In situ velocities in pelagic carbonates: New insights from Ocean Drilling Program Leg 130, Ontong Java Plateau

Abstract

Ocean Drilling Program Leg 130 returned an extensive suite of in situ borehole logging and closely spaced shipboard physical properties measurements from four sites drilled in carbonate‐rich pelagic oozes and chalks on the Ontong Java Plateau. Using these data, we evaluated factors responsible for observed differences between laboratory data and in situ values. Laboratory and log differences for bulk density/porosity are small and nearly constant and can be accounted for by hydraulic rebound of pore fluid following sediment recovery. Evidence for mechanical rebound of sediments was not observed. Velocity‐porosity relationships cannot be used to indirectly correct laboratory velocities to in situ because fundamental assumptions required for rebound corrections are not satisfied. Velocity‐porosity systematics are also complicated by foraminiferal intraparticle porosity variations. We derived empirical laboratory velocity corrections using log and laboratory velocity differences expressed as functions of depth and effective pressure. We tested the velocity corrections using data for carbonates recovered from Ocean Drilling Program Sites 704, 722, and 762; comparisons with logs show that corrected velocities accurately estimate in situ values for oozes and chalks (CaCO3 >60%). For applications to unlogged Deep Sea Drilling Project Leg 85 sites, igneous basement travel times calculated using corrected velocities agree with acoustic basement travel times from seismic records, implying that corrected velocities are realistic in situ estimates. At Site 574, we used our corrected velocities to reevaluate previous lithostratigraphic and seismostratigraphic correlations based on mechanical rebound‐corrected velocities. The new velocities correlate reflectors to progressively greater core depths. Although changes are minor above 200 m below seafloor, the lowest assigned reflector is shifted 36 m deeper to more closely coincide with a diagenetic boundary and acoustic basement correlates with igneous basement instead of with postulated high‐impedance reflectors within the sediments.

Application of differential effective medium, magnetic pore fabric analysis, and X-ray microtomography to calculate elastic properties of porous and anisotropic rock aggregates

“…In contrast for porosities typical of reservoir rocks (20–40%) V P and V S changes are already significant for aspect ratios 1 to 5. For high porosities characteristic of poorly compacted sediments (40–80%) the effect of the low aspect ratio is of particular importance for dV S , which may be significant for carbonate reservoir rocks, such as chalk and mudrocks [ Urmos and Wilkens , 1993; Røgen et al , 2001; Mallon et al , 2005]. Figures 3a and 3c display a scenario where we consider an aggregate of crystals, which all have the same orientation, effectively producing the same result as the elastic tensor of a single crystal.…”

A compaction front in North Sea chalk

“…The Ekofisk field is located about 5 km north of the upper left corner of the map. (left) Porosity and (right) P wave sound velocity data from well logs and core samples versus various parameters for the chalk in North Sea wells Jette-1 and Sine-1 and from a sequence of pelagic carbonate of Albian-Aptian to Pleistocene age from Ocean Drilling Program (ODP) site 807 (Leg 130 [Urmos and Wilkens, 1993]) (a, b) versus depth, (c, d) versus effective depth [cf. Japsen et al, 2005b;Fabricius et al, 2008], (e, f) versus temperature.…”

“…Log data are averaged over intervals of 10 m. The chalk porosity log was estimated from density log data assuming a pure calcite density of 2.71 g/cm 3 . The sonic log data for the ODP well are noisy for depths greater than 980 m below the seabed [Urmos and Wilkens, 1993]; thus only the well data from above that depth are used here. The location of North Sea wells is given in Figure 1, and pressure and temperature data are given in Tables 1a and 1b.…”

Proceedings of the Coastal Benthic Boundary Layer Key West Workshop

“…In contrast, compressional wave velocity measured and corrected for in situ temperature, pressure and intraparticle porosity (Urmos and Wilkens, 1993) using transducers mounted in a Hamilton Frame in the laboratory on cores recovered from northern Little Bahama Bank (ODP Leg 101, Site 630) shows minimal increase with depth and significantly lower values of compressional wave velocity than those measured during laboratory testing (Lavoie 1986). Results from Site 630 suggest that the carbonate sediment (predominantly foraminifers and coccoliths) consolidate much less in situ than in the laboratory.…”

“…Time and temperature conditions necessary for pressure solution and cementation in the laboratory cannot be adequately simulated, thus the Dry Torugas and Marquesas sediments are easily consolidated during laboratory testiing. Urmos and Wilkens, 1993 Figure 4. Shear wave velocity measured during consolidation testing and estimated from measured porosity using Bryan and Stoll's (1988) Bryan and St ll, 1988).…”