Water saturation in organic-rich mudstones is a crucial parameter for characterizing and evaluating reservoir hydrocarbon in-place. The presence of organic matter and complex shale pore system create difficulty for wireline log-based water saturation interpretation. Many practitioners use conventional methods that are inappropriately applied due to the presence of organic matter. This paper offers a method for evaluating organic-rich shale water saturation and applied it to a case study of North Sumatra Basin.

In general, the interpretations were conducted by integrating the conventional and NMR logs, special and routine core analysis, petrography analysis, and geochemical analysis. We started to determine the total and effective porosities and also the total porosity considering the presence of kerogen. These parameters were estimated by using the conventional and NMR log and validated by core data. Then, the water saturation equation of Indonesia model was modified by involving the effect of total organic carbon (TOC). TOC was estimated in advance by averaging three results from the correlation of TOC-Density, Passey's ΔlogR and modified Carbolog methods. The modified Carbolog method is a technique for measuring TOC by using a resistivity and acoustic log crossplot. Equally important, we used saturation exponent (n), cementation factor (m), and the tortuosity factor (a) which were obtained from laboratory measurement of formation resistivity factor and resistivity index (FFRI).

Finally, the interpretation result of water saturation under this developed method was made. We compared results to other interpretation methods (the revised water saturation-TOC and some conventional water saturation methods). In conclusion, the water saturation from the proposed method gives a reasonable fluid saturation distribution result for both free gas and absorbed gas while the conventional methods only consider to the free gas filled in the fracture and/or matrix pore spaces.

This study demonstrates how to integrate and maximize the petrophysical, geological and geochemical data in order to solve problems in estimating water saturation of unconventional organic mudstone. This analysis improves understanding of water saturation for better interpretation in organic-rich shale reservoirs.

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