Effect of pore-lining chlorite on petrophysical properties of low-resistivity sandstone reservoirs

Chlorite-bearing sandstones usually give low resistivity signals and are thus erroneously identified as nonpay zones, even if they exhibit good preserved porosities at depth. The purpose of this paper is to provide petrophysical and mineralogical laboratory measurements that help improve the log interpretation of these sandstones. The main results were obtained from a selection of reservoir cores. For sandstones having an amount of chlorite between 3 and 11 wt%, they show that the cementation index and saturation exponents (m and n) have values lower than 2, with n frequently around 1.5. These low values have been checked carefully to avoid experimental artifacts. Cationic Exchange Capacities (CEC) and Specific Surface Areas (SSA) both have low values. as expected from the clay structure. The distribution of pore throats is bi- or trimodal, with a large contribution of microporosity. The interpretation suggested by these results is that the amount and distribution of microporosity associated with pose-lining clay is the key to the chlorite sandstone electrical behavior. The values of CEC or SSA are too low to fully explain the low values of the saturation index n. Use of these low values in log interpretation has the effect of increasing the interpreted oil in place. Advantages of a multidisciplinary approach for better evaluation of such complex argillaceous sandstone reservoirs is highlighted.