Lacustrine Basin Research Articles

Climatic and anthropogenic influences on vegetation changes during the last 5000 years in a seasonal dry tropical forest at the northern limits of the Neotropics

Seasonal dry tropical forest (STDF) is a widespread vegetation type in western Mexico. Generally, this type of forest develops in semihumid habitats where lacustrine basins are scarce, preventing documentation of the vegetation history and forest dynamics using a paleoecological approach. Here, we present a palynological record from the Santa Maria del Oro crater lake, located within the distribution area of the STDF and the adjacent dry oak forest which gives insight into the changes in diversity and human impact at this tropical site. Pollen data, in combination with geochemical analysis and microcharcoal data from a lacustrine sequence, are used to discuss vegetation change and diversity in the plant assemblages related to drought or anthropogenic activity over the last 5000 years. Our results show three distinct periods of drought in the basin, from 4200 to 3850, 3100 to 2300, 1570 to 1100 and 300 cal year BP, with changes in vegetation composition mainly related to a decrease in taxa diversity during these periods. Based on the presence of maize pollen, two periods of human activity (3790–2160 and 280 cal year BP to present) were detected with an increase in herbaceous pollen used as a surrogate for deforestation. These two disturbance periods, pre-Columbian and postcolonial, occurred during wet conditions in the basin. Our results highlight the correlation between El Niño events, drought and fire with changes in the composition and diversity of STDF.

Geochemical characteristics and source analysis of natural gas in the saline lacustrine basin in the Western Qaidam basin

This paper analyzed the geochemical characteristics and the sources of natural gas in the western Qaidam Basin based on isotopes and gas composition. The natural gas in the western Qaidam Basin is mainly comprised of hydrocarbon gases, including sapropelic, humic and mixed gas. In the southwestern Qaidam Basin, the main gas is sapropel gas, with some mixed gas and a small amount of humic gas. In contrast, the northwestern Qaidam Basin contains mainly humic gas, some mixed gas, and a small amount of sapropel gas. The dryness coefficient varies between 0.6 and 0.95. The carbon isotopes of methane, ethane, and propane are distributed between −32.9‰ ~ −51.4‰, −34.2‰ ~ −22.4‰, −30‰ ~ −17.4‰, respectively. The calculated Ro value of the natural gas is between 0.5% and 1.87%. In general, the natural gas shows the characteristics of high dryness coefficient, heavy carbon isotopes, and high thermal maturity. The gas-source correlation makes it clear that the main hydrocarbon source rocks in the western Qaidam Basin are the lower Ganchaigou (E3), the upper Ganchaigou (N1) and the lower Youshashan formations (N21). The gas-producing layers in the northwestern Qaidam Basin are primarily from the lower Ganchaigou (E3), the lower Youshashan (N21), and the upper Youshashan formations (N22). The primary gas source rocks are the lower Ganchaigou (E3), the upper Ganchaigou (N1), and the lower Youshashan formations (N21). The main gas-producing layers in the southwestern Qaidam Basin are the lower Ganchaigou (E3), the upper Ganchaigou (N1), the lower Youshashan (N21) and the upper Youshashan formations (N22), with the main gas source rocks being the lower Ganchaigou Formation (E3).

Corrigendum to “Forced or normal regression signals in a lacustrine basin? Insights from 3D stratigraphic forward modeling in the SW Pannonian Basin” [Global and Planetary Change, 196 (2021), in press

Corrigendum to “Forced or normal regression signals in a lacustrine basin? Insights from 3D stratigraphic forward modeling in the SW Pannonian Basin” [Global and Planetary Change, 196 (2021), in press

Gas Generation Potential and Characteristics of Oil-Prone Shale in the Saline Lacustrine Rifting Basins: A Case Study of the Dongpu Depression, Bohai Bay Basin

In comparison to the continuous breakthrough in shale gas of marine basins, the pace of study of lacustrine basins is relatively slow. The latest studies have shown the saline lacustrine rifting ba...

Micro/Nanopore Systems in Lacustrine Tight Oil Reservoirs, China.

A nanoscale pore throat system develops extensively in rocks of unconventional reservoirs serving as both source and reservoir rock. The nanoscale pores provide the main storage spaces, accounting for 70% to 80% of the total unconventional tight reservoirs in China. As one of most important unconventional petroleum accumulations, tight oil has accumulated in more than 20 lacustrine strata since the Permian in China. Three types of tight oil reservoirs were identified based on the lithology and provenance in the lacustrine basins, including terrigenous sandstone, endogenous carbonate rocks and mixed sedimentary rocks. The micro/nanopore structures of these tight rocks were investigated with the application of optical microscopy, scanning electron microscopy (SEM), mercury injection capillary pressure (MICP), gas adsorption (GA) and nuclear magnetic resonance (NMR). The results indicated that the pore systems were connected by nanoscale throats dominated the storage spaces of the lacustrine tight oil reservoirs, while there were obvious differences among these three tight rocks, including pore types, pore size and movable fluid distribution. (i) The terrigenous sandstones, which were represented by the Triassic Chang 7 tight sandstones in the Ordos Basin and Cretaceous Quantou tight sandstones in the Songliao Basin, were mainly arkoses, and their storage space was mainly composed of dissolution pores and intraclay mineral pores. Feldspar, rock fragments and carbonate cements were the majority of the dissolved components, and the diameter of dissolution pores ranged from 1 micron to 50 microns. Abundant intrakaolinite and illite/smectite mixed layers pores were developed, and the pore size was 10 nm to 500 nm. The MICP and GA data suggested that storage spaces were connected by throats with diameters of 10 nm˜300 nm. (ii) The endogenous carbonate rocks, which were represented by the Jurassic Da'anzhai limestones in the Sichuan Basin, were the tightest rocks with porosities of less than 5% and permeabilities of less than 0.01×10-3 μm². The calcite dissolution pores and fractures with diameters of 10 nm˜500 nm were the most important storage spaces. The majority of pore systems were connected by throats with diameters of 6 nm˜100 nm based on the MICP and GA data. (iii) The tight mixed sedimentary rocks, which were represented by the Permian Lucaogou Formation in the Junggar Basin, were complex in lithologic composition, and dolostones and dolomite sandstones were the most important exploration targets. The interdolomite pores were the dominant storage spaces, in which abundant illite/smectite mixed layers were filled, and the pore size ranged from 50 nm to 50 microns. The MICP and GA data showed that the storage space was dominated by throats with diameters of 10 nm˜200 nm, and their volumetric contributions could reach over 70%. These results could provide a reference for future tight oil research and exploration in China.

Seismic-scale geometries and sequence-stratigraphic architecture of Early Cretaceous syn-post rift carbonate systems, Presalt Section, Brazil

Abstract Regional and detailed seismic stratigraphic analyses of Early Cretaceous (Aptian) presalt carbonate sections from offshore Brazil reveal the complex stratigraphic architecture of late- and post-rift lacustrine carbonate systems. The lateral and vertical distribution of calibrated seismic facies within this framework highlights the evolution through time of the carbonate system and bathymetry of the host lacustrine basin. Despite the simple, largely abiotic and microbial components, lacustrine carbonate accumulations formed complex geometries that closely resemble those observed from marine systems, suggesting that a downward-tapering carbonate production profile must have occurred. The complexity of the stratigraphic architecture reflects lateral variations in subsidence patterns combined with the interference of the basement topography, palaeowind directions and basinal filling patterns. Well-imaged clinoforms several hundred metres high attest to both the existence of significant lake-bottom topography, locally in excess of 800 m, and the occurrence of deep water at time of deposition. Platform margin trajectory and vertical and lateral architecture of clinoform packages through time reveal distinct sequence boundaries that can be correlated in detail only locally, demonstrating the impact of syndepositional tectonics, and possibly recurrent isolation of smaller lakes during lowstands. Depositional models from this study fill a gap in current understanding of lacustrine carbonate systems and offer a template for exploration and appraisal of the presalt play.

Развитие Солонцовских озер как показатель динамики увлажнения в Центральном Сихотэ-Алине в позднем голоцене

The stages of development of small Solontsovskie (Shanduyskie) Lakes located in the middle mountains of the Central Sikhote-Alin within large landslides, formed on the slopes of the paleovolcano, are identified on the basis of complex study of the sediment section of the Nizhnee Lake. The ecological-taxonomic composition of the diatom flora, the botanical composition of peat have been analyzed, and the tendencies of lacustrine sedimentation depending on the different scale of hydroclimatic changes in the Late Holocene have been established. The age model is based on 6 radiocarbon dates. The temporary resolution for the reconstructions is 30–60 years. A comparison of the development of Nizhnee and Izyubrinye Solontsi Lakes was carried out, the stages of watering and shallowing of lakes were identified on the basis of their dynamics, which made it possible to restore the change in moisture in the middle mountains. Organogenic deposits in lacustrine basins accumulated at high rates (up to 1.7–1.9 mm/ year). The most detailed data were obtained for the last 2.6 thousand cal. yr BP based on the study of the sediment section of the Nizhnee Lake, which responded more sensitively to changing climatic conditions. Frequent changes in diatom assemblages and peat-forming plants indicate unstable hydroclimatic conditions with varying degrees of watering and drainage up to complete overgrowth of water bodies. According to the data of diatom analysis, a successive change in the trophicity of the lake was traced. A frequent change of sphagnum mosses of different sections with different trophic preferences was established. The main reason for the change in the hydrological regime of the lakes was variations in precipitation during the short-term climatic changes. The correlation of the identified paleoclimatic events with global data has been carried out. Cooling periods, as a rule, were accompanied by a decrease in moisture, but the Little Ice Age was wet due to an increase in precipitation.

Characteristics of Milankovitch cycles recorded in Eocene strata in the eastern depression of North Yellow Sea Basin, North China

As cyclical orbital movements of Earth, Milankovitch cycles can be recorded in sedimentary strata. The time they reflect can be used to accurately divide and compare strata. Milankovitch cycles recorded in strata enrich the stratigraphic theory, especially the theories of cycle stratigraphy, and thus they are widely used in geological survey engineering nowadays. This study explored the characteristics of the Milankovitch cycles recorded in the eastern depression of the North Yellow Sea Basin, highlighting their control over high-frequency stratigraphic sequences. The Eocene Milankovitch cycles in the depression were calculated based on the method proposed by J. Laskar, and their parameters primarily include eccentricity cycles of 125 ka and 99 ka, obliquity cycles of 51 ka and 39 ka, and precession cycles of 23 ka and 19 ka. Spectral analysis of gamma-ray (GR) and spontaneous potential (SP) log curves of the Eocene strata was carried out to divide and compare stratigraphic sequences, revealing that the spectral peaks correspond well to astronomical cycles. This indicates that the strata in the depression fully record Milankovitch cycles. Furthermore, there are long-, medium-, and short-term stratigraphic cycles in the eastern depression, with a thickness of 13.03–15.89 m, 3.70–5.21 m, and 2.17–2.94 m, respectively. The sedimentation rates of the Eocene strata were calculated to be 121.2–127.12 m/Ma accordingly. From the uplift to the center of the lacustrine basin along the slope in the eastern depression, both the sedimentation duration and the sediment thickness increase, while the sedimentation rate remains relatively stable. The Eocene strata can be divided into six stages of high-frequency sequences by continuous wavelet transformation, namely E6–E1 from bottom to top. The sedimentation duration and sedimentation rates of the sequences were calculated using spectral analysis with each of the sequences as a separate window. Moreover, the impacts of climate change on the sedimentary environment in the eastern depression were analyzed. It can be concluded that E6 was a lowstand system tract, E5 and E4 were lacustrine expansion system tracts, E3 was a highstand system tract, and E2 and E1 were lacustrine contraction system tracts. All these verify that Milankovitch cycles serve as an effective approach for the analysis of sedimentary cycles.

Lacustrine carbonate tufa facies of Winnemucca Dry Lake Basin, Nevada, U.S.A.

ABSTRACT Lacustrine carbonate tufa deposits are common in present-day lakes and dry pans of the western United States, and large-scale deposits (> 100 m high) are found throughout the subbasins of Pleistocene Lake Lahontan. This study presents a depositional model for very well exposed tufa in Winnemucca Dry Lake, a subbasin of Lake Lahontan, that incorporates new observations of tufa growth over length scales of 10–4–102 m. Tufa depositional facies are defined on the basis of outcrop morphology and texture. Deposits were mapped using satellite imagery and field observations. Tufa facies and volumes were quantified for seven tufa exposures across the basin using digital outcrop and elevation models from aerial images acquired from a small uncrewed aerial system (sUAS). Tufa thin sections were examined using transmitted-light petrography and scanning electron microscopy and combined with measurements of porosity and permeability to define small-scale facies characteristics. Both porosity and permeability are highly variable across textures; average values for both (ϕ = 29%, k = 5.5 D) indicate that all tufa types may exhibit excellent reservoir properties. The age and distribution of these facies across the basin are directly linked to hydroclimate and variations in lake level. The most important controls on tufa distribution at the basin scale are basin hydrology and pathways of groundwater inflow. Groundwater flow into the basin is largely concentrated along the western flexural margin along the contact between volcanic and volcaniclastic bedrock and alluvial sediments, rather than concentrated along the border fault margin, in contrast to other models which predict strong fault control of tufa occurrence. Microbially influenced tufa textures and morphologies are the most volumetrically significant tufas in the basin, composing between 77% and 100% of tufa volume at individual exposures; these are inferred to form during times when lake waters were warmer and levels higher, while physico-chemical processes dominate during early tufa formation, and generally in colder waters and under conditions of lower lake level. Deposition of tufas is a result of combined physical, chemical, and biological factors that are directly related to the basin geology and hydroclimate; however, the importance of each controlling factor is highly variable both spatially and temporally, complicating the development of effective and predictive depositional models. This case study describes tufa deposition intrinsically linked to basinal hydroclimatic histories, and understanding these relationships may assist in predicting volumes, physical properties, and stacking patterns of petroleum reservoir facies in lacustrine basins.

Paleocene storm-related event beds in the Gaoyou Sag of the Subei Basin, eastern China: A new interpretation for these deep lacustrine sandstones

Lacustrine storm deposits in hydrocarbon-rich lake basins can be excellent reservoirs; therefore, the identification and interpretation of storm deposits have great significance for future petroleum exploration. Recent progress in facies analysis helps to discriminate the sedimentary effects of combined flow and oscillatory flow in the depositional processes. Through core and well log data and thin-section analysis, the sandy deposits of Paleocene E1F3 member in the middle part of the Gaoyou Sag, Subei Basin, are interpreted as storm-induced deposits. Petrologic analysis shows that the coarser sediments are mostly fine-grained to silt-sized feldspathic litharenite. Five lithofacies were identified based on core observation, including massive sandstone, cross-bedded sandstone, hummocky cross-stratification (HCS) sandstone, wave rippled sandstone and siltstone, and massive mudstone. Lithofacies associations revealed a storm “sequence” beginning with an erosional surface and then fining upward, indicating hydrodynamic conditions that ranged from gravity flow, unidirectional flow, combined flow to oscillatory flow, which are attributed to storm-related processes. According to the different lithofacies associations and depositional processes, a depositional model including four facies tracts of storm-related deposits are recognized in plane view: (1) storm-induced turbidity channel facies characterized by coarser particles and formed by storm-triggered turbidity current, (2) proximal facies dominated by gravity, unidirectional, and combined flows, (3) transitional facies influenced by all the flow regimes and exhibiting a relatively complete storm “sequence”, (4) distal facies formed by low-energy deep-lake turbidity flow. Because storm deposits have petroleum significance in an oil-bearing basin such as Subei Basin, this study provides practical examples and guidance for future hydrocarbon exploration in other ancient lacustrine basins.

Pre-glacial and post-glacial history of the Scandinavian Ice Sheet in NW Russia – Evidence from Lake Ladoga

Lacustrine sedimentary records are valuable archives of regional paleoenvironmental and paleoclimatic changes. High-resolution seismic reflection data collected in 2013, in conjunction with data from sediment core Co1309, provide a detailed reconstruction of the preglacial and postglacial environmental and sedimentological history of Lake Ladoga (NW Russia) during the late Quaternary. The exceptionally good quality of the seismic data reveals an up to 90-m-thick sedimentary infill in undisturbed places, containing four distinct sequences documented in unprecedented detail. The deepest sequence S1 most likely represents the bedrock of the lacustrine basin, which was reshaped during the subsequent glaciations. The overlying sequence S2 contains large-scale mounds of variable dimension, shape and internal architecture, which suggest a complex history of formation and deformation. We interpret these mounds as a moraine-mound complex resulting from thrusting and bulldozing processes of pre-existing, subglacial sediments during the overall retreat of the Scandinavian Ice Sheet following the Last Glacial Maximum. The finely-layered sequence S3 coincides with varved sediments in core Co1309 that mark the Scandinavian Ice Sheet deglaciation in the Ladoga basin. The youngest sequence S4, characterised by a semi-transparent facies in the acoustic data, coincides with lacustrine sediments in core Co1309 that were deposited during the Holocene. Our results also document major thoroughgoing fault systems in the northwestern part of the lake basin, along with unconformities related to lake-level fluctuations, notably in the eastern-, western- and southernmost regions. By coupling hydroacoustic and sedimentological data, we are able to pinpoint lateral variations and modifications of sedimentary inputs through time during the evolution of the Ladoga basin, along with unravelling the paleoenvironmental history of Lake Ladoga since the pre-LGM.

Study of some soils South El-Amiria, Alexandria Governorate, Egypt using two soil classification systems

A pilot area south El-Amiria, Alexandria Governorate, Egypt was studied by using remote sensing techniques to define its main physiographic units. The soil studies were carried out by two classification systems; USDA (USDA, 2014) and FAO (FAO, 2006 b). The area locates between longitudes 29o47`55``and 30o 30`05`` East and latitudes 29o29`30`` and 30o30` 05`` North and comprise an area of about 571168 Feddan. Two physiographic units were recognized in the studied area as follows: I) Lacustrine plains, which includes four sub-units differ in their soil depths, i.e., moderately deep, deep, very deep, and rock outcrops. 2)Windblown sand, which includes three sub-units, i.e.sand sheets soils, BarchanDunes and Barchan dunes with partial CaCO3 cementations. By comparing the two systems of soil classification used in this study, it is observed that FAO system (FAO, 2006 b) includes different levels of soil properties which have a direct effect on soil reclamation, suitability and land use. On the other hand, USDA system (USDA, 2014) gives importance for texture, mineralogy, soil temperature …etc. in family level which adds more detailed information for soil management. Consequently, applications of the two classification systems, is necessary or preferable due to their compilation.

Geological theory and exploration & development practice of hydrocarbon accumulation inside continental source kitchens

Abstract Continental shale strata in China are rich in petroleum resources and are an important area to strengthen domestic oil exploration and development. Based on the latest progress in geological research and exploration and development of petroleum inside continental source kitchens of China National Petroleum Corporation exploration areas in recent two years, we have achieved the following results: (1) The geological connotations of continental hydrocarbon accumulation inside source kitchen and “sweet spot” have been proposed. The intra-source petroleum accumulation refers to the accumulation of liquid-rich hydrocarbons retained or captured in the continental organic-rich shale strata, and “sweet spot” refers to the favorable reservoir with higher oil content, better physical properties, easier to stimulate and higher in commercial development value in the overall oil-bearing continental source rock series, they can be divided into three types, interlayer, hybrid sediment and shale. (2) High-quality shale formations in both salt water and freshwater lacustrine basins can generate hydrocarbons on a large scale, shale strata have multiple types of favorable reservoirs with large-scale storage capacity, the intra-source shale strata are overall oil-bearing and large in resource scale, and there are multiple favorable shale series for development. The exploration and development practice is propelling the formation of a series of exploration and development key technologies with “sweet spot exploration” and “volume development” as the core. Some pilot tests of these technologies have provided an important scientific basis for the economic and effective development of hydrocarbon accumulation inside source kitchen, and popularization of these technologies have achieved encouraging results preliminarily. (3) Two types of continental intra-source petroleum resources in China have great potential, including medium-high maturity with liquid-rich hydrocarbons and medium-low maturity with organic-rich matter. The Ordos, Songliao, Bohai Bay and Junggar basins are the main areas of these resources. By addressing the theoretical and technical challenges in the exploration and development, the two types of resources inside continental source kitchens will become the realistic and major strategic replacement oil resources respectively in the future.

Forced or normal regression signals in a lacustrine basin? Insights from 3D stratigraphic forward modeling in the SW Pannonian Basin

Abstract The overall geometry of sedimentary basins is driven by a complex interplay between subsidence, water-level variations, sedimentation rates and basin-margin topography. Seaward movement of the shoreline is driven by either high sediment input or by base-level drop. The influence of the specific driving mechanisms is often overwritten by subsequent burial and compaction effects or it is below the resolution of observational data. Our main aim is to describe and discuss the influence of these internal and external forcing factors and demonstrate their sedimentary response, particularly the related unconformities and clinoform geometries in a deep lacustrine setting. To this aim, 3D numerical simulations were performed using DionisosFlow stratigraphic forward modeling software. Models are constrained by seismic and well data from the Drava Basin part of the SW Pannonian Basin, Central Europe. A wide range of parameters, such as subsidence rates, lake-level curves and sedimentation rates were compared and analyzed. Post-rift subsidence and inversion of the basin have been simulated together with the main sedimentary transport routes. Our calculation infers that paleo-water depth could have reached ca. 1300 m in the deepest parts of the basin. Furthermore, our models show that differential compaction, subsidence and lateral variations of the sediment supply result in apparently descending shelf-edge trajectories and onlapping clinoform surfaces that are often misinterpreted as base-level drops. By analyzing the sedimentary response to different climatic variations, we argue that there is no direct indication of a major base-level drop in the Drava Basin, which was much larger than the seismic resolution, i.e. 40–50 m. Modeling also infers that autoretreat and autocyclic variations are more effective at low sediment supply and higher amplitude lake-level variations.

The sedimentary architecture of hyperpycnites produced by transient turbulent flows in a shallow lacustrine environment

Hyperpycnal flows are river-derived extrabasinal turbidity currents transporting both sand and clay to lacustrine, coastal, shelf and deepwater sedimentary environments. Experimental research in the past twenty years has shown that the presence of clay in sediment-laden flows promotes a transitional behavior between fully turbulent flow and a quasi-laminar plug flow regime. However, to date, most work concerning the fluid dynamic interpretation of lacustrine hyperpycnal flows has been founded on concepts based on fully turbulent flows, and relatively little is known about the influence of clay on the rheological and turbulence characteristics of these flows. With the help of 3D seismic volume and wire-line logging data, the present study undertakes a description and interpretation of subsurface mixed sandstone–mudstone bedforms observed in cores from the Upper Cretaceous Heidimiao Sandstone, Nenjiang Formation, Songliao Basin, NE China. Six lithofacies and five lithofacies associations are recognized from the cores and interpreted as formed under both turbulent and transitional flows. Typical bedforms of transitional flows include large current ripples and low-amplitude bedwaves. The delta-fed shallow lacustrine hyperpycnal flow deposits are characterized by proximal sinuous channels (extending for c. 20 km) and lobe deposits (extending for >20 km), showing a fan-like geometry associated with distributary channel extension in the down-dip direction. Basal erosion in the proximal channels is produced by the initial turbulent and turbulence-enhanced transitional flows, with channel infill dominated by upper transitional plug flows or quasi-laminar plug flows. The lobe deposits possess a fan-like geometry with large aspect ratio, and were formed from turbulence-enhanced and lower transitional plug flows. The downdip transformation of sediment-laden flows in the lacustrine basin varies from an initial turbulent flow, via quasi laminar plug flows to upper and lower transitional plug flows, to turbulence enhanced transitional flows. These transitional flows are interpreted to have experienced gradual dilution and deceleration and eventually transformed to turbulent flows in their distal regions. A new model for delta-fed shallow lacustrine hyperpycnal flow deposits is presented incorporating decelerated transitional flows and flow transformations. This new model can aid understanding of the depositional processes of lacustrine transitional flows and the facies distribution of hyperpynal flows in both modern and ancient sediments.

Lake basin closure and episodic inflow as recorded by radiogenic Sr isotopes: Eocene Green River Formation in the Piceance Creek Basin, Colorado

Lacustrine basins are excellent archives of lake evolution, and deposits record the uplift and weathering histories of the surrounding terrain. The application of Sr isotopes has been tested in several lacustrine basins, both modern and ancient, based on the premise that lakes are well mixed, and shifting Sr isotopes may suggest changes in lake provenance. In the Eocene lacustrine Green River Formation in the Piceance Creek Basin of Colorado, Sr isotope analysis of carbonate mudstones indicates that radiogenic Sr in the center of the Piceance lake decreased during the evolution of the lake, from 52.8–48.4 Ma. Because deposition in the basin center occurred away from the influence of episodic alluvial inflow at the basin margin, Sr isotope evolution in the Piceance lake after basin closure is recorded in the John Savage #24-1 core deposits, not the Anvil Points deposits. Sr isotope analysis of carbonate mudstones at Anvil Points below 55 m shows fluctuating radiogenic Sr values that record episodic inflow from the White River Uplift. This inflow is responsible for the difference in radiogenic Sr trends recorded between the basin center and margin. Above 55 m, fluctuating Sr isotope values at Anvil Points record episodic inflow from the White River Uplift, without inflow of Paleozoic and Mesozoic carbonates. The boundary at 55 m records the hydrologic closure of the Uinta and Piceance lakes around 52 Ma, when lake level lowered beneath the basin sill and the lakes were no longer connected across the Douglas Creek Arch. A significant increase in radiogenic Sr across the 55-m-boundary records this transition from open to closed hydrology, reflecting a loss of dissolved Sr sourced from Paleozoic and Mesozoic carbonates.

Sedimentary architecture and provenance analysis of a sublacustrine fan system in a half-graben rift depression of the South China Sea

Research on the sedimentary architecture and provenance of sublacustrine fans is crucial not only to better understand the gravity flow evolution in lacustrine basins but to successfully locate hydrocarbon reservoirs. This study targets at a sublacustrine fan with a large petroleum reserve that has been revealed by drilling within an Eocene lacustrine succession in the Fushan Depression, northern South China Sea. The sedimentary architecture of the sublacustrine fan was investigated through the integration of core, well logs and 3D seismic data, and its provenance was identified by detrital zircon UPb dating. The examination of 117 m long continuous drilling core reveals four major lithofacies in the sublacustrine fan deposits, which are interpreted as deposits of two types of gravity flows (cohesive debris flows and turbidity currents). The cohesive debris flows developed in the lower section are mainly composed of massive gravelly mudstones with various floating clasts and high clay content, whereas the turbidity currents in the upper section are characterized by parallel to ripple bedding and normal graded bedding. These observations reflect a transformation from cohesive debrites to turbidites from deep to shallow water environments. In addition, provenance analysis of 648 detrital zircon UPb ages yield a well-defined relationship between potential provenance areas and sublacustrine fan deposits in the deep lake. The UPb age pattern of the fan is similar to that of northern fan delta deposits, but different from sediments from the western and southern deltas, suggesting that the sublacustrine fan was sourced from the north. Therefore, a deep-lacustrine depositional model is proposed for the sublacustrine fan, showing a gravity-driven evolution process from fan delta front in the northern slope into cohesive debrites and then turbidites in the lake center. This was mainly controlled by fault-related topography and sediment supply. This study highlights the effective use of the detrital zircon UPb dating method to identify the provenance direction of sublacustrine fans, and the proposed depositional model may be applicable to reservoir prediction in other lacustrine basins.

Compositional relationship between the source-to-sink segments and their sedimentary response to diverse geomorphology types in the intrabasinal lower uplift of continental basins

Investigating on new method for Source-to-Sink study by integrating modern system and subsurface interpretation, establishment of the quantitative coupling model from source to sink, and form industrial application standard have long been the research trends of Source-to-Sink system in continental basins. Here the symmetrical and asymmetrical conceptual source-to-sink coupling models (i.e., Symmetrical Dumbbell Type and Asymmetrical Dumbbell Type) are proposed to respond to diverse geomorphology types (i.e., convergent and unitary incised type) based on the morphological and digital proportion between catchment-drainage unit of upstream and sediment-fan of downstream, then summarize and extract the two models by contrasting a modern source-to-sink system of Hovd Central Uplift in northwestern Mongolia as well as an ancient source-to-sink system of Huizhou-Lufeng Lower Uplift in Pearl River Mouth Basin, South China Sea. In the practical study, the satellite remote sensing and Google geological map are used to deconstruct the modern case and combine the Google-topographic-map, well log data, core data and seismic-geomorphology analysis to dissect the ancient case analogously. This paper mainly discusses morphological proportional ranges of catchment acreage (Asource) and sediment fan acreage (Ssink), the relativity between topographic gradient (Gsource) and the source ratio (Rsource=AsourceAsource+Ssink), and the physical properties of sedimentary sand-body associated with diverse geomorphologic types in continental lacustrine basins. Therefrom, it is concluded that the proportion (P=Asource:Ssink) between catchment-drainage segment of source area (Asource) and sediment-fan of depositional sink (Ssink) is over 1:1 in Symmetrical Dumbbell Type source-to-sink coupling model, which is commonly between 1:1.1 and 1:1.6; However, it is below 1:1 in Asymmetrical Dumbbell Type source-to-sink coupling model, which is generally between 1:0.6 and 1:0.9; and there is an evident negative correlation between topographic gradient (Gsource) and the source ratio (Rsource). Finally, these results reveal that the reservoir formed by Symmetrical Dumbbell Type source-to-sink coupling model performs much more superiority. In summary, the two coupling models have not only supplied a new insight into the research on sandy-sediments transport and distribution, but also defined the directions for better finding favourable sand bodies (reservoir) when exploring oil and gas.

Reservoirs properties of slump-type sub-lacustrine fans and their main control factors in first member of Paleogene Shahejie Formation in Binhai area, Bohai Bay Basin, China

Abstract High-yielding oil wells were recently found in the first member of Paleogene Shahejie Formation, the Binhai area of Qikou Sag, providing an example of medium- and deep-buried high-quality reservoirs in the central part of a faulted lacustrine basin. By using data of cores, cast thin sections, scanning electron microscope and physical property tests, the sedimentary facies, physical properties and main control factors of the high-quality reservoirs were analyzed. The reservoirs are identified as deposits of slump-type sub-lacustrine fans, which are marked by muddy fragments, slump deformation structure and Bouma sequences in sandstones. They present mostly medium porosity and low permeability, and slightly medium porosity and high permeability. They have primary intergranular pores, intergranular and intragranular dissolution pores in feldspar and detritus grains, and structural microcracks as storage space. The main factors controlling the high quality reservoirs are as follows: (1) Favorable sedimentary microfacies of main and proximal distributary gravity flow channels. The microfacies with coarse sediment were dominated by transportation and deposition of sandy debris flow, and the effect of deposition on reservoir properties decreases with the increase of depth. (2) Medium texture maturity. It is shown by medium-sorted sandstones that were formed by beach bar sediment collapsing and redepositing, and was good for the formation of the primary intergranular pores. (3) High content of intermediate-acid volcanic rock detritus. The reservoir sandstone has high content of detritus of various components, especially intermediate-acid volcanic rock detritus, which is good for the formation of dissolution pores. (4) Organic acid corrosion. It was attributed to hydrocarbon maturity during mesodiagenetic A substage. (5) Early-forming and long lasting overpressure. A large-scale overpressure compartment was caused by under-compaction and hydrocarbon generation pressurization related to thick deep-lacustrine mudstone, and is responsible for the preservation of abundant primary pores. (6) Regional transtensional tectonic action. It resulted in the structural microcracks.

Microbially induced dolomite precipitates in Eocene lacustrine siliciclastic sequences in the Dongying depression, Bohai Bay Basin, China: Evidence from petrology, geochemistry, and numerical modeling

This study investigates the origin of dolomite by examining thick lacustrine sequences of evaporites, mudstones, and deep-water sublacustrine fan sandstones in the lower unit of the fourth member of the Eocene Shahejie Formation (Es4x) in Bohai Bay Basin, China. This petrographic, geochemical, and numerical modeling study recognizes evidence for microbially mediated dolomite precipitates in the Es4x sandstones, including the following: (1) spherical or elliptical dolomite aggregates consisting of submicrometer-sized crystals; (2) negative δ13CVPDB (VPDB refers to Vienna Peedee belemnite) values of the dolomite cements (–6.56‰ to –3.35‰); (3) relatively low precipitation temperatures of dolomite cements (39°C–65°C [102°F–149°F]) based on δ18OVPDB values from –8.76‰ to –4.65‰; and (4) negative 34SVCDT (VCDT refers to Vienna Canon Diablo troilite) values of associated framboidal pyrite (–3.9‰ to +5.7‰) with respect to an anhydrite precursor (+21.2‰ to +37.8‰). One-dimensional reactive transport modeling incorporating biodegradation kinetics was conducted to evaluate the link between biodegradation of organic matter and geochemical reactions of inorganic minerals during microbial sulfate reduction (MSR). Microbial metabolism in the model creates favorable geochemical conditions for dolomite precipitation by increasing pH, carbonate alkalinity, and Mg2+/Ca2+ ratio. The modeling results reproduce petrographic observations associated with the MSR process and indicate that the kinetics of microbial growth can significantly modify the geochemistry of simulated waters and enhance microbially mediated dolomite precipitation. Therefore, this study illustrates the effect of MSR in dolomite precipitation in a semiclosed and reducing continental lacustrine basin.