Largo

Largo uniquely integrates well log data analyses and rock physics modeling with seismic analysis. Largo is the processing and quality control link between petrophysics, rock physics, and the Jason™ suite seismic inversions.

Rock Physics and Fluid Substitution

Some key features of Largo are:

  • Powerful rock physics modeling tools for the calibration and synthesis of Vp, Vs, and density logs for fluid substitution modeling.
  • Support for multiple well processing and well zoning.
  • Use of lithology types and generation of them from a crossplot analysis of logs.
  • Auto-update of log plots and crossplots when well data changes.

Technical Background

Compressional sonic well logs are common while (reliable) shear sonic logs are not. There is therefore often a need to generate synthetic shear sonic logs that correspond to the compressional sonic and density logs. There exist a number of empirical, usually linear relations (such as the Mudrock line) that transform compressional into shear velocity data. Largo fully supports these types of empirical approaches. However, the propagation of seismic waves in a fluid-filled porous rock depends on rock matrix composition and structure, as well as the properties of the pore fluids. A correct velocity estimation must therefore also depend on these factors. Empirical relationships, however, are not capable of including all necessary parameters.

Largo takes a more fundamental modeling approach: first use theoretical rock models to derive the effective elastic properties from rock and fluid composition and mineral parameters, and then calibrate the model parameters by comparison of the synthetic to the available density and compressional sonic log. A good comparison between the logs allows the parameters thus derived to be applied to shear sonic synthesis. If measured shear sonic is available, then that data can also be used for calibration.

Largo contains the following rock physics algorithms:

  • Xu & White’s model
  • A faster approximation of Xu & White’s model
  • A self-consistent model following Berryman
  • A model for dispersed clay
  • A grain supported model
  • A matrix supported model
  • Critical porosity model
  • Greenberg & Castagna’s relation

Largo also has a number of simple averaging methods (e.g., Wyllie, Voigt, Reuss, Hashin-Shtrikman). In addition, fluid properties can be estimated based on the formulas from Batzle & Wang.

The rock models are based on the Kuster-Toksöz inclusions model where pores with a given aspect-ratio are mixed into a solid matrix. The empty pores are then filled with an effective fluid, assuming low sonic frequencies (Gassmann theory). The models compose the solid matrix of two rock members. In case more than two rock members are present, the algorithms can be applied interactively to any desired level of complexity.

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