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The western coast of India is composed of two major sedimentary basins, the Mumbai or Bombay Basin in the north and the Konkan-Kerala-Lakshadweep Basin (and its deep water continuation) in the central and southern parts of the margin.
Geohistory
The evolution of the western continental margin basins of India is related to the break up of Gondwana and the subsequent spreading history of the eastern Indian Ocean. Rifting and the development of rift-generated depocenters did not begin until the Late Cretaceous in these basins. A series of linear horsts and grabens resulted in response to the rifting along dominant basement trends (NNW-SSE (Dharwar Trend), NE-SW (Delhi-Aravalli Trend) and ENE-WSW (Satpura Trend)). Late Cretaceous syn-rift sediments are known in the Konkan-Kerala-Lakshadweep Basin, as well as in the Cambay Basin to the north of the Bombay Basin, but have not been encountered in the Bombay Basin itself, either as a result of complete absence or restricted distribution below the thick Deccan Trap basalts. Early syn-rift deposition in the Konkan-Kerala-Lakshadweep Basin consisted of sandstone dominated continental deposition, which gradually changed to more sandstone, shale and coal dominated deltaic/marginal deposition and eventually to pulsating marine conditions characterised by sandstones, shales and limestones.
Having crossing the equator, the western margin of the Indian Plate passed over the Reunion hotspot/mantle plume towards the end of the Cretaceous. This intense hotspot activity resulted in a sustained period of intrusive and extrusive igneous activity during the Maastrichtian and Early Paleocene, coupled with uplift and initiation of further rift activity along the NNW-SSE Dharwar Trend. The thick Deccan Trap volcanics were extruded along the western margin of the Indian Plate, occurring throughout the Bombay Basin and to a lesser extent in the Konkan-Kerala-Lakshadweep Basin.
Syn-rift continental to paralic deposition occurred in the Bombay Basin during the Early to Middle Paleocene, with the first marine incursions not appearing until the Late Paleocene/Early Eocene. Sandstones, shales and coals dominated the continental/paralic succession (e.g. Panna Formation), with limestones becoming more abundant with the marine incursions occurred (Jafarabad Formation). Deposition within the Konkan-Kerala-Lakshadweep Basin was essentially similar during this time, with continental, marginal marine and shelf deposition resulting in the succession of sandstones, shales and limestones of the Kasargod Formation. During the Middle Eocene, as a passive margin regime prevailed, the western margin of India underwent major transgression and inundation. Over the shelf of both the Bombay and Konkan-Kerala-Lakshadweep basin, carbonate deposition was widespread (e.g. Bassein and Karwar formations) deposited in various environments from lagoonal and restricted marginal marine on the basin margins, to more open platform and deeper basinal deposition towards the west. In these deeper basinal areas, deposition of marls, limestones and shales occurred as pelagic and hemi pelagic deposits. Reefal limestones developed on structural highs, whilst shelf margin and slope instability resulted in the development of submarine fans and debris flows. In the Bombay Basin these deposits are named the Deep Continental Shelf Group (Angaria Group), whilst in the Konkan-Kerala-Lakshadweep Basin they remain generally unnamed.
A major margin wide unconformity occurred at the end of the Eocene, which was followed by a major transgression in the Early Oligocene, which inundated the basinal areas and much of the shelf areas. Extensive carbonate deposition resumed over much of the outer shelf and deep basinal areas, the Mukta Formation in the Bombay Basin and the Calicut Formation in the Konkan-Kerala-Lakshadweep Basin. Carbonate sand shoals were particularly common during this time. Towards the basin margins, more marginal marine systems developed, which were often confined to residual trough topography within the basins. Sandstones, shales, coals and thin limestones were deposited in marginal marine, estuarine, deltaic and swampy environments, with the Mahuva and Heera formations being deposited in the Bombay Basin, and the Mayyanad Formation in the Konkan-Kerala-Lakshadweep Basin. Deep water slope and basinal deposition continued in the western extremes of these basins. These depositional systems began to exhibit a more regressive nature towards the end of the Early Oligocene, and were eventually terminated by a widespread unconformity.
Upon resumption of widespread sedimentation in the Late Oligocene, Carbonate deposition in the Bombay Basin was suppressed by the development of a deltaic system that originated in the northeastern corner of the basin and prograded out westwards and southwestwards. Sandstones, shales and coals of the Daman Formation were deposited within this delta system, whilst further out shelfal deposition consisted of shales and thin limestones and sandstones of the Alibag Formation. Clastic deposition of the Mayyanad Formation continued on the shallow shelf in the Konkan-Kerala-Lakshadweep Basin during this time, whilst in the outer shelf and deeper waters, limestones and shales of the Quilon Formation were deposited.
In response to a sea level rise during the Early Miocene, large areas of the coastal tracts and basin fringes were inundated. The Oligocene delta which was prograding in the northeastern part of the Bombay Basin, receded north and northeastwards as a result of the fast subsiding shelf margin. Shallow marine shales, limestones and siltstones of the Mahim, Ratnagiri and Bombay Formation were deposited in the Bombay Basin. A widespread unconformity occurred towards the end of the Early Miocene. Further south, in the Konkan-Kerala-Lakshadweep Basin, shallow shelf clastic deposition continued with the Mayyanad Formation, whilst the limestones and shales of the Quilon Formation continued in the outer shelf and deeper waters.
The marine transgression which occurred in the Middle Miocene represented the last phase of widespread carbonate sedimentation in the Bombay Basin, with the limestone and shale deposition becoming almost basinwide (Ratnagiri and Bandra formations), with lagoonal systems being common. Shallow marine/deltaic clastics were very restricted to the northeastern margin (Tapti Formation). This transgression also drowned the clastic system (Mayyanad Formation) in the Konkan-Kerala-Lakshadweep Basin, with the shallow marine limestones of the Quilon Formation becoming widespread.
Minor tectonic movements began at the end of the Middle Miocene, with the tectonic movements causing these readjustments resulting in a general westward tilting. A regional unconformity had also developed at this time, resulting from a major sea-level fall. The tectonic event, which coincided with the onset of the main Himalayan orogenic phase, caused a deepening of the west coast basins, as well as a major influx of terrigenous clastic sediment into the basins, resulting in the development of a thick montonous shale/clay sequence. Due to this heavy influx of clastics, the shelf edge rapidly shifted basinward towards its present day position. In the Bombay Basin, this marginal marine to shallow marine system deposited the Chinchini (Tarapur) Formation, whilst in the Konkan-Kerala-Lakshadweep Basin, the Warkali, Mangalore, Vemband and Trichur formations were deposited. This excessive increase in clastic supply and deepening basins inhibited further carbonate sedimentation except in the form of isolated off shelf carbonate build-ups. A thick succession of prograding slope facies also developed in the deep basinal areas, with localised retrogradation of the shelf edge due to gravity faulting and slumping resulting in the development of submarine fans.
Source Rocks
The numerous fields and discoveries in the Bombay Basin attest to the presence of proven source rock intervals. In the Konkan-Kerala-Lakshadweep Basin however, the presence of source rock intervals may only be postulated, primarily on the basis of the contiguous relationship with the Bombay Basin.
The main source rocks in the Bombay Basin are in the Paleocene to Eocene sequence and consist of shales and coals primarily of continental to paralic origin, although shallow marine shales may also have source rock characteristics. The principal source rock unit is the Paleocene to Lower Eocene Panna Formation. This contains Type III, oil prone organic matter and locally has Total Organic Carbon (TOC) contents of 70.5%, although averages tend to be around 4%. However, the distribution of these source rock units is not basin wide. They tend to be restricted to certain major depressions or troughs, with the presence of structural highs causing sediment starvation in some areas, and restricted thicknesses in many areas. The principal areas of source rock development in the Bombay Basin are in the Surat Depression, Central Graben and Vijaydurg Graben. Oligocene and Miocene units are known to have some source rock characteristics. Oligocene units (Alibag Formation) tend to have the richer organic matter than the overlying Miocene units. Oligocene pro-delta clays with TOC contents greater than 1% are located in the Surat Depression and Ratnagiri areas. Miocene calcareous shales and minor limestones usually have TOC values less than 0.7%, but on occasion values over 1% are encountered in some areas.
Modelling suggests that the Panna Formation entered the oil window during the Middle Eocene (about 45 Ma) in the Surat Depression, and during the Early Oligocene (about 33 Ma) in the Central Graben. Peak oil generation occurred during the Late Eocene (about 38 Ma) in the Surat Depression and during the latest Oligocene (about 25 Ma) in the Central Graben. Modelling also suggests that the formation is likely to be presently overmature in the Surat Depression, but may partly still be in the dry gas window in the Central Graben. Areas contiguous to these source rock depocentres and also areas connected by faults with the depocentres should have hydrocarbon accumulations. Hydrocarbon generation and migration may have been influenced by overpressure. Abnormally high formation pressures are known to exist in parts of the basin, and hydrocarbon expulsion pressure may not have exceeded the ambient hyperpressure within the formation thereby inhibiting primary migration. It is not clear whether Oligocene and younger sediments, even with a few intervals rich in Type-II kerogen, have reached the oil window and attained peak generation potential. Rapidly deposited Oligo-Miocene units are generally overpressured, and this may also have inhibited any potential within them.
The sparse information regarding source rock units in the Konkan-Kerala-Lakshadweep Basin is based on data from the small number of wells in the basin, and by correlation with intervals within the Bombay Basin. As with the Bombay Basin the principle source rock intervals are thought to be Paleocene (Kasargod Formation) and Eocene to ?Oligocene (Karwar Formation) units. The Paleocene units consist of lignitic claystones and shales deposited in restricted marginal marine environments, whilst the Eocene units consist of limestones, shales and coals deposited in relatively similar environments. TOC contents within these units range between 0.1 – 51%. Modelling suggests that oil generation began in the Early Eocene (about 55 Ma) in the deepest parts of the basin. Peak oil generation may have begun in the Late Eocene to Early Oligocene (about 35 Ma). The onset of gas generation is suggested to have occurred during the Early Miocene (about 21 Ma). Other source rock intervals have been identified within the sedimentary succession of the Konkan-Kerala-Lakshadweep Basin. Basal Miocene clays and lignites have TOC contents ranging from 0.15 - 23.92% (av. 9.75%), Early to Middle Miocene clays and limestones have TOC contents of 0.06 – 1.82% (av. 1.33%) and Late Miocene claystones have TOC contents of 0.9 – 1.4%. However, it is generally not thought that these units have undergone sufficient burial to have attained maturity. Furthermore, the main source rocks recognised in the basin are those that occur in the main part of the Konkan-Kerala-Lakshadweep Basin. The presence and nature of source rock intervals in the deeper water continuation of the basin are unclear.
Reservoirs
Proven sedimentary reservoirs in the Bombay Basin range from Paleocene to Middle Miocene in age. Less conventional reservoirs are also located in the crystalline basement and the Maastrichtian age volcanics of the Deccan Trap. Of these, the most productive reservoirs to date are the Bombay and Bassein formations.
Limestones and sandstones of the Lower to Middle Miocene Bombay Formation are the most productive in the basin, and form the principal reservoirs in the Bombay High Field, within which several stacked separate layers have been identified (L-I, L-II, S-I and L-III). The limestones are typically micrites and biomictrites, although the L-I interval primarily consists of shell debris. Both primary and secondary porosity occurs within these units, and are very facies and palaeotopographically dependant. Thin, fine grained sandstones (S-I) relating to longshore currents also provide a reservoir interval. Biomicritic limestones of the Ratnagiri Formation, which contain both mouldic and vuggy secondary porosity, are also closely associated with the Bombay Formation reservoirs.
Limestones of the Middle to Upper Eocene Bassein Formation form the principal reservoir to the east of the Bombay High. Porosity in the Bassein limestones varies from moderate to good and is controlled by diagenesis and microfacies type. Secondary porosity is dominant and the main porosity types are solution vugs, channels and moulds. Intermicritic porosity is often observed and is enhanced by solution activity.
Sandstones of the Paleocene to Lower Eocene Panna Formation are also becoming increasingly important as reservoirs in the northern part of the basin. However, having been deposited in a sheltered shelf environment as well as continental to paralic environments, the sand bodies are likely to be thin, laterally discontinuous and moderately to poorly sorted. The Basal Sandstone Unit of the Panna Formation is however, more widespread and continuous. This unit occurs directly above the basement and consists of a thin sand bed of 2 - 3 m thick. It is a medium to coarse grained and poorly sorted sand which appears to be a basement derivative.
The Lower Oligocene Mahuva and Mukta formations also appear to be increasingly important reservoirs, particularly around the Surat Depression. Shallow marine limestones of the Mukta Formation and the more transitional limestones, sandstones and siltstones of the Mahuva Formation contain both primary and secondary porosities. Similar lithologies and facies in the overlying Upper Oligocene to Lower Miocene Alibag and Daman formations also provide reservoirs. These Oligo-Miocene units are considered to have significant further potential. Lower Miocene carbonates of the DCS Group also form minor reservoir at present. Reefal bodies with good primary porosities developed during the Aquitanian, although the DCS Group and these facies do have a much greater age span. The deeper water continuation of the DCS Group is likely to possess greater potential, not only in terms of the reefal occurrences but also in terms of clastics units that spilled over the shelf margin into the basinal areas, as slope and basin floor fans, not only in the Late Tertiary, but also in Paleocene and Eocene age units.
Given the absence of discoveries within the Konkan-Kerala-Lakshadweep Basin, potential reservoirs are identified on the basis of analogues with the Bombay, and by the presence of limited hydrocarbon shows in some exploratory wells. Continental, marginal marine and shelf sandstones and limestones of the Upper Paleocene to Lower Eocene Kasargod Formation form the lateral equivalent of the Panna Formation and may have similar reservoir properties. The overlying Eocene shelf carbonates of the Karwar Formation forma lateral equivalents to the important Bassein Formation.
The Oligocene Calicut Formation, the Oligo-Miocene Mayyanad Formation and the Miocene Quilon Formation show similar facies to the Mahuva/Mukta, Alibag/Daman and the Bombay formations, and may thus also have similar potential. It is not known for example, whether the specific facies and environmental conditions that developed over the Bombay High during deposition of the Bombay Formation occurred in the Konkan-Kerala-Lakshadweep Basin at this time, but similarities do exist with the various troughs and highs within the basin. In the deeper waters, reefal development may have occurred, as evident in the DCS Group in the Bombay Basin, but generally during the development of the basin, the deep waters of the Konkan-Kerala-Lakshadweep Basin were starved of suitable clastic influx and thus reservoir development may be limited. However, the high influx of sediment during the Middle and Late Miocene may have created suitable deep water fan and slope systems in this area.
Traps
The majority of structural traps in the Bombay Basin, and thus those most likely in the shelf areas of the Konkan-Kerala-Lakshadweep Basin, are anticlinal features that developed as a result of the draping of sediments over structural highs, either residual basement features, or as fault blocks developing during the rift events. Faulting during the development of the Panna and Kasargod formations may have lead to the development of fault blocks or fault related traps. Stratigraphic trapping occurs in various forms. These occur as stratigraphic pinchouts in the clastic units and reefal and porosity/permeability pinchouts within the carbonate units. In the more deep water sediment, the development of feeder channels and stacked fan deposits on the slope and basin floor are likely to provide stratigraphic trapping sites. Combinations of stratigraphic and structural traps are common in the Bombay Basin and may also have potential in the Konkan-Kerala-Lakshadweep Basin. |