Low Resisitivity Pay; the Role of Chlorite in Controlling Resistivity Responses

Petrophysics traditionally uses Archie’s equation to estimate the amount of hydrocarbons initially in place. This relies on the increase in resistivity when non-conductive hydrocarbons replace conductive saline fluids in the pore space. However, if clay minerals are present in sufficient abundance, they can lower the resistivity to such an extent as to compensate for the increase in resistivity caused by the presence of hydrocarbons. The study reservoir (A), of the Berkine Basin, Algeria, is an example of this “low resistivity contrast”. No discernable change in the resistivity between the water-bearing and the hydrocarbon-bearing sections of the sandstone reservoir results in a continuous overestimation of the water saturation. Chlorite is a known cause of “low resistivity contrast” and is prevalent throughout the study reservoir sandstones. The low cation exchange capacity of chlorites means known shaly-sand models do not apply. Therefore it is necessary to develop an alternative method for estimating saturation in the study reservoir. To understand where the resistivity may be most affected the distribution of the chlorite within the reservoir must be determined. Detailed analysis of the sedimentary data identified a link between the chlorite-rich sandstones and the upper shoreface depositional environment. Discriminant statistical analysis of the log data was successful in identifying the upper shoreface, chloritic sandstones from the lower shoreface sandstones and offshore mudstones. This provided a classification scheme to identify the chlorite-rich intervals from log data in uncored wells. Analysis of capillary pressure curves, with respect to the depositional environments, identified a strong correlation between the chlorite occurrence and core-based petrophysical characteristics. This allowed for Leverett-J saturation height functions to be developed for the upper shoreface, chlorite-rich sandstones and lower shoreface sandstones. Transformation of these Leverett-J functions to the wireline log scale allowed saturation estimations to be calculated that account for the chlorite presence and don’t require the resistivity measurement.