- API Specification 13A Section 19 (OCMA grade Bentonite) Calcium Carbonate. Drilling grade. Micronized (D50) - Calcium Carbonate 10 microns - Calcium Carbonate 25 microns ... (Locally Manufactured in Oman) Fertilizer Industry. Bentonite Fertilizer grade (Locally Manufactured in Oman) Civil Construction Industry.