Acoustic Properties of "Young" Carbonate Rocks, ODP Leg 166 and Boreholes Clino and Unda, Western Great Bahama Bank

Abstract

Acoustic properties determined from logs and discrete measurements along a borehole transect across the slope of the western Great Bahama Bank margin (ODP Leg 166) show a distinct relationship with primary sediment composition and noncarbonate content.Dominant sediment types are alternating cemented and uncompacted calciturbidites with shallow-water derived grains, and poorly to uncemented and compacted periplatform ooze of globigerinid-dominated wackestone to packstone. Comparison of the velocity-porosity variation from logs and discrete measurements reveals that both tend to follow the trend of generally accepted velocity transforms. Primary control on the acoustic velocity is exerted by total porosity, whereas the remaining variation in velocity is explained by differential diagenesis. The less cemented to uncemented and compacted background sediments have velocities that, at a given porosity, fall below the timeaverage equation for calcite. The cemented turbidites, however, have velocities that are higher than those predicted by the time-average equation. Although it is unclear whether the original volume of aragonite or presence of organic matter controlled this difference, it is suggested that the acoustic properties reflect an input signal. The presence of noncarbonate material, some of which is clay, increases basinward and further reduces the acoustic velocities in the background sediment. In contrast, earlier work on the more proximal slope, and nearly pure carbonate, intervals in Clino and Unda cores documents diagenetic differentiation in compositionally similar sediment, resulting in alternating acoustically fast and slow layers. Marginal and nearly pure limestones have a more complex acoustic behavior and hence are difficult to predict.Though both velocity logs and discrete velocity measurements show comparable general trends, logging data show a considerably larger variation at a given porosity value (up to 3.5 km/s) than discrete data, which ranges between 1.0 and 2.5 km/s. To improve the quality of velocity logs it is suggested to groundtruth these data with other logging data, general velocity transforms, and, ideally, discrete measurements on core plugs.

Acoustic and reservoir properties of microporous carbonate rocks: Implication of micrite particle size and morphology

“…Recent studies have shown that seismic properties of carbonate rocks are affected by many parameters such as rock fabric, pore type and shape, porosity, and pore fluid, making it difficult to attribute changes in seismic expression to any one parameter [ Anselmetti and Eberli , , ; Kenter et al ., , , ; Assefa et al ., ; Verwer et al ., ; Weger et al ., ; Brigaud et al ., ; Casteleyn et al ., , ]. These studies have mainly been focused on grain‐supported limestones that can have a high macropore space development.…”

“…Some studies also provide petrophysical ranges of the porosity and permeability properties associated with micrite morphologies and proposed morphological classification partly related to those properties [Lambert et al, 2006;Richard et al, 2007;Volery et al, 2010;Deville de Periere et al, 2011]. Theories and relationships that describe sonic wave propagation in siliciclastic media [Wyllie et al, 1956[Wyllie et al, , 1958Gardner et al, 1974;and Raymer et al, 1980] are hard to apply in the complex systems of pure carbonates because of their textural complexity and their several diagenetic pathways [Kenter et al, 1997[Kenter et al, , 2002Eberli et al, 2003;Brigaud et al, 2010]. More precisely, the classic velocity-porosity transforms and the Gardner's empirical relation allow the prediction of P wave velocity from porosity or bulk density but fail to account for the variation in velocity at a given porosity value.…”

“…More precisely, the classic velocity-porosity transforms and the Gardner's empirical relation allow the prediction of P wave velocity from porosity or bulk density but fail to account for the variation in velocity at a given porosity value. Those empirical relationships still have substantial limitations in carbonates media because they do not take into account the pore space parameters or mixed mineralogy [Kenter et al, 2002]. This paper reports a detailed rock physics study coupled with microstructural observations from cores of the Late Jurassic Limestones located in the eastern part of the Paris Basin.…”

NMR relaxation of neritic carbonates: An integrated petrophysical and petrographical approach

“…In particular the process of early diagenetic (eogenesis) alteration of the primary sedimentary facies appears to be a crucial factor in understanding the rock-fabric properties (e.g. Kenter et al, 2002;Melim et al, 2001) and the NMR signal of some carbonates.…”

Limestone–marl alternations as environmental archives and the role of early diagenesis: a critical review

“…The regular and laterally continuous intercalation of the limestone beds, which are more resistant to weathering, and the softer interlayers, renders micritic calcareous rhythmites conspicuous in the field. In cores, a typical expression of calcareous rhythmites is the regular alternation of lighter and darker (e.g., Betzler et al 1999;Frank et al 1999), or of more and less porous layers (e.g., Westphal et al 2000;Kenter et al 2001Kenter et al , 2002.…”