Seismic Stratigraphy and Seismic Geomorphology

Although reservoir characterization has been carried out by many researchers on the sedimentary package of the Bengal basin hydrocarbon province, integration of petrophysical and seismic sequence-based reservoir evaluation is rarely taken into account. This paper focuses on the identification of gas zones, reserve estimation and identification of new prospects in Srikail gas field within the eastern fold belt of Bengal basin integrating four wireline logs and 2D seismic data. Our study finds seven hydrocarbon-bearing zones (A, B, C, D, E, F and G) within the measured depth between 2429.5 and 3501 m. Petrophysical properties of seven hydrocarbon-bearing zones indicate that they are good quality reservoir sands. The gas horizons were mapped on seismic sections which reveal that the NW-SE anticlinal structure is largely affected by channels in the crest and western flank. The channels are infilled by fine-grained sediments which act as cap rock on northern and western parts of the structure. Thus, the anticlinal structure and fine-grained sediments make a potential trap for hydrocarbon accumulation and laterally and vertically well-distributed sequence remnants are the main reservoir rocks in this area. Volumetric reserve estimation of these sands provided a total gas initially in place as 552 billion cubic feet. Moreover, all the four wells are drilled in the southern block of the structure, and since there is a structural continuity from south to the north, it is highly recommended to drill a well up to 3000 m depth in the northern block to test its hydrocarbon potentiality. Overall, the outcomes of this study contribute new insights for reservoir characterization and identification of new prospects in an efficient way.

We use two-dimensional pre-stack depth migrated seismic reflection profiles and seafloor bathymetry to describe the continental margin structure and a massive mass-transport deposit off the west coast of India. This giant slide runs from the Gujurat–Saurashtra margin to the Laxmi Basin. It is over 330km long, amaximum of 190km wide and its run-out basal gradient is 1.2◦. We name this giant mass wasting deposit the Nataraja Submarine Slide. This slide covers 49 ±16 ×103km2and represents avolume of 19 ×103±4 ×103km3, making it the second by volume of any passive margin landslide/mass-transport deposit. Seismic facies analysis allows the internal structure of the mass-transport deposit to be described as far as the toe. This slide has been able to circumvent massive seamounts, thus highlighting the capacity of the flow and its potential energy during emplacement in a funnel between the slope of the Western Indian passive margin and the Laxmi Ridge. Stratigraphically, the emplacement of the Nataraja Slide predates the main pulse of sedimentation during the late Miocene–Recent associated with the Indus Fan but follows rapid sedimentation across S and SE Asia during the Early–Middle Miocene. The margin architecture at the head of this slide is associated with a gravity-controlled fold and thrust belt that may have caused slope steepening and triggering of the slide.

New 3D seismic data collected over 4870 km2 in the 3°45'–12°30'S Peruvian segment of the East Pacific subduction system image seafloor erosional surfaces that can be mapped across the forearc basins. Forearc basins experience various stresses, from their base where basal tectonic erosion acts to the seafloor which is influenced by aerial, shallow and deep water currents driven by waves or thermohaline oceanic currents. Previously there has been little interest in stresses on the upper layer and there is a lack of documentation of unconformities and the erosive processes in certain bathymetric domains in forearc basins. We address this with the study of examples sourced from 3D seismic reflection surveys of the seafloor offshore Peru. Unconformities occur in two distinctive bathymetric domains associated with the continental shelf and the upper slope of the margin. Identification and characterization of unconformity surfaces yield estimates of the amount of erosion at the modern seafloor that range from 18 to 100%. Regional physical oceanography allows us to calibrate potential candidates for these two distinctive domains. The first control on erosion is the dynamics of deep to intermediate oceanic currents related to the Humboldt-Peru Chile water masses, while the second is wave action in the shallower erosional surfaces. This study illustrates the unseen landscape of the forearc basins of South America and helps to highlight the importance of erosive surficial processes in subduction landscapes.

Quaternary reef development and demise have been shown to correspond to worldwide sea-level fluctuations and related environmental changes, yet the mechanisms and rates affecting this relationship are not well resolved. A set of high-resolution seismic reflection and multibeam bathymetric data were interpreted. Six distinct systems of relict fringing reefs embedded within the sedimentary cover of the northern shelf of the Gulf of Elat/Aqaba were identified. The two uppermost relict fringing reefs are also exposed on the sea floor in water depths of ca 20 m and ca 60 m, along the north-western corner of the northern Gulf of Elat/Aqaba and sub-parallel to the current northern coast, respectively. Two other relict fringing reefs are laterally correlated to each of the last two transgressive cycles, and are inferred to have formed during decelerations in sea-level rise during the last two deglaciations. These reefs and the units to which they are laterally correlated portray a repeating stratigraphic pattern of reef development during deposition of seismically homogeneous sediment and an ensuing demise during deposition of heterogeneous sediment. Correlation of the reef stratigraphy with rates of Late Quaternary sea-level rise and a sediment accumulation rate from one shelf core provide age estimations for the seismic stratigraphy. Two phases of fringing reef generation occurred during the last deglaciation (since ca 18 ka); the older reef probably developed between 12
8 ka and 11
5 ka at ca 60 m below present sea-level and the younger reef after 8
4 to 8
0 ka at ca 20 m below present sea-level. These last two phases of reef generation are separated by a lobate seismic unit that is interpreted as fluvial-deltaic deposits that backstepped across the shelf during the Early Holocene transgression. Results suggest that fringing reefs evolved along the northern shelf of the Gulf of Elat/Aqaba only during relative decelerations in sea-level rise, contemporaneous with low input of terrigenous sediment and probably during a period of aridity. At present, no fringing reef grows along the northern coast of the Gulf of Elat/Aqaba, suggesting that these relatively arid periods may have been drier than Recent. The pattern of slowdown in sea-level rise contemporaneous with conditions of increased aridity seems to have repeated during the last two sea-level rises, approximately at the same sea-levels, suggesting a common mechanism of short (millennium-scale) phases of eustatic and
climatic alteration during deglaciations.

Identification of a previously undocumented hydrate system in the Tumbes Basin, localized off the north Peruvian margin at latitude of 3°20' – 4°10'S, allows us to better understand gas hydrates of convergent margins, and complement the 36 hydrate sites already identified around the Pacific Ocean. Using a combined 2D-3D seismic dataset, we present a detailed analysis of seismic amplitude anomalies related to the presence of gas hydrates and/or free gas in sediments. Our observations identify the occurrence of a widespread Bottom Simulating Reflector (BSR), under which we observed, at several sites, the succession of one or two BSR-type reflections of variable amplitude, and vertical acoustic discontinuities associated with fluid flow and gas chimneys. We conclude that the uppermost BSR marks the current base of the hydrate stability field, for a gas composition comprised between 96% methane and 4% of ethane, propane and pure methane. Three hypotheses are developed to explain the nature of the multiple BSRs. They may refer to the base of hydrates of different gas composition, a remnant of an older BSR in the process of dispersion/dissociation or a diagenetically induced permeability barrier formed when the active BSR existed stably at that level for an extended period. The multiple BSRs have been interpreted as three events of steady state in the pressure and temperature conditions. They might be produced by climatic episodes since the last glaciation associated with tectonic activity, essentially tectonic subsidence, one of the main parameters that control the evolution of the Tumbes Basin.

The structural framework and stratigraphy of the Ariki Field in the Western Shallow Offshore Depobelt, Niger Delta Basin, Nigeria are interpreted using well logs and 3-D seismic data, and the control of rift on hydrocarbon accumulation is analyzed. Five maximum flooding surfaces and five sequence boundaries are identified in the major reservoir succession of Agbada Formation in the field; and the structural framework of the field is controlled by the major growth faults, minor faults and rollover anticlines formed by synsedimentary tectonic movements. There are a total of seven faults identified across the field, including three major faults and four minor faults. On the western flank of the field, graben and horst-like structures were bounded by minor faults. Fault throw analysis shows that below 1800 ms, the major faults with large fault throws act as hydrocarbon migration pathways, while above 1 800 ms, faults with small throws act as seals of hydrocarbon traps. There are four-way dip rollover anticlines and three-way downdip traps created by the minor faults. Six other prospective traps (drilling targets) were identified, which have promising oil accumulation potential.

Keywords: mass wasting slide seismic reflection West India margin Arabian Sea We use two-dimensional pre-stack depth migrated seismic reflection profiles and seafloor bathymetry to describe the continental margin structure and a massive mass-transport deposit off the west coast of India. This giant slide runs from the Gujurat–Saurashtra margin to the Laxmi Basin. It is over 330 km long, a maximum of 190 km wide and its run-out basal gradient is 1.2 •. We name this giant mass wasting deposit the Nataraja Submarine Slide. This slide covers 49 ± 16 × 10 3 km 2 and represents a volume of 19 × 10 3 ± 4 × 10 3 km 3 , making it the second by volume of any passive margin landslide/mass-transport deposit. Seismic facies analysis allows the internal structure of the mass-transport deposit to be described as far as the toe. This slide has been able to circumvent massive seamounts, thus highlighting the capacity of the flow and its potential energy during emplacement in a funnel between the slope of the Western Indian passive margin and the Laxmi Ridge. Stratigraphically, the emplacement of the Nataraja Slide predates the main pulse of sedimentation during the late Miocene–Recent associated with the Indus Fan but follows rapid sedimentation across S and SE Asia during the Early–Middle Miocene. The margin architecture at the head of this slide is associated with a gravity-controlled fold and thrust belt that may have caused slope steepening and triggering of the slide.

Durante sua evolução, a margem brasileira foi cenário de
inúmeros eventos de fluxos gravitacionais turbulentos, os
quais geraram os mais importantes reservatórios
petrolíferos em produção no Brasil na atualidade.
Os diferentes cenários deposicionais desenvolvidos nos
últimos milhões de anos na Bacia do Espírito Santo
demonstram um imenso potencial para aplicação de
modernas técnicas de reconhecimento de análogos de
reservatórios (Hercos & Schreiner, 2007).
A crescente melhoria das ferramentas e técnicas
geofísicas vem possibilitando significativos avanços na
caracterização de sistemas turbidíticos, principalmente
em relação às geometrias deposicionais, às dimensões
dos corpos rochosos e seu empilhamento estratigráfico.
A partir de dados de sísmica de reflexão de uma região
da referida bacia, diversas imagens foram compostas
demonstrando a evolução de sistemas turbidíticos
desenvolvidos entre o Mioceno Superior e o Holoceno.
Os principais estilos arquiteturais (apud Moraes et al.,
2006) reconhecidos para estes sistemas foram: a:
complexos de canais sinuosos; b: complexos de canais
amalgamados; c: complexos de canais discretos e d:
complexos de canais distributários e lobos.
Estes estilos arquiteturais são recorrentes no tempo
geológico e seu desenvolvimento está estritamente
relacionado com a variação relativa do nível do mar nos
últimos 10 M.a. Deste modo, foi possível reconhecer
quatro distintos estágios deposicionais: Estágio 1:
desenvolvimento de complexos de canais sinuosos
conectados a vales incisos; Estágio 2: desenvolvimento
de complexos de canais amalgamados e complexos de
canais discretos; Estágio 3: desenvolvimento de
complexos de canais distributários e lobos conectados a
ravinas; Estágio 4: desenvolvimento de complexo de
canais discretos intercalados com sedimentação
hemiplégica.
Concomitante à sedimentação, ocorreram processos
halocinéticos que influenciaram nos padrões
deposicionais de todos os sistemas, principalmente em
relação ao desenvolvimento e migração dos canais.

Lake Estanya is a small (19 ha), freshwater to brackish, monomictic lake formed by the coalescence of two karstic sinkholes with maximum water depths of 12 and 20 m, located in the Pre-Pyrenean Ranges (North-eastern Spain). The lake is hydrologically closed and the water balance is controlled mostly by groundwater input and evaporation. Three main modern depositional sub-environments can be recognized as: (i) a carbonate-producing ‘littoral platform’; (ii) a steep ‘talus’ dominated by reworking of littoral sediments and mass-wasting processes; and (iii) an ‘offshore, distal area’, seasonally affected by anoxia with fine-grained, clastic sediment deposition. A seismic survey identified up to 15 m thick sedimentary infill comprising: (i) a ‘basal unit’, seismically transparent and restricted to the depocentres of both sub-basins; (ii) an ‘intermediate unit’ characterized by continuous high-amplitude reflections; and (iii) an ‘upper unit’ with strong parallel reflectors. Several mass-wasting deposits occur in both sub-basins. Five sediment cores were analysed using sedimentological, microscopic, geochemical and physical techniques. The chronological model for the sediment sequence is based on 17 accelerator mass spectrometry 14C dates. Five depositional environments were characterized by their respective sedimentary facies associations. The depositional history of Lake Estanya during the last ca 21 kyr comprises five stages: (i) a brackish, shallow, calcite-producing lake during full glacial times (21 to 17·3 kyr bp); (ii) a saline, permanent, relatively deep lake during the late glacial (17·3 to 11·6 kyr bp); (iii) an ephemeral, saline lake and saline mudflat complex during the transition to the Holocene (11·6 to 9·4 kyr bp); (iv) a saline lake with gypsum-rich, laminated facies and abundant microbial mats punctuated by periods of more frequent flooding episodes and clastic-dominated deposition during the Holocene (9·4 to 0·8 kyr bp); and (v) a deep, freshwater to brackish lake with high clastic input during the last 800 years. Climate-driven hydrological fluctuations are the main internal control in the evolution of the lake during the last 21 kyr, affecting water salinity, lake-level changes and water stratification. However, external factors, such as karstic processes, clastic input and the occurrence of mass-flows, are also significant. The facies model defined for Lake Estanya is an essential tool for deciphering the main factors influencing lake deposition and to evaluate the most suitable proxies for lake level, climate and environmental reconstructions, and it is applicable to modern karstic lakes and to ancient lacustrine formations.

This study applies modern seismic geomorphology techniques to deep-water collapse features in the Orange Basin (Namibian margin, Southwest Africa) in order to provide unprecedented insights into the segmentation and degradation processes of gravity-driven linked systems. The seismic analysis was carried out using a high-quality, depth-migrated 3D volume that images the Upper Cretaceous post-rift succession of the basin, where two buried collapse features with strongly contrasting seismic expression are observed. The lower Megaslide Complex is a typical margin-scale, extensional-contractional gravity-driven linked system that deformed at least 2 km of post-rift section. The complex is laterally segmented into scoop-shaped megaslides up to 20 km wide that extend downdip for distances in excess of 30 km. The megaslides comprise extensional headwall fault systems with associated 3D rollover structures and thrust imbricates at their toes. Lateral segmentation occurs along sidewall fault systems which, in the proximal part of the megaslides, exhibit oblique extensional motion and define horst structures up to 6 km wide between individual megaslides. In the toe areas, reverse slip along these same sidewall faults, creates lateral ramps with hanginwall thrust-related folds up to 2 km wide. Headwall rollover anticlines, sidewall horsts and ramp anticlines may represent novel traps for hydrocarbon exploration on the Namibian margin.

The Megaslide Complex is unconformably overlain by few hundreds of metres of highly contorted strata which define an upper Slump Complex. Combined seismic attributes and detailed seismic facies analysis allowed mapping of headscarps, thrust imbrications and longitudinal shear zones within the Slump Complex that indicate a dominantly downslope movement of a number of coalesced collapse systems. Spatial and stratal relationships between these shallow failures and the underlying megaslides suggest that the Slump Complex was likely triggered by the development of topography created by the activation of the main structural elements of the lower Megaslide Complex.

This study reveals that gravity-driven linked systems undergo lateral segmentation during their evolution, and that their upper section can become unstable, favouring the initiation of a number of shallow failures that produce widespread degradation of the underlying megaslide structures. Gravity-driven linked systems along other margins are likely to share similar processes of segmentation and degradation, implying that the megaslide-related, hydrocarbon trapping structures discovered in the Namibian margin may be common elsewhere, making megaslides an attractive element of deep-water exploration along other gravitationally unstable margins.

The prograding shelf margin of Lake Pannon reached and ran across the Northern Somogy (Hungary) area between 8.8 and 8 Ma in an overall NNW–SSE direction.The aim of this study was to understand the controlling effects of relative lake-level changes and structural movements on slope sedimentation, based on the interpretation of approximately 1800 km of 2D seismic reflection.The retro-deformation of the seismic sections revealed the characteristic morphological features, deep basins and elevated highs, of the present-day pre-Pannonian (Late Miocene) basement. This dissected morphology is the result of the complex structural evolution of the area that included complex transpressional–transtensional deformation before and during slope progradation. This fundamentally influenced the Pannonian sedimentation, including the overall thickness of the lacustrine strata, the distribution of deep-water and slope mudstones, turbiditic sediments, and deltaic deposits, and the direction of slope progradation. The shelf margin is constructed of alternating aggradational and progradational units, which indicates repeated rising and stagnant lake levels.