Gwchrmrca PI Cosmochrmrca Acro Vol. 41. pp. 267-215 0 Per8amon F’ra Ltd. 1983. Printed m U.S.A. 0016.7037/83/020267-09$03.00/0 Correiation of crude oils from the San Jorges Basin, Argentina R. P. PHILP CSIRO Division of Fossil Fuels, P.O. Box 136. North Ryde, NSW 2 113. Australia (Received May 21, 1982; accepted in revisedform November 2. 1982) Abstract-The aliphatic hydrocarbon fractions of sixty oils from the San Jorges Basin, Argentina have been analysed by computerized-gas chromatography-mass spectrometry. The initial aim of this study was the correlation of the oils using sterane and triterpane biomarkers. The oils could be divided into four groups which were distinguished by the relative proportions of regular to demethylated hopanes. Although it has been previously suggested that the demethylated hopanes found in oils could have resulted from biotransformation of the oils in the reservoir, in this basin the possibility that these compounds originated directly from the source beds cannot be entirely eliminated since the oils do not appear to be extensively biodegraded on the basis of their aliphatic hydrocarbon distributions. An alternative theory for the hydrocarbon distributions observed in these oils, is initially biodegradation of the oils in the reservoir followed by addition of non-degraded oil to produce a mixture of degraded and non-degraded oil. INTRODUCX-ION EXPERIMENTAL THE SAN JORGESBasin is Argentina’s oldest and most prolific oil province. Production was established at Comodoro Rivadavia in 1907 and more than 1.4 billion barrels of oil have been produced since that time. The basin is in Patagonia and extends through much of Chubut and Santa Crux provinces and onto the continental platform (Fig. 1). The principal zones of hydrocarbon formation are thought to be in the Chubut Group and the main source rocks are be- lieved to be=Neocomian in age and the largest fields in the basin are in the eastern zone of the continental platform (Fig. 2). The accumulation of oil appears to depend on a combination of structure and strati- graphic factors based on a wedge out of the sand updip or an updip fault seal. Despite the abundance and relative ease of discovery of the oil no detailed organic geochemical survey has ever been published on oils or source rocks from this basin. In this paper the distribution of steranes and ,hopane-type triter- panes in sixty oils from the San Jorges Basin as de- termined by computerized-gas chromatography-mass spectrometry (C-GC-MS) are reported. The possibil- ity of using C-GC-MS as an analytical tool for the purpose of oil-oil or oil-source rock correlations has been described many times in recent years (i.e. SEI- FERT and MOLDOWAN, 198 1 and refs. therein) and will not be discussed again in this paper. Sixty crude oils collected from the on-shore part of the San Jorges Basin were fractionated into aliphatic, aromatic and NSO fractions using silica-gel chromatography. The aliphatic fractions were analyscd initially by GC using a packed eutectic column ar 4 subsequently subjected to anal- ysis by C-CC-MS. For the latter analyses, an HP 5985A C- GC-MS system was used and employed a fused silica col- umn (25 m) coated with 3% OVlOl and operated from 180 to 280” at S”/min with a split ratio of 2&l and with the column connected directly to the ion source of the mass spectrometer. For the majority of the analyses, the system was run in the multiple ion detection (MID) mode, mon- itoring four ions during each analysis. Normally the ions monitoredwerem/z71. 177,191,217.lnselectedcasesthe analyses were repeated and the mass spectrometer scanned continuously from mass 50 to 450 in order to confirm the identity of the various components which had been tenta- tively identified on basis of relative retention times and rel- ative intensities of key ions in the MID mode. A selected number of the aliphatic hydrocarbon fractions were rean- alysed using a Finnigan 4023 GC-MS system in the MID mode. This was done to enable a larger number of ions to be monitored with greater sensitivity and enhanced chro- matographic resolution. A 20 m fused silica OV 10 1 column was used and operated from 10” to 200” at 20”/min. and then to 270’ at 4”/min. Filament current was 350 @A, elec- tron energy 70 eV and IS temp. 26O’C. Analysis of the aliphatic hydrocarbon fractions by GC alone was performed on an HP 57 10 gas chromatograph equipped with a OVlOl fused silica column and operated from 10” to 260” at 4”/min. The oils in this basin can be arbitrarily divided into four groups on the basis of their relative distri- butions of regular and nuclear demethylated ho- panes. Various theories are proposed in this paper to account for the apparent inconsistency of finding both demethylated hopanes, previously associated with heavily biodegraded oils (SEIFERT and MOL- DOWAN, 1979) and asphalt (RULLK&TER and WEN- DISCH, 1982) and n&canes which are normally ab- sent from heavily biodegraded oils. RESULTS AND DECUS!SION The main aim of this work was to determine how many different groups of oils were present in the San Jorges Basin on the basis of fingerprints of two classes of biological markers: steranes and triterpanes. It has been well-documented that the distributions of tri- terpanes are particularly useful for oil-oil and oil- source rock correlation studies (SEIFERT and MOL- DOWAN, 198 1). Hence in the first instance, the dis- tributions of hopane-type triterpanes in the aliphatic 267