ABSTRACT/Two marine bacterial isolates from drill mud cuttings obtained from Agbara oilfield, Staphylococcus sp. and Bacillus sp., were cultured aerobically in the presence of varying concentrations (0, 25, 50, and 75 i.Lg/ml) of drilling fluids to determine the effects of concentration of toxicants on their growth. With the exception of Clairsol, Enviromul, and Baroid mineral oil, which had little or no effect, the exponential growth of Bacillus sp. was depressed by all other test chemicals. Additionally, all test chemicals except Clairsol had no effect on lag phase of growth of Bacillus sp. With Staphylococcus sp. the depressive effect on the exponential phase of growth was shown by almost all test chemicals. There was enhancement of both growth rate and generation times of Staphylococcus sp. and decrease of those of Bacillus sp.with increasing concentrations of drilling fluids. These results show that while some drilling fluids may be stimulatory or depressive to bacterial growth, others may be without effect.According to Muhlemann (1986), nine distinct mud systems are defined, the first seven of which are water-based, and the eighth oil-based. The final category is a specialized one in which air or gas is the basic circulating medium. The seven water-based muds include nondispersed, dispersed, calcium-treated, polymer, low solids, saturated salt, and workover muds. Polymer muds generally incorporate long-chain, high-molecular-weight chemicals that are effective in flocculating muds, increasing viscosity, reducing filtrate loss, and stabilizing the formation. Various types of polymers are available for this purpose, including bentonite extenders, which have higher acid solubilities than bentonites and thus reduce the amount of clay needed to maintain viscosity, Completion and workover fluids are specialized systems designed to minimize formation damage, be compatible with acidizing and fracturing operations (acid soluble), and be capable of inhibiting swelling clays that reduce formation permeability (Muh[emann 1986).Oil-based fluids are used for a variety of applications such as high temperature wells, deep holes, wells where sticking and bole stabilization are problems, and many other areas. They consist of two types of systems: invert emulsion muds and oil muds.In the Nigerian oil industry, water-based mud is predominantly used in oil drilling operations, Oilbased mud is hardly utilized except in some special
