This crossplot displays how effective porosity and sand grain volumes change with shale volume in clastic reservoirs. Dispersed, laminated and structural shale models are presented. Solid lines are sand grain volumes and dashed lines are porosity volumes. All models are based on Katahara's work (see reference below).
With the disperesed shale model for a clean sandstone, shale progressively fills intergranular pore space, and when all pore space is filled, at VSH=PHIEmax, shale begins to replace sand grains. With the laminated shale model for a clean sand, shale replaces clean sand from left to the right. With the structural shale model for a clean sand, shale replaces only sand grains until all the sand has been replaced. Once all the sand has been replaced, shale then replaces intergranular porosity.
Inputs are in yellow and outputs are in green. Enter the maximum porosity for a clean sand reservoir interval, PHIEmax, when VSH=0. Next, enter a VSH value from a specific interval. Effective porosities, tied to input values and each shale distribution model, are displayed as outputs.
Katahara, K.W., "Gamm Ray Log Response In Shaly Sand." The Log Analyst Volume 36, Number 4, 1995: 50-55.