HR: 1330h
AN: T43A-03    [Abstracts]
TI: Pore structure effects on elastic moduli - porosity relationships in carbonate rocks
AU: * Baechle, G T
EM: gbaechle@rsmas.miami.edu
AF: Gregor Baechle, MGG, RSMAS Comparative Sedimentology Laboratory 4600 Rickenbacker Cswy, Miami, FL 33149 United States
AU: Weger, R
EM: rweger@rsmas.miami.edu
AF: Gregor Baechle, MGG, RSMAS Comparative Sedimentology Laboratory 4600 Rickenbacker Cswy, Miami, FL 33149 United States
AU: Eberli, G
EM: geberli@rsmas.miami.edu
AF: Gregor Baechle, MGG, RSMAS Comparative Sedimentology Laboratory 4600 Rickenbacker Cswy, Miami, FL 33149 United States
AU: Massaferro, J L
EM: j@repsol
AF: Repsol, Carbonate Group, , Madrid, Spain
AB: One characteristic of carbonate rocks is the lack of correlation between velocity and porosity. This is often due to diagenetic changes in the pore structure. Understanding the effect of rock texture on acoustic properties of carbonate rocks is vital for reliable porosity determination from seismic signatures. In carbonates, the pore type and porosity mainly controls the velocity and herewith the impedance contrast. The carbonate pore type combines information about the pore structure geometry and grain to grain contacts. In this study we investigate systematically the effect of pore types on velocity on a dataset of 127 samples from 3 different pure carbonate formations. Their porosity covers a range from 3% to 50%, the compressional velocity span from 2900 m/s to 6300 m/s. Experimental results show that theoretically impedance contrasts can be solely caused by changes in pore types, without the need for changes in porosity or mineralogy. In order to highlight the importance of pore structure, the data is presented in normalized bulk moduli-porosity space, hereby eliminating effect of differing calcite and dolomite velocities. Samples of the same pore types shows distinctive moduli-porosity best-fit curves. Empirical relationships between moduli and porosity have been derived for carbonates with high amount of microporosity and vuggy dolostones. Vuggy Miocene dolostones from ODP Leg194 show a considerably stiffer rock frame then their microporous counterparts. Vuggy dolostones show non-linear moduli-porosity relationship, with bulk modulus stiffness outside the Nur's modified Voigt boundary. Qualitative image analyses objectively characterize the pore structure and reduce the uncertainties present in velocity scatter. We measured macroporosity from digital image analysis and related it to total plug porosity to derive microporosity. Image macropores are all pores visible in thin section, generally pores bigger than 30 microns in diameter. The microporosity gives a good correlation to the velocity and constrains the microporosity uncertainty. Furthermore, the velocity-microporosity best-fit curve is in close vicinity of the Wyllie's time average trend line. Image macroporosity shows a positive correlation with velocity deviation calculated from velocities of Wyllie's time average equation. Pore space stiffness calculated from the poroelastic properties alone, uniquely quantify the velocity deviation at a given porosity. Recrystallized dolomites with secondary vuggy porosity show distinctive higher pore space stiffness values of around 0.2 compared to high microporosity limestones with lower pore space stiffness (0.1). These results indicate that there is hope for pore structure inversion in carbonate rocks.
DE: 3909 Elasticity and anelasticity
SC: Tectonophysics [T]
MN: 2005 Joint Assembly