"Oil is floating (on the water) as far as it can be seen," said forest conservator Karttik Chandra Sarkar.
The oil tanker sank at Chandpai in the Shela River on Tuesday, causing a major oil spill. It was carrying over 350,000 litres of furnace oil.
Forest officials said oil from the vessel had spread across 34,000 hectares of forest area.
The experts fear the spill could have catastrophic effects on the Sundarbans heritage.
The officials said an oil slick was visible on the waters around Mrigmari, Andharmanik, Tambulbunia, Harintana, Jiwdhara, Dhansagar, Nandabala, Harbarhia, Chandpai Forest Station, Marapashur, Jangrha, Karamjal Wildlife Reproduction Centre, Dhangmari, Ghaghramari and Laudob area.
Dolphins used to roam in the waters of Jaimani, known to be a sanctuary for them around the banks of the Shela River. But the creatures have not been seen since the spill, said local fishermen.
The oil was spotted in the water of canals in Jaimani, Mrigmari, Badamtala, Kharma, Ulubunia, Battala, Tetulbunia, Bhaijora, Hetalmari and Jinnat.
Forestry official Karttik said those areas were filled with Sundari trees. Leaves of trees, like Sundari, Kaora, Bain, which are adjacent to river Shela were coated with oil.
In some places, the rising tidal waters had left oil on the aerial mangrove roots and a black layer on the ground.
Nearly 300 families of the Bagerhat's Chila Union live by fishing in the river.
Fisherman Imran Sheikh, 30, told bdnews24.com: "Shela River is overflowing with oil. We are unable to get into the water. Fishing has stopped now."
He feared the catch might not be good in future because of the spill.