Method to evaluate the R.sub.wb parameter of the dual water log interpretation model

A method for determining the resistivity parameter R.sub.wb of waters adsorbed to a plurality of shale cores taken from an oil reservoir rock, which parameter is useful for estimating the reservoir rock hydrocarbon content, comprising the steps of:

• (1) separately milling the shale of the respective shale cores to retain the fraction passing through a 200 mesh Tyler screen;
• (2) preparing a plurality of brines varying in NaCl concentration from about 25,000 to about 250,000 ppm;
• (3) measuring the resistivity R.sub.w of the plurality of the brines as prepared in (2);
• (4) separately preparing a plurality of muds having a solids content of about 20% and a liquids content of about 80% to provide a density of about 1.4 g/cm.sup.3 from the milled shale cores of Step (1) and the brines of Step (2), and then allowing the muds thus prepared to settle for at least 72 hours;
• (5) filtering each of the muds prepared in Step (4) at a pressure of about 7 kg/cm.sup.2, collecting the respective filtrates and measuring the resistivities R.sub.mf thereof;
• (6) plotting R.sub.w as a function of the salt filtration efficiency SFE(R), where SFE(R) is given by:
• ##EQU5## for each of the muds prepared in Step (4);
• (7) extrapolating the data plotted in Step (6) to find the value of R.sub.w where SFE(R)=0;
• and
• (8) determining R.sub.wb as being equal to the value of R.sub.w where SFE(R) extrapolates to zero, at which value the resistivity of the brine used to prepare a mud is equal to the resistivity of the filtrate obtained from that mud or R.sub.wb =R.sub.w =R.sub.mf.