Effects Of Diagenesis Research Articles

Effects of organic richness, mineral composition, diagenesis, and thermal maturity on the viscoelastic properties of organic-rich Cretaceous mudstones in the Middle Magdalena Valley basin, Colombia

A major geological risk factor for engineering and energy-related applications, such as CO2 storage and unconventional hydrocarbon production, is the geomechanical integrity of mudrocks. The goal of this work is to better constrain the macroscopic mechanical properties resulting from microstructural components by investigating the effects of thermal maturity, mineral composition, diagenesis and organic richness on the mechanical properties of Cretaceous mudstones in two surface sections (San Vicente de Chucurí and La Cristalina Creek) and one well (La Luna-1) from the Middle Magdalena Valley Basin (MMVB), Colombia. The thermal maturity of 22 rock samples was evaluated via organic source-rock screening and determination of illite crystallinity (Reichweite). Both geothermometers placed the La Luna Formation rock samples from San Vicente de Chucurí and the La Luna-1 well within the oil generation window (80°C–120 °C). In contrast, the rock samples of the Guaguaquí Group from La Cristalina Creek in the southern part of the basin are within the gas window (>180 °C). The mineralogy of organic-rich mudstones in the MMVB is dominated by quartz and calcite. Moreover, mudstones from the MMVB have a wide range of organic richness and clay contents, with organic abundances comparable to and clay contents lower than those found in other source-rock reservoirs. The Young's modulus (EIT), Martens hardness (HM), and creep (CIT), as determined through microindentation experiments, highly varied, but overall, the findings suggest that the Cretaceous rocks of the MMVB present conditions favorable for hydraulic fracturing. Furthermore, the viscoelastic properties of Cretaceous rocks in the MMVB are influenced primarily by the concentrations of strong minerals (quartz and calcite) and weak components (clay and TOC). Specifically, an increase in the concentration of weak components compared with that of strong minerals leads to an increase in CIT and a decrease in HM and EIT, and vice versa. Unlike other source-rock reservoirs, there are no indications of a strong correlation between thermal maturity and mechanical properties, suggesting a decoupling of these parameters in rocks of the MMVB. A close scanning electron microscopy examination of the rocks indicated that the observed weak to moderate correlation between thermal maturity and mechanical properties appears to result from rock silicification and calcite cementation caused by the precipitation of these minerals at early stages of diagenesis. This observation implies that the impact of diagenesis on rock geomechanics of silica-rich and calcareous-rich mudstones may override or be more important than the effects of thermal maturity, which, along with clay and organic matter contents, are the greatest controls in the geomechanics of argillaceous source-rock reservoirs.

Reconstruction of Phanerozoic climate using carbonate clumped isotopes and implications for the oxygen isotopic composition of seawater

The oxygen isotope ratio 18O/16O (expressed as a δ18OVSMOW value) in marine sedimentary rocks has increased by ~8‰ from the early Paleozoic to modern times. Interpretation of this trend is hindered by ambiguities in the temperature of formation of the carbonate, the δ18Oseawater, and the effects of postdepositional diagenesis. Carbonate clumped isotope measurements, a temperature proxy, offer constraints on this problem. This thermometer is thermodynamically controlled in cases where carbonate achieves an equilibrium internal distribution of isotopes and is independent of the δ18O of the water from which the carbonate grew; therefore, it has a relatively rigorous chemical-physics foundation and can be applied to settings where the δ18O of the water is not known. We apply this technique to an exceptionally well-preserved Ordovician carbonate record from the Baltic Basin and present a framework for interpreting clumped isotope results and for reconstructing past δ18Oseawater. We find that the seawater in the Ordovician had lower δ18Oseawater values than previously estimated, highlighting the need to reassess climate records based on oxygen-isotopes, particularly where interpretations are based on assumptions regarding either the δ18Oseawater or the temperature of deposition or diagenesis. We argue that an increase in δ18Oseawater contributed to the long-term rise in the δ18O of marine sedimentary rocks since the early Paleozoic. This rise might have been driven by a change in the proportion of high- versus low-temperature water-rock interaction in the earth's hydrosphere as a whole.

A study of the effects of early diagenesis on the geotechnical properties of carbonate sediments (North West Shelf, Australia)

Carbonate sediments are often regarded as problematic in geotechnical engineering due to the high variability of their properties. Understanding and quantifying this variability will become increasingly critical in the years ahead, notably with respect to upcoming developments in offshore renewable energy, for which limited in-situ data are typically available to characterise large areas. Here, six intervals from the North West Shelf of Australia, each composed of similar carbonate grains but accumulated in different environments, are investigated to better understand how the post-depositional cementation, alteration and dissolution of sediments, known as diagenesis, impact their geotechnical properties. Intervals are primarily affected by mineralogy-driven meteoric diagenesis, comprising in-situ dissolution of metastable grains and subsequent precipitation of cement that occurred when the shelf was exposed during lower sea-levels, and by marine diagenesis. In both cases, increased diagenesis results in a higher cement-to-solid ratio and compressive strength. However, while marine diagenesis is associated with a reduction in void ratio, this is not initially observed with mineralogy-driven meteoric diagenesis. Additionally, for a similar cement-to-solid ratio, microcrystalline cement results in higher compressive strength than sparite cement. The data further reveal that the level of meteoric cementation and the compressive strength increase as a function of the duration of exposure and of the regional climate, along with a reduction of the specific gravity related to the replacement of aragonite by calcite. However, increased meteoric diagenesis also leads to the formation of macro-scale heterogeneities such as calcrete layers and karsts that can affect the holistic geotechnical behaviour of such deposits.

Abu Roash F reservoir hydrocarbon potential, Western Desert, Egypt

The Western Desert of Egypt is recognized for its significant potential in oil and gas reserves, making it a prominent hydrocarbon-producing area with conventional reservoirs. Recent findings from various wells suggest that unconventional reservoirs in the Western Desert may hold considerable promise. The primary objective of this study was to explore this potential. Accordingly, a comprehensive and multidisciplinary approach has been employed to assess the Abu Roash/F (AR/F) reservoir. The methodology involved meticulously examining formation characteristics and integrating borehole images with existing core data and thin sections. This integration aimed to analyse and interpret the lithofacies of the reservoir, with a specific focus on identifying fractures and decoding potential diagenesis effects. As a result, we successfully identified wells exhibiting optimal reservoir quality, unveiling the presence of fractured dolomite or vuggy dolomite in wells containing promising oil and gas pay intervals. Notably, some wells displayed low porosity (3%), yet seismic attribute maps in their vicinity indicated a high density of fractures. This underscores the importance of combining borehole images with seismic attributes to evaluate carbonate formations effectively. The fractures, predominantly oriented NW–SE, were found to be associated with deformation zones. Our findings emphasize the critical role of this integrated approach in unlocking the full potential of hydrocarbon reservoirs in the Western Desert.

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.

The Miocene lacustrine carbonate factory of the Ñirihuau Formation, Ñirihuau Basin, North Patagonian Andes, Argentina

AbstractFacies, microfacies and stable isotope analyses of limestone beds in the northernmost Ñirihuau Basin, North Patagonian Andes, Argentina, document and constrain the past hydrological, sedimentological and climate conditions that prevailed during the deposition of a lacustrine system between ca 15 and 13 Ma. This palaeoenvironment is recorded in the middle section of the Ñirihuau Formation, which holds significance because: (1) It was deposited during a transition from an extensional to a compressional tectonic regime; (2) it spans the Middle Miocene Climatic Optimum and the beginning of the Middle Miocene Climatic Transition; and (3) it contains limestone beds interbedded within a 600 m thick interval of mudstones and siltstones, along with intercalated sandstone and volcaniclastic bodies. Two detailed sedimentary logs were surveyed along the Arroyo Las Bayas, at the western and eastern flank of the David Syncline. Limestones from both stratigraphic sections were sampled as well as isolated limestone beds from two other sites. One facies association was defined and interpreted as a perennial lake associated with a deltaic system and dominated by detrital clastic material. It comprises Facies 1 (Marginal lacustrine) and Facies 2 (Lower delta plain); in both, the presence of grainstones and calcimudstones stands out. Through petrography and cathodoluminescence studies of these continental carbonates, nine microfacies were identified: (a) Intraclastic grainstone, (b) Homogeneous calcimudstone, (c) Silty grainstone, (d) Disrupted micrite, (e) Birds eye micrite, (f) Bioclastic mudstone, (g) Calcimudstone with sparse detrital grains, (h) Fenestral micrite, (i) Stromatolitic boundstone. These indicate mainly bio‐induced subaqueous carbonate precipitation and subordinate deposition by tractive flows with short‐distance transport on a littoral lacustrine environment. Most of these microfacies exhibit very early diagenesis (eogenesis) effects. These features, and the geochemistry results, indicate that they were deposited in a palaeolake system under temperate to warm and humid conditions.

Diverse Behaviors of Feldspar Grains during Sandstone Diagenesis: Example from the Xujiahe Formation in the Western Sichuan Basin, China

Feldspar alteration is among the most important processes in clastic rocks during diagenesis, but uncertainty remains about the factors that control feldspar diagenesis under subsurface conditions. Hence, the Upper Triassic Xujiahe formation of the Western Sichuan Basin were examined by an integrated petrographic, mineralogical and geochemical approach to unravel the causes and effects of feldspar diagenesis, with implication for mass transfer and openness of the geochemical system. The sandstones at various depths demonstrate three distinct, separate diagenetic behaviors of detrital feldspar within a single formation including (1) the complete dissolution of both plagioclase and K-feldspar in the upper member; (2) conservation of abundant detrital feldspar grains with minor albitization or overgrowths within the lower member of depths greater than 5 km; and (3) complete disappearance of K-feldspar within the uppermost horizons of the lower member, while plagioclase have survived in significant amounts. The exceptional disappearance of K-feldspar is the result of selective dissolution of K-feldspar during burial, accompanied by illite cementation and substantial K transfer at a scale of tens of meters. It is apparent that the clay diagenesis in the overlying mudstones, rather than porewater chemistry, is the major control of the reactivity of K-feldspar in adjacent deeply buried sandstones.

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.

Research on the Diagenetic Facies Division and Pore Structure Characteristics of the Chang 63 Member in the Huaqing Oilfield, Ordos Basin.

In tight sandstone reservoirs, diagenesis has a significant impact on the development of reservoirs and pore structures. To clarify the effect of diagenesis on the pore structure of tight sandstone, 12 samples of the Yanchang Formation in the basin were studied based on experiments such as high-pressure mercury intrusion and low-temperature nitrogen adsorption. The diagenetic facies in the study area are divided into two categories: strong cementation facies of carbonate minerals and strong compaction facies of soft component minerals, which are relatively unfavorable diagenetic facies, and stable facies of felsic minerals and strong dissolution facies of feldspar minerals, which are dominant diagenetic facies. The pore structure of the Chang 63 reservoir in the study area has obvious fractal characteristics, with a fractal dimension D 1 greater than D 2 and a greater heterogeneity of large pore throats. Compared to compaction and cementation, dissolution has a stronger controlling effect on the pore structure of reservoirs. In tight sandstone reservoirs with low porosity and permeability, dissolution has a more important impact on reservoir transformation and development. The intensity of different types of diagenesis in the Chang 63 reservoir affects reservoir heterogeneity, and the level of the reservoir heterogeneity affects the complexity of reservoir pore structure. In tight sandstone reservoirs, cementation has a stronger controlling effect on the structural complexity of large pores, while dissolution has a stronger controlling effect on the structural complexity of small pores. The dissolution has a strong control effect on the physical properties of the reservoir. This study provides insights into the relationships among the diagenetic facies, reservoir quality, and pore structure of tight sandstone reservoirs. This study has reference significance for the exploration and development of tight oil in the research area.

Evolution of Pore Spaces in Marine Organic-Rich Shale: Insights from Multi-Scale Analysis of a Permian–Pennsylvanian Sample

The quantitative evolution pattern of pore space and genetic pore types along the maturation process in organic-rich shale reservoirs is unclear, which affects the assessment of shale storage capacity and petroleum production. A black shale outcrop sample from Kansas that is of Permian–Pennsylvanian age was collected and subjected to thermal simulation experiments at 10 different maturity stages to understand the pore sizes and pore types. Scanning electron microscopy (SEM) and image processing were used to characterize the full-scale pore-size distribution and volume evolution of this shale sample by combining low-temperature gas (CO2 and N2) physisorption and mercury intrusion porosimetry (MIP) in order to discuss the effects of hydrocarbon generation and diagenesis (HG&D) on pore development at different pore sizes. The study showed that the original shale sample is dominated by slit-like pores, with mainly organic matter (OM) pores distributed in 0–100 nm, intraparticle pores (Intra-P) of clays distributed in 30–100 nm, and interparticle pores (Inter-P) distributed in 100–1000 nm. With the increase in maturity or Ro, the OM pores increased gradually, and the OM pore-size distribution diverged to the two poles. In the oil generation stage, the OM pores were distributed in the range of 30–100 nm, while in the gas generation stage, the OM-hosted pores were mainly distributed in the range of 10–20 nm and 100–500 nm. Further into the over-maturity stage, the OM pores were mainly distributed in the range of 0–10 nm and >100 nm. The pore volume distribution across the whole pore sizes showed that the pore volume of low-maturity shale samples was mainly provided by 100–1000 nm (macropores), and the pore volumes of 0–2 nm, 30–100 nm and 1000+ nm pores gradually increase with increasing thermal maturity, with the final pore-size distribution having four peaks at 0–2, 30–100, 500–1000 nm, and 10–100 µm. Hydrocarbon generation mainly affects the pore volume in the 0–2 nm and 100–1000 nm intervals, with a positive correlation. The 2–30 nm and 30–100 nm pores were likely controlled by diagenesis, such as mineral transformation, illitization, and cementation during the maturation process.

Diagenetic controls over reservoir quality of tight sandstone in the lower Jurassic reservoir in the Lenghu area, the north margin of Qaidam basin

The Lower Jurassic Reservoir (LJR) in the Lenghu area on the northern margin of the Qaidam Basin (NMoQB) has become the most promising target for hydrocarbon exploration. The reservoir has experienced complex diagenesis; however, the porosity evolution and the influence of different diagenesis events on reservoir densification remains unclear. In this study, various analytical methods was first used to clarify the diagenetic stage and sequence, establish a porosity evolution model, quantitatively analyze the time and influence of different diagenesis events on reservoir densification, and illustrate the densification mechanism of tight sandstone reservoir in the Lenghu area. The results showed that the dominant rock types in the LJR were feldspathic litharenite, followed by litharenite, lithic arkose, and a small amount of subarkose and sublitharenite. The reservoir is a typical tight sandstone reservoir (TSR), with average porosity and permeability of 5.5% and 0.08 mD, respectively. The pore types were primarily secondary pores, followed by residual intergranular pores and occasional fractures. Diagenesis of LJR has entered the meso-diagenetic stage A, with minor progression into meso-diagenetic stage B. Quantitative calculation showed that the initial porosity of LJR is 32.2%. In eo-diagenetic stage A, compaction is the main factor for porosity reduction. In eo-diagenetic stage B, the porosity loss rates caused by compaction and cementation were 81.5% and 11.8%, respectively. In this stage, the LJR in the Lenghu area has almost been densified, mainly affected by compaction, calcite cementation and clay minerals cementation. Additionally, cementation is an important factor causing reservoir heterogeneity and is dominated by calcite and clay mineral cementation. After entering the meso-diagenetic stage A, dissolution is the main diagenesis event, which can increase porosity by 6.0%. This stage is the critical period for forming high-quality reservoir of LJR in the Lenghu area, mainly in the 4,400 m depth. In meso-diagenetic stage B, the influence of dissolution gradually decreased. Simultaneously, in this stage, the cementation of iron-bearing calcite further strengthened the densification of the LJR in the Lenghu area.

Seismic Prediction for Formation Pressure Considering Diagenesis Effect

Seismic Prediction for Formation Pressure Considering Diagenesis Effect

U–Pb dating of belemnites and rugose corals: The potential for absolute dating of calcitic invertebrate fossils

The purpose of this work is to test the ability of laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to provide absolute ages of useful precision on calcitic fossils. Data on the belemnite Belemnitella americana, from the Peedee Formation (North Carolina, USA) define an isochron with a Campanian/Maastrichtian age of 72 ± 2 Ma. Two other specimens from the same area show highly clustered data suggesting an effect of late diagenesis involving recrystallization of primary aragonite. A specimen of Belemnitella mucronata from the Mons Basin (Belgium) gives an age of 76 ± 2 Ma, which is consistent with the stratigraphy. U content (5–15 ppm) is highest in the apical line of belemnite, where it may be associated with phosphate-calcite rich in organic matter. Rugose coral specimens from the Devonian Hungry Hollow Formation (Ontario, Canada) give indistinguishable ages of 368 ± 14 Ma, 364 ± 14 Ma and 359 ± 12 Ma. Clustered data from a brachiopod specimen in the overlaying Widder Formation gives a less precise age of 366 ± 42 Ma. These results imply that the biostrome at the top of the Hungry Hollow Formation, previously thought to be Givetian (388–383 Ma), may be coeval with Famennian (372–359 Ma) volcanism and mass extinction. UPb dating with percent measurement precision is possible on calcitic invertebrate fossils, but accuracy is limited to more than this by the requirement of correcting for ablation bias using secondary standards. Nevertheless, it can be an important tool for paleontology.

Proterozoic to Phanerozoic case studies of laser ablation microanalysis for microbial carbonate U–Pb geochronology

Some of the earliest bio-sedimentary records of life on Earth are represented by microbial carbonates, which are also critical geochemical archives of ancient seawater chemistry and the environmental circumstances in which they precipitated. Reconstructing paleo-microbial environments on Earth and potentially other planets requires precise determination of the depositional ages of these materials. The (abiogenic) carbonate geochemistry communities can now use developments in in-situ laser ablation U-Pb dating using inductively coupled plasma mass spectrometry (LA-ICP-MS). Due to the effects of impurity mixing and diagenesis, microbial carbonates have received little geochronological study despite their broad relevance for understanding ancient seawater's environmental conditions and geochemical compositions. This study demonstrates using time-of-flight mass spectrometry (TOF-MS) to perform quick, quantitative elemental mapping before U-Pb spot dating to improve experiment success rates and data reliability and offers four practical application examples.

Sedimentary Environment Interpretation and Organic Matter Enrichment of the Lower Cambrian Shuijingtuo Shale in the Yichang Slope, South China: Insight from Sedimentary Geochemical Proxies with Major/Trace Elements

The vertical geochemical variations in total organic carbon (TOC) content and major and trace elements of the Lower Cambrian Shuijingtuo Formation from the Yichang Slope in the Upper Yangtze were investigated to assess the environmental conditions (redox conditions, water mass restriction, terrigenous input, relative sea-level terrestrial, and paleoproductivity) and to determine the primary controlling factors for organic matter enrichment. The Shuijingtuo Shale is divided into three intervals. The lithofacies of interval I are mainly black siliceous shale with high TOC content, and interval II is mainly black siliceous shale with moderate TOC content. Interval III consists of black, clay-rich siliceous shale and dark-gray calcareous shale and is characterized by a low TOC content. The effects of diagenesis and hydrothermal activity on the elements were evaluated prior to analyzing the environmental condition. There are good positive correlations between TOC and U/Al or Mo/Al ratios, suggesting that major/trace elements still retain the geochemical signature of the sedimentary environment. Meanwhile, the geochemical proxies consisting of Al, Fe, Mn, and Ti indicate that the study area did not experience hydrothermal deposits. The redox proxies (U/Th, Corg:Ptot, and MoEF–UEF) indicate that the interval I samples were formed under a strong reducing condition. The diagram of TOC vs. Mo indicates that the water mass was moderately restricted during the deposition of interval I shales. Proxies of terrigenous input and relative sea-level (Zr/Al and Zr/Rb) suggest that the organic-rich shales at the bottom were deposited under a relatively high sea-level and experienced minimal input of terrigenous debris from the source area. Additionally, the paleoproductivity indicated by Sibio and (Ni + Cu + Zn)/Al was high for interval I samples. During the interval II period, the relative sea-level began to decline, the seawater still remained in reducing conditions, and there was no change in the productivity, but the input of terrigenous debris increased significantly. In the interval III depositional period, the relative sea-level continued to decrease, the seawater shifted to a dysoxic condition, and the paleoproductivity was also at a lower level. The evolution of the sedimentary environment indicates that the high TOC content in the interval I samples is mainly attributed to the strong reducing condition, the preservation condition and debris dilution together control the organic matter content within the interval II samples, and the low TOC content within the interval III samples is constrained by a combination of the poor preservation conditions and lower paleoproductivity.

Reservoir Quality Related to Diagenetic Development in the Carbonate Mishrif Formation: A Case Study from the X Oilfield, Southern Iraq

The Middle Cretaceous Mishrif Formation is considered as the main productive reservoir, southern of Iraq. The rudist-bearing productive zones are characterized by high heterogeneity due to the effects of diagenesis and the depositional environment. The Mishrif Formation represents a carbonate succession which deposited through the Middle Cretaceous (Cenomanian-Early Turonian) within the main retrogressive depositional period succession. This research is identifying the effect of each diagenetic process on the reservoir properties by using the utility of core data and thin sections collaborated with the well logs data along the wells X-46, X-41 and X-42 in the X oilfield. The Mishrif Formation have been subjected into various diagenetic processes such as micritization, neomorphism, dissolution, cementation, dolomitization and pressure solution or stylolitization. Effect of eight diagenetic processes have been recognized in the Mishrif Formation, these processes have affected constructively and destructively on the reservoir quality of the Mishrif Formation; dissolution and fractures have been affected positively, while micritization, cementation, dolomitization, compaction, bioturbation and neomorphism have a negative affect. The porosity logs (Neutron, Density and Sonic) and Gamma ray log are interpreted to find the porosity and clay content to determine the reservoir quality. The porosity results show a wide variation, the primary porosity is decreased in the well X-46 at the depth of 2300-2450 due to the cementation while the secondary porosity is enhanced due to the dissolution and fractures. In well X-42, at the depth of 2225-2400 m there is an increasing in the porosity due to the increasing of grains to clay ratio, while in the X-42 at the middle to the upper parts, the porosity is very low due to cementation, compaction and micritization processes as well as the high content of clay. In general, the porosity is enhanced in the reefal zones due to the geometry nature of the pore throat which increasing both of the effective porosity and permeability.

Carbonate-associated sulfate as an archive of sulfur and oxygen stable isotope composition of seawater sulfate: Evidence from reef carbonate rocks of the southern South China Sea

Carbonate-associated sulfate (CAS) is a useful proxy for the reconstruction of the isotopic composition of ancient seawater sulfate, archiving information on the global sulfur cycle, Earth's surface redox evolution, and biological activity. The CAS proxy is possibly compromised by early diagenetic alteration, which may cause secondary inhomogeneities in the isotopic composition of pore water sulfate achieved in the form of CAS. However, the effects of early diagenesis on the retention of primary δ34S signals of CAS in reef carbonates are not well understood, and it is unknown whether reef carbonate deposits can faithfully record the coeval isotopic composition of marine sulfate. To provide new constraints on the resilience of CAS isotopic signatures of reef carbonate, we have measured and analyzed CAS content and CAS isotopic composition of reef carbonates from the Meiji Atoll (well NK-1) of the South China Sea, representing carbonates affected by meteoric and marine diagenesis including dolomitization. CAS contents were found to decrease rapidly during meteoric and marine diagenesis, with minimum CAS contents caused by prolonged subaerial exposure. Despite low contents, the corresponding δ34SCAS values were found to vary within a narrow range from 21.5 to 24.1‰, compared to values of 20.9 to 22.7‰ of coeval barite, suggesting that the original δ34SCAS values are preserved and largely unaffected by recrystallization. However, δ18OCAS values were found to vary widely, ranging from 7.1‰ to 15.0‰, only recording the same oxygen isotope signal as coeval marine barite for reef carbonates younger than 5 Ma. Our data reveal that reef carbonates can be used as an archive of paleoceanic change, faithfully tracking δ34S changes in coeval seawater regardless of whether reef carbonates underwent recrystallization, including dolomitization. In contrast to sulfur isotopes, the oxygen isotope composition of CAS is more susceptible to alteration during diagenesis. Overall, this study provides new insight for the future reconstruction of the sulfate sulfur and oxygen isotope record of ancient seawater.

Simulations of the effect of smectite‐to‐illite transition in shales on permeability and overpressures using a stochastic approach, a Norwegian margin case study

AbstractThe smectite‐illite transition in shales due to subsidence, temperature changes and diagenesis influences many processes in a sedimentary basin that can contribute to overpressure build up like reducing the shale permeability. The smectite‐rich layers can form sealing barriers to fluid flows that will influence pore pressure prognosis for drilling campaigns, contribute to sealing caprocks for possible CO2 storage and to sealing of plugging and abandonment wells. In this work, we have included the diagenetic smectite‐illite transition into a three‐dimensional pressure simulation model to simulate its effect on pressure build‐up due to reduced shale permeabilities over geological time scale. We have also tested effect of thermal history and potassium concentration on the process of smectite‐illite transition and the associated smectite‐illite correction on permeability. A new smectite‐illite correction has been introduced, to mimic how shale permeability will vary dependent on the smectite‐illite transition. Stochastic Monte Carlo simulations have been carried out to test the sensitivity of the new correction parameters. Finally, a 3D Monte Carlo pore pressure simulation with 1000 drawings has been carried out on a case study covering Skarv Field, and Dønna Terrace offshore Mid‐Norway. The simulated mean overpressures are in range with observed overpressures from exploration wells in the area for the Cretaceous sandy Lysing Formation and for the two Cretaceous Intra Lange Formation sandstones. The simulated smectite content versus depth is in line with published XRD dataset from wells. The corresponding modelled present‐day permeabilities for the shales including the smectite‐illite transition are two magnitudes higher than measured permeabilities on small samples in the laboratory using transient decay method. The measured permeabilities are in the range of 2.66·10−18 to 3.94·10−22 m2 (2695 to 0.39 nD) for the North Sea database and represent the end members for shales‐permeabilities with the lowest values, since the small samples are selected with no or minor natural fractures. This work shows that by upscaling shale permeabilities from mm‐scale to km scale, natural fractures and sedimentary heterogeneities will increase the shale permeabilities with a factor of two and that by including permeability correction controlled by the smectite fraction, pressure ramp can be simulated due to diagenesis effect in shales.

Geological modeling of diagenetic logs of the Sarvak reservoir in Dezful Embayment, southwestern Iran: implications for geostatistical simulation and reservoir quality assessment

Reservoir quality in carbonate reservoirs is significantly influenced by diagenetic processes. Although diagenesis is studied as a common reservoir quality damaging/enhancing process in many previous studies, literature is limited about the spatial modeling of diagenesis processes using advanced geostatistical algorithms. In the current study, 3D models of the main diagenetic processes which affect the reservoir quality of the Sarvak reservoir in an Iranian oilfield located in the north Dezful Embayment were constructed using geostatistics. According to the petrographic studies, a total of 10 microfacies were identified. In addition, the significant diagenetic processes in this reservoir include dolomitization, cementation, dissolution, and compaction. In this study, diagenetic electrofacies were determined using the “multi-resolution graph clustering” method based on the quantitative results of the petrographic studies. The results of spatial modeling and provided average maps were used to investigate the lateral variation of those properties and their relationship with effective porosity. It shows that trends of the secondary porosity and velocity deviation log (VDL) maps are generally correlatable with the effective porosity maps confirming the impact of dissolution as a main significant diagenetic process on reservoir quality enhancement. The most impact of the dissolution on porosity is observed in Lower Sarvak-E2 zone where the correlation coefficient is 0.75. The correlation coefficient between porosity and VDL in some zones is high indicating the effect of diagenesis on reservoir quality as it exceeds 0.61 in Lower Sarvak-A1 zone. In the occurrence of dolomitization, it has dual constructive and destructive effects on the reservoir quality. The most constructive and destructive effects of dolomitization were observed in Lower Sarvak-E1 and Lower Sarvak-F zones in which the correlation coefficients were 0.476 and − 0.456, respectively. In addition, low porosity zones are correlatable with developing cementation, stylolites, and solution seams.

Study on pore structure characterization of strong diagenesis sandstones and the logging response characteristics in Tazhong area, Tarim Basin

The original framework of pore structure is determined by sedimentation; however, reservoirs with different rock components undergo different diagenetic processes with different intensities, so they have different pore structure characteristics. The logging response characteristics of strong diagenetic sandstone show a series of geophysical responses, such as acoustic, nuclear, and electrical, of the final pore structure style formed by diagenetic transformation after primary sedimentary structures. In low permeability reservoirs, the response characteristics are more reflected as the signal coupling of different geological factors instead of a linear correlation with porosity or permeability like in conventional reservoirs. The formation studied, the Silurian system in the Tazhong area, is mainly characterized by a large area and low abundance of lithological reservoirs with complex geological characteristics such as variable lithology, large physical property differences, and strong heterogeneity. Due to the early formation of the paleo-uplift in the Tazhong area, the Indosinian and Yanshan orogeny had negligible effects and formed a stable tectonic background. Furthermore, the evolution of pore structure is reflected in the transformation from primary pores to secondary or the remaining pores by cementation and dissolution in different diagenetic stages of different sand bodies in tidal flat faces. Therefore, the analysis and characterization of the pore structure system are particularly important. Using data based on 314 MICP samples, continuous changes of the pore structure, and the logging response in single layer and correlated layers, we conducted observations and analytics in the study. Based on the continuous MICP data, the relationship between the changes in pore structure, the geological principles, and the logging response characteristics is analysed. Our results show that the pore system heterogeneity is mainly caused by dissolution and cementation and differs in distribution. The main controlling factors of pore structure changes are transformed from sedimentation controls mainly to sedimentation–diagenesis or mainly diagenesis effects from the bottom-up case. Six types of pore structures are characterized, and we analysed the coupling effect of sedimentation and diagenesis factors on the logging response in the inconsistency features of curves. The classification of pore structure can help to analyse the law of pore structure change in sand body, and quantitatively characterize it from the perspective of control factors and logging response characteristics, which lays a foundation for the subsequent pore structure prediction.