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The article examines the Cenozoic Song Hong and Beibuwan basins in Vietnam and their petroleum potential. It notes that the exposures of the Palaeogene syn-rift succession of the basins provides an insight into the Cenozoic syn-rift succession of the region. It relates how the Song Hong Basin evolved due to the movements along the Vietnamese margin. It concludes that the regional syn-rift system is linked to the syn-rift kitchen areas where petroleum and condensate have been discovered.

Reconstruction of non-marine syn-rift deposition in ancient basins is challenged by intense deformation, rapid lateral facies variation and limited biostratigraphic resolution. This affects petroleum resource assessments that relies on prediction of basin fill. The northern Gulf of Tonkin contains such non-marine-dominated rift system. The area is in an early stage of exploration. Depositional facies analysis based on all existing 2D seismic and well data suggests that Eocene-Oligocene deposition in grabens and half-grabens occurred in three steps. This resulted in formation of three successive packages, namely the rift initiation-, the rift development- and the rift termination succession. Rift initiation deposits are mainly composed by alluvial-fluvial strata deposited in segmented and often isolated grabens and half-grabens. As rifting progressed, faults joined and rift depressions merged into larger, coherent areas of deposition. Faulting peaked, which resulted in rapid creation of accommodation space, relief, and the development of deep lakes, alluvial fans and fan deltas. Elsewhere, floodplain, deltaic and shallow lake deposits comprise most of the rift development succession. As rift-subsidence declined, floodplain, deltaic and shallow lake deposits filled accommodation space and therefore dominate the rift termination succession. Structuring by strike-slip faulting and inversion complicates sequence stratigraphic analysis. Moreover, diachronic rift culmination caused the rift initiation, -development and -termination successions to vary laterally in age, further obscuring sequence stratigraphic analysis. Even so, tectonostratigraphic grouping into these three units allows a meaningful mapping of gross-depositional facies. The locally developed deep lake successions, potentially containing petroleum source rocks and seals, together with areas including potential deltaic, fan-delta, fluvial and turbidite reservoir sand suggest play types similar to those working in the Chinese part of the Beibuwan Basin. The most prospective areas are least explored suggesting an overlooked petroleum potential in the northern Gulf of Tonkin. • Eocene–Oligocene syn-rift strata in northern Gulf of Tonkin is mainly continental. • Structuration, lateral variation and data limitations challenge syn-rift analysis. • The syn-rift includes rift initiation, rift development and rift termination units. • Subdivision into these three units enables depicting deposition through time. • Rift development deposits include lacustrine source rocks and sandstone reservoirs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hue Sub-basin (HSB) is located in the western Song Hong Basin (SHB). • HSB is distinct in structural development from SHB, despite earlier beliefs. • The HSB opened in Late Oligocene – Early Miocene, driven by onshore strike-slip faults. • Transpression in HSB and SHB might be resulted from SE Asia-Australia collision. Cenozoic strike-slip deformation and associated basin formation in Indochina provide critical clues on crustal response during India-Asia collision. Typically, Indochina is considered a rigid block during continental extrusion. We demonstrate that the Song Ca-Rao Nay Fault System (SCRNFS) in north central Vietnam and its offshore extension, the Hue Sub-basin, subdivided Indochina into discrete blocks. Using an integrated dataset including topographic maps, geologic maps, onshore fieldworks, and offshore seismic and well interpretation, the structural evolution of the SCRNFS and Hue Sub-basin is investigated. During Late Oligocene, the SCRNFS initiated with right-lateral motion, causing pull-apart onshore and Hue Sub-basin opening offshore. The End-Oligocene inversion affecting the northern Song Hong Basin also caused a major NE–SW reverse fault in the Hue Sub-basin. In Early Miocene, rifting resumed in the Hue Sub-basin with accelerated faulting and westward rift migration in the south. This is distinct from the Song Hong Basin, where the main rift period was Eocene(?) – Oligocene, and the Early Miocene only features mild extension. During latest Early Miocene – earliest Middle Miocene, the SCRNFS switched to left-lateral transpression. This caused inversion and prolonged uplift in the northern-most Hue Sub-basin. The inversion associated unconformity can be traced onshore where it separates a compositionally immature conglomerate from an overlying quartz conglomerate. Left-lateral transpression in the Hue Sub-basin coincides with that in the Song Hong Basin and other inversion events across SE Asia. This may have been caused by Australia-SE Asia collision restricting escape movement of Indochina away from the India-Asia collision zone. [ABSTRACT FROM AUTHOR]

Novel C 34 and C 35 hexacyclic hopanes having side chains with a pentacyclic or hexacyclic ring at C-22 are abundant in lacustrine oils and source rocks but almost absent from marine oils. The corresponding C 34 and C 35 hexacyclic hop-17(21)-enes were identified in thermally immature mudstones from Vietnam cored in the 500 m deep ENRECA-3 well. All the novel hexacyclic compounds were unambiguously identified by comparison with synthetic standards. Traces of heptacyclic hopanes were detected in a lacustrine oil from Thailand. The C 34 and C 35 hexacyclic hopanes have much longer gas chromatography (GC) retention times than the corresponding pentacyclic hopanes. They are isomerized faster at C-22 than the pentacyclic hopanes and their 22S/(22S + 22R) ratio values at equilibrium are much higher (0.8 for C 34 ). Precursors could be penta- and hexafunctionalised hopanoids in cyanobacteria and methanotrophs. [ABSTRACT FROM AUTHOR]

Located at the junction between the Beibuwan Basin and the Song Hong Basin in the Gulf of Tonkin, the Bach Long Vi (BLV) island offers a rare window into the Cenozoic geology of Indochina and surrounding region. The island exposes 5 km-long continuous outcrop of weakly to moderately dipping Oligocene deep-lake deposits along the coastline. Using integrated UAV imaging and modern field methods, we characterized the deformation on the island at unprecedented level of details, and then related these features to regional tectonic events. BLV's mudstone-rich succession was intruded by pervasive sand injectites which consist of three main types: linear dikes, irregular dikes, and irregular sandstone bodies. Linear dikes and irregular dikes are found throughout most of the island. Linear dikes are steeply dipping, trend dominantly ENE, and tends to be thicker close to the top of the island stratigraphy. Irregular dikes have contorted shapes both in plan view and cross-section, and often connect adjacent linear dike segments. Irregular sandstone bodies are located toward the highest part of the island stratigraphy, and are characterized by depositional sands modified by sand injection processes. Most of the sand injectites on BLV constitute an interconnected complex that formed at the same time, probably in the late Oligocene during syn-rift. Sand injectites are later cross-cut by NW-SE left-lateral and its conjugate NE-SW right-lateral strike-slip faulting. The inferred ENE-trending maximum principal stress and NNW-trending minimum principal stress of this deformation is compatible with the regional end-Oligocene inversion as indicated by seismic data. • Bach Long Vi island with its 5-km long Oligocene exposure provides a rare window to Cenozoic geology of Indochina • Unprecedented details of deformation have been characterized through integrated UAV imaging and smartphone measurements. • Oligocene extension triggered sand intrusion into deep-lake mudstones • End-Oligocene inversion led to NW-SE left-lateral and NE-SW right-lateral strike-slip faults offsetting the sand injectites. [ABSTRACT FROM AUTHOR]

[Display omitted] • Zircon dates are presented for Cretaceous, Jurassic, Triassic and Permian granites and volcanics from S Cambodia and SW Vietnam. • Cretaceous and Jurassic granites in Vietnam & S Cambodia were generated during Paleo-Pacific subduction. • The origin of sparse Triassic igneous rocks is unclear. • Linking Cambodian Permian volcanism to Paleo-Tethys subduction is problematic. • Permian magmatism in the region was likely related to Paleo-Pacific subduction. Zircon ages and geochemistry are presented for igneous rocks from SW Vietnam and S Cambodia. Four main age groupings occur: Cretaceous (107–75 Ma), Early Jurassic (195 Ma), Late Triassic (230–222 Ma), and Permian (294–265 Ma). Cretaceous and Jurassic samples are amphibole-bearing biotite granodiorites to biotite granites and occur mainly east of the Kampot Fold Belt. Pre-Cretaceous samples occur within the Kampot Fold Belt and are dominantly felsic volcanics. The rocks are primarily high-K calc-alkaline, weakly peraluminous rhyolites or granites, with similar arc-like trace element patterns. Cretaceous granites are similar to Dalat Zone Granites and formed during Pacific subduction. Two Cretaceous granites have adakite-like signatures. The youngest Cretaceous granite has an A-type signature and may represent post-collision activity following the cessation of subduction. Jurassic granites are also linked to Pacific subduction. Two Triassic samples could be affiliated to similarly aged rocks in the Chanthaburi Terrane, or linked to the Loei Fold Belt and related to closure to the Sa Kaeo back arc. We cannot exclude that they are related to Pacific Plate subduction. Correlation of Permian volcanics to paleo-Tethys subduction and the Sukhothai-Chanthaburi arc is not clear as the Cambodian volcanics are older than Chanthaburi rocks, and there are stratigraphic contrasts between S Cambodia and Chanthaburi. Furthermore, such a correlation requires extension of the Sa Kaeo suture into S Cambodia and SW Vietnam at a high angle to regional structures. We propose that the Permian volcanics are also related to Paleo-Pacific subduction as shown in regional reconstructions of the region. [ABSTRACT FROM AUTHOR]

Highly oil prone source rocks containing sapropelic Type I and mixed Types I and III kerogen are found in 233 m of the 500 m deep, fully cored ENRECA-3 well drilled on Bach Long Vi Island, located at the junction between the Beibuwan and the Song Hong basins offshore northern Vietnam. The cored succession is made up entirely by Oligocene, deep lacustrine deposits. The origin of the organic matter (OM) in 62 samples is evaluated on the basis of gas chromatography-mass spectrometry (GC-MS and GC-MS/MS). OM in 55 samples is sourced primarily from lacustrine organisms and contains abundant 4-methylsteranes, tetracyclic polyprenoids (TPP) and lycopane, and no markers for marine organic matter. The proportion of triterpanes derived from angiosperms is low in most saturated fractions and only traces of gymnosperm diterpanes are detected. In contrast, all aromatic fractions have a high proportion of compounds having an origin from both gymnosperms and angiosperms. Very high concentrations of hopanoids in both fractions suggest significant bacterial alteration of the original OM. Most samples have a high concentration of rare C 31 diaromatic 8,14-secohopanes with a fluorene moiety. Low pristane/phytane (Pr/Ph) ratios indicate that anoxic/suboxic bottom water conditions existed during OM accumulation. The presence of β-carotane, gammacerane and traces of arylisoprenoids could indicate a stratified water column. OM in a few samples with lower than average hydrogen index (HI) is mainly of terrestrial origin with a higher proportion of C 29 steranes, diterpanes and oleanoids and lower concentrations of both saturated and aromatic hopanes. Higher Pr/Ph, a higher C 34 isohopane ratio (IHR34) and a lower homohopane index (HHI) indicate that this low-HI OM was originally deposited under more oxic conditions and subsequently transported to deeper water. Low sterane and hopane isomerization ratios show that no samples have reached the oil-window. Some novel maturity parameters for immature lacustrine samples are proposed. • Biomarkers in 62 samples from a 500 m long core is studied using GC-MS and GC-MS/MS. • Compounds related to fresh water algae are abundant. • A few samples have a high concentration of compounds from higher plants. • Samples are immature but the usual maturity indicators show little change from top to base. • Some new maturity indicators are proposed. [ABSTRACT FROM AUTHOR]