The oldest and still producing oil field in India is the Digboi oil field, which is located in the state of Assam. Let's discuss why Digboi oil field is considered the oldest and still functioning oil field in India:

1. Discovery and History:
- The Digboi oil field was discovered in 1889 by Assam Railways and Trading Company (ARTC) while drilling for water.
- The presence of oil was confirmed in 1890 when the first oil well was drilled.
- The field was initially operated by Assam Oil Company (AOC), which later became part of Indian Oil Corporation Limited (IOCL).

2. Operational Continuity:
- Digboi oil field has been in continuous production since its discovery, making it the oldest continuously producing oil field in India.
- It has witnessed more than a century of oil extraction and remains active to this day.
- The field has faced numerous challenges, including natural disasters like earthquakes and floods, but it has managed to sustain production.

3. Geological Significance:
- The Digboi oil field is located in the Upper Assam Basin, which is known for its rich hydrocarbon deposits.
- The field is situated in the eastern part of the basin, which is characterized by folded structures and anticlines.
- The presence of these geological features has created favorable conditions for the accumulation of oil and gas reserves.

4. Production and Reserves:
- Digboi oil field has produced more than 6.5 million tonnes of crude oil since its inception.
- It has also contributed significantly to India's oil production, especially during the early years.
- Despite being an old field, it still holds significant reserves and continues to produce oil at a steady rate.

5. Importance and Legacy:
- Digboi oil field holds historical significance as it played a crucial role in the development of India's petroleum industry.
- It was the first oil field in South Asia and played a key role in meeting the country's energy needs during the early 20th century.
- The field has also been recognized as a heritage site by the Indian Oil Corporation due to its historical and cultural importance.

In conclusion, the Digboi oil field in Assam is the oldest and still producing oil field in India. Its discovery in 1889, continuous production, geological significance, and historical legacy make it a significant landmark in the country's petroleum industry.