Sedimentary, Diagenetic and Accumulation Characteristics of an Offshore Oil-gas Field—a Case Study of Huizhou Depression, Pearl River Mouth Basin, South China Sea

In the Z21 oil-gas field, a total of six depositional lithofacies and two depositional elements were identified based on core observation. Three main diagenetic processes, namely mechanical compaction, cementation, and dissolution of Miocene Zhujiang Formation sandstones were identified according to thin section and scanning electron microscope (SEM) of core samples. Cementations mainly contain silica cementation, carbonate cementation, clay minerals and pyrite. A total of three main pore types, residual primary intergranular pores, secondary dissolution pores and micropores, were identified. Sand sheet deposited in low-energy environment and is characterized by relatively low porosity and permeability values. Lager grain-sized sandstones are of higher quality compared to smaller-sized sandstones. Mechanical compaction, calcite cementation and clay mineral cementation play a key role in reducing porosity and permeability, whereas dissolution of feldspar and debris contribute significantly to improving the reservoir quality. The gas charge occurs prior to oil charge, forming a gas cap in the structural high and an oil ring in the lower formation. Irreducible water stored in the lenticular sandstone of low-porosity and permeability reservoir may convert to movable water as the drill and production perform.

Geological evidence of an unreported historical Chilean tsunami reveals more frequent inundation

Assessing tsunami hazards commonly relies on historical accounts of past inundations, but such chronicles may be biased by temporal gaps due to historical circumstances. As a possible example, the lack of reports of tsunami inundation from the 1737 south-central Chile earthquake has been attributed to either civil unrest or a small tsunami due to deep fault slip below land. Here we conduct sedimentological and diatom analyses of tidal marsh sediments within the 1737 rupture area and find evidence for a locally-sourced tsunami consistent in age with this event. The evidence is a laterally-extensive sand sheet coincident with abrupt, decimetric subsidence. Coupled dislocation-tsunami models place the causative fault slip mostly offshore rather than below land. Whether associated or not with the 1737 earthquake, our findings reduce the average recurrence interval of tsunami inundation derived from historical records alone, highlighting the importance of combining geological and historical records in tsunami hazard assessment.

Eolian stratigraphic record of environmental change through geological time

The terrestrial sedimentary record provides a valuable archive of how ancient depositional systems responded to and recorded changes in Earth’s atmosphere, biosphere, and geosphere. However, the record of these environmental changes in eolian sedimentary successions is poorly constrained and largely unquantified. Our study presents the first global-scale, quantitative investigation of the architecture of eolian systems through geological time via analysis of 55 case studies of eolian successions. Eolian deposits accumulating (1) under greenhouse conditions, (2) in the presence of vascular plants and grasses, and (3) in rapidly subsiding basins associated with the rifting of supercontinents are represented by significantly thicker eolian dune-set, sand-sheet, and interdune architectural elements. Pre-vegetation eolian systems are also associated with more frequent interactions with non-eolian environments. The interplay of these forcings has resulted in dune-set thicknesses that tend to be smallest and largest in Proterozoic and Mesozoic successions, respectively. In the Proterozoic, the absence of sediment-binding plant roots rendered eolian deposits susceptible to post-depositional wind deflation and reworking by fluvial systems, whereby highly mobile channels reworked contiguous eolian deposits. During the Mesozoic, humid greenhouse conditions (associated with relatively elevated water tables) and high rates of basin subsidence (associated with the breakup of Pangea) favored the rapid transfer of eolian sediment beneath the erosional baseline. The common presence of vegetation promoted accumulation of stabilizing eolian systems. These factors acted to limit post-depositional reworking. Eolian sedimentary deposits record a fingerprint of major environmental changes in Earth history: climate, continental configuration, tectonics, and land-plant evolution.

Tectonic, hydrogeologic, and climatic controls on Late Holocene dune formation, China Lake basin, Indian Wells Valley, California, USA

Analysis of patterns of faulting and hydrogeology, stratigraphic and sedimentologic studies, and luminescence dating of aeolian deposits in China Lake basin provide new perspectives on the origins and development of Late Holocene dunes and sand ramps in the seismically active Indian Wells Valley of eastern California. Aeolian dune and sand sheet deposits were sourced from alluvial material derived from granitic rocks of the south-eastern Sierra Nevada and are concentrated in areas with sand-stabilizing phreatophyte vegetation influenced by high groundwater levels along the active oblique-normal Little Lake and Paxton Ranch faults, which locally form barriers to groundwater flow. Three episodes of sand accumulation are recognized (2.1 ± 0.1 to 2.0 ± 0.1 ka, 1.8 ± 0.2 to 1.6 ± 0.2 ka, and 1.2 ± 0.1 to 0.9 ± 0.1 ka) during conditions in which sediment supplied to the basin during periods of enhanced rainfall and runoff was subsequently reworked by wind into dunes and sand ramps at the transition to more arid periods. Understanding the role tectonics plays in influencing the hydrogeology of seismically active lake basins provides insights to accurately interpret landscape evolution and any inferences made on past hydroclimate variability in a region.

Channel, dune and sand sheet architectures of strait-adjacent deltas

Sea strait geographies amplify tidal currents, which can result in the formation of tidal strait deposits with a symmetrical facies arrangement. It can be problematic to distinguish such confined tidal strait deposits from strait systems that developed in less constricted settings. To push a more robust differentiation between the confined tidal strait model and a model for less constricted tidal deposits, this study presents an example of a strait-adjacent delta and compares it to the existing model of confined tidal straits. The strait-adjacent delta interpretations are based on an exposed succession in Northern Morocco, that formed in the Miocene Rifian Corridor. The multi-km, seismic-scale exposures at the Ben Allou locality, formed in a region with a largely unconstrained coastline. Clayey and silty portions dominate the distal offshore and prodelta facies, while the proximal delta front and delta plain are comprised of carbonate-rich sandstones. These sandstones exhibit complex architectures of stacked channels and dunes in the delta front, and mud drape-bearing sand sheets on the delta plain. It is shown that the strait-adjacent delta model that is presented herein, is different from a confined tidal strait deposit as it has an asymmetric facies arrangement, and a basinward reduction in depositional energy.

Quantitative analysis of the sedimentary architecture of eolian successions developed under icehouse and greenhouse climatic conditions

The continental terrestrial record preserves an archive of how ancient sedimentary systems respond to and record changes in global climate. A database-driven quantitative assessment reveals differences in the preserved sedimentary architectures of siliciclastic eolian systems with broad geographic and stratigraphic distribution that developed under icehouse versus greenhouse climatic conditions. Over 5600 geological entities, including architectural elements, facies, sediment textures, and bounding surfaces, have been analyzed from 34 eolian systems of Paleoproterozoic to Cenozoic ages. Statistical analyses have been performed on the abundance, composition, preserved thickness, and arrangement of different eolian lithofacies, architectural elements, and bounding surfaces. Results demonstrate that preserved sedimentary architectures of icehouse and greenhouse systems differ markedly. Eolian dune, sand sheet, and interdune architectural elements that accumulated under icehouse conditions are significantly thinner relative to their greenhouse counterparts; this is observed across all basin settings, supercontinents, geological ages, and dune field physiographic settings. However, this difference between icehouse and greenhouse eolian systems is exclusively observed for paleolatitudes <30°, which suggests that climate-induced changes in the strength and circulation patterns of trade winds may have partly controlled eolian sand accumulation. These changes acted in combination with variations in water table levels, sand supply, and sand transport, ultimately influencing the nature of long-term sediment preservation. During icehouse episodes, Milankovitch cyclicity resulted in deposits typified by glacial accumulation and interglacial deflation. Greenhouse conditions promoted the accumulation of eolian elements into the geological record due to elevated water tables and biogenic- and chemical-stabilizing agents, which could protect deposits from wind-driven deflation. In the context of a rapidly changing climate, the results presented here can help predict the impact of climate change on Earth surface processes.

Contextualized sedimentation rates for large floods along the lower Mississippi River: the importance of flood duration

<p>Flood sedimentary deposits vary due to upper basin and lower basin controls. In this study we focus on overbank sediment thickness, which over longer periods drives changes to riparian aquatic habitat and floodplain construction. The study setting is a ~25 km long segment of the lower Mississippi alluvial valley, between Natchez, MS and Red River Landing, LA. We report new field data for overbank sedimentation generated by compound flooding over 2018 and 2019 hydrologic years, and compare with sedimentation data from prior large flood events. Overbank conditions in 2018 and 2019 persisted for 286 days (at Natchez, MS). During the 2019 hydrologic year the Mississippi was overbank for a record duration of 216 days, resulting in a much greater duration of overbank sedimentation than the 2011 (53 days) and 1973 (90 days) flood events.</p><p>The thickness of overbank deposits are reported for 48 field sites across a range of depositional environments typical of large lowland meandering river floodplains. Flood deposits were sampled in October 2019 using conventional field sampling procedures, including sedimentation traps (artificial grass mats installed in October 2017) and recognition of recent sediment deposited atop buried organic layers. The thickness of each reported sample is an average of three measurements obtained at each field site.</p><p>The average thickness of flood deposit samples over 2018-2019 hydrologic years is 71 mm, with variability according to distance from channel and floodplain depositional environment. Maximum sedimentation was associated with crevasse (750 mm) and sand sheet (1,430 mm) deposition along the crest of natural levees. Sedimentation thickness decreases within ~250 m of the channel, but remains high at a distance of ~3.5 km (30 mm). Beyond the range of sand sheet deposition, overbank deposition is likely influenced by variability in floodplain hydrology and geomorphology across natural levee (181 mm), meander scroll (30 mm), old channel (77 mm), and backswamp (108 mm) environments. High backswamp sedimentation at the study site is likely influenced by historic hydraulic engineering for flood control, which has altered local sedimentation patterns.</p><p>The 2018-2019 sedimentation data are contextualized by comparison with field data from the record 2011 magnitude flood (peak Q of 65,978 m<sup>3</sup>/s at Vicksburg, MS, USGS 0728900) and the historic 1973 flood (55,558 m<sup>3</sup>/s).  Average sediment thickness for the 2011 and 1973 overbank deposits was 42 mm (n=49) and 230 mm (n=31), respectively. The 2018-2019 daily sedimentation rate (0.25 mm/day) is much less than 2011 (0.75 mm/day). Thus, the much thicker sedimentary deposits for the 2018-2019 events suggests the greater importance of flood duration – rather than flood magnitude – to overall floodplain processes and alluvial fill chronologies along lowland rivers. The much lower flood sedimentation rate for 2018-2019 in comparison with 1973 (2.49 mm/day) may reveal the persistent decline in Mississippi suspended sediment loads since the early 1950s. Study results are further contextualized by considering corresponding event-based discharge – suspended sediment dynamics, sediment province, as well as flood hydroclimatology.</p>