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Characterization of Carbonate Heterogeneities at Outcrop Scale: An Integrated Approach Coupling Geophysics, Petrophysics and Geostatistics

C. Matonti, Y. Guglielmi, S. Viseur, P-O. Bruna, and C. Dahl
Aix–Marseille Univ, CEREGE Centre Saint-Charles, Marseille, France

Carbonate rocks are highly heterogeneous and complex as they are induced from superimposed biological, chemical and mechanical processes. Few studies have been dedicated to the characterization of their petrophysical heterogeneity at meso-scale, i.e. above the laboratory scale and under the common seismic resolution. In this paper, petrophysics, geophysics and geostatistics are combined at outcrop scale to characterize the relationships between heterogeneities and the spatial distribution of petrophysical properties. Thus, three decameter-scale outcrops (Cassis, Calissane and Grignantes outcrops) presenting contrasted faciologic, structural and sedimentary heterogeneities were chosen to support this analysis. For each outcrop, the following workflow is proposed:

    (1) Mapping of geological and mechanical structures (stylolithes, joints, fractures, karst, etc.); characterized by morphological and qualitative attributes as fracture aperture, strike and dip, karstification level.
    (2) Geophysical and petrophysical measurements at inframeter scale (Vp on outcrop surface and porosity on plugs). P-wave velocities were measured on the field with a portative device (Pundit7 CNS Farnell Ltd.) composed of two piezoelectric transducers. A step by step iterative and directionnal method was used. At each measurement step, emitter is placed on the previous receiver location along the horizontal and vertical directions. Finally, Vp values were artificially attributed to the centre of the emitter/receptor segment along horizontal or vertical transects.
    (3) Statistical and geostatistical analyses of the measured petrophysical and geophysical properties. Histogram statistics as well as aerial variograms (8 directions) were calculated on Vp values. The variogram analysis was performed using Gringarten et al. 1999 methodology.

Wide differences were noticed between the three outcrops, concerning the nature and the spatial repartitions of the geological and mechanical structures. The Cassis outcrop is composed of lower Barremian tight grainstone from inner platform facies. It presents a high fracture density and a karstified area on its eastern part. Two fracture sets affect the rock mass, striking around N030 and N090 respectively. The N090 striking set shows evidences of reactivation associated with a principal stress striking N030. Fracture aperture measurements show an increasing trend toward the eastern part of the outcrop. This may result from the fracture reactivation or the dissolution of fracture walls by water circulation.

 

AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012