Geomorphology and sedimentary sequence evolution during the buried stage of paleo-uplift in the Lower Cretaceous Qingshuihe Formation,Junggar Basin,northwestern China: Implications for reservoir lithofacies and hydrocarbon distribution

The evolution of large-scale paleo-uplifts within sedimentary basins controls the sedimentary provenance, depositional systems and hydrocarbon distributions. This study aims to unravel changes in paleo-geomorphology, interpret sedimentary sequence evolution, and investigate favourable reservoir types and the hydrocarbon distribution during the buried stage of a long-term eroded paleo-uplift, taking the Lower Cretaceous Qingshuihe Formation (K₁q) in the Junggar Basin as an example. These research topics have rarely been studied or are poorly understood. This study integrates current drilling production data with outcrop and core analyses, drilling well logs, 3D seismic data interpretations, grading data, physical property comparisons and identified hydrocarbon distributions.After more than 20 million years of differential river erosion and weathering in arid conditions, the large-scale Chemo paleo-uplift within the hinterland area of the basin formed a distinctive valley–monadnock paleo-geomorphology prior to the deposition of K₁q. Since the Early Cretaceous, tectonic subsidence and humid conditions have caused the base level (lake level) to rise, leading to backfilling of valleys and burial processes. Two systems tracts in the target strata of K₁q, consisting of distinctive depositional systems, can be identified: (1) a lowstand systems tract (LST), which is confined within incised valleys and is mainly composed of gravelly braided rivers and rarely occurring debris flows and (2) an extensive transgressive systems tract (TST), which developed into an almost flat landform and consists of braided river delta to lacustrine depositional systems. Overall, the physical properties of braided river reservoirs in the LST are better than those of the braided river delta reservoirs in the TST. However, the inhomogeneous distributions of carbonate cements cause differences in the physical properties of conglomerate reservoirs in the LST. However, for sandstones in both the LST and TST, coarser grain sizes and better sorting result in better physical properties. Altogether, four types of reservoir can be identified in the study area: Jurassic inner monadnock reservoirs, K₁q LST stratigraphic onlap reservoirs, LST structural reservoirs and TST structural reservoirs.     

Lithological and palynological evidence of late Quaternary depositional environments in the subaqueous Yangtze delta, China

AMS ¹⁴C ages of post-glacial core sediments from the subaqueous Yangtze delta, along with sedimentary structures and distributions of grain size, pollen spores, and dinoflagellate cysts, show an estuarine depositional system from 13 to 8.4 cal ka BP and a deltaic system from 5.9 cal ka BP to the present. The estuarine system consists of intertidal to subtidal flat, estuarine, and estuarine-front facies, characterized by sand–mud couplets and a high sedimentation rate. The deltaic system includes nearshore shelf and prodelta mud featured by lower sedimentation rate, markedly fewer coastal wetland herbaceous pollens, and more dinoflagellate cysts. We explain the extremely high sedimentation rate during 9.2–8.4 cal ka BP at the study site as a result of rapid sea-level rise, high sediment load due to the unstable monsoonal climate, and subaqueous decrease of elevation from inner to outer estuary. A depositional hiatus occurred during 8.2–5.9 cal ka BP, the transition from estuarine to deltaic system, caused possibly by a shortage of sediment supply resulting from delta initiation in paleo-incised Yangtze valley and strong tidal or storm-related reworking in offshore areas. The subsequent development of deltaic system at the study site indicates accelerated progradation of Yangtze delta post-5.9 cal ka BP.     

Hierarchical dynamic stratigraphy in various Quaternary gravel deposits,Rhine glacier area (SW Germany): implications for hydrostratigraphy

The approach of dynamic stratigraphy aims to understand genetic processes that form stratigraphic units in a hierarchy of spatial and temporal scales. This approach was used to investigate Quaternary gravel deposits in terms of their sedimentology and in order to characterize the various sedimentary units in terms of their hydrogeological properties. Facies analysis within 62 gravel pits, laboratory permeability measurements of field samples and geophysical surveys (3-D georadar, 2-D seismic reflection) led to the detection and classification of sedimentary heterogeneity according to the following six scales whereby each scale can be translated into defined hydrostratigraphic units. (1) Particles and pores (micro scale) that reflect depositional and diagenetic fluid dynamics as well as source material behaviour (e.g. grain-size, roundness, lithological composition). This was found to be important for the hydrogeochemistry of groundwater in gravel aquifers (e.g. higher sorption capacity of carbon-rich limestone particles for organic pollutants). (2) Strata (meso scale) contain the recognition of sorting, fabric, texture and stratinomic features, which can give an indication of transport and depositional dynamics. Five major lithofacies groups, for example, were distinguished within fluvial gravel-bed deposits. Their variable hydraulic properties led to their subdivision into 12 hydrofacies types. They form the smallest mappable hydrostratigraphic units, which may result in either preferred pathways for fluid flow or flow barriers. (3) Depositional elements (macro scale) enable reconstruction of sedimentary/geomorphic elements and their dynamics within a depositional system (e.g. gravel-bed braided river systems are dominated by gravel sheet, gravel dunes and scour pool depositional elements). Hydrostratigraphically, the architecture of depositional elements influences the hydraulic connectivity and local permeability structure/distribution within an aquifer body. Five types of depositional elements in fluvial gravel-bed deposits were distinguished and their geometries/dimensions quantified. (4) Facies bodies (mega scale) composed of a stack of depositional elements and strata recording distinct environmental systems and their dynamics (e.g. a coarse-grained prograding delta system). Hydrostratigraphically, facies bodies represent major compartments of an aquifer. Six major types of meltwater-controlled facies bodies were identified in the study area. (5) Genetic sequences (mega scale) reflect the shifts of depositional environments caused by allocyclic changes (e.g. glacial advance recorded by a coarsening upward sequence) or autocyclic changes of landscape shaping events. These sequences may form separate hydrostratigraphic units or aquifer storeys. (6) Basin fill (giga scale) comprising the lateral and vertical stacking of facies bodies and genetic sequences controlled by either long-term glacier dynamics or short term flood events. The regional distribution of permeable gravel units and, for example, less permeable diamicts builds the larger scale hydrostratigraphy.     

Distribution of diagenetic alterations in glaciogenic sandstones within a depositional facies and sequence stratigraphic framework: Evidence from the Upper Ordovician of the Murzuq Basin, SW Libya

The spatial and temporal distribution of diagenetic alterations has been constrained in relationship to depositional facies and sequence stratigraphy of the Upper Ordovician glaciogenic quartzarenite sandstones in the Murzuq Basin, SW Libya, which were deposited during the Haritanian glaciation when the basin was laying along the continental margin of Gondwana. Eogenetic alterations encountered include: (i) replacement of detrital silicates, mud matrix and pseudomatrix by kaolinite in paraglacial, tide-dominated deltaic, in foreshore to shoreface (highstand systems tract; HST) and in post-glacial, Gilbert-type deltaic (lowstand systems tract; LST) sandstones, particularly below the sequence boundaries (SB). Kaolinite formation is attributed to the influx of meteoric water during relative sea level fall and basinward shift of the shoreline. (ii) Cementation by calcite (δ¹⁸O_VPDB = − 3.1‰ to + 1.1‰ and δ¹³C_VPDB = + 1.7‰ to + 3.5‰) and Mg-rich siderite in the paraglacial, tide-dominated deltaic and foreshore to shoreface HST sandstones, in the glacial, tide-dominated estuarine (transgressive systems tract; TST) sandstones and in the post-glacial, shoreface TST sandstones is interpreted to have occurred from marine pore-waters. (iii) Cementation by Mg-poor siderite, which occurs in the post-glacial, Gilbert-type deltaic LST sandstones and in the paraglacial, tide-dominated deltaic and foreshore to shoreface HST sandstones, is interpreted to have occurred from meteoric waters during relative sea level fall and basinward shift of the shoreline. (iv) Pervasive cementation by iron oxides has occurred in the glacial, shoreface–offshore TST sandstones and post-glacial, shoreface TST sandstones immediately below the maximum flooding surfaces (MFS), which was presumably enhanced by prolonged residence time of the sediments under oxic diagenetic conditions at the seafloor. (v) Formation of grain-coating infiltrated clays mainly in the glacial, fluvial incised-valley LST sandstones and in the post-glacial, Gilbert-type deltaic LST sandstones as well as, less commonly, in the paraglacial, foreshore to shoreface HST sandstones and in the tide-dominated deltaic HST sandstones below the SBs.

Mesogenetic alterations include mainly the formation of abundant quartz overgrowths in the glacial, fluvial incised-valley LST sandstones, post-glacial, Gilbert-type deltaic LST sandstones and glacial, shoreface TST sandstones, in which early carbonate cements are lacking. Illite, chlorite and albitized feldspars, which occur in small amounts, are most common in the glacial, tide-dominated estuarine TST sandstones and paraglacial, shoreface HST sandstones. This study demonstrates that the spatial and temporal distribution of diagenetic alterations and their impact on reservoir-quality evolution in glacial, paraglacial and post-glacial sandstones can be better elucidated when linked to the depositional facies and sequence stratigraphic framework.     

GPR reflection characteristics and depositional models of mud volcanic sediments—Wushanting mud volcano field, southwestern Taiwan

Ground penetrating radar (GPR) survey was conducted in the Wushanting mud volcano field (Yanchao, Kaohsiung) using a 500 MHz antennae, which allowed high-resolution imaging of subsurface structures. Seven GPR reflection characteristics are recognized. Sigmoid GPR reflection patterns resulted from a recent mud lobe deposited above an underlying older mud lobe front. Contorted GPR facies resulted from recent mud flow which encountered obstacles. Subparallel reflections resulted from mud volcano deposits of limited flowability, low velocity and gentle gradient. Hummocky reflection patterns are formed by interfingering of recent mud lobes building onto low land. Disrupted GPR facies were due to lateral breaks of continuity from mud cracks, which, according to field observation, can provide channels for erosion and form deeper erosion gullies. GPR time slices of different depths are rendered as a three-dimensional model. Approximately orbicular GPR reflection characteristics can indicate arcuate stacked mud lobe fronts of different periods. Some depositional models to explain GPR reflection characteristics can be founded upon observations of recent sedimentary phenomena. The models of this study may be applied to paleoenvironments and the depositional evolution of mud volcanoes in similar geological settings.     

Organic geochemical characteristics and depositional environments of the Jurassic coals in the eastern Taurus of Southern Turkey

In this study, organic matter content, type and maturity as well as some petrographic and physical characteristics of the Jurassic coals exposed in the eastern Taurus were investigated and their depositional environments were interpreted.The total organic carbon (TOC) contents of coals in the Feke–Akkaya, Kozan–Gedikli and Kozan–Kizilinc areas are 24.54, 66.78 and 49.15%, respectively. The Feke–Akkaya and Kozan–Kizilinc coals have low Hydrogen Index (HI) values while the Kozan–Gedikli coals show moderate HI values. All coal samples display very low Oxygen Index (OI) values. The Kozan–Gedikli coals contain Type II organic matter (OM), the Feke–Akkaya coals contain a mixture of type II and type III OM; and the Kozan–Kizilinc coals are composed of Type III OM. Sterane distribution was calculated as C₂₇ > C₂₉ > C₂₈ from the m/z 217 mass chromatogram for all coal samples.T_max values for the Feke–Akkaya, Kozan–Gedikli and Kozan–Kizilinc coals are 439, 412 and 427 °C. Vitrinite reflectance values (%R_o) for the Feke–Akkaya and Kozan–Kizilinc coal samples were measured as 0.65 and 0.51 and these values reveal that the Feke–Akkaya and Kozan–Kizilinc coals are at subbituminous A or high volatile C bituminous coal stage. On the basis of biomarker maturity parameters, these coals have a low maturity.The pristane/phytane (Pr/Ph) ratios for the Feke–Akkaya, Kozan–Gedikli and Kozan–Kizilinc coals are 1.53, 1.13 and 1.25, respectively. In addition, all coals show a homohopane distribution which is dominated by low carbon numbers, and C₃₅ homohopane index is very low for all coal samples. All these features may indicate that these coals were deposited in a suboxic environment.The high sterane/hopane ratios with high concentrations of steranes, low Pr/Ph ratios and C₂₅/C₂₆ tricyclic ratios > 1 may indicate that these coals formed in a swamp environment were temporarily influenced by marine conditions.     

河北平原曲流河沉积特征及其模式

本文研究了河流沉积构造、结构、石英颗粒表面特征和沉积相特征,并提出组成曲流点坝标志的两个重要要素及沉积模式。有助于认识古代岩相中点坝层序和对此模式。     

赣江入湖三角洲上的网状河流体系研究

中国南方的冲积河流有许多属于分汊河流，这已被许多研究者进行过比较深入的分析研究，但赣江在其入湖三角洲上的多河道体系与分汊河道有着明显的不同，它的形成是河流自发调整的结果，而不象分汊河流那样由节点控制。它具有网状河流所具有的地貌和沉积物特征，属于典型的网状河流体系。虽然是低含沙河流，但由于其水动力较弱及汛期基准面的上升，洪泛频繁，可输入河间地以大量的泥沙并在低能环境中发生沉积，使河道及河间地能够协调加积升高，并维持多河道体系的稳定性。     

Organic facies and depositional palaeoenvironment of lignites from Rio Maior Basin (Portugal)

The Rio Maior Basin (Portugal) is a tectonic depression, filled by a Pliocene sequence that comprises, from floor to roof: (i) kaoliniferous fine sands, (ii) diatomites and lignites, (iii) recent deposits of sandstone and clay. The diatomites and lignites form a small dissimetric syncline with alternating seams. Ten lignite seams were identified and named from floor to roof as F, E, D, C.2, C.1, C, B, A, a and a′. Seams A, D, E and F are considered to be the main seams.The organic fraction consists mainly of macerals of the huminite group, with small percentages of inertinite and liptinite groups. However, the petrographic composition of each seam is distinct, particularly with regards to macerals of the huminite and liptinite groups.Calculation of petrographic indices permitted to plot the coals in facies and palaeoenvironment diagrams. Five facies have been defined: (i) aquatic, (ii) herbaceous swamp, (iii) mixed swamp with forest and herbaceous vegetation, (vi) forest swamp (wetter) and (v) forest swamp (drier).These lignites are humic coals formed from organic matter of terrestrial origin. The peat biomass at the origin of these coals formed from a very diverse vegetation comprising gymnosperms and angiosperms. In seams F, and occasionally in seams E and D, Botryococcus algae have also contributed to the biomass. Peat deposition corresponded to a rheotrophic hydrological regime: the water level always remained above the topographic surface of the basin. Nevertheless, during the deposition of seam A in the northern part of the basin, the water level was slightly below the topographic surface. The organic matter was preserved in anaeorobic conditions.     

Early Permian to Middle Jurassic rifting and sedimentation in East Africa and Madagascar

Pre-drift sediments of Madagascar (Early Permian-Middle Jurassic) have been studied palynologically. These studies resulted in a more precise dating of individual stratigraphic units and the recognition of minor and major breaks in the succession. Palynostratigraphic and physical evidence of unconformities have been used to subdivide the pre-drift sediments into depositional cycles and to infer rifting events. A comparison with equivalent strata of Mozambique, Tanzania, Kenya, Ethiopia and Somalia shows a general correspondence and provides additional information for the construction of a generalized framework for the East African/Madagascan region which demonstrates the relationships between rifting and sedimentation and elucidates the prehistory of the break-up of Gondwana into a western and eastern fragment during the Middle Jurassic. The predrift succession of East Africa/Madagascar can be subdivided into two major sequences, a Karoo sequence (cycles 1–5, Asselian-early Smithian) and a transitional sequence (cycles 6–9, Ladinian-early Bajocian). The two sequences are separeted by a late Scythian to Anisian hiatus which indicates extensive uplift and erosion before a major rifting event initiated the resumption of sedimentation in the Ladinian. This Middle Triassic event marks the transition from the intracratonic rift stage to the pericratonic basin stage and correlates with a significant event in the Pangaean history, the transition from final coalescence to initial dispersal. The onset of the southward drift of Madagascar is believed to have occurred about 60 Ma later near the Aalenian-Bajocian boundary, contemporaneous with or immediately after the deposition of syntectonic sediments of cycle 9.