Porosity/formation‐factor relationships for high‐porosity siliciclastic sediments from Amazon Fan

Abstract

Abstract. The electrical resistivity of siliciclastic rocks is a function both of pore-fluid resistivity and of formation factor (FF), an intrinsic rock property. For low-porosity rocks, FF depends on clay conduction and porosity. In contrast, we find that FF of high-porosity sediments (fractional porosities of 0.3-0.6) from Amazon Fan is controlled primarily by porosity and pore geometry; clay conduction is a minor effect. Porosity vs. formation factor (FF) plots for Amazon Fan well logs demonstrate two distinct trends which are dependent on the amount of shale present. Muds, with more than about 0.4 shale fxaction, follow a trend that increases to much higher FF as porosity decreases, compared to sands and slightly muddy sands. This separation is of opposite sign to the separation between low-porosity sandstones and shales attributable to clay conduction. We conclude that in high-porosity muds, presence of clays reduces formation conductivity by increasing the tortuosity of pores.

Universal scaling of the formation factor in porous media derived by combining percolation and effective medium theories

“…Similar to Winsauer et al [], Carothers [] presented the power law equation F = 0.85 φ −2.14 for limestones. In a separate study, Erickson and Jarrard [] found F = 0.76 φ −2.51 for shale‐like muds having shale fraction greater than 0.4. As mentioned above, when the actual value of the threshold porosity is greater than zero, setting its value equal to zero would force the exponent determined from experiments to be greater than 2 (e.g., 2.14 in Carothers [] and 2.51 in Erickson and Jarrard []).…”

“…In a separate study, Erickson and Jarrard [] found F = 0.76 φ −2.51 for shale‐like muds having shale fraction greater than 0.4. As mentioned above, when the actual value of the threshold porosity is greater than zero, setting its value equal to zero would force the exponent determined from experiments to be greater than 2 (e.g., 2.14 in Carothers [] and 2.51 in Erickson and Jarrard []). Again assuming φ t = 0.1 φ [ Hunt , ], fitting equation gave φ x = 0.86 for the Carothers [] data and φ x = 1 for the Erickson and Jarrard [] data (results not shown).…”

Fracture-controlled gas hydrate systems in the northern Gulf of Mexico

“…In shales or clay-rich sediments, modifications of Archie's equation are often used because clay-associated ions are thought to contribute to the measured conductivity. Erickson and Jarrard (1998) determined that shallow, high-porosity water saturated clays do not display any conductivity increase due to clay content and that it is appropriate to apply Archie's equation without modification. However, Archie's equation must be used with caution in fractured media.…”

Rapid sedimentation and overpressure in shallow sediments of the Bering Trough, offshore southern Alaska

“…This method yielded a = 0.99 ± 0.16 and m = 1.92 ± 0.01 where the reported errors are ±1 σ (Figure ). Note that the presence of clay minerals contributes additional conductivity to the formation and may affect the porosity estimates for porosities less than 0.30 [ Erickson and Jarrard , ]. The low value of m we obtained, as well as the small error on the value, suggests that this effect was not significant in these data [ Revil et al , ].…”

“…However, electrical resistivity logs were acquired from 92 to 699 m wireline depth below seafloor [ Expedition 341 Scientists , ], which allow estimations of porosity continuously over this interval. Following Erickson and Jarrard [], we used Archie's equation [ Archie , ] to calculate porosity ϕ from resistivity as…”