The Cambay Basin is located in Gujarat State, on the western margin of India. The basin lies predominantly onshore, with only the southwestern corner offshore in the Gulf of Cambay. The Cambay basin is rich petroleum province, with active exploration history. The basin is a narrow elongated, intra-cratonic rift basin of late Cretaceous age and contains different sub-basins with varying sediment fills.
The Cambay basin is a north-south trending graben with an average width of 50 km, 450 km long and a maximum depth of about 7 km. The maximum depth of the basin may exceed 11 km if one includes the Deccan Trap lava flows. The basin has more than 40 years of active hydrocarbon exploration history. The total area of the basin is about 53,500 sq. km including 6,880 sq. km in the shallow waters (Gulf of Cambay). The basin is roughly limited by latitudes 21° 00’ and 25° 00’ N and longitudes 71° 30’ and 73° 30’ E.
The origin of the Cambay and other basins on the western margin of India are related to the break up of the Gondwana super-continent in the Late-Triassic to Early-Jurassic (215 m.y.a.). As India drifted away from Africa and Madagascar, rift grabens began to form on the west coast of India. As a result of movement, the boundary faults of the grabens were initiated through reactivation of Pre-Cambrian faulting. On the north-western margin of the Indian shield, three craton-margin-embayed basins came into existence – Kutch, Cambay and Narmada – along three rift systems aligned with major Pre-Cambrian tectonic trends – Aravalli, Dharwar and Satpura.
The Evolution of the Cambay basin can be broadly divided into two main structural stages – (1) a lower, Upper Jurassic-Lower Cretaceous platform stage (pre-rift) and
(2) an upper, Upper Cretaceous Tertiary composite (syn-rift/post-rift) stage. These two main evolutionary stages have been further sub-divided into the following multiple phases:
• The Upper Jurassic-Lower Cretaceous Platform Phase-Pre-rift
• The Upper Cretaceous First Rift Phase
• A Paleocene Formative Phase
• A Lower-Middle Eocene Second Rift Phase
• An Upper Eocene-Oligocene Inversion Phase, and
• A Miocene-Holocene Post-Rift Phase
Pre- Rift (Platform Phase)
The Pre–Cambrian and Mesozoic sequence form the pre-rift evolutionary stage for the basin. This stage is not considered here in detail, although some important controls on basin formations have been briefly described below
The structural disposition of the Pre-Cambrian basement – a complex of igneous and metamorphic rocks – exposed in the vicinity of the Cambay basin, control its architecture. The principal lineaments in the basin are aligned NE-SE, ENE-WSW and NNW-SSE.
The NE-SW trending lineament, in the northern part of the basin, is related to the Aravalli–Delhi fold belt. Another major lineament trending NNW-SSE is related to the Dharwar orogenic belt. The third major lineament trending ENE-WNW is aligned to the Satpura orogenic belt, and is dominant in the Narmada basin
The pre-rift stage also called the lower stage, mainly appeared during Upper Jurassic–Lower Cretaceous time. During this period the Cambay Basin was gentle shelf bounded on the east by the Indian shield. A thin sequence of Upper Jurassic- Lower Cretaceous sediments was deposited on this shelf in environments alternating between brackish, deltaic and shallow marine with sediments derived by rivers draining the Aravalli hills and adjacent uplands. This stage is completely masked by thick Deccan Trap volcanic flows which took place during Upper Cretaceous. The thickness of Mesozoics varies from 20 to 30 meters on the basin’s eastern margin to more than 750 meters on the western flank of the basin (between the western margin fault and the Saurashtra craton).
The Mesozoic succession is overlain by an almost complete sequence of Tertiary sediments with an intervening succession of thick Deccan Trap basalts of Late Cretaceous age.
The Late Cretaceous and Tertiary together form Syn–rift, the second structural stage of the basin. During this structural stage, an intra-cratonic graben was formed due to the reactivation of marginal faults in late Cretaceous, striking NNW-SSE to almost N-S. These marginal faults remained active during late Cretaceous to early Palaeogene.
The syn–rift stage has been further divided into two sub-rifting episodes with intervening Formative phase and also a final inversion phase.
• First Rift Phase (episode)
The first phase of rifting took place during Deccan Trap Volcanism in the late Cretaceous. In this phase a half-graben came into existence along marginal faults aligned NNW-SSE. In order to accommodate huge flow of Deccan Trap basalts, it is believed that the Cambay graben might have subsided more than two thousand meter during the first phase of rifting.
• Formative Phase
First rift phase is followed by formative syn-rift phase. During this phase intra basinal horst and rifts formation along with deposition of trap-derived clastics material took place. Reactivation of basement faults along NNW-SSE trend produced considerable relief on the Deccan trap basement floor. On this uneven basin floor a thick sedimentary section consisting of volcanic conglomerate, sandstone, siltstone, and other trap-derived detritus material was formed. This formation known as “Olpad Formation” was deposited under sub aerial to fluvial conditions as cones, fans and gravel beds.
• Second Rift Phase (episode)
Following the first rift and formative phases, the second rift phase took place in Lower Eocene during the deposition of thick Cambay Shale. This phase accommodated more than 1500 meters of Cambay Shale in the graben. It is during this sub-phase that the basin showed a long history of subsidence synchronous with marine transgression with sedimentation over a wide area. This is typical of intra-cratonic basins and of high relevance from the hydrocarbon generation standpoint.
In this stage of rifting, the half-graben has transformed into a full graben during subsequent periods of sedimentation. Towards the later part of this second stage, during Middle Eocene, the regression took over in the northern part of the basin, resulting in deltaic environments (Kadi formation). These have acted as good reservoir facies in the area.
The early Eocene marine transgression did not reach to the extreme north of the Cambay basin or the Patan-Tharad block. In this block, the arenaceous Tharad Formation sediments were deposited under fluvial-lower delta plain to delta front conditions.
In the northern part of basin, a master river flowing along the basin axis brought in coarse clastic material and deposited a sand-shale-coal sequence coeval with the fully marine Cambay Shale which was being deposited in the southern part of the basin.
• Inversion Phase
The succeeding syn-rift phase is termed ‘Oscillatory’ or ‘Stage of inversion’. This phase was one of regression punctuated by marine transgressive pulses during the Upper Eocene-Oligocene. This caused widespread development of fluvial and deltaic systems in the basin.
In the northern part of the basin, during this phase, the basin slope became gentler and the sand-shale-coal sequences of the Kalol Formation were deposited in a vast area extending southwards at least as far as Cambay Field (Cambay-Tarapur block).
In the southern part of the basin, during the inversion phase, sand and shale sequences were deposited in a deltaic to paralic facies with episodes of shallow marine facies in an alternating transgressive-regressive environment (Ankleshvar and Dadhar formations).
Post – Rift
Post-Upper Eocene was characterized by only mild tectonic activity and this phase of basin evolution is termed as post – rift. From this period on, fault-controlled differential subsidence essentially ceased, and the basin has since, behaved as a single tectonic unit. Subsidence continued, but at a far slower rate; this stage too, has major implications on evolutionary aspect of the basin.
During the Late Eocene to Early Oligocene period, in conformity with global eustatic (sea level) changes, there was a widespread transgression, which resulted in deposition of the Tarapur Shale. This acts regionally as a cap rock. Towards the later part of Early Oligocene, when the sea started receding, sedimentation took place under prograding deltaic conditions in the southern part of the basin (Ardol and Dhadhar units).
Following this, a withdrawal of the sea during the end of early Oligocene resulted in a basin-wide unconformity at the top of Tarapur Shale. A marine transgression during early Miocene resulted then, in the deposition of Kathana formation in the southern Cambay Basin. In the same period, in the northern part of the basin, even the earlier strata of Oligocene were eroded away, and the Miocene formations rest directly upon the Eocene shale.
During the Lower Miocene period, there was a heavy clastic influx resulting in the deposition of the Babaguru formation. Subsequent deposition during Miocene has resulted in fluvial clastic formations represented by Kand and Jhagadia. Pliocene depositions are represented by Broach and Jambusar formations, which are succeeded by Gujarat Alluvium over the entire basin.
The Cambay rift valley is bounded by well demarcated basin margin step faults. Based on the cross trends the basin has been divided into five tectonic blocks. From north to south, the blocks are:
• Patan-Sanchor block
• Mehsana-Ahmedabad block
• Cambay-Tarapur block
• Jambusar-Broach block
• Narmada-Tapti block