Balance de materiales para yacimientos de petróleo subsaturado 37

The study area is located in the Indian part of the Bengal Basin and is situated in the Himalayan foreland at the junction of the Indian, Eurasian and Burmese Plates. On the west and northwest, the Bengal Basin is bounded by the Rajmahal Hills, and from northeast to east, it is flanked by the Shillong Plateau, the Tripura Hills and the Indo-Burmese Fold Belt (Figure 3.1). The southern delta and the adjacent floodplain are bounded by the Bay of Bengal, which forms the world’s largest submarine fan (MUKHERJEE et al. 2009). Today’s basin is the largest fluvio-deltaic sedimentary system on earth, occupying an area of about 200,000 km², where approximately 120 million people live in three federal states of India (Bihar, West Bengal, Assam) and the entire state of Bangladesh (ALAM et al. 2003) (Figure 3.2).

Present geomorphologic characteristics divide the Bengal Basin into Holocene lowland and Pleistocene uplands. Pleistocene units include four terraces, which are interpreted as remains of former floodplains. Two of them flank the basin in front of the Rajmahal Hills and the Tripura Hills, while the Barind and Madhupur terraces are situated inside the basin (Figure 3.1). Although outcropping Pleistocene formations are similar to Holocene units, they are characterised by a reddish brown colour, are mottled and relatively dry, have low organic contents and contain nodules of secondary iron phases and carbonates (BGS & DPHE 2001). The Holocene lowland can be subdivided into the Holocene alluvial floodplain (fluviatile, freshwater dominated floodplains) and the intergradient delta (the saline lower delta in the south), which are often aggregated as the Ganges-Brahmaputra-Meghna flood and delta plain, or simply as the Bengal Delta Plain, henceforth referred to as BDP (MUKHERJEE et al. 2009).

Four different Holocene geomorphologic main units can be distinguished in the Bengal Basin:

 The alluvial fans located at the foothills of the Himalaya, which are mainly composed of fresh coarse sand and gravel;

 The Tippera Surface near the Tripura Hills;

 The Sylhet Basin located in the northeast;

 The central floodplain and the southern delta of the BDP that cover together an area of about 10⁵ km² and represent the central

formation (MUKHERJEE et al. 2009).

In the central floodplain and the southern delta, numerous sub-deltas of the three rivers (Ganges, Brahmaputra and Meghna) overlap and create a dense system of adjacent alluvial flood- plains. The elevation of this areal reaches from 15-20 m above the sea level (asl) in the northwest, to 1-2 m asl near the southern shoreline (Figure 3.2). Meanwhile inactive Quaternary faults had changed the flow of important tributaries for several times, and created numerous interconnected abandoned tributary channels that are distributed among the entire floodplain and delta complex (ACHARYYA et al. 2000).

Figure 3.1: Pseudo colour mosaic picture of the Bengal Basin created with NASA World Wind (Landsat 7 image). Coloration emphasizes the presence of water (blue), vegetation (green) and bare soil (purple). The picture is completed by landform features, location of the investigation area and principal regional flow directions of groundwater (after MICHEAL & VOSS 2009a,b and MUKHERJEE et al. 2007, 2009). Underlying pseudo colour image provided by MDA Federal Information Systems for NASA World Wind.

Figure 3.2: Satellite imaginary (Landsat 7) and transect elevation profiles (super-elevated) of the Bengal Basin. Image created with NASA World Wind.

Resolution of satellite image is 15 m. Elevation profiles generated with NASA World Wind Terrain Profiler, based on the SRTM-30 PLUS digital elevation model (resolution: 3 arc-seconds).

3.2 GEOLOGY

The geological history of the Bengal Basin is relatively complex according to a large variety of geological processes involved in the basin formation (ALAM et al. 2003). Evolution of the proto-basin began with the break-up of Gondwanaland during the mid to late Mesozoic, followed by massive basalt extrusions at the Rajmahal and Shillong areas during the late Jurassic to the early Cretaceous (LINDSAY et al. 1991). Slow subsidence of the Bengal Shelf caused marine transgressions into the south-eastern part of the proto-basin and repeated sequences of submergence and transgression caused deposition of the first deltaic sediments. Increased tectonic activity generated a massive transgression in the middle Eocene, which covered the whole basin and formed the Sylhet limestone formation (see Table 3.1). Due to varying tectonic movement rates, evolution of the eastern and western part differed. A permanent change in the local sedimentation environment arose from the collision of the Indian and Eurasian Plates that induced uplift of the basin’s stable shelf and finally formation of the Himalayan Mountains. While the eastern part remained under marine influence, the uplift changed the sedimentation environment in the rest of the basin during the middle to late Eocene, super-imposing preliminary marine carbonatic-clastic sediments by fluvio-clastic sediments (LINDSAY et al. 1991).

Modern deltaic basin evolution was initiated by intensive tectonic activities along the Dauki fault zone (ranging from east to west in front of the Shillong Plateau) and the Naga thrust zone (parallel to the Indo-Burmese Fold Belt, see Figure 3.1) during the middle and late Miocene (LINDSAY et al. 1991). A basin-wide regression followed and changed the previously marine-estuarine environment in the eastern part into a fluvial-tidal dominated system.

Table 3.1: Geology of the western part of the Bengal Basin where West Bengal is located (after ALAM et al. 2003, MUKHERJEE et al. 2009, SARKA et al. 2009).

Age

Group Formation (thickness) Lithology

Alluvium (~30-60 m) Silt, clay, sand, gravel

Barind Barind (~200 m) Brown clay, silty clay, silty sand

Bhagirathi

Debagram / Ranaghat (~1200 m) Sandstone, siltstone, shale

Pandua / Matla (~400 m) Sandstone, siltstone, silty mudstone Memari / Burdwan (~150 m) Sandy mudstone

Jainitia

Kopili (>100 m) Shale, mudstone, sandstone

Sylhet Limestone (~300 m) Fossiliferous limestone, interbedded sandstone Upper Jalangi (~230 m) Sandstone, sandy mudstone Lower Jalangi (~400 m) Sandstone

Rajmahal

Ghatal (~150 m) Sandstone, limestone, shale Bolpur (100 m) Sandstone, mudstone Rajmahal (~250 m) Basalt, andesite, shale Gondwana

Beginning in the late Quaternary, basin development was mainly controlled by tectonic activities. This differs from traditional models of delta formation, which base on sea level fluctuations as controlling factor (GOODBRED et al. 2003). Tectonic movement mainly influenced the north-west of the basin close to the stable shelf, causing alternating fluvio-dynamic processes. In contrast, sedimentation at the coast was primarily affected by changes in the eustatic sea level during the ice ages, where mangrove forests and swamps followed the active coastline. Last chapter in the Modern-Delta evolution began at the onset of the last glacial maximum

(LGM) at around 20 ka BP, when the sea level was approximately 120 m lower than today. To this time, the lowest units of the delta sediments were exposed to erosion and weathering. Channel erosion incised broad valleys, while soils of several metres developed on top of the interfluves

(ACHARYYA et al. 2000, ISLAM & TOOLEY 1999, LAMBECK et al. 2002).

A period of highest delta progradation followed between 7 to 9 ka BP, when changes in the regional climate intensified the seasonal monsoon precipitation. Hence, fluviatile transport and sedimentation rate of the three rivers Ganges, Brahmaputra and Meghna reached a maximum of approximately 2.4 x 10⁹ t a^-1 (GOODBRED et al. 2003). Due to sea-level rise at the early Holocene, the shoreline shifted again gradually in northern direction, where a huge estuary formed. Mangrove forests followed the coastline and organic-rich sediment deposit developed. During this time, fine sediments were deposited up to the northern part of the basin that filled the incised Pleistocene channels and buried the older interfluves (MC ARTHUR et al. 2008, WRIGHT & MARRIOTT 1993). After a rapid decrease, the annual sediment discharge stagnated at around 5 ka BP, but a constant sedimentation flux of about 1.0 x 10⁹ t a^-1 allows the active delta to gradually progradate into the Bay of Bengal to the present day (GOODBRED et al. 2003). As a result, a more fluviatile influenced floodplain environment developed in the hinterland, where thick sediment packages deposited (ALAM et al. 2003). Today, Holocene sediments can be divided into a clay- and silt-rich top, and an underlying sandy part with locally increased OM contents, especially in the southern dellta and the central floodplain (located in the western part of the Bengal Basin).

3.3 CLIMATE

The tropical monsoon climate of the Bengal Basin is mainly controlled by the southeast monsoon regime that transports moist air masses inland from the Bay of Bengal from early June to mid of October (BGS & DPHE 2001).

Average air temperatures range from 10°C during winter to 35°C in summer, and annual rainfall varies between <1,200 mm in the flat western and central basin and >1,600 mm towards the Himalaya Foreland (MUKHERJEE et al. 2007). A total of 82.2 % of the annual average rainfall (AAR) falls during the monsoon season, while the dry season can be subdivided into the pre-monsoon season (between January and May with 16.2 % of the AAR) and the post-monsoon season (November to December with 1.57 % of the AAR) (MUKHERJEE et al. 2007). Extensive irrigation is supposed to exceed the absolute recharge from precipitation in parts that are under intensive agricultural use (MUKHERJEE et al. 2007). When heavy monsoon rains meet discharge water from the Himalayan snowmelt, heavy flood events occur in the flat areas of Bangladesh.