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Petrophysical Modeling for Evaluating Moldic Porosity in Carbonate Reservoirs

Rourke, Marvin *1; Soleiman, Wael 1; Truax, Jerome 2; Quirein, John 2
(1) Halliburton, Al Khobar, Saudi Arabia. (2) Halliburton, Houston, TX.

Predicting reservoir quality and productivity from petrophysical log measurements in carbonates is a well known challenge. Heterogeneous carbonate pore systems often defy petrophysical correlations such as porosity-perm which are routinely used in silici-clastic environments. Diagenetic processes of dissolution, precipitation and recrystalisation add further complexity. In some extreme cases the leaching of peloid and ooid in carbonate enhances porosity and then subsequent calcite crystallization forms interparticle cement that occludes much of this porosity. This moldic porosity drastically reduces permeability despite there being reasonably good porosity as measured by density, neutron, sonic and NMR logs. In addition resistivity log characteristics are altered due to the lack of pore connectivity. The non-connected pores do not contribute to electrical conductivity and lead to increased values of Archie’s “m” cementation exponent. . If this geology is not recognized a conventional petrophysical analysis would flag these intervals as potential pay regardless of pore fluid type.

Sonic derived porosity has long been known to measure interparticle porosity and be largely insensitive to fractures and vugs. Hence an undercall in sonic porosity compared to total porosity from other logs is an indicator of secondary porosity. The difference can be quite distinctive over intervals where moldic porosity dominates. In this presentation we present examples of using a probabilistic model that has proven beneficial in detecting moldic porosity and hence identify poor reservoir intervals. The petrophysical evaluations were confirmed by thin-section studies of wireline acquired cores which clearly indicate the presence of moldic porosity.

The dielectric log is making a comeback within the industry after nearly a 20 year hiatus. The primary measurement from a Dielectric log is water filled porosity and when combined with convenient total porosity logs, a flushed zone saturation (Sxo) independent of resistivity can be determined. In carbonates this measurement when coupled with shallow resistivity can be used to estimate Archie’s cementation exponent “m” value which can be applied directly to the saturation calculation in a deterministic model. In this paper we discuss and present some examples of the integration of both sonic and dielectric measurements to improve carbonate petrophysical evaluations.


AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain