Give the distribution of petroliferous basins of India. Discuss the geology of Cambay Basin.

Distribution of Petroliferous Basins in India

India's 26 sedimentary basins are classified into four categories based on their hydrocarbon prospectivity and the maturity of resources. As of 2024, only about 10% of India's sedimentary basin area is under active exploration, with plans to increase it to 16% by the end of 2024 through initiatives like the Open Acreage Licensing Policy (OALP).

The distribution is as follows:

• Category-I Basins (Proven Commercial Productivity): These basins have established reserves and are currently producing hydrocarbons. They cover about 30% of the total basinal area and hold 85% of the total unrisked conventional hydrocarbon in-place.
  • Onshore: Assam Shelf, Rajasthan, Cambay
  • Offshore: Mumbai Offshore, Krishna-Godavari (KG), Cauvery, Assam-Arakan Fold Belt
• Category-II Basins (Identified Prospectivity, Contingent Resources): These basins have proved occurrences of hydrocarbons but are yet to achieve commercial production. They cover 23% of the total basinal area.
  • Saurashtra, Kutch, Vindhyan, Mahanadi, Andaman Nicobar
• Category-III Basins (Prospective Resources Awaiting Discovery): These basins are considered prospective based on geological considerations but have no significant oil and gas shows yet. They cover 47% of the total basinal area.
  • Kerala-Konkan, Bengal-Purnea, Ganga-Punjab, Pranhita-Godavari (PG), Satpura-South Rewa-Damodar, Himalayan Foreland, Chhattisgarh, Narmada, Spiti-Zanskar, Deccan Syneclise, Cuddapah, Karewa, Bhima-Kaladgi, Bastar
• Category-IV Basins (Potentially Prospective, Requiring Basic Data): These basins have uncertain prospects and require extensive basic data for prognosis.

Geology of Cambay Basin

The Cambay Basin is a significant intracratonic rift graben located in northwestern India, primarily in the state of Gujarat. It is one of India's most prolific onshore hydrocarbon provinces, known for its substantial oil and gas fields. The basin's formation and evolution are closely linked to the Deccan Traps volcanic event in the late Cretaceous and the subsequent rifting of the Indian Plate.

1. Tectonic Evolution and Structure:

• The Cambay Basin originated as a continental rift graben during the Paleocene-Early Eocene due to the northward drift of the Indian plate following the Deccan Traps volcanism.
• It is an elongated, NNW-SSE trending basin, approximately 500 km long and 50 km wide, covering an area of about 53,500 sq km.
• The basin is structurally divided into five major tectonic blocks (e.g., Sanchor-Patan, Ahmedabad-Mehsana, Tarapur, Broach, Narmada) separated by major cross-trends and basement faults. These faults played a crucial role in controlling sedimentation and hydrocarbon migration and accumulation.
• The tectonic evolution can be broadly categorized into three stages:
  • Paleocene – Early Eocene Rift Stage (Syn-rift): Characterized by active extension, deposition of trap derivatives, alluvial fan deposits, and lacustrine claystones in fault-controlled half-grabens. The early Eocene witnessed a significant marine transgression.
  • Middle Eocene – Early Miocene Post-rift Stage (Thermal Subsidence): Dominated by thermal subsidence, leading to deposition of deltaic and marginal marine sediments.
  • Middle Miocene and Younger Stage (Post-rift Structural Inversion/Compression): Led to the formation of numerous structural traps, notably the giant Ankleshwar Oil Field.

2. Stratigraphy and Sedimentary Succession:

The sedimentary infill of the Cambay Basin rests unconformably over the Deccan Trap basalts, which form the basement. The stratigraphy primarily consists of Tertiary sediments.

Formation Name	Age	Lithology/Characteristics	Significance
Olpad Formation	Paleocene	Volcanic conglomerate, sandstone, siltstone, claystone (trap wacke), derived from Deccan Traps. Erratic distribution.	Basal sedimentary unit, directly overlies Deccan Traps.
Cambay Shale Formation	Early Eocene	Thick, dark grey to black carbonaceous shales, claystones with siltstone interbeds, deposited in restricted marine/pro-delta to lagoonal-paludal environments. Divided into Older Cambay Shale (OCS) and Younger Cambay Shale (YCS).	Main Source Rock for conventional oil and gas. Contains Type II and Type III organic matter. Also a target for shale oil/gas exploration.
Kadi Formation	Early Eocene (Northern part)	Paralic-deltaic deposits, sandstones, siltstones, coal. Lateral equivalent/interfingers with Cambay Shale in the north.	Important reservoir facies in the northern part of the basin (e.g., Mehsana, Patan sub-blocks).
Kalol Formation	Middle Eocene	Deltaic sequence with fining-upward sandstone, carbonaceous shale, siltstones, and thick coal.	Major Reservoir Rock in the Ahmedabad-Mehsana block (e.g., Kalol, Sobhasan, Mehsana fields). Multiple hydrocarbon-bearing sands (e.g., K-X pay).
Anklesvar Formation	Middle Eocene - Early Oligocene (Southern part)	Similar deltaic and marginal marine deposits.	Important reservoir and source rock in the southern part of the basin.
Tarapur Formation	Early Miocene	Marine transgression deposits, intercalation of shale, claystone, and thin sandstone beds.	Caprock and minor reservoir intervals.
Babaguru/Dhadhar Formations	Late Miocene - Pliocene	Fluvial sediments.	Regional uplift and change in drainage patterns.

3. Petroleum System Elements:

• Source Rocks: The primary source rock is the Early Eocene Cambay Shale Formation, particularly the Older Cambay Shale (OCS) and Younger Cambay Shale (YCS) members. These shales are organically rich, thermally mature, and have generated significant oil and gas. In the northern Ahmedabad-Mehsana Block, coal within the deltaic Kalol Formation also acts as a source rock.
• Reservoir Rocks: Major reservoir rocks are sandstones and siltstones of Eocene and Oligocene age. Key reservoirs include:
  • Kalol Formation sands: Prolific reservoirs in the Ahmedabad-Mehsana block.
  • Kadi Formation sands: Important in the northern parts.
  • Hazad Formation (Middle Eocene): Recent discoveries like West Matar-2 have found commercial oil and gas in Hazad Formation sands (GS-6 and GS-8).
  • Olpad Formation: While primarily basal clastics, specific facies can act as reservoirs.
• Cap Rocks: The shales and claystones within and overlying the reservoir formations (e.g., intra-formational shales, Tarapur Formation shales) act as effective seals.
• Trapping Mechanisms: The basin features diverse traps, including:
  • Structural Traps: Domal closures, fault-bounded anticlines, and tilted fault blocks are common, often resulting from the structural inversion phase. The Ankleshwar field is a classic example of a structural trap.
  • Stratigraphic Traps: Lenticular sand bodies, pinch-outs, and deltaic channel fills also form significant traps, especially around Mehsana.
  • Combination Traps: A combination of structural and stratigraphic elements is prevalent.
• Migration: Hydrocarbon generation began in the Middle Eocene in the southern part of the basin, with basin-wide migration occurring in the Early Miocene.

4. Major Oil and Gas Fields:

The Cambay Basin is home to numerous productive fields. Some of the prominent ones include:

• Ankleshwar: One of India's oldest and largest oil fields, discovered in 1960.
• Cambay field: Discovered in 1957 by ONGC, marking India's first hydrocarbon discovery outside Assam. It contains hydrocarbons in Miocene, Oligocene, and Eocene zones.
• Kalol, Sanand, Sobhasan, Mehsana, Nawagam, Gandhar: Significant fields primarily producing from Eocene sandstones.
• Recent discoveries by ONGC in FY 2023-24 include Jaya-1 (gas) and FB-1 (oil) in the Cambay Basin, and West Amod#1 (oil) in March 2024, demonstrating continued potential.