Calcite Cement Research Articles

New constraints from in-situ U-Pb ages and fluid inclusions of calcite cement and structural analysis on multiple stages of strike-slip fault activities in the northern Tarim Basin, NW China

Dating the activity of complex faults and fracture systems is crucial for creating reliable geological models for various tectonics and subsurface engineering applications. This study presents a comprehensive study integrating U–Pb fracture cement dating, trace elements, and fluid inclusion temperature analysis with seismic analysis of faults, fault diagenesis, and burial history studies to better constrain faulting and fracture activities in an intra-cratonic strike-slip fault system in the northern Tarim Basin. Seismic profiles indicate at least three distinct phases of fault activity corresponding to the Middle Ordovician, Permian, and Paleogene periods. Fracture cementation and crosscutting relationships corroborate the identification of three fracturing stages. U–Pb dating of fractured cement has widely detected Middle Ordovician and Early Permian age intervals. Fluid inclusion homogenization temperatures from the fractured cements, ranging across < 50 °C, 70–130 °C, and 150–180 °C, correspond to three episodes of rapid subsidence during the Ordovician, Permian, and Neogene, respectively. These results suggest three phases of fault/fracture reactivation in the Middle Ordovician (prior to 470 Ma), Early Permian (prior to 295 Ma), and Cenozoic. The fault/fracture reactivation in the Ordovician is closely related to the regional tectonic transition from extension to compression, while fault and fracture reactivation in the Early Permian may be related to hydrothermal activity associated with large-scale igneous province and oil emplacement. Fault/fracture activity in the Cenozoic may be related to a reduction in subsidence, gradual reduction of geothermal gradients, and massive oil emplacement. This research underscores the significance of integrating geochemical and subsurface datasets for accurately determining the timing of faulting and fracturing in sedimentary basins.

Dual clumped isotopes (Δ47 and Δ48) reveal non-equilibrium formation of freshwater cements

Low-Mg calcite, precipitating from meteoric fluids, is a common mineral that forms in a variety of near-surface diagenetic environments. However, recent studies, based on a combination of analyses of δ18O and Δ47 values, have suggested that this mineral might form in disequilibrium and consequently yield kinetic bias in Δ47-derived temperatures and fluid δ18O values. Here, we use dual clumped isotope proxies (Δ47 and Δ48) to investigate the influence of kinetic isotope effects within different meteoric diagenetic zones of Holocene and Pleistocene carbonates from the southern Florida and the Dominican Republic. In the Miami Oolite, the primary aragonite ooids and secondary low-Mg calcite cements in the bulk sample were separated from each other and their isotopic compositions (δ13C, δ18O, Δ47 and Δ48 values) were measured. The Δ47 and Δ48 values of the separated aragonite are consistent with the modern ooid sediments and in approximate equilibrium with the surface seawater. In contrast, the low-Mg calcite cement shows the higher Δ48 and lower Δ47 values, than expected, with the disequilibrium arising as a result of CO2 degassing in the vadose zone. Such deviations of Δ47 and Δ48 values are also observed in low-Mg calcite vadose cements in the Dominican Republic. While low-Mg calcites formed in the lower freshwater phreatic zone in the Dominican Republic have the Δ47- and Δ48-derived temperatures close to expected, the same mineral forming near the water-table and upper phreatic zone shows much higher Δ48-derived temperatures (up to ∼ 90 °C). The possible origin of such elevated temperatures can be attributed to non-equilibrium processes caused by changes in pH and pCO2, mediated by microbial sulfate reduction. Such differential kinetic behavior of Δ48 values between vadose and phreatic zones could be used as a proxy marker for the presence and the location of a water-table. This study demonstrates the great potential of dual clumped isotopes in the investigation of meteoric diagenesis and will help understand the alteration of ancient sequences and the interpretation of stable C isotope trends that they contain.

Determining the age and origin of a Tertiary karstic system by in situ U-Pb geochronology on speleothems

Abstract Dating the onset of ancient (&amp;gt;1 Ma) karstification is a challenge. One approach is to date the earliest calcite cements in speleothems. We show the benefits of in situ U-Pb dating directly on thin sections from ancient (ca. 30 Ma) thin (&amp;lt;1 mm) speleothems in the karstified Lower Oligocene lacustrine-palustrine carbonates of the Paris Basin (France), which cannot be dated using other methods. We dated 32 calcite rafts (a type of speleothem), one geopetal cement, and 10 calcite cements precipitated along the karstic walls. The ages of the calcite rafts and cements at 29 ± 1 Ma (Lower Oligocene) fall within the age range of the host deposits (≈29 Ma) previously deduced from palaeontological evidence. We demonstrate that cementation of the carbonate host rock, its dissolution, and the speleothem precipitation occurred within 2 m.y. after deposition. Ostracods and intraclasts trapped within Rupelian calcite rafts clearly indicate that the karst developed deep underground shortly before a phase of lacustrine-palustrine sedimentation at the surface. This very early dissolution episode is attributed to the uplift of the Paris Basin as a result of the far-field intraplate deformation induced by the alpine orogenesis. This study shows that in situ U-Pb geochronology on ancient calcite rafts is a promising technique for the indirect dating of karstification and, more broadly, for dating geodynamic events and diagenetic evolution of sedimentary basins.

Fractal Dimension Analysis of Pore Throat Structure in Tight Sandstone Reservoirs of Huagang Formation: Jiaxing Area of East China Sea Basin

The reservoir quality of tight sandstone is usually affected by pore throat structures, and understanding pore throat structures and their fractal characteristics is crucial for the exploration and development of tight sandstone gas. In this study, fractal dimensions of pore throat structures and the effect of diagenesis on the fractal dimension of tight sandstone sweet spot in Huagang Formation, Jiaxing area, East China Sea Basin were studied by means of thin sections, scanning electron microscopes, X-ray diffraction analysis, scanning electron microscope quantitative mineral evaluation, and high pressure mercury injection experiments. The results show that the total fractal dimension ranges of type I, type II, and type III sweet spots were 2.62–2.87, 2.22–2.56, and 2.71–2.77, respectively. The negative correlation between total fractal dimensions, porosity, and permeability of type I sweet spots was different from those of type II and type III sweet spots. The negative correlation between total fractal dimensions of type II and type III sweet spots and maximum mercury saturation, average pore throat radius, and skewness were significant, whereas the correlation between total fractal dimensions of type I sweet spots, and maximum mercury saturation, average pore throat radius and skewness were not significant. The positive correlation between the total fractal dimensions of type II and type III sweet spots and the relative sorting coefficient, displacement pressure, and efficiency of mercury withdrawal were significant, whereas the correlation between the total fractal dimension of type I sweet spots and relative sorting coefficients, displacement pressures and efficiency of mercury withdrawal were not significant. The effect of diagenesis on fractal dimensions was investigated. Compaction reduced the pore space of tight sandstone and increased fractal dimensions. Quartz cementation and calcite cementation blocked pores and throats, reduced pore space, and increased fractal dimensions. Chlorite coat can inhibit compaction, protect pore throat structures, and maintain fractal dimensions. Most clay minerals filled primary pores and secondary pores and increased fractal dimensions. Dissolution increased the pore space of tight sandstone and decreased the fractal dimensions of the pore throat structures. The pore throat structures of type I sweet spots were mainly composed of macropores, mesopores, transitional pores, and micropores, and the fractal dimension of type I sweet spots was chiefly controlled by chlorite coat formation, dissolution, and a small amount of compaction. This study provides a reference for pore throat structure and fractal dimension analysis of tight sandstone sweet spots.

Diagenetic archives of the deeply-buried Cambrian Xiaoerbulake Formation microbialite reservoirs, Bachu-Tazhong area, Tarim Basin, China

The deep-burial Cambrian Xiaoerbulake Formation microbialites reservoir hosts significant hydrocarbon in the Tarim Basin. However, limited understandings of fluid-rock interactions from deposition to deep-burial regimes hamper petroleum exploration. Here, petrological observations, in-situ elemental contents and porosity-permeability analyses were performed to understand the complex interactions and the microbialites performance. Dolo-mudstone, crystalline dolostone, and four kinds of microbialites are distinguished as dolostones types. In the depositional stage, microbialites are characterized by (evaporated) seawater dolomitization with the mediation of microbial activity. The dolostone matrix shows seawater-like δ13C, δ18O and REY patterns. Thrombolitic and foam spongy dolostones from high-energy subtidal to intertidal zones show large framework pores compared to stratiform stromatolitic dolostones from low-energy intertidal zones. In near-surface settings, meteoric diagenesis generates secondary pores, while fine-crystalline dolomite cements partially fill pores. During the shallow to intermediate burial regimes, medium-to coarse-crystalline dolomite cements yield negative δ18O values and slightly flat REY patterns. The precipitation of this dolomite phase further decreases reservoir porosity. In deep burial settings, one type of saddle dolomite is likely to be formed by replacement from carbonate precursors, while the second type might be directly precipitated from hydrothermal fluids. These two types of saddle dolomites show different cathodoluminescent characteristics and geochemical data. Subsequently, two types of calcite cements are formed by the calcification of dolomite and yield significant negative δ13C values due to thermochemical sulfate reduction. Finally, the third calcite phase is likely to be a direct precipitate from burial brines. Hydrothermal activity and thermochemical sulfate reduction generate and/or re-distribute secondary pores. Based on the plethora of depositional environments and fluid-rock interactions, thrombolitic dolostone generally yields higher porosity-permeability than other types of microbialites. Thrombolite reservoir has high potential for petroleum exploration and is significant for those concerned with microbialite hydrocarbon fields.

Evaluation of Reservoir Properties of Sylhet Limestone of Jaintia Group, North-Eastern Sylhet, Bangladesh

The Sylhet Limestone in the Bengal Basin, formed in the Eocene Epoch and known for its fossil content, is significant in understanding the reservoir characteristics. Despite exposure in Jaflong and Takerghat of the Bengal Basin, little research has been conducted on reservoir characteristics. This study aimed to comprehensively examine the Sylhet Limestone Formation, encompassing its crystalline upper and fossiliferous lower sections. It utilized field investigations and laboratory analyses to address the gaps of sporadic or insufficient earlier studies. A thorough examination of thin sections from the Sylhet Limestone, exposed in the Dauki River area, provides insights into the textural and mineralogical attributes and the presence of skeletal fossils within the limestone. Based on the analysis of thin sections, the limestones are categorized as Rudestones and Packstones. The porosity observed in the exposed rocks ranges from 5% to 12%, with most pores associated with interconnected fractures and joints. However, thin-section studies also indicate evidence of diagenetic recrystallization and calcite cementation. Hence, closely spaced, interconnected joints and fractures filled with diagenetic calcite might deteriorate the reservoir quality. Notably, this limestone exhibits fossilized specimens such as Nummulite, Discocyclina, Alveolina, Assilina, and Ostracoda, among others. The combination of the fossil assemblage, limestone texture, and composition strongly suggests that this limestone formation was deposited in a shallow marine environment with minimal sediment input under a warm and humid climate. The petrographic analysis of the limestones concludes that the upper portion of the formation is fine-grained while the lower part is coarse-grained. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 12(2), 2023, P 119-137

The interplay between dolomitizing fluids, tectonically-controlled saddle dolomite and calcite cements in lower Cambrian to Furongian strata in the Tazhong Uplift

The interplay between dolomitizing fluids, tectonically-controlled saddle dolomite and calcite cements in Lower-Cambrian to Furongian strata in the Tazhong Uplift, Tarim Basin were interpreted from petrography, geochemistry and fluid inclusions microthermometry. Stochiometric replacement dolomites (RD1, RD2 and RD3) and saddle dolomite (DC) cement were identified. Equant-drusy calcite (ECAL) and blocky-to columnar calcite (LCAL) cements were identified. Weak relationships exist between δ13C, δ18O, 87Sr/86Sr, Mg/Ca versus Mn/Sr ratios, which are tools used to evaluate the impact of diagenetic environment on C–O–Sr isotope composition of the carbonates. The replacement dolomites and DC with high Mg/Ca ratios (0.97–1.21) have low depleted δ18O, δ13C values, compared to the ECAL and LCAL with more depleted δ18O, δ13C values, indicating that the C–O–Sr isotope composition suffered diagenetic alteration by higher-temperature and equilibrium isotopic fractionation. The carbonates fluid system is characterized by negative shift in C–O-isotopic composition with hypersaline signatures compared to Early Paleozoic evaporative seawater. Moreover, the increasing trend in (Cl, Br, I)/(CaO + MgO) ratios with Mg/Ca ratios, couple with positive relationship between Cl and Br, suggests fluids contamination by halogen-rich inclusions. The RD1, RD2 and RD3 were precipitated by relatively higher-temperature saline fluids at depth during shallow-deep burial dolomitization. The DC have average Th-values (141.5–199.2 °C) mostly higher than the estimated ambient temperatures (165 °C) of the studied strata, meaning their growth in fractures resulted from higher-temperature Mg-enriched saline basinal fluids conveyed by hydrothermal convection and short-lived geothermal squeegee-type flow. The LCAL were precipitated by Ca/Mg-enriched basinal brine fluids diluted by influx of meteoric water, cementing fractures in host rocks, as dolomite was replaced by calcite (i.e., calcitization). The conceptual model reveals that dolomitizing fluids and calcitization fluids were conveyed through permeable horizons resulting from strong extensional tectonics and weak compressional tectonics, forming irregular networks of saddle dolomite and calcite cement in host rocks.

Diagenetic evolution of Upper Cretaceous carbonate sequences (Sarvak Formation) in the Abadan Plain, Iran: evidence from petrographic, elemental, stable, and strontium isotopic data

ABSTRACT This study investigates the diagenetic evolution and geochemical variations within the Sarvak Formation by integrating the core, thin-section, scanning electron microscopy, X-ray diffraction, trace elements, carbon-oxygen, and strontium isotopic data from three wells in the Abadan Plain. The formation displays diverse diagenetic processes, influenced by multiple exposure surface condition including the Cenomanian-Turonian boundary (CT-ES) and middle Turonian (mT-ES). Meteoric diagenesis, shaped by tectonic activities and eustatic sea level changes, played a pivotal role, leading to dissolution (karstification), brecciation, cementation, and palaeosol formation. Geochemical analysis revealed significant elemental variations across the studied wells. Magnesium (Mg) exhibited peaks at sequence boundaries, responding to facies changes, while iron (Fe) and manganese (Mn) concentrations increased below disconformities, reflecting changes in redox conditions and karstification. Sodium (Na) concentrations varied with facies changes, showing peaks at sequence boundaries, indicative of palaeoexposure events. Strontium (Sr) isotopes aided in delineating stratigraphic positions of palaeoexposure surfaces and absolute dating of sequences. Increases in87Sr/86Sr ratios below disconformities suggested subaerial exposure events, with shorter durations at CT-ES and longer durations at mT-ES, contributing to dissolution features development. The δ13Ccarb and δ18Ocarb values indicated severe meteoric diagenesis below sequence boundaries, characterized by dissolution and reprecipitation of calcium carbonate as low-magnesium calcite (LMC) cements. Cross plots of δ13Ccarb vs. δ18Ocarb displayed inverted-J patterns, indicative of meteoric diagenesis effects. Comparison with previous studies highlighted similar trends in geochemical variations, with notable differences in element concentrations between wells. This study enhances our understanding of diagenetic history of the Sarvak Formation in the Abadan Plain and, in a larger scale, provides insights into diagenetic evolution of palaeoexposure-related carbonate sequences.

U‐Pb Geochronology of Paleosol Carbonate Cements by LA‐ICP‐MS: A Proof of Concept and Strategy for Dating the Terrestrial Record

AbstractThis study investigates the potential of laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) U‐Pb dating for carbonate nodules in the Late Triassic Ischigualasto Formation of northwestern Argentina. We establish a fully characterized paragenetic sequence to guide the analysis of three pedogenic carbonates and compare the U‐Pb ages with published geochronology from volcanic ashes within the sedimentary succession. Our findings demonstrate the importance of interpreting U‐Pb data within a well‐defined paragenetic framework for accurate age interpretation of pedogenic carbonates. We observe variations in U‐Pb isotopic signatures across different generations of carbonate precipitates and identify syn‐pedogenic and early burial calcite cements as most suitable for precise dating. Respectively, these two calcite cements are interpreted as microcodium and crack‐lining calcite cements formed early in the paragenetic sequence during pedogenesis to early burial of the paleosols as they transitioned from the unsaturated vadose to saturated phreatic zone below the water table. The U‐Pb ages obtained from the carbonate nodules agree with the radioisotopic ages of volcanic ashes, supporting the validity of our dating strategy. These results contribute to advancing U‐Pb carbonate geochronology and highlight its increased potential for dating sedimentary records in the terrestrial realm. Future research should focus on replicating similar work on different carbonate nodules within the Ischigualasto Fm and expanding the application of LA‐ICP‐MS U‐Pb dating to other carbonate‐bearing formations, especially in successions with limited absolute ages or where volcanic ashes are sparse or absent.

Shear Modulus and Damping Ratio of a Calcite Cemented Gravelly Sand

Shear Modulus and Damping Ratio of a Calcite Cemented Gravelly Sand

Time-calibration of carbonate diagenesis and regional tectonism in the Norwegian Barents Sea

Diagenesis plays a crucial role in carbonate reservoir properties, for example through the dissolution or precipitation of carbonate minerals, with burial history and fluid migration thought to play an important role in the timing of these events. To better understand these relationships and the local manifestation of regional events, we study carbonate sedimentary rocks and associated diagenetic cements from the Loppa High and Finnmark Platform using in-situ U–Pb carbonate geochronology and C–O stable isotope ratios, combined with burial history modelling. The results indicate a complex history of diagenesis: analyzed dolomicrite samples from the Loppa High typically yield ages that are older than their biostratigraphic age, in contrast to dolomicrite samples from the Finnmark Platform that yielded younger ages; this regional offset may be reflective of different styles of early diagenesis, as well as heterogeneous and re-deposited origin of some studied materials. While many diagenetic calcite cements coincide with modelled burial or uplift events, other events have no cements associated with them, although the possibility that some diagenetic carbonate phases were unsampled cannot be ruled out. Some calcite cements are not associated with burial events at all and may instead be related to hydrocarbon ‘charging’, supported by strongly negative δ13C values recorded in these cements. Broadly, these results highlight the value of integrating petrographic observations, burial history modelling, carbonate U–Pb geochronology, and C–O isotope ratios, as well as the complexity of untangling diagenetic histories.

Assessment of the Upper Cretaceous Abu Roash carbonate source rocks from the Beni Suef field, Western Desert, Egypt

Global sea-level rise during the Early Turonian-Late Cenomanian Bonarelli event resulted in oceanic anoxia and deposition of organic-rich source rocks across Northern Africa, such as the Late Cretaceous Abu Roash-F carbonates. Here, using thin sections, SEM, XRF, standard core analysis, and Rock-Eval pyrolysis data, we examine the petrographic and geochemical properties of the Abu Roash-F (AR-F) carbonate source rocks. These deep marine carbonates consist of dominantly planktonic foraminifera and calcispheres and are classified as wackestone. Extensive micritization, calcite cementation, and ferroan dolomite cement replacement (filling the bioclastic tests and chambers) are identified as the principal diagenetic factors. Core measurements indicate that these carbonates have very low porosity below 3% and horizontal permeability of 0.003 mD (3x10-18 m2), which is also supported by the observed isolated nanopores in SEM. The elemental concentration from the XRF data confirms a highly reducing depositional environment that facilitated the organic richness during the Early Turonian Oceanic Anoxia Event 2 (OAE2). The studied carbonate interval consists of Type-II kerogen in the oil window with a Tmax of about 440 °C and up to 4.2 wt.% TOC indicating ‘fair’ to ‘excellent’ organic richness and a very high probability of active oil generation and expulsion. Following the wireline logs and core data, the self-sourcing unconventional reservoir potential of the AR-F carbonates has been emphasized.

Carbonate dissolution enhances particulate iron sulfide mobilization during water-shale interaction

The interaction between shale and water in the environment can lead to the mobilization of micrometer-sized iron sulfide (pyrite) grains. Because these grains often contain high concentrations of toxic elements, including arsenic, this process can pose a significant hazard. While previous studies have suggested that calcite dissolution plays a major role in iron sulfide mobilization, the relationship between rock composition and the release of particulate matter is unclear. Here, we performed laboratory experiments that simulated water–rock interaction in the subsurface on 5 different shale formations: Eagle Ford, Marcellus, Mancos, and Barnett from the USA, and Ein Zeitim from Israel. We used high resolution imaging to evaluate the impact of a reactive fluid on the shale surface, and used image analysis software to determine the rate of iron sulfide grain mobilization. Comparison of the shale surfaces before and after the experiments showed that the dissolution of calcite cement had a major impact on sulfide mobilization: grains that were primarily surrounded by calcite cement were up to 85 times more likely to be mobilized than grains embedded in the shale matrix, which compromises a complex mixture of submicrometer-scale phases, including phyllosilicates, organics, and carbonates. By contrast, iron sulfide embedded in organic matter typically remained cemented in place. This suggests that while calcite dissolution is a crucial phase in facilitating iron sulfide detachment in shales, organic matter might act as an adhesive that suppresses particulate mobilization. However, the adhesive effect of organic matter is only likely to be of secondary importance. Overall, our results indicate that during hydraulic fracturing operations, which involve the injection of fluid into shales in the subsurface, formations with high levels of carbonate cement are more likely to release particulate iron sulfide, thereby reducing the quality of flowback water and potentially increasing treatment costs.

Integrated petrophysical, sedimentological and well-log study of the Mangahewa Formation, Taranaki Basin, New Zealand

This study aims to address the problem of porosity preservation in the Mangahewa Formation of five main hydrocarbon fields covering onshore and offshore of the Taranaki Basin. An integrated reservoir characterization of the Middle to Late Eocene Mangahewa Formation is achieved through petrophysical evaluation, sedimentological and petrographical descriptions, and well log analysis methods. Petrophysical parameters (porosity and permeability) were acquired from the available core analysis and using mathematical equations to obtain other petrophysical matrices such as normalized porosity index (NPI) and reservoir quality index (RQI). Factors that affected Mangahewa reservoir were studied through thin-section microscopy and well-log analysis helped to measure the reservoir and hydrocarbon potentiality in the Mangahewa Formation. The Mangahewa Formation is dominated by sandstone and a range of marginal to shallow marine facies with varying hydraulic flow units (HFU). The Mangahewa Formation has a high positive correlation in porosity-permeability relationship and has a maximum of 4.67 μm RQI and 20.08 μm FZI (Well Kapuni-14) which reflect potential reservoir. The Mangahewa Formation observed from Wells Kapuni-14, Maui-A1G, McKee-16A, and Mokau-1 are dominated with 59.6%, 56.4%, 79.3%, and 68% of macro- and megapores, respectively. The presence of authigenic clay and calcite cement has greatly reduced the reservoir quality; however, primary and secondary pores are still observed within the Mangahewa sands. Moreover, well log analysis was carried out on four wells in Taranaki Basin, to run a qualitative and quantitative analysis of the Mangahewa reservoir. Eight potential reservoir zones were examined, revealing that the Mangahewa Formation has a very low shale volume of less than 6%, good effective porosity ranging between 11.0% and 13.3%, up to 36.2% of average water saturation and maximum of 69.8% average hydrocarbon saturation. In conclusion, from this comprehensive study, it can be deduced that the Mangahewa Formation possesses fair to good reservoir quality and hydrocarbon potentiality.

Initiation and development of tectonic stylolite – Vein system in micritic limestone (Les Matelles, France)

In this study, we describe the characteristics of tectonic stylolites and related veins affecting a low-porosity micritic limestone (Jurassic carbonates, Les Matelles, South of France) in order to unravel the conditions of initiation and interaction between pressure-solution and fracturing in such rock. Field description, various petrographic and microstructural investigations (cathodoluminescence, SEM imaging, EBSD analysis), and petrophysical/geochemical analyses (Hg porosimetry, XRD, EPMA) are used. We document that pressure-solution initiates at micropores and propagates along calcite grain contacts, connecting surrounding stylolite micro-segments, and progressively concentrates insoluble material such as clays and siliceous particles. The dissolved material is evacuated to the veins where the newly-formed porous space is progressively filled by calcite cement. These deformation processes are strictly restricted to the stylolitic interface and veins, as no modification of porosity or grain deformation is detected in the neighboring host rock. This is due to the low-permeability of the surrounding host rock impeding the evacuation of dissolved material and fluids through interstitial porosity around the pressure-solution zone, leading to overpressure and veins formation. The water release and microporosity caused by diagenesis of the clay fraction (smectite-illite transformation) are discussed as key diagenetic processes instigating the conditions of pressure-solution initiation, then tectonic stylolite formation in low-porosity limestones.

Three-stage hydrocarbon accumulations in the Middle Permian in the Central Sichuan Basin

The Middle Permain Maokou Formation (P2m) is a new region of natural gas exploration in the Sichuan Basin, is characterized by bioclastic limestone with localized dolomitization, and karst fractured-vuggy reservoirs. Currently, on the gas source, hydrocarbon accumulation process and control factors in the Sichuan Basin during the Permian are lacking. To bridge this gap, herein, we identified the filling sequence minerals inside the pores/vugs, along with the oil charge of the Maokou Formation using drill cores, thin sections, oil inclusion analysis, and U-Pb dating of calcite cements. The results showed that the reservoir space of the Maokou Formation was predominated by the residual dissolved pores/vugs, fractures, and dissolved fractures. The pores/vugs underwent four stages of mineral filling by very fine-fine (-crystalline, CC1) calcite → fine-medium calcite (CC2: from 256.4 ± 1.7 to 244.1 ± 6.3 Ma) → fibrous calcite (FC; ∼183.9 ± 8.2 Ma) → coarse-macro calcite (CC3; ∼171.5 ± 5.3 Ma). Combined with the homogenization temperature and salty of fluid inclusion, we considered that three stages of oil charge were present in the Maokou Formation reservoirs. The first stage involved the formation of paleo-oil reservoirs during the Late Permian to Early Triassic, corresponding to the high-maturity aqueous inclusions in CC2, with a homogenization temperature of 106.7°C–137.8°C. At that time, the oil generation from the Lower Cambrian Qiongzhusi Formation rocks peaked, and the generated hydrocarbons migrated upward into the Maokou Formation through the strike-slip faults in the basin center. The second stage involved the formation of paleo-oil reservoirs during the Early Jurassic. The Permian source rocks reached the oil generation window with hydrocarbon expulsion, which was consistent with the oil inclusions in FC. The third stage involved the formation of paleo-gas reservoirs during the Middle Jurassic to Early Cretaceous, corresponding to the high-density methane inclusions and bitumen inclusions occurring in CC3, with the homogenization temperature peaking at 151.9°C–178°C. The natural gas in the Middle Permian of the Central Sichuan Basin is predominantly sourced from the Lower Cambrian Qiongzhusi Formation mudstone and partially from the source rocks of the Middle Permian, indicating a significant source-reservoir conduit of the strike-slip faults in the basin center. The findings provide considerable baseline data to advance further research in the Sichuan Basin.

Diagenetic evolution and origin of cements of the Cretaceous carbonates in the Akal Block, southeast Gulf of Mexico

The Cretaceous succession of deep marine facies (580 m-thick) of the Akal Block, southeast of the Gulf of Mexico, is dominated by lime mudstones that have been extensively affected by dolomitization. Petrographic examination, including cathodoluminescence (CL), and microthermometry reveal three phases of dolomitization that occurred as replacement and pore-/fracture-filling cements. These are (1) an early replacement dolomicrite (D1, <50 μm) with dull orange CL, (2) eu-to subhedral dolomite (D2, 60 μm–150 μm) with distinctive cloudy cores (dull CL) and clean edges (non-luminescent to dull CL), and intercrystalline porosity (6 %), and (3) anhedral vug-/fracture-filling dolomite (D3, 200 μm–500 μm) exhibiting CL zonation. The mean δ18O values of those dolomites show a trend of slight decrease from D1 to D3 (D1 = 0.0 ± 1.3‰ VPDB, D2 = −0.2 ± 1.1‰ VPDB, D3 = −1.2 ± 1.6‰ VPDB), which reflects the influence of progressive burial. The low Sr contents (101 ± 29 ppm) of D1, along with its near-micritic grain size and estimated δ18OD1fluid of parent dolomitizing fluid (−3.6‰–1.9‰ SMOW), support early-stage dolomitization at low temperature of near-surface conditions from solutions of mixed meteoric and seawaters. The higher estimated δ18Ofluid values of D2 and D3 (3.0–13.9‰ SMOW) are expected for fluids in burial settings of high temperatures. The homogenization temperatures of the primary two-phase fluid inclusions of D2 (Th = 99.1 ± 7.9 °C) and D3 (Th = 89.5 ± 11.2 °C) suggest precipitation at deeper burial settings, which is also consistent with their lower Sr contents (80 ± 14 ppm and 98 ± 33 ppm, respectively). The lower mean Th value of D3 than that of D2 likely reflects the influence of tectonic uplift that affected the Akal Block. The shale-normalized (REESN) patterns of the dolomites mimic that of modern seawater and those of D1 and D2, in particular, coincide with that of the lime mudstones (C1), thus suggesting that C1 was their precursor and also D3 originated almost from the same parent fluid that its geochemical composition evolved with progressive burial. The latest fracture-filling calcite cement (C2) postdated D3. The investigation of the products of burial diagenesis, such as fracturing, dissolution, and dolomitization events and their relationship with the tectonic uplift of the Akal Block allows a better understanding of the factors controlling the quality of Cretaceous reservoir rocks.

Fluid inclusion records of oil-cracked wet gas in Permian carbonate reservoirs from the Eastern Sichuan Basin, China

Oil-cracked gas is an important gas resource in petroliferous basins. In theory, oil-cracked gas should be rich in C2+ (i.e., wet gas). However, gas reservoirs of oil cracking origin in the Sichuan Basin (South China) are mainly composed of dry gas. Fluid inclusions record hydrocarbon generation and evolution. Obviously, identifying wet gas inclusions is the key to solving the above controversy. Here, we calibrate the Raman spectroscopic detection limits of ethane and propane (0.007 and 0.016 MPa, respectively) and demonstrate that Raman spectroscopy is a powerful method to identify C2+-bearing inclusions. Based on this technique, C1–C2–C3-bearing wet gas inclusions (C1/ƩCi = 0.90–0.93) are identified in calcite cements of the Permian Changxing Formation carbonate reservoir in the Eastern Sichuan Basin. These inclusions are direct geological evidence of oil-cracked gas because the calcite cement precipitated after the formation of pyrobitumen. Furthermore, reconstruction of the temperature-pressure-composition evolutions of calcite-hosted (Changxing Formation) and quartz-hosted gas inclusions in the Ordovician Wufeng–Silurian Longmaxi Formation black shale indicates that high pressure can effectively retard the cracking of C2+ gas hydrocarbons. Differences in gas composition and δ13CCH4 signature between reservoir gas and inclusion gas lead us to conclude that large-scale oil cracking occurred during the Early–Middle Jurassic in the Permian Changxing carbonate reservoir. Nevertheless, oil-cracked gas escaped during the evolution of the paleo-gas reservoir. Formation of the current dry gas reservoir can be attributed to the continuous mixing of late-stage kerogen pyrolysis gas.

Diagenetic Impact on High-Pressure High-Temperature Reservoirs in Deep-Water Submarine Fan Sandstone of Qiongdongnan Basin, South China Sea

The diagenetic evolution of sandstone is very complicated under the conditions of high temperatures and pressures in deep-water, deep-buried regimes, which have great influence on reservoir quality. This study investigates the typical reservoir target of Neogene deep-water, submarine-fan sandstones under high-temperature, high-pressure regimes in the Qiongdongnan Basin, South China Sea. Utilizing a thin section, scanning electron microscope (SEM), mineral geochemistry combined with burial history evolution, complex diagenetic events, and main controlling factors of the sandstone in the Neogene Meishan Formation were determined. The results show that the evolution of sandstone reservoirs is initially controlled by depositional framework compositions and subsequently modified by eogenetic and mesogenetic alterations during progressive burial. Eogenetic alterations mainly include the following: (1) mechanical compaction; (2) dissolution of feldspar; (3) low-Fe calcite cementation. Mesogenetic events were identified as the following: (1) dissolution of feldspar; (2) ferroan calcite and ankerite formation; (3) precipitation of quartz and clay mineral. Mechanical compaction is greatly influenced by the original depositional framework composition, and sandstone samples enriched in high contents of detrital clay matrix always experienced extensive mechanical compaction. Different phases of carbonate cement during different diagenetic regimes lead to continuous destruction on reservoir porosity. The dissolution of unstable feldspar minerals during eogenetic and mesogenetic environments leads to the development of secondary porosities and would enhance the quality of the reservoir. Overpressure formation is pervasively developed owing to early disequilibrium compaction and subsequent natural gas charging. Only well-sorted sandstones with low contents of detrital clay matrix could resist early mechanical compaction, lead to ample residual original porosities, and then undergo extensive mineral dissolution to generate sufficient secondary porosities. Subsequently, these porosities would be effectively protected by overpressure formation. Poor-sorted sandstones with high contents of detrital clay matrix would experience strong mechanical compaction and extensive destruction of original porosities. Thus, these sandstones are difficult to have significant dissolution and are unable to be effectively protected by overpressure formation. Therefore, the interplay between the original framework composition and the corresponding diagenetic pathways coupled with overpressure formation would result in strong reservoir heterogeneity for the deep-buried sandstones during progressive burial.

Coevolutionary Diagenesis in Tight Sandstone and Shale Reservoirs within Lacustrine-Delta Systems: A Case Study from the Lianggaoshan Formation in the Eastern Sichuan Basin, Southwest China

Tight sandstone and shale oil and gas are the key targets of unconventional oil and gas exploration in the lake-delta sedimentary systems of China. Understanding the coevolutionary diagenesis of sandstone and shale reservoirs is crucial for the prediction of reservoir quality, ahead of drilling, in such systems. Thin-section description, scanning electron microscopy (SEM), X-ray diffraction (XRD), fluid inclusion analysis, porosity and permeability tests, high-pressure mercury intrusion (HPMI) measurements and nuclear magnetic resonance tests (NMR) were used to reveal the coevolutionary diagenetic mechanisms of a sandstone and shale reservoir in the Lianggaoshan Formation of the Eastern Sichuan Basin, China. The thermally mature, organic-matter-rich, dark shale of layer3 is the most important source rock within the Lianggaoshan Formation. It started to generate abundant organic acids at the early stage of mesodiagenesis and produced abundant hydrocarbons in the early Cretaceous. Porewater with high concentrations of Ca2+ and CO32− entered the sandstone reservoir from dark shale as the shale was compacted during burial. Potassium feldspar dissolution at the boundary of the sandstone was more pervasive than at the center of the sandstone. The K+ released by potassium feldspar dissolution migrated from the sandstone into mudstone. Grain-rimming chlorite coats occurred mainly in the center of the sandstone. Some silica exported from the shale was imported by the sandstone boundary and precipitated close to the shale/sandstone boundary. Some intergranular dissolution pores and intercrystal pores were formed in the shale due to dissolution during the early stages of mesodiagenesis. Chlorite coats, which precipitated during eodiagenesis, were beneficial to the protection of primary pore space in the sandstone. Calcite cement, which preferentially precipitated at the boundary of sandstone, was not conducive to reservoir development. Dissolution mainly occurred at the early stage of mesodiagenesis due to organic acids derived from the dark shale. Calcite cement could also protect some primary pores from compaction and release pore space following dissolution. The porosity of sandstone and shale was mainly controlled by the thickness of sandstone and dark shale.