Aside from its large identified natural gas reserves, Bangladesh had few mineral resources. The Bay of Bengal area was being explored for oil, and in some offshore areas, drilling was being conducted by international companies. Bangladesh had reserves of good-quality coal in the northern districts, but extraction has been difficult since many deposits were located at a depth of more than 900 m. Production estimates of mineral commodities in Bangladesh in 2000 included hydraulic cement, 980,000 tons; marine salt, 350,000 tons; and limestone (mined in the Sylhet and Chittagong regions), 28,000 tons.