Microfracture classification, quantification, and petrophysical behavior in the Tengiz and Korolev carbonate reservoirs, Kazakhstan

Abstract

The purpose of this study is to classify and quantify predominantly open microfractures in the subsurface Tengiz and Korolev carbonate reservoirs of western Kazakhstan. Microfractures, previously unknown in these fields, occur in nearly all of hundreds of the thin sections examined using transmitted-light (TL) and confocal microscopy. This study, done in carbonate rocks, differs from most previous work on microfractures, which mainly concentrates on silica-bearing rocks (sandstones and granites) and sealed (cemented), rather than open, microfractures.Microfractures in Tengiz and Korolev rocks are obscure or invisible using TL petrography. However, they are clearly visible in high-resolution (0.2 μm/pixel) laser-scanning confocal images. In decreasing order of abundance, we observed the following microfracture types: (1) dissolution-enlarged microfractures, (2) mosaic microfractures, (3) sealed and partially sealed microfractures lined with carbonate cement and/or bitumen, (4) intercrystalline microfractures in echinoderm fragments and coarse blocky calcite cements, (5) en echelon microfractures, (6) sediment-filled microfractures, (7) microfaults and deformation bands, (8) microfracture ganglia, and (9) reaction halos in the matrix near open microfractures. All microfracture types are interpreted to be natural, as opposed to induced features.Microfracture quantification, used here to determine the amount of open microfracture porosity in two-dimensional confocal images, has been applied to more than 300 samples. Microfracture porosity typically ranges from 0.2 to 1.0 porosity units. Aperture widths are generally on the order of 0.2 to 10 μm.A unique feature of this study is the relationship between dynamic properties and the amounts and types of microfractures observed in the rocks. Specifically, microfracture abundance relates to the effect of variable confining pressure on porosity and permeability in core plugs. Samples in which total confocal porosity is dominated (>80%) by microfractures, mainly upper-slope boundstones, showed moderate decreases in porosity and major decreases in permeability as a function of increased confining pressure. Samples in which microfractures are a relatively minor component (<20%) of total confocal porosity, mainly middle-slope breccias and grainstones and outer-platform grainstones, showed significantly smaller decreases in porosity and permeability as a function of increased confining pressure. Hysteresis effects, especially for permeability, are common. In conclusion, in reservoirs with open microfractures, core-plug scale permeability is subject to change with variable confining pressure. This mode of permeability variation should be considered in reservoir models conditioned by core-plug results.