Deep tar-mat layers, which occur between the aquifer column and the reservoir-oil column, contain substantial quantities of extremely heavy oil and are found in various parts of the world, most prominently in Middle Eastern nations such as Saudi Arabia, Qatar, south Iraq, and Kuwait. Tar mats are thick layers with oil gravity of less than 10 °API; they isolate conventional oil reservoirs from their aquifers either partially or completely, causing numerous problems when applying oil recovery technology. However, in this study, tar mats are considered as a potential source from which to extract difficult oil. Researchers have attempted to experimentally evaluate the effect of toluene, hot water, and surfactant solution extractions on several carbonate tar-mat rock samples at several different temperatures (25°C, 135°C, 225°C and 315°C). In the study presented in this paper, samples were selected from a tar-mat carbonate reservoir in Kuwait. The physical and chemical geneses of tar-mat oil were analyzed utilizing several experimental geochemical techniques, such as rock evaluation pyrolysis, liquid extracts for Saturates, Aromatics, Resins, and Asphaltenes (SARA analysis), and elemental fingerprinting of Carbon, Hydrogen, Nitrogen, Sulfur, and Oxygen (CHNSO). These geochemical techniques were integrated in order to quantitatively evaluate the tar-mat recovery patterns of the carbonate reservoir at different temperatures. The results showed that the oil recovery significantly increased as the temperature increased. A comparison of all three methods indicates that for light and heavy tar mats, toluene produced the highest oil recovery at all temperatures tested, reaching as high as 47.1% at 315 °C. The surfactant treatment yielded less oil recovery than toluene, which reached as high as 7.5% at 315 °C. The temperature had a significant effect on light tar-mat oil recovery when using water methods; the oil recovery reached as high as 6.2% when using water at 315 °C. However, high-temperature water had little effect on extremely heavy tar mats. The paper provides a systematic method by which to quantitatively characterize tar-mat crudes in carbonate reservoirs.

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