The investigation of fluid presence in sedimentary basins holds immense significance among geoscientists and geotechnicians. The existence of fluids within sediments controls their shear strength and can possess substantial commercial value to hydrocarbon industries (Andresen et al., 2012). Reflection seismology is often used to decipher fluid accumulations (Carlson et al., 1985) as this technique yields unparallel insights into the distribution of fluid reserves and their association with deeper underlying faults. Studies reported evidence of fluid migration from deeper to shallower depths through permeable pathways such as faults, fractures, buried channels, or igneous intrusions (Cartwright et al., 2007; Davies et al., 2012; Schofield et al., 2020). Seismic studies reported evidence of fluid flow through polygonally faulted strata in the Canterbury Basin of New Zealand (Holford et al., 2017). The seismic imaging of faults and fractures, which control the fluid migration pathways from overpressured compartments, has been further studied in the Yinggehai Basin of the South China Sea (Xie et al., 2003). Additionally, various geophysical inversion techniques have been employed for the characterization and quantification of gas hydrates and free gas in Indian offshore basins (Sain and Gupta, 2012; Dewangan et al., 2011), to model fluid/gas expulsion signatures (Dewangan et al., 2010) and to study sediment loading and transportation mechanisms (Biksham et al., 1988). One of the most well studied Indian basin is the Krishna-Godavari basin due to ongoing gravity-driven shale tectonism / neotectonism (Dewangan et al., 2010) and its effect on the slope instability associated with huge sediment influx in the basin (Ramprasad et al., 2011). Evidence of fluids is also reported from Mahanadi (Sain and Gupta, 2012) and Kerala Konkan basins (Mishra et al., 2021), but, to date, very little studies are carried out in the Andaman offshore to characterize its basinal deposits and their fluid reserves and migratory signatures through associated fault systems.
Polygonal fault systems, which are characterized by normal faulting (Goulty, 2002; Frankowicz and McClay, 2010) with relatively small displacements of 10 to 100 m, are reported to play important roles in fluid migration (Cartwright & Lonergan 1996). These fault systems can be regular or irregular shapes depending on the specific geological conditions and the nature of the stress field (Goulty, 2002) and are frequently observed in fine-grained sedimentary sequences and have been documented in numerous basins worldwide (Berndt et al., 2012). Various genetic mechanisms have been proposed to explain their formation, including gravity collapse, density inversion, syneresis, and compactional loading (Cartwright and Dewhurst, 1998). Among these mechanisms, syneresis is considered the most likely mode (Cartwright et al., 2003) that occurs during the burial and compaction of sedimentary layers, elevating fluid pressures within the porous rock and subsequently expelling fluids from the sediment matrix.
In this study, the results derived from a single 2D reflection marine seismic profile extracted from a 3D survey carried out by ONGC between East Narcondam offshore and the northern edge of Alcock Rise are presented to investigate the sedimentary deposits and associated fluid contents and its migratory pathways at Narcondam offshore. The conventional semblance-based processing technique is employed to obtain the migrated image followed by AVA analysis to validate the presence of fluid charged layers. The bathymetric data enables us to understand the geomorphic features of the basin, volcanic structures, and major and minor fault systems including polygonal fault setting. The outcome of the study contributes to understanding the development of fluid saturated deposits within the basin.
Background Tectonics
The Andaman-Sumatra subduction zone associates convergence between the Indo-Australian plate and the Eurasian plate and exhibits varying rates of convergence, obliquity, sedimentary thickness, and age, along with distinct geomorphic features along the Sunda arc (Curray, 1989; Sieh and Natawidjaja, 2000). The angle of obliquity of the subducting plate increases towards the Andaman and Nicobar Islands and becomes parallel to the trench further north (Rao and Kumar, 1999; Nielsen et al., 2004). In contrast, the rate of convergence decreases from 40-50 mm/yr off Sumatra to 20 mm/yr near the Andaman-Nicobar region (McCaffrey, 2009). The oblique subduction leads to the partitioning of slip at the plate interface into trench-normal and trench-parallel components, with the latter resulting in the formation of the Andaman Sea Transform Fault (ASTF), Andaman Sea Spreading Center (ASSC), and Andaman Nicobar Fault (ANF) (Fitch, 1972). The normal component leads to the subduction of the Indian plate and thrust faulting in the offshore Sumatra-Andaman forearc. The Andaman Sea Spreading Center (ASSC), which is a mid-ocean ridge marking the boundary between the Indian Plate and the Burma Microplate, is responsible for the extension and spreading of the oceanic crust in the region. The Andaman Sea exhibits several significant morphological features, including Barren and Narcondam islands, as well as the Alcock and Sewell rises. Narcondam Island is a dormant volcano and hosts andesitic lava dating back to ~0.55-0.56 million years ago (Streck et al., 2011).
Study area
The study area encompasses an offshore basin bordered by Narcondam volcanic Island (Pal et al., 2007) to the west, several seamounts to the east, Myanmar to the north, and the Alcock Rise to the south (Figure 1). The Alcock Rise is a volcanic ridge (Morley and Alvey, 2014). The volcanic arc in the Andaman Sea is primarily associated with Narcondam Island and Barren Island. Barren Island is located ~ 135 km northeast of Port Blair, the capital city of the Andaman and Nicobar Islands, at coordinates around 12.278°N latitude and 93.858°E longitude. Narcondam Island is situated northeast of Barren Island, with a diameter of 2 km and a height of 710 m above mean sea level (Rodolfo et al., 1969). The seismic profile extends between East Narcondam and North Alcock Rise, where the water depths in the basin range from ~1.2-1.6 km along Line 1.
Data
A 3D seismic reflection dataset was acquired over the basin, situated adjacent to the Alcock Rise and ~30 km eastward from Narcondam Island. The data collection involved utilizing a 6 km-long streamer with 478 channels, employing a sampling interval of 2 ms. Receiver groups were positioned at intervals of 12.5 m, while shots were fired at 25 m intervals. The seismic profile had a length of ~ 30 km, with a recording length of 10,238 ms two-way travel time. The near-offset was set to ~ 273 m, and the far-offset to ~ 6,273 m. The recorded signal bandwidth ranged from 2 to 100 Hz, with a dominant frequency varying between 35 and 45 Hz. Seismic 2D profiles with a record length of 4.5 s two-way travel time are extracted from the original 3D volume for the processing and only results from Line 1 are presented in this study. In addition, the GEBCO 2022 bathymetry maps with 15 arc-second intervals are employed to gain insights into the morphotectonics of the offshore Narcondam region and to establish correlations with the findings from the seismic datasets.