Deep Burial Conditions Research Articles

Simulation of lower Cambrian oil and gas generation and phase evolution process in ZH1 of Tazhong area

The Lower Cambrian petroleum system in the Tarim Basin has undergone multiple periods of tectonic movements, leading to successive hydrocarbon expulsion events and adjustments. The complex process of hydrocarbon accumulation occurred under deep burial conditions, persisting at depths of nearly 6,000 m over an extended period. This has resulted in the current occurrence of various phases including light oil, condensate, and gas in the deep-ultradeep strata of the Tarim Basin. The intricacies of formation of hydrocarbon accumulations and phase evolution have posed challenges to understanding the accumulation mechanisms and enrichment patterns of the Cambrian in the Tarim Basin, consequently lowering the success rate of oil and gas exploration. Such characteristics of multi-stage accumulation and adjustment are prevalent in deep hydrocarbon systems. Therefore, based on data from Well ZH1 in the Tazhong Uplift, combined with basin simulation, Compositional kinetics model, and PVT performance simulation, this study investigates the hydrocarbon generation and phase evolution processes in the deep hydrocarbon systems of the Tazhong Uplift. The results indicate that Well ZH1 entered the hydrocarbon generation peak during the Late Ordovician, followed by secondary cracking as the predominant hydrocarbon evolution process. Hydrocarbon fluids within the Lower Cambrian reservoir transitioned into the condensate phase towards the end of the Cambrian, with increasing depth resulting in higher gas-oil ratios and a decreasing trend in viscosity and density.

Synchronous Grouting Analysis of Shield Tunneling through High Water Pressure Fault Fracture Zone

Using numerical software to establish a three-dimensional geological model of shield tunneling through fault fracture zones, considering the synchronous grouting effect at the shield tail, the disturbance of various geotechnical structures in the fault fracture zone under deep burial and high water pressure conditions is studied. According to the comparison between the numerical model analysis results and the on-site inspection results of the entire process of Shiziyang B3 section, synchronous grouting at the shield tail is the main reason for the changes in soil and water pressure in the fault fracture zone area. The soil and water pressure changes strongly during shield tunneling, and the maximum excess pore pressure value occurs when the shield tail passes. When the shield tunneling machine is far away, the soil and water stress shows a downward trend. The impact of synchronous grouting decreases with the increase in tunneling distance. Due to the difference in permeability, the variation law of pore pressure in the fault fracture zone is significantly different from the original rock, leading to a faster increase in pore pressure in the fracture zone area and a wider range of influence.

Tectonic Control on Shale Pore Structure and Gas Content from the Longmaxi Formation Shale in Southern Sichuan Basin, China: Insights from Fractal Analysis and Low-Pressure Gas Adsorption

Tectonic deformation of different intensities significantly controls shale pore structure, seepage channels, and gas content. The Longmaxi Formation shales in the southern Sichuan Basin have experienced multi-stage tectonic movements, resulting in a diverse fracture system and tectonic deformation. This study focuses on three representative tectonic morphologies: deeply buried strongly deformed (DBSD), deeply buried weakly deformed (DBWD) and shallowly buried weakly deformed (LBWD). We investigated the pore structure characteristics and heterogeneity of these shales under various tectonic conditions using total organic carbon (TOC) content, X-ray diffraction (XRD), scanning electron microscopy (SEM), a low-pressure N2/CO2 adsorption experiment (LP-N2/CO2 GA), and multi-scale fractal theory. The results reveal that strong tectonic compression and deformation conditions lead to the compression and flattening of organic pores by brittle minerals, resulting in long, oriented OM pores. Fracturing of brittle pore creates multiple internal fracture systems linked to dissolution pores, forming a complex micro-fracture–pore network. With intense tectonic deformation, mesopores tend to be compressed, increasing micropore pore volume (PV) and surface area (SA). The DBSD shale exhibits the highest micropore heterogeneity, while the LBWD shale shows the lowest heterogeneity. Fractal analysis indicates a significant decrease in micropore fractal dimension (Df) with increasing burial depth. In contrast, the surface and matrix fractal dimensions (Ds and Dm) of low-buried shale micropores and meso-macropores align vertically. Shale reservoirs in tectonically stable regions exhibit more favourable gas-bearing characteristics than strongly tectonically deformed areas. The LBWD has stable tectonic conditions that are favourable for shale gas preservation. Conversely, slip faults under deep burial conditions lead to extrusion and deformation of shale pore space, ultimately compromising the original reservoir capacity and hindering shale gas enrichment. These findings contribute significantly to our understanding of pore structure and heterogeneity in tectonically deformed shale reservoirs, providing invaluable guidance for the exploration, development, and prediction of shale gas resources.

Research on the Effect of Burial Depth on The Bearing Characteristics of Three Helical Piles Jacket Foundation for Offshore Wind Turbines

With global offshore wind power gradually moving to deep and distant waters, there is a clear trend towards larger-scale units, posing significant challenges for future offshore wind power foundations. In this paper, a helical pile jacket foundation type is proposed by combining the advantages of the current jacket foundation, which is suitable for deep and distant seas and offers high stiffness, with the excellent bearing performance of helical pile foundations. The influence of buried depth on the bearing characteristics of this foundation type is discussed through a physical model test. The results obtained from the study reveal the distinct bearing characteristics and damage modes exhibited by the foundation under deep and shallow burial conditions. These findings clearly indicate that the overall bearing characteristics and damage modes are superior in deep burial conditions compared to shallow burial conditions. Furthermore, it is observed that the damage and displacement of the foundation are more concentrated in localized areas when subjected to shallow burial.

The Impact of Hydrothermal Fluids on Porosity Enhancement and Hydrocarbon Migration in Qamchuqa Formation, Lower Cretaceous, Kirkuk Oil Company

The Lower Cretaceous reservoir core samples from the upper part of Qamchuqa Formation, Baba Dome, Kirkuk Oil Company, show evidence for multistage episodes of dolomitization and a complex diagenetic history. Optical microscope reveals muti-phase of diagenesis: an early stage of diagenesis and its alteration, later, by evaporated seawater under near-surface setting conditions, followed by different event of dolomitization. The stylolite microstructures postdate anhydrite and early matrix dolomite crystals (DI) and predated the coarse rhombohedral (DII) and saddle dolomite crystals (SD), which were formed under a deep burial realm. High-resolution data from stable isotopes integrated with intensive optical observation, ImageJ software, and litho-log are utilized to establish a qualified methods for mapping a better image of hydrothermal diagenesis under subsurface conditions. These methods revealed different types of dolomites, mostly focused on fractures and void spaces, and the paragenetic sequence shows the complex history of diagenetic carbonate rocks hosted in the limestone of Qamchuqa Formation. The sequence is started from older to younger as follow: Micritization, early anhydrite mineral formation, early dolomite, stylolization, rhombohedral dolomite, and saddle dolomite crystals. The early dolomite phase is usually corroded by hydrocarbon phase, and, geometrically, the hydrocarbon phase is overgrown by the early dolomite. Therefore, the dolomitizing fluids enhanced the porosity system and had positive impact on the hydrocarbon movement. This phase of dolomite and anhydrite formation were associated with the first groups of δ18OVPDB and δ13CVPDB data, a narrow range of oxygen values, and inverse Js of Lohmann curve fits towards the near-surface and shallow diagenetic settings. Detailed optical microscope and supportive data from oxygen-carbon isotopes of saddle dolomite confirm the presence of hot fluids under subsurface condition. The latter data were supported by light δ18OVPDB and constant heavy δ13CVPDB, which indicates a hot fluid possibly circulated in deep burial conditions, and this is channeled along the fracture and pore spaces, consistent with hydrocarbon migration. These pore spaces influenced by leaching were hydrocarbon migrations associated with hot fluids under deep sitting conditions. However, a remarkable part of pristine microfacies of host limestone was preserved. In summary, this study will add a new understanding and insight into the origin, genesis, and timing of these dolomites and their direct connection to hydrocarbon exploration and development in most reservoir oil rocks, which are exposed to hydrothermal fluids. Additionally, the study adds new data on hydrothermal fluids in subsurface conditions, whereas most of the previous reported work has mostly focused on exposed rock.

Diagenesis and reservoir quality evolution of the Lower Cretaceous turbidite sandstones of the Agat Formation (Norwegian North Sea): Impact of clay grain coating and carbonate cement

The turbidite sandstone of the Agat Formation is one of the most important hydrocarbon reservoirs in the Lower Cretaceous succession of the Norwegian North Sea. The diagenetic history of these sandstones has greatly impacted the reservoir quality, resulting in a high heterogeneity in porosity and permeability. To investigate this impact, sixty thin sections and rock samples were investigated through petrographic observations, geochemical microanalysis, and X-ray diffraction. Three main diagenetic events occurred during burial and influenced the reservoir properties: (1) mutual growth of chlorite, siderite, and apatite, (2) calcite cementation, and (3) dissolution of K-feldspar and calcite. Early diagenesis occurs under strongly reducing conditions, leading to the formation of Fe-chlorite/berthierine, siderite, pyrite, and apatite forming coatings around the detrital grains. Chlorite coatings have a key role in controlling the reservoir quality in the Agat Formation. In the lower part of the reservoir, chlorite forms thick coats (15–25 μm) that strongly reduce permeability. Conversely, in the upper part of the reservoir, chlorite coats are thinner (4–10 μm), favoring porosity preservation by inhibiting quartz overgrowth. X-ray diffraction and electron microscope observations indicate Fe-rich chlorite of Ib polytype, suggesting solid-state transformation from a berthierine precursor. The kinetics of berthierine/chlorite growth are governed by the dissolution of Fe-rich grains and fine fraction materials found as matrix and early coats. These grains are abundant in the lower part of the reservoir favoring the formation of thick chlorite coats. Chlorite was followed by pervasive calcite cementation in deep burial conditions creating low permeability barriers in the reservoir. The source of this calcite is internal and probably related to the dissolution of an early diagenetic calcite cement forming at shallow depth, with a minor contribution from marine carbonates. During late diagenesis, K-feldspar grains and calcite cements underwent intense dissolution creating secondary porosity and consequently increasing permeability. The input of acidic fluids associated with the maturation of the source rocks could have facilitated these dissolution reactions. Our results highlight the importance of inherited mineralogy in controlling the distribution of chlorite, carbonates, and secondary porosity in the Agat Formation. This study provides useful indicators to help predict diagenetic reactions that can occur in deep marine siliciclastic reservoirs.

Active failure characteristics and earth pressure distribution around deep buried shield tunnel in dry sand stratum

The stability of the tunnel face is a key issue in shield tunnel engineering. In this paper, a series of 1 g model tests were conducted in dry sand for various buried depth ratios H/D = 2, 3, 4, 5, and 6 to study the failure characteristics, earth pressure distribution and the soil arching effect. Each test provided a measurement of the earth pressure with the face withdrawal displacement. And the soil deformation and the failure characteristics were investigated by Particle Image Velocimetry (PIV) system. The development of the failure characteristics was discussed for the buried depth ratios H/D = 3. The relationships between the failure characteristic and buried depth ratios were investigated. The variation of limit support pressure, soil stress distribution at different depths and earth pressure around the tunnel are analyzed. The soil arching effect around the tunnel at deep burial conditions is revealed. With the withdrawal displacement increasing, the tunnel face is in close contact with the support panel at first, and then gradually separated from the support panel. The larger the H/D is, the smaller the increment of horizontal soil stress is. It could be concluded that the increase of tunnel depth directly leads to the soil arching effect.

Origin of dolomites in oolitic carbonates of the Middle Jurassic Dorgali Formation, eastern Sardinia, Italy: Petrographic and geochemical constraints

The Bathonian-Callovian (Middle Jurassic) Dorgali Formation in eastern Sardinia (Italy) consists of dolomitized oolitic grainstone and records a complex diagenetic history. Petrographic examinations revealed three main phases of replacive (D1, D2) and fracture-filling (D3) dolomites with crystal-size ranges of <4–50 μm (dolomicrite to dolomicrosparite D1), 50–200 μm (eu-to subhedral dolomite D2), and 50–400 μm (sub-to anhedral crystals of dolomite D3). The early D1 exhibits dull luminescence (CL) under cathodoluminoscope, whereas the later D2 shows dull to zoned CL, and D3 consistently zoned CL. The near-micritic grainsize, low Sr contents (136 ± 43 ppm) and estimated δ18O values of the parent dolomitizing fluids (−1.3‰ to −0.9‰ VSMOW) suggest an early-diagenesis origin for D1 at low temperatures of near-surface conditions. In contrast, the larger crystal size and microthermometric measurements of primary two-phase fluid inclusions (Th = 101.7 ± 8.0 °C) reveal that D2 precipitated during later stages at higher temperatures of mid-to deeper burial settings, which is supported by the high calculated δ18O values of D2 parent dolomitizing fluids (10.0‰–10.7‰ VSMOW). Petrographic examinations suggest that the vug- and fracture-filling D3 and coarse calcite cement (C3, up to 2 mm) precipitated at deeper burial conditions, which is consistent with the low δ18O (−5.2 ± 1.0‰ VPDB) and δ13C (−5.1 ± 2.6‰ VPDB) compositions of C3. The similarity in shale-normalized REESN patterns, strong negative Ce anomalies and lack of Eu anomalies, suggest a seawater origin for the parent dolomitizing fluids.

Dolomitization micro-conditions constraint on dolomite stoichiometry: A case study from the Miocene Huangliu Formation, Xisha Islands, South China Sea

The micro-conditions that control dolomitization processes were still not well known in natural environments. Cenozoic “island dolostones” are ideal for examining the processes since they are geologically young and have not experienced deep burial conditions. Miocene dolostones from the Xisha Islands, located in the South China Sea, are similar to those found on Grand Cayman Island, located in the Caribbean Sea. These dolostones are composed of low-Ca dolomite (LCD, < 55 mol % CaCO3; hereafter referred to as %Ca) and high-Ca dolomite (HCD, > 55 %Ca). Individual dolomite crystals are commonly formed entirely of LCD or HCD, and/or HCD cores that are encased by LCD and alternating zones of HCD and LCD (1–10 μm thick). The crystal textures indicate that the LCD and HCD are primary growth features (no obvious recrystallization) that formed as the dolomite replaced the limestone precursor. The compositional features (%Ca in LCD and HCD, average %Ca, and proportion of LCD and HCD) and geochemical parameters (O/C isotopes, Sr, Fe and Mn) of dolomite indicate that the Mg was incorporated into dolomite crystals in an orderly manner that was mainly controlled by the interaction of fluid properties and crystal structure. The fact that dolomite stoichiometry varied laterally and vertically throughout the successions shows that micro-scale physicochemical conditions played a critical role in the evolution of individual dolomite crystals. The similarities between the dolostones of the Xisha Islands and Grand Cayman, which are 15,795 km apart, indicate that the factors that control the micro- and large-scale features of these dolostones must have undergone similar processes or experienced similar physicochemical conditions despite their disparate settings.

Experimental study of trench effect on lateral failure mechanisms around the pipeline buried in clay

Pipelines are usually trenched and backfilled using the pre-excavated soil as cost-effective protection against the environmental, constructional, and operational loads. The design of buried pipelines for potential lateral displacements caused by ground movements is a challenging aspect that is usually simplified by assuming a uniform soil stratum. However, the remoulded backfilling material and its lower stiffness compared with the native ground can significantly affect the soil failure mechanisms and the mobilized lateral soil resistance. In this study, a qualitative experimental assessment was conducted to investigate the lateral pipeline-backfill-trench interaction effect on soil failure mechanisms and the resultant lateral soil resistance in shallow and deep burial conditions. Particle image velocimetry (PIV) was used to observe the interactive soil displacements directly. The partially drained condition was adopted to magnify the pipeline-backfill-trench interaction effects. It was observed that the interactive effects of pipeline, backfill, and trench precede their individual shear strengths and reduce the ultimate soil resistance. Also, the study revealed the significance of the pipeline/trenchbed interaction in the mobilization of the lateral soil resistance and several other mechanisms not yet addressed in the literature. As a result, several new research avenues were identified, and the ground was prepared for proposing cost-effective solutions to improve the prediction of the lateral response of buried pipelines in the near future.

Fault-controlled carbonate cementation: A case study from Eocene turbidite-delta sandstones (Dongying Depression) and implication for hydrocarbon migration

The subsurface distribution of carbonate cements in sandstones is cornerstone in understanding the fluid regime and origin of cements to shed the light on hydrocarbon migration. Petrography, cuttings, cores, well logs, and seismic data were integrated to better understand the distribution of carbonate cements in the Eocene turbidite-delta sandstones of the Dongying Depression. Petrographic examinations (including cathodoluminescence, CL) reveal that the carbonate cements are mainly calcite with minor ankerite that precipitated in deep burial settings. The significant abundance of carbonate cements near fault zones implies migration of fluids from deep settings through late fractures developed by compaction under deep burial conditions. The extent of carbonate-cemented zones may reach hundreds of meters away from the faults, while scatter carbonate cements are much more extensive. The relationship of pervasive and scattered calcites with oil-bearing sandstones suggests that the pervasive calcite cementation started as soon as the hydrocarbon and brine mixed fluid were displaced while the scatter calcite cements came after. The occurrence of abundant carbonate-cemented zones in the delta front oil-free sandstone intervals suggests possible hydrocarbon migration to the overlying sandstone units.

CO2 storage capacity of anthracite coal in deep burial depth conditions and its potential uncertainty analysis: a case study of the No. 3 coal seam in the Zhengzhuang Block in Qinshui Basin, China

The storage of CO2 in deep unminable coal seams can mitigate greenhouse gas emissions. However, CO2 storage in deep anthracite coal is complex with some uncertainties in the estimation of CO2 storage capacity. Based on isothermal adsorption experiments and gas solubility experiments under high temperature and pressure conditions, the total storage capacity of CO2 in anthracite coal is discussed. The results show that the absolute adsorption amount is over 44 cm3/g at temperatures of 318.15, 335.65, and 353.15 K as well as adsorption equilibrium pressures of 10 MPa. The storage capacity of adsorbed and free gas is 35–70 cm3/g and 5–8 cm3/g, respectively, within a depth range of 1000–2000 m. The soluble gas can be ignored for its low content between 0.22 cm3/g and 0.28 cm3/g with a proportion of less than 1%. The storage capacity of CO2 may be estimated inaccurately because of the heterogeneity and uncertainty of the macroscopic geological conditions and coal reservoir parameters. Taking the No. 3 coal seam in Zhengzhuang block as an example, the storage priority area was divided into supercritical area and subcritical area with five sub-areas according to storage conditions, and the storage capacity was calculated, showing a relatively good storage potential.

Diagenesis of the Vaca Muerta Formation, Neuquén Basin: Evidence from petrography, microthermometry and geochemistry

The Tithonian Vaca Muerta Formation (500 m-thick marine carbonates) in the Pampa de Tril area is a potential unconventional hydrocarbon reservoir but earlier studies rarely focused on its diagenetic history. It constitutes the upper part of the lower Mendoza Group in the Neuquén Basin, western Argentina. Petrographic examinations reveal four generations of calcite (C1, C2, Cb and C3). The microbial lime mudstone (C1, <4 μm), which exhibits dull cathodoluminescence (CL), is the most abundant with mean δ18O value of −7.2 ± 1.3‰ (VPDB) and δ13C of −1 ± 3.3‰ (VPDB) and deemed to have deposited in tropical warm shallow-water settings. The equant calcite cement (C2, 25 μm–140 μm) exhibits red CL and is associated with recrystallization of bioclasts and micritic matrix. The fibrous calcite (Cb, 35 μm–125 μm) fills parallel-to-bedding antitaxial veins (beefs) and shows bright orange CL. Its δ18O and δ13C values are more depleted (−10.7 ± 2.2‰ VPDB and −4.8 ± 3.9‰ VPDB, respectively) relative to those of C1, which is consistent with the occurrence of hydrocarbon fluid inclusions and may imply precipitation during a mid-to late stage of diagenesis within the oil window. The fracture-filling C3 consists of coarse subhedral crystals (40 μm–5 mm) showing bright orange CL, which has been interpreted as precipitated during deep burial conditions at temperatures around 118 ± 26.8 °C based on fluid-inclusion microthermometry. The relative depletion of Fe and Mn from C1 to C3 and the low concentration of U, may imply a relatively closed system of low water/rock interaction and restricted meteoric alteration. The shale normalized (REESN) patterns of C1, Cb and C3 generally exhibit parallel patterns with a distinct positive Eu anomaly and progressive enrichment towards the MREE and HREE. The positive Eu (Eu/Eu*)SN anomalies reflect the impact of dissolution of plagioclase feldspars by circulation of the diagenetic fluids through the surrounding siliciclastics in the basin.The Ce (Ce/Ce*)SN and La (La/La*)SN anomalies of Cb and C3 are consistent with precipitation under reducing conditions of deep burial settings. The Vaca Muerta carbonates are enriched in organic matter (up to 12%) that fills the pores but permeability is insignificant.

Palaeogeographical reconstruction and provenance of Oxfordian aeolian sandstone reservoirs in Mexico offshore areas: comparison to the Norphlet aeolian system of the northern Gulf of Mexico

Abstract Palaeogeographical reconstruction of the Oxfordian-age (Jurassic) depositional systems in the southern Gulf of Mexico is supported by detailed sedimentological analyses, detrital zircon geothermochronology and plate tectonic restorations. This integrated approach departs from prior local studies by placing the Bacab Sandstone, a major reservoir in offshore Mexico, in a larger basin- to regional-scale context. Sedimentary characteristics derived from detailed core description show how remarkably similar the Bacab Sandstone is to coeval sandstones of the Norphlet Formation of the northern Gulf of Mexico. A comparison of lithofacies associations suggests similar depositional processes and palaeoclimate regimes, with aeolian dunes forming near base level with adequate wind, sediment supply and arid climatic conditions promoting development of dunes and adjacent sabhkas. Construction of prominent ergs (aeolian sand seas) with lateral transitions to updip fluvial–wadi systems is envisioned in Mexico, comparable to the Norphlet of the northern Gulf of Mexico. Reservoir quality over the southern Mexican offshore area is variable, as numerous well penetrations over several decades have demonstrated. However, in the core area of the Ek-Balam Field, the best reservoirs have surprisingly good porosity and permeability for their age (Jurassic) and present-day depth of burial (&gt;4000 m). However, published information is not definitive on the factors mitigating subsurface reservoir quality destruction. It is possible that similar processes preserving or enhancing porosity under deep burial conditions for the Norphlet are likely to have operated in the Ek and Balam well locations. Available petrographical data on framework grains are similar for Oxfordian sandstones in both the northern and southern Gulf of Mexico. Detrital zircon U–Pb geochronology, however, indicates that different source terranes supplied clastics to the Oxfordian Bacab and Norphlet sandstones. Detrital zircon geothermochronology age spectra indicate that the Mayan (Yucatán) Block was the primary terrane for the Bacab Sandstone. This is separate and distinct from Norphlet source areas that vary from Appalachian (Laurentian) to Pan-African (Gondwanan and peri-Gondwanan) terranes. While it possible that the Bacab and Norphlet sandstones were continuous and connected across the Yucatán margin that lies between the two areas, detrital zircon provenance results do not show any indication of common source terranes. Dimensional considerations, such as the contrasting ratios of Bacab and Norphlet source areas to their respective documented areas of deposition, also support the notion of separated aeolian palaeoenvironments.

Molecular weight variations of kerogen during maturation with MALDI-TOF-MS

Kerogen is an amorphous organic matter (AOM) in fine grain sediments, which produces petroleum and other byproducts when subjected to adequate pressure and temperature (deep burial conditions). Chemical characteristics of kerogen by considering its biogenic origin, depositional environment, and thermal maturity has been studied extensively with different analytical methods, though its molecular structure is still not fully known. In this study, conventional geochemical methods were used to screen bulk rock aliquots from the Bakken Shale with varying thermal maturities. Organic matter was isolated from the mineral matrix and then a mass spectrometry method was utilized to quantify molecular weight distribution (MWD) of four different kerogens at various thermal maturity levels (immature to late mature). Furthermore, to complement mass spectrometry, Fourier transform infrared (FTIR) spectroscopy was employed as a qualitative chemical and structural investigation technique. The MWD of four samples was obtained by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, and the results are correlated with the absorption indices (CH3/CH2 ratio and aromaticity) calculated from the FTIR attenuated total reflectance (ATR) method. The results showed when the degree of maturity increases, the aliphatic length shortens, and the branching develops, as well as the aromatic structure becomes more abundant. Moreover, based on the MWD results, higher maturity kerogen samples would consist of larger size molecular structures, which are recognized as more developed aromatic, and aliphatic branching stretches. The combination of infrared spectroscopy (AFT-FTIR) and mass spectrometry (MALDI-TOF) provided MWD variations in organic matter samples as a function of maturity based on varying absorption indices and revealed the rate of change in molecular mass populations as a function of thermal maturity in a type II kerogen.

Deep-burial hydrothermal alteration of the Pre-Salt carbonate reservoirs from northern Campos Basin, offshore Brazil: Evidence from petrography, fluid inclusions, Sr, C and O isotopes

Petrographic, mineralogical, elemental, isotopic and fluid inclusion analyses were integrated to unravel the diagenetic evolution of Brazilian Pre-Salt lacustrine carbonate reservoirs of northern Campos Basin, southeast Brazilian margin. Detailed thin section and cathodoluminescence petrography, scanning electron microscopy and electron microprobe analyses established a paragenetic evolution of diagenetic processes and products, comprising extensive dolomitization, silicification, and dissolution. A paragenesis including saddle dolomite, macrocrystalline calcite, mega-quartz, Sr-barite, celestine, fluorite, dickite, sphalerite, galena, and other metallic sulfides filling fractures and dissolution porosity, and aqueous fluid inclusions with homogenization temperatures of 92–152 °C and salinities between 13 and 26 wt % eq. NaCl characterized a hydrothermal system with some analogy to carbonate-hosted Pb–Zn Mississippi Valley (MVT) and Irish-type deposits. Petroleum inclusions and solid bitumen testify atypical oil generation and migration, associated with the hydrothermal flow. The host Pre-Salt spherulitic and fascicular carbonates present highly radiogenic 87 Sr/ 86 Sr ratios, indicating strong interaction with continental crust materials. Hydrothermal phases show δ 18 O values more negative than syngenetic and diagenetic carbonates. The δ 13 C values are interpreted as result of interaction between the hydrothermal fluids and the host rocks. The combined data set provides clear evidence of intense hydrothermal alteration of northern Campos Basin Pre-Salt reservoirs at deep-burial conditions (>2 km), possibly related to Late Cretaceous or more probably Paleogene magmatic activity. Mixed-sourced fluids bearing a basinal signature fed the hydrothermal system and promoted dissolution of the host rocks. The hydrothermal alterations had strong impact on the porosity, permeability, and heterogeneity, contributing, together with the associated fracturing, to the excellent production performance of the Pre-Salt reservoirs. • A deep-burial hydrothermal system is recognized in the Campos Basin Pre-Salt carbonates. • Hydrothermal phases show low δ 18 O values, and high Th and salinity fluid inclusions. • The geochemical and fluid inclusion signatures are similar to MVT deposits. • Hydrothermal fluids are the result of mixing between different sources. • Atypical oil generation and migration occurred associated with the hydrothermal action.

Experimental evidence that water-exchange unevenness affects individual characteristics of two wetland macrophytes Phalaris arundinacea and Polygonum hydropiper

Water-exchange unevenness (WEU) is the temporal variation in water exchange rate that can be characterized by the coefficient of variation of water exchange rates across time. Although WEU is recognized as an important hydrological feature, little is known about its effects on aquatic ecosystems. Herein, two laboratory experiments were conducted to test the effects of WEU on 1) environmental conditions in the absence of plants, and 2) both environmental conditions and plant growth in the presence of two wetland macrophytes Phalaris arundinacea and Polygonum hydropiper. A two-factor factorial design comprising WEU (low-, intermediate-, and high-level of unevenness) and burial depth (0 and 5 cm) was used for both experiments. Compared to turbidity, pH, and dissolved carbon dioxide concentration of the waterbody, dissolved oxygen concentration showed the highest sensitivity to WEU, which decreased significantly with increasing WEU regardless of the absence or presence of plants. For the sediment, oxidation-reduction potential (ORP), NO3-N, total N, available P, and total P decreased consistently with increasing WEU, regardless of burial depth and the absence or presence of plants. Furthermore, deeper burial led to further decreases in soil ORP for all three WEU levels. Biomass accumulation decreased with increasing WEU and burial depth in both plant species. Additionally, plants developed shorter and thicker roots, and the root alcohol dehydrogenase activity increased, but there was a decrease in the root mass ratio, likely due to the plants adapting to anaerobic stress under higher WEU levels and/or deeper burial conditions. However, under deeper burial conditions, increased WEU generally did not lead to further root adjustments due to the limited nutrient absorption capacity. Consequently, plant biomass decreased by 49.82% for P. arundinacea and 76.94% for P. hydropiper from low to high WEU levels at 5 cm burial depth compared to values of 39.68% and 45.71% at 0 cm burial depth, respectively. Overall, our results demonstrated that WEU strongly influences the performances of wetland macrophytes through modulation of oxygen and nutrient availability. Considering that anthropogenic disturbance and climate change are markedly altering the WEU of lake ecosystems, we call for more research into this hydrological element to assess its potential ecological impacts.

Dolomitization and its impact on porosity development and preservation in the deeply burial Lower Ordovician carbonate rocks of Tarim Basin, NW China

The deeply buried dolostones (>6000 m) of the Lower Ordovician Penglaiba Formation are currently the key targets of hydrocarbon exploration in the Tarim Basin. Petrographic, isotopic, and fluid inclusion microthermometric data were obtained in an attempt to determine the origin of dolostones and its effect on porosity development. Based on the petrography examination, three matrix dolomites and two dolomite cements were identified: very finely to finely crystalline, nonplanar-a to planar-s (D1) dolomite; finely to medium crystalline, planar-s to planar-e (D2) dolomite; medium to coarse crystalline, nonplanar-a (D3) dolomite; medium to coarse crystalline dolomite cements (Cd and Sd). D1 dolomite, which commonly occurs widely distributed throughout the study area, is characterized with δ13CVPDB values of −2.4 to −0.1‰, δ18OVPDB values of −8.1 to −4.1‰, and 87Sr/86Sr ratios of 0.70899–0.70979, inferring the seawater origin. D2 dolomite, merely confined to one side of the stylolite, has δ13CVPDB values of −1.8 to −0.4‰, δ18OVPDB values of −8.3 to −3.7‰, and 87Sr/86Sr ratios of 0.70871–0.70955, suggesting that D2 is precipitated in shallow-middle burial conditions and the dolomitizing fluids is derived from the residual primitive seawater in the formation. D3 dolomite is considered to be formed at higher temperatures under deep burial conditions considering its crystal textures and the δ18OVPDB values of −9.0 to −6.8‰. The two dolomite cements, Cd and Sd dolomites, are characterized with the inclusion Th values of 119–163 °C and 87Sr/86Sr ratios of 0.70941–0.70993, inferring the hydrothermal origin. Based on the distribution, petrography and geochemistry data of D1, the mainly hydrological system for dolomitization in the Penglaiba Formation is interpreted to be the seepage-reflux model. Intercrystalline pores and vugs are two types of pores associated with dolomitization, the latter being the most dominant type that is related to hydrothermal dolomitization.

Petrological and geochemical constraints on the origin of dolomites: a case study from the early Cambrian Qingxudong Formation, Sichuan Basin, South China

Several types of dolomite had occurred pervasively in the lower Cambrian Qingxudong Formation in the Sichuan Basin. However, the origin of these dolomites remains particularly controversial until now. Here, we describe the petrographic and geochemical characteristics of dolomites from the lower Cambrian Qingxudong Formation in the southeastern Sichuan Basin, which can potentially provide useful information to constrain the origin and evolution of dolomitization fluid. Three fabric types and five dolomite generations were identified in this study: fabric-retentive dolomites (dolomicrite, Dol-1, and dolomitic microbialite, Dol-2), fabric-destructive dolomites (sucrosic dolomite, Dol-3) and pore-filling dolomites (medium- to coarse-crystalline dolomite, Dol-4, and saddle dolomite, Dol-5). The δ13C and δ18O values, 87Sr/86Sr ratios and rare earth element (REE) pattern of Dol-1, Dol-2 (except the δ13C values) and Dol-3 are approximately consistent with original seawater values, suggesting that the dolomitization fluids were mediated by seawater. The calculated dolomitization temperature (25.66 °C) and Z values (125.2) of Dol-1 with evaporite minerals indicate that the dolomitization fluid was high-salinity concentrated seawater derived from evaporation. The calculated dolomitization temperature (24.20 °C) and Z value (121.8) of Dol-2, and no traces of evaporite minerals in Dol-2 imply that the dolomitization fluid was slightly hypersaline seawater. The more negative δ13C and abundant microbial features of Dol-2 indicate that microbial mats were involved in dolomite formation. The dolomitization temperature (30.55 °C) and Z value (125) of Dol-3 also indicate that the dolomitization fluid was high-salinity concentrated seawater. However, Dol-3 is commonly distributed in reef and shoal facies (high porosity and permeability) throughout the Qingxudong Formation. Moreover, the abundant organic matters in the dolomitization fluid likely overcame the kinetic and thermodynamic barriers in the process of dolomitization. The geochemical data (except the δ18O values) of Dol-4 are approximately consistent with fabric-retentive dolomites, suggesting that the Dol-4 had been inherited from precursor carbonates. The negative δ18O values of Dol-4 revealed a further burial environment. High LREE/HREE ratios, positive europium anomalies and negative δ18O values of Dol-5 suggest that these dolomites were affected by hydrothermal fluids under deep burial conditions. Thus, Dol-1, Dol-2 and Dol-3 are likely attributed to sabkha, organogenic (microbial) and seepage-reflux dolomitization models during early-stage diagenesis, respectively. Dol-4 and Dol-5 are commonly attributed to burial and hydrothermal dolomitization models during deep burial diagenesis. This study provides new insights into the origin of dolomites and is especially useful for the further understanding of dolomite reservoir formation in the Qingxudong Formation.

Responses to Sedimentation in Ramet Populations of the Clonal Plant Carex brevicuspis.

In aquatic ecosystems, sedimentation is an important factor that affects plant growth, mainly due to sediment depth. Clonal morphological plasticity is an effective strategy in clonal plants for acclimatization to sediment burial. To date, few studies have examined growth responses to sedimentation on the clonal plants at the ramet population level. This study aimed to explore the interactive effects of population size and burial depth on growth and clonal morphology of Carex brevicuspis. Three population sizes (2, 8, and 32 ramets) and 3 burial depths (0 cm, 5 cm, and 10 cm) were used in this experiment. Under shallow (5 cm) and deep (10 cm) burial conditions, biomass accumulation and relative growth rate (RGR) were lower than in the no burial treatment (P < 0.05). RGR of the small and medium populations was especially high compared to the large populations (P < 0.05). Biomass allocation was higher to belowground parts than aboveground parts, except for the small populations in the 5 cm burial treatments. Both shallow burial and smaller populations led to more biomass being allocated to aboveground parts. Deep burial elongated the first order spacer more than shallow burial, and sedimentation had negative effects on the second order spacer length. The number of new ramets did not decrease in the 5 or 10 cm burial treatments compared to the unburial treatment, and larger populations usually had more ramets than smaller ones; the proportion of clumping ramets was higher than the proportion of spreading ramets, and deeper burial and smaller populations led to higher proportions of spreading ramets. These results indicated that the growth of C. brevicuspis was limited by sediment burial at the ramet population level. Smaller populations enable C. brevicuspis to adjust its escape response to burial stress, may allow this species to effectively survive and widely distribute in Dongting Lake wetland.