europe, aiming at explanation of the phenomena of its formation and characteristics (Tokarski, 1905; Hurai et al., 1989; Dudok et al., 1997; Jarmolowicz- Szulc, 2001a).

Methods used Double-sided-polished thin sections of the sandstones, and quartz crystals themselves, were prepared for fluid inclusion studies using two preparation techniques (Jarmołowicz-Szulc 2001a,b). The microscopic studies performed in the Petrological Department of the Polish Geological Institute in Warsaw, Poland comprised: - a microscope analysis of fluid inclusions (Leitz Orthoplan); - fluorescence studies together with a microphotography (Nikon microscope and UV device); - cold cathodoluminescence studies (CCL 8200 mK3); - microthermometric analyses (homogenisation and freezing using Fluid Inc. System).

Inclusions in the quartz cement Occur in the early and late quartz cements. HCFI display fluorescence in white – blue colour. Oil is light and mature. Densities from 41 to 45°API. AQFI are at the boundary of the detrital and the authigenic quartz, in the early cement and in the late filling of microfractures. The HCFI are most frequent in the late filling of the fissures.

Isotope studies in quartz overgrowths & detrital grains Quartz overgrowths studied by means of SIMS point method display delta oxygen values in the interval of 23.0 to 25.6 per mille.

Results for sandstones The AQFI are at the boundary of the detrital and the authigenic quartz, in the early cement and in the late filling of microfractures. The hydrocarbon inclusions are most frequent in the late filling of the fissures, thus pointing out the late hydrocarbon migration to the reservoir

Conclusions Oil is present in the late cements in the Cambrian sandstones, mostly in the fillings of tectonic microfractures and stylolitic sutures. It migrated together with water and was trapped in the late quartz in the conditions of 107  C and 317 bars. It is light and mature. The abundance of HCFI is restricted to some boreholes and the upper parts of the profiles.

Features of the Marmarosh diamonds

The isotope studies were conducted by means of the method of a point analysis in quartz. SIMS isotope analysis of the oxygen was performed at the Arizona State University in Tempe (AZ, USA) – the procedure of Hervig et al. (1992) and Williams et al. (1997). Three grains of quartz (the Marmarosh diamonds) have been studied.

Isotopic point analysis in quartz (the Marmarosh diamonds) The δ 18 O SMOW values for quartz range from 15.7‰ and 26.7‰ (SIMS). Data for the crystals from the eastern part are most scattered and lower. Average

The Marmarosh diamonds from the Western and Eastern Carpathians Represent a type of the bi-pyramidal quartz in euhedral, transparent crystals of an ideal crystal habit and a perfect reflection (Karwowski & Dorda, 1974). The inclusions in this type of quartz are either of primary or of secondary origin.

Presentation of types of inclusions

The first type of the quartz filling in the porous space has been the object of numerous petrological analyses for long years (e.g. Sikorska & Paczesna, 1997). The present case concerns the Cambrian sandstones from the Polish segment of the Baltic Sea, which are the reservoir rocks for oil

Microthermometry results and interpretation for the Marmarosh diamonds

Fluid iclusions The quartz crystals come from the tectonic zones and contain a spectrum of fluids - from methane, through oil to aqueous solutions.

Conclusions The fluid inclusions and isotopic studies in both the quartz cement and the quartz crystals point to the hydrocarbon migration and the character of the palaeofluids responsible for the cement formation and/or the quartz crystallisation. The areas of the occurrence of the hydrocarbons in the inclusions in the quartz may be mostly prospective for the search for oil and gas

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