Lacustrine Basin Research Articles

Deep hydrocarbons in the northwestern Junggar Basin (NW China): Geochemistry, origin, and implications for the oil vs. gas generation potential of post-mature saline lacustrine source rocks

Abstract Deep (>4500 m) sedimentary units in petroliferous basins are a frontier field for petroleum exploration and exploitation worldwide. This study focuses on the northwestern Junggar Basin of NW China and presents the results of a comprehensive geochemical analysis of the source rocks and hydrocarbons hosted by deep reservoirs in this region, including data obtained from Rock-Eval pyrolysis, carbon isotope, biomarker, and light hydrocarbon analyses. These data provide insights into the exploration potential of deep hydrocarbons in the study area and in other areas with similar geologic settings (i.e., saline lacustrine petroleum systems). Results indicate that the deep hydrocarbons in the northwestern Junggar Basin are dominated by oil, with both the oil and natural gas being derived mainly from the lower Permian Fengcheng Formation. This formation was deposited in a heterogeneous saline lacustrine environment, which led to the generation of geochemically diverse oil compositions. Consequently, Fengcheng-derived oils can be divided into subcategories based on source rock lithology; i.e., hypersaline alkaline muddy dolomites, saline (dolomitic) mudstones, and brackish mudstones. A small number of deep oils in the study area were derived from the Carboniferous–lower Permian Jiamuhe Formation. The deep hydrocarbons in the study area generally accumulated in near-source environments, indicating that prospective areas are present in the form of Fengcheng-derived oils in the depocenter of the Mahu sag, especially oils derived from hypersaline alkaline muddy dolomites and saline (dolomitic) mudstones. These source rocks are likely to have generated oil even under the highly mature conditions associated with deep burial. This might explain why discoveries within saline lacustrine basins are generally dominated by oil, implying that these conditions might also exist within the deeper parts of saline lacustrine basins worldwide.

Miocene fragmentation of the Central Andean foreland basins between 26 and 28°S

We present new U-Pb LA-ICP-MS data from the Central Andean foreland basins combined with new and published stratigraphic information in order to reconstruct the Miocene fragmentation of the Andean foreland between 26 and 28°S. The disruption of this foreland basin and the subsequent development of elevated intermountain basins have been the focus of several studies. However, the absence of temporal constraints in the Miocene to Pliocene sedimentary record of the low elevation Choromoro and Tucuman foreland basins has presented an obstacle for precise paleogeographic reconstructions. We describe 11 discontinuous stratigraphic sections and use the U-Pb LA-ICP-MS method to date 10 pyroclastic-bearing sediments in order to reconstruct the stratigraphic evolution of the Choromoro and Tucuman basins. We combine our results with published stratigraphic and thermochronologic data from adjacent basins to present a refined Miocene paleogeographic model. In a first stage, a continuous Early Miocene foreland lacustrine basin developed, filling up the preexisting Paleogene topography. The second stage is characterized by basin unroofing around ∼12 Ma; the easily eroded sedimentary cover was removed, leading to the uplift of the underlying basement rocks and the segmentation of the lacustrine system. In the third stage, relief increase took place after ∼6 Ma due to the low erodibility of the basement blocks; as a result, stable fluvial systems developed. Progressive relief development caused pronounced unconformities in the basins and the development of proximal fluvial-gravitational depositional systems after 3 Ma. This model emphasizes on the relations between tectonics, climate, and erodibility, and their control on the evolution of the depositional systems and relief.

1 billion-year-old cell contents preserved in monazite and xenotime

Exceptional microfossil preservation, whereby sub-cellular details of an organism are conserved, remains extremely rare in the Precambrian rock record. We here report the first occurrence of exceptional cellular preservation by the rare earth element (REE) phosphates monazite and xenotime. This occurs in ~1 billion-year-old lake sediments where REEs were likely concentrated by local erosion and drainage into a closed lacustrine basin. Monazite and xenotime preferentially occur inside planktonic cells where they preserve spheroidal masses of plasmolyzed cell contents, and occasionally also membranous fragments. They have not been observed associated with cell walls or sheaths, which are instead preserved by clay minerals or francolite. REE phosphates are interpreted to be the earliest minerals precipitated in these cells after death, with their loci controlled by the micro-scale availability of inorganic phosphate (Pi) and REEs, probably sourced from polyphosphate granules within the cells. The strong affinity of REEs for phosphate and the insolubility of these minerals once formed means that REE phosphates have the potential for rapid preservation of cellular morphology after death and durability in the rock record. Hence, authigenic REE phosphates provide a promising new target in the search for the preservation of intra-cellular components of fossilised microorganisms.

Stratigraphic charactertistic and tectonic control on the paleogene sedimentary evolution of the Baxian Half-graben, Bohai Bay Basin, east China

The depositional and structural development of a deltaic dominated succession, interpreted from seismic data, cores, well curves and lab data, is discussed. The case is from a comprehensive analysis of stratigraphy and the depositional process of a deltaic to continental succession in the early to late Paleogene in the Baxian Half-graben, Bohai Bay Basin, East China. The evolution of three genetic successions with a total thickness of 1000–7000 m, comprising three facies associations, including fan delta, braided-river to shallow water delta and lacustrine deposit, records: a, initial rift stage, b, late rift stage, and c, post-rift stage. (1) The initial rift stage is made up of several fining-upward successions, mainly stacked in retrogradation style that pass from the fan delta plain, through the fan delta front to pro-delta in a deep lacustrine environment. This stage is characterized by high intensity rifting and the deposition was keeping up with subsidence. (2) The late rift stage is made up of several successions stacked in both progradation and retrogradation styles. The succession is defined by braided-river deltas, distributary channels, subaqueous channels, swamp, levees, and fluvial-dominated mouth bar, which are mainly identified using core data. This stage is characterized by a decrease in the intensity of rifting and finer-grained deposits. (3) The post-rift stage is made up of several successions stacked mainly in an aggradation style. Shallow water delta developed in the early phase, and lacustrine deposits developed in the late phase. The transition from initial-rift to late-rift phase is represented by a seismically identifiable regional angular unconformity and an overall upward decrease in the quantity of syn-depositional faults. The late-rift to post-rift evolution is represented by an overall decrease in grain size, decrease in fluvial influence and increase in wave influence. The research process can be a reference for the study of stratigraphic architecture and sedimentary process in other lacustrine basins where tectonic movements are well developed. The achievements of this article can be an important basis for the further exploration in the half-graben.

Source-to-sink analysis in an Eocene rifted lacustrine basin margin of western Shaleitian Uplift area, offshore Bohai Bay Basin, eastern China

During the Eocene, the western Shaleitian Uplift and its adjacent sags constituted a source-to-sink (S2S) system on an active lacustrine basin margin with a steeply dipping boundary fault. We integrated a range of data types with subsurface observations from the western offshore Bohai Bay Basin (OBBB) that allowed us to quantify the size and classification of catchment-fluvial segments, and analyze the sedimentary architecture and sediment fluxes of the proximal fan-delta system. We then discovered quantitative coupling relationships among the parameters of ancient source-to-sink segments. Results show that in the study area the bedrock of the western Shaleitian Uplift is composed of Proterozoic migmatized granites or granites. On the basis of paleogeomorphology reconstruction, we established classification standards for multi-scale catchment units, divided the provenance into four third-order catchment units, obtained quantitative parameters of the catchment-unit area (A), catchment relief (R), and total sediment discharge (Q s , calculated by BQART model) and then confirmed the scales and distribution (cross-sectional area, S cs ) of sediment-transport pathways. Furthermore, we analyzed tectonic subsidence, sediment-dispersal patterns, and sediment fluxes of fan-delta systems (Q f ) using seismic sedimentology in the third member of the Eocene Shahejie Formation (Es 3 ). Finally, we used quantitative, multivariate, statistical analysis of the parameters involved in the source-to-sink segments to (1) show the proportional range between total sediment fluxes of fan delta (Q f ) and sediment discharge (Q s ) from catchment (largest rate of Q f /Q s in the lower part of Es 3 , smallest rate of Q f /Q s in the middle part of Es 3 , and the range from 30% to 70%) and (2) arrange the mainly controlled parameters in terms of their importance (i.e., rate of tectonic subsidence, catchment area, catchment relief, and cross-sectional area of sediment-transport pathway are listed in order of importance). This study can provide an effective way of quantifying and ordering the parameters important in that controlling a complex ancient source-to-sink system, thus helping to achieve an understanding of the process of tectono-sedimentary evolution on a rifted, lacustrine-basin margin. • Divide the catchment segment into different orders and pick up the main parameters. • Indicate the sediment-dispersal patterns and sediment flux of fan-deltas in an active extensional setting. • Reveal the correlation and ordering of multiple important parameters of ancient source-to-sink in a rift basin margin.

Development characteristics and controlling factor analysis of the Neogene Minghuazhen Formation shallow water delta in Huanghekou area, Bohai offshore basin

Based on a combination of high resolution 3-D seismic, drilling and well logging and core data, this study focuses on describing the depositional features of the Neogene Minghuazhen Formation shallow water delta in Huanghekou area (HHKA), Bohai offshore basin and discussing the evolution and controlling factors of shallow water delta sandbody. An obvious meandering fluvial delta system developed in sequence 1 (SQ1) of the Neogene in HHKA with thinner sandbody of distributary channels and poor development of mouth bar. The sequence texture obviously influences the vertical development and stacking pattern of sandbodies and controls the distribution of sandbodies in plain view as well. In shallow water lacustrine basins, relative topographic height difference leads to change of distribution of accommodation space, and sandbodies of distributary channels usually develop well in local low-lying areas where accommodation space increases. The delta is dominated by distributary channel sandbodies during the early period of base level rising. Sandbodies contact with each other in a lateral stacking pattern and are characterized by a fan shape in plain view. Distributary channels gradually narrow and tend to shift during the mid- late-period of base level rising, while the sandbodies are characterized by a net shape in plain view. During the period of base level slow falling, the multistory/multilateral channel sandbodies dominated the inner front of shallow-water delta and the delta sand dispersal distributes as a lobe shape.

Sedimentary characteristics and depositional models of two types of homogenites in an Eocene continental lake basin, Shulu Sag, eastern China

Homogenites are homogeneous and structureless, fine-grained rocks. They have been studied extensively in marine settings and are typically interpreted as being related to catastrophic events such as earthquakes. However, studies on lacustrine homogenite-like rocks are limited. New core data from the Shulu Sag in China reveal that thick massive calcilutites exist in the lower Shahejie 3 Formation, which formed during the middle Eocene rifting stage. Based on petrographic, geochemical, and mineralogical data, these massive calcilutites are interpreted as homogenites and grouped into two types. Type A homogenite is composed of intrabasinal materials characterized by high calcite, total organic carbon (TOC) content, and δ13C value. Type A homogenite has two facies successions, the first of which is characterized by homogeneous appearance capped by an erosional interface and the second is characterized by repeated massive-laminated calcilutite successions. Type B homogenite contains abundant allochthonous materials displaying relatively high dolomite content and low calcite, TOC, and δ13C content. Type B homogenite represents the topmost finer portion of a graded megabed which includes, from base to top, grain supported carbonate conglomerates, slumped deformed portions, and debrites. The whole unit can be considered as a megaturbidite. Earthquakes are the most reasonable triggers of homogenite formation in the Shulu Sag. Type A homogenites are thought to be derived by fluidification, resuspension, and decantation of lacustrine fines due to shock waves generated by an earthquake. Type B homogenites might be deposits of distal turbidity currents and turbid clouds transformed from other gravity flows. The results of this study provide a better understanding of the formation of homogenites caused by earthquakes in a confined lacustrine basin.

Critical differences in sediment delivery and partitioning between marine and lacustrine basins: A comparison of marine and lacustrine aggradational to progradational clinothem pairs

Critical differences in sediment delivery and partitioning between marine and lacustrine basins: A comparison of marine and lacustrine aggradational to progradational clinothem pairs

How marine incursion influences the quality of lacustrine source rocks: The Paleogene Nanxiang Basin, eastern China

The impact of marine incursions during transgression (i.e., sea level rises and the shoreline moves landward) on the formation and quality of lacustrine source rocks is an important and contentious issue. In this study, we present a case study of the Paleogene Hetaoyuan Formation in the Biyang sag, Nanxiang Basin, eastern China. Paleontological, trace-element, and biomarker data indicate that the Hetaoyuan source rocks in this region were influenced by a marine incursion. The paleontological evidence indicates that the marine incursion resulted in the introduction of red and brown algae, which commonly inhabit marine environments. Trace-element analyses yielded representative evidence of marine incursion (e.g., equivalent boron content >300 ppm and B/Ga ratio >4.2). Biomarker evidence for marine incursion includes C26/C25 tricyclic terpanes ratios of 1:3, which is the threshold for distinguishing marine organic matter from lacustrine. Using the B/Ga ratio as a typical paleosalinity indicator, it was determined that the influence of marine incursion decreased from the Biye 1 to Cheng 2 to An 3006 wells, with the B/Ga ratio average decreasing from 7.51 to 6.81 to 3.73, respectively. With an increasing extent of marine incursion (e.g., distance landward, overall water depth, and marine–freshwater mixing), the primary productivity of organic matter increased, and the preservational environment became more reducing. These changes resulted in higher contents of organic matter (total organic carbon e 2–8 wt. %) and a more favorable type of organic matter for oil generation (kerogen type I–II), indicating that the marine incursion had a positive effect on the formation of source rocks. Therefore, the formation mechanism of high-quality source rocks in coastal lacustrine basins during high sea-level periods and associated resource potential might need to be reevaluated (e.g., the Campanian lower Neslen Formation along the margins of the Western Interior Seaway of North America and the terminal Oligocene–early Miocene in the fluvial Saldanha Bay at the southwestern tip of Africa). The results also provide useful data for regional oil and gas exploration.

Distribution of mineral phases in the Eocene Green River Formation, Piceance Basin, Colorado—Implications for the evolution of Lake Uinta

The mineralogy of the Eocene Green River Formation in the Piceance Basin, Colorado, has been the subject of numerous studies since the 1920s. Most previous work has focused on the resource potential of these lacustrine mudrocks, which in addition to substantial oil shale potential (in-place resources of 353 billion barrels of synthetic crude oil for rocks yielding at least 25 gallons per ton, GPT), includes nahcolite, a currently utilized soda ash resource, and dawsonite, a potential alternative source of aluminum. Another reason to study the mineralogy in this system is that the geographic and stratigraphic distribution of various authigenic minerals may provide insights into the geochemistry and depositional environment of the long-lived Eocene Lake Uinta. In this study, legacy non-quantitative (presence/absence) X-ray diffraction (XRD) data recently published by the U.S. Geological Survey (USGS) for more than 9,000 samples collected from 30 coreholes in the Green River Formation, Piceance Basin were examined. These data were used to better define the stratigraphic and paleogeographic extent of a set of indicator minerals (illite, analcime, albite, dawsonite, and nahcolite) within the Piceance Basin lacustrine strata. This set of minerals was selected based on observations from previous work and variability in their occurrence and co-occurrence within the Piceance Basin. The USGS database has been used to (1) construct maps showing geographic variations in mineral occurrences for 14 stratigraphically defined rich and lean oil shale zones; (2) assess co-occurrences of indicator minerals; and (3) compare occurrence results with quantitative XRD datasets collected on Piceance Basin oil shales. Occurrences of many authigenic minerals (analcime, dawsonite, and nahcolite) varied in the lacustrine strata near and around the depocenter, but others, like quartz, dolomite, and feldspar (potassium + undifferentiated), were widely and consistently present (>90% of samples) across the basin. Shifts in the distribution of indicator mineral occurrences generally coincide with changes identified in previous lake history descriptions and indicate that the water chemistry of Lake Uinta varied significantly going from near-shore to the depocenter and through time.

Origin of Miocene halite from Tuz Gölü basin in Central Anatolia, Turkey: Evidences from the pure halite and fluid inclusion geochemistry

The study area is located in the south of Konya Closed Basin which is the largest Miocene inner basin in Central Anatolia. The aim of this study is to determine the chemical composition of the Miocene halite deposits and depositional environments of Tuz Gölü lacustrine basin. For this scope, fluid inclusion analyses (K+, Mg2+, SO42-) of nine pure halites were performed and twenty six pure halite samples have been examined. Additionally, K+, Mg2+, SO42- ion concentrations of the fluid inclusions and pure halite samples were compared to Gulf of Suez (GS), Red Sea (RS) and other Cenozoic seawaters (CS).Results from analyses show that K+ content of the fluid inclusions of the halites in the Tuz Gölü basin, which are considerably higher than the K+ contents both of the pure halites and the GS and RS and CS, result from dissolution of the primary K+-salts. The Br− content of the pure halite samples is ranging from 5 to 637 ppm. The high Br− (>40 ppm) content indicates the marine origin while the low Br− content is related to dissolution and recycled of the previously deposited marine halites. Mg2+ and SO42- contents of the Tuz Gölü basin samples are higher than those of the GS and RS and CS except for TG4 samples. The high Mg2+ and SO42- contents were resulted from the dissolution of previously deposited gypsum and de-dolomitization process. δ34S isotope (18.80 to 21.00‰) and low δ18O (8.10 to 9.50‰) values of the Tuz Gölü basin samples, which are compatible with isotope values of the CS, indicate marine origin. In addition, the high δ18O values (16.20 to 20.50‰) and the δ34S content can be explained by recycling of marine sulphate as well as dissolution and re-crystalization of previously deposited sulphates.The possible effective mechanisms causing chemical differences in the Tuz Gölü basin, which is a sedimentary basin formed under highly complex different physico-chemical mechanisms, have been detected as (1) the changes in chemistry of halites due to dissolution and re-crystallization processes of previously deposited halites and evaporites, (2) de-dolomitization processes and (3) seawater input.

Palaeoenvironmental and palaeohydrological variability of mountain areas in the central Mediterranean region: A 190 ka-long chronicle from the independently dated Fucino palaeolake record (central Italy)

Proxy records of past climate change spanning beyond the radiocarbon range commonly derive their chronologies from orbital tuning strategies, thus bounding our spatio-temporal reconstructions to a priori assumptions that can not be directly tested. Here we present a tephrochronologically constrained framework of past environmental and climatic changes in the central Mediterranean region during the last ca. 190 ka. Our research is based on a high-resolution, multi-proxy study of a sedimentary record (cores F1-F3) retrieved from the Fucino Basin lacustrine succession, central Italy, in 2015. We update the existing tephrostratigraphic framework of the F1-F3 record with the finding of the widespread Campanian Ignimbrite tephra marker layer and produce a robust and independent chronology based on new and published 40Ar/39Ar and 14C dating of 17 tephra layers. Observed palaeoenvironmental changes are tracked in other lacustrine, marine and speleothem records across the Mediterranean and North Atlantic regions via tephrostratigraphic correlations and chronological matching providing a robust assessment of age uncertainties. Results show a complex interplay between local environmental changes and broad-scale climatic processes highlighting a strong orbital forcing on glacial-interglacial changes. Along with these major changes we detect prominent millennial-scale variability. During times of intermediate global ice volumes, Mediterranean mountain ecosystems oscillated around an “interglacial” state suggesting that climatic shifts, although large, did not exceed the local environmental tolerance-resilience threshold. Conversely, during periods of large global ice volume, we observe subdued millennial-scale variability with ecosystems operating in a strictly ruled glacial environment.

Sequence palaeogeography, lacustrine basin evolution, and coal accumulation in the Lower Cretaceous Fuxin continental faulted basin, China

The Fuxin Basin is one of the major coal‐bearing basins in northeastern China with the coal seams mainly preserved in the Lower Cretaceous Fuxin Formation. The sequence stratigraphy, palaeogeography, basin evolution, and coal accumulation of the lacustrine coal‐bearing sequences in the Fuxin Formation were analysed based on borehole cores, well logs, and outcrop sections. Fifteen distinct lithofacies types were recognized with lithologies ranging from conglomerates, sandstones, siltstones, and mudstones to coals. Lithofacies are arranged into five facies associations representing alluvial fan, braided fluvial, meandering fluvial, fan delta, and lacustrine depositional systems. These facies associations collectively revealed a continental faulted basin setting. Based on the regional unconformities, basal scour surfaces of the incised valley fills sandstones, paleosols, or seat earths of coal seam on interfluves, and surfaces that mark abrupt depositional facies shifts, the Fuxin Formation was subdivided in two third‐order sequences. Palaeogeographic maps of each sequence have been reconstructed based on the contours of lithological parameters. The palaeogeographic units include alluvial fan, alluvial plain, channel (braided and meandering fluvial), fan delta, and lake. Preferred coal‐forming environments were mainly developed in the fan delta plain and fluvial back‐swamp. Compared with Sequence 1, the activity of basin‐margin faults and basement subsidence of the Sequence 2 gradually decreased, resulting in the lower rate of potential accommodation and less favourable conditions for peat accumulation. This background provided a basic understanding for the lacustrine basin evolution and coal accumulation. A six‐phase evolution model was proposed, which includes lowstand systems tract, early transgressive systems tract, late transgressive systems tract, early highstand systems tract, late highstand systems tract, and falling stage systems tract. The early and late TST of Sequence 1 and Sequence 2 were relatively beneficial stages for peat accumulation and coal development in the Fuxin Basin.

Rift activity and sequence stratigraphy of the Oligocene Dongying Formation in the Nanpu Sag, eastern China: Implications for rift sequence stratigraphy in lacustrine basins

Rifting plays an important role in accommodation variations in rift basins. However, fluvial discharge as another important factor affecting lacustrine rift sequence stratigraphy is not well explored. In this work, seismic and well data are used to investigate the stratigraphic features, rift activity, and effect of fluvial discharge in the Nanpu Sag, Bohai Bay Basin. The syn‐rift deposition of the Dongying Formation (28.5–23.8 Ma) is divided into four sequences (SQ1, SQ2, SQ3, and SQ4), and the rift‐related subsidence rate along two parallel boundary faults is calculated. The results indicate that the rift activity weakens from SQ1 to SQ4 and that the two parallel boundary faults consist of a total of four fault segments. As the fault segmented, three propagation folds formed intrabasin highs on which the Beipu, Laoyemiao, and Gaoliu outlets developed. Well logs and 3D seismic data show that from SQ1 to SQ3, retrogradational stacking patterns occur in Beipu, Laoyemiao, and the hanging‐wall ramp, while progradational stacking patterns occur in the Gaoliu outlet. In SQ4, the sequence stacking patterns are progradation‐dominated. By evaluating channel erosion and sediment supply, the fluvial discharge can be estimated, and the result indicates that the Gaoliu outlet has the strongest fluvial discharge from SQ1 to SQ3 with strong channel erosion, and in SQ4, the fluvial discharge of each outlet is comparable and relatively low. The high fluvial discharge of the Gaoliu outlet from SQ1 to SQ3 is considered to be the primary reason for the lake level rise in the intervals between the pulses of rift activities. Sequence stacking patterns and depositional trends in our study intervals are not in line with the rift subsidence rate; this indicates that rifting is not the only factor controlling the rift sequence stratigraphy and variations in the accommodation of lacustrine rift basins. While the Gaoliu outlet has predominant fluvial discharge from SQ1 to SQ3, progradational parasequence sets deposit in the Gaoliu outlet. In contrast, the retrogradational parasequence sets occurred in the Beipu outlet, Laoyemiao outlet, and the hanging‐wall ramp. As the fluvial discharge of the Gaoliu outlet decreased in SQ4, the fluvial discharge in each outlet became low, and the stacking patterns were dominated by progradation. Consequently, this research suggests that fluvial discharge is a significant factor controlling the accommodation creation in lacustrine rift basins, and a sequence model which gives a full consideration of fluvial discharge is constructed for lacustrine rift basins.

Lacustrine Basin Fills in an Early Cretaceous Half-Graben, Jiuquan Basin, NW China: Controlling Factors and Implications for Source Rock Depositional Processes and Heterogeneity

Studies on basin fills have provided significant insights into reservoir distribution and prediction in petroliferous basins, however, the effect of basin fills on source rock properties has been underexplored. This paper documents basin filling characteristics and their implications for depositional processes and heterogeneity of source rock in the Qingnan subsag of the Jiuquan Basin, by using subsurface geological data from recent hydrocarbon exploration efforts in this area. Drill core data reveals that the basin fill of the Qingnan subsag was dominated by fan delta-lacustrine systems, in which deposition of the fan deltas along the basin margin was mainly through gravity flows. The temporal and spatial evolution of the depositional systems indicates that the basin fill was characterized by a continuously retrogradational process, with decreasing extent of fan deltas in vertical succession. Weakening of tectonic activities and climate change from humid to semi-arid are interpreted to be the main control factors that were responsible for the retrogradational basin fill. The different depositional environments in the early stage and late stage of the retrogradational basin filling history resulted in the different depositional processes and properties of source rocks. This study suggests that source rock heterogeneity associated with basin fills in lacustrine basins should be considered in hydrocarbon exploration.

Characteristics and formation mechanism of multi-source mixed sedimentary rocks in a saline lake, a case study of the Permian Lucaogou Formation in the Jimusaer Sag, northwest China

Abstract The Permian Lucaogou Formation in the Jimusaer Sag of the southeastern Junggar Basin in China is an uncommon set of mixed sedimentary rocks. The mixed sediment content of the Lucaogou Formation is unusual due to the large amounts of tuffaceous material, which makes the system more complex than the commonly mixed siliciclastic-carbonate rocks. In this study, the volcanic material acts as a third end-member component in a multi-source mixed sedimentary system. Therefore, a new type of mixed siliciclastic-carbonate-tuffaceous rocks has been introduced. Methods including core observations, thin section analysis, grain size measurements, and X-ray diffraction are used to investigate the characteristics of mineral components, sedimentary structures and lithological assemblages in upper and lower sections of the formation. These analyses are then used to determine the lateral distribution area of mixed sedimentation. Sedimentary mixing processes originally described for siliciclastic-carbonate systems are used and expanded here for the multi-source mixed rocks in a saline lacustrine basin. Controlling factors on mixed sedimentation included climate change, lake level fluctuation, tectonic activity and sediment supply rate mentioned in previous studies, as well as variations of energy levels in different parts of the lake. Mixed-depositional models for the lower and upper sections of the Lucaogou Formation in saline lacustrine basin have been established, respectively.

First Peopling and Lithic Raw Material Use in Lacustrine Basins and Highlands of Central-Western Santa Cruz Province (Argentine Patagonia)

ABSTRACTIn the steppe and forest ecotone of central-western Santa Cruz province, Argentina, there are three lithic raw materials whose primary and secondary sources have been recognized. These are the obsidian, the siltstone, and a silicified green tuff. Moreover, the characteristics of their use during the late Holocene are well known. In this paper, we evaluate diagnostic and comparative information regarding their use during the early stages of hunter-gatherer settlement in this southern region. The analysis is based on debitage, tools, and cores recovered in contexts with early and middle Holocene chronologies from the low basins of Tar, San Martín, and Cardiel lakes as well as the highlands of the Guitarra plateau and Perito Moreno National Park. We also discuss the importance of these knappable materials in the human settlement processes and the initial learning of the landscape.

Paleogeomorphic Influence on Facies Characteristics and Reservoir Prediction in Lacustrine Basin<br/>—By Taking Cretaceous Reservoirs in the Western Slope of SL Basin, China as an Example

The West Slope of SL Basin, China, mainly targets on lithologic reservoirs for oil and gas. The paleogeomorphology and provenance direction analysis has great significance for predicting the type and distribution of sand body. According to the analysis of well logging, core and seismic data, braided delta and gravity sediment mainly develop in the study area. A backs tripping method is used to reconstruct the topography of Qingshankou to Yaojia Formation, Cretaceous. The West Slope is mainly controlled by two slope belts in Qingshankou Formation, with the width of upper slope 12 - 18 km and the gradient 0.7 - 0.8 degrees, the width of lower slope 13 - 15 km and the gradient 1.0 degrees. The West Slope is controlled by a single slope belt in Yaojia Formation, with the lower slope width of 13 - 16 km and the gradient 0.4 degrees. The relationship between the slope belt, provenance direction and sand body distribution is analyzed, and result shows that the combination of upper and lower slope controls the distribution of reservoir types, and the provenance supply rate controls the scale of sand body. The gravity flow develops when the provenance direction is parallel to slope direction, and the slope becomes transporting channel in vertical to provenance direction. The paleogeomorphology, lake level and provenance direction are the main control factors of sand body in West Slope.

Genesis and depositional model of subaqueous sediment gravity-flow deposits in a lacustrine rift basin as exemplified by the Eocene Shahejie Formation in the Jiyang Depression, Eastern China

Abstract Subaqueous sediment gravity-flow (SSGF) deposits have recently been recognized as a major reservoir plays in lacustrine basins. Research on their genesis and depositional models are crucial to understand gravity flow evolution and the distribution of their deposits. This study analysed SSGF deposits in the third member of the Shahejie Formation in the Jiyang Depression, Eastern China, based on 3-D seismic data, well-log data, and core observations. The types and origins of SSGF, the distribution patterns and depositional models of SSGF deposits have been investigated. The dominant types of SSGF include: muddy debris flows, sandy debris flows, high density turbidity currents, surge-like low density turbidity currents, and quasi-steady low density turbidity currents, which are mainly caused by river floods and sediment failures. The overall distribution of SSGF deposits is controlled by the sediment supplies, basin structure, and sequence stratigraphic framework. Paleotopography, flood energy, triggering mechanism, and the slope angle of the delta front are also important. SSGF deposits associated with river floods can be further subdivided into elongated and fan-shaped deposits. Elongated sandstones are composed predominantly of bedload-dominated and suspension load-dominated hyperpycnal flow deposits. The fan-shaped sandstones can be subdivided into an inner fan, a middle fan, and an outer fan. Inner fan deposits are dominated by gully-filling debrites. Middle fan deposits are composed predominantly of channel-fill deposits, while outer fan deposits consist of thin bedded and laterally extensive lobe intervals. SSGF deposits triggered by sediment failures can be further subdivided according to their proximity to slumping: proximal, intermediate and distal parts. The proximal deposits are composed predominantly of slide and slump deposits. The intermediate deposits are dominated by sandy debrite tongues, while the distal deposits are composed predominantly of thin-bedded, surge-like, low-density turbidites. This study offers some insight into the types, origin of gravity-flow, as well as the distribution of their deposits in a lacustrine rift basin.

Influence of sedimentary environment on the shale gas reservoir of Sahai formation in Fuxin Basin, northeast China

ABSTRACTFuxin Basin is a continental faulted lacustrine basin in northeast China with significant shale gas resources. The shale reservoir mainly developed in the sedimentary environment of the deep and semi-deep lake, swamp, and the interdistributary bay. Deep and semi-deep lake shale has considerable thickness, high clay content, dominant type II1 kerogen, and more pyrite framboid pores; swamp shale has the highest TOC content, type III kerogen, high clay content, and more organic pores; interdistributary bay shale shows high brittle mineral contents, type II2 kerogen, and more intergranular pores. The methane adsorption capacity is highest in swamp shale, followed by the deep and semi-deep lake and interdistributary bay.