Application of Petroleum Markets to Geochemical and Environmental Investigations

M. A. ABU_ELGHEIT

M. SH. EL-GAYAR

A. H. HEGAZI

Department of Chemistry

Faculty of Science

Alexandria University

Alexandria, Egypt

Application of trace-metal and biological markers to geochemical studies has shown that crude oils could be correlated or differentiated according to their geologic age. The V/Ni, V/E Ni, Mg, Fe), and Pr/Ph markers were almost uniform in Culf of Suez crude oils, revealing their same origin, yet showing marked differences in Western Desert crude oils, reflecting varying degrees of their maturity and migrational history. The significance of petroleum markers was extended to monitoring of oil spill sources. Weathering of spills usually renders their source identification questionable by infraredor gas chromatography profiles. Since evaporative loss light petroleum fractions does not appreciably affect the high-Molecular Weight components with which trace metals, isoprenoids, hopanes, and steranes are associated, V/Ni, Pr/Ph, m/z 217 mass chromatogram fragments were found reliable in fingerprinting oil spill sources in Mediterranean waters.

Trace-metal and biological markers play an indisputable role in petroleum geochemistry. In spite of the undesirable problems they bring about in the petroleum industry, trace metals are significant for evaluating several petroleum markers. Various techniques, including atomic absorption spectroscopy (AAS), neutron activation analysis (NAA), X-ray fluorescence (XRF)< inductively coupled plasma-atomic emission spectroscopy (ICP-AES), and emission spectrography, have been reported for the determination of those metals (Bermejo Barrera et al., 1001; Oluwole et al., 1993; Mohamed et al., 1989; Botto 1991; Abu-Elgheit et al., 1981). The relevance of biological markers in petroleum is also well recognized. The development of high-precision instrumentation and refined techniques made it possible to determine such markers. In this connection, much use has been made of gas chromatography (GC) in conjunction with mass spectrometry (MS) (Peters & Moldowan, 1993).

In view of this, the foregoing types of markers were applied to some crude oils for geochemical studies related to their origin, maturation, and geologic age. Moreover, for environmental considerations, oil spills in territorial waters resulting from il tanker accidents or their illegal discharges are of constant concern. Monitoring the source of these oil slicks is very important for pollution control.

Received 27 August 1996; accepted 23 September 1996.

Address correspondence to M.A. Abu-Elgheit, Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426, Alexandria, Egypt.

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Energe Sources,20:3-10,1998

Copyright ©1998 Taylor & Francis

0090-8312/98$12.00+.00

4 M.A. Abu-Elgheit et al.

Weathering of oil spills usually renders their source identification questionable by infrared (IR) or GC profiles because of evaporative loss of the light hydrocarbons. As this does not appreciably affect the high-MW components with which trace metals, isoprenoids, hopanoids, and steranoids are associated, trace metal and biological markers should be reliable in fingerprinting the crude oil responsible for water pollution.

Experiment

Crude Oils

Ten crude oils have been used for geochemical studies. The oil samples were obtained from oil wells in the Gulf of Suez and in the Western Desert. Their geologic age extends from the Miocence to the Cretaceous.

Standards

These standards were aqueous solutions of reagent-grade nitrates of the elements. For vanadium, ammonium metavanadate was used. An acid concentration similar to that of the oil solutions was maintained in all cases.

Weathered Oils

Three oils in Mediterranean waters were each exposed in the laboratory to simulated weathering for 20 days at room temperature. Light was applied through an ultraviolet (UV) lamp, and airflow was provided by a fan.

Trace Metal Analysis

About 30 g of crude oil sample contained in a silica crucible were ashed according to a reported procedure (ASTM D482-63 1968). The ash content was dissolved in dilute mineral acid. The solutions were then analyzed for their V, Ni, Mg, Fe, Co, and Cu content by 3110 Perkin – Elmer flame AAS. For Ni, Mg, Fe, Co, and Cu, air/acetylene was used, while nitrous oxide/acetylene was adopted for V. The measurements were conducted at the resonance lines V 318.0 nm, Ni 232.0 nm, Mg 285.2 nm, Fe 248.3 nm, Co 240.7 nm, and Cu 324.8 nm.

Fractionation of Oils

Asphaltenes were precipitated from the crude by n-hexane. The deasphalted oils were separated into saturated, aromatic, and polar fractions using a chromatographic column packed with equal amounts of activated silica and alumina. The saturate fraction was examined by capillary column GC and by GC/MS. GC analyses for the saturates were performed on a Perkin-Elmer 9600 gas chromatograph equipped with an on-column injector, a fused silica capillary column (60 m × 0.25 mm) coated with SE-54, and a flame ionization detector. The temperature was programmed from 60 to 80 °C at 30°C/min and from 80 to 300°C at 4°C/min with an initial time of 1 min and a final time of 30 min. GC/MS measurements were carried out on Hewlett Packard 5890 gas chromatograph linked to Hewlett Packard 5972 mass spectrometer.

Petroleum Markers for Geochemical Investigations 5

Identification of individual hydrocarbons was based upon comparison of their retention times or MS data with literature values.

Results and Discussion

The distribution of trace metals in crude oils has been of great interest to many authors. Their correlations with the refinery properties of oils provide clues pertaining to the metals related to oil genesis and those introduced as contaminants.

Petroleum samples from local oil fields were analyzed for several trace metals by AAS. Of these, V, Ni, Mg, Fe, Cu and Co were selected for the present study. The first two metals are the most abundant and the most significant. The others were also considered because of their relationship to the original source materials.

The levels of metals are within the following ppm ranges: 0.01 – 71.4 in the Gulf of Suez crude oils and 0.01 -21.7 in the Western Desert crude oils. The metal contents are in the decreasing sequence V, Ni, Mg, Fe, Cu and Co. The variance in distribution is interpreted in terms of petroleum genesis and the conditions during the transformation of source materials into crude petroleum. It is also explained to be due to the direction and duration of oil migration. The geochemical marker V/Ni was then evaluated and set up in Table 1. its constancy throughout the Gulf of Suez crude oils is an indication of similar source materials and hence a common origin. The Western Desert crude oils have an average V/Ni marker of 1.00. It is not comparable to that of the Gulf of Suez oils(2.23). The two sets of crude oils with their dissimilar markers cannot therefore be correlated. Crude oils are only correlated on the basis of a constant, or nearly constant, V/Ni marker (Scott et al., 1954). Furthermore, a correlation between the V/Ni marker and the geologic age of oil has thus been established (abu-Elgheit & Ijam, 1980).