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The Bengal Fan is a mud-rich, deep-water sediment system. A series of recently discovered biogenic gas reservoirs in the northeastern Bay of Bengal are fine-grained. The Pliocene gas discovered in the study area accumulated in the levee element of a deep-water channel complex identified by high-resolution three-dimensional (3D) seismic interpretation integrated with well logs and sidewall cores (SWC). The thickness and porosity of the levee gas reservoir are of good quality, but the permeability is relatively low owing to the high mud content. The discovery of the levee sandstone indicates various reservoirs in this mud-rich sediment system. Moreover, gas discovery implies that the gravity-flow sediment systems "link" the source rock in the study area. Gas reservoirs were discovered in Pliocene gravity-flow sediment systems, where the palaeo-climate experienced significant changes with a dramatic increase in total organic carbon (TOC) from the Miocene to the Pliocene. TOC-rich pelagic mudstones and deep-water gravity sediments act as source rocks for petroleum systems. Therefore, gravity-flow sediment systems have developed self-accumulating petroleum plays that exhibit enormous potential for exploration. Given the structural framework in the northeastern part of the Bay of Bengal, stratigraphic or structure-stratigraphic traps could have developed in the Pliocene deep-water sediment systems in the Bengal Fan. • Fine-grained deep-water turbidite gas reservoirs in upper Bengal Fan. • Bengal Fan is with Pliocene deep-water sediment systems. • Stratigraphic or structure-stratigraphic traps for exploration. • A close relationship between petroleum and deep-water gravity systems. [ABSTRACT FROM AUTHOR]

The characteristics and evolution of different types of channel-complex sets (CCSs) have long attracted attention from both academia and the oil industry. 3D seismic data from the world's largest submarine fan in the Bay of Bengal allow the exploration of morphology, architecture, and evolution of a Pleistocene submarine channel system from inception to abandonment. Four types of architectural elements are recognized, including feeder and distributary channel-complex set (CCS), levees, bend cutoffs, and incised and unincised crevasse splays. Three types of CCSs are recognized: (1) erosional non-leveed CCSs with the lowest values of channel width (W), thickness (T), and cross-sectional area (C a) (mean values of W = 153 m, T = 20 m, and C a = 2613 m2); (2) graded CCSs with the highest values of W , T and C a (mean values of W = 745 m, T = 89 m, and C a = 49183 m2); (3) aggradational leveed CCSs with intermediate values of W , T and C a (mean values of W = 476 m, T = 56 m, and C a = 21968 m2). Graded CCSs and aggradational leveed CCSs have overbank levees, crevasse splays and bend cutoffs, whereas erosional non-leveed CCSs lack these features. Such architectural difference in overbank deposits suggests a channel evolution pattern of initial incision (erosional non-leveed CCSs) and then aggradation (graded and aggradational leveed CCSs), resulting in a pattern of incision-to-aggradation channel evolution. An increase-then-decrease in channel morphometrics (represented by W , T , and C a) from erosional non-leveed CCSs with the lowest mean values of W , T , and C a , to graded CCSs with the highest mean values of W , T , and C a , and finally to aggradational leveed CCSs with the intermediate mean values of W , T , and C a suggest that the incision-to-aggradation channel evolution is related to waxing-then-waning energy cyclicity. Submarine channel turbidity flows during the waxing energy phase became progressively more energetic and increasingly more erosional, producing erosional non-leveed CCSs. Instead, submarine channel turbidity flows during the waning energy phase were more diluted and increasingly more depositional, thereby producing aggradational leveed CCSs. The phase of peak environmental energy was most likely accompanied by most energetic turbidity currents, resulting in graded CCSs. Results and observations from the current study contribute to a better understanding of architectural complexity and evolution of submarine channels. • Channel morphometrics increase-then-decrease from erosional CCSs, to graded CCSs, and finally to aggradational CCSs. • Graded and aggradational CCSs have overbank levees, crevasse splays and bend cutoffs, but erosional CCSs don't. • Channel evolution from erosion, to gradation, finally to aggradation was related to a waxing-then-waning energy cyclicity. [ABSTRACT FROM AUTHOR]

Avulsions and splays of submarine channels are important autogenic responses affecting sediment distribution in deep-water areas; however, their formation and development are still not well understood. Using high-resolution 3D seismic data and spectral decomposition red–green–blue (RGB) color blends, two seismically well-imaged submarine channel systems (SCS1 and SCS2) on the northeastern Bengal Fan are used to document the formation and development of avulsions and splays. The results suggest that avulsions occur mainly during the early evolution stage of submarine channel systems. SCS1 and SCS2 are respectively accompanied by one and four avulsions and accordingly produce two and five independent submarine channels. During this early phase, these channels collectively incise into the underlying stratigraphy but lack levee deposits. Splays, in contrast, develop consistently during the late evolution stage of the submarine channel systems. Three crevasse splays and three overbank splays develop on the overbank areas of SCS1 and SCS2. During that late stage, channel and levee deposits build over the site of former erosion, thus forming the aggradational phase of channel evolution. Crevasse and overbank splays are two different types of splays observed on this part of the Bengal Fan. The former are created by flow breaching of the adjacent levees, producing large-scale elongated shapes; they are related to relatively stable sediment gravity flows, a high-degree of channel instability, and steep levee topography. Overbank splays, however, are created by flow overtopping adjacent levees and produce small-scale, fan-like shapes; they are related to gradually-weaking sediment gravity flows, a low-degree channel unstability, and gentle levee topography. • Avulsions and splays are integral components of submarine channel evolution. • Splays of crevasse and overbank types are highlighted in this study. • Three main factors affect the formation of crevasse and overbank splays. [ABSTRACT FROM AUTHOR]

As very common deep-water phenomena, submarine channel avulsions have been paid considerable attention recently. The differences among them are, however, rarely noted, nor their relationship to the evolution of submarine channels. A seismically well-imaged channel-levee system from the Pliocene Bengal Fan, named target channels (TCs) and consisting of three independent channels (Tc1-Tc3), has been selected as a case to address these issues. Using high-resolution 3D seismic data along with spectral decomposition and RGB color blending techniques, this study images TCs from initiation to abandonment and recognizes three different types of avulsion events: regional avulsion, local avulsion and bank crevasse. In the early evolution stage of TCs, the regional avulsion and the local avulsion successively occurred, promoting TCs to obtain a stable flow course and then undergo a significant evolution. Moreover, coupled with those regional and local avulsion events, TCs were growing respectively in the form of individual channel and amalgamated stacking. In the late evolution stage of TCs, only bank crevasses occurred and led to the formation of crevasse splays in the overbank setting, which coincided with the aggradational stacking of TCs developing. This new evolution model of channel-levee systems, which acknowledges different avulsion events, may provide insights on the understanding of analogue deep-water deposits and in turn can be applied to guide the hydrocarbon exploration and development. • Three different types of avulsions are recognized in deep-water channels. • Regional and local avulsions occur in the early stage and correspond to the individual channel and amalgamated channel. • Bank crevasses occur in the late stage and develop in the aggradational channel system. [ABSTRACT FROM AUTHOR]