Physical and chemical characterization and evaluation of organic matter from tar-mat samples: Case study of Kuwaiti carbonate reservoir.

Abstract

Tar-mat columns exist in many carbonate reservoirs in the Middle East. The presence of tar mats has become one of the most serious problems in the oil industry, as it impacts the extraction of primary oil and the application of improved oil recovery (IOR) technologies On the other hand, tar mats can be considered potential oil reserves in and of themselves. Tar mats, which are generally dark brown to black semi-solids, can isolate the aquifer from an oil reservoir. Understanding tar mats and being able to characterize and identify them is critical for minimizing production costs and strategically producing crude oil. The objective of the study presented here was to characterize the physical and chemical properties of extremely viscous tar-mat oil and to evaluate the properties of the organic matter before and after the extraction. Five tar-mat cores were collected from a Kuwaiti carbonate reservoir, and 13 samples were prepared from each tar-mat core. One sample from each core was used for the evaluation before the extraction, while the other 12 were used for the evaluation after the extraction by toluene, hot water, and surfactant under different temperatures (25°C, 135°C, 225°C, and 315°C). The chemical genesis of tar-mat oil samples has been analyzed using novel techniques such as Vario Macro Elemental Analysis 106, Rock-Eval 6 pyrolysis, the Soxhlet apparatus, and thin-layer chromatography (TLC). The results of the geochemical analysis using Rock-Eval 6 pyrolysis showed that the Kuwaiti carbonate reservoir was oil-prone and capable of oil/gas production (type ll and ll-lll kerogen). Most of the samples were thermally mature and good in terms of hydrocarbon generation. However, oil could not have been produced from these samples naturally. Also, the ratio of H/C increased as the API decreased. Moreover, the results showed that toluene had more of an impact on most of the parameter values, while hot water and surfactant only slightly affected them.