Advanced digital shaly sand analysis techniques have been recently developedwhich are based on the Waxman-Smits model and variations in the basicproperties of various clay minerals.

Based on density, neutron, and natural gamma ray spectral data, two importantformation parameters, clay density (?Cl) and neutron response to100% clay (NCl), can be determined at every depth level over thecomputed interval of interest. No average clay property values from adjacentshale formations are required as input. Hence, the constraints inherent toother techniques, i.e., clay properties in the clastic reservoir and adjacentshales assumed to be identical, do not apply.

Having determined the two critical clay parameters, ?Cl and NCl the clay volume(VCl) is calculated simultaneously from both the potassium and thorium values. This VCl value is essentially independent of the clay types.

These three parameters (?Cl, NCl, VCl) thenallow for the calculation of two important reservoir parameters at each depthlevel, 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, andkaolinite/chlorite are grouped at three separate, clearly defined positions.Application of these log-derived clay-typing techniques in exploration, drilling operations, and detailed reservoir characterizations isillustrated.


Few hydrocarbon-bearing clastic reservoirs are essentially free of clayminerals. The basic objective in shaly sand formation evaluation then is arealistic log-derived description of reservoir quality in terms ofpetrophysical parameters, type and volume of hydrocarbon resources-in-place, and the expected production behavior. Such evaluation methods may be simpleempirical rules, standard analysis concepts. digital well site "quick look"techniques, or advanced digital interpretive models such as the Waxman-Smitsmodel (1968).

A typical shaly clastic reservoir rock and/or a typical shale formation maycontain different clay minerals in varying amounts. No single clay parametercan therefore be used universally to characterize a specific type ofargillaceous sediment or shaly reservoir rock. The most common clay minerals, their chemical composition, matrix density, hydrogen index, cation exchangecapacity, and distribution of potassium, thorium, and uranium, based on naturalgamma ray spectral information, are summarized Table 1 (Fertl and Frost,1980).

Shaliness and hydrocarbon effects on the generalized Response of the variouslogs is illustrated in Table 2 (Fertl, 1983).

a) Basic Concepts

As shown in Table 1, various clay minerals have different potassium (K), uranium (U), and thorium (Th) concentration. The geological significance andapplications of natural gamma ray spectral ratio values are listed in Table 3(Fertl, 1979) and, with special emphasis on clay typing, illustrated in Fig. 1(Hassan, et a1., 1976).

Initial attempts to recognize clay types from well logs have focused on naturalgamma ray spectral logging information (Fertl, 1979; Juhasz, 1981).Subsequently, a generalized model was proposed, which relates available loggingmeasurements via analytical regression type techniques to the expected claytypes (Quirein, et a1., 1981).

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