The main purpose of the project so far is to find bentonite in quantity and quality suitable, in its natural state or, as is more likely, after conversion from the. commoner non-swelling to the swelling form, for use as oil well drilling fluid material. In the preceding phase of the project Mr David Ker, with Geological Survey of Indonesia (G.S.I.) staff, located, surveyed and sampled a number of prospects; but assessment of. their quality was hampered by lack of reliable chemical and rheological testing and was, in fact, based almost entirely on detection of smectite * group minerals by x~ray diffraction (Ker, 1977}.

The major constituent of bentonite of such high grade as is here in question is clay mineral of the smectite group; the minerals of this group have cation exchange properties and, when sodium is present as the main exchangeable cation, they confer on their aqueous suspensions the rheological properties - viscosity and thixotropy - that are highly valued in bentonite for use in oil well drilling. Bentonite most commonly occurs in nature with calcium and magnesium as the preponderant exchangeable cations and in this form it is "non-swelling11 - it does not exhibit the desirable rheological properties; however, a simple chemical .treatment leads •to replacement of the divalent cations in ~~e smectite with sodium ions: the bentonite then exhibits the rheological properties to a greater or less degree depending on a number of factors - only one of these is the proportion of smectite and not all of the others are understood.

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