Factors controlling elastic properties in carbonate sediments and rocks

Abstract

Carbonate sediments are prone to rapid and pervasive diagenetic alterations that change the mineralogy and pore structure within carbonate rocks. In particular, cementation and dissolution processes continuously modify the pore structure to create or destroy porosity. In extreme cases these modifications can completely change the mineralogy from aragonite/calcite to dolomite, or reverse the pore distribution whereby original grains are dissolved to produce pores as the original pore space is filled with cement to form the rock (Figure 1). All these modifications alter the elastic properties of the rock and, therefore, the sonic velocity. The result is a dynamic relationship among diagenesis, porosity, poretype, and sonic velocity. The result is a wide range of sonic velocity in carbonates, in which compressional-wave velocity (V P ) ranges from 1700 to 6600 m/s and shear-wave velocity (V S ) from 600 to 3500 m/s.Porosity is the main controlling factor in determining the sonic velocity in rocks but in carbonates the pore type is nearly equally important in the elastic behavior and resultant sonic velocity (Anselmetti and Eberli, 1993, 1997). Most of the current theoretical equations do not, or insufficiently, account for this modification of the elastic behavior by the pore type. Consequently, seismic inversion, AVO analysis, and calculations of pore volumes that are based on these equations are prone to large uncertainties in carbonates.We measured acoustic velocities on modern carbonate sediments and rocks in various stages of diagenesis to reveal the relationships between original composition, porosity, pore type, and velocity. The apparatus for these laboratory experiments, constructed by VerdeGeoScience, consists of an oil-filled pressure vessel that contains the ultrasonic transmitter-receiver pair with piezoelectric transducers and the sample. Miniplug samples of one inch (2.5 cm) diameter and 1-2 inches in length are positioned between two piezoelectric transducers and sealed from the confining oil in the pressure vessel. Confining and pore-fluid pressures are chosen independently to simulate most accurately insitu stress conditions of buried rocks. The pore-fluid pressure is kept stable at 2 MPa and the confining pressure is varied between 3 and 100 MPa, resulting in an effective pressure of up to 98 MPa. The pair of transducers generates one compressional wave signal (V P ) and two perpendicularly polarized shear wave signals (V S 1, V S 2) at central frequencies close to 1 MHz.Sonic velocity of carbonate sediment. Grain size and shape, sorting, and the ratio between grain and matrix influence acoustic velocity in unconsolidated carbonate sediment. Pure carbonate mud has an average porosity of 60% and V P of ~1700 m/s. At a compression of 170 MPa, the porosity is reduced to 29% and V P increases to 2250 m/s, while V S is between 900-1200 m/s. These mud samples have a low shear modulus and, thus, a behavior similar to materials that have no rigidity (liquids). Carbonate sand (ooids and skeletal grains) s...

Stress sensitivity of stylolite morphology

“…We do not think that our observed trend in the stylolite crossover is a function of varying elastic parameters. In order to explain such a smooth variation of the crossover over the whole profile by a variation of one of the parameters (Young's modulus, porosity or density) would require that these parameters vary smoothly and linearly with depth, which is not very often the case (compare Eberli et al, 2003). For example a decrease in porosity down-section, i.e.…”

“…(4) shows that the crossover length is a function of the elastic properties, which are strongly influenced by the rock porosity (e.g. Eberli et al, 2003). It is likely that the elastic parameters evolve with the compaction process, consequently the elastic parameters may change with depth.…”

Characterization of a complex near-surface structure using well logging and passive seismic measurements

“…On the one hand, Upper Cretaceous materials are mainly limestone rocks where porosity, pore types and insoluble content control the variation of physical parameters (Eberli et al, 2003). Regarding elastic properties, there is not a direct correlation between seismic velocities and depth of burial or age.…”

“…7a delineates this cluster interpreted as pure or massive limestone. Fast velocities (> 4000 m s −1 ) are reached if the clay content is below 5 % (Eberli et al, 2003). The rest of the points are related to marly limestones and marls.…”

“…Since cementation can add stiffness to shallow carbonate rocks, increasing the elastic moduli and hence velocity, velocity inversions with depth are common (Eberli et al, 2003). On the other hand, many carbonates contain a small amount of insoluble components (e.g.…”