Subsurface and Petrophysical Studies of Shaly-Sand Reservoir Targets in Apete Field, Niger Delta

Conventional departures from Archie conditions for petrophysical attribute delineation include shaliness, fresh formation waters, thin-bed reservoirs, and combinations of these cases. If these departures are unrecognized, water saturation can be overestimated, and this can result in loss of opportunity. Wireline logs of four (4) wells from Apete field were studied to delineate petrophysical attributes of shaly-sand reservoirs in the field. Shale volume and porosities were calculated, water saturations were determined by the dual water model, and net pay was estimated using field-specific pay criteria. Ten sand units within the Agbada formation penetrated by the wells were delineated and correlated and their continuity was observed across the studied wells. The reservoirs had high volume of shale (Vcl), high hydrocarbon saturation, low water saturation, and good effective porosity ranging 12.50–46.90%, 54.00–98.39%, 1.61–46.0%, and 10.40–26.80%, respectively. The pay zones are relatively inhomogeneous reservoirs as revealed from the buckle’s plot except in Apete 05. The direction of deposition of the sands was thus inferred to be east west. Empirical relationships apply with variable levels of accuracy with observation of the porosity-depth, water saturation-depth, and water saturation-porosity trends. Core data is recommended for better characterization of these reservoirs. 1. Introduction Shales can cause complications for the petrophysicist because they are generally conductive and may therefore mask the high resistance characteristic of hydrocarbons [1]. Several factors are to be considered when delineating petrophysical attributes for shaly-sand reservoirs because clay minerals add conductivity to the formation especially at low water saturations. Clay minerals attract water that is adsorbed onto the surface, as well as cations (e.g., sodium) that are themselves surrounded by hydration water. This gives rise to an excess conductivity compared with rock, in which clay crystals are not present, and this space might otherwise be filled with hydrocarbon. Using Archie’s equation in shaly sands results in very high water saturation values and may lead to potentially hydrocarbon bearing zones being missed. Moreover, in clean sands, the irreducible water volume is a function of the surface area of the sand grains and therefore the grain size, but for shaly sands the addition of silt and clay usually decreases effective porosity due to poorer sorting and increases the irreducible water volume with the finer grain size [2]. Archie’s equation was developed for