Rock-Fabric Petrophysical Analysis of Core and Wireline-Log Data Haradh 101 and Haradh 106 Ghawar Field, Saudi Arabia

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Analysis of thin sections and core data demonstrates that rock fabrics can be grouped into petrophysical fields defined by porosity and permeability. Grainstones and large crystal dolostones fall within the petrophysical class 1 field of Lucia (1995). Permeability increases with increasing dolomite crystal size. The class 1 field is enlarged slightly to include large crystal dolostones with crystal sizes ranging up to 300 microns. Grain-dominated and dolomitic mud-dominated fabrics containing more than 25 percent dolomite fall into the petrophysical class 2 field. The dolomitic mud-dominated fabrics plot in the class 2 field because progressive dolomitization increases pore size by increasing porosity in the intercrystal mud and by creating intercrystal pore space. Mud-dominated fabrics having less than 25 percent dolomite are mostly dense but, when permeable, plot in the petrophysical class 3 field. A global relationship between rock-fabric petrophysical class, interparticle porosity, and permeability that does not require fabrics being divided into specific petrophysical classes has been developed and is used in this analysis. Permeability can be estimated from wireline logs according to the rock-fabric method. Interparticle porosity is estimated by subtracting total porosity from separate-vug porosity, which, in turn, is estimated from transit-time-porosity cross plots. Petrophysical classes can be identified from a cross plot of water saturation and porosity. The wells are far enough above the free-water level that reservoir height is not an important consideration. We identified boundaries between rock-fabric classes and multiple-regression analysis, using equations of the boundary lines, resulted in a relationship between petrophysical class, saturation, and porosity. Permeability is calculated by substituting log-calculated interparticle porosity and log-calculated petrophysical class into the global transform equation. These calculations compare well with core data and retain high and low permeability values.


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