Assessing the dynamics of Bhasan Char is very crucial, as the Government of Bangladesh (GoB) has recently selected the island as the accommodation of the FDMN. This article critically evaluates the spatiotemporal morphological variations due to erosion, accretion, and subsurface deformation of the island through multi-temporal geospatial and geophysical data analysis, groundwater quality-quantity, and also determines the nature and rate of changes from 2003 to 2020. This is the first study in this island on which multi-temporal Landsat Satellite Imagery and seismic data have been used with geospatial techniques with Digital Shoreline Analysis System (DSAS) and petrel platform, respectively. The analysis of satellite images suggests that the island first appeared in 2003 in the Bay of Bengal, then progressively evolved to the present stable condition. Significant changes have taken place in the morphological and geographical conditions of the island since its inception. Since 2012, the island has been constantly accreted by insignificant erosion. It receives tidally influenced fluvial sediments from the Ganges-Brahmaputra-Meghna (GBM) river system and the sedimentary accretion, in this case, is higher than the erosion due to relatively weaker wave action and longshore currents. It has gained approximately 68 km² area, mostly in the northern part and because of erosion in the south. Although the migration of the Bhasan Char was ubiquitous during 2003–2012, it has been concentrated in a small area to the east since 2018. The net shoreline movements (NSM) suggest that the length of the shoreline enlarged significantly by around 39 km in 2020 from its first appearance. Seismic and GPS data clearly indicate that the island is located on the crest of a slowly uplifting low-amplitude anticline, which may result in a stable landform around the island. Based on the analysis of historical data, it has been assessed that the current configuration of Bhasan Char would not be severely affected by 10–15-foot-high cyclone. Therefore, FDMN rehabilitation here might be safer that would be a good example for future geo-environmental assessment for any areas around the world for rehabilitation of human in remote and vulnerable island. The findings of this research will facilitate the government’s decision to rehabilitate FDMN refugees to the island and also contribute to future research in this area.

Unlike other structures in the vicinity of the Chittagong Tripura Fold Belt, the Changotaung anticline is one of Bangladesh's least explored structures. An attempt has been made for the first time to understand and document sedimentary deposits, environments, structure, and tectonic activity based on the high-resolution outcrop and reconnaissance study with the knowledge of broad-brush geology. We found that Changotaung is a symmetrical box-folded structure with an extensive western flank where the amount of dip varies between 11° and 45°. The exposed Cenozoic succession was categorized into three separate sedimentary sequences and correlated with the conventional stratigraphic unit. A first-order simple Markovian approach was presented for the exposed litho-section in an effort to illustrate vertical facies variations in the Upper Surma group. We quantified that heterolithic bed mostly overlies both trough cross-bedding ( $${P}_{ij}U$$P ij U = 0.706) and parallel laminated bed ( $${P}_{ij}U=0.955)$$P ij U = 0.955 ) according to the facies transition probability matrix. According to the results of the stationary distribution, there is a 40% chance of coming across heterolithic beds within the Upper Surma group during any given event that is completely random whereas trough cross-bedding, parallel laminated bed, cumulative sandstone facies, and cumulative shale facies shows around 10.8%, 15.2%, 20.6%, 13.4% probability. We hypothesized, based on the interpretive facies analysis, that the Chittagong Tripura fold belt region's Upper Surma Group underwent three interrelated depositional settings (wave-dominated shallow marine, tide-dominated shallow marine, and fluvio-deltaic distributary).

This research focuses on studying the Surma Group sediments with an integrated approach of using wireline logs and 2D seismic data from Sylhet structure and outcrop analogy from the Zaflong-Tamabil-Jaintiapur area to explain the geological evolution in the context of sequence stratigraphic framework and its implications to identify the hidden potential of hydrocarbon traps that remains overlooked. The 'SYL-07' well logs divided the encountered sedimentary strata by sequence boundaries (SB) and flooding surfaces (FS) into an upper third-order Lowstand System Tract (3_LST), followed by the third-order Highstand System Tract (3_HST), and underlying third-order Transgressive System Tract (3_TST) which correspond to the Tipam, Boka Bil, and Bhuban formations, respectively. The 3_HST has further been subdivided into fourth-order sequences. Well-to-seismic ties helped in identifying the sequence boundaries in the regional context. Three new prospects in the nose and flank area of the Sylhet structure have been detected based on seismic attributes and log responses. Seismic sweetness attribute anomalies confirmed the existence of these prospects. Rigorous fieldworks provided nineteen facies and seven sets of sedimentary facies associations and helped to establish the sedimentological stacking patterns and corresponding depositional environments. About 2250m lithocolumn constructed from field data revealed four third-order sequence stratigraphic system tracts roughly corresponding to Renji (3_LST), Bhuban (3_TST), Boka Bil (3_HST), and Tipam (3_LST) formations in the bottom-up order, while the obtained facies associations aid in identifying four fourth-order sequences including fourth-order system tracts in the 3_HST. The lithologic column corresponds well with the seismic sections and validates the seismically interpreted third-order system tracts and fourth-order sequences. However, the high-frequency fourth-order system tracts can only be correlated with the log motif patterns due to sub-seismic resolution. The erosional remains of the fourth-order sequences in 3_HST, for instance, the coarsening upward shoreface and shelfal facies topped by transgressive shale, have yielded potential future drilling sites based on seismic attribute and log response analysis. The analogy of field data with well log and seismic data helped the overall evaluation of the hydrocarbon prospectivity of the area. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 11(1), 2022: 95-110

This study focuses on the source rock, a critical component of the petroleum system, within the Eastern Fold Belt (EFB) of Bangladesh. It aims to assess the occurrence and quality of source rocks by employing a comprehensive approach that includes fieldwork, laboratory analysis, seismic interpretation, and well data. The research was conducted in the Sylhet and Khagrachari regions, where prior source rock evaluations were inadequate. Organic carbon content evaluation using TOC analysis reveals a substantial presence of source rock in the EFB, which can produce hydrocarbon upon maturation. Depositional environment analysis suggests the source rocks were deposited in geological settings, including the continental shelf, prodeltaic, shallow marine, and deltaic sub-environments. Laboratory studies indicate the total organic carbon (TOC) content varied, ranging from 0.24% to 1.812%, with an average TOC of 0.65% to 0.7% in Sylhet and an average TOC of 0.28% to 0.33% in Khagrachari where the Kopili Shale of Sylhet exhibiting the highest amount of organic content. The samples from the northern part of the Bengal Basin were observed to be richer in organic content than those from the southern regions. Well-log data from Fenchuganj show that the source rocks are located at depths ranging from 3500 to 4200 m, which is correlated by seismic data. This depth suggests that the source rocks have undergone sufficient thermal maturation (in the temperature window of 105 -140℃) to generate hydrocarbons. Seismic sections further demonstrate the regional distribution of source rock, reinforcing the potential for gas-prone hydrocarbon prospects in the EFB. The integrated approach provides valuable insights into the petroleum potential, emphasizing the crucial role of source rocks in the EFB for future hydrocarbon exploration efforts in the Bengal Basin, offering hope for future discoveries. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 13(2), 2024, P 77-104