Elsevier

Marine and Petroleum Geology

Volume 27, Issue 9, October 2010, Pages 2051-2061
Marine and Petroleum Geology

Pre- and post-collisional depositional history in the upper and middle Bengal fan and evaluation of deepwater reservoir potential along the northeast Continental Margin of India

https://doi.org/10.1016/j.marpetgeo.2010.04.007Get rights and content

Abstract

The Bengal fan is the largest submarine fan in the world that has formed as a result of high sediment transport from the Himalaya by the Ganga–Brahmaputra river system. The Himalaya was formed as a result of the collision between the Eurasian and Indian plates. The initiation of this collision known as “soft” collision occurred around 59 Ma, whereas, the major collision, known as “hard” collision took place around 15 Ma ago. Prior to the collision, sediments into the Bay of Bengal were derived from the northwest by relatively smaller river system like Mahanadi–Godavari. The switching of river systems with time was not distinct but gradational. In the post- collision period, the sediment input from the NW was masked in most instances because of rapid sediment supply from the Himalaya to the north. Pre-collisional sediment dispersal pattern from the NW was largely affected by pre-existing basement high known as 85°E Ridge; this ridge was submerged during the post-collisional period. Post-collisional sediments are commonly referred to as the Bengal fan sediments and show huge accumulation along the shelf and beyond. High resolution 2D seismic data acquired along a corridor covering the upper, middle and distal parts of the present day active Bengal fan system indicates that the fan has prograded southward with time because of continuously increasing sediment supply and has, therefore, masked the effect of eustacy. The present day geometry of the fan shows a single active canyon and an associated single active fan. The active channel shows typical meandering pattern that shifts laterally with time. The seismic facies analysis indicates that both the pre- and post-collision basin has significant hydrocarbon potential. The thermogenic model is best suited for modeling source rock maturity in the pre-collision basin whereas both biogenic and thermogenic models best explain source rock maturity in the post-collision, younger Bengal fan. The wedge out against the 85°E Ridge is considered to be one of the important play types for hydrocarbon exploration in the deeper part of the basin. On the other hand, the channel levee complexes and frontal splay/basin floor fan are the possible target areas for petroleum exploration in relatively younger Bengal fan deposits.

Introduction

Submarine fan systems form the largest deep-water sediment bodies near continental margins and the depositional history of these sediments contains useful record of past land and marine climate, sea level changes, local and regional tectonic activity. Because of their huge sediment repository, very often, submarine fans are known to be potential areas of hydrocarbon exploration (Lopez, 2001). On the basis of tectonic setting, Shanmugam and Moiola (1988) classified submarine fans into four types: (i) immature passive-margin fans (North Sea type); (ii) mature passive-margin fans (Atlantic type); (iii) active-margin fans (Pacific type); and (iv) mixed-setting fans (Bay of Bengal). Based on the sediment supply, they can also be broadly divided into four types such as the mud rich (Amazon fan, Flood et al., 1991), sand rich (Sierra Nevada, California, Busby-Spera, 1985), mud-sand rich (California deep sea basin fan, Normark, 1970) and gravel rich fans (Upper Jurassic system in North Sea, Hurst et al., 2005)

The Bengal Fan is bordered by the Indian continental shelf in the west, continental shelf of Bangladesh in the north and Sunda trench in the east (Fig. 1). It is one of the largest submarine fans in the world covering the entire Bay of Bengal from 20°N in 1400 m water depth to 7°S at 5000 m water depth over a N–S length of 3000 km (Curray and Moore, 1971). Though some of the major rivers from eastern India feed into the Bay of Bengal from the west, sediments of the fan are largely contributed by the erosion of the Himalayas and transported by the Ganga–Brahmaputra River system (Curray and Moore, 1971, Curray et al., 2003). Although soft collision (Curray et al., 2003) between India and Asia can be considered as the time of the initiation of the fan, the major sediment supply started after the hard collision during Mid-Miocene which caused continuous progradation of the Bengal fan southward. Detailed seismic reflection and refraction studies as well as echo sounding investigations in the Bengal Fan by several previous workers gave rise to valuable information on the stratigraphic development, sedimentation history of the fan as a whole (Curray and Moore, 1974, Curray et al., 1982, Curray et al., 2003) and evolution of channel–levee system (Curray et al., 2003, Schwenk et al., 2003, Schwenk et al., 2005). The time of initiation of the fan in the northern part is considered to be Early Eocene (Curray, 1994) and the regional unconformity at Early Eocene (55 Ma) is considered to differentiate between pre- and post-collisional deposition. The timing of initiation of fan becomes younger towards south pointing to progressive progradation of the Bengal fan. However, an Early Miocene age for initiation of the fan has been observed in southern part of the fan (Cochran, 1990). This indicates that the possibility of deposition of fan sediments in Pre-Miocene period in the northern proximal part of the fan cannot be ruled out.

This paper presents results obtained from the analysis of high resolution seismic data acquired along a corridor covering the upper, middle and distal parts of the present day active Bengal fan system (Fig. 1) to understand the development of the depositional fairway in pre- to post-Bengal fan deposition. In addition, we characterize different types of sediment flows for constructing the paleogeography during Mio-Pliocene time. High-resolution multibeam bathymetry data has been used to reinterpret parts of the present-day active channel system. Finally, the results of this study have been integrated to explore the development of possible petroleum system during pre- and post-collision history of the Bengal fan.

Section snippets

Regional geologic setting

The Bengal fan covers the entire Bay of Bengal from 20°N to 7°S over a length of 3000 km. The width of the fan varies between 1430 km at 15°N and 830 km at 6°N with an area of approximately 3 × 106 km2 excluding the area of Nicobar fan (Curray et al., 2003). Towards north, the deepest sedimentary section of over 22 km is observed below the Bangladesh shelf and the sediment thickness progressively decreases towards south (Curray, 1994).

Data

Hydrocarbon exploration during the last decade along the eastern continental margin of India in both shallow and deep water areas has resulted in the generation of a large volume of high-quality 2D and 3D seismic data. In the present study, some of the processed seismic sections covering the active channel–levee system in the upper-to-lower fan sub-divisions of the Bengal fan have been utilized. Available high-density seismic data is further used to prepare isochronopach maps in order to

Results and discussion

The interpretation presented in this section is made mainly based on available 2-D and 3-D seismic data integrated with regional tectono-stratigraphic information.

Conclusions

From the above analysis, the following conclusions can be drawn.

  • Pre-collisional depositional pattern was controlled by emplacement of 85°E ridge that divided the basin into mini sub-basins. The restricted environment between basement highs provides good opportunity to develop hydrocarbon source rock facies.

  • Post-collisional depositional pattern in the study area is interplay of different system from N–S and NW–SE. While, the N–S trending Bengal fan trend became more prominent after post-Mid

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