The occurrence of tar or bitumen has been reported in several Middle East carbonate reservoirs. There is a lot of speculation about the genesis of this bitumen; however, its impact on pressure support and fluid flow in the reservoir has been recognized for quite some time. Hence, for proper reservoir management, it is necessary to quantify the pore volume occupied by the bitumen as well as to determine the nature of all the fluids coexisting with it at the same depth in the reservoir.

Several methods have been proposed in the literature for the identification of tar zones based on conventional triple-combo log data. We show that these methods might be successful in identifying the major tar zones, but are less reliable for quantifying the amount of tar present. They also do not help determine the nature of all the fluids present in the pore space.

We demonstrate the use of borehole nuclear magnetic resonance (NMR) logs to quantify the tar-filled pore volume and determine the presence, if any, of liquid hydrocarbons coexisting in the pore space. We also demonstrate the use of 3D-NMR methods to further characterize the nature of the fluids in the pore space.

Extensive core data is presented to substantiate the findings from the log analysis.

Our study shows that:

  • Comparison of total porosity from nuclear and NMR logs is a definitive method to identify the presence of bitumen in the pore space.

  • The tar-filled pore volume can be accurately quantified using NMR logs.

  • Viscous liquid hydrocarbon is often present in conjunction with the tar in the pore space, and 3D-NMR provides a reliable means to quantify its presence and characteristics.

The methods presented in this paper are equally applicable to other carbonate and clastic reservoirs world-wide.

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