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

Chlorite-bearing sandstones usually give low resistivity signals and are th us erroneously identified as nonpay zones, even if they exhibit good preser ved porosities at depth. The purpose of this paper is to provide petrophysi cal and mineralogical laboratory measurements that help improve the log int erpretation of these sandstones. The main results were obtained from a selection of reservoir cores. For san dstones having an amount of chlorite between 3 and 11 wt%, they show that t he cementation index and saturation exponents (m and n) have values lower t han 2, with n frequently around 1.5. These low values have been checked car efully 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 distri bution of microporosity associated with pose-lining clay is the key to the chlorite sandstone electrical behavior. The values of CEC or SSA are too lo w 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 fo r better evaluation of such complex argillaceous sandstone reservoirs is hi ghlighted.