AAPG Geoscience Technology Workshop

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The Role of Detrital Components in the Petrophysical Parameters of Paleogene Calcareous-Dominated Hemiplegic Deposits


The role of detrital components in influencing the petrophysical parameters of predominantly calcareous rocks is essential to understanding its influence on the porous space, which in turn controls reservoir quality. A long (340 m) core encompassing primarily Eocene carbonate sediments from southern Israel has been made available for conducting high resolution petrophysical analyses. The sedimentary record was separated into three units (Units 1 to 3) based on variations in the lithology and associated measured petrophysical parameters (density, magnetic susceptibility, and sonic velocity). The most significant differences in the petrophysical parameters occur in the lowermost Unit 1 (late Palaeocene), which displayed erratic variations in measured sonic velocity, density, magnetic susceptibility, porosity, and insoluble residual contents greater than 10%. Unit 1 is composed primarily of an intercalation of dark grey clay, hard white chalk, and grey dark marl, with smectite as the dominant clay mineral. The overlying units 2 and 3 (Early to Middle Eocene) are mainly composed of dark chalk with some chert units, occasionally intercalated with grey chalk. Sonic velocity, bulk-density, magnetic susceptibility, and insoluble residual measurements in these two units gradually increase with depth parallel, with a decrease in porosity. The authors suggest that the main factors influencing variations in the petrophysical parameters are the presence of clay content, and carbonate dissolution. The area is predominantly composed of terrigenous sediments, the deeper buried marly clay lithology is probably sourced from the overlaying sediments, and the source of high magnetic susceptibility values observed in Unit 1 is unclear. The authors have developed a useful empirical relation for the prediction of acoustic impedance in pure carbonate sediments.