هادی کرمانشاهی

Asmari and Sarvak formations are the two main reservoir elements in the Cretaceous and Paleogene hydrocarbon petroleum systems of the Zagros sedimentary basin. The lithological composition, sedimentological features, stratigraphic column and geological events affecting these two formations in the studied area are somewhat different from other parts of the Zagros sedimentary basin, and this is one of the factors affecting the chemical composition of reserved oils and their hydrocarbon families. The reservoir parts of these two formations and their time equivalents in the neighboring countries are considered as the main hydrocarbon reservoirs of the region and therefore have always been the focus of researchers in this field. The oil in these two reservoirs is not of a single origin and is often identified as a mixture of oils of different origins in different fields. This issue is much more common in the case of Asmari reservoir hydrocarbons. Therefore, here it has been tried to determine the exact origin of the oils in these two reservoirs and their affinity with the oils produced from Stones of different origins.

In order to carry out geochemical studies on the oil of Asmari reservoirs (Ghar) and Sarvak Maidan in the studied field, first the extracted oil of both reservoirs was prepared. In this regard, first, the separation process of asphaltene cutting was done with normal pentane solvent, then by column chromatography technique with stationary phase of alumina or silica gel, and also using normal solvents of hexane, benzene and ethanol, other hydrocarbon cuttings of oils. separated and their percentage was determined. After preparing different hydrocarbon fractions from oil samples, the saturated fractions were subjected to gas chromatography-mass spectrometry analysis at the same time as gas chromatography analysis. The analysis of the results of these laboratory methods can be used to study the sedimentary environment, determine the degree of maturity of organic materials, oil and identify microbial degradation (Bordenave et al, 1993). In general, the laboratory methods of gas chromatography and gas chromatography-mass spectrometry contain valuable information regarding biomarkers, which are widely used in studies of oil-to-oil compatibility and oil-to-source rock compatibility.
 

Based on the results, it seems that the studied oil samples are the same in terms of chemical composition, and except for a small difference in the aromatic, resin and asphaltene fractions, no significant difference is observed in the hydrocarbon fractions of these oils. Also, in order to investigate the sedimentation environment conditions of the source rock producing these oils, the ratios of Pr/ Ph and Pr/ Ph versus CV or standard variable and the curve of changes in Pr/nC17 parameter against Ph/nC18 have been used. The results indicate that all the oil in two reservoirs was produced from the same source rock with an open sea facies and probably deltaic environment. In addition, the investigation of the maturity indices of the aromatic, hopane and aromatic biomarkers related to the oil samples of both formations showed the same characteristic and indicates a degree of maturity in the early to middle of the oil generation window. Accurate biomarker studies indicate the Jurassic age and older for the studied oil producing rocks.

The results of the experiments show that the oil reserved in these two formations can be considered the same in terms of chemical characteristics. Examining the relative frequency of C27, C29, and C28 strains using a triangle diagram shows that the oil of the two reservoirs studied was produced from open marine facies and possibly a deltaic environment with the introduction of some continental organic matter. Accordingly, by using different methods of geochemical studies, it can be said that the oils accumulated in the Asmari reservoirs (Ghar) and Sarvak Maidan in the studied field are produced from a formation with the same sedimentation conditions and with an age of Jurassic and older. The set of studied biomarker parameters is indicative of the carbonate lithology of the source rock of these two formations.
The study of the maturity indices of strane, hopane and aromatic biomarkers belonging to the Asmari (Ghar) and Sarvak reservoirs, indicate that the same characteristics and the maturity of oil-producing organic matter in the early to middle of the oil generation window.

With respect to distribution of Surmeh Formation in Persian Gulf region, this formation may act as candidate source rock for hydrocarbon accumulation in this region. A comprehensive geochemical analysis including Rock-eval Pyrolysis, Gas Chromatography (GC), Gas Chromatography - Mass Spectrometry (GC-MS), were carried out on cuttings, in order to determine its hydrocarbon potentiality. Based on Rock-Eval data, the organic matter of the Surmeh formation is type II and III kerogen, with low maturity being at the end of diagensis stage. It exhibits a poor source rock characterization with low content of TOC. A close similarity can be seen, between organic matter extracted from Surmeh sample with that of oil sample collected from Upper Dariyan, Lower Dariyan and Maudud reservoir in the field. Biomarker results indicate that an open marine and few terrestrial input source rocks deposited in an anoxic condition with carbonate origin. Based on geochemical investigations Surmeh Formation isnt suitable source rock in the study area locally. Biomarker derived from rock extracts and trapped oils indicated that they share the same origin and were stained by laterally migrating oils from distant area.

In this study، in order to determine the geochemical properties of condensates from South Pars Field، 4 samples from Kangan (Early Triassic) and Dalan (Middle- Late Permian) reservoir were subjected to geochemical analyses. Concentration and Identification of biomarkers and their fingerprint were achieved by successive treatments of condensate samples. These analyses were; i) Mild evaporation of light hydrocarbons، ii) Mild oil topping of samples and iii) Urea adduction. Based on different biomarkers fingerprints، the accumulated condensates were generated from a carbonate-clastic source rock containing organic matters with mainly kerogen type II and little terrestrial inputs، with marine origin، which has been deposited in anoxic conditions. Maturity of condensate indicates، hydrocarbon generation from a source rock with late oil window and early gas generation stage. Further investigations revealed that، gas and condensates were originated from highly reach organic matter، Silurian shales (Sarchahan Formation) deposited in the Fars and offshore of Persian Gulf region.