The birthplace of India's oil — a unique double-plate-boundary geology once made Assam the country's only oil-producing state.
For about a century, Assam was India’s leading oil-producing state — and for the first five decades after the first commercially successful well was drilled in 1890, it was the only state producing oil in India. Digboi’s refinery, running since 1901, is the oldest continuously operating refinery in the world.
The easy accessibility of that oil is thanks to Assam’s unique geology, sandwiched between two major tectonic boundaries: the Indian–Eurasian plate boundary to the north and the Indian–Burmese boundary to the east. The Assam–Arakan Basin was ocean floor for much of the Eocene and Oligocene, when large amounts of organic matter accumulated on the seafloor; massive erosion of the rising Himalaya then buried it beneath thick layers of sediment, and deep underground, rising heat and pressure matured it into petroleum.
The oil became far more accessible because the continued movement of the Indian Plate beneath the Burmese Plate folded the rock layers into anticlines (upward folds), where oil naturally accumulates. Near the Naga Thrust, these rocks were also broken and stacked on top of one another, forming the Naga Schuppen Belt.
Simplified for clarity: petroleum actually forms in source rocks and migrates into reservoir rocks where it accumulates — it doesn’t sit in fixed layers.
Sources
- Assam–Arakan Basin report, National Data Repository (NDR), Government of India
- Oil India Limited