Mesoproterozoic Xiamaling Research Articles

A protracted Mesoproterozoic carbon cycle perturbation in response to volcanism at ∼ 1.39 Ga

The Earth's carbon cycle throughout the Mesoproterozoic Eon remains a long-standing problem in geology. Recent studies have reported vigorous tectonic and volcanic activities and co-appearances of multiple black shale depositions and large igneous provinces (LIPs) events of this time. These events might have greatly affected the Mesoproterozoic carbon cycle. Additionally, compared with the Phanerozoic, the Mesoproterozoic Eon frequently recorded peculiarly long-lasting carbon isotope disruptions, suggesting specific driven systems behind them. Yet, the internal links between carbon isotope perturbations, black shale depositions and magmatism during this period are poorly understood. Here we identify a ∼ 5.7‰, lengthy (10.6 Myr) negative carbon isotope excursion event recorded in the Mesoproterozoic Xiamaling Formation of North China Craton (XML-NCIE). Laboratory and modeling work indicate that, dissimilar to most Phanerozoic NCIEs, this event is a two-stepped, multi-factor-controlled case. Step 1-NCIE is caused by massive light carbon injection (125,000 Gt, -30‰) from the coexisting strong volcanism (LIPs) and methanogenesis. After step 1, although the volcanism is terminated, it probably sparks some follow-up activities and brings about another episode of carbon input (30,000 Gt, -37‰). In summary, volcanism is the primary triggering factor of XML-NCIE and further acts to motivate successive reactions. Meanwhile, the Mesoproterozoic less forceful volcanism is conducive to the particularly long XML-NCIE duration. This finding may also bring fresh perspectives on the Mesoproterozoic black shale depositions, suggesting that both volcanism and microbial reworking processes might have remarkably affected the Mesoproterozoic carbon cycle and promoted organic carbon burial at that time.

P-T paths of contact metamorphism constraints on emplacement mechanism of magmas of the zoned Fangshan granitic Plutons, Zhoukoudian, Western Hills of Beijing, China

P-T path of a contact aureole can provide important insights into the magma emplacement mechanism for nearly concentrically zoned granitic plutons. The Mesoproterozoic Xiamaling formation immediately adjacent to the southern Fangshan granitic Plutons in the Western Hills of Beijing, China, developed three contact metamorphic prograde staurolite zone, sillimanite zone and K-feldspar zone. These zones preserved corroded pink-andalusite wrapped by pale-andalusite, corroded garnet inclusion inside the K-feldspar porphyroblasts and staurolite, which add to well-established sequences of mineral growth. These sequences allow reconstruction of the complete P-T path for each prograde zone on the pseudosection diagram obtained by full system composition-based phase equilibrium modeling The resultant prograde P-T paths are composed of two segments, with the first one isobaric heating and the second distinct compression and heating. The staurolite zone shows the P-T path initiating from ∼2.0 kbar/350 °C to ∼1.8 kbar/510 °C and ending at ∼3.3 kbar/580 °C. The sillimanite zone path began from ∼2.0 kbar/350 °C to ∼1.5 kbar/500 °C then ended at ∼3.8 kbar/580 °C. The K-feldspar zone path started from ∼2.0 kbar/350 °C to ∼1.7 kbar/510 °C, then terminated at ∼3.5 kbar/670 °C, approaching the initial partial melting temperature condition, then documented a retrograde path cooling down to ∼630 °C. The two prograde P-T path segments of the three zones suggest a in situ heating followed by a ductile return flow from ∼7 km down to ∼11 km depth of the metapelites from the Xiamaling formation, responsable to a diapiric emplacement of the magmas for the zoned composite Fangshan Plutons.

Construction of multidimensional structure model of the Mesoproterozoic Xiamaling shale kerogen, northern North China

In the southeastern Xuanlong sag of Northern China, the organic-rich black shale of the Mesoproterozoic Xiamaling Formation holds significant potential as a source rock. Through advanced analytical techniques including elemental analysis, 13C NMR, XPS, FTIR, and XRD, the elemental composition and chemical structure were thoroughly characterized, and the molecular structure of the Xiamaling shale kerogen was established. The 13C NMR analysis revealed the carbon composition of kerogen, indicating 75.19 % aliphatic carbon, 22.06 % aromatic carbon, and 2.75 % carbonyl carbon, with quantitative support from FTIR and XRD spectra. The dominant methylene carbons exhibited a low average methylene chain length (Cn) of 3.87, suggesting that they are primarily short-branched or alicyclic compounds rather than long straight chains. This conclusion was verified by the absence of a sharp peak at a wavenumber of 719.95 cm−1 in the FTIR spectrum. Oxygen-containing functional groups observed in 13C NMR spectra were also identified in FTIR spectra at a wavenumber range of 3500–1000 cm−1. XPS analysis indicated that nitrogen-containing groups were amino, quaternary, pyridine, and primary pyrrole groups, and sulfur existed as sulfoxide, aromatic sulfur, aliphatic sulfur, and sulfone. Finally, a relatively rational 2D molecular structure with the chemical formula of C278H346N6O26S2 was established by comparing the predicted and experimental 13C NMR spectra. Using MD simulation, five optimized 2D molecules were combined to construct a robust 3D kerogen model with a physical density of 1.067 g/cm3. Comparatively, the aliphatic structure of the Mesoproterozoic Xiamaling kerogen model consists of some monocyclic alkanes and polycyclic saturated structures, resembling Paleozoic kerogen models but differing from Mesozoic counterparts. The constructed kerogen model addresses the gap in the Precambrian kerogen modeling and sets the stage for in-depth exploration of pyrolysis mechanisms in the future.

Organic matter accumulation mechanisms and source rock potential of the Mesoproterozoic Xiamaling Formation, Yanliao Basin, North China

The Mesoproterozoic Xiamaling Formation (∼1.4 Ga) is considered to be a set of promising strata for shale oil and gas exploration and production in the Yanliao Basin, North China Craton. This study aimed to investigate the organic matter accumulation mechanism, and hydrocarbon generation and expulsion characteristics of the Xiamaling Formation, using various methods, including scanning electron microscopy, rock-eval pyrolysis, and elemental geochemistry. Multiple geochemical proxies indicate that Unit 3—Unit 1 of the Xiamaling Formation was deposited in a climate that alternated between highly humid and semiarid conditions, with the strong-to-weak terrigenous influx. The redox conditions of bottom water were undulatory oxic in Unit 3 and the upper part of Unit 1, but anoxic in Unit 2 and the lower part of Unit 1. Sulfurization of organic matter was the main reason for the high TOC content in Unit 3. However, the high TOC content in Unit 2 and the lower part of Unit 1 was caused by an oxygen-deficient water column. Unit 2 and the lower part of Unit 1 exhibit a significant shale oil potential owing to low maturity, high hydrogen index (HI) values, and prevalence of type Ⅰ, II1, and II2 kerogens.

Process and Mechanisms of Deep Fluid Effects on Hydrocarbon Generation and Pore Space in Shale: A Case Study from the Mesoproterozoic Xiamaling Formation in the Zhaojiashan Section

AbstractHeat carried by deep fluid might greatly affect hydrocarbon generation and pore space in shale. Dyke intrusion carrying high levels of heat may be a means by which to explore the influence of deep fluid on shale reservoirs. This study evaluates hydrocarbon generation and analyzed the evolution of shale storage space in the third member of the Xiamaling Formation in the Zhaojiashan section, Hebei Province, based on experimental data such as TOC, SEM, VRo, low‐temperature N2 adsorption and high‐pressure mercury injection. The results show that the dyke intrusion reduced the shale TOC content drastically—by up to 77%—and also induced instantaneous hydrocarbon generation over a range about 1.4 times the thickness of the intrusion. Furthermore, the dyke intrusion might transform organic pores in surrounding shales into inorganic pores. There were two shale porosity peaks: one appeared when VRo = 2.0%, caused by the increase of organic pores as thermal maturity increased, the other occurred when the VRo value was between 3% and 4%, caused by the increase of inorganic mineral pores. It can be concluded that dyke intrusion can be an effective tool with which to study how deep fluid affects instantaneous hydrocarbon generation and pore space in shale.

Geochemistry and depositional environment of the Mesoproterozoic Xiamaling shales, northern North China

The Mesoproterozoic Xiamaling shales from Xiahuayuan area, southeastern Xuanlong sag, Northern China, were examined the elemental analysis by applying the inductively coupled plasma-mass spectrometry (ICP–MS). The Xiamaling shales have good to excellent oil hydrocarbon generation potential with type II kerogenas demonstrated by high content of alginate macerals, TOC, HI, S1 + S2 and extractable organic matter (EOM). The Xiamaling shales were deposited under a marine environment according to boron (B) (56–111 ppm) and other trace elements concentration, which is consistent with the geological settings. These shales can be classified into three types: (1) Type I is a coastal facies, which is influenced by fresh riverine water and has low B content of 56–61 ppm; (2) Type II is a marginal marine with B content of 73–79 ppm; and (3) type III has high B content of 111 ppm, typical of a brackish, and open marine environment.The Xiamaling shales have high B (>50 ppm), low Na, Ca, Ce, Mn, and rare earth elements (REEs). According to the elemental composition and ratios, the Xiamaling sediments were derived from the felsic source, which has suffered from moderate to strong chemical weathering based on the CIA, ICV, and PIA values. The Xiamaling marine shales were mostly deposited under dysoxic-anoxic conditions. The paleoclimate during the deposition of the Xiamaling shales was warm and humid based on the cross plots of Sr/Ca vs Mn/Ca values and Sr/Cu vs Rb/Sr ratios. Their total REE (ΣREE) contents increase from coastal towards the open marine zone. The Xiamaling shales display a negative Ce anomaly, representative of marine shale, and show a negative Eu anomaly, which is representative of lacustrine oil shale and the Post-Archean Australian Shale (PAAS) and is different from typical marine shale of younger age, indicating the influx of terrestrial sediment.

Fields and directions for shale gas exploration in China

China is highly dependent on foreign oil and gas, and the exploration and exploitation of shale gas is an important way to alleviate the energy and environmental pressure of China. Although a series of significant progress has been made in a short time, China is still at the crossroads of shale gas exploration direction. Therefore, this paper systematically reviews the particularity of shale gas geological conditions in China, and points out China's exploration direction of shale gas in different fields combined with the research achievements of shale gas accumulation mechanism. The following results were obtained. (1) China's shale gas exploration fields can be divided into three major parts, i.e., marine, continental and marine–continental transitional facies. The geological characteristics of shale gas in North China, Yangtze and Tarim plates are different. The interaction of multiple factors, including tectonic and sedimentary evolution, shale gas accumulation, late-stage transformation and etc., has led the complexity of the geological distribution and the diversity of exploration fields of shale gas in China. (2) The organic pores and fractures of marine shale are developed. The Ordovician Wufeng-Silurian Longmaxi Formation will still be the primary target of shale gas exploration in future. Meanwhile, the Cambrian Niutitang and Sinian Doushantuo Formation in the Yangtze region, the Devonian Luofu and Carboniferous Jiusi Formation in the Yunnan-Guizhou-Guangxi region, the Jixianian Hongshuizhuang and Mesoproterozoic Xiamaling Formation will be the targets of strategic breakthrough in shale gas exploration. (3) The structural deep and slope of middle-large basins and the sedimentation centers of small-middle basins are the main directions of continental shale gas exploration. Inter-laminar cracks and intergranular pores are well developed in continental shale. The coupling of various organic matter types and thermal evolution results in the symbiosis of shale oil and shale gas, which will become an important field for further exploration of continental shale gas. (4) The marine–continental transitional shale, which mainly formed in Late Carboniferous-Permian, is characterized by cyclic association with sandstone, mudstone, coal and carbonate. Diagenetic fractures are well developed in transitional shale. The favorable structural and diagenetic preservation are the two major factors for the enrichment of this type of shale gas. The flat-lagoon and delta sedimentary systems, middle-large superimposed and small-middle residual basins, and deep layers with quality caprock and good structural preservation are the favorable directions for shale gas exploration in transitional facies.

Pore evolution model and diagenetic evolution sequence of the Mesoproterozoic Xiamaling shale in Zhangjiakou, Hebei

Current research on diagenesis and its evolution focuses mainly on sandstone reservoirs. Conventional oil and gas reservoir observation techniques for characterizing shale reservoirs are limited, and comprehensive research on nanoscale pore variations, diagenesis and evolution of shale reservoirs, especially immature and low-maturity marine shale reservoirs, is relatively scarce. To reveal the diagenesis and evolution of low-maturity marine shale, geochemical and mineral analyses, quantitative characterization of the pore structure and qualitative characterization of the pore morphology were performed on the Xiamaling shale. With the burial and geothermal history, the Xiamaling diagenetic evolution sequence and the nanoscale pore system evolution were studied. The results show that smectite continually transforms to an illite-smectite (I/S) mixed layer and then quickly transforms to illite and chlorite; the kaolinite content gradually decreases as maturity increases. The hydrocarbon expulsion threshold of the Xiamaling shale is EqRo = 1.0%, and the porosity decreases to a minimum value of EqRo = 1.0% and then increases. The pore structure evolution model shows that the pore volumes (PVs) of micropores and macropores decrease to minima at approximately EqRo = 0.92% and 0.85%, respectively, and then increase; the mesopore PVs continuously decrease. Micropore and mesopore surface areas (SAs) decrease to their minima at approximately EqRo = 0.85% and 1.0%, respectively, and then increase; simultaneously, the macropore SAs continuously decrease. The main diagenetic types of the Xiamaling shale are compaction, dissolution, cementation, thermal maturation of organic matter (OM), and clay mineral transformation; it reaches period B of the middle diagenetic stage, and the diagenetic evolution sequence is determined. The pore system composition and variation characteristics of the Xiamaling shale are analysed, and a corresponding evolution model is established.

Diagenetic evolution sequence and pore evolution model of Mesoproterozoic Xiamaling organic-rich shale in Zhangjiakou, Hebei, based on pyrolysis simulation experiments

Organic-rich shale is an unconventional and complex petroliferous system that integrates a “source-reservoir-cap”, and the coupled evolution of hydrocarbon generation, diagenesis and nanoscale pores is the key problem that affects shale gas accumulation. Current research methods for this issue mainly include direct observation and physical simulation. Direct observation ignores the heterogeneity and regional differences of the natural samples, and physical simulation mostly lacks an intuitive characterization and cannot clearly show the relationship between minerals and pore evolution characteristics in the same region. To avoid the effect of heterogeneity on the researched samples and clearly and intuitively reveal the coupled evolutionary relationship among the hydrocarbon generation of thermal maturation, diagenesis and nanopore structures in shale, this research included hydrous pyrolysis experiments on low mature marine shale samples to achieve various maturities and diagenesis stages. The pyrolysis products at each stage were recovered and subjected to an ongoing multidisciplinary analytical program. The results show that an increased temperature intensifies the evolution of dissolution pores in unstable brittle minerals, promotes clay mineral conversion, and accelerates the development of clay mineral pores and organic pores. The pore volume (PV) of the nanoscale pores reached its minimum at 300 °C, increased to its maximum at 500 °C and decreased thereafter. The surface area (SA) of the nanoscale pores reached their minimum at 300 °C and then continuously increased, while those of macropores reached their maximum at 500 °C and then decreased. The diagenesis of the pyrolysis products were mainly dissolution, clay mineral transformation, the thermal maturation of organic matter (OM), compaction and cement filling. The diagenetic evolution was divided into four stages, and the diagenetic evolution sequence and pore evolution model of the shale were established. Hydrocarbon generation, diagenesis and nanoscale pore evolution have synergistic effects, which have great significance for shale reservoir evaluation and shale gas accumulation, exploration and exploitation. The conceptual evolution model provides a quantitative prediction method for hydrocarbon generation, diagenesis and nanoscale pore variations.

A diverse organic-walled microfossil assemblage from the Mesoproterozoic Xiamaling Formation, North China

The mid-Proterozoic (ca. 1.85–0.85 Ga) might have been environmentally “boring”, but was a crucial interval of time for the early establishment of eukaryotic life on Earth. The Mesoproterozoic shale-dominated Xiamaling Formation (ca. 1.4–1.35 Ga) in North China preserved abundant organic-walled microfossils, some of which were of eukaryotic affinity but their taxonomic diversity and evolutionary implications remain unclear. In this paper, we present a palynological study of the lower Xiamaling Formation in the northern Tianjin Municipality. Similar to other Mesoproterozoic microbiotas, this assemblage consists of well-preserved and diverse spheroidal to ellipsoidal vesicles, filamentous microfossils and cellular aggregates. In total, 36 species belonging to 28 genera are identified, including 1 new species Quadrimurus clavatus gen. et sp. nov., and 5 unnamed forms. Majority of these taxa are recognised for the first time from this formation, indicating a richer assemblage than previously known. Although most taxa are of unknown biological affinities, this microbiota is thought to be composed of prokaryotic and eukaryotic microorganisms, in which 12 taxa are inferred to be eukaryotic. These microfossils indicate a moderate diversity of eukaryotic life documented in the Xiamaling Formation, which is consistent with other microbiotas in the mid-Proterozoic. The presence of Jixiania lineata Yan, 1986 (=Lineaforma elongata Vorob’eva and Sergeev, 2015) in North China extends its geographic distribution and further enhances its potential as index fossil for the early Mesoproterozoic strata. The Xiamaling assemblage provides one of the broader pictures of the Mesoproterozoic biosphere, in which eukaryotes were predominantly protistan-grade microorganisms.

Organic-induced nanoscale pore structure and adsorption capacity variability during artificial thermal maturation: Pyrolysis study of the Mesoproterozoic Xiamaling marine shale from Zhangjiakou, Hebei, China

This research analyzed two low-maturity Mesoproterozoic Xiamaling marine shale samples with different TOC contents in Zhangjiakou of Hebei, China. Artificial anhydrous pyrolysis experiment simulated the thermal evolution process of the shale sample from low maturity to over maturity. The evolution characteristics of geochemical parameters and mineralogy of shales with the increase in maturity were investigated. A general evolution model of nanoscale pore structure and methane adsorption content (MSC) was established. Results show that hydrocarbon generation and mineral conversion promote the generation, development, combination, and rearrangement of nanoscale pores in shale samples with the increase in thermal maturation. The maturity increase helps facilitate the generation and evolution of pores in shales. Heating can promote the generation, development, and evolution of nanoscale pores, and this condition can contribute to forming large pore volume and surface area. The formation and evolution of the nanoscale pores in shale can be roughly subdivided into adjustment, development, combination, and rearrangement stages. Nanoscale pore structure parameter and MSC decrease and reach the minimum value as the temperature reaches 350 °C in the adjustment stage. Then, they rise and reach the maximum value at approximately 650 °C during the development stage. Finally, they decline again as the pyrolysis temperature rises at the combination and rearrangement stages. The general evolution model of nanoscale pore structure and MSC was proposed and subdivided into three stages by combining the hydrocarbon generation, mineral conversion, nanoscale pore structure, and MSC evolution of the thermal simulation shale samples. The differences in organic geochemical index, mineral characteristics, nanoscale pore structure, and MSC evolution characteristics of the simulated samples are mainly due to the difference in their TOC content.

Full-Scale Pore Structure Characteristics and the Main Controlling Factors of Mesoproterozoic Xiamaling Shale in Zhangjiakou, Hebei, China.

Nanoscale pore structure characteristics and their main controlling factors are key elements affecting the gas storage capacity, permeability, and the accumulation mechanism of shale. A multidisciplinary analytical program was applied to quantify the pore structure of all sizes of Xiamaling shale from Zhangjiakou, Hebei. The result implies that Mercury injection porosimetry (MIP) and low-pressure N2 curves of the samples can be divided into three and four types, respectively, reflecting different connectivity performances. The maximum CO2 adsorbing capacity increases with increasing total organic carbon (TOC) content, pore volume (PV), and surface area (SA) of the micropores are distributed in a three-peak type. The full-scale pore structure distribution characteristics reveal the coexistence of multiple peaks with multiple dominant scales and bi-peak forms with mesopores and micropores. The porosity positively correlates with the TOC and quartz content, but negatively correlates with clay mineral content. Organic matter (OM) is the main contributor to micropore and mesopore development. Smectite and illite/smectite (I/S) assist the development of the PV and SA of pores with different size. Illite promotes the development of the nanoscale PV, but is detrimental to the development of the SA. Thermal maturity controls the evolution of pores with different size, and the evolution model for the TOC-normalized PVs of different diameter scales is established. Residual hydrocarbon is mainly accumulated in micropores sized 0.3 to 1.0 nm and mesopores sized 40 nm, 2 nm and less than 10 nm. Since the samples were extracted, the pore space occupied by residual hydrocarbon was released, resulting in a remarkable increase in PV and SA.

Evolution Mechanism of Material Composition–Pore Structure–Adsorption Property in Marine Shale Based on Pyrolysis Experiments: A Typical Case of the Mesoproterozoic Xiamaling Formation

The evolution of shale gas reservoirs is highly complicated, especially when the reservoir is at a great depth and has undergone complex diagenesis and multiple stages of tectonic activity. However, the evolution mechanism of material composition–pore structure–adsorption property remains unclear. In this study, pyrolysis experiments were used to obtain products of different maturities and determine the evolution processes that took place in a reservoir and total organic carbon (TOC), X-ray diffraction, field emission scanning electron microscopy, low-pressure nitrogen/carbon dioxide gas adsorption, and methane isothermal adsorption experiments were employed to analyze the material composition, pore morphology and structure, and adsorption capacity. The results showed that the TOC value of the pyrolysis samples decreased in comparison to the original sample because some organic matter was converted into hydrocarbons. As diagenesis strengthened, unstable minerals, such as feldspar and carbonate, were converted into clay minerals, while quartz remained basically unchanged. The transformation between clay minerals was quite substantial and inheritable in terms of pore structures and adsorption capacity. The change in the pore structure was mainly caused by mesopores. The evolution of the pore structure and the adsorption property were controlled by clay minerals (mainly illite–smectite mixed layer and illite) and organic matter. Both micro- and mesopores had a controlling effect on the adsorption capacity. When Ro exceeds 2.48%, there was a dramatic change in material composition, pore structure, and adsorption property, which should be given more attention in future research.

Using cyclostratigraphic evidence to define the unconformity caused by the Mesoproterozoic Qinyu Uplift in the North China Craton

In the Yanliao Basin of the North China Craton, the Mesoproterozoic Xiamaling Formation contacted unconformably with the underlying Tieling Formation. This unconformity was caused by the Qinyu Uplift. Underneath this unconformity, the Tieling Formation experienced variably weathering with more extensive erosion of carbonates in the west than in the northeast of the basin. To constrain the timing of the Qinyu Uplift and the duration of subsequent unconformity, cyclostratigraphic analysis was conducted on the Tieling carbonate rocks. Referencing the periodicities of the eccentricity, obliquity and precession obtained from the Mesoproterozoic Xiamaling and Hongshuizhuang Formations, Milankovitch cycles during the Tieling Formation were recognized. A bentonite located at the middle of the Tieling Formation was recognized from the core at the Beizhangzi section, and was considered to be contemporaneous with those found at the Jixian and Liujiagou sections, which had a consistent age of 1441 Ma. Then a deposition rate from the bentonite layer to the top interface of the Tieling Formation was calculated to be 1.65 ± 0.22 cm kyr−1, and the duration time were ca. 9 million years. So the minimum age of the Tieling Formation was ca. 1432 Ma. Combined with our new 207Pb/206Pb weighted average age of 1418 ± 14 Ma obtained from the bottom of the Xiamaling Formation, the Qinyu Uplift was constrained to occur between 1432 Ma and 1418 Ma, older than the previously suggested age of 1400 Ma. The duration of this unconformity was suggested to be no less 14 million years, and might be longer in the western part.

Oil generation character of Mesoproterozoic Xiamaling formation during stepwise pyrolysis

The exploration potential of Mesoproterozoic petroleum system has getting increasingly attention and the oil generation character of Mesoproterozoic source rock is still uncertain. To solve the problem, this study choose sample from Mesoproterozoic Xiamaling formation from north China, using the stepwise gold tube pyrolysis method to study the oil generation character and the composition of liquid pyrolysates, and choose sample from Mesozoic Yanchang formation in middle China as a comparison to study their differences in oil generation character. The results showed that oil generation window of Mesoproterozoic Xiamaling sample in this research was earlier and narrower, with an earlier oil generation peak, this maybe the result of the single source of organic matter and its labile chemical bound. Furthermore, mainly source of organic carbon, like cyanobacteria and acritarchs, were lack of lipid compounds, perhaps this could make the relative concentration of saturated hydrocarbon in liquid pyrolysates of Mesoproterozoic Xiamaling sample much lower than the Mesozoic Yanchang sample with the same kerogen type, nonpolar fraction of liquid pyrolysates in the oil generation window were mainly aromatic hydrocarbon. Maybe the results mean that although Mesoproterozoic Xiamaling formation is still in low maturity stage, it may has already started oil generation, and type of oil generated by Mesoproterozoic source rock in oil generation window may have significant difference from the Phanerozoic source rock.

Growth structures and growth strata of the Qianjiadian Basin in the western Yanshan fold and thrust belt, North China

The northwestward subduction of the Izanagi Plate beneath the eastern Eurasian Plate during the Late Mesozoic caused a series of compressional deformation events in the Yanshan fold and thrust belt (YFTB), but the tectonic deformation timing, tectonic properties and relationship between tectonic uplift and sediment accumulation in the intermontane basins continue to be debated. For this reason, the sedimentology, sediment provenance, and basin structure of the Qianjiadian Basin (QJB) in the northern Beijing region during the Late Jurassic and Early Cretaceous were studied in detail. The results suggest that a fault-propagation fold-type (FPFT) growth structure and growth strata developed on the western edge of the QJB and that the top part of Member 2 and Member 3 of the Tuchengzi Formation are growth strata controlled by the limb rotation mechanism. In two small thrust faults in the QJB, the Mesoproterozoic Xiamaling Formation is thrust over the Tuchengzi Formation, and these faults may have controlled the development of the fault-bend fold-type growth strata. An analysis of the “source-to-sink” process suggests that the hanging wall succession of the Shangyi-Pingquan fault (SPF) was the main source area of the Late Jurassic to early Early Cretaceous strata in the QJB. A zircon 206Pb/238U age of 140.8±2.4 Ma for the volcanic rocks at the bottom of the FPFT growth strata represents the timing of the initiation of FPFT growth structure development. The discovery of the FPFT growth structure and growth strata in the QJB indicate that the QJB was an intermontane flexural basin controlled by fold and thrust structures during the early Early Cretaceous. Near the E-W-trending SPF, the NE-SW-trending Qianjiadian thrust fault (QJTF) and two small intrabasinal thrust faults may constitute a unified right-lateral strike-slip system that formed in response to the northwestward flat subduction of the Izanagi Plate beneath the East Asian continent during the Jurassic-Early Cretaceous.

Evolution characteristics and model of nanopore structure and adsorption capacity in organic-rich shale during artificial thermal maturation: A pyrolysis study of the Mesoproterozoic Xiamaling marine shale with type II kerogen from Zhangjiakou, Hebei, China

Thermal maturity has a considerable impact on hydrocarbon generation, mineral conversion, nanopore structure, and adsorption capacity evolution of shale, but that impact on organic-rich marine shales containing type II kerogen has been rarely subjected to explicit and quantitative characterization. This study aims to obtain information regarding the effects of thermal maturation on organic matter, mineral content, pore structure, and adsorption capacity evolution of marine shale. Mesoproterozoic Xiamaling immaturity marine oil shale with type II kerogen in Zhangjiakou of Hebei, China, was chosen for anhydrous pyrolysis to simulate the maturation process. With increasing simulation temperature, hydrocarbon generation and mineral transformation promote the formation, development, and evolution of pores in the shale. The original and simulated samples consist of closed microspores and one-end closed pores of the slit throat, all-opened wedge-shaped capillaries, and fractured or lamellar pores, which are related to the plate particles of clay. The increase in maturity can promote the formation and development of pores in the shale. Heating can also promote the accumulation, formation, and development of pores, leading to a large pore volume and surface area. The temperature increase can promote the development of pore volume and surface area of 1–10 and 40-nm diameter pores. The formation and development of pore volume and surface area of 1–10 nm diameter pores are more substantial than that of 40-nm diameter pores. The pore structure evolution of the sample can be divided into pore adjustment (T < 350°C, EqRo < 0.86%), development (350°C < T < 650°C, 0.86% < EqRo < 3.28%), and conversion or destruction stages (T > 650°C, EqRo > 3.28%). Along with the increase in maturity, the methane adsorption content decreases in the initial simulation stage, increases in the middle simulation stage, and reaches the maximum value at 650°C, after which it gradually decreases. A general evolution model is proposed by combining the nanopore structure and the adsorption capacity evolution characteristics of the oil shale.

Evolution of organic pores in marine shales undergoing thermocompression: A simulation experiment using hydrocarbon generation and expulsion

To evaluate the evolution and significance of organic pores in marine shales, samples with high organic matter and low thermal maturity were collected from the Mesoproterozoic Xiamaling Formation in the Xiahuayuan region, Hebei province, northern China. Samples underwent a thermocompression simulation experiment. Scanning electron microscope (SEM) observations, N2/CO2 gas adsorption analyses, and statistical calculations assured the qualitative and quantitative characterization of organic pores during kerogen maturation. Results demonstrated that pores largely evolve from macro- and meso-sizes to meso- and micro-sizes along with increasing pore volumes. As the total proportion of organic pores reached over 50% from the maturation (345 °C and 375 °C) to high maturation stages (440 °C), they provided the main space for gas storage. Organic pore types changed from hydrocarbon shrinkage cracks to hydrocarbon bubble pores and hydrocarbon dissolution pores. Additionally, organic porosity sharply increased initially followed by as low decrease as the samples were heated to the post maturation (500 °C) stage.

Stratiform siderites from the Mesoproterozoic Xiamaling Formation in North China: Genesis and environmental implications

Stratiform or layered siderites are much less common than siderite concretions in the stratigraphic record. In the middle part of the Mesoproterozoic Xiamaling Formation (ca. 1.40–1.35Ga), layered siderites are widespread across the North China Platform. Facies analysis indicates that the Xiamaling siderites were formed in shallow-water environments above the storm wave base. High Fe/Al and V/Al ratios and the lack of pyrites in siderite-bearing strata indicate ferruginous bottom-water conditions during the formation of siderites. The low thermal maturity of organic matter and clay mineral structures suggest that the layered siderites were formed at burial temperatures ≤90°C. The negative δ13C values (−24‰ to −12‰) of siderites and the presence of μm-scale hematite inclusions in siderite grains suggest that dissimilatory iron reduction (DIR) during early diagenesis has played critical roles in siderite formation. The source of Fe-oxyhydroxides required for DIR could be from Fe(II) oxidation in shallow seawater above the redoxcline in response to enhanced hydrothermal activities or, alternatively, directly from continental Fe-oxide input. The positive cerium (Ce) and europium (Eu) anomalies of clay minerals in siderite-bearing strata and their positive correlation with total Fe contents (TFe2O3) seem to favor a hydrothermal origin of iron. Syndepositional hyalophane concretions, intermittent tuff layers, and remarkable enrichment of Cu and Zn in some of the siderite-hosting rocks provide further support to hydrothermally sourced iron. The enhanced hydrothermal activity for the development of stratiform siderites in the Xiamaling Formation probably related to the final breakup of the North China Craton from the Columbia supercontinent, during which active hydrothermal systems provided sufficient Fe(II) for Fe-redox recycling and formation of layered siderites.

Ferruginous seawater facilitates the transformation of glauconite to chamosite: An example from the Mesoproterozoic Xiamaling Formation of North China

Berthierine and chamosite are iron-rich clay minerals that share similar chemical compositions. Berthierine forms at low temperature (25–45 °C) during early diagenesis and may transfer to chamosite at temperatures of ≥70 °C. Because the formation of berthierine and chamosite requires significant amount of Fe 2+ supply, their presence in marine sediments is often used as a mineral proxy for ferruginous conditions in porewater. Recent studies reveal that the Precambrian oceans were characterized by pervasive ferruginous water-column conditions that may favor the formation of iron-rich clay minerals like berthierine and chamosite. To evaluate if ferruginous water-column conditions in the Precambrian ocean played a role on iron-rich clay mineral formation, we conducted an integrated petrographic, mineralogical, and geochemical study on the chamosite- and glauconite-bearing strata of the Mesoproterozoic Xiamaling Formation (~1.40–1.35 Ga) in North China. Petrographic, XRD, SEM, and EDS analyses show that the chamosites of the Xiamaling Formation was transferred from glauconite, with berthierine as an intermediate mineral phase during early diagenesis. Geochemical analyses indicate that a complete transformation from glauconite-dominated to chamosite-dominated end-members (samples) requires an addition of a large amount of Fe (16.9 wt%), Mg (2.4 wt%), and a small amount of Al (1.4 wt%), but a simultaneous release of Si (11.8 wt%) and K (6.0 wt%). Considering that the glauconite- and chamosite-bearing strata are devoid of iron-rich detrital minerals (e.g., biotite and iron oxides) and lack evidence of hydrothermal alteration, the required Fe 2+ for glauconite-berthierine-chamosite transformation was most likely from Fe 2+ -rich (ferruginous) seawater, which may have promoted glauconite-berthierine transformation at the very early diagenetic stage when Fe 2+ exchange between porewater and seawater was still available. This interpretation is consistent with the high Fe HR /Fe T (but low Fe py /Fe HR ), Fe/Al, and V/Al ratios from the hosting strata that support ferruginous depositional environments. Because most Precambrian strata have passed the oil window temperature (>50–150 °C), the preservation of berthierine would be rare and chamosite should be the representative iron-rich clay mineral. Thus, the abundance of chamosite in fine-grained, marine siliciclastic sediments may be used as a mineral indicator of ferruginous water-column conditions.