This phase signifies the early syn-rift stage & is represented by trap-derived clastics contributed by the then existing paleo-highs essentially in continental to fluvial environment in its lower part (Panna Formation). It is overlain by grey to dark grey shales with subordinate sands possibly representing the first marine transgression into the basin. Presence of carbonaceous shale and coal at a few places suggest localized restricted conditions.

Main clastic depocenters like Surat Depression and the contiguous southward lows like Central Graben (Panna Bassien block) and Vijayadurg Graben (Ratnagiri Block) received these sediments in considerable thickness aided by syn-sedimentary activity of the bounding faults. A few localized depressions in Mumbai Platform and over some other horst blocks also received these sediments. Panna Formation is wide spread in the basin except over the crestal parts of prominent paleo-highs like Mumbai High, Heera etc. Its thickness varies from almost nil to hundreds of meters in deep sinks.

Shelf Margin block, though under deep marine realm seem to have received lesser quantities of sediments which were either derived from the Diu Arch (?) or from localized provenances. The facies developed in this block are mainly claystone, argillaceous and carbonates with some amount of pelagic fauna.

Carbonate facies (Devgarh Formation) development is observed towards the southern edge of Mumbai High in the form of muddy foraminiferal- algal banks; Deep Continental Shelf area and isolated off-shelf carbonate build-ups at a few places in Shelf Margin and Ratnagiri.

The syn-rift stage of Late Paleocene-Early Eocene period got terminated with a basin wide regression and development of an unconformity

After a period of peneplanation, the basin witnessed a major transgression. Extensive carbonate sedimentation occurred in the shallow shelf area of Mumbai Platform, Panna-Bassein-Heera block and Ratnagiri block (Bassein Formation). However the period witnessed essentially clastic sedimentation in Surat Depression (Belapur and Diu formations) with occasional carbonate bands and a few sand stringers and argillaceous carbonates and shales in Central and Vijayadurg Grabens (Panna-Bassein-Heera block and Ratnagiri block). Shelf margin was generally starved of clastics with deposition of minor claystone and carbonates of mixed middle shelf to bathyal origin ( Belapur Formation)

Bassein Formation also indicates a wide range of environments – restricted platform, shelf lagoon with isolated shoals in Bassein area to open carbonate shelf in DCS and Ratnagiri and finally deep water carbonates in Shelf Margin area. It also formed wedge outs against the rising flanks of Mumbai High and Heera, which can be considered as potential exploration targets.

During this period, Surat Depression experienced the maximum subsidence-accumulating thick under compacted claystone relating to the prograding delta from northeast (Mahuva Formation). The Mumbai platform experienced generally shallower water depths and shale interbeds within limestone becoming more frequent. In Shelf Margin area thinner carbonates are deposited under relatively deeper conditions.

End of Early Oligocene also witnessed initiation of the westerly tilt of the basin.

Close of Early Oligocene is marked by a minor period of non-deposition except in Shelf Margin area. A few brief spells of transgression followed by continuous eustatic rise in sea level up to Early Miocene marked this period. Crestal part of Mumbai High that hitherto remained a positive area also got submerged during this period.

Surat Depression witnessed reduced subsidence resulting in a regressive coastline. A package consisting of sand bodies deposited in distributary channels, coastal bars, tidal deltas and other transitional environments encased in marginal marine normally pressured silty and carbonaceous shale overlying over pressured prodelta clay stone of Early Oligocene. (Daman Formation) The reservoir facies within this Formation have assumed great importance as they have been found to host significant amounts of hydrocarbons.

There was faster subsidence in Shelf Margin to accommodate the increased sediment load supplied by the westward prograding delta system. The finer clastics reaching the Shelf Margin block were mainly deposited in the depression between Kori High and the carbonate platform. (Alibag Formation)

Southward Close of Early Oligocene is marked by a minor period of non-deposition except in Shelf Margin area. A few brief spells of transgression followed by continuous eustatic rise in sea level up to Early Miocene marked this period. Crestal part of Mumbai High that hitherto remained a positive area also got submerged during this period.

Southward from Surat Depression, clays got dispersed over Panna-Bassein-Heera block, including the crestal areas and the northern part of Ratnagiri block as well as Bombay Platform. While in Mumbai High-DCS area and southern part of Ratnagiri, the unit is termed as Panvel Formation, in Panna-Bassein-Heera and northern part of Ratnagiri, the unit is named as Alibag Formation.)

It was a period of eustatic rise in sea level punctuated by a brief spell. The finer clastics entering into Surat Depression got mostly dispersed westward into Saurashtra basin and Shelf Margin area. Limited quantity of clastics got dispersed southward and entered Mumbai platform at its southeast and also up to Heera area. In response to the rising sea level, the delta being formed in Surat Depression in Late Oligocene shifted eastward.

Bassein and the area to its south that experienced shoaling conditions during Eocene was the site for fine clastic deposition during Early Miocene. Mumbai High and its western part (DCS) underwent fairly thick carbonate sedimentation. In fact the major reservoir of Mumbai High that hosts major part of the Country’s hydrocarbon reserves belongs to this unit. While over the Mumbai High area the facies are low energy, very fine grained to clayey carbonate reservoirs, the DCS area represents high-energy bio-clastic grainstone facies along with minor mudstone and wackestone.

The sea level continued to rise during this period. Clastic supply also continued into the basin. However much of the clastic material got dispersed westward into Saurashtra and Shelf Margin areas. Considerable quantity of clastics got dispersed southward also covering the entire Panna-Bassein area and also the Mumbai High and its immediate surroundings to the west and south. This clastic unit over Mumbai High includes sheet like sand, which has also been found to be hydrocarbon bearing. Carbonate sedimentation continued in Ratnagiri and DCS areas. Toward the later part of Middle Miocene, clastic deposition almost came to a halt in Mumbai High and other areas and consequently carbonates got deposited over many areas. Uppermost part of the Middle Miocene Limestone in Heera field has been found to be hydrocarbon bearing. Close of Middle Miocene was marked by a very pronounced unconformity.

Post Middle Miocene witnessed a major transgression covering the entire basin coupled with spectacular increase in clastic supply. The earlier initiated westerly tilt of the basin also became more pronounced. All these events brought the carbonate sedimentation to a total halt. The increased clastic supply also resulted in a significant progradation of Miocene shelf at places up to 80 km (Chinchni Formation)

Shallow protected shelf facies consisting of organic rich shales (Panna Formation- Paleocene to early eocene & Belapur Formation- Middle Eocene)

3-11% organic carbon and the kerogene type is mixed Type II and Type III.

Expected oil window is around 3000 m

The bounding faults of this tectonic unit have been continuously active accommodating huge pile of sediments that are being brought by the Narmada/Tapti fluvial systems

The enclosure provided by the Diu Arch and Mumbai High could have prevented free open marine circulation and coupled with optimum subsidence appears to have helped in preservation of organic matter.

Several layers of shale/claystone in a few wells are reported to have requisite TOC and have reached the oil window (Panna Formation & Belapur Formation)

The oil window from the available geochemical data appears to be between 2900m and 3850m.

Possible reasons for the exploration setbacks could be the speculative nature of reservoir rocks and hydrocarbon expulsion pressure did not exceed the ambient hyper pressure within the formation inhibiting primary migration .

The finer clastics entering into Surat Depression through Narmada /Tapti systems have been getting partially dispersed southward and entering these two prominent lows that appear to be an arm of the Depression extending to the south. Syn depositional sinking of these two lows accommodating the huge clastic influx from north is evident from the seismic data.

It is widely perceived that the Central Graben in Panna-Bassein Heera block and Vijayadurg Graben in Ratnagiri block had contributed to huge hydrocarbon accumulations in many major structural features like Panna, Bassein, Heera, South Heera, etc. lying on the western horst block suggesting a major westward hydrocarbon migration. However discovery of Neelam field within Central Graben indicated hydrocarbon opportunities within the graben itself provided better reservoir facies coupled with proper entrapment condition is available.