Shallow Lacustrine Research Articles

Evaluation of source rocks and prediction of oil and gas resources distribution in Baiyun Sag, Pearl River Mouth Basin, China

By conducting organic geochemical analysis of the samples taken from the drilled wells in Baiyun Sag of Pearl River Mouth Basin, China, the development characteristics of hydrocarbon source rocks in the sag are clarified. Reconstruct the current geothermal field of the sag and restore the tectonic-thermal evolution process to predict the type, scale, and distribution of resources in Baiyun Sag through thermal pressure simulation experiments and numerical simulation. The Baiyun Sag is characterized by the development of Paleogene shallow lacustrine source rocks, which are deposited in a slightly oxidizing environment. The source rocks are mainly composed of terrestrial higher plants, with algae making a certain contribution, and are oil and gas source rocks. Current geothermal field of the sag was reconstructed, in which the range of geothermal gradients is (3.5–5.2) °C/100 m, showing an overall increasing trend from northwest to southeast, with significant differences in geothermal gradients across different sub-sags. Baiyun Sag has undergone two distinct periods of extensional process, the Eocene and Miocene, since the Cenozoic era. These two periods of heating and warming events have been identified, accelerating the maturation and evolution of source rocks. The main body of ancient basal heat flow value reached its highest at 13.82 Ma. The basin modelling results show that the maturity of source rocks is significantly higher in Baiyun main sub-sag than that in other sub-sags. The Eocene Wenchang Formation is currently in the stage of high maturity to over maturity, while the Eocene Enping Formation has reached the stage of maturity to high maturity. The rock thermal simulation experiment shows that the shallow lacustrine mudstone of the Wenchang Formation has a good potential of generating gas from kerogen cracking with high gas yield and long period of gas window. Shallow lacustrine mudstone of the Enping Formation has a good ability to generate light oil, and has ability to generate kerogen cracking gas in the late stage. The gas yield of shallow lacustrine mudstone of the Enping Formation is less than that of shallow lacustrine mudstone of the Wenchang Formation and the delta coal-bearing mudstone of the Enping Formation. The numerical simulation results indicate that the source rocks of Baiyun main sub-sag generate hydrocarbons earlier and have significantly higher hydrocarbon generation intensity than other sub-sags, with an average of about 1 200×104 t/km2. Oil and gas resources were mainly distributed in Baiyun main sub-sag and the main source rocks are distributed in the 3rd and 4th members of Wenchang Formation. Four favorable zones are selected for the division and evaluation of migration and aggregation units: No. ▪ Panyu 30 nose-shaped structural belt, No. ▪ Liuhua 29 nose-shaped uplift belt and Liwan 3 nose-shaped uplift belt, No. ▪ gentle slope belt of Baiyun east sag, and No. ▪ Baiyun 1 low-uplift.

Prediction method for the porosity of tight sandstone constrained by lithofacies and logging resolution

The resource potential of the Upper Triassic Yanchang Formation in the Ordos Basin is considerably large. However, owing to the low porosity and permeability, poor connectivity, and strong heterogeneity of tight sandstone, predicting the porosity of tight sandstone in this formation poses substantial challenges. The lithology of Chang 71 sub-member is dominated by arkose and lithic arkose, wherein the porosities mainly ranged between 3 and 13% and pore types were mainly feldspar dissolution pores. Based on core sample experiments and thin section analysis, combined with conventional logging curves, we proposed a three-porosity weighted average prediction method under lithological control, and a novel method for reading logging curves controlled by logging resolution is developed. We used principal component analysis to optimise the logging curves and reduce the impact of complex coupling relationships between logging curves on lithofacies identification. The stacking algorithm, which is a combination of the random forest and extreme gradient boosting models, was applied to divide the lithofacies into five categories: homogeneous-distributary-channel fine sandstone, heterogeneous-distributary-channel fine sandstone, homogeneous-mouth-bar fine sandstone, heterogeneous-mouth-bar fine sandstone, and shallow lacustrine mudstone. Compared with the results of manual classification based on logs, the accuracy of lithofacies recognition was approximately 94.2%. Additionally, sensitivity analysis of porosity curves was conducted on four types of lithofacies (i.e., except for shallow lacustrine mudstone), and porosity models for each lithofacies were established, providing an effective and objective method for the accurate prediction of porosity. This prediction method comprehensively considers sedimentary factors and incorporates statistics that are fitted using multiple linear regression, which is highly reliable. In the Chang 71 sub-member, the fitting degree between the predicted and core porosities reached 0.912, indicating that this three-porosity weighted average method based on lithofacies constraints is reasonable, reliable, and has stronger adaptability than the those reported previously. The prediction method based on lithofacies control and weight analysis can be applied to other tight sandstones, providing reliable technical support for oil and gas exploration and development.

Basin-scale exhumed Cretaceous fluvial systems (Chubut Group, Cañadón Asfalto basin), central Patagonia, Argentina

This contribution describes basin-scale, Cretaceous (Late Aptian to Cenomanian-Early Turonian) fluvial systems outcropped over ∼30,000 km2 that contain more than 7000 exhumed paleochannels in the Chubut Group, Cañadón Asfalto basin (central Patagonia), Argentina. Paleochannels are encased within a ∼350 m thick stratigraphic interval (∼20 My) including volcaniclastic and epiclastic fluvial units (Cerro Barcino and Puesto Manuel Arce formations). Geomorphologically, the exhumed paleochannels conform to isolated sandstones/conglomeratic ridges, a swarm of paleochannels, and elongated/equidimensional mesas (plateaus), characterizing much of the “mesetiform” landscape in central Chubut. The plan-view analysis of exhumed paleochannels mainly shows a mean WNW-ESE orientation (112°–292°) and predominance of narrow (83.1%) and very narrow (14.4%), low-sinuosity channels (96%). A plan-view multi-scale architectural analysis of sandbodies allows us to recognize different fluvial hierarchies: a) 3rd-order lithosomes (macroform growth increments), b) 4th-order lithosomes (point bars and crevasse-channels), b) 5th-order lithosomes (main channels), and c) 6th-order lithosomes (channel belts). Two main types of channel arrangements in the exhumed fluvial systems were recognized: fixed-channel and mobile channel belts. We observed a broad relationship between geomorphologic configurations and fluvial architectural units. Some macro spatiotemporal variation in the fluvial architecture was observed towards the youngest positions, including wider channels and a greater relative abundance of high-sinuosity channels with point bars. The basin-scale distribution of exhumed paleochannels would allow us to infer the predominance of shallow lacustrine or lagoon deposits in the western basin region, probably controlled by the inherited topography of the Jurassic- Early Cretaceous depocenters.

Paleogene-Neogene ring-shaped sedimentary system and reservoir characteristics in the Western depression of the Qaidam Basin

The Paleogene-Neogene strata in the Western Depression of the Qaidam Basin represent a primary focus for oil and gas exploration and development. Influenced by both terrigenous clastic influx and endogenic carbonate deposition, these strata exhibit significant variation in sedimentary systems and reservoir characteristics. This study comprehensively examines the depositional patterns and reservoir properties of the Paleogene-Neogene sequence across the inner, middle, and outer belts of the basin, employing core analysis, thin section petrography, and physical property assessment of reservoirs. Key findings include 1) The development of a concentric sedimentary system in the Western Depression during the Paleogene-Neogene period, characterized by increased carbonate mineral content and decreased clastic material from the periphery to the center of the basin. 2) Varied sedimentary facies associations across different zones, with the outer belt dominated by fan delta and braided river delta deposits, and the middle and inner belts characterized by near-shore shallow lacustrine carbonates and algal mat deposits, and offshore semi-to deep-lacustrine fine sediments, respectively. 3) The outer belt exhibits reservoirs with favorable physical properties and connectivity, while the inner and middle belts show high heterogeneity, indicating potential for lithological traps and shale oil exploration. These insights offer scientific guidance for further investigation into the depositional systems of lacustrine basins in the Western Depression of the Qaidam Basin and for identifying promising reservoirs.

Holocene paleoenvironmental history of Jackson Lake (Grand Teton National Park, USA) deduced from CHIRP seismic reflection and radiocarbon-dated sediment cores

Sedimentary records from Jackson Lake (Grand Teton National Park, Wyoming, USA) provide an opportunity to investigate the Late Quaternary tectonic and hydroclimatic history of a region known to have considerable earthquake hazard potential. Here, we present a new paleoenvironmental reconstruction for Jackson Lake using high-resolution Compressed High-Intensity Radar Pulse (CHIRP) seismic reflection data and sediment cores from Moran Bay, which is adjacent to the active Teton fault on the southwestern lake margin. Three seismic units (SU) are present within Moran Bay. The lowermost SU-1 defines the acoustic basement, which is characterized by a sharp reflector of variable relief above material with a very low signal-to-noise seismic response; direct sampling reveals this unit to be glaciogenic in origin. SU-2 contains diverse acoustic facies that onlap, downlap, and drape underlying strata. Prominent in SU-2 are faulted, folded, and slumped reflections with mixed amplitude characteristics, as well as reflections organized into prograding clinoforms, which suggest a history of deformation and slope instability in fluvio-deltaic and shallow lacustrine environments. A local unconformity separates SU-2 and SU-3, and above this boundary clinoform packages backsteps towards the hinterland. A bi-directionally tapering package of low-moderate amplitude, continuous to semi-continuous reflections onlap the clinoform; this unit was fully cored (13.8 m), allowing the unit boundary to be sampled and dated. Radiocarbon ages indicate that the core spans ca. 10.4 calibrated kiloyears before present (cal ka BP) to present. Retrogradational stacking of deltaic clinoforms are best explained by increased accommodation in Moran Bay around ca. 10 ka, consistent with a major earthquake and hanging wall subsidence on the Teton fault. Two turbidites identified in the strata align temporally (ca. 10.3 ± 0.4 cal ka BP and ca. 8.3 ± 0.3 cal ka BP) with earthquakes on the Teton fault that have been identified in terrestrial and lacustrine paleoseismic records. From ca. 10.4–6.6 cal ka BP core sediments are relatively rich in clay with highly variable magnetic susceptibility and organic carbon content. Deposition during this period was influenced by an evolving stream network and a relatively dry paleoclimate. From ca. 6.6–2.7 cal ka BP core sediments have higher sand and organic carbon contents, suggesting variable fluvial discharge and climate instability. The youngest core records produced a sharp increase in sedimentation rate, sand and organic carbon content, propelled by greater effective moisture, higher lake levels, and increasing watershed vegetation density. Based on clinoform rollover position, water levels at ∼2,046, ∼2,049, ∼2,053, and 2063 m asl characterized Moran Bay during the Holocene. Together, the seismic and core data records indicate a complex and dynamic Holocene lacustrine evolution controlled by fault motion and water level changes.

Whole petroleum system in Jurassic coal measures of Taibei Sag in Tuha Basin, NW China

Based on the latest results of near-source exploration in the Middle and Lower Jurassic of the Tuha Basin, a new understanding of the source rocks, reservoir conditions, and source-reservoir-cap rock combinations of the Jurassic Shuixigou Group in the Taibei Sag is established using the concept of the whole petroleum system, and the coal-measure whole petroleum system is analyzed thoroughly. The results are obtained in three aspects. First, the coal-measure source rocks of the Badaowan Formation and Xishanyao Formation and the argillaceous source rocks of the Sangonghe Formation in the Shuixigou Group exhibit the characteristics of long-term hydrocarbon generation, multiple hydrocarbon generation peaks, and simultaneous oil and gas generation, providing sufficient oil and gas sources for the whole petroleum system in the Jurassic coal-bearing basin. Second, multi-phase shallow braided river delta–shallow lacustrine deposits contribute multiple types of reservoirs, e.g. sandstone, tight sandstone, shale and coal rock, in slope and depression areas, providing effective storage space for the petroleum reservoir formation in coal-measure strata. Third, three phases of hydrocarbon charging and structural evolution, as well as effective configuration of multiple types of reservoirs, result in the sequential accumulation of conventional-unconventional hydrocarbons. From high structural positions to depression, there are conventional structural and structural-lithological reservoirs far from the source, low-saturation structural-lithological reservoirs near the source, and tight sandstone gas, coal rock gas and shale oil accumulations within the source. Typically, the tight sandstone gas and coal rock gas are the key options for further exploration, and the shale oil and gas in the depression area is worth of more attention. The new understanding of the whole petroleum system in the coal measures could further enrich and improve the geological theory of the whole petroleum system, and provide new ideas for the overall exploration of oil and gas resources in the Tuha Basin.

Flood events in the lower reaches of the Yellow River: OSL and radiocarbon dating on the Anshang site

The lower reaches of the Yellow River have witnessed numerous episodes of flooding and considerable shifts in its course. Examining flood records within the Yellow River basin offers invaluable insights into ancient human settlements, agricultural practices, trade routes, and technological advancements of ancient Chinese civilizations. However, dating these fluvial sediments poses a considerable challenge due to either the scarcity of organic material for radiocarbon (14C) dating or the potential incomplete bleaching of sediments for optically stimulated luminescence (OSL) techniques. This study focused on the Anshang site located in Henan Province, we obtained OSL ages from distinct grain size fractions of quartz and cross-verified them with independent acceleration mass spectrometry (AMS) 14C ages. The findings revealed that both coarse quartz grains (CG) and medium quartz grains (MG) underwent complete bleaching in silt-dominated flood deposits and shallow lacustrine deposits. Conversely, the OSL ages of CG and MG quartz were notably overestimated by approximately 3–6 ka in silty clay-dominated flood deposits. Upon comparing the OSL ages with AMS 14C ages, it was evident that bulk organic matter AMS 14C ages aligned with CG OSL ages in shallow lacustrine deposits, but they appeared older than CG OSL ages in flood deposits. However, the AMS 14C ages from plant demonstrated good consistency. Consequently, we successfully identified flood-lacustrine sediments dating back to approximately ∼13 ka, ∼8–3 ka, ∼2.6 ka, ∼1 ka, and the last few hundred years, aligning well with historical documentation and earlier research. The regional evidence presented herein enhances our understanding of the applicability of OSL dating and AMS 14C dating in fluvial-lacustrine sediments. Armed with these new ages, we constructed a reference chronology for archaeological localities situated in the fluvial sediments of the lower reaches of the Yellow River.

Tectonic and high-latitude climate controls on Quaternary sedimentary processes on the northern coast of Bohai Bay

Separating the relative roles of tectonics and climate change in geological evolution is complicated. The northern coast of Bohai Bay (NCBB), with a thick accumulation of Quaternary lacustrine sediments, is a subsiding area and is well suited to studying tectonic versus climatic controls on sedimentary processes. We used magnetostratigraphy and sedimentological analyses of borehole core JL01 from the NCBB to provide new insights into the effects of tectonic processes and climate changes in this region. The results show the following: (1) A shallow lacustrine delta, a deep-water lacustrine sedimentary system and a subaqueous fan-delta sedimentary system developed during 2.1–1.4 Ma, 1.4–0.78 Ma, and from 0.78 Ma to the present, respectively, corresponding to the rift-initiation, rift-climax, and late-rift tectonic stages of the evolution of an extensional basin, respectively. These stages constitute a low-order tectono-sedimentary cycle. (2) Tectonic and orbital-scale climate changes can be distinguished, particularly the 2.5 Ma climatic transition and a significant shift from 41- to 100-kyr cyclicity across the Mid-Pleistocene climatic transition. These changes indicate that the sedimentary processes of the NCBB were influenced by high-latitude climate forcing since the Late Pliocene. Our results demonstrate that detailed sedimentary facies analyses can provide valuable information about the effects of tectonics and climate changes on basin sedimentary evolution. This approach can be applied to other regional basins worldwide.

The effect of the faults to the tectono-stratigraphy evolution in the Panyu low uplift

Introduction: The Panyu low uplift is located in the middle of the Central Uplift Belt of the Pearl River Mouth Basin, which has experienced multiple episodes of tectonic movements since the Cenozoic, and the faults are extensively developed there. The fault activities had a significant influence on the migration and accumulation of oil and gas, as well as the sedimentation.Methods: In order to recognize the effect of the faults on the tectono-straigraphy evolution in the Panyu low uplift, based on the high-resolution seismic and latest drilling well data, the study has comprehensively analyzed the geometry and kinematics of these faults, as well as the sedimentary filling evolution of the residual depressions.Results: The results show that two major fault systems has developed in the Panyu low uplift: the deep and shallow fault systems. The former was dominated by a series of NEE and NW-trending high-angle listric/plane faults, along with several low-angle detachment normal faults, which were under the joint control of the NW-SE and NS-trending extensional stress during the Eocene. The latter was dominated by NW-NWW trending strike-slip normal faults with tension-shear properties under the NEE trending dextral strike-slip stress field in the late Miocene. The uplift had undergone five tectonic evolution stages: the initial rifting stage (Tg-T83), the intensive rifting stage (T83-T80), the rifting and depression transitional stage (T80-T70), the thermal subsidence depression stage (T70-T35), and the tectonic reactivation stage (T35-T10).Discussion: Based on the 3D seismic phase interpretation and drilling sample analyses, the study also indicates that during the initial rifting stage, the Panyu low uplift manifested as multistage depositional center. The sedimentary strata were distributed in the downthrown wall of the depression-controlling faults, exhibiting multiple subsidence and sedimentary centers, and the depressions was supplied by multi-provenance systems. During the intense rifting period, the depression widened, the lacustrine basins were connected with each other, and the sedimentary center migrated. During the rifting and depression transitional stage, the faults controlling on deposition weakened. The thick Enping formation from the northwest provenance direction is unconformably overlaid on the Panyu low uplift. Meanwhile, the depressions was characterized by the sedimentary deposits of lacustrine basin. Various types of sedimentary facies were developed, including the delta, semi-deep lacustrine, shallow lacustrine, and shore-shallow lacustrine deposits, and the mudstone of the semi-deep lacustrine facies could serve as a high-quality source rock for the hydrocarbon production.

The lower Barra Velha formation (Aptian) in the Atapu field, Santos basin: Geological model for a pre-salt succession

The characterization of facies, microfacies and facies successions allowed to develop a conceptual geological model for the lower succession of the Barra Velha Formation in the Atapu field, Aptian of the Santos Basin. The obtained data and interpretations were based on the detailed description (scale 1:10) of ca. 90 m of two almost continuous well-cores and petrographic analysis. The facies were defined based on their primary depositional attributes, which are well recognized despite diagenetic modifications. Lutites, microbialites, tufa-rocks, calcirudites constitute the main recognized facies. The clastic lutites recorded transport and deposition of terrigenous and intraformational clasts by hydrodynamic and gravitational processes in low energy and shallow lacustrine environment. Microbialites and tufa-rocks record in situ carbonate deposition in alkaline saturated waters. Injectites, diastasis cracks, loop bedding, slump folds, many of the normal and listric faults and fractures are syn-depositional structures interpreted as primarily induced by tectonics. Facies changes allow to divide the succession in three stratigraphic intervals. Lutites predominates in intervals 1 and 3 whereas tufa-rocks constitute the interval 2. In the lower part of interval 1 labile fresh siliciclastic components and tractive and gravitational structures are abundant, indicating dry climate but with sporadic rainfall during a period of relative high water level of the lake. The upward decreasing of siliciclastic fragments, the appearance of coarse microbialite and a fish mortality level in the upper part of interval 1, mark the transition to the tufa-rocks of interval 2. These latter features denote a relative dryer climate and higher alkalinity waters, and a low water level of the lake. The interval 3 lutites cover abruptly the tufa-rocks recording the drowning of the succession. This may be related to renewed rifting or to the beginning of the basin thermal subsidence.

Origin, Migration, and Characterization of Gas in the Xinglongtai Area, Liaohe Subbasin (Northeast China): Insight from Geochemical Evidence and Basin Modeling

Buried hill zones in the rift basins have a significant impact on the enrichment of natural gas resources, and this is of great significance for exploration and development. This study aims to unravel the origins, migration, and dynamic accumulation process of natural gas in the Xinglongtai structural belt, Liaohe Subbasin. A comprehensive geological and geochemical analysis was performed on source rocks and natural gas samples from various geological structures within the Xinglongtai structural belt. Moreover, basin modeling techniques were employed to trace the genesis and migration of natural gas, offering an in-depth understanding of the dynamic process of accumulation. We identified the Fourth Mbr (Es4) and Third Mbr (Es3) of the Shahejie Fm as the main source rocks in the Qingshui and Chenjia Sags. The Es4, primarily Shallow Lacustrine Mudstones, contributed mainly type II organic matter, while the Es3, consisting of Nearshore Subaqueous Fan and Deep Lacustrine Mudstones, contributed mainly type III and type II organic matter, respectively. Two distinct hydrocarbon accumulation systems were observed, one inside and one outside the buried hills. The system outside the buried hill is governed by a complex fault system within the lacustrine basin, resulting in dual-source directions, dual-source rock types, two migration phases, and late-stage accumulation. In contrast, the system within the buried hill primarily involves reservoirs nested in the basement, exhibiting dual-source directions, dual-source rock types, a single migration phase, and early-stage charging. The understanding of this interplay, alongside dynamic simulation of generation, migration, and accumulation, can provide valuable insights for predicting natural gas distribution and accumulation patterns in terrestrial faulted lacustrine basins. This knowledge can guide more effective exploration and development strategies for natural gas.

Linking environmental changes and organic matter enrichment in the middle part of the Yanchang Formation (Ordos Basin, China) to the rollback of an oceanic slab in the eastern Paleo-Tethys

The Yanchang Formation, which records the whole evolutionary history of the Middle–Late Triassic lacustrine basin in the western part of the North China Block, provides an opportunity to understand the relationship between the tectono-sedimentary evolution and massive organic matter burial in lacustrine systems. The present study investigates the changes in the lithofacies association, provenance, regional tectonism, depositional environment, and organic matter accumulation in the lower-middle part of the Yanchang Formation, aiming to unravel their links. As a result of the rapid deepening of the southern Ordos Basin, the deposits in the depocenter of the basin shifted dramatically from shallow lacustrine sandstones of the lower part of the Yanchang Formation (i.e., Chang 8 Member) to deep lacustrine shales and gravity flow deposits of the middle part of the Yanchang Formation (i.e., Chang 7 Member). The provenance of the overlying Chang 7 Member is distinct from that of the underlying Chang 8 Member. It is proposed that crustal extension caused by the southwestward rollback of an oceanic slab in the eastern Paleo-Tethys may have triggered the rapid extensional subsidence in the southern Ordos Basin during the early Ladinian. In response to this deep geodynamic process, the depositional environment of the contemporaneous Ordos Basin changed significantly, as reflected by improved primary productivity, enhanced bottom-water anoxia, as well as reduced terrigenous clastic supply in the Ordos Lake. The coupling of these processes further led to the enrichment of organic matter in the Chang 7 Member.

How to distinguish between marine and lacustrine sedimentary environments? —A case study of Lishui Sag, East China Sea Shelf Basin

The identification and characterization of marine versus lacustrine sedimentary environments can aid in the assessment of source and reservoir rock distribution, which is important for hydrocarbon exploration. Analysis of sedimentary facies, trace fossils, micropaleontology, geochemical characteristics, and relative water-levels was conducted in this study. Analysis of the core revealed typical sedimentary structures, including fourteen lithofacies and eleven lithofacies associations, which are indicative of wave-influenced origins and tidal processes. The sediments of the Yueguifeng Formation and upper Mingyuefeng Formation were deposited in shallow water settings, which later transitioned into the semi-deep water environments of the Lingfeng Formation and lower Mingyuefeng Formation. Sediments of the lower Mingyuefeng Formation and Lingfeng Formation were deposited in semi-brackish water or brackish water environments, whilst from the lower Mingyuefeng Formation to upper Mingyuefeng Formation and lower Lingfeng Formation to Yueguifeng Formation, the environment was semi-brackish to freshwater. The Lishui Sag experienced a large-scale and rapid relative water-level increase from the Yueguifeng Formation to the lower Mingyuefeng Formation and a decrease during the upper Mingyuefeng Formation. The relative water-level of the Lingfeng Formation and the lower Mingyuefeng Formation was consistent with the inferred eustatic global sea-level, and the relative water-level of the Yueguifeng Formation and the upper Mingyuefeng Formation deviated from the inferred global eustatic sea-level. The typical lithofacies associations, micropaleontology, and geochemical characteristics are identified as the key indicators to determine the sedimentary environments. The sedimentary environment analysis results show that the Lingfeng Formation to the lower Mingyuefeng Formation developed in a marine setting, whereas the Yueguifeng Formation and the upper Mingyuefeng Formation developed in a lacustrine sedimentary environment. The sediments deposited within shallow marine settings form good source rocks, the coastal sediments form good reservoirs, sediments of shallow lacustrine can generate source rock, and the deep-lacustrine sediments form good source rocks that produce waxy crude. Therefore, accurately distinguishing the difference between marine and lacustrine basins is important in terms of the overall understanding of the basin and the hydrocarbon play elements contained within it.

Paleosol-derived paleoclimate and paleoenvironment reconstruction of the Rukwa Rift Basin, Tanzania: implications for faunal dispersal in the Miocene–Pliocene

ABSTRACT The East African Rift System records a key interval in the evolution of modern African ecosystems, documenting significant floral changes and faunal dispersals in the context of environmental shifts. To date, Miocene-to-Pliocene data from eastern Africa have been derived primarily from richly fossiliferous rift basins along the far north of the Eastern Branch of the rift, with more limited windows emerging from the Malawi Rift and more recently, coastal Mozambique. Here, we present the first quantitative paleoclimate data for the Miocene–Pliocene transition from the Western Branch of the East African Rift System, based on analyses of paleosols from the Rukwa Rift Basin. Paleosols derived from the fossiliferous late Miocene–early Pliocene lower Lake Beds succession in southwestern Tanzania preserve a shallow lacustrine setting grading into a system of alluvial fans and braided rivers with abundant floodplain deposits. Paleoclimate reconstructions using bulk geochemistry and clay mineralogy reveal a highly seasonal, semiarid, mesic climate during the late Miocene, with increased moisture availability in the early Pliocene resulting in a shift to subhumid conditions. Stable-carbon-isotope composition of pedogenic carbonates document a woodland/bushland/shrubland paleoenvironment across the Miocene–Pliocene transition. Results support the presence of Pliocene subhumid to humid habitats, dominated by woody vegetation offering shade, food, and water for faunal dispersal along an inland corridor connecting northern segments of the East African Rift System with southern Africa.

Paleobiodiversity and paleoenvironments of the eastern Paratethys Pleistocene lacustrine-palustrine sequence in the Baklan Basin (SW Anatolia, Turkey)

The Lower-Upper Pleistocene sedimentary record of the Baklan Basin, a long-lived continental half-graben basin in SW Turkey, is characterized by shallow lacustrine and palustrine deposits. The paleoenvironmental changes recorded in the basin succession allow for a multiproxy approach in reconstructing the paleoclimatic, paleoecological, and paleobiogeographical evolution of southwestern Anatolia during the Early-Late Pleistocene. Based on sedimentological, paleontological, and geochemical data, three main types of depositional intervals have been identified, corresponding to different phases of a lake expansion cycle: The first interval is characterized by the perennial shallow lake environment (PSL deposits), which represents the very early stage of the Early Expansion System Tract (VEEST). This suggests a very early stage of lake transgression in arid climate conditions. The second interval is represented by the palustrine carbonate lake center environment (PLC deposits), which corresponds to the late stage of the Early Expansion System Tract (LEEST). This indicates a late early stage of lake transgression in semiarid to subhumid climates. The third interval is marked by the palustrine lake margin environment (PLM deposits), which represents the Late Expansion System Tract (LEST) under humid conditions.The Lower-Upper Pleistocene successions of the Baklan Basin provide an excellent example of lacustrine and palustrine deposition in a laterally extensive, low-gradient, shallow lake system in the semi-isolated Pontocaspian freshwater to slightly brackish water (oligohaline-low mesohaline) long-lived lake. The presence of Pontocaspian ostracod and mollusc faunas in the studied successions indicates that the largest major Caspian transgression around 2.6 millon years ago extended to SW Anatolia. The studied successions represent a rich archive of landscape, climate, and biotic development in the eastern Paratethys region during the Early-Late Pleistocene. The biogeographic signature of fossil faunas (mammals, ostracods, molluscs, and fishes) and floras (Characeae) is predominantly modern Palearctic and Holarctic, with a minor amount of endemic Pontocaspian elements. This study presents the Pleistocene Pontocaspian species of the Anatolian lakes that may have served as refugia for the Palearctic taxa during adverse time intervals. Consequently, this study shows that Lower-Upper Pleistocene lacustrine to palustrine sedimentation in the Baklan Basin has been controlled by the combination of tectonics, climate changes, and the largest major Caspian Sea transgression. The findings of this study could be used to evaluate the impact of similar allocyclic factors on the sedimentological, hydrological, and geochemical development of other intermontane lake basins.

Evolution of the Pristane/Phytane Ratio with Maturity of Lacustrine Source Rocks: Implications of a Thermal Simulation Experiment

The pristane/phytane ratio (Pr/Ph) of source rocks has been extensively used for the identification of paleosedimentary environments and oil–source rock correlations. However, the influences of maturity on the Pr/Ph ratio are not fully understood, which largely limits the applicability of this parameter under geological conditions. Based on a thermal simulation experiment performed on typical lacustrine source rock samples and systematic research on geological source rock samples, the evolution of the Pr/Ph ratio with increasing maturity was investigated in this study. The results indicate that the Pr/Ph ratios of the two source rock extracts progressively decrease with increasing maturity, and the decrease in this parameter is more significant for shallow lacustrine samples than for deep lacustrine samples. The Pr/Ph ratios of the two types of source rocks are different at the oil generation stage with EasyRo less than 1.02% but are similar at the oil cracking stage with EasyRo larger than 1.02%. The Pr and Ph of source rocks are contributed by both biogenic and thermal origins, which are largely determined by the sedimentary environment and maturity, respectively, and the contributions of the two origins are diverse at different maturity stages. The Pr/Ph ratio is mainly controlled by the sedimentary environment at the diagenetic stage, by both sedimentary and thermal maturity at the oil generation stage, and by thermal maturity at the oil cracking stage. The Pr/Ph ratio of source rocks can be used to identify sedimentary environments and oil–source rock correlations at diagenetic and oil generation stages.

Early Cretaceous (late Barremian−Early Albian) expanding aeolian activity in East Asia: Evidence from the stratigraphic evolution of aeolian deposition in the Baiyin-Jingyuan Basin, northern China

Cretaceous aeolian deposition is important for understanding variations of the tropical−subtropical atmospheric circulation under the greenhouse background. Nevertheless, detailed records revealing the response of aeolian deposition to rising atmospheric CO2 are still lacking. To shed light on their linkage, we examined the Early Cretaceous stratigraphic evolution of the Hekou Group in the Baiyin-Jingyuan Basin, northern China, using multiple methods. The results indicate that the lower Hekou Group is characterized by a transition from submerged alluvial to shallow lacustrine deposition. Overlying these facies is loess-like deposition without bedding, distinguished by grain-size distribution, surface microtextures, and geochemical analysis, which is followed by palustrine deposition. In the palustrine deposition, aeolian sand-dune and sand-sheet deposition can be identified, which gradually developed upward in the Hekou Group, with intercalated wet interdunes, damp interdunes, and pebble-sand sheetflood beds. Finally, aeolian sand dunes and sand sheets dominated the upper Hekou Group. Overall, the upward-changing facies indicated the development of aeolian deposition from an aqueous environment, revealing a long-term paleoclimatic shift from semi-humid, semi-arid to extremely arid, which reflects the expanded aeolian activity in East Asia during the Early Cretaceous. Furthermore, various records were combined to investigate the expanding aeolian activity in East Asia, which indicates a west−east-trending arid belt that was wider than the modern counterpart during the late Barremian−Early Aptian and late Aptian−Early Albian, respectively. These stepwise expansions of aeolian activity were mainly driven by rising atmospheric CO2, which strengthened the subsiding branches of Hadley circulation and the subtropical high, eventually leading to intensified aridification.

Morphological Typology and Petrography of Plio-Quaternary Continental Carbonates of Aït Chaïb, Skoura Basin, Middle Atlas, Morocco

The Plio-Quaternary Ait Chaib basin occupies the northwestern part of the Skoura Syncline, which is part of the northern Middle Atlas Folded Zone. This intracontinental basin was formed probably in the Pliocene after the Tortono-Messinian regression sea. The Plio-Quaternary deposits are bound by fluvial conglomerates and continental carbonates. They consist of barrage travertines; travertine limestones and oncholitic travertine silts and sands of the upstream and downstream deposits. These travertines laterally pass to lacustrine limestones at the bottom of the basin. The genesis of these continental carbonates, correlative to phases of karstification, has affected the Meso-Cenozoic carbonate deposits of the folded Middle Atlas, and is linked to favorable paleo-environmental conditions of geological, geomorphological, hydrogeological, hydrological and bioclimatic order. The petrographic study of these lacustrine limestones shows laminar structure allowing to assimilate into continental stromatolites. They were formed by cyanobacterial mats in a shallow lacustrine environment in which water were was non-turbid and supersaturated in limestone solution, coming from karstic springs staking out the Middle Atlasic North Accident. The important diagenesis evolution is linked to the surface water table contain, which has as a floor gypsiferous lagoonal marl of the Upper Miocene age.

Sequence architecture, depositional system evolution and controlling factors in a lacustrine basin: a case study from Changdi Area, Bohai Bay Basin, Eastern China

AbstractThe Paleogene Shahejie and Dongying formations of the eastern Bohai Bay Basin contain >500 m fluvial-deltaic deposits and >600 m deltaic-lacustrine deposits, both of which are important for oil and gas exploration. This study aims to figure out the deposition system distribution within the sequence stratigraphy and predict the exploration target. Sequence boundaries are recognized by unconformities in the seismic profiles and abrupt changes in the well-logging curves (acoustic, spontaneous potential, natural gamma-ray and resistivity), and two second-order and five third-order sequences (SQs1, SQs3 in the Shahejie Formation and SQd1-SQd3 in Dongying Formation) are identified. Four types of depositional system and distribution are determined by well-logging profiles, cores, relative sand content and root-mean-square seismic attributes. The SQs3 sequence is predominantly fan deltas and lacustrine deposits, the SQs1 sequence is predominantly shallow lacustrine deposits, the SQd3 sequence is predominantly braided-river delta, semi-deep lacustrine, and turbidite fan, and the SQd2 sequence is predominantly braided-river delta deposits, whereas the SQd1 sequence is predominantly braided-river deposits. The controlling factors are determined by basin subsidence simulation and fossil data of biological palynology, indicating that tectonic activity is the main controlling factor in SQs3 and SQd3 while climate is the main controlling factor in the SQs1, SQd2 and SQd1 sequences. Combined with the physical properties of sandstone, it is concluded that the beach bar in the SQs1 sequence, turbidite fan in the SQs3 and SQd3 sequences, and thick sandstone in the SQd2 sequence are the important exploration targets.

The Influence of Continental Mixosedimentite Sedimentation on Shale Oil Sweet Spot: A Case Study on the Permian Lucaogou Formation in the Jimsar Sag of the Junggar Basin, NW China

Over the years, shale oil has been at the leading edge of oil exploration and also become the hotspot of reserve increases for various oil fields. The Permian Lucaogou Formation in the Jimsar Sag has been less extensively researched in terms of, among others, lithologic interpretation of fine-grained mixosedimentite, identification and classification of sedimentary microfacies, and sweet spot evaluation. The present study identifies the classification of sedimentary microfacies, characteristic distribution of sedimentary reservoir, and spatial configuration distribution of facies-controlled sweet spot body based on core logging, cutting logging, and data while drilling, combined with logging information, experimental analysis data, and production data with respect to formation test and production test and through the lithology identification based on element logging. This study indicated the following: 1) Microfacies of the reservoir body include a shallow lacustrine sand bar, mixing beach, and dolomitic flat. Different microfacies are adapted to different lithologies or lithological associations. 2) The lower sweet spots P2l12−3, P2l12−2, and P2l12−1 evolved into the dolomitic flat from mixing beach from NE to W. Also, the dolomitic flat is mostly developed in the west of the work area, and the mixing beach is better developed in the east. The shallow lacustrine sand bar is highly developed in the middle location, and the mixing beach and shallow lacustrine sand bar are highly developed on the northern and southern sides. 3) Sedimentary microfacies have evident controls over sweet spots. The upper sweet spots P2l22−3 and P2l22−2 are dominated by Class I sweet spots followed by Class II and IV sweet spots, which are major sweet spot reservoirs. The upper sweet spot P2l22−1 is mainly suffused with Class IV sweet spots, and the Class I reservoir is feldspathic litharenite (shallow lacustrine sand bar microfacies accordingly).