Tar mats are extra heavy oil zones sandwiched between aquifers and adjoining oil columns. They isolate either partially or completely an oil reservoir from its aquifer. This results in a rapid pressure drop, a premature high gas-oil ratio and a low primary oil recovery; all of which point to some form of pressure maintenance early in a field's life.

Eventhough tar mats represent considerable hydrocarbon reserves as they are rather common in the Middle East and Africa, it is their impact on oil recovery from adjoining oil columns which is of interest for the time being. Tar must be characterized to evaluate its mobility and ways of establishing contact between an oil column and its aquifer as well as to design an optimum water injection scheme.

The present paper discusses a detailed chemical characterization of tar from a carbonate reservoir in Saudi Arabia. Thermal stability variation was evaluated by thermal gravimetry (TG), and differential thermal analysis (DTA). Elemental analysis of preserved and non preserved samples were carried out with a Carlo Erba 1106 elemental analyser. The sulfur content was also determined by two different ASTM methods. The presence of Nickel, Vanadium and Iron, the major metals usually found in hydrocarbons, was investigated by X-ray fluorescence (XRF) spectroscopy. The tar major hydrocarbon group components were separated and quantified by high performance liquid chromatography (HPLC).

Experimental results showed variation, in tar properties, with depth and area within the same field. The carbon to hydrogen ratio increased systematically with a decrease in API gravity. The sulfur contents obtained with the Carlo Erba elemental analyser were in good agreement with those obtained by the general bomb method (ASTM D-129-64). The content of Hexane insolubles was relatively high at about 38 % by weight. The polar compounds ranged between 5 and 9 % by weight.

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