Step 1: Understanding the Concept:
Petroleum is a key commercial energy source in India, used as fuel for transport, industry and household needs, and also as a raw material for the petrochemical industry.
Step 2: Onshore fields.
Major onshore petroleum producing areas include Digboi, Naharkatiya and Moran in Assam (India's oldest oil producing region), and fields in Gujarat around Ankleshwar, Cambay and Kalol.
Step 3: Offshore fields.
Bombay High, off the coast of Mumbai in the Arabian Sea, is India's single largest petroleum producing field and was discovered in the late 1960s, transforming India's offshore oil output. Fields in the Krishna-Godavari and Cauvery basins off the east coast are other important offshore sources.
Step 4: Refining infrastructure.
Crude oil is processed into usable fuels at refineries spread across the country, with major complexes such as Jamnagar and Koyali in Gujarat, Mathura in Uttar Pradesh, Haldia in West Bengal, and Vishakhapatnam in Andhra Pradesh.
Step 5: Challenges.
Domestic production still falls short of India's total demand, so the country imports a large share of its crude oil from the Middle East and other regions, making energy security and price stability important national concerns.
Final Answer:
India's petroleum resources come mainly from Assam and Gujarat onshore, and Bombay High and the Krishna-Godavari basin offshore, refined through plants across the country, though heavy imports remain needed to meet total demand.\[ \boxed{\text{Assam, Gujarat, Bombay High, KG basin}} \]