STUDY OF IMPORTANT PETROLIFEROUS BASINS OF INDIA :

BOMBAY HIGH OFFSHORE BASIN

Group members:ABHIJEET BANERJEE

CARLTON NAZARETHNAVNEET SHARMA

SAVIO VALES

TABLE OF CONTENTS

INTRODUCTION TECTONIC SETUP STRUCTURAL FRAMEWORK LITHOSTRATIGRAPHY HYDRO-CARBON OCCURRENCES SOURCE ROCKS SEAL TRAP RESERVOIR ROCKS PETROLEUM SYSTEM MATURITY MODELING CORRELATION STUDIES CONCLUSION REFERENCES

Bombay offshore basin accounts for nearly two thirds of the annual petroleum production of India. The mature source rocks are present in the lower Eocene Paleocene Panna formation. Further, marginally mature potential source rocks within the Oligocene in Tapti- Daman are and within Neogene in Deep Continental shelf and deeper part of the basin also exist. Hydrocarbons have been discovered in multiple reservoirs in this basin, ranging from fractured basement to middle Miocene. The Bombay offshore basin has three major depressions

1. Surat and its southward extension to Ratnagiri in the East.2. Saurashtra low in the northwest,3. Murud and Rajpur lows in the southwest.

Due to multiplicity of depressions, source rocks and the reservoirs and oil source genetic relationship is a challenge.

The Bombay offshore basin is proved to be the most important petroliferous basin in the west

coast sedimentary province of India. Virtually all the commercially exploitable hydrocarbon

fields discovered so far are located in this basin. It covers an area of over 120,000 sq km up to

200 m isobaths. It is separated from the Kutch basin in the north by Saurashtra arch and from

Kerala Konkan basin in the south by Vengurla arch (Rao and Talukdar 1978). To its west lies the

deep sea basin.

TECTONIC SETUP

The continental shelf is the widest (about 1300) off Bombay, while both to the north towards Saurashtra and the South towards Ratnagiri, it is narrow (about 60 Km). The Palenoshelf is of low relief and shelf break occurring at about 90 mts. (Hari Narain et al. 1974). The basin formation started in the Upper Cretaceous-Paleocene time with rifting. Synchronous with this movement, out pouring of the Deccan trap volcanic flows started. The basin might have been formed by regional down warping accompanied by a series of faults. The floor of the tertiary sequence is the volcanic flows of the Deccan Trap. At places, the tertiary rocks rest directly over the Archean Inliers especially over the pronounced paleoreliefs. Lagoonal to continental rocks with occasional and sparse thin marine limestone from the Early Eocene to Late Paleocene basal clastic sequence. Middle Eocene to middle Miocene units is dominated by a thick carbonate sequence with minor shale. A major unconformity between Middle Eocene to Early Oligocene and another near Lower Eocene/ Middle Eocene level is noticed throughout the shelf. The section from the Middle Miocene upwards is represented by dominant shale section with minor limestone bands and carbonate stringers.

The entire shelf from the coast towards shelf margin zone is segmented by basement controlled NW-SE to N-S faults giving rise to many horst-graben features. Seismic features associated with carbonate buildups are noticed in the Miocene-Oligocene section. Bassein, Ratnagiri, Bombay High DCS trends and the early formed structures follow the Dharwad trend. The structures in the Surat Depression may be related to Satpura trend and appear younger in age than the structures following the Dharwad trend.

STRUCTURAL FRAMEWORK

Structural framework of the basin spells out the salient characteristics of the structural units and submits. Following are amongst the significant ones:

1. Surat depression is one of the major clastic filled basinal areas. It has a Paleocene Eocene clastic fill often in excess of 3 kms.

2. North south trending Diu arch separates Surat depression from Saurashtra basin. It provided sealed basin conditions to Surat depression during Paleocene Eocene.

3. The Murud depression appears to have subsided continuously from the Late Paleocene to Middle Miocene. However the period from Late Oligocene to Holocene witnessed maximum subsidence.

4. Rajapur depression had maximum subsidence during post Middle Miocene times.5. The Saurashtra basin and shelf margin basin, although formed in Paleocene, had maximum

subsidence during late Oligocene to middle Miocene and later.6. Bombay Platform includes deeper part of continental shelf, a monocline rising area and the

uplifted block of Bombay High.7. Bombay Platform, Heera- Bassein and Ratnagiri platform were relatively stable areas with

dominance of carbonate tills.8. The Heera-Bassein block is longitudinally divided into western platform arch separated from the

eastern homoclinal area by the central graben and its southward extension.9. Northeast trending Ratnagiri arch separating the Murud depression from Rajapur depression is

traceable from the shelf area deep sea. It is a significant tectonic element in the tectonics and sedimentation to the north and south of this ridge are vastly different. The superimposition of the NNW structural elements in the shelf.

10. The north Ratnagiri block also has a three-fold division similar to Heera – Bassein block, namely Jagagadh homocline, Vijaydurg graben, Shrivardhan Horst. This structural style has great relevance to the paleogeography and distribution of facies. The central Graben and Vijaydurg Graben were more of less continuous in the beginning and has been intimately connected with the Surat depression. Due to several ENE or NE trending strike slip faults this depression bordering the carbonate platform now appears en-echelon and discontionous with complicated fault pattern. The Ratnagiri arch terminates the structural elements of the North Ratnagiri block.

11. The central graben and Vijaydurg graben have substantial Paleocene – Eocene clastic fill like Surat Depression.

12. The South Ratnagiri block has NNW trending horst graben structure similar to north Ratnagiri block. But the Paleocene – Eocene clastic fill is substantially dissimilar to that of North Ratnagiri block.

13. The kori Arch separating the shelf Margin Basin form the Deep sea Basin is a semi-continous regional arch with several culminations separated by well marked lows.

STRATIGRAPHY

Bombay-offshore basin, a divergent passive continental margin basin, is located on the

continental shelf off the west coast of India. The basin is bounded by the western coastline of

India in the east, Saurashtra arch in the north, Vengurla arch in the south and west margin

basement arch in the west. The basin was formed due to extensional tectonics at the time of

rifting of the Indian Plate from Madagascar during Late Jurassic – Early Cretaceous period.

Large-scale volcanic eruptions, which covered most of the basin, followed this episode. As the

rifting continued, the immature sediments deposited at the toe of faults as alluvial fans, filled

the initial morph tectonic depressions during Paleocene and beginning of early Eocene. Thus,

early Eocene marks a widespread transgression. Sediments were deposited in deltaic to

restricted marine to shallow marine environments. Sedimentation during this period caused

some adjustments in the basin. The early Oligocene transgression covered most parts of the

basinal are and inundated parts of Bombay High. A major unconformity is noted at the top of

the lower Oligocene. Sea level rise during early Miocene submerged large areas of the basin

and terminated the Oligocene delta progradation. The Middle Miocene transgression marks

the last phase of the widespread carbonate sedimentation in the Bombay High. The basin has

NW-SE trending Horst – Graben geometry. The grabens are bounded by normal faults, and the

horsts/ ridges are dissected by NE-SW trending cross faults. On the basis of its structural

configuration and its nature, as well as the type of sediment fill, the basin is divided into Six

tectonic blocks: Tapti – Daman, Diu, Heera – Panna – Bassein, BombayDCS, Ratnagiri and shelf

Margin Blocks.

Lithostratigraphy of Bombay Offshore Basin :

1. BASEMENTDeccan Trap, in general, forms the floor of Bombay Offshore Basin with an exception of a few Archaean inliers. The crystalline basement is reported from the wells drilled over paleo-highs. The basement is of varied lithology in different parts of basin, viz. biotite gneiss, chlorite schist, syenite and granodiorite. So far, Mesozoic sediments, below the trap, have not been encountered in this basin. The Deccan Trap lavas are dark grey to greenish grey, hard fractured with abundant cavities filled with calcite and zeolite minerals.

2. PANNA FormationPanna Formation overlies Deccan Trap or crystalline basement and is overlain unconformably by Bassein/Belapur/Pipavav Formation. The upper boundary of Panna Formation coincides with H-4 seismic horizon. The formation is spread over the entire Bombay Offshore Basin excepting the paleo- highs and is composed of sandstone and claystone at the bottom, overlain by a section of coal-shale alternation in the middle and succeeded by shale unit at the top. The sandstone is white to yellowish, coarse to medium grained and poorly sorted. The shale is grey to bluish grey, moderately hard, carbonaceous and pyritiferous. The formation displays considerable fades variation across the different tectono-sedimentary blocks. In the Tapti- Daman block, a good sand developed part of this formation is represented by thick sandstone unit in the Panna East area. In the south Ratnagiri, this formation is represented by a fairly thick trap-wash sequence consisting of reddish brown claystone and siltstone. A fluvial to shallow marine depositional environment is deciphered for this format ion. Paleocene - Lower Eocene age has been assigned to Panna Formation.

3. JAFARABAD FormationJafarabad Formation is homotaxial of Panna Formation. It unconformably overlies Deccan Trap and is overlain by Belapur Formation. The aerial extent of this formation is restricted to Diu block where it consists of an alternating sequence of limestone and shale with a few quartzitic sandstone layers in its lower part. It grades into limestone of Devgarh Formation the formation was deposited under shallow marine environment. Paleocene- Lower Eocene age has been assigned to it.

4. DEVGARH FORMATIONDevgarh Formation is unconformably overlain by Bassein Formation. Its contact with the underlying Panna Formation is conformable and is marked at the change of lithology from limestone to clastics. The formation extends over the southern fringe of Bassein block, most of stone overlain by 20-25 m thick radioactive, carbonaceous shale inter-bedded with thin limestone layers. The upper shale unit, however, thins out toward north Ratnagiri and finally disappears in the DCS area. Paleocene-Lower Eocene age has been assigned to this formation.

5. BELAPUR FormationThe formation extends over most of the Tapti-Daman block, north eastern part of Panna Bassein block (Mahim graben), Shelf Margin block and Diu block, It unconformably overlies the Panna Formation in moat of the area except Diu block where It Overlies Jafarabad Formation. It is overlain by Diu Formation in Tapti-Daman and Diu blocks and Bassein Formation. In Shelf Margin block and Panna Bassein block (Mahim graben). In some of the wills of Tapti Daman area, the upper boundary of Belapur Formation coincides with highest occurrence level of Cribrohantkenina Inflata. It consists dominantly of calcareous shale, claystone and a limestone marker bed at the bottom. It is distinguished from the overlying Diu Formation by Its calcareous nature and increased resistivity. Th. faunal content of this formation suggests Inner to middle neritic depositional regime.

6. Diu FormationDiu Formation unconformably overlies Belapur Formation and is overlain unconformably by Mahuva Formation. The contact between Diu and Mahuva Formations corresponds to the seismic horizon H-38. The formation extends over most of Tapti-Daman block and Diu block. It is composed of silty shales which have distinct log characters as compared to underlying calcareous shales of Belapur Formation and silt free, low resistivity shales of overlying Mahuva Formation. In Tarapur area, a few limestone bands appear in this formation whereas, due north it undergoes a gradual fades change to silty shills and finally to sandstones of Pipavav Formation. Upper Eocene age has been assigned to this formation.

7. PIPAVAV FormationUnconformably overlying Panna Formation and underlying Mahuva Formation, this sand dominated unit in the Saurashtra homocline area, has been designated as Pipavav Formation. The formation has a very limited aerial extend and is only 94 meters thick in the type section. It is expected to thin out in the northwest direction, towards

Saurashtra coast and it undergoes a facies change to the shales of Diu Formation over the rest of the Tapti-Daman & Diu blocks. The formation consists of medium to find grimed moderately to poorly sorted sandstones interspersed with thin layers of calcareous shales. The lithological and faunal content of the formation Indicates marginal marine to shallow marine depositional regime. Upper Eocene age has been assigned to this formation.

8. BASSEIN FormationThis formation is present over the entire Bombay Offshore Basin except Tapti-Daman block. Diu block, Bombay H1 and eastern part of south Ratnagiri. In the Panna-Bassein block, Bassein Formation Is unconformably underlain by Panna Formation and the contact between the two formations coincides with H-4 seismic horizon. In the Shelf Margin block and Mahim graben it overlies the shales of Belapur Formation. Over Ratnagiri and DCS area, it unconformably overlies the Devgarh Formation. A regional unconformity separates the Bassein Formation from the overlying Mukta Formation and this contact corresponds to H-3B seismic horizon. Over most of the shelf area, base of a 15-2O m thick high gamma log marker defines the boundary. In the type area, the formation consists of porous wackestone in the upper part, echinodermal wackestone in the middle part and hard packstone with pelloidal fades towards bottom. The microfiches of Bassein Formation Indicate a wide range of environments like restricted platform with isolated shoals In the Bassein area to open carbonate platform in DCS and Ratnagiri area. Middle-Upper Eocene age has been assigned to this formation.

9. MAHUVA FormationThe formation extends over Tapti Daman and Diu blocks. It unconformably overlies the Diu Pipavav Formation and is overlain by Daman Formation, in the Tapti-Daman block. Westward over the Diu block, its contact with overlying Mukta Formation is gradational. Mahuva Formation consists dominantly of shale with thin limestone bands and occasional sandstone stringers towards its upper pert. Towards the northern part of Tapti- Daman area, the limestone bands totally disappear and the unit becomes siltier Westward, over the Diu saddle, the upper part of this formation changes to limestone of Mukta Formation. Deltaic to inner neritic depositional environment has been inferred for this formation. It has been assigned Lower Oligocene age.

10. MUKTA FormationThe formation extends over most of Bombay Offshore Basin except Tapti-Daman block. It is unconformably underlain by Bassein Formation In major part of the basin. However, In Diu block, its contact with underlying Mahuva Formation is gradational. In the Panna-

Bassein and the eastern part of Ratnagiri blocks the contact of Mukta Formation with the overlying Heera Formation is sharp but conformable. Its contact with the overlying Panvel Formation in the DCS and Ratnagiri blocks as well as Alibag Formation over rest of the area is unconformable. The formation consists of argillaceous limestone underlain by a 15-20 m thick zone characterized by radioactive shales. The limestone is white to dirty white, hard, compact, argillaceous and fossiliferous. Lower Oligocene age has been assigned to the formation.

11. HEERA FormationThe formation extends over the Panna-Bassein block, Shelf Margin block and most of Ratnagiri block, it conformably overlies the Mukta Formation and is unconformably overlain by the Alibag Formation. The boundary between Heera Formation and Alibag Formation coincides with H-30 seismic horizon. The formation consists of an alternating limestone-shale sequence which does not show any drastic variation over its areal extent laterally it grades into the shales of Mahuva Formation and limestone of Mukta Formation. Lower Oligocene age has been assigned to this formation.

12. DAMAN FormationThe formation is present over most of the Tapti-Daman block. It unconformably overlies the Mahuva Formation and its contact with the overlying Mahim Formation is also unconformable and coincides with the H-3CGG seismic horizon. The formation consists of sandstone-shale alternations with a few coal Ienticles. The sandstones are of different types, ranging from shallow marine to deltaic. The formation contains abundant globular bodies. Towards the southern part of Tapti Daman block this formation becomes more argillaceous, and finally it grades into shale of Alibag Formation. Upper Oligocene Basal Miocene age has been assigned to this formation.

13. ALIBAG FormationThe formation unconformably overlies Heera Formation and is overlain by Mahim Bombay/Ratnagiri Formation. It extends over the Panna-Bassein block, eastern and central parts of Ratnagiri block, eastern part of Bombay High-DCS block, western and southern margin of Tapti Daman block, shelf Margin block and most of Diu block. The formation consists of dominantly greenish grey, calcareous, splintery shales interspersed with thin limestone bands. The limestone is generally hard, tight & fossiliferous micrite. Due south and west, over the Ratnagiri and Bombay High-DCS blocks, the limestone percentage of this formation progressively increases until the equivalent section is represented by dominantly limestone fades of Panvel Formation.

Inversely in the north and northeasterly direction, the limestone bands completely shale out. Further north it grades into the sandy fades of Daman Formation. The formation has been assigned Upper Oligocene to Basal Miocene age.

14. PANVEL FormationThe formation unconformably overlies Mukta Formation and is overlain by Ratnagiri Formation. It extends along a NW-SE trending belt constituting the western part of Diu block, most of Bombay High-DCS block and western part of Ratnagiri block. Lithologically it consists dominantly of limestone interspersed with a few thin shale layers which are more common in its upper part. Generally, argillaceous nature of the limestone, differentiates this formation from the relatively shale free limestones of the underlying and the overlying formations. The formation has been assigned Upper Oligocene to Basal Miocene age.

15. MAHIM FormationThe formation is unconformably underlain by Daman/Alibag Formation and is overlain by Tapti Formation. However, in certain areas, it is underlain by the Bombay Formation and the contact between the two is conformable & gradational. The upper contact of this formation corresponds to the seismic horizon H-1C and is marked at the bottom of a siltstone marker bed, which very closely follows a biostratigraphically defined regional unconformity. This marker is referred as S1 sand over the Bombay High area. The formation consists of dominantly shale with subordinate limestone and a few siltstone layers. Thin limestone bands are generally restricted to the lower part of the Mahim Formation and their presence help in differentiating this from the underlying Alibag Formation. Lower Miocene age has been assigned to this formation.

16. BOMBAY FormationThe formation is underlain unconformably by Alibag Formation. Its contact with overlying Mahim Formation is however conformable. It extends over the eastern part of Bombay High-DCS block, southern & western parts of Panna-Bassein block and most of Diu block. The formation consists dominantly of limestone interspersed with thin shale layers. The limestone is generally biomicrite to micrite type and shows fairly good development of intergranular, intragranular, moldic & vugular porosity. Over Diu block, this limestone is generally tight. Eastward over the Panna-Bassein block, the formation gradually shales out, resulting in its progressive thinning. Lower Miocene age has been assigned to this formation.

17. RATNAGIRI FormationIn the western part of Bombay High/DCS block and southern part of Bassein block, the shales overlying Bombay Formation and underlying Bandra Formation are completely replaced by limestone and the two formations become almost Indistinguishable. This monotonous limestone sequence bounded by seismic horizons H-3CGG and H-1A has been designated as Ratnagiri Formation. The formation is underlain unconformably by the Alibag Formation over most of Ratnagiri block and Panvel Formation in DCS block. Its contact with overlying clastic fades of Chinchini Formation is sharp and disconformable. The formation consists dominantly of biomicritic limestone occasionally interspersed with thin layers of grey fissile shales. Lower-Middle Miocene age has been assigned to this.

18. TAPTI FormationThe formation is unconformably underlain by the Mahim Formation. A basin-wise disconformity marks the upper boundary of this formation with the Chinchini Formation. The formation extends over the Tapti-Daman block. Most of Diu & Panna-Bassein block and Shelf Margin block. It consists dominantly of shales with minor siltstone layers. A few thin limestone bands are encountered in the upper part of this formation which helps in differentiating this from the overlying Chinchini Formation. Middle Miocene age has been assigned to this formation.

19. BANDRA FormationBandra Formation overlies Tapti Formation with a gradational boundary. A regional disconformity separates it from the overlying Chinchini Formation. It extends along a NW- SE trending belt covering Diu block, Bombay High, eastern part of DCS and southern part of Panna-Bassein block. The formation consists dominantly of limestones interspersed with a few thin shale layers. The limestones belonging to this formation gradually shales out over south Bombay High and northern parts of Panna- Bassein areas. Middle Miocene age has been assigned to this formation.

20. CHINCHINI FormationThe sediment cover overlying Ratnagiri/Bandra/Tapti Formation in different parts of the basin has been designated as Chinchini Formation. As mentioned earlier, its contact with underlying rock units is characterized by a regional unconformity mapped as seismic horizon H-1A. The formation consists of greenish grey to bluish grey soft sticky clays with frequent Intercalations of shell debris. The basal part of the formation is represented by grey to greenish grey shales occasionally interspersed with thin limestone layers, particularly in the areas, where it is underlain by Ratnagiri Formation.

The depositional environment of this formation has been inferred to be shallow marine, varying from inner neritic to outer neritic. Upper Miocene- Recent age has been assigned to this formation.

21. ANGARIA GROUPThe Shelf Margin block is dotted with a few isolated carbonate build ups which developed in the deep sea as banks or reefs over the paleo highs. Some of these banks continue their vertical accretion even today whereas others got buried and welded to the present day shell by the prograding clastics during post Mid Miocene. These reefoidal bodies consist of thick monotonous limestone sections, rarely interspersed with thin shale layers. Thoi4, limestone sections which sometimes ranges in age from Lower Eocene to Recent, Is interspersed with biostratigraphically well defined regional unconformities; it is rather difficult to subdivide this sequence into laterally correlatable litho units. The entire limestone section belonging to these isolated carbonate build ups has been designated as Angaria Group.

Hydro-Carbon Occurrences

Oils mainly occur in Limestone horizons of lower Miocene age(L-III) in major hydrocarbon fields; Bombay High, Panna , S.Bassien, Heera and Ratna, Mahuva and Daman Pay (Oligocene) in Tapti area, Bassien Pay (Middle Eocene – Upper Eocene) in Panna-Bassien-Herra and Ratna areas and Ratna pay (Mid Eocene – Lower Eocene) in Ratnagiri area. With the recent oil occurrences in the sands of Panna formation in Vasai East, the Panna pay is emerging as the essential pay zone. Few oils also occur in clastic/fractured basemant reservoirs and middle Miocene L-II ans SI pays. The majority o these oils show moderate API gravity(25-40°),high pour point(27-33°),signifigant wax(7-20%),and low sulphur(0.1-0.3%) contents. These oils are predominantly aliphatic,having high saturate/aromatic ratio(>1.5) and saturate content(>40%).

Source Rocks

The clastic sediments in the lower Eocene to Paleocene sedimentary sequences are principle source rocks across the basin. Thickness of the source rock varies from 30m-1000m.The excellent source rock of restricted marine to lagoonal deposits within the Panna Formation in the central graben and adjoining area are the principle source of hydrocarbon accumulation in the basin. In the Mahim graben, a 400m thick sequence in the Panna formation contains very good/excellent oil-prone effective source rock facies, which account for commercial petroleum reservoirs within Bassien, Mukta and Heera formations in the east of Panna and Bassien Fields. Organic rich source rock sequences in the Panna formation occur in depressions across the DCS area and west-south-west of Bombay High.Source rock data from the deepest exploratory well in the Vijaydurg graben of Ratan depression show good, mature source rock section in the lowermost unit of the Panna formation and thin coal and coaly shale layers with very good source-rock quality at the Panna Formation. In the Tapti-Daman area,two exploratory wells, located in the eastern flank of Navsari low, contain about 70m thick oils and gas prone source rocks layers , and better source rocks are more likely to occur in distal environments in the Purna graben and west Daman low corresponding to these layers. The sedimentary column in Shelf Margin areas is dominated by clastics, except in middle Eocene, which has corbonates. Source rock potential of the Paleogene sediments is moderate, but some good organic carbon-rich source-rock layers are present in Neogene sediments.

Seal

Shale as well as carbonates is found to act as seals for the hydrocarbon accumulation on Bombay offshore Basin. Effective seal for the sand of Panna formation and its equivalents and for those in the Oligocene sequence in Tapi-Daman areas is provided by the enclosing shales/claystones. The upper Oligocene shale provides the cap rock for virtually all the fields having oil or gas in the A member of Bassien formation. Another sahel of seal facies having considerable lateral extent occurs in Saurashtra formation immediately overlying the Bombay formation in the Bombay High area. The shales of Tarapur formation in Bombay High and elsewhere constitute perhaps the most widespread seal facies. The limestone immediately underlying is hydrocarbon bearing in Bombay High.

Carbonates also act as seal in numerous cases on outstanding example of which is Bassien Formation. It appears that carbonates get preferentially cemented below zones of vugular porosity associated with exposure surfaces. The excessive cementation within these tight layers could be attributable to.

1) CaCO3 dissolved from the leached zone getting precipitated below and adjacent to leached zone where the percolating meteoric water reaches saturation.

2) Early Digenetic cementation in which calcite is derived from vadose zone.3) Compaction of beds with clay admixture.

In Bombay High, the cyclically deposited carbonate reservoirs have inter-bedded transgressive shale bands or regressive carbonaceous shale bands which act as seals.

Trap

In carbonate platform areas, majority of the fields have been discovered in anticlinal reversals or faults clusters. While this approach has largely been successful in cases of major structural features like Bassien, Panna , Neelam ,etc. It has been beset with problems in smaller structures. There is sufficient evidence to indicate that besides the anticlinal reversals there have been additional factors for the formation of pools. Some of these are updip changes of facies from limestone to shale as in case of Bombay High, D-18 and D-12 development of tight limestone, truncation by faulting.

Most of the hydrocarbon pools discovered so far in sands of the Surat Depression, Heera-Bassien block and Ratnagiri Block are encased by shale and therefore are primarily stratigraphy features. While paleogene and Eogene sands could be assumed to have their hydrocarbon from enclosing shales, the same may not always be applicable to the Oligocene sands. It may be

useful to establish possible migration conduits connecting these traps with the generation centre for systematic exploration.

Reservoir Rocks

Surat Depression

This basin contains sands at various levels ranging from Late Palaeocene to Miocene. In the syn-rift paleogene to lower Eocene section, sands associated with regressive phases or low stands of sea level, as encountered is ED-5 and TSA, are possible widespread in occurrence. These sands possibly have a dual origin. Sand along the south-eastern half of the Surat Depression are likely to have been bought in by Narmada-Tapi river system. The sands along the north-western part of the depression might have their origin from the onshore Cambay Basin.

Having been deposited in sheltered shelf environmental as well as continental to paretic environment the sand bodies are likely to be thin, laterally discontinuous and moderately to poorly sort. Exploration for hydrocarbons in these reservoir basins will have to be carried out with the help of good quality seismic data and has to be supported by effective sedimentological and palaeoecological studies.

Sands in the Lower and Upper Oligocene sections deposited in continental to paretic and shallow shelf environments, mainly under the influence of a fast propagating delta, are similarly in the form of laterally discontinuous thin layers.

Middle to Late Eocene was a period of extremely restricted clastic supply. During this time even parts of Surat Depression witnessed carbonate sedimentation. Being contiguous to know source rocks, these limestones should be considered as potential reservoirs.

Heera-Bassein Block

There are three structural settings which influenced palaeogeography and paleogeomorphology of the block. During Paleocene to Eocene, the Central graben and other structurally low areas witnessed deposition of shales, sands, coal seams. The sands, which are invariably arkosic and clayey, could have been derived from localized positive blocks in the vicinity as well as partly from the sedimants load bought in by the Narmada-Tapi syatem. Several hydrocarbon finds has been made in these sands during the recent past.

Middle Eocene to Early Oligocene was a period of extensive carbonate sedimentation in this block. The carbonate platform was bordered to the north by inner to middle shelf sea with a highly indented platform to basin margin. To the east ,lay the marine embayment along the central trend which became progressively shallow southward up to the ENE trending Heera uplift zone. Under the influence of this embayment and the shelf sea, carbonate sand shoals. The interior of the platform upto Bombay High witnessed relatively less wave and current agitations as a result of which carbonates with mud supported fabric developed.

Porosity in Bassien formation is essentially associated with three or four spells of unconformities or diastem during which Karstic or Caliches surfaces developed. The sequence resulting in the formation of porous zones consist of:

-Development of carbonate sand shoals on platform edges overlooking marine embayment’s or open sea. A paleostructure does not seem to be an essential prerequisite for developments of these shoals.

-Repeated low stands of a sea level leading to unconformities or diastems during which paleogeomorphically elevated areas such as shoals mentioned above and the paleostructurally influenced features in the platform interior underwent landing.

-Rocks with grain supported fabrics developed moldic porosity as a result of dissolution of foraminiferal sand. Similar process in the platform interior having rocks with mud supported fabrics developed porosites by leaching of larger grains, leaching of comminute grains in the lime mud matrix and aggrading neomorphism of micrite followed by dissolution of lime mud particles.

This has probably resulted in the development of a dual or multiple pore system with vastly different pore throat geometry and effective permeability. Therefore, exploration and development of hydrocarbons in reservoir rocks with grains supported and the mud supported fabrics have to be distinctly differentiated and understood. In the carbonated sand shoals, the porosity and hydrocarbon accumulation follow predictable patterns. Structures in the platform interiors show complex pore system resulting in less predictable hydrocarbon distribution.

The area to the east of central graben appears to have been a shelf lagoon with a mix of clastics and carbonates. The carbonates interbedded, with shale here are generally clayey and much less porous than elsewhere. Localized exception to this could be found in small sized carbonate build ups which defy systematic mapping except when these have taken place on structural anomalies.

The Miocene carbonate equivalent to L-IV of Bombay Formation in Bombay High extends into Panna – Bassein area. The exact pale gradient along the zone of facies change to shale futher

east is not clearly understood. Available data shows that there are multiple layers of porosity along the facies change boundary.

The Heera area witnessed fairly well agitated environment of carbon deposition from Middle Eocene to Middle Miocene. This facilitated by communication with open sea to the south of Bombay Platform. Here again while Middle Eocene to Lower Oligocene sequence is fairly well understood in terms of their reservoir characteristics, the Miocene carbonates have not been studied well.

Bombay Platform

In the Bombay Platform, as indicated earlier, there are localized depressions in which shales, sands and coals of Paleocene and Eocene age are found. snads within this sequence which often contain hydrocarbons are derived from gneissic rocks in nearby positive areas. Systematic exploration for hydrocarbons in these sands is usually different.

Thick piles of carbonate ranging in age from Eocene to Middle Miocene have developed in continental shelf areas around the platform margins to the west and south. The lithology indicates that in most cases the limestone has grain supported fabrics. Limestones in the western margin were probably deposited as carbonate sand shoals overlooking the open sea. It appears that the process of exposure and leaching was experienced by these carbonate also. But mesogenetic cementation has destroyed much of the porosity in these rocks. Lower Eocene to Lower Oligocene carbonates along the southern slopes of Bombay Platform had a dominantly mud supported fabric. The frequency of unconformities and the resultant porous zones is much lower here than elsewere. Porosity in these carbonates is considered to be generally low. However a few wells have indicated dolomitization to various degrees in the Eocene section although its effect on creation of porosity is yet to be established.

The younger carbonates have reservoired hydrocarbon in the southwestern and southern margins as seen in structure D-18(Oligocene), D-12 and D-1(Miocene). The reservoir rocks are thin usually seperated by shales or tight limestones. Here also the porosity is described as due to vugs or channels. Several other structures drilled along the southern slope of the platform have porous intervals in Oligocene-Miocene sections but proved to be devoid of hydrocarbons. This shows that the hydrocarbon accumulation here is mainly controlled by other factors like proximity to generation centre and availability of trapping mechanism.

The homoclinal area between DCS and Bombay High contains carbonates from Middle Eocene to Middle Miocene age with variable intervals of shale. A few hydrocarbon strikes have been made in carbonates of Bassein Formation and Alibaug Formation with entrapment controls similar to DCS area.

The Lower Oligocene-Early Miocene carbonates of Bombay Formation are the main hydrocarbon reservoirs in Bombay High(L-III & L-IV). These carbonates are essentially deposited

in a cyclic fashion. The general carbonate texture, composition and the multiplicity of cycles make them distinctly different from the carbonates of the Bassein and Alibaug Formations. The Lower Miocene carbonate depocentre in Bombay High appears to have been dissected by diastemising channels with fills of shale or lime mud. Reservoir properties of limestone exhibit complexities because of the shale or lime mud in the vicinity.

In Saurashtra Formation the S1 sand represents a low stand of a sea level or brief spell of increased coarser clastics dispersed over Bombay High from east. The uppermost part of the formation in Bombay High represented by L-II limestone which is similar to L-III unit of Bombay Formation.

Northern Ratnagiri Block

Sands in the Paleocene-Eocene sequence encased within shales are becoming increasingly important as reservoirs in those blocks. These sands are similar to those found in central graben. Principle reservoirs are in carbonates of Bassein Formation. Porosity development in this case is also related to unconformities or diastems developed at the end of various depositional cycles. Complexities in porosity development similar to Heera-Bassein block are noticeable here too.

Southern Ratnagiri Block

In these blocks, the sands in basal clastics equivalent to Panna Formation are similar to those in the north. Carbonates of Early Eocene to Middle Miocene age have been encountered here also. However, they are more porous with possible cavernous zones resulting in severe mud loss. Thus, despite the presence of porosity in all the Formations from Paleocene to Middle Miocene, the wells in southern Ratnagiri have not struck any commercial accumulation of hydrocarbons and reasons for this should obviously be sought elsewere.

Saurashtra Basin

Several wells have been drilled in saurashtra Basin. Most of them have encountered carbonates ranging in age from Middle Eocene to Middle Miocene. However, the most widespread phase of carbonate sedimentation seems to be Oligo-Miocene. The carbonates are essentially foraminiferal lime wackestone to packstone with frequent intercalation of claystones. In several cases intervals of fair porosities have been observed, particularly in association with regional unconformities. Pressures in these limestones have been found to be higher than hydrostatic to the extent of 20% - 50%. Thus the basin has adequate reservoir facies and the reason for the negative result of exploration is not attributable to this factor.

Shelf Margin

To the west and southeast of Bombay Platform, two wells have been drilled which shows foraminiferal lime wackestone to packstone in several layers within Lower Eocene to Lower

Oligocene section. Compaction and mesogenetic cementation have destroyed porosity to a great extent.

In the off shelf, carbonate banks like SM-79 and R-1, there are numerous intervals with excellent to fair porosities. The wells in these structures experienced extensive mud loss suggesting high porosity including possible cavernous zones. Thus, it seems that lack of porosity is not a contributory factor here for the negative results of exploration.

From the foregoing, it may be seen that reservoir facies are very widespread in the basin although local variation and discontinuities occur in porous zones of different lithologic units. Their capability of hosting hydrocarbons is mainly controlled by the proximity to a generation centre, availability of migration conduits and trapping mechanism.

PETROLEUM SYSTEM

Based on the reservoirs and the source combinations, the various petroleum systems in the basin are: the Panna-Daman, Panna-Mahuva (only in Tapti-Daman block) and Panna L-III, Panna-Mukta, Panna-Bassein and Panna-Panna.

MATURITY MODELING

1D-thermal maturity modeling, using Genex 1D-basin modeling software, indicates that the sedimentary sequences of the Panna Formation from a well near the Mahim graben have generated substantial oil. These source-rock sequences started expelling oil from late Oligocene(30Ma) with peak expulsion spreading from 12 Ma to present day along the flank of graben.

These source rock layers distributed in Mahim graben area are overmature, generating gas in the centre of the depression, as the maturation level is high(Ro=1.3-2.0%). Thus, the entire section appears to be the major gas source for the giant Bassein gas field adjacent to this hydrocarbon kitchen. Maturity modeling of a well from the low of Bassein platform also shows significant generation, which began 20 Ma, and peak expulsion started taking place from 6 Ma and continues to present day. Approximately 120m thick dominantly marine organic-rich source-rock section from a well near the depocentre of DCS area is predicted to have begun significant generation 12 Ma, with peak expulsion occuring from 5 Ma and continuing to present day. In Ratna depression, thick lowermost source-rock section began significant generation 18 Ma.

CORRELATION STUDIES

Based on several bulk parameters, conventional biomarkers, and stable carbon isotopic composition, oils of the Bombay Offshore Basin, irrespective of their pay zones, can be broadly categorized in two groups with some overlaps. Group I oils show high values of Pr/Ph ratio

(>3.0), high canonical variables (C.V. >0.47), relatively low abundances of bicadinanes, oleanane, oleanoid triterpanes, tricyclic terpanes, predominance of C29 over C27, and C28 regular steranes, and the presence of diasteranes. However, the group II oils have low Pr/Ph (<3.0), low Pr/nC17 (<1) ratios, low values of C.V. (<0.47), relatively high abundances of bicadinanes, oleanane, oleanoid triterpanes,presence of C30 steranes (both 24-n-propyl cholestane and 4-methyl steranes), and are isotropically heavier than the group I oils. The occurrences of these genetically dissimilar oils are not following any distinct pattern and are present in all the blocks in different pay zones. Group I oils are generated from predominantly type III organic matter and deposited under fluvial conditions whereas the group II oils are derived from mixed organic matter input with significant marine organic matter contribution and deposited under marginal marine conditions. However, all these oils have been generated at similar maturity levels.Studies also indicate that source rock extracts of the Lower Eocene-Paleocene sediments from the peripheral part of the various lows in different blocks contain mainly terrestrial organic matter deposited in fluctuating fluvial/fluvio-deltaic to marginal marine environments. These source rocks are adequately mature to generate hydrocarbons and are genetically correlatable with the group I oils. Several thin streaks in the upper layers of the Lower Eocene-paleocene sediments in the central part of these depressions also show marine organic matter and these source rocks are genetically correlatable with the group II oils. The difference in the relative abundance of the several biomarkers; i.e., bicadinanes, oleanane, Pr/Ph ratio, etc., in two different groups oils seems to be controlled mainly by the change in the depositional environment from the basinal part (more anoxic) towards the peripheral area(less anoxic). These two groups of oils and their probable source rocks are clearly differentiated by a plot of bicadinanes/C30 hopane versus oleanane/C30 hopane ratios and stable carbon isotopic composition. Though some lacustrine facies were also present in a few wells in Mahim graben and DCS low, no oil with dominant lacustrine biomarker characteristics was found in the basin so far. This may be due to charging of reservoirs from the multiple source rocks. Results also show that the studied oils and the potential source rocks have been generated at similar maturity level (moderate to peak oil window). Presence of oleanoid triterpanes in both oils and source rock extracts and also the carbazoles distribution in oils support the conclusion that, except for the oils of Bombayhigh, most of the oils in different blocks are locally generated and have not experienced a long distance migration.

CONCLUSION

Lower Eocene-Paleocene sediments have very good source-rock characteristics and good hydrocarbon generation potential throughout the basin, but quality and quantity of the organic matter deteriorated towards the peripheral area from the basinal lows. The organic matter is dominated by input from terrigenous higher land plants. However, two potential source sequences are identified in the Lower Eocene-Paleocene sediments. Specific biomarkers, particularly the relative contributed from resinous plants, flowering plants, carbon isotopic composition and marine organic matter diagnostic biomarkers indicate that these sediments exhibit intervals with different organofacies.

Oils of the Bombay Offshore Basin can be broadly categorized into two groups based on their

biomarkers distribution and stable isotopic composition.

Group I oils containing only terrestrial organic matter from fluvial/fluvio-deltaic depositional environment generally correlate with the Lower Eocene-Paleocene source-rock extracts from the wells located in the peripheral area of the lows in each block. On the other hand, group II oils, which contain mixed type organic matter deposited in marginal marine conditions, correlate with the upper layers of the Lower Eocene-Paleocene source sequences in the wells located in the central parts of the lows in each block.

All oils in the field are at approximately the same level of thermal maturity and were expelled from sources that still retain significant generative potential. These oils are generated locally and have not undergone long distance migration. Maturity modeling of vitrinite indicates that in general the sedimentary sequences of the Panna Formation started oil expulsion 18-30 Ma and peak oil expulsion 12 Ma and continues to present day.

REFERENCES

Basu, D.N., A. Banerjee, and D.M. Tamhane, 1982, Facies distribution and petroleum geology of Bombay Offshore basin, India: Journal of Petroleum Geology, v.5, p.57-75.

Zutshi, P.L., A. Sood, P. Mohapatra, K.K.V. Ramani, A.K. Dwivedi, and H.C. Srivastava, 1993, Bombay Offshore basin-lithostratigraphy of Indian Petroliferous basins (document-V).

Wandrey, C.J., 2004, Bombay geologic province Eocene to Miocene composite total petroleum system, India: U.S. Geological Survey Bulletin 2208-F, 26p.