An extensive environmental assessment study was conducted in 1990 around six drilling and production platforms in the mid-Gulf of Thailand to assess the impacts of petroleum hydrocarbons arising from the discharge of low toxicity oil-based drilling muds. Studies focused on chemical contamination from the discharge of drilling mud and cuttings, and the effects on the biological community structure of the local bottom sediments. Sampling was performed to assess chemical and biological conditions in bottom sediments surrounding these platforms, measurements of physical oceanography and currents aided interpretation of distributional patterns. The main findings of the studies included: (1) Surface sediments surrounding wellhead (drilling) platforms were enriched by the discharge of barite-laden drilling mud/cuttings containing Mentor 26, a low-toxicity oil used in Gulf of Thailand drilling operations. Components of discharge materials were noted to 1,000 m, but the primary area of influence was restricted to within 100-250 m, where elevated levels of petroleum hydrocarbons (PHC) and barium (Ba) were detected along with enrichment by coarse-grained sediment. Spatial patterns of mud/cuttings components were consistent with physical oceanography, in particular, at each platform investigated, PHC/solids were further dispersed in the direction of dominant currents, (2) Biological response, as indicated by the benthic macrofaunal community, was strongly related to PHC concentration and the response was similar at platforms where drilling muds and cuttings were discharged, (3) Further from platforms (250-1,000 m), more subtle biological changes were noted that seem related to low-level enrichment in PHC, Ba, and fine-grained particles, (4) From the detailed comparisons of two platforms drilled over a 5-year period, biological differences were statistically demonstrable and the changes implied over time were in the direction of recovery.