So far, veteranarians have examined only two of the dolphins. Authorities say autopsies will be performed on the rest.
"The preliminary results show that the animals did not have trace of hydrocarbon in their respiratory system, nor in their skin, throat or stomach," the report said. Both of the dolphins examined so far bore signs of injuries, it said.
The Japanese-owned MV Wakashio, a bulk carrier, ran aground on 25 July and began to spill oil about a week later. The ship was scuttled on Monday.
The autopsy on the first two was conducted by the government-run Albion Fisheries Research Centre.
"The autopsy will be carried out on all other dolphins," said Jasvin Sok Appadu from the fisheries ministry.
Local environmental group Eco-Sud called for the full autopsy results to be released publicly. International environmental pressure group Greenpeace called on the government of Mauritius to launch an inquiry to determine whether the deaths were linked to the oil spill.