Summary of Key Concepts

The goal of reservoir characterization is to describe the spatial distribution of petrophysical parameters such as porosity, permeability, and saturation. The rock fabric approach presented here is based on the premise that pore-size distribution controls the engineering parameters of permeability and saturation and that pore-size distribution is related to rock fabric, a product of geologic processes. Thus, rock fabric integrates geologic interpretation with engineering numerical measurements.

To determine the relationships between rock fabric and petrophysical parameters it is necessary to define and classify pore space as it exists today in terms of petrophysical properties. This is best accomplished by dividing pore space into pore space into three types.

The petrophysical properties of interparticle porosity are related to particle size, sorting and interparticle porosity. Grain size and sorting is based on Dunham's classification, modified to describe current conditions. Instead of dividing fabrics into grain support and mud support, fabrics are divided into grain-dominated and mud-dominated.

Grain-dominated fabrics = grain supported with open or occluded intergrain porosity. Grainstone = no intergrain lime mud Grain-dominated packstone = intergrain lime mud and open or occluded intergrain pore space. Mud-dominated fabrics = grain or mud supported with areas between the grains are filled with mud. Mud-dominated packstone = grain support with intergrain volume filled with lime mud. Wackestone = mud-supported with more than 10 percent grains. Mudstone = mud-supported with less than 10 percent grains.

The important fabric elements to recognize for petrophysical classification of dolostones are precursor grain size and sorting, dolomite crystal size, and inter-crystal porosity. Important dolomite crystal size boundaries are 20 and 100. Dolomite crystal size has little effect on the petrophysical properties of grain-dominated dolostones. The petrophysical properties of mud-dominated dolostones, however, are significantly improved when the dolomite crystal size is >20 .

Rock fabrics are grouped into three petrophysical classes each with a unique interparticle-porosity permeability transform.

Class 1 is composed of (1) limestone and dolomitized grainstones and (2) large crystalline grain-dominated dolopackstones and mud-dominated dolostones.

Class 2 is composed of (1) grain-dominated packstones, (2) fine to medium crystalline grain-dominated dolopackstones, and (3) medium crystalline mud-dominated dolostones.

Class 3 is composed of (1) mud-dominated limestone and (2) fine crystalline mud-dominated dolostones.

A global transform has been developed based on the assumption that grain size and sorting change continuously from mudstone through packstone to grainstone, and that dolomite crystal size changes continuously from very fine to very large. This equation has its principal use in log calculations and not in visual description.

The addition of separate-vug porosity to interparticle porosity increases total porosity but does not significantly increase permeability. Therefore, separate-vug porosity is not included in porosity-permeability transforms. Interparticle porosity can be reasonably estimated by subtracting separate-vug porosity from total porosity.

Touching-vug pore systems cannot be related to porosity. Therefore, the rock-fabric approach cannot be used to characterize touching-vug reservoirs.

The key to constructing a geologic model that can be quantified in petrophysical terms is to select facies or units that have unique petrophysical qualities for mapping. In non-touching vug reservoirs, the most important fabric elements to describe and map are (1) grain size and sorting using the modified Dunham classification, (2) dolomite crystal size using 20 and 100 as size boundaries, (3) interparticle porosity, (4) separate-vug type with special attention to intragrain microporosity, and (5) separate-vug porosity. In touching vug reservoirs, the rock fabric approach can not be used, but the recognition of the presence of a touching-vug pore system is paramount because it may dominate the flow characteristics of the reservoir.