Rock-Fabric/Petrophysical Classification of Carbonate Pore Space for Reservoir Characterization

ABSTRACT This paper defines the important geologic parameters that can be described and mapped to allow accurate petrophysical quantification of carbonate geologic models. All pore space is divided into interparticle (intergrain and intercrystal) and vuggy pores. In nonvuggy carbonate rocks, permeability and capillary properties can be described in terms of particle size, sorting, and interparticle porosity (total porosity minus vuggy porosity). Particle size and sorting in limestones can be described using a modified Dunham approach, classifying packstone as grain dominated or mud dominated, depending on the presence or absence of intergrain pore space. To describe particle size and sorting in dolostones, dolomite crystal size must be added to the modified Dunham terminology. Larger dolomite crystal size improves petrophysical properties in mud-dominated fabrics, whereas variations in dolomite crystal size have little effect on the petrophysical properties of grain-dominated fabrics. A description of vuggy pore space that relates to petrophysical properties must be added to the description of interparticle pore space to complete the petrophysical characterization. Vuggy pore space is divided into separate vugs and touching vugs on the basis of vug interconnection. Separate vugs are fabric selective and are connected only through the interparticle pore network. Separate-vug porosity contributes little to permeability and should be subtracted from total porosity to obtain interparticle porosity for permeability estimation. Separate-vug pore space is generally considered to be hydrocarbon filled in reservoirs; however, intragranular microporosity is composed of small pore sizes and may contain capillary-held connate water within the reservoir. Touching vugs are nonfabric selective and form an interconnected pore system independent of the interparticle system.