| Low resistivity pay zone has low resistivity contrast with sandstone and adjacent mudstone. They are
widespread in the global petroleum basins, and play a critical role in increasing petroleum production in China as well.
The sandstone reservoir of the Paleogene Enping Formation is the main oil producing zone in Lufeng 13-1 Oilfield, Pearl
River Mouth Basin. The pay zones are typical low resistivity reservoirs because the resistivity of the pay zone is usually
lower than that of the up and below mudstone layers: resistivity of pay zone is about 2~8 Ω·m, and resistivity of nonreservoir
mudstones is about 5~20 Ω · m. Even though these reservoirs have been exploited for nearly 30 years, the
resistivity lower limit of pay zone is still uncertain. It makes great difficulties in discriminating oil and water in reservoir.
Based on the study of reservoir parameters, the relationship between oil saturation and formation resistivity in common
logging interpretation models are compared in this paper. Then the Indonesia formula is optimized to theoretically analyze
the influencing factors of reservoir resistivity including shale content, porosity, formation water resistivity and water
saturation. Calculation method of resistivity lower limit under different physical properties and at different depth points is
proposed. The results show that the high shale content, well porosity, low formation water resistivity and high water
saturation would result in the low resistivity of the reservoir. The resistivity lower limit is a dynamic parameter: in
different physical property condition, the resistivity lower limit is different. Under the worst physical property with shale
content of 35% and porosity of 12%, the maximum resistivity lower limit of reservoir is 4.6 Ω·m; under the average
physical property with shale content of 10% and porosity of 18%, the average resistivity lower limit of reservoir is 3.2 Ω·m;
and under the best physical property with shale content of 3% and porosity of 22%, the minimum resistivity lower limit of
reservoir is 2.5 Ω·m. The average resistivity lower limit is proved by production data. At different depth points, the
resistivity lower limit is calculated based on corresponding reservoir parameters. The ratio between resistivity logging
values and resistivity lower limit of oil, water-oil and water layers are ＞0.85, 0.7~0.85 and ＜0.75 respectively. The
differentiation criterion is applied in perforated intervals and its validity and accuracy are proved by the oil test result. The
proposed method would be beneficial to identify oil and water layer fastly. |
