Resistivity Logs High deep resistivity means: Hydrocarbons Tight streaks (low porosity) Low deep resistivity means: Shale Wet sand Separation between resistivities means: The formation fluid is different from the drilling fluid The formation is permeable to the drilling fluid Deep, Medium, and Shallow refers to how far into the formation the resistivity is reading (4 ft, 2 ft, few in) ILD (deep) MSFL SFL Formation Fluid different from Drilling Fluid SLIDE 9 Another common log measures the electrical resistivity of the formation Tools are designed to investigate different distances into the rocks Shallow = a few inches into the formation Medium = about 2 feet into the formation Deep = about 4 feet into the formation If the deep resistivity is high = either HCs or low porosity tight streaks If the deep resistivity is low = shale or wet sand If there is separation between the medium and deep measurements, it means The formation fluid is different from the drilling fluid, and The formation is permeable to the drilling fluid On the log that is shown, deep = black; red = medium The region boxed in red – the curves are separated, hence formation fluid different from drilling fluid E.g., if drilling fluid = water, then interval does not have water in the pore space The region boxed in green – the curves are NOT separated, hence formation fluid same as drilling fluid Formation Fluid similar to Drilling Fluid Courtesy of ExxonMobil L 4 - Well Log Data