Diagenetic Stage Research Articles

Depositional environment and diagenesis of early Miocene Nyalau Formation, Sarawak, Malaysia

Diagenetic understandings tied to depositional sequences that developed over the past decades help to predict the reservoir quality in oil and gas exploration. Understanding the lithologic characteristics and diagenetic alterations is key to decipher the various complex controls on its porosity and permeability. This diagenetic heterogeneity is a product of a complex history of diagenetic modifications, however, there are not many studies addressing this problem. The results show that the exposed sections around Bintulu town comprise of massive sandstones intervals, laminated clays, and brackish-shales and lignite. Five main types of lithologies are identified in the stratigraphic sections: sandstones, shales, coal, siltstones, and mudstones. These rock types are further sub-divided into fifteen lithofacies based on sedimentary structures and other characteristics. Petrographic studies revealed that the sandstones are well-sorted quartz arenite, with fine to medium grained sand. The diagenetic processes recognized include compaction, dissolution and cementation. The early, middle, and late diagenetic stages are present in the lithostratigraphy. Bioturbation marks the start of post-depositional changes, and allowed rapid exchange of pore waters with overlying depositional water. Iron oxide occurred in early diagenesis rapidly after the sediments were buried. However, later grain dissolution provided the secondary porosity and permeability. Compaction rates were high in muddy sediments. Sandstone samples with high mud content have less diagenetic signatures. Later, quartz and calcite cement stabilizes the grains and decreases porosity generated by early dissolution. Middle stages of diagenesis involved the growth of chlorite perpendicular to quartz grains. The late diagenetic changes involved the growth of pyrite crystals associated with chemical composition of deep burial pore fluids. The cement types that have been observed in this formation include clay, calcite, quartz overgrowth and iron oxides whereas, the later phases destroyed the early generated porosity. The calcite cement in the early stage of diagenesis, though, resisted the compaction of sediments and aids to preserve the porosity. Dominant porosity is interparticle porosity that is present depositionally and later enhanced by secondary grain dissolution.

Diagenesis and hydrocarbon charging period in the Lower Cretaceous Bayingebi Formation, Hari sag, Yin`E Basin, Northern China

The Lower Cretaceous Bayingebi Formation at the Hari sag, Northern China is a significant bearing oil-gas tight reservoir in the Yin`E Basin. In order to understand petrophysical properties of the reservoirs, the diagenesis and hydrocarbon accumulation phases were investigated using casting thin sections, scanning electron microscopy (SEM), X-ray diffraction and fluid inclusion analysis. Additionally, the histories of burial evolution, organic matter thermal evolution and hydrocarbon charge are investigated. The results showed that the pore types include primary pore, intergranular pore and dissolution intragranular pore. The diagenesis of sandstone reservoirs in the process of burial and diagenesis mainly includes compaction, fracture, cementation, metasomatism and dissolution. The comprehensive analysis of various diagenetic stage indexes shows that Bayingebi Formation is in the middle diagenetic stage A-B. The homogenization temperature and salinity of fluid inclusions are low temperature and low salinity (120 °C ∼ 130 °C, 0% ∼ 9%), medium temperature and high salinity (140 °C ∼ 150 °C, 10% ∼ 17%) and high temperature and medium salinity (160 °C ∼ 170 °C, 9% ∼ 11%) respectively. According to the homogenization temperature of the inclusions and the history of burial evolution, the Bayingebi Formation has mainly three phases of hydrocarbon accumulation (100 ∼ 97Ma, 93 ∼ 87Ma, 84 ∼ 81Ma).

Occurrence, Genesis, and Significance of Analcime in Fine-Grained Sedimentary Rocks

Natural analcime, an aluminosilicate mineral with multiple genetic mechanisms, widely occurs in fine-grained sedimentary reservoirs rich in oil and gas. Researchers have discussed the source and formation mechanism of reservoirs and the influence of morphology formation, occurrence characteristics associated with minerals, geochemical data, and Si/Al ratio on reservoir properties. The occurrence location, particle size, automorphism, purity, and fracture development can indicate the source of analcime macroscopically. The correlation between the enrichment of associated minerals and the content of analcime indicates that the associated mineral assemblage or correlation provides a material source for the formation of analcime or effectively improves the formation environment. Geochemical data are often used to identify analcime related to primary magmatic crystallization and hydrothermal processes. The genetic source grouping scheme based on the Si/Al ratio, which is a traditional means to identify the source of analcime, has been widely used in the research on analcime. After more than 200 years of study, research has shown that analcime distributed in fine-grained sedimentary rocks was mainly formed by burial alteration of volcanic materials (V-type analcime), conversion of nontuffaceous materials (N-type analcime), hydrothermal deposition mineralization (H-type analcime), and precipitation directly from an alkaline lake or pore water (P-type analcime). Based on reservoir properties, analcime that formed before the organic acid release stage of source rocks can effectively improve the porosity through precipitation–dissolution mechanisms after the release of massive organic acid, whereas the cementation formed by the transition of the fluid from acid to alkaline during the intermediate diagenetic stage would reduce porosity to some extent.

Diagenesis and Its Impact on the Reservoir Quality of Continental Shales: A Case Study of the Lower Jurassic Da’anzhai Member of the Ziliujing Formation in the Sichuan Basin, China

The Jurassic continental shale oil (gas) is a favorable unconventional resource in the Sichuan Basin of China. In this paper, analysis methods such as core and outcrops observation, thin section identification, scanning electron microscopy (SEM), and X-ray diffraction (XRD) are used to describe the characteristics of lacustrine fine-grained rock reservoir in Da’anzhai Member of the Ziliujing Formation in Sichuan Basin, and it discussed the influence and control of lacustrine fine-grained rock diagenesis on the quality of the reservoir, in order to determine the shale reservoir control factors. The results show that there are three types of rocks in the Da’anzhai Member, which are mudstone, siltstone, and limestone, and the rock combination of shell shale intercalated with shell limestone is developed in the Da 2 submember. Fine-grained sedimentary rocks in this section have undergone compaction, cementation, dissolution, metasomatism, transformation of clay minerals, and hydrocarbon generation of organic matter, which are currently in the middle diagenetic stage A or B substage. Compaction and cementation are the main factors that control the physical use of shale and limestone, and strong cementation is the main reason for the tightness of limestone reservoirs in the Da’anzhai Member in the northern and eastern parts of Sichuan Basin. The difference between dissolution and cementation is the main control factor for the formation of limestone or argillaceous limestone reservoirs in the same section in central Sichuan Basin. The organic-rich shale and shell shale of the Yuanba and Fuling area are the most favorable reservoirs of the Ziliujing Formation in this region. The shell limestone that experienced favorable dissolution in the Da’anzhai Member in the central of the Sichuan Basin has become a limestone reservoir. Discussing the impact of continental fine-grained rock diagenesis on the reservoir can better explore and develop similar intervals, enrich unconventional shale oil and gas accumulation and storage theories, and provide basic theoretical support for finding favorable unconventional shale oil and gas reservoirs.

Genesis of the Gaolong gold deposit in Northwest Guangxi Province, South China: Insights from in situ trace elements and sulfur isotopes of pyrite

The Guangxi-Yunnan-Guizhou district, also named Dian-Qian-Gui “Golden Triangle”, in Southwest China has a large amount of Carlin-type Au deposits. Gaolong is one source of Carlin-type Au deposit in the northwestern Guangxi Province. Pyrites are the major Au-bearing sulfides. Majority of Au are structurally bound Au+, with a few nanoparticles (Au0) occurring in the sulfides. In this study, in situ chemical and S isotopes of pyrites in the Gaolong Au deposit were analyzed by laser ablation inductively-coupled plasma mass spectrometry and laser ablation multi-collector inductively-coupled plasma mass spectrometry, respectively. Backscattered electron (BSE) images and chemical composition analysis showed that the pyrites were divided into four main types: (1) As-poor Py1 (pre-ore sedimentary or diagenetic stage, with prominent spotted and aggregate structures. These are fine-grained pyrite aggregates with rough surfaces and low brightness under the BSE, with edges often eroded by hydrothermal fluid); (2) As-weak Py2 (early ore stage, the core of the rim pyrite, with good crystallization and characteristic corrosion spots in the particle, and with the edge dissolved to show irregular structure); (3) As-rich Py3 (main ore stage, the narrow pyrite rims overgrowing Py1 and Py2, irregularly-shaped and less than 50 μm in width); and (4) As-rich Py4 (fine-grained pyrite, with good crystallization, high brightness, and symbiotic with Py3 and Py1, or an isolated distribution). Py1 contains higher concentrations of S, Fe, and W, and lower Co/Ni ratios (average of 0.91), suggesting a pre-ore origin. The ore-stage pyrites Py2, Py3, and Py4, have elevated mineralization elements—Au of 0.25–72 ppm, As of 2214–40613 ppm, Tl of 0.95–109 ppm, Sb of 31.16–1851 ppm, Cu of 49.09–910 ppm, with average Co/Ni ratios of 0.43. Au was positively correlated with As, Tl, Cu, and Ni in these pyrites. Electron micro probe analyzer (EMPA) elemental mappings show that: 1) Py1 has a low Au content, while Py2, Py3, and Py4 have high Au, As, Cu, Ti, and Mg contents; 2) Au occurs in As-rich pyrite in the form of invisible gold; 3) Rim pyrite contains high concentrations of Cu, which has a good positive correlation with As, while As and Au are decoupled on a smaller scale. The Au contents and Au/As ratios show that the Au-carrying fluids are unsaturated with Au. In situ S isotopes show that the δ34S values range from 10.19‰ to 13.79‰ with a mean value of 12.07‰ for Py1, 8.60‰ to 10.78‰ with a mean value of 9.77‰ for Py2, 1.42‰ to 8.29‰ with a mean value of 6.81‰ for Py3, and − 0.49‰ to 4.42‰ with a mean value of 1.67‰ for Py4. The δ34S data decreased from Py1 to Py4, whereas the As content increased significantly. This indicates that Au and As are enriched in ore-forming fluids, and the ore-forming fluids have low S isotope values. Based on a comprehensive study of geological and geochemical data, we believe that the magmatic hydrothermal genetic model can explain the genesis of the Gaolong gold deposit. We conclude that the S isotope of pyrites resulted from the mixing of magmatic S and sedimentary S, and the Au-bearing fluid came from deep-concealed granite, establishing a magma genesis of the Gaolong Au deposit.

Study on Hydrocarbon Accumulation Periods Based on Fluid Inclusions and Diagenetic Sequence of the Subsalt Carbonate Reservoirs in the Amu Darya Right Bank Block

In order to provide paleofluid evidence of hydrocarbon accumulation periods in the Amu Darya Right Bank Block, microexperiments and simulations related to the Middle-Upper Jurassic Callovian-Oxfordian carbonate reservoirs were performed. On the basis of petrographic observation, the diagenetic stages were divided by cathodoluminescence, and the entrapment stages of fluid inclusions were divided by laser Raman experiment and UV epifluorescence. The hydrocarbon generation (expulsion) curve and burial (thermal) history curve of source rocks were simulated by using real drilling data coupled with geochemical parameters of source rocks, such as total organic carbon (TOC) and vitrinite reflectance ( R o ). The above results were integrated with microthermometry of fluid inclusions by inference the timing of hydrocarbon migration into the carbonate reservoirs. The horizon-flattening technique was used to process the measured seismic profile and restore the structural evolution profile. Four diagenetic periods and three hydrocarbon accumulation periods were identified. (i) For Syntaxial stage, the fluid captured by the overgrowing cement around particles is mainly seawater; (ii) for (Early) Mesogenetic burial stage, the calcite cements began to capture hydrocarbon fluids and show yellow fluorescence under UV illumination; (iii) for (Late) Mesogenetic burial stage, two sets of cleavage fissures developed in massive calcite cements, and oil inclusions with green fluorescence were entrapped in the crystal; (iv) for Telogenetic burial stage, blue fluorescent inclusions along with hydrocarbon gas inclusions developed in dully luminescent calcite veins. Based on the accurate division of hydrocarbon migration and charging stages, combined with the structural evolution history of the traps, the hydrocarbon accumulation model was established. Because two of the three sets of source rocks are of marine origin, resulting in the lack of vitrinite in the kerogen of those source rocks, there may be some deviation between the measured value of R o and the real value. Some systematic errors may occur in the thermal history and hydrocarbon generation (expulsion) history of the two sets of source rocks. Due to the limitations of seismic horizon-flattening technique—such as the inability to accurately recover the inclined strata thickness and horizontal expansion of strata—the final shape of the evolution process of structural profile may also deviate from the real state in geological history. The accumulation model established in this study was based upon the fluid inclusion experiments, which can effectively characterize the forming process of large condensate gas reservoirs in the Amu Darya Right Bank Block and quantify the timing of hydrocarbon charging. However, the hydrocarbon migration and accumulation model does not take the oil-source correlation into account, but only the relationship between the mature state of source rocks and the timing of hydrocarbon charging into the reservoirs. Subsequent research needs to conduct refined oil-source correlation to reveal the relationship between gas, condensate, source rocks, and recently discovered crude oil and more strictly constrain and modify the accumulation model, so as to finally disclose the origin of the crude oil and oil reservoir forming process in the Amu Darya Right Bank Block, evaluate the future exploration potential, and point out the direction of various hydrocarbon resources (condensate gas and crude oil).

Occurrence of uranium minerals in the Xiaomeigou Formation in northern Qaidam Basin, Northwest China

The Lenghu sandstone type uranium deposit as industrial value in the northern Qaidam Basin, Northwest China. This study analyzed the LA-ICP-MS U-Pb dating of detrital zircons, mineralogical features, electron probe, backscatter electron imaging and track etch of the altered minerals from uranium-bearing strata in the Xiaomeigou Formation of Lower Jurassic on the basis of field investigation in order to trace the source, parent rock lithology, geochronological age, uranium minerals, compositional characteristics, occurrences of uranium mineralization.The U-Pb ages of detrital zircons from the ore-bearing member are ranging from 117 to 270 Ma and 370–480 Ma, respectively, indicating that the provenance and uranium source were mainly derived from the Permian-Triassic intermediate-acid igneous rocks in the eastern Altyn Tagh and western Saishiteng Mountains, a small amount of them are from the Ordovician-Silurian intermediate-acid igneous rocks in the Xiaosaishiteng Mountains. Two types of ores in Xiaomigou Formation in Lenghu region are mainly composed of graywacke and carbonaceous sandstone with main uranium minerals are coffinite, pitchblende and trace amounts of lead selenite. They are closely associated with clay minerals, organics and pyrite, which lie in the intergranular space of quartz, feldspar and other altered lithic mineral grains or in cleavage joints of altered clay minerals that occur in fine columnar, gel-like or irregular sawtooth shape. It is inferred that the Fe2+ in feldspar and biotite reacts with H2S or S2- bearing minerals participated in the formation of pyrite, where the U6+ was captured by late multiple fluid processes occurred reduction-precipitation-adsorption uranium mineralization in the reduction environment provided by Fe2+. Based on the analysis of the existence of high Y and low Y types of uranium minerals, it is inferred that uranium in uranium minerals is the residual of original uranium minerals derived from the pre-enrichment of uranium-bearing detrital particles in sedimentary and diagenetic stages, and at the later stage, the high radioactivity abnormal geological bodies migrated to the inner basin through the multi-stage reformation of uranium-containing and oxygen-containing fluids without Y or at very low contents.

Petrography, geochemistry and genesis of dolomites in the upper Cambrian Sanshanzi Formation of the western Ordos Basin, northern China

The Cambrian Sanshanzi Formation is an important dolomite horizon in the Ordos Basin of northern China, and is well exposed in the western part of that basin. This paper focuses on the petrography and geochemistry of dolomites in this formation with an aim to provide a better understanding of their genesis. Reflux seepage, burial, and hydrothermal dolomites are identified in the Sa nshanzi Formation. Reflux seepage dolomites include dolomitic grainstone and powder crystal dolomite, and have high Sr and low Mn contents, a high CaO/MgO ratio and low δ13 C values, and the reflux seepage dolomitization was formed by high-salinity and Mg2+-enriched fluid penetrates downward into the limestone. Burial dolomites are the main type of dolomite distributed through the Sanshanzi Formation. Burial dolomites are composed of fine- to coarse-crystal dolomite and have low Sr and high Mn contents, a low CaO/MgO ratio, and high δ13C values. The features of high salinity, strong reducibility and negative δ18O values indicate the burial dolomites formed under high-temperature and high-pressure conditions, the burial dolomitization was due to the limestone deeply buried during the later diagenetic stage. Hydrothermal dolomites are mainly characterized by saddle dolomite features, a positive δEu anomaly, and low δ18O values, and the hydrothermal dolomitization occurred during Mg2+-rich hydrothermal fluid invaded. The reflux seepage and hydrothermal dolomite developed in local areas, with the hydrothermal dolomite affected by the distribution of faults.

Mineralogy, geochemistry, and genesis of glauconite mineral from paleotsunami deposit in Lebak, Banten, Indonesia

Bagedur coast in Lebak regency, Banten province, Indonesia is part of lowland-swampy swale area that consist of various sediment deposit. Based on its sediment characteristics and geomorphology profile, Bagedur coast is assumed as depositional environment of paleotsunami deposit. Hand Auger drilling method is used to collecting loose sediment sample from eight observation stations. The depth of hand drilling in BG 01 station to BG 08 station is consecutively as follows: 115 cm, 100 cm, 80 cm, 700 cm, 400 cm, 450 cm, 160 cm, and 143 cm. This research is carried out to understand the diagenetic process of glauconite mineral in presumption paleotsunami deposit. The diagenetic process of glauconite mineral is identified based on its mineralogical and geochemical characteristics by petrography and X-Ray Fluorescence (XRF) methods. Observation samples is taken from green sand sediment bed in hand drilling core. Petrographic analysis under polarized microscope shows various glauconite mineral which recognized as matrix, oval pellets, replacement of precursor mineral, and bioclastic shell infillings. XRF analysis result show that glauconite in observation site are classified as glauconitic-illite type. The diagenetic stage is interpreted as early stage of diagenetic (nascent) which characterized by low Fe2O3 and K2O content relative to Al2O3 content.

In situ trace element and sulfur isotope of pyrite constraints the mineralization process of haoyaoerhudong gold deposit in Inner Mongolia, China

AbstractWith more than 11,000 t of gold resources, the Central Asian orogenic belt is the most significant gold deposit belt in the world. The majority of the orogenic belt's gold resources are found in a layer of carbon‐rich black shale. However, there is disagreement regarding the origin and metallogenic process of such a significant quantity of gold. The Haoyaoerhudong gold deposit is located where the northern margin of the North China Craton and the southern margin of the Central Asian orogenic belt converge. It is the most significant black shale gold deposit in the northern margin of North China Craton gold province. The pyrite that typically develops in the Haoyaoerhudong gold deposit has been categorized into five varieties through comprehensive field investigation and mineralogical research, which correspond to five metallogenic stages: Stage 1, sedimentary diagenesis; Stage 2, tectonic deformation; Stage 3, hydrothermal; Stage 4, hydrothermal transformation; Stage 5, late metallogenic. For pyrite in the previous four metallogenic stages, in situ LA‐ICP‐MS trace element analysis and pyrite sulfide isotope analyses were performed. The results suggest that: The average Au contents in the pyrite of sedimentary diagenesis stage is 0.098 ppm; the average Au contents in the pyrite of the tectonic deformation stage and hydrothermal stage, is below the detection limit mostly; the average Au contents in the pyrite of hydrothermal transformation stage is 0.12 ppm. The results indicate that only 22.4% more gold is present in hydrothermal transformation stage than in sedimentary diagenetic stage overall. It may be inferred that the gold enrichment of Haoyaoerhudong gold deposit mostly took place during the sedimentary diagenetic stage; subsequent brittle‐ductile shear and hydrothermal fluid activity did not result in a further enrichment of gold. The sulfur isotope test results of various metallogenic stages in the deposit can be analyzed, and they are generally consistent. The δ34S values range from +10.15% to +16.47%, with an average value of +13.02%. It suggests that there might be a single source of sulfur. According to extensive analysis, the Haoyaoerhudong gold deposit formed a relatively low‐grade ore body during the sedimentary diagenesis stage, and the subsequent tectonic deformation stage and hydrothermal stage provided physical conditions for further activation of gold metal but did not bring corresponding material sources for mineralization.

Diagenetic fluid controls chemical compositions of authigenic chlorite in clastic reservoirs

Authigenic chlorite is widely developed in clastic reservoirs, and its chemical compositions should reflect diagenetic fluid and reservoir properties, although this has been little studied. Here we carried out an investigation of sandy conglomerate reservoirs in the upper Permian upper-Wuerhe Formation in the Zhongguai Uplift of the Junggar Basin, northwestern China. Results show that authigenic chlorite in the sandy conglomerate reservoirs can be divided into grain-coating and pore-filling types, and the former can be further divided into grain-coating with cement and without cement. The chlorite coats are mainly brunsvigite and diabanite, in which calcite, laumontite, and quartz cements are typically developed. The presence of cements in chlorite coats had little effect on the chlorite geochemistry. In the early diagenetic stage, grain-coating chlorite was derived from trioctahedral smectite by solid-state transformation in an alkaline environment with the presence of large numbers of Fe and Mg ions. During mesodiagenesis, with increasing burial depth and temperature, the grain-coating chlorite was transformed into pore-filling chlorite, resulting in a higher Fe content and the formation of brunsvigite. At this stage, the chlorite crystal structure became more stable. The grain-coating chlorite partially preserved primary pores by inhibiting quartz cementation and limiting compaction; however, the pore-filling chlorite was not conducive to reservoir development. The clay coats in the study area are indicative of high-quality reservoirs, which contain mainly dissolution pores after the removal of soluble cements. Therefore, reservoirs far from their sediment source and with chlorite coats are of relatively high-quality, which may be a common feature of clastic reservoirs.

Diagenetic stages of oil-saturated sandstones of the pashyisky horizon at the Romashkinskoye oil field

It was identified three stages of reservoir rock formation of the Pashyisky horizon of the Frasnian stage of the Upper Devonian at the Romashkinskoye field, based on optical microscopic studies. The first stage, associated with clastic deposits sedimentation and marked by clastic grains dense structural packing formation, close to cubic. The second diagenetic stage of quartz sandstones is associated with the subsidence stage of sediments into the burial zone. During this period were actively proceeding the processes of grains mechanical deformation, blastesis of quartz clasts, the formation of siderite fragments, and fibrous chalcedony, partially metasomatic replacing clay layers in sandstones. The third diagenetic stage in quartz sandstones is associated with the migration of underground gas-water solutions. Analysis of the transformation degree of the Pashyisky horizon quartz sandstones at different areas of the Romashkinskoye field revealed the relationship between the intensity of secondary diagenetic processes and the degree of rocks oil saturation. Keywords: pashyisky horizon; oil; sandstone; reservoir; diagenesis.

Morphology of framboidal pyrite and its textural evolution: Evidence from the Logatchev area, Mid-Atlantic Ridge

The special and representative polymorphous pyrites of the Logatchev area, Mid-Atlantic Ridge, have a close evolution relationship. There are relatively complete pyrite intermediates undergoing a series of evolutions from disordered nano-micron crystals → framboids → colloidal and subhedral structures. This study focuses on a sulfide-bearing tuff sample of the Logatchev area and presents the detailed mineralogical, textural and geochemical results of a series of intermediate products, from pyrite framboids to subhedral crystals, using results from secondary ion mass spectrometry, field emission scanning electron microscopy, electron backscattered diffraction, and electron microprobes. Framboids packed with nanocrystals are significantly with negative δ34S (−28.3 to −17.4‰), probably formed via organoclastic sulfate reduction (OSR) in the seawater column or shallow marine sediment. And their associated microcrystal framboids have less-negative δ34S value (−22.3 to −8.8‰) and high Cu/Zn ratio, which are interpreted as framboids continuous growth by venting fluids with high temperature in an open system. During the diagenetic stage, the evolution of framboidal pyrites to subhedral crystals occurs mainly through two ways: cementation and rearrangement after growth. As an intermediate, the δ34S values of cemented framboids (−7.6 to +8.7‰) are close to the colloidal and subhedral structure pyrites (+9.1 to +11.2‰), reflecting the subsequent precipitation of pyrite and secondary overgrowth in a closed environment after burial. During the recrystallization process from nano-micron crystals to cemented framboids to colloidal or subhedral structures, the decreasing trend of SiO2 contents (come from the surrounding basement) and increasing Fe, S, Mo contents revealed the consequence of continuous expulsion of silicate in the intergranular pore space, which provides evidence for framboids aggregated and evolving to euhedral pyrite. Some intermediates from framboids to massive pyrites have a certain local ordering. Regulated by surface energy, grains in some framboidal pyrites are physically rotated and repositioned during the growth process, which show either 〈100〉 or 〈111〉 preferred orientations. The planes with higher energy are combined to form a regular arrangement and assembled into a polygonal frame until it evolves into a subhedral structure. The size uniformity of grains derived from framboidal pyrite is key to aggregation and self-organization. The research results are helpful for understanding the genesis mechanism of framboidal pyrite and its evolution process to a euhedral structure, which is of great significance to explore the structural growth of pyrite aggregates and the changes in the environmental response.

The pore structures of different lithofacies in low-permeability sandy conglomerate reservoirs and their diagenetic impacts: A case study from the Es4 member of the northern steep slope in Dongying Depression, Bohai Bay Basin, NE China

Sandy conglomerates are an important reservoir type in the Bohai Bay Basin, China. These reservoirs are characterized by complex lithofacies distribution, low permeability and heterogeneous pore structures that significantly impact reservoir quality and field development strategies. This study evaluated the Es4 member of the Eocene Shahejie Formation based on cores from three wells in the Dongying Depression. The analytical methods include thin section analyses, scanning electron microscopy (SEM), pressure-controlled mercury injection (PMI), rate-controlled mercury injection (RMI) and X-ray computed tomography (CT). The results show that the microscopic pore structures among various lithofacies of the Es4 sandy conglomerate reservoirs are controlled by different levels of diagenetic alteration, resulting in the heterogeneity of macroscopic reservoir quality. The high content of matrix in Mc (matrix-supported conglomerate) reservoirs leads to low primary porosity. Due to the poor grain sorting, Gc (grain-supported fine conglomerate) and Ps (pebbly sandstone) reservoirs lose a great number of primary pores during mechanical compaction process, whereas Ms (massive medium sandstone) reservoirs preserves abundant intergranular pores. Highly mineralized pore fluids, resulting from massive dissolution of volcanic rock fragments and carbonate fragments, contributes to extensive ferroan carbonate cementation during the late diagenetic stage. Powder crystal ankerite, with a grain radius of about 0.02 mm, blocks most pore throats within Gc reservoirs. Authigenic illite and mixed-layer illite/smectite bridges the throats and reduces the pore throat radius exponentially. Moderate dissolution of rock fragments and feldspars generates a number of secondary pores in Gc, Ps and Ms reservoirs, slightly enhancing permeability. With greater residual primary intergranular porosity and more large-size throats, Ms reservoirs possess the best reservoir quality, followed by Ps reservoirs. These research results are crucial to reservoir evaluation and predication in similar sandy conglomerate reservoirs.

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

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

Reservoir characteristics of the first member of Middle Permian Maokou Formation in Sichuan Basin and its periphery and inspirations to petroleum exploration, SW China

Based on a large number of field outcrops and cores taken systematically from boreholes, by microscopic observation, physical property analysis, mineralogy analysis, geochemical analysis etc., reservoir characteristics of the first member of Middle Permian Maokou Formation in Sichuan Basin (“Mao 1 Member” for short) are analyzed. (1) Rhythmic limestone-marl reservoirs of this member mostly exist in marl layers are a set of tight carbonate fracture-pore type reservoir with low porosity and low permeability, with multiple types of storage space, mainly secondary dissolution pores and fissures of clay minerals. (2) The clay minerals are mainly diagenetic clay minerals, such as sepiolite, talc and their intermediate products, aliettite, with hardly terrigenous clay minerals, and the reservoir in different regions have significant differences in the types of clay minerals. (3) The formation of high quality tight carbonate reservoir with limestone-marl interbeds is related to the differential diagenesis in the early seawater burial stage and the exposure karstification in the early diagenetic stage. It is inferred through the study that the inner ramp of southwestern Sichuan Basin is more likely to have sweet spots with high production, while the outer ramp in eastern Sichuan Basin is more likely to have large scale contiguous reservoir with low production.

Genetic Mechanism and Environment Implications of Siderites in the Lopingian Coal-Bearing Series, Western Guizhou of China: Constrained by Whole-Rock and In Situ Geochemistry

A large number of siderites have been found in the Lopingian (Late Permian) coal-bearing series in western Guizhou, which occurs in various microscopic morphologies and has potential insights into the sedimentary and diagenetic environments. An integrated set of analyses, such as microscopic observation; X-ray diffraction; whole-rock major and trace element, carbon, and oxygen isotope; and in situ major and trace element, has been carried out to unravel the genetic mechanism of the siderites and their environmental implications. According to the microscopic morphology, the siderites can be generally divided into three types and six subtypes, including gelatinous siderites (I), microcrystal-silty siderite [II; microlite siderites (II1), powder crystal siderites (II2)], and spheroidal siderite [III, petal-like siderite (III1), radiating fibrous siderite (III2) and concentric siderite (III3)]. Whole-rock geochemical results show that the iron source for the formation of the siderites was mainly from extensive weathering of the Emeishan high-titanium basalts in hot climate conditions. The carbon and oxygen isotopic results indicate that the origin of CO2 in type I siderites is derived from the dehydroxylation of organic matter. The CO2 in types II1 and II2 siderites is mainly derived from deposited organic matter and marine carbonate rocks, respectively. The CO2 source of type III siderites is sedimentary organic matter and marine carbonate rocks and is affected by different fluids during diagenesis. The whole-rock and in situ geochemical characteristics further point to that type I siderites were formed in the synsedimentary period most strongly affected by seawater. Redox proxies, such as V/Sc, V/(V+Ni), and δ Ce, constrained their formation in a stable and weakly reduced condition. Type II siderites could have been developed in saltwater. Among them, type II1 siderites were formed in the early diagenetic stage, whereas type II2 siderites originated from recrystallization of type II1 siderites and accompanied by metasomatism with calcites under diagenetic fluids of weak reduction to weak oxidation conditions. Type III siderites were formed under the influence of multistage diagenetic fluids. Among them, type III1 siderites formed by the growth of powder crystal siderites (II2) under diagenetic fluids with a weak reducing condition. Type III2 siderites formed by growth around microlite siderites under weak reducing diagenetic fluids. Type III3 siderites formed by concentric growth in diagenetic fluids with weak reduction to weak oxidation conditions and relatively active conditions.

Tracking the Origin and Evolution of Diagenetic Fluids of Upper Jurassic Carbonate Rocks in the Zagros Thrust Fold Belt, NE-Iraq

Utilizing sophisticated tools in carbonate rocks is crucial to interpretating the origin and evolution of diagenetic fluids from the Upper Jurassic carbonate rocks along the Zagros thrust-fold Belt. The origin and evolution of the paleofluids utilizing in-situ strontium isotope ratios by high resolution laser ablation ICP-MS, integrated with stable isotopes, petrography and fieldwork are constrained. Due to the lack of information on the origin of the chemistry of the fluids, the cements that filled the Jurassic carbonate rocks were analysed from the fractures and pores. This allowed us to trace the origin of fluids along a diagenetic sequence, which is defined at the beginning from the sediment deposition (pristine facies). Based on petrography and geochemistry (oxygen-, carbon- and strontium-isotope compositions) two major diagenetic stages involving the fluids were identified. The initial stage, characterized by negative δ13CVPDB values (reaching −10.67‰), involved evaporated seawater deposited with the sediments, mixed with the input of freshwater. The second stage involved a mixture of meteoric water and hot fluids that precipitated as late diagenetic cements. The late diagenetic cements have higher depleted O–C isotope compositions compared to seawater. The diagenetic cements display a positive covariance and were associated with extra- δ13CVPDB and δ18OVPDB values (−12.87‰ to −0.82‰ for δ18OVPDB and −11.66‰ to −1.40‰ for δ13CVPDB respectively). The distinction between seawater and the secondary fluids is also evident in the 87Sr/86Sr of the host limestone versus cements. The limestones have 87Sr/86Sr up to 0.72859, indicative of riverine input, while the cements have 87Sr/86Sr of (0.70772), indicative of hot fluid circulation interacting with meteoric water during late diagenesis.

Petrographical-geochemical characteristics and floral-faunal compositions of the Valia lignite deposits from Cambay Basin (Gujarat), western India

Lignite-bearing sequence of the Valia mine, associated with the Cambay Shale Formation (early Eocene) in Cambay Basin of western Indian State Gujarat, has been studied. An integrated approach (including organic petrography, palynofacies, palynology, entomology, geochemistry) has been adopted to assess the origin and characters of lignite deposits and their potential for the hydrocarbon generation, besides to understand the evolution and interaction of different biotic (floral-faunal) groups of the geological time. The studied samples (lignites/shales) are characterized by the domination of huminite group (av. 67 vol.%)/phytoclasts (av. 58%) along with the abundance of short-chain n-alkanes (bacterial activity). The frequencies of detrohuminite sub-group, non-biostructured elements, macrophyte-derived amorphous organic matter (AOM, mainly), and high values of proxy aqueous are indicating the elevated bacterial degradation of the OM and/or also suggest the significant contribution of the herbaceous vegetation/aquatic macrophytes/algal input to the palaeo-peat. The relative abundance of hopanes and low values of the steroids/hopanoids ratios further indicate a high bacterial activity. On the other hand, the contents of telohuminite, liptinite group, and cuticle elements together with the high carbon preference index, low proxy wax, and terrigenous/aquatic ratios confirm both terrestrial and macrophyte contributions during the peat accumulation. Fascinatingly, we have found Dipterocarpus-derived amber (fossil resin) within the lignite seam (lower) with remarkably well-preserved arthropod inclusions consisting of arachnids and insects. Generally, insects have an association with flora and provide important ecosystem functions. In the studied amber samples, the insects to arachnids (spiders and pseudoscorpions) ratio is 3:1. Their quantitative analysis indicates the dominance of the Chironomidae (non-biting midges 32%) followed by Araneae, Myrmicinae, Cecidomyiidae, and Mycetophilidae. A rare entity such as pseudoscorpions has also been found during the investigation.The derived petrographic indices indicate that the peat-forming limno-telmatic vegetation was accumulated mainly in back-barrier settings with high groundwater (mesotrophic to rheotrophic) condition. The contents of pyrite (mainly framboidal) mineral and sulphur (0.69–2.24 wt.%) suggesting that peat had experienced the marine influx. Further, the high occurrence of funginite maceral (among inertinite group), and spores of family Microthyriaceae point towards a warm and moist condition. The plotting of palynofacies data on Tyson's APP diagram, and the values of pristane/phytane ratio suggest that the OM was primarily deposited in dysoxic to suboxic settings. The high incidence of family Chironomidae, non-biostructured and low cuticle contents indicate the proximal settings and a low energy environment like lake/pond or marshlands. However, the occurrence of granular-AOM along with high (relatively) opaque phytoclast contents in the upper part indicates that distal settings were formed at the termination of the OM accumulation. The total organic carbon (TOC) contents and hydrogen index values in the studied samples indicate a good to excellent source for the hydrocarbon generation. However, the low gross calorific value (av. 4467 cal/g), values of Tmax (mostly below 435 °C), and H/C and O/C atomic ratios showing that the samples are of immature (early diagenetic stage; huminite reflectance value: av. 0.28%) nature of the OM. Moreover, the different cross plots (hydrogen index (HI) vs. oxygen index (OI), S2vs. TOC, HI vs. Tmax) indicate that the Valia lignite-bearing succession contains admixed type-III/II kerogen, and can generate gaseous and some liquid hydrocarbons upon maturation or with additional heating.

Effects of diabase intrusions on surrounding clastic rock reservoir in early diagenetic stages: A case study of the Early Permian Fengcheng Formation in the Dabasong uplift, Junggar Basin

A stage of mafic magmatic activity occurred in Early Permian in the Dabasong Uplift of Junggar Basin, part of the magma intruded into the normal sedimentary and shallow buried fine sandstone to form diabase, and part of the magma erupted to form basalt. The surrounding fine sandstone just entered in the early diagenetic stage A when the magma intruded. The compaction of the surrounding clastic rock and rupture of a small number of clastic grains were caused by the extrusion of the magma intrusion. The presence of chemically deposited alkaline minerals such as calcite, dolomite, shortite, natural alkali, and northupite indicates an alkali lake sedimentary environment for the Fengcheng Formation. Primary alkaline minerals dissolved from the surrounding rocks were subsequently transported and precipitated to form cements. The formation of the calcite cements and calcite metasomatism resulted in considerable densification of the surrounding rock during early diagenesis and destruction of the reservoir quality. The mafic magma had abundant Fe2+ and Mg2+ ions and was deficient in K+ ions, resulting in large amounts of chlorite and iron precipitation in the surrounding rock mainly composed of clay. We have analyzed the influence of an ultrashallow intrusion on the surrounding clastic rock during the early diagenetic period, which provided a typical reference for establishing a systematic mechanistic model of how magmatic intrusions affect the surrounding rock.