Lower Middle Jurassic Research Articles

River‐dominated and tide‐influenced shelf‐edge delta systems: Coarse‐grained deltas straddling the Early–Middle Jurassic shelf–slope break and transforming downslope, Lajas–Los Molles formations, Neuquén Basin, Argentina

AbstractThe three‐dimensional facies and architecture variability of shelf‐edge deltaic units cropping out at the transition between the Lower–Middle Jurassic Lajas and Los Molles formations of southern Neuquén Basin, Argentina, is presented here, as well as their stratigraphic relationship to uppermost deep‐water slope channel systems. Deep‐water, slope mudstones with thin turbidite beds merge upward with prodelta mudstones and thin sandstones, which are truncated by delta‐front to mouth‐bar sandstones. The latter sandstones are then downcut by large‐scale, trough cross‐stratified coarse‐grained sandstones and conglomerates of distributary channel systems and along‐strike, amalgamated with cross‐bedded sandy units showing evidence of tidal reworking. Proximal–distal facies and architecture variability within a shelf‐edge deltaic succession demonstrates that distributary channel‐complexes become wider and deeper basinward, forming channelized river‐dominated distributary fairways separated by tidally reworked inter‐distributary sand belts at the shelf edge. Evidence from depositional‐dip oriented outcrops shows a lack of collapsed and slumped strata at the shelf edge, and that the coarse shelf‐edge distributary channel fills continue far down the deepwater slope, and conglomerates transform to become high‐density turbidites to mainly thick‐bedded, sand‐matrix‐supported debrites. The interplay between flood tides and river currents is interpreted to have primarily modulated the focusing of river drainages, and consequently coarse‐grained sediment transport, along preferential routes on the outer‐shelf to shelf‐edge and down onto the slope. This contribution documents a unique example of coarse‐grained (mostly conglomeratic) shelf‐edge delta systems, tying bed‐scale facies and architecture data to a seismic‐scale shelf‐margin morphology, thus providing outcrop analogue data for the characterization of shelf‐edge delta systems in the subsurface.

Sandstone characteristics and hydrocarbon exploration significance of the Middle-Lower Jurassic of the central-eastern part of the northern tectonic belt of the Kuqa Depression, Tarim Basin, China

The Jurassic at the northern tectonic belt of the Kuqa Depression, Tarim Basin, is the key frontier of replacement for oil and gas exploration in the 13th five-year plan of China. The 4th national resource evaluation of China demonstrates that this area possesses resources amounting to 565 × 106 t of oil equivalent, and yet the proven resource accounts for only 3.7% of the total. To determine the exploration target of the Mid-Lower Jurassic and identify the favorable zone and trap type, studies on sedimentary microfacies and sand body configuration have been carried out based on data from outcrops, drilling, and well logging. It is clarified that the braided river delta plain-front-shore-shallow lake sedimentary system deposited during lake transgression in the wide and gentle lake basin occurs in the Middle and Lower Jurassic. The Ahe Formation is developed with the ultra-thick sand body of the large braided river channel in the braided river upper delta plain. Meanwhile, the Yangxia and Kezilenuer formations are found with the mid-thin sand body of the medium and small fluvial channel of the braided river lower delta plain-front sedimentation. There are three sand bodybuilding models favorable for hydrocarbon accumulation that has been developed. Moreover, it is proposed that the sand body of the Ahe Formation, originating from vertical accretion of the braided river channel upper delta plain fluvial, is the exploration zone for structural reservoirs; the lateral imbricated sand body of the braided river channel lower delta plain fluvial of the Yangxia Formation, and the lenticular sand body resulting from lateral accretion of the delta front fluvial channel of the Kezilenuer Formation, are the exploration zone for structural-lithological and lithological reservoirs. The Kezilenuer and Yangxia formations at the eastern, northern, and western flanks of the Tugeerming anticline are the most favorable for structural-lithological reservoirs, and a favorable zone of 700 km2 at the flanks has been demarcated. The platform zone at the Dibei slope is the most preferable for structural reservoirs, where in the average porosity of the Ahe Formation reservoir rock is 5.6%, the average permeability is 0.75 × 10−3μm2, and the favorable zone covers 750 km2. Solid evidence has been provided for the determination of the favorable zone and trap type in the Middle and Lower Jurassic at the northern tectonic belt of the Kuqa Depression.

Features of the hydrogeological stratification of oil and gas bearing deposits in the western part of the Yenisei-Khatanga basin

The article presents results and findings of the research into stratification of the hydrogeological section of western areas of the Yenisei-Khatanga basin localized in the Siberian Arctic zone. The identified two hydrogeological structural stages in the study area include the Paleozoic (undivided), Triassic, Lower-Middle Jurassic, Upper Jurassic, Neocomian, Aptian-Albian-Cenomanian, Upper Cretaceous and Paleogene-Quaternary water-bearing complexes. There have been recognized as many as eleven aquifers and eleven confining beds formed within the area where the established hydrogeological units are found not ubiquitously distributed, which is associated with the sedimentary basin evolution. A considerable part of the horizons have failed to be included in the hydrogeological section either due to the erosion processes within the Messoyakha inclined ridge or their pinching out in the borders of the basin.

Discovery of the high-yield well GT1 in the deep strata of the southern margin of the Junggar Basin, China: Implications for liquid petroleum potential in deep assemblage

The accumulation assemblages in the deep Jurassic and Cretaceous strata of the southern margin of the Junggar Basin (SMJB) have favorable petroleum accumulation conditions. Due to the huge burial depth, gas/condensate gas reservoirs are expected as the main targets, however, barely any exploration breakthrough has been achieved. The recently drilled well GT1 has obtained a high oil production of >1200 tons/day in the deep Cretaceous strata, which was classified as low gas-to-oil ratio (GOR) light oil. This is the first significant breakthrough in the deep assemblages in the SMJB which brings to the question of its genesis and preservation and the true petroleum potential in the area. In this study, we have presented integrated geochemical analyses including GC × GC-TOFMS, GC-MS and stable carbon isotopes and thus to conclude that the GT1 oil was sourced from Jurassic coal-measure source rocks with maturity of 0.7–0.9%Ro. As only a small amount of diamondoids and ethanodiamondoids were detected, the GT1 oil was classified as normal mature oil well preserved from thermal cracking despite its huge burial depth. The associated gas is coal-type gas with relatively high wetness and the similar maturity to that of the oil. According to the hydrocarbon accumulation analyses, the main source rocks were developed in the Lower-Middle Jurassic with present-day maturity of 1.0%–2.0%Ro. The sandstones developed in the transitional facies in the overlying Lower Cretaceous Qingshuihe Formation were impacted by the tectonic compression and preserved as belt-like distribution in anticline traps. Regional Cretaceous Tugulu Group also overlie these sandstone facies. This forms a favorable source-reservoir-seal assemblage for oil and gas accumulations in the area. Therefore, the deep assemblage in the SMJB is favorable for the accumulation and preservation of large-scale oil reservoirs and shows great potential for liquid petroleum exploration.

Geophysical Methods for Determining the Reservoir Properties of Structurally Complex Lower–Middle Jurassic Reservoirs within Hydrocarbon Fields in the Latitudinal Ob Region

The use of geophysical well logging for interpreting structurally complex oil-saturated intervals of the Lower and Middle Jurassic horizons that are characterized by a high average number of permeable intervals and by the presence of semiconductor minerals (pyrite, siderite, glauconite, and others) is considered. The complex of geological and geophysical information that has been aggregated and research methodology that has been developed for areas with hard-to-recover reserves is developed based on the example of deposits that have been in operation for many years.

Detrital zircon U-Pb dating in the southern Hefei Basin: Evidence for exhumation of HP-UHP rocks of the Dabie Orogen

The Hefei Basin of eastern China developed in response to uplift of the Dabie Orogen, and zircon dating can be used to assess the exhumation history of the orogen. Zircons were collected from samples of the Lower Jurassic Fanghushan Formation and Middle Jurassic Sanjianpu Formation in the southern Hefei Basin, and mica-quartz schist and biotite granite gneiss from the Susong Complex of the Dabie Orogen. The zircon U-Pb dating was undertaken using laser ablation-inductively coupled plasma-mass spectrometry. The detrital zircons from conglomerates of the Fanghushan Formation and from clasts within the conglomerates have age-frequency distributions with the main clusters between 2.0 and 1.8 Ga, similar to age data of the Susong Complex. On the other hand, the zircons of the Fanghushan Formation do not show the age cluster at 1000-900 Ma that characterizes zircons in the underlying metasediments of the lower Paleozoic Foziling Group. A cluster of Triassic zircon ages also appears in the arkosic sandstones of the Fanghushan Formation. These data indicate that the provenance of the Fanghushan Formation was a mixture of high-pressure (HP) and ultrahigh-pressure (UHP) Triassic metamorphic rocks, Paleozoic magmatic rocks, and the Susong Complex, but not the lower Paleozoic Foziling Group even though it directly underlies the sediments of the Hefei Basin. Two samples from the Sanjianpu Formation show zircon age clusters at 797 and 791 Ma (middle Neoproterozoic) and 226 Ma (Triassic), and again, these are markedly different from the age clusters that characterize the Foziling Group. It seems, therefore, that despite the Foziling Group being at the surface in the underwater depositional area of the Hefei Basin, it was not exposed in the source area of the Hefei basinal sediments during the Jurassic, and there are two possible reasons for this. First, the exhumation of the Dabie Orogen was directed partly towards the north, in the process of which some of the Foziling Group was covered. Second, the Susong Complex rocks became involved in the development of an accretionary wedge, thus covering some of the Foziling Group during the process of subduction.

Overview of potential geothermal reservoirs in Denmark

AbstractThe Danish onshore subsurface contains very large geothermal resources that have the potential to make a significant contribution to transforming Danish energy consumption toward a more sustainable energy mix. Presently, only a minor fraction of this green energy is exploited in three small plants. The main factors that have hampered and delayed larger-scale deployment are related to uncertainties in the geological models, which inevitably lead to high economic risks that are difficult for smaller district heating companies to mitigate without support from a compensation scheme. To facilitate and stimulate much wider use of the Danish geothermal resources, the Geological Survey of Denmark and Greenland (GEUS) and other research institutes have conducted several regional research projects focusing on the geological and geochemical obstacles with the principal objective of reducing the exploration risks by selecting the best geological reservoirs.One of the most important geological factors causing uncertainty is the quality of the reservoirs and their ability to produce the expected volume of warm geothermal brine. Thus, great emphasis has been placed on investigating and understanding the relationships between reservoir sandstone, porosity, permeability, petrography, diagenetic processes and alterations related to variable sediment sources, basin entry points, depositional systems and climate, burial and thermal history. Mesozoic sandstones comprise the most important geothermal reservoirs in Denmark. Details concerning the reservoir quality are compiled and compared for the Lower Triassic Bunter Sandstone, Triassic Skagerrak, Upper Triassic – Lower Jurassic Gassum and Middle Jurassic Haldager Sand formations. The Bunter Sandstone Formation contains extensive aeolian and more confined fluvial sandstones with high porosity and permeability. However, highly saline formation water could be unfavourable. The Skagerrak Formation comprises well-sorted braided stream sandstones in the centre of the basin, and is otherwise characterised by muddy sandstones and alluvial fan conglomerates. An immature mineralogical composition has caused intensive diagenetic changes in the deepest buried parts of the basin. The Gassum Formation consists of shoreface, fluvial and estuarine sandstones interbedded with marine and lacustrine mudstones. In the upper part of the formation, the sandstone beds pinch out into mudstones towards the basin centre. Pervasive siderite- and calcite cement occurs locally in shallowly buried sandstones, and with burial depth the maximum abundances of quartz and ankerite cement increase. Sandstones of shallow burial represent excellent reservoirs. The relatively coarse grain size of the Haldager Sand Formation results in high porosity and permeability even at deep burial, so the formation comprises a high-quality geothermal reservoir.Substantial progress has been made, and a well-established regional geological model combined with reservoir quality is now available for areas with cored wells. This has enabled an improved estimation of reservoir quality between wells for exploration of geothermal reservoirs.

Managing and Surveying the Geological Garden at Tata (Northern Transdanubia, Hungary)

The Geological Garden at Tata is an open-air geological museum where a spectacular succession of Tethyan Mesozoic sedimentary rocks, characteristic of the Alpine-Carpathian region, is finely exposed in abandoned quarries and cleaned rock surfaces. In addition to geological values, the area houses copper-age chert mines, the only ones accessible to the wider public in Hungary. Due to financial problems, however, maintenance of the site was more or less confined to mowing for more than 20 years. Renewal of the Geological Garden began in 2015 when large-scale cleaning as well as detailed surveying was carried out. Removing the soil and loose rock debris provided opportunity to study the effects of weathering and growth of vegetation. Near 60 m3 of rock debris and soil had been accumulated on approx. 5000 m2 rock surface over the decades of obligate deterioration. Lower Jurassic marl and Middle Jurassic radiolarite chert beds as well as Middle Jurassic limestones proved to be especially deeply weathered. The cleaning action raised the question whether the sub-horizontal rock surfaces can be conserved for a long time in their present state or not. Lessons drawn from the latter can serve as a basis for future maintenance activities as well as for plans aiming at developing geosites.

A neornithischian dinosaur from the Middle Jurassic Xintiangou Formation of Yunyang, Chongqing, China: the earliest record in Asia

ABSTRACT A new neornithischian dinosaur, Sanxiasaurus modaoxiensis gen. et sp. nov., is described based on a partial postcranial skeleton from the Middle Jurassic Xintiangou Formation of Yunyang, Chongqing Municipality, China. It is the first neornithischian dinosaur from the Middle Jurassic Xintiangou Formation of Sichuan Basin, and also the earliest record of a neornithischian dinosaur in Asia. It differs from other neornithischians in possessing the following unique combination of characters: at least four sacrals; humerus strongly bowed laterally along its length; obturator process located at mid-length of ischial shaft; lesser trochanter of femur broadened, wing shaped and subequal in anteroposterior width to greater trochanter; distal tibia with elongate posterolateral process. Phylogenetic analysis shows that Sanxiasaurus modaoxiensis is positioned near the base of neornithischia. This find extends the Asia neornithischian stratigraphic horizon to the lower Middle Jurassic.

Identifying multiple sources of petroleum fluids in Browse Basin accumulations using diamondoids and semi-volatile aromatic compounds

Abstract Hydrocarbon fluid correlation studies often rely on compounds that are source-specific, but only represent a minor fraction of the bulk fluid (e.g. steranes). The correlation in the present study utilises compounds that represent a significant proportion of the bulk fluid, namely semi-volatile aromatics (SVA), and diamondoids. Comprehensive two dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC × GC-TOFMS) was used to analyse diamondoids and SVA in whole oil/condensate samples from the Browse Basin, North West Shelf, Australia. Together with aromatic maturity ratios and the stable carbon isotopic composition for selected diamondoids and alkylbenzenes, these analyses demonstrate that numerous accumulations within the Browse Basin contain hydrocarbons from mixed sources. The diamondoid and SVA data provides further information on the current hydrocarbon fluid discrimination, which is primarily based on saturated biomarker compositions. Results of this study reveal a complex fill history by identifying the presence of a high maturity fluid(s) in the biodegraded accumulations on the Yampi Shelf, i.e. in the greater Cornea field and at Gwydion-1. This high maturity fluid is most likely derived from the Lower–Middle Jurassic Plover Formation (J10–J20 supersequences) in the greater Cornea area and, in the case of Gwydion-1, additional hydrocarbons may also be derived from the Upper Jurassic Vulcan Formation (J30–K10 supersequences). Other accumulations that comprise mixtures include Mimia-1 and Concerto-1 ST1 Deep, located to the north of the Ichthys field, and could represent charge from several Jurassic source rocks. The most mature fluids analysed in this basin are family II Plover-sourced and reservoired condensates from Kronos-1, Dinichthys North-1 and Gorgonichthys-1, as demonstrated by their elevated concentrations of the methyladamantanes and methyldiamantanes. The Dinichthys North-1 and Gorgonichthys-1 condensates also show a significant enrichment in the δ13C compound-specific values of the di-, tri- and tetra-methylbenzenes, whereas the Kronos-1 condensate does not, suggesting derivation from different source kitchens.

ЭВОЛЮЦИЯ ГИДРОГЕОХИМИЧЕСКОГО ПОЛЯ СЕВЕРНЫХ И АРКТИЧЕСКИХ РАЙОНОВ ЗАПАДНО-СИБИРСКОГО ОСАДОЧНОГО БАССЕЙНА В МЕЗОЗОЕ

Актуальность исследования состоит в выявлении фундаментальной роли палеогидрогеологических факторов в эволюции гидрогеохимического поля северных и арктических районов Западно-Сибирского осадочного бассейна в мезозойскую эру. Впервые предложен механизм, объясняющий их вклад в формирование гидрогеохимических аномалий в оксфордском резервуаре (верхнеюрском водоносном комплексе) изучаемого региона, где широко развита гидрогеохимическая инверсия. Цель: выявить особенности состава сингенетичных вод, захораниваемых одновременно с осадком на элизионных и инфильтрационных этапах развития осадочного бассейна на основе установленной цикличности гидрогеологической истории северных и арктических районов Западно-Сибирского осадочного бассейна; выполнить сравнительных анализ состава современных и сингенетичных вод и установить природу гидрогеохимических аномалий. Методы. Использованы методические приемы А.А. Карцева, С.Б. Вагина, Е.А. Баскова и других исследователей по палеогидрогеологическим реконструкциям осадочных бассейнов, выполнена периодизация гидрогеологической истории с выделением основных гидрогеологических циклов и инфильтрационных и элизионных этапов. На основе методики восстановления солевого состава вод древних морских и озерно-аллювиальных бассейнов, палеогеографических карт на юрский и меловой периоды, составленных сотрудниками ИНГГ СО РАН, информационного банка данных лаборатории гидрогеологии осадочных бассейнов Сибири ИНГГ СО РАН по арктическим районам Западно-Сибирского осадочного бассейна проведены палеогидрогеохимические реконструкции юрско-мелового разреза. Результаты. Приводятся результаты палеогидрогеологических реконструкций северных и арктических районов Западно-Сибирского осадочного бассейна. Установлено, что в гидрогеологическом разрезе исследуемого региона доминируют седиментогенные воды, в некоторых случаях разбавленные инфильтрогенными, проникшими при регрессии морского бассейна. Сравнительный анализ захороненных сингенетичных вод оксфордского бассейна и данных по современной гидрогеохимии подземных вод выявил ряд отрицательных и положительных аномалий. Отрицательные аномалии закономерно трассируют границу глинизации оксфордского регионального резервуара и связаны с развитием процессов элизионного водообмена. Геостатическая (литостатическая) водонапорная система на глубинах 2‒2,5 км переходит в термодегидратационную, что сопровождается аномально высокими пластовыми давлениями и появлением в водонапорной системе пресных химически-связанных вод. Большинство положительных аномалий приурочены к структурам Южно-Надымской и Предъенисейской мегамоноклиз, где широко проявлены процессы вертикальной разгрузки подземных вод нижне-среднеюрского и палеозойского водоносных комплексов в зонах тектонических нарушений. Системе «вода–порода–газ–органическое вещество» принадлежит ключевая роль в процессах формирования химического состава подземных вод нефтегазоносных отложений.

Late Mesozoic tectonostratigraphic division and correlation of the Bohai Bay basin: Implications for the Yanshanian Orogeny

We present the results of Mesozoic sequences of the Bohai Bay basin in North China, based mainly on geochronology and interpretations of seismic profiles and logging data. Five tectono-stratigraphic sequences are defined: Lower-Middle Triassic, Lower-Middle Jurassic, Upper Jurassic, Lower Cretaceous and Upper Cretaceous. Based on an analysis of detrital zircons, the clastic rocks recorded two intervals of Jurassic magmatic activity, during 180–175 and 160–152 Ma, which can be correlated to the Nandaling and Tiaojishan Formations in the Yanshan area, respectively. However, since Jurassic volcanic rocks have not yet been found in the Bohai Bay basin, we speculate that these two stages of activity were mainly concentrated around the periphery of the North China Craton (NCC) rather than within. Based on an analysis of zircons from volcanic rocks and pyroclastics, the early Cretaceous magmatism in the Bohai Bay basin can be divided into two stages, 125–120 and 110–100 Ma, which can be correlated to magmatism in the eastern part of the NCC. The zircon ages indicate an absence of volcanic activity during the late Tuchengzi and Zhangjiakou periods which may correlate to the uplift of the Bohai Bay basin in the late Late Jurassic. Comparison of the development of Mesozoic basins and sedimentary strata in the central-eastern part of Yanshan tectonic belt and the the Bohai Bay basin indicates that the two areas are generally comparable, but with substantial differences. The central-eastern part of the Yanshan structural belt lacks Early-Middle Triassic strata, and the Bohai Bay basin lacks late Jurassic-early Cretaceous strata. Based on research results from late Mesozoic sedimentary structures in the central and eastern parts of the Yanshan tectonic belt, we infer that episode A of the Yanshanian Orogeny was weak in the Bohai Bay basin and its periphery, while episode B of the Yanshanian Orogeny had a strong influence on the Bohai Bay basin and its periphery. The available data reveal differences in the expression of these episodes: the Bohai Bay basin is characterized by vertical uplift, and the northern margin of NCC is characterized by horizontal compression uplift.

A growing sandstone type uranium district in South Yili Basin, NW China as a result of extension of Tien Shan Orogen: Evidences from geochronology and hydrology

The Mesozoic Yili Basin of NW China represents the largest known concentration of U deposits in China and contains five major deposits, namely (from west to east) the 512 (Kujie’ertai), 513, 511, 510 (Mengqiguer), and 509 deposits. Pre-mining resources within the explored sandstone-type uranium deposits in this area are reportedly as much as 20,000 t contained U. The mineralization is hosted by the Middle–Lower Jurassic Shuixigou Group, which (from base to top) is divided into the Badaowan, Sangonghe, and Xishanyao formations. The U-Pb isotopic analysis of ores from the Kujie’ertai and Mengqiguer deposits indicate that they contain high and variable amounts of initial (common) Pb, meaning that the only possible way to date these deposits is by using U-Pb isochrons. Two major stages of uranium mineralization have been identified by the U-Pb isotope dating of uranium ores in this region. The Kujie’ertai deposit apparently formed between 23.4 ± 3.3 and 20.18 ± 0.49 Ma, corresponding to a period of crustal thickening and uplift within the Tien Shan Orogen. This event (35–21 Ma) accommodated the majority of the strain generated by the northward collision of the Indian Plate with the Asian Plate. However, the dating of samples from the Mengqiguer deposit yielded much younger ages (between 0.61 ± 0.24 and 0.347 ± 0.0048 Ma). The western Tien Shan mountains expanded until the Pliocene as a result of the far-field influence of continuous penetration of the Indian Plate into the Asian Plate. This activated reverse faults and folds in the piedmont of the Tien Shan mountains and caused the continuous uplift of the southern flank of the Yili Basin. The uplift caused the erosion of anticline hinge zones, introducing significant amounts of oxidizing water into the Shuixigou Group, generating a second stage of uranium mineralization. Hydrological sampling also suggests that the Mengqiguer deposit continues to grow, indicating a possible third stage of uranium mineralization (∼0 Ma). This also indicates that the U within these deposits is derived not only from U-bearing sediments but from the Tien Shan mountains as a result of groundwater cycling. The evolution of the U contents of groundwater that was initially derived from cold springs that flow into the mineralized units indicates that these cold springs have an essential role in mobilizing U from the Tien Shan mountains, with rivers flowing through areas of outcropping mineralized units acting as a source of mineralizing fluids during the formation of the Mengqiguer deposit.

Characteristics and controlling factors of the Lower–Middle Jurassic sandstone reservoirs in Amu Darya right bank area, Turkmenistan

Based on laboratory analysis and core description, characteristics and the controlling factors of the Lower–Middle Jurassic reservoirs in Amu Darya right bank area are examined in this article. The study reveals that fine-grained lithic sandstones, feldspar–lithic sandstones, and siltstones are the main types of reservoir rocks in the central area, while fine-grained lithic–quartzose sandstones are predominant in the eastern area. The major pore types are intergranular and intragranular dissolved pores; the predominant pore throats are lamellar throats. Fractures develop poorly in the study region, but relatively well in the eastern area. The reservoirs are pore-dominated sandstone reservoirs with limited fractures, characterized by very low porosity and permeability. Reservoir evolution is jointly controlled by sedimentation, diagenesis, and tectonic movements. Sedimentation laid the foundation to porosity evolution. Subaqueous distributary channels and mouth bars are the beneficial lithofacies for reservoir development. Poor compressive strength of sandstones, deep paleoburial depth, and destructive subsidence style of the strata led to strong compaction. Primary pores are almost entirely destroyed by compaction and eogenetic calcite cementation. Ferroan dolomite cements and quartz overgrowths occupied the remaining intergranular pores furtherly. However, dissolution played an effective role on the generation of secondary pores, and the influences of authigenic clay minerals are minor on reservoirs. The Neogene–Quaternary tectonic movements caused intensive lateral compressional stress thus made further damage to reservoir pore structure. Nevertheless, fractures generated by tectonic movements increased reservoir permeability and provided valid migration pathways for the late dissolution fluids, therefore improved reservoir physical properties obviously.

<p><strong>Late Triassic fossil insects from the Laohugou Formation of northern China (Qinhuangdao, Hebei Province)</strong></p>

Triassic insect fossils from China are very limited. Here we report on numerous insect fossils discovered in the Upper Triassic Laohugou Formation at Heishanyao, Qinhuangdao, Hebei Province. These fossils are represented mainly by forewings of cockroaches and coleopteran elytra. The insect assemblage is most similar to that from the Upper Triassic Momonoki Formation of Japan. Fossils of the Laohugou Formation have been poorly known, so our discovery of new fossil insects bear significance for understanding the biota and sedimentary environment of this formation. The Mesozoic stratigraphic division in the Liujiang Basin has been controversial, and the usage of stratigraphic units has been inconsistent. Here we refine the stratigraphic sequence, from the bottom to the top, of the Upper Triassic Laohugou Formation, the Lower-Middle Jurassic Yaopo Formation, the Upper-Middle Jurassic Longmen Formation, the Upper Jurassic Tiaojishan Formation, and the Lower Cretaceous Zhangjiakou Formation. The Laohugou Formation is scattered in various basins in western Liaoning and northern Hebei, with the lower part mostly characterized by thick layers of complex conglomerates, suggesting a regional tectonic uplift. There is a sedimentary discontinuity between the Laohugou Formation and the Yaopo/Beipiao Formation, reflecting the uplifting activities during the late Late Triassic-early Early Jurassic in eastern China. The Laohugou Formation is overlaying on various ancient strata, representing the first regional unconformity of the northeastern margin of the North China Craton.

Paleohydrogeochemistry of the Upper Jurassic Deposits of the Arctic Regions of the West Siberian Megabasin

The paper considers the paleohydrogeochemistry of the Upper Jurassic sediments of the Arctic regions of Western Siberia. On the territory under investigation during the late Jurassic marine sedimentation was predominate, the sea depths reached 400 m in the Volga time. Under these conditions, syngenetic sodium thalassogenic chloride waters were buried, in some places with a high content of magnesium, the salinity of which reached 35-40 g/dm3. A comparative analysis of the buried syngenetic waters of the Oxford basin and data on the modern hydrogeochemistry of groundwater of the Upper Jurassic aquiferous complex revealed very significant differences. The analysis of distribution value the total mineralization allowed to establish areas of positive and negative anomalies. In the first case, most of the anomalies are associated with the processes of vertical discharge of groundwater of the Lower Middle Jurassic and Paleozoic aquifers in the zones of tectonic disturbances. Negative anomalies regularly trace the clay boundary of the Oxford regional reservoir, expanding significantly in the north-east direction. Their nature is associated with the development of processes of elision water exchange and thermal dehydration of clay minerals.

The Carbon Isotope Fluctuations across the Lower–Middle Jurassic Boundary and the Paleoclimate Changes

The Carbon Isotope Fluctuations across the Lower–Middle Jurassic Boundary and the Paleoclimate Changes

Characteristics and evaluation of Mesozoic source rocks in the southeastern East China Sea continental shelf

ABSTRACT Source rocks are the material basis of oil and gas generation and determine the potential resources of exploration blocks and have important research value. This paper studies the lithology, thickness, and geochemistry of Mesozoic source rocks in the southeastern East China Sea continental shelf. The results show that the Mesozoic source rocks are mainly dark mudstone and coal-bearing strata. The total thickness of Lower–Middle Jurassic source rocks ranges from 100 m to 700 m, and that of Lower Cretaceous source rocks ranges from 50 m to 350 m. The overall thickness of Mesozoic source rocks is distributed in the NE direction and their thickness center is located in the Jilong Depression. The Lower–Middle Jurassic source rocks are mainly developed shallow marine dark mudstone and transitional coal measure strata. Those of the Lower Cretaceous are mainly mudstone of a fan delta front. Lower–Middle Jurassic and Lower Cretaceous hydrocarbon source rocks are dominated by type III kerogen, with Lower–Middle Jurassic hydrocarbon source rocks having high organic matter abundance and being medium–good hydrocarbon source rocks, while Lower Cretaceous hydrocarbon source rocks have relatively poor quality. From northwest to southeast, the vitrinite reflectance Ro of Mesozoic source rocks increases gradually. Source rocks in the study area are divided into three types. The first hydrocarbon-generating area is mainly located in the southeastern region of the study area, and the Jilong Depression is the hydrocarbon-generating center. The results of this study can provide a basis for exploration of Mesozoic oil and gas resources in the southeastern East China Sea continental shelf. © 2019 China Geology Editorial Office.

Discovery of Middle Jurassic trench deposits in the Bangong-Nujiang suture zone: Implications for the timing of Lhasa-Qiangtang initial collision

The Mesozoic trench deposits in the Bangong-Nujiang suture zone in central Tibet can provide critical information for reconstructing the paleotectonic evolution of the Bangong-Nujiang Ocean and constraining the timing of the Lhasa-Qiangtang initial collision. In this paper, the Gamulong Formation, which is well exposed in the Dong Co area, western Bangong-Nujiang suture, central Tibet, was studied. The sedimentological analysis shows that the Gamulong Formation was deposited in a submarine fan environment. The youngest detrital zircon age and fossils from limestone pebbles within the Gamulong Formation combine to determine a Middle Jurassic age. The pebbles in the conglomerate are dominated by limestones and sandstones, which is consistent with the sandstone petrology showing limestone and sandstone fragments as the main framework components. Detrital zircon U-Pb ages from the sandstones yield age populations of 164–178 Ma, 200–500 Ma, 700–1000 Ma, 1700–2100 Ma and ~2500 Ma. The corresponding zircon εHf(t) values are distributed between −23.5 and +11.1. These zircon features are similar to those from the Qiangtang terrane. Accordingly, the provenance results imply that the Gamulong Formation was recycled from the accretionary complex (Mugagangri Group mélange) in the Bangong-Nujiang suture zone and the forearc basin deposits (Lower-Middle Jurassic Sewa/Shaqiaomu/Jiebuqu formations) in the southern Qiangtang subterrane. The Gamulong Formation is characterized by sedimentary recycling in the trench, likely in response to a tectonic event (possibly the continental collision). Together with the previous studies, this work on the occurrence of the Gamulong Formation indicates that the Lhasa-Qiangtang initial collision could have occurred in Middle Jurassic time (~166 Ma).

Regional Jurassic sediment depositional architecture, Browse Basin: Implications for petroleum systems

The Browse Basin hosts considerable gas and condensate resources, including the Ichthys and Prelude fields that are being developed for liquefied natural gas (LNG) production. Oil discoveries are sub-economic. This multi-disciplinary study integrating sequence stratigraphy, palaeogeography and geochemical data has mapped the spatial and temporal distribution of Jurassic to earliest Cretaceous source rocks. This study allows a better understanding of the source rocks contribution to the known hydrocarbon accumulations and charge history in the basin, including in underexplored areas. The Jurassic to earliest Cretaceous source rocks have been identified as being the primary sources of the gases and condensates recovered from accumulations in the Browse Basin as follows: - The Lower–Middle Jurassic J10–J20 (Plover Formation) organic-rich source rocks have been deposited along the northeast-southwest trending fluvial-deltaic system associated with a phase of pre-breakup extension. They have charged gas reservoired within J10–J20 accumulations on the Scott Reef Trend and in the central Caswell Sub-basin at Ichthys/Prelude, and in the Lower Cretaceous K40 supersequence on the Yampi Shelf. - Late Jurassic–earliest Cretaceous J30–K10 source rocks are interpreted to have been deposited in a rift, north of the Scott Reef Trend and along the Heywood Fault System (e.g. Callovian–Tithonian J30–J50 supersequences, lower Vulcan Formation). The J30–K10 shales are believed to have sourced wet gas reservoired in the K10 sandstone (Brewster Member) in the Ichthys/Prelude and Burnside accumulations, and potentially similar plays in the southern Caswell Sub-basin. - The organic-rich source rocks observed in the Heywood Graben may be associated with deeper water marine shales with higher plant input into the isolated inboard rift. They are the potential source of fluids reservoired within the Crux accumulation, which has a geochemical composition more closely resembling a petroleum system in the southern Bonaparte Basin.