Determination of Volume, Type, and Distribution Modes of Clay Minerals from Well Logging Data

Abstract

ABSTRACT Typical shaly clastic reservoir rocks and/or shale intervals frequently contain varying amounts of different clay minerals. Most common clay minerals exhibit significant differences in their basic properties, including chemical composition, matrix density (g/cc), photoelectric cross section (Pe, barns/electron), hydrogen index (HI), cation exchange capacity (CEC, meq/lOOg), potassium (percent), uranium (ppm), and thorium (ppm). These different properties and associated interpretive models allow the identification and quantitative evaluation of the major clay mineral groups such as smectite, illite, kaolinite, and chlorite via geophysical well log responses. Based on density, neutron, and natural gamma ray spectral data, two important formation parameters, clay density (ρcl) and neutron response to 100% clay (Ncl), can be determined at every depth level over the computed interval of interest. No average clay property values from adjacent shale formations are required as input. Hence, the constraints inherent to other techniques, e.g., the assumption that clay properties in the clastic reservoir and adjacent shales are identical, do not apply. Having determined the two critical clay parameters, ρcl and Nd, the clay volume (Vd) is calculated simultaneously from both the potassium and thorium values. This Vd is essentially independent of the clay types. These three parameters (ρcl, Nd, Vd) then allow for the calculation of two important reservoir parameters at each depth level, namely the cation exchange capacity (CEC) and the hydrogen index (HI). The resulting CEC and HI values then define the types of clay minerals present. On a CEC versus HI crossplot, the smectite (montmorillonite), illite, and kaolinite/chlorite are grouped at three separate, clearly defined positions. Recently developed interpretive refinements incorporate the photoelectric cross section value, Pe, which also allows quantitative differentiation of kaolinite and chlorite. Hence, all four major clay mineral groups (smectite, illite, kaolinite, chlorite) can be evaluated via modern logging measurements. Furthermore, it is well known that the type of clay mineral distribution in a reservoir rock significantly affects the effective reservoir porosity, effective water saturation, permeability, and hence, intervalproducibility. Different clay mineral distributions, such as clay dispersion, clay lamination or structural clay, will affect effective reservoir porosity (ϕc) and permeability in a drastically different manner. These different clay distribution modes also can be determined from the above mentioned well logs. Several field examples illustrate these concepts as applied to well completion, formation damage, and detailed clastic reservoir characteristics.