POROSITY FORMATION-FACTOR RELATIONSHIPS FOR HIGH-POROSITY SILICICLASTIC SEDIMENTS FROM AMAZON FAN/

The electrical resistivity of siliciclastic rocks is a function both o f pore-fluid resistivity and of formation factor (FF), an intrinsic ro ck property. For low-porosity rocks, FF depends on clay conduction and porosity. In contrast, we find that FF of high-porosity sediments (fr actional porosities of 0.3-0.6) from Amazon Fan is controlled primaril y by porosity and pore geometry; clay conduction is a minor effect. Po rosity vs. formation factor (FF) plots for Amazon Fan well logs demons trate two distinct trends which are dependent on the amount of shale p resent. Muds, with more than about 0.4 shale fraction, follow a trend that increases to much higher FF as porosity decreases, compared to sa nds and slightly muddy sands. This separation is of opposite sign to t he separation between low-porosity sandstones and shales attributable to clay conduction. We conclude that in high-porosity muds, presence o f clays reduces formation conductivity by increasing the tortuosity of pores.