Deriving mineral moduli of the noncarbonate fraction in a marly chalk reservoir using petrophysical logging data and an isoframe model

Abstract

The knowledge of the elastic properties of clays and clay-rich sediments is essential when performing quantitative seismic interpretation and geomechanical modelling of reservoirs where they are present. Such is the case of the marly chalk reservoirs of Lower Cretaceous age in the Danish North Sea. We present a new method to derive elastic properties of the clay-rich insoluble residue (IR) fraction in marly chalks, using petrophysical logs and rock physical modelling. We calculated compressional ( M), shear ( G), and bulk ( K) moduli of the Formation along the borehole using the density and sonic (shear and compressional) logs, and modelled the elastic properties of minerals using the Isoframe rock physics model. In our model, the marly chalk is composed of two materials: calcite, with well-known elastic properties, and the IR fraction, whose elastic properties are the sought variable. We systematically vary the assumed elastic properties of the IR fraction, each time achieving equality between measured and modelled elastic moduli solving for the Isoframe values of calcite ( IF c) and the IR fraction ( IF ir) for each of the three elastic parameters one at a time, deriving the modelled values for the other two. We add up the error between modelled and logged values for each set of assumed elastic properties of the IR fraction. We adopt the values of K ir and G ir that minimize the compound error as the actual elastic properties of the IR fraction. We ran the algorithm in the Boje-2c and Valdemar-2H wells in different stratigraphic units of the Lower Cretaceous: the Fanø Formation and the marly chalk of the Tuxen and Sola Formations. Elastic properties derived for the IR fraction compare well to data for wet clays from ultrasonic velocity laboratory tests. In a clean chalk section (nearly 100% calcite), the model outputs values similar to calcite.