Introduction

The paper provides comprehensive description of seaward dipping reflectors (SDRs) along Indian coastline to ascertain their relevance for the development of hydrocarbon system and formulation of exploration strategy. International scenario is also discussed, and fairly extensive hydrocarbon prospects, associated with them, have been described. So far, very few attempts have been made to map SDRs along the Indian coast (Kaul et al. 2008; Ajay et al. 2010; Corfield et al. 2010; Phukan et al. 2010; Saha et al. 2015; Ismaiel et al. 2019, Siawal et al. 2019). Seaward dipping reflectors are observed along Indian coasts—more along east coast than west coast. For the present work, regional seismic lines along east and west coast of India were scanned to look for SDRs. Along East Coast, SDRs have been mapped and consequently, continent-oceanic boundary (COB) has been interpreted.

Occurrence of SDRs is related to volcanism. Seismically imaged SDR units are found in offshore regions of many rifted continental margins. Keeping in view that volcanic basins are no more perceived to be non-prospective, as a number of hydrocarbon discoveries have been made all across the globe from volcanic margin basins, the volcanic characteristics of Indian continent margin have been critically studied. Attempt has also been made to relate SDRs to hydrocarbon prospectivity along volcanic margin of Indian offshore.

Seaward dipping reflectors (SDRs)

Seaward dipping reflectors are characterized by strong amplitude seismic reflectors, steepening and diverging downward to dips of more than 15 degrees. Seaward dipping reflectors are found in volcanic rifted margins (Fig. 1) and were first recognized along North Atlantic margin. Initially, SDRs were gently dipping. However, later due to subsidence of volcano-sedimentary sequence, SDRs acquire steeper dips while progressing towards oceans. These reflectors are found to extend up to 10 km in thickness, 200 km orthogonal to the margin and < 4000 km along strike. Seaward dipping reflectors diverge and increase in dip towards the ocean (Fig. 1).

Fig. 1
figure 1

Typical example of seaward dipping reflectors (Indian west coast regional line)

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Normally, SDR sequences are generated from sub-aerial eruptions. Typical termination of SDRs towards sea is interpreted as transition to submarine eruptions (Menzies et al. 2002). The sequences generally are volcanic rocks with association of sedimentary rocks (Urdaneta et al. 2020). Flood basalts, extruded rapidly during initial sub-aerial sea floor spreading or rifting, are represented by SDRs. These form major characteristics of volcanic rifted margin. Internal reflector pattern in the SDRs may represent pounded or thick lava units or very fragmented or hydroclastic units deposited in a shallow marine environment (Planke et al. 1999).

Transitional continental crust is generally overlain by inner SDRs. They consist of varying combinations of sub-aerial volcanic flows along with volcanic clastic and non-volcanic sediments, while oceanic crust is overlain by outer SDRs and consist of submarine basaltic flows, ranging in thickness from 3 to 9 km. It is believed that they lie at or near the continent-ocean boundary and hence are helpful in providing information regarding evolution and structures of passive margins (Mutter 1985; Harkin et al. 2020).

Seaward dipping reflectors are characteristic features of volcanic rifted margin which are formed basically in four stages: (i) movement of plates in opposite directions; (ii) extrusion of lava on the surface; (iii) extrusion of more layers as plates move apart; (iv) extra weight, particularly near extrusion point, causes material to subside and the layers to dip (Chauvet et al. 2021). Seaward dipping reflectors are recognized along most of the passive continental margins all over the globe. Several models have been proposed regarding their formation. Model proposed by Geoffroy (2005) suggests that development of mantle plume is largely responsible for tectonics and volcanism. However, highly linear nature, repeated volcanism and long duration of geological events extending for several hundred million years raise doubt about mantle plume theory (Misra 2008, 2010, 2012). To initiate rifting, it is not always required to have a mantle plume. Rifting is quite common along spreading axis and is accompanied by volcanism along them due to decompression melting.

Importance of SDRs

It is observed that SDRs are confined to continental crust in proximity of continental-ocean transition zone. These zones are identified by extensional tectonics and accompanied with volcanic effusion from weak zones. These weak zones might be the continental-continental break-up zones (Geoffroy 2005). Our studies facilitated identifications of SDRs by seismic reflectors having strong amplitudes and characteristic dipping surfaces. This led to mapping of continent-oceanic boundary (COB). Furthermore, the studies brought out relationship between SDRs and volcanism, suggesting several phases of volcanism continued during long period of time from beginning to the end of Cretaceous.

Heat provided by volcanism created suitable warm environment for deposition of carbonates. This carbonate-volcanic sequence has later been uplifted by subsequent volcanism. In Paleogeographic determination of ancient shorelines and paleomagnetic orientation of grains in basaltic lava with high magnetic susceptibility, SDRs are helpful. These attributes are required for identification of litho-facies and consequently hydrocarbon prospectivity assessment. Seaward dipping reflectors being basaltic are impermeable and thus provide suitable trapping mechanism and seal for hydrocarbon accumulation. Lava has very high temperature, and due to its extrusion, SDRs are formed. Therefore, suitable temperature for hydrocarbon maturation, also known as secondary induced maturity, is provided by SDRs.

Seaward dipping reflectors are one of the largest volcanic constructs on earth that have been imaged by seismic methods and are found offshore of many rifted continental margins with many of them around the Atlantic Ocean (Buck 2017). Analogy of SDRs from Eastern Gulf of Mexico and Norwegian margin with the East coast of India depicts that their presence and implications for basin opening are beyond doubt. Below the Jurassic Louann salt, in the Eastern Gulf of Mexico, deep reflectors dipping towards sea have been known for a long time. They were interpreted as half-graben fills before the opening of Gulf in the Callovian. As seismic technology advanced, new data helped to delineate their geometry and suggest that they are SDRs, representing sub-aerial flood basalts associated with the early opening of the Gulf of Mexico (GOM).

Seaward dipping reflectors in Indian sedimentary basins

Seaward dipping reflectors are the basaltic feature overlain by sediments. As acoustic impedance contrast between sediments and SDRs is very high, we get large amplitude at SDRs (Planke et al. 1999). As SDRs are typical geological features having distinct seismic characteristic, it is easily observable on seismic section. These are basaltic and basalts have high magnetic susceptibility (Davis 2018). Therefore, high value of magnetic susceptibility above SDRs is observed. Being basaltic, SDRs have high density. So, positive gravity anomaly above SDRs is observed (Saha et al. 2015).

Seaward dipping reflectors have been observed along east coast of India in Cauvery, Krishna-Godavari (KG) and Mahanadi Basins. For this paper, the SDRs have been identified and mapped in the regional seismic lines along east coast of India. Along west coast, SDRs could be observed on very few lines and so mapping or any comprehensive interpretation could not be done. In west coast, SDRs observed along Kutch-Saurashtra offshore and Kerala-Konkan offshore are illustrated in Figs. 2, 3 and 4 (Kaul et al. 2008; Ajay et al. 2010; Saha et al. 2015; Siawal et al. 2019).

Fig. 2
figure 2

Seismic line and interpreted diagram along part of the line RE-19, on western flank of the Laccadive Ridge. SDRs have been interpreted below sedimentary sequence on the right side (modified from Ajay et al. 2010)

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Fig. 3
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The presence of SDRs shown in a regional seismic line across Kutch offshore (inset: zoomed part) (modified from Siawal et al. 2019)

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Fig. 4
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The presence of SDRs shown in a regional seismic line across Mumbai offshore (inset: zoomed part) (modified from Siawal et al. 2019)

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Hydrocarbon prospectivity

Many discoveries and research on volcanic margins and hydrocarbon prospectivity in basaltic regions have proved the possibility of hydrocarbon prospects in these zones (Nathaniel 2013). Earliest exploratory efforts in volcanic province took place in Hara Oil field in Japan as early as 1900. In Paz oilfield in Venezuela, drilling evaluation for volcanic rocks was carried out by Venezuela in 1953. It opened a prelude to the understanding of volcanic reservoirs. In spite of the fact that prospect delineation in volcanic basins is constrained by poor sub-basalt imaging, following basins have produced hydrocarbon or are under active exploration. Other basins/sub-basins along with the in-place volumes are summarized in Table 1.

Table 1 Oil and gas fields along volcanic margins
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The reserves data illustrated in Table 1 have been compiled from various sources in the public domain. For example, Corrib field data are retrieved from Rohrman (2007); Samgori from C&C Reservoirs (2005).

Petroleum system

In Indian sedimentary basins, petroleum exploration has been mainly concentrated on Tertiary targets, particularly in Western Offshore, Western onland, Assam-Arakan Fold belt, Krishna-Godavari, Cauvery and Mahanadi Basins. However, volcanics have also contributed to hydrocarbon accumulations in various parts, such as, trap fractures and Mesozoics in Kutch & Saurashtra offshore, and in onland part of India, Rageshwari field in Barmer, Ingoli, Padra in Cambay and Rajol formation in Krishna-Godavari (KG) basin have proved to be hydrocarbon bearing. Of these Rageshwari, Razole, Ingoli and Padra are prolific hydrocarbon producers.

Basalt being volcanic is not a source rock. However, effusion provides heat and facilitates maturation of the source rock. Moreover, Basalt acts as reservoir in fractured and altered rocks (Gas from Kutch-Saurashtra basin, India). Mostly structural plays are identified. Inter-trappeans within the basalt flows produce hydrocarbon in Padra field in Cambay basin, fractured basalt is producing gas in Razole formation in KG basin, and vesicles present within basalt are hosting oil in West Greenland field. Igneous intrusion into surrounding country rocks have resulted in closed trapping structures in Ohama Dome field in Illinois basin, USA. Igneous rocks, altered to clay and impermeable intruded rock (e.g. Laccolith or massive basalt) also act as oil seals in many fields.

Sub-basalt prospectivity in Indian offshore

In Western Offshore of India, during Upper Cretaceous to Paleocene, multiple lava flows laid down the Deccan Trap basalts, extensively in the western part of Indian Peninsula. These Deccan basalts now serves as the technical basement of petroliferous Cambay basin. The same basaltic rocks serve as commercial oil producers in a small field Padra situated at the eastern margin of this basin, with varying depth to the basement from about 400 m in the east to about 800 m in the west towards the basin axis (Jamkhindikar et al. 2013). Commercial production of oil from Deccan Trap in Padra and Gamij fields, near the eastern basin margin, proves that Deccan basalt can constitute a viable hydrocarbon reservoir in Cambay Basin (Gupta et al. 2012). Interpretation and integration of 2D and 3D seismic data with well data have brought out possible fracture trends and the probable locales for exploration of Deccan basalt reservoir in Cambay Basin. Similarly, Ingoli field of the Cambay basin is hydrocarbon bearing from fractured trap/intra-trappean reservoirs. A series of wells in the Ingoli field encountered thick Deccan basalt sections with intermittent fine-grained intra-trappeans (Farooqui et al. 2011).

West coast offshore

Along the west coast offshore of India, thick basalt layer overlies the Mesozoic sediments. Considering hydrocarbon fields, all across the world, more than half of reservoir and source rocks belong to Mesozoics (Kendall et al. 2009). In the western offshore, although not many wells have penetrated the entire section of basalt overlying the Mesozoic sequence, the results from such wells are encouraging, as many of these wells have proved to be hydrocarbon bearing. The distribution pattern of Cretaceous volcanic in the Indian sedimentary basins has contributed to our understanding of their origin and has revealed a new scope for exploration of hydrocarbon in vast sub-basalt regions (Srivastava and Mittal 2018). However, no commercial production has begun from any of the discoveries, from Mesozoic sediments, in western offshore.

Sub-basalt Mesozoic sediments have been encountered in the Kutch and Saurashtra Basins and in Seychelles to the south-west, where drilling results, subsequent to studies of cores and cuttings, proved the presence of thick Mesozoic sequence, and where hydrocarbon presence has confirmed the Mesozoic source potential, having adequate organic matter richness, (Kalra et al. 2014). The organic matter is oil and gas prone Type II and Type III. The values observed from hydrocarbon generation capacity are highly encouraging for Jurassic sediments, whereas Early Cretaceous is just on threshold of maturity in Kutch (Rammurthy et al. 2017). The reported hydrocarbon has same oil characteristics as found in numerous tar balls stranded around the islands of the Seychelles Archipelago (Fig. 5). Potential source and reservoir rocks of the West coast offshore of India range from Triassic Karoo fluvial sandstones and shales, through Jurassic marine shales and sands, to Early Cretaceous restricted marine shales and shelfal sands, which persisted into the Late Cretaceous between India and the Seychelles (Roberts et al. 2008). In Kutch offshore, a number of wells have been drilled down to Cretaceous sediments and hydrocarbon discoveries have been made from sub basalt Mesozoic sediments (Fig. 6) (Singh et al. 2006). Out of several test wells, Well B-1 reported gas from Lower Cretaceous sediments from a depth of 3500 m, well D-1 flowed gas from Upper Cretaceous sediments from 3700 m, whereas Well E-1 flowed oil from Lower Cretaceous sediments at depth 4000 m (Singh et al. 2006). Similarly, in Saurashtra offshore, one well flowed gas on testing, from sub-basalt Mesozoic sediments after drilling through a section of over 2400 m basalt.

Fig. 5
figure 5

Showing the structural-stratigraphic correlation of three Seychelles wells; Owen Bank-1, Seagull Shoals-1 and Reith Bank-1. Occurrence of Jurassic and Cretaceous shallow marine sediments in graben troughs are seen (modified from Roberts et al. 2008)

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Fig. 6
figure 6

Location map of Kutch Offshore showing wells B1, D1 and E1 and Structure Contour map on top of Late Cretaceous (modified from Singh et al. 2006)

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Likewise, in Kerala Konkan offshore, wells indicated gas presence in the Upper Cretaceous basalt and clastic layers. A significant anticline, within Mesozoics, is seen in RIL operated block in Kerala Konkan basin and shown in Fig. 7. However, in well KKMZ-E-A, drilled in Kerala-Konkan offshore in 2014 (drilled depth: 4874 m), limestone sequence was encountered as inter-trappean within two basaltic horizons. The HR Strontium Isotope Stratigraphy (SIS) studies of these samples of Cochin-equivalent Formation showed that ages of both lower and upper basalt unit appeared to be same (Early Paleocene, ~ 62 Ma), indicating possibility of encountering Mesozoic sequences in the Kerala-Konkan Basin to be very weak (Rathore et al. 2020) (Figs. 5, 6 and 7).

Fig. 7
figure 7

Seismic sections showing Anticline within Mesozoics in Kerala Konkan basin (modified from Nathaniel 2013)

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In addition to this, considerable interest has been generated by deepwater Mesozoic hydrocarbon discovery in the adjoining Mannar Offshore Basin of Sri Lanka (Nathaniel 2013). Towards further southwest, in Seychelles Platform, a thick Mesozoic succession, including Triassic and Jurassic sediments are found. The thick source sequences which have charged the exhumed super-giant oil fields onshore Madagascar (Roberts et al. 2008) are believed to be contained in these Mesozoics also. Taking analogy from all this, a similar Mesozoic sequence is perceived to exist in west coast offshore of India (Venkatraman et al. 2010).

East coast offshore

Along the east coast offshore of India, in the coastal belt of KG basin, in situ origin of lava and eruption of volcanics are generally described as Rajahmundri volcanics on land and Razole formation in subsurface and offshore region (Misra et al. 2010). Correlation of large subcrop extensions of Rajahmundry Traps with the Deccan volcanism has been found (Reddy et al. 2002) that indicates Rajahmundry Traps were erupted through a volcanic vent or a zone of basement faults along the present East Coast, possibly in and around Razole. This further implies that the mantle plume responsible for the Deccan volcanism had similar or bigger dimensions than that of the Indian Peninsular Shield. Occurrence of this basalt flow had been reported to be of Cretaceous age in the offshore region KG Basin and in the Bay of Bengal (Gupta 2019). It is interesting to add that occurrence of hydrocarbons, although occasional, is found within Razole Formation itself, indicating its positive reservoir characteristics. In Krishna-Godavari (KG) Basin, Tirupati, Raghavpuram and Golapalli Formations of Mesozoic age are hydrocarbon producers, apart from hydrocarbon producing sediments of Tertiary age.

The Cauvery basin came into existence due to fragmentation of the Gondwana land at the time when India-Sri Lanka landmass system drifted away from Antarctica/Australia continental plate during Late Jurassic/Early Cretaceous. The initial rifting caused the formation of NE-SW horst-graben features. Subsequent drifting and rotation caused the development of NW–SE cross faults. In southern Mannar area, the Cretaceous volcanics are encountered in drill hole data. However, so far, these volcanics are not reported from neither land nor offshore parts of Cauvery and Palar areas (Misra et al. 2010). In Cauvery Basin, Nannilam, Bhuvanagiri and Andimadam Formations of Mesozoic age are hydrocarbon producers. In Cauvery basin, Tertiary sediments are also hydrocarbon producers.

In Mahanadi basin, Cretaceous volcanics are found in offshore area. However, no volcanics are seen in land areas, except one doleritic dyke near Naraj. The Mahanadi Basin, situated on the East Coast of India and evolved as an outcome of rifting and break up of Gondwana Land, is a basin with significant unexplored hydrocarbon potential. Significant gas finds have been reported from Miocene and Pliocene sediments in Mahanadi basin, but all these have turned out to be non-economical.

Hydrocarbon potential of the rift basins

Both western and eastern margin basins of India peninsula are rifted basins. Rifting has a tendency to produce restricted as well as open marine settings, both of which are favourable to accumulation of source rocks and so, hydrocarbon potential of Rift Basin is considered to be high. Conversion of organic matter in the source rocks to hydrocarbons is helped by relatively high geothermal gradients. Clastic systems from the mature shield terranes are produced by subsequent rotational faulting and marginal uplifting which form shoal areas making way for development of potential reservoirs such as porous reef build-ups and dolomitized limestones. Structural traps are produced by rotational faulting of these units. Onlapping basinal mudrocks or evaporites may be seal for these units when thermal subsidence of the rift occurs later on.

Tectonic set up

In order to understand the pattern of hydrocarbon habitat along Indian offshore, it is imperative to understand the tectonic setting with reference to Plate Tectonics. Indian Rifting of West India from Madagascar took place about 90 million years ago (Mid Cretaceous) (Lal et al. 2009) which led to start a series of stages of volcanism, one after one and also rifting, which shaped the present-day structure of the Western offshore of India. After Madagascar separated from India during Late Cretaceous, Seychelles remained attached with India. Hence development of Mesozoic basin (Kerala Basin) remained restricted to the southern part of western continental margin of India (WCMI) (Fig. 8) (Siawal et al. 2018). The absence of Upper Jurassic sediments in Kerala basin, though present over Sri Lanka (Tabbowa Group) and Cauvery basin (Sivaganga beds), is suggestive of late opening for Kerala Basin. During Late Cretaceous period, an arm of the rift extended between Seychelles and Indian landmass. It remained active until outburst of Deccan lava. However, its south-eastward extent of this arm remained speculative. Deccan lava outpour was accompanied with complete plate reorganization as the spreading ridge jumped southward between Seychelles and Laxmi ridge with the formation of proto Carlsberg Ridge. Out pour of lava also resulted in detachment of Saurashtra from mainland with initiation of Cambay Rift. With this Saurashtra moved north-westward with clockwise rotation resulting in acute compression in the areas of Kutch. This also explains present compressive environment in Kutch area (Biswas 1987) (Fig. 8).

Fig. 8
figure 8

Tectonic evolution summary of WCMI (modified from Siawal et al. 2019)

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Extensive magmatism during the Late Cretaceous-Paleocene generated KT Flood Basalts, blanketed the major parts of western continental margins of India (Nathaniel 2013). In peninsular geological setting of India, the Phanerozoic sedimentary sequences are inter-layered with Cretaceous basaltic flows (Misra et al. 2008). The thickness pattern of Cretaceous volcanics varies greatly in different areas (> 3000 m). Deccan Trap flood basalts covered much of western India and extended offshore to the Seychelles and beyond.

Many classic characteristics of a volcanic-type margin are shown in the WCMI (Western Continental Margin India). Along the margin, a number of volcanic facies are identified and mapped which comprises seaward-dipping reflectors, clinoform packages interpreted as lava and volcaniclastic delta systems, sub-aerial seamounts and thick, seismically layered sequences interpreted as volcanically derived sediment deposited in both marine and fluvial environments. Major variation in the overall thickness and style of volcanism across the margin is shown in the results that may be related to variability under the influence of Deccan Plume/Reunion Plume, in addition to localization along structural features, inherited from older tectonic events. Hydrocarbon potential, distribution and preservation of the pre-rift sequence across the margin are impacted by rapid lateral variation in the thickness of extrusive volcanism (Corfield et al. 2010).

A hot spot idea has also been advocated along the west coast of India. This is believed to have moved from north to south and caused Deccan Volcanism in early phase and later moved over to ocean to form Lakshadweep chain of islands to Chargos islands to cross over to Carlsberg ridge right up at Reunion Island in southern part of Indian Ocean. The volcanic units encountered in Kerala-Konkan basin is believed to be due to this (Misra et al. 2010).

Along the east coast of India, the rift related crustal extension between India and Sri Lanka, which was result of Africa-Antarctica rifting and development of Natal basin, marked the evolution of East Coast. An arm of this rift resulted in initial extension in the Cauvery Basin and failed. Later, propagation of the Indian-West Australia rift continued further in SW direction, causing initiation of KG and Mahanadi Basin (Siawal and Kaul 2019). As Nayudupeta high was acting as a pivot, this extension was an oblique one. Transpression was developed along India Sri Lanka and Antarctica junction due to the oblique extension followed by asymmetrical sea floor spreading. It resulted in a NNW-SSE trending transcurrent fault along which movement of Antarctica towards south occurred. Subsequently, evolution of entire east coast took place through a more or less post-rift stage.

The offshore basins of Peninsular India were developed in response to the tectonics along the rifts and grabens. This extensional tectonics was the main basin forming tectonics in offshore regions. In offshore extensions, different basins particularly Krishna-Godavari along Pranhita–Godavari graben, Mahanadi basin along Mahanadi graben and Bengal basin along extension of Damodar valley and Narmada-Tapti tectonic zone have developed (Misra et al. 2010).

Methodology

Some high dipping strong seismic reflectors seen below the basalt cannot miss the eyes of an interpreter. The presence of such conspicuous seismic signatures at such deep level is bound to generate interest, like El Saoud et al. (2022), Halleland et al. (2007), Ismail (2020, 2021) utilized seismic attributes to identify gas bearing zones. Seismic attributes enhance information that might be more subtle in a traditional seismic image (El Saoud et al. 2022; Halleland et al. 2007; Ismail 2020, 2021).

Regional seismic lines along east coast of India were scouted in detail to look for the SDRs particularly. The SDRs were observed clearly that are illustrated here.

In east coast, there were 18 dip lines (shown in Fig. 9 with identity from A to R) and 1 cross line (which was divided in 3 parts and shown on base map with identity as CL1, CL2, CL3). First, basement was recognized and correlated on all the lines. The SDRs along east coast on the regional seismic lines were mapped (Figs. 10, 11, 12 and 14). Seamounts are visible in Fig. 13. Basis of recognizing SDRs on seismic section was: (a) high amplitude, (b) dip > 15°. Based on SDRs mapping, attempt was made to draw the continental ocean boundary (COB) (Fig. 9).

Fig. 9
figure 9

Seismic location map: east coast offshore basins

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Fig. 10
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Line A2 showing seaward dipping reflectors

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Fig. 11
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Line A2 (zoomed part) showing seaward dipping reflectors

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Fig. 12
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Line I showing seaward dipping reflectors

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Fig. 13
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Line F showing faults and seamounts

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Results and discussion

Tectonic history of East Coast of India is complex, involving rifting, diachronous drifting, igneous activity and substantial post-rift deformation. The formation of passive margin since Late Cretaceous has setup the East Coast of India as an ideal location for development of SDRs.

Seaward dipping reflectors are high amplitude seismic reflectors with a dip > 15°, identified on continental plates. They (SDRs) represent flood basalts, which are rapidly extruded during either rifting or initially subaerial sea floor spreading. They form the main characteristic features of volcanic rifted margin (seamounts in Fig. 13). In the present study, SDRs are identified based on their property of high amplitude and dip angle, similar to any other seismic attribute which is based on variation in amplitude, frequency or attenuation, etc. El-Saoud et al. (2022) and Ismail et al. (2020, 2021) applied seismic attribute to identify gas bearing zones, and Halleland et al. (2007) used seismic attributes to map deep water reservoir successfully because of pattern and variation in behaviour of seismic waves upon interaction of different physical property. These attributes or properties, if identified correctly and utilized judiciously, could be applied easily to locate or map any feature associated with structure or stratigraphy (Ismail et al. 2020).

In this study, SDRs are identified along east coast of India on regional seismic lines, Figs. 10 and 12 show the presence of SDRs identified between 6 and 6.5 s (TWT) in line A2 and I, respectively. Subsequently, basement was recognized and correlated on all lines and thereafter, COB was marked (Fig. 9). Seaward dipping reflectors are characterized by reflections that become more convex-upward in the downdip direction and terminate against a sub-horizontal base (Fig. 14). The sequences of SDRs were first recognized along the North Atlantic margin. The petroleum world has been watching these features with immense curiosity. For the first time perhaps, such comprehensive work for SDRs has been done for Indian offshore. Seismic interpretation of east coast seismic lines with focus on capturing highly dipping strong amplitudes resulted in identifications of interesting seaward dipping reflectors. High dipping strong reflectors were seen in the west coast also, but they were observed on very few lines and so, any significant interpretation (mapping, etc.) could not be done.

Fig. 14
figure 14

Line B showing seaward dipping reflectors

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When such interesting seismic events are further analysed, its relationship with basement becomes clear. Keeping in view the relevance of SDRs with reference to geological understanding about volcanism and consequently the hydrocarbon prospectivity, the present work assumes high importance. They (SDRs) form the main characteristic features of volcanic rifted margin. Seaward dipping reflectors, composed of impervious, non-porous material, are expected to provide formidable seal in the migration pathways of hydrocarbon. They are formed due to lava extrusion and lava has very high temperature. So, SDRs provide suitable temperature for hydrocarbon maturation, also known as Secondary induced maturity. The presence of SDRs is useful while studying the petroleum system of an area.

The presence of SDRs at deeper levels may prove to be quite useful for the scientists dedicated for the study of volcanic margins and future explorationists searching for hydrocarbon accumulations in new areas.

Although SDRs are easy to recognize on seismic sections along regional seismic lines in Indian east coast as they have high amplitudes and dipping events, yet, as SDRs are found at deeper depth, it is recommended that long offset broadband seismic data, using advanced technology, should be acquired to capture these deep events clearly. Ismail et al. (2020) also stated that attribute might not help to identify different targets correctly if the seismic data is noisy or lacks quality. Further, detailing of such interesting prospects at deeper levels should be carried out through 3D seismic. Keeping in view the recent success in sub-basalt sediments in west coast and fractured basalt/basement reservoirs in Kutch and Cambay (Padra, Ingoli), sub-basalt and basalt exploration should be intensified.

Conclusions

  1. a.

    Seaward dipping reflectors are important for (i) understanding basement architecture, (ii) locating COB and (iii) assessment of hydrocarbon prospectivity at deeper levels.

  2. b.

    The SDRs are useful to study petroleum system as it provides (i) formidable seal and (ii) secondary induced maturity.

  3. c.

    Identification of COB is critical for focused exploratory efforts considering that sediments over continental crust are important for hydrocarbon exploration.

  4. d.

    The SDRs are observed along coasts of Peninsular India. Mapping of COB through identification of SDRs by interpreting long offset seismic profiles is the significant finding of the present work.

  5. e.

    Better imaging of SDRs using new technology like Broad Band seismic would provide significant lead for sub basalt exploration in western offshore of India.