The Permo-Triassic Dehram Group (Kangan & Dalan formations) is a giant reservoir consisting of relatively good quality carbonate rocks in Qatar/Fars High. Recently, a well completed in the crestal part of the Khayam structure in coastal Fars area unusually yielded oil and liquid of higher asphaltene content in basal part of upper Dalan reservoir. In order to assess the origin of the heavier fluid and possible occurrence of tar mat, four gas and liquid samples from different DSTs were analyzed for their biomarker and isotope values. In addition, 27 cutting samples from the reservoir column were analyzed for their Extractable Organic Matter (EOM) and asphaltene content. The biomarker and carbon isotope data showed relatively the same lithology and depositional environment for their source rock of all fluid samples which is believed to be Silurian marine shale. The biomarker maturity related parameters such as C29Ts/ (C29Ts+C29H), Ts/C30H, C27 Diasterane/Sterane and 4-MDBT/1-MDBT ratios showed slightly lower thermal maturity for DST#3 (oil sample) relative to other DSTs (gas condensate). The Liquid chromatography of 27 cutting samples showed increased amount of asphaltene content towards the base of upper Dalan reservoir. The rock extract of around 20 m of the lowermost part of upper Dalan formation has relatively high asphaltene content (>30%) which identified as minimat or small tar mat. The high asphaltene content and the presence of slight UCM hump in Gas chromatogram of the DST3 oil sample together with the δ13CCO2 and δ13C3 values of gas samples may indicate slight biodegradation and possible incorporation of secondary microbial methane in this reservoir. Combining the data, it seems that the reservoir was originally filled with a fluid of medium maturity which then subjected to possible slight anaerobic (bio) degradation. Finally, it is charged with light fluid of higher maturity which masks the effect of possible slight (bio) degradation and resulted in deposition of asphaltene in lower part of the upper Dalan reservoir and formation of tar mat near possible water oil contact. This study showed possible occurrence of anaerobic biodegradation should be taken into account for further exploration and development and there exist possible water oil contact in lower most part of upper Dalan reservoir below the tar mat in this field.


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