An Integrated Approach to Calculate Vertical and Horizontal Resistivity Utilizing Conventional Logs in Low Resistivity Pay Reservoirs in Gulf of Suez: Case Study

Abstract

The occurrence of Low Resistivity Pay (LRP) have been widely reported. The conventional induction resistivity log does not recognized LRP, because this phenomenon affects the conventional resistivity log to read lower response. This problem typically occurs because of the shoulder effect of shale laminae on resistivity measurements of oil saturated sand intervals and as a result, conventional petro-physical interpretations would be unable to identify pay intervals in such low-resistivity measurements. Conventional well log evaluation usually considers these intervals water-bearing (60-70%) interval, which completely conceal oil occurrence. Numerous case studies showed water-free oil production from low resistivity laminated reservoirs. When these laminations thickness are less than the vertical resolution of conventional resistivity measuring device itself, the measured resistivity values is no longer representative to these laminations, however it becomes an average of the resistivity of these thin laminations, dominated by the laminations with lower resistivity. These apparent average values of resistivity curves may mask the recognition of hydrocarbon presence. This formation evaluation challenge is usually addressed using simultaneous measurements of horizontal (Rh) and vertical (Rv) resistivity measurements. Such measurements are not conventionally used and therefore unconventional interpretation would be impossible or at least difficult. In the present paper, an innovative technique is developed to calculate the resistivity of sand intervals within thin laminated reservoirs from conventional apparent resistivity measurements and lithology logs. This is achieved considering two main concepts; separating apparent resistivity in Rh and Rv using Moran and Gianzero (1979) formula and calculating sand resistivity using sand - shale resistivity connections (parallel and series connections). In North Morgan Belayiem reservoir located in the Gulf of Suez in Egypt, which was discovered since 1965, a new detailed interpretation and formation evaluation analysis have been done to re-interpret thin sand shale laminations having low conventional resistivity curves. Pay was identified using Rv and Rh calculations in petro-physical interpretation instead of using pervious conventional resistivity and lithology logs. After validating the petro-physical analysis and making an integration with available production data, the potential of the LRP zones in this reservoir was confirmed and quantified using petro-physical attribute.