Porosity‐formation factor and porosity‐velocity relationships in Barbados prism

Abstract

Abstract. Ocean Drilling Program Leg 171A collected logging while drilling (LWD) data to investigate the porosity and velocity structure of Barbados accretionary complex. Porosity and velocity measurements were needed for interpretation of prism dynamics. However, the LWD tool string did not measure velocity, and previous velocity data from Barbados prism are scarce and generally of low quality. Furthermore, most density-based porosities are subject to errors associated with variations in smectite content. To determine porosities more accurately, we use smectite-corrected, density-based porosities to calibrate a porosity-formation factor (FF) transform, then calculate porosities from resistivity logs. We used cation exchange capacity (CEC) measurements and LWD logs to evaluate the possibility that resistivity is affected by clay conduction. The CEC data warn of potential clay conduction, especially in smectite-rich zones, but LWD data indicate that clay conduction is minor in these high-porosity sediments. Instead, tortuosity is the secondary control of FF, after porosity. By comparing resistivity-based porosities to velocities from vertical seismic profiles, we determine a porosity-velocity transform applicable to Barbados. We then use this transform to calculate velocity logs from resistivity-based porosity logs.

Velocity‐porosity relationships in smectite‐rich sediments: Shikoku Basin, Japan

“…This is consistent with the absence of a smectite to illite transition seen within the sediments at these sites [ Underwood et al ., ] and poor consolidation in the sediments despite reaching a depth of ∼950 mbsf, the equivalent of 8.4 MPa differential pressure. Considering this trend, the critical porosity of 0.30–0.39 used in previous studies [ Erickson and Jarrard , ; Hoffman and Tobin , ] is clearly too high for these sediments. In addition, the low critical porosity (0.20) chosen, based on the addition of published petrophysical data for other mudstones, suggests that the nature of this inflection point does not follow the definition of critical porosity as outlined by Mavko et al ., [].…”

Spatial variations of the décollement/protodécollement zone and their implications: A 3‐D seismic inversion study of the northern Barbados accretionary prism

“…The quality of this log was relatively poor, especially at the DZ because of a collapse of the borehole ( Shipboard Scientific Party 1995). Erickson & Jarrard (1999) recently made an effort to create velocity logs using LWD density, resistivity, cation exchange capacity measurements and vertical seismic profile (VSP) data. The accuracy of the created velocity may still need to be tested.…”

Northern Barbados accretionary prism: Structure, deformation, and fluid flow interpreted from 3D seismic and well‐log data

“…Median matrix densities (Table 1) measured from the cores taken at the ODP sites (Shipboard Scientific Party 1998) were used for matrix density. The density of fluid was assumed to be 1.02 g/cm 3 (Erickson & Jarrard 1999).…”

“…Because clay minerals such as smectite contain a substantial amount of interlayer water, the matrix density of clay‐rich sediments, determined from dried samples, can be too high. Erickson and Jarrard (1999) constructed the smectite block model (their figure 3) for the LWD sites in the study area in which smectite contents (wt%) were converted to matrix densities using the equation:…”

“…Smectite and all other minerals were assumed to have a density of 2.1 g/cm 3 and a median density of 2.8 g/cm 3 , respectively (Erickson & Jarrard 1999). We followed Erickson and Jarrard's (1999) method to determine the matrix densities corrected for smectite (Table 2), and then the corrected matrix densities were used in Equation 1 to compute the porosities corrected for smectite (Fig. 4c).…”