Abstract:
Accurate porosity determinations are vital to estimation of permeability, exhumation, compaction, and fluid flux, as well as reservoir evaluations. Well logs provide in situ physical properties measurements that can be used to determine porosity. This study analyzes well logs to determine density-based porosity-FF and porosity-velocity relationships from a variety of geologic settings, primarily the Ferron Sandstone in southern Utah, the Amazon submarine fan, and the Barbados accretionary prism. The fluvial-deltaic sandstones of the Cretaceous Ferron Sandstone, Utah, demonstrate petrophysical responses of exhumation. Porosity increases with decreasing depth due to leaching, while velocity logs show a corresponding sonic velocity decrease that is even greater than expected based on the porosity change, attributable to the reduction of framework modulus caused by exhumation-induced opening of microcracks. Exhumation is also common in the accretionary prism setting. A porosity-velocity relationship is presented for well logs obtained by the Ocean Drilling Program (ODP) from unconsolidated, high porosity siliciclastic sediments at the Amazon fan. Based on combining the Amazon fan observations with a variety of other datasets, new empirical porosity-velocity transforms are proposed, which can be used for marine siliciclastic sediments of any porosity. It is well known that the resistivity of low porosity rocks depends on clay conduction and porosity. In contrast, we find that the resistivity of high porosity sediments (fractional porosities of 0.3-0.6) from the Amazon fan is controlled by porosity and pore geometry; clay conduction is minor. Porosity vs. formation factor (FF) plots for the Amazon fan well logs demonstrate two distinct trends which are dependent on the amount of shale present; muds have higher FF than sands of the same porosity, because of more tortuous current paths in the muds. ODP Leg 171A collected Logging While Drilling data to study porosity and fluid flux in Barbados accretionary complex. Porosity and velocity are determined, based on an analysis of local porosity-FF and porosity-velocity relationships and a comparison to the Amazon fan relationships.