Type and distribution modes of clay minerals from well logging data

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 −1), photoelectric cross section ( Pe, barns per electron), hydrogen index ( HI), cation exchange capacity ( CEC, meq 100g −1), potassium (%), 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 chloride via geophysical well-log responses. Based on density, neutron, and natural gamma-ray spectral data, two important formation parameters, clay density ( P cl) and neutron response to 100% clay ( N cl), 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, P cl and N cl, the clay volume ( V cl) is calculated simultaneously from both the potassium and thorium values. This V cl is essentially independent of the clay types. These three parameters ( p cl, N cl, V cl) 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, allite, kaolinite, chlorite) can be evaluated via modern logging measurements. Field examples illustrate these concepts as applied to well completion, formation damage, and detailed clastic reservoir characteristics.