“OCMA bentonite” appears in drilling tenders across the Middle East, Africa, and Asia decades after the body that coined it dissolved. Most explanations on the web get the definition backwards. Here is what OCMA-grade bentonite actually is, how it sits inside API Spec 13A, and how it differs from standard API bentonite.
OCMA is the Oil Companies Materials Association, a British body that set drilling-material specifications for oil companies operating mainly in the Eastern Hemisphere. It is defunct. Its committees were declared defunct and its specifications were handed to the American Petroleum Institute (API) in 1983, after which they were carried into the API — and later the joint API/ISO — standards. (Primary sources spell the name slightly differently — “Company” or “Companies,” “Material” or “Materials” — a sign of how old the term is.)
The organizationis not — it has been gone since 1983. But “OCMA-grade bentonite” survived as a named category inside API Spec 13A, which is why tenders still use it. Its section number has moved between editions, which is the source of a lot of confusion:
So a supplier page saying “Section 11” and another saying “Section 19” are both right — for different editions. When a tender says “API 13A Section 11 OCMA,” it is using the legacy numbering for the same grade.
This is where most of the web is wrong. API Spec 13A defines three separate bentonite categories, and OCMA-grade is not the premium one:
| Category | What it is |
|---|---|
| Bentonite (Section 9) | The higher rheological-performance grade — the standard API drilling bentonite. |
| Non-treated bentonite (Section 10) | Natural sodium bentonite with no chemical treatment. This — not OCMA-grade — is the truly untreated grade. |
| OCMA-grade bentonite (Section 19) | A grade for natural clays that cannot meet all of Section 9, and which is explicitly permitted to be treated (soda ash, polymer) to improve suspension. |
Two common claims are therefore false. OCMA-grade is not a premium high-yield grade — by definition it is for source clays that fall short of Section 9. And OCMA-grade is not“untreated natural” bentonite — it is allowed to be chemically treated; the untreated grade is the separate Non-treated category. The grades differ in profile, not simply in quality:
| Property | OCMA-grade (Section 19, Table 18) | API bentonite (Section 9) |
|---|---|---|
| Viscometer dial reading, 600 r/min | minimum 30 | minimum 30 † |
| Yield point / plastic viscosity ratio | maximum 6 | tighter (commonly published as ≤3) † |
| Filtrate volume | maximum 16.0 mL | tighter (commonly published as ≤15.0 mL) † |
| Residue coarser than 75 µm | maximum 2.5% (cleaner) | looser (commonly published as ≤4.0%) † |
API bentonite (Section 9) holds a tighter yield-point-to-plastic-viscosity ratio and a slightly tighter filtrate limit — a higher performance bar. OCMA-grade is more forgiving on those, but holds a cleanergrit limit (less coarse residue). Neither is simply “better”; they are different profiles for different source clays and uses.
† The OCMA-grade figures are from API Spec 13A Table 18 (published in the free 2020 addendum). The Section 9 API-bentonite figures are the values commonly published across the trade; confirm them against a current copy of API Spec 13A, 19th edition, before relying on the exact numbers.
Read the tender for the category name, not just “bentonite.” If it says OCMA-grade(or “Section 11”/“Section 19 OCMA”), it wants the Table 18 profile above — a workable choice for water-well and general drilling where full Section 9 performance is not needed, and usually the more available and lower-cost tier. If it says API 13A bentonite without qualification, it means the Section 9 grade. If it says non-treated, it specifically wants natural sodium bentonite with no chemicals added. Send us the wording and we quote to the exact grade, with a certificate of analysis.
“OCMA grade” is sometimes written for barite, not bentonite — there OCMA set a weighting-agent specification (specific gravity at least 4.20), close to the API barite grade. On this page “OCMA” means the bentonite grade. If your tender pairs “OCMA” with a 4.20 specific-gravity figure, it is referring to barite.