Stages Of Diagenesis Research Articles

Chlorite authigenesis and its impact on reservoir quality in tight sandstone reservoirs of the Triassic Yanchang formation, southwestern Ordos basin, China

Authigenic chlorite plays an important controlling role in reservoir quality, and is widely distributed in the tight sandstone reservoirs of the Triassic Yanchang Formation in the Southwestern Ordos Basin, China. The occurrence and origins of authigenic chlorite were investigated using an integrated approach of optical microscopy, scanning electron microscopy, X-ray diffraction, electron probe microanalysis, and fluid inclusion analysis. The sandstones are fine-grained lithic arkoses and feldspathic litharenite with 11.5% volcanic rock fragments. Authigenic chlorite occurs in two morphotypes, including pore-lining and pore-filling chlorite. Pore-lining chlorite occurs as coatings on the framework grains and covers the entire fining-upward sandstones. Pore-filling chlorite is restricted to the middle to upper part of the fining-upward sandstones and appears as discrete flaky plates or rosettes in the pores or attaches to the surfaces of pore-lining chlorite. Pore-lining chlorite originated from smectite precursors that were derived from volcanic material alteration in the early stage of diagenesis, with temperatures primarily about 90 °C. In contrast, pore-filling chlorite was precipitated in pore spaces with higher concentrations of Mg2+ and Fe2+ released from volcanic rock fragments and mica debris dissolution. Moreover, the chemical composition of pore-filling chlorite was affected by early calcite cementation which consumed some ions at temperatures lower than about 110 °C. The thin and continuous pore-lining chlorite is the only chlorite type with a positive effect on reservoir quality, whereas the very thick pore-lining chlorite (>6.7 μm) or pore-filling chlorite significantly reduce the porosity and permeability. Systematic studies of authigenic chlorite can provide important insights into the reservoir heterogeneity and are applicable to the exploration and development of tight oil.

Constrains of eruption environment and hydrothermal fluid on the Permian pyroclastic reservoirs in the Sichuan Basin, SW China

A breakthrough has been recently made in the hydrocarbon exploration of the Permian pyroclastic rocks in the Jianyang Area, western Sichuan Basin, China. With an aim to decipher the impacts of the eruption environment, the temperature of hydrothermal fluids, and the paleo-salinity on the formation of authigenic mineral assemblage and pores, this study implements comprehensive petrographic and geochemical studies through the integrated core and thin section analyses. The data presented demonstrate that the Permian volcanic intervals are intensively affected by an event of Emei taphrogeny. During basaltic magma upward migration, fractional crystallization of anorthose results in slightly-alkaline magma. The specific pyroclastic rocks are formed by the eruption of slightly-alkaline magma in the sea or a salt lake and subsequent hydrothermal alteration. During deposition and diagenesis, the authigenic mineral association is constrained jointly by the sodium-rich and high salinity water environment, and mid-high temperature, high-salinity hydrothermal fluid. Specifically, the sodium-rich hydrothermal fluid, which may sustain till the late diagenesis stage, caused pervasive albitization of pyroclastic rocks, then leading to mineral transformation and formation of a series of mineral associations. Therefore, zeolitization of volcanic glass and vesicle-infillings of zeolite is an essential condition for later mineral transformation and dissolution. Albitization of analcite, recrystallization induced by deep hydrothermal fluids, and both meteoric and deep burial dissolution expanding the micro-pore space ultimately formed porous pyroclastic reservoirs.

Sedimentary facies control on sandstone reservoir properties: A case study from the permian Shanxi Formation in the southern Ordos basin, central China

During the Upper Paleozoic large-scale tight sandstone oil and gas reservoirs developed in the southern Ordos Basin of China. Due to the heterogeneity of sedimentary facies and diagenesis, the reservoir quality of these deposits is extremely uneven. Based on field profile observations, conventional thin section, cast thin section and scanning electron microscope (SEM) analysis, combined with general physical analysis, the sedimentary characteristics, physical properties and main controlling factors of the reservoirs of the Upper Paleozoic Shanxi Formation of the southern Ordos Basin were determined. The results of this contribution show that the Shanxi Formation is dominated by delta facies, with small-scale lacustrine facies. The delta facies can be further divided into distributary channel, natural levee, interdistributary swamp, underwater distributary channel, underwater distributary bay, mouth bar and sand sheet microfacies. Diagenesis within the study area has generally entered the mesodiagenetic B phase. Compaction and cementation were the main destructive stages of diagenesis. While cementation blocked the reservoir pores and reduced the physical properties, dissolution effectively improved the physical properties of the reservoir. The physical reservoir properties of the studied sandstone reservoirs are controlled by different sedimentary microfacies that are mainly, represented by distributary channel, underwater distributary channel and mouth bar microfacies that also show the best properties. These microfacies are followed by sand sheets. The lowest properties can be observed in the natural levees.

Influence of sedimentation and diagenesis on reservoir physical properties: a case study of the Funing Formation, Subei Basin, eastern China

The sandstone of the third member of the Funing Formation (E1f3) in the northern slope zone of the Gaoyou Sag has the typical characteristics of high porosity and ultralow permeability, which makes it difficult for oil to flow. In this study, the lithological characteristics, sedimentary facies, diagenetic characteristics, pore structure, and seepage ability of this sandstone are characterized in detail. Correlation analysis is used to reveal the reason for the sandstone high porosity-low permeability phenomenon in the study area. The results indicate that this phenomenon is controlled mainly by the following three factors: 1) the sedimentary environment is the initial affecting factor, whereby the deposition of a large number of fine-grained materials reduces the primary pores of sandstone. 2) The Funing Formation has undergone strong compaction and cementation, which have led to the removal of most of the primary pores and a reduction in size of the throat channels. 3) Owing to fluid activity during the later stage of diagenesis, sandstone underwent intense dissolution and a large number of particles (feldspar and lithic debris) formed many dissolution pores (accounting for nearly 60% of the total pore space). Among these factors, dissolution has contributed the most to the development of high porosity-low permeability phenomenon. This is mainly attributed to the inhomogeneous dissolution process, whereby the degree of particle dissolution (e.g. feldspar) exceeds that of cementing minerals (clay and carbonate minerals). The secondary dissolution pores have increased the porosity of sandstone in the study area; however, the pore connectivity (permeability) has not been significantly improved, thus resulting in the special high porosity-low permeability characteristics of this sandstone.

Diagenesis and pore evolution model of tight sandstone gas reservoirs of Benxi Formation in the south-central Ordos Basin, China

ABSTRACT In recent years, gas tests in Benxi Formation in the south-central Ordos Basin have shown good oil and gas displays. In 2009, a gas reservoir was encountered in the Benxi Formation during drilling by the Shanxi Yanchang Petroleum Company, and the open flow potential was estimated at over a million cubic meter per day. Thus, studying the reservoirs in this area is of great exploration and guidance significance. In the paper, reservoirs were studied by casting thin sections, scanning electron microscopy, and high-pressure mercury intrusion experiments, and then, a diagenetic evolution sequence was established. Combined with the recovery model of the initial porosity of the sandstone and a quantitative pore analysis technique, the quantitative porosity evolution model of the Benxi Formation reservoir was established. The results show that i) the Benxi Formation mainly includes quartz sandstone, with less lithic quartz sandstone and lithic sandstone; ii) In general, five types of pores are generally developed in the Benxi Formation, namely, residual intergranular pores, intergranular dissolution pores, intercrystalline pores, micropores and microfractures; iii) Reservoir development was mainly affected by sedimentation and diagenesis; iv) At present, the main reservoir has currently reached the end of the mid-diagenesis of the B stage or late diagenesis stage. The average primary porosity of the sandstone was approximately 38.1%. The reservoir underwent a porosity reduction stage due to strong compaction, a porosity reduction stage due to early cementation, a porosity enhancement stage due to dissolution and a porosity reduction stage due to late cementation. The average porosity of the reservoir is now 4.89%.

Microbial and thermochemical controlled sulfur cycle in the Early Triassic sediments of the Western Canadian Sedimentary Basin

Pyrite is one of three main sulfur reservoirs and one of the biggest fluxes in the global sulfur cycle. The sulfur isotopic signature of hydrogen sulfide, pyrite and their parent sulfate is widely used as a proxy for tracking sulfur cycle variations in diagenetic environments. The Early Triassic Montney Formation in the Western Canadian Sedimentary Basin is characterized by distinct regional variations in pyrite abundance, type, sulfur isotopic signature and H 2 S concentrations in natural gas. Two main types of framboidal and crystalline pyrite were identified to have formed during various stages of diagenesis. The wide range of δ 34 S pyrite values (−34.4 to + 57.8‰ V-CDT) demonstrates that the sulfur cycle in the Montney Formation is governed by both microbial and thermochemical processes. The comparison of δ 34 S of the produced-gas H 2 S with pyrite, anhydrite and solid bitumen of the Montney, and underlying and overlying formations, suggests a mixture of dominantly in situ and minor migrated H 2 S with thermochemical sulfate reduction origin in the Montney Formation. The large diagenetic variations in pyrite types and δ 34 S pyrite values suggest a lack of direct biogeochemical connection to the sulfur cycle and emphasize the importance of careful petrographic observations and micro-scale isotopic analysis of sedimentary units to accurately reconstruct paleoenvironmental conditions. Supplementary material: Methodology and δ 34 S results are available at https://doi.org/10.6084/m9.figshare.c.5272484

Carbonate cementation patterns and diagenetic reservoir facies of the triassic Yanchang Formation deep-water sandstone in the huangling area of the Ordos Basin, northwest China

The Chang 6 member sandstone is one of the main oil reservoirs of the Ordos basin, China, and consists of a lacustrine gravity flow deposited in Huangling area. Carbonate cement is volumetrically the most important diagenetic parameter controlling reservoir quality in the sandstones.Three generations of carbonate cements were recognized. The massive microcrystalline calcite, poikilotopic calcite and microcrystalline dolomite from the supersaturated alkaline pore water precipitate with basement cements at an early diagenesis stage. The ferroan calcite is mainly pore-filling and replacement cementation in the middle diagenesis stage, and ferroan dolomite is pore-filling cementation in the late diagenesis stage.The four types of reservoir facies and the distribution patterns were identified in the Chang 6 member sandstones. (1) Massive carbonate cemented facies with high contents of carbonate cements are distributed along the edges of the sandstone bodies, which are in the channels of the turbidite and sandy debris flow deposits. (2) Partial carbonate cemented facies occur along the margins of the massive carbonate cemented facies. (3) The porous facies with the lowest percentages of carbonate cements in sandstone are distributed in the middle of the sand-bodies of the sandy debris flow deposits. (4) High muddy debris facies that have high percentages of carbonate-cemented and high muddy debris sandstones occur essentially in the proximal overbank and lobe of the turbidite deposits.The reservoir quality of sandstones depended upon time of precipitation of carbonate cements on and diagenetic processes. Massive carbonate cements and the high muddy debris of sandstones undergo lots of carbonate cements and strong compaction in the early diagenesis stage, respectively. There was weak water rock reaction of dissolution in the later diagenesis process. Partial carbonate cemented facies and the porous facies of sandstones have sufficient water rock reaction and more dissolution pores.

On the origin of bleaching phenomena in red bed sediments of Triassic Buntsandstein deposits in Central Germany

The Lower Triassic Buntsandstein in Central Germany comprises one of the major clastic, lithified hydrocarbon reservoirs, it is a major groundwater aquifer and it is considered as a potential underground gas storage lithology. Thus, sandstones of the Buntsandstein from Thuringia and NE Hesse were used to analyse their geochemical composition combined with mineralogical data in order to investigate the origin of several bleaching phenomena in primary red beds, since these give a hint for mineral alteration, mineral formation and mineral reactivity in the course of fluid-rock-interactions. This is relevant for e.g. recent Carbon Capture and Storage (CCS) efforts. The origin of the observed bleaching phenomena is diverse and it is linked to certain stages of diagenesis and different episodes of fluid flow: (1) eodiagenetic bleaching coupled with palaeosoil formation and iron mobilisation, (2) mesodiagenetic bleaching as a result of hydrocarbon migration, (3) Tertiary bleaching due to CO2 degassing from Miocene volcanism, (4) Tertiary telodiagenetic bleaching due to weathering under humid climate conditions, and (5) weathering processes related to recent telodiagenesis/pedogenesis. This study emphasizes the multiple nature of fluids, which can induce iron mobilisation and bleaching of red beds.

Генезис вантгофіту, левеїту й кізериту в родовищах калійних солей Передкарпаття за даними експериментальних досліджень

The genesis of vanthoffite, loeweite and kieserite in the potassium salts deposits of Precarpathian region is characterized on the basis of mineralogical, geochemical and experimental studies. Vanthoffite and loeweite are rare minerals. Vanthoffite associates with astrakhanite and loeweite. It is proved due to experimental studies that dehydration of astrakhanite occurs at the stages of late diagenesis–katagenesis under conditions of increasing temperature. From the products of its decomposition, loeweite is formed first, and then vanthoffite. This is facilitated by an increase of pressure, under which minerals with a higher density are more stable (the density of vanthoffite (2.69 g/cm3 ) is much higher than that of loeweite (2.37) and astrakhanite (2.32)). Kieserite is found in sylvinite, kainite and kainite-langbeinite rocks in the amount of small impurities up to 10–33 %. It is unstable on the day surface: it absorbs water from the atmosphere and transforms into hexahydrite. Experimental studies have shown that kieserite does not precipitate in salt sedimentary basins, but is formed from epsomite (hexahydrite) due to its dehydration Петро Білоніжка 60 ISSN 2078-6220. Мінералогічний збірник. 2021. № 71 under conditions of increasing temperature and pressure at the stages of late diagenesis and katagenesis. Increasing temperature values (up to 200 °С), which existed in the potassium salts deposits of Precarpathian region during the thrust of the Skybova zone to the Precarpathian Foredeep Inner zone and the formation of folding in salt deposits, are sufficient for phase transformation of astrakhanite to loeweite and vanthoffite, and epsomite (hexahydrite) – to kieserite.

Modelovanje geotermičke evolucije sedimentnih stena istočne Hercegovine na osnovu refleksije vitrinita

This paper presents the first results of the geothermal evolution modeling in East Herzegovina based on vitrinite reflectance calculations and measurements. Modeling of burial and geothermal was performed using "SimpleMat" software with well known kinetic model of vitrinite reflectance - EASY % Ro. By fitting the calculated reflectance values with mean measured vitrinite reflectances, it was concluded that there was erosion of some 900 m in the area, and that first possible petroleum generation could have started some 100 Ma ago. According to final calibration model that includes additional thickness and subsequent erosion of Paleogene sequence, the Upper Triassic sediments should have maturity from 0.70 to 1.1 % Rr, Lower Jurassic from 0.62 to 0.70 % Rr, whereas Upper Cretaceous sediments should have values from 0.40 to 0.60 % Rr, which is in accordance with measured vitrinite reflectance in 23 samples. Assuming that beginning of oil window in the East Herzegovina correspond to the vitrinite reflectance of 0.50 % Rr, all analyzed Mesozoic samples are at the initial stage of catagenesis ("oil window"), while Cenozoic sediments are at the stage of diagenesis.

LA-ICPMS trace elements of pyrite from the super-large Dulong Sn-Zn polymetallic deposit, southeastern Yunnan, China

滇东南马关都龙是一个以锡锌为主，共-伴生铟、铜、铅、钨、铁、银等多种元素的锡锌多金属超大型矿床。虽然前人从矿物学、矿床地球化学、年代学等不同角度开展了较多的研究，该矿床锡锌多金属矿化为燕山晚期岩浆热液活动的产物已是不争的事实，但关于该矿床是否存在热水沉积作用及其与锡锌多金属成矿作用的关系依然存在较大争议。本文选取都龙矿区广泛存在的黄铁矿作为主要研究对象，在矿相学基础上利用LA-ICPMS对不同阶段黄铁矿的微量元素组成开展了系统的研究。野外及显微鉴定结果表明，矿区存在四种类型（期次）的黄铁矿，即：鲕状黄铁矿Py1；穿切或交代Py1的细脉状黄铁矿Py2；与闪锌矿等硫化物共生的自形黄铁矿Py3；包裹早期黄铁矿或闪锌矿等硫化物的他形黄铁矿Py4。LA-ICPMS分析结果表明，该矿床黄铁矿中富集多种微量元素，其中Co、Ni、As、Ge等元素以类质同象的形式存在黄铁矿晶格中，而其余元素多以显微矿物包体形式赋存于黄铁矿中。上述四期黄铁矿微量元素组成存在较大差别，Py1相对富集Zn和As，而其余微量元素含量较低，Co与Ni含量较低，Co/Ni比值远低于1.00，其微量元素组成与典型沉积作用形成黄铁矿基本一致；Py2与Py1具有相似的微量元素组成特征，其Co/Ni比值接近Py1变化范围；Py3和Py4除富集Zn、As外，Mn、Co、Ni、Cu、Sb、Pb、Bi元素含量也相对较高，其Co/Ni比值相对较高，多大于1，与典型岩浆热液型黄铁矿微量元素组成相似，而与沉积型黄铁矿差异明显。结合各阶段黄铁矿产出地质特征，对比不同类型黄铁矿微量元素组成，本研究认为：Py1鲕状黄铁矿为热水沉积作用形成；Py2为Py1变质改造形成的细脉状黄铁矿，其微量元素继承了Py1；Py3为岩浆热液活动形成的自形黄铁矿；Py4为岩浆热液活动晚期形成的他形黄铁矿，Ag和Bi组成作为区分不同成因类型黄铁矿的化学指标的潜力。矿区早期沉积作用形成鲕状黄铁矿过程可能为后期成矿作用提供了部分硫源及少量Zn等成矿物质，海西-印支期区域变质改造作用对矿区成矿作用影响不大，而燕山晚期岩浆热液活动才是矿区锡多金属大规模成矿作用的主导因素。;The Dulong Sn-Zn polymetallic deposit, located in the southeastern Yunnan Province, is a super-large Sn-Zn polymetallic deposit with by-products of In, Cu, Pb, W, Fe, Ag, etc. Many studies on mineralogy, ore deposit geochemistry and chronology have been carried out on this deposit, confirming a genetical relationship of its Sn-Zn mineralization to the Late Yanshanian magmatic activity. However, whether the deposit underwent exhalative sedimentary Sn-Zn mineralization is still controversial. Based on detailed petrographic observation, the LA-ICPMS trace element compositions of pyrite that widely occur in the Dulong deposit are investigated systematically in this paper. Field and microscopic observations show that four types of pyrite have been identified in Dulong, including the oolitic pyrite (Py1), fine vein pyrite (Py2) crosscutting and/or replacing Py1, euhedral pyrite (Py3) coexisting with sphalerite and other sulfides, anhedral pyrite (Py4) enclosing early generated pyrite and sphalerite. LA-ICPMS analysis results suggest that pyrite is enriched in Co, Ni, As and Ge, which are mainly incorporated into the pyrite lattice in the form of isomorphism. Other elements occur mostly as mineral micro-inclusions in pyrite. The trace element compositions of the four types' pyrite exhibit significant difference from each other: Py1 is enriched in Zn and As but depleted in Co, Ni and Cu with low Co/Ni ratios (<1.0), which share similar features with typical sedimentary pyrite; Py2 is similar in its trace element compositions and Co/Ni ratios to those in Py1; while Py3 and Py4 have elevated Zn, As, Mn, Co, Ni, Cu, Sb, Pb and Bi contents with high Co/Ni ratios (mostly >1.0), which is consistent with that of euhedral pyrite formed by magmatic-hydrothermal activity rather than sedimentary pyrite. Comparing with the trace element compositions of pyrite in different types, we consider that Py1 is of sedimentary origin, Py2 that inherits the trace element features of Py1 is formed by metamorphic reworking of Py1, while Py3 and Py4 are formed by late magmatic-hydrothermal activity. Ag and Bi in pyrite display potential probobilities as a geochemical indicator to distinguish pyrite of different genetic types. Oolitic pyrite formed in the depositional diagenesis stage may provide some sulfur and a small amount of Zn for the later Sn-Zn mineralization. The regional metamorphic reworking in the Hercynian to Indosinian displays a limited effect on the Sn-Zn mineralization, while Late Yanshanian magmatic-hydrothermal activity is the controlling factor for the formation of the super-large scale Dulong Sn-Zn polymetallic deposit.

Porosity Development Controlled by Deep-Burial Diagenetic Process in Lacustrine Sandstones Deposited in a Back-Arc Basin (Makó Trough, Pannonian Basin, Hungary)

Deeply buried Pannonian (Upper Miocene) siliciclastic deposits show evidence of secondary porosity development via dissolution processes at a late stage of diagenesis. This is demonstrated by detailed petrographic (optical, cathodoluminescence, fluorescence, and scanning electron microscopy) as well as elemental and stable isotope geochemical investigations of lacustrine deposits from the Makó Trough, the deepest depression within the extensional Pannonian back-arc basin. The analyses were carried out on core samples from six wells located in various positions from centre to margins of the trough. The paragenetic sequence of three formations was reconstructed with special emphasis on sandstone beds in a depth interval between ca 2700 and 5500 m. The three formations consist, from bottom to top, of (1) open-water marls of the Endrőd Formation, which is a hydrocarbon source rock with locally derived coarse clastics and (2) a confined and (3) an unconfined turbidite system (respectively, the Szolnok and the Algyő Formation). In the sandstones, detrital grains consist of quartz, feldspar, and mica, as well as sedimentary and metamorphic rock fragments. The quartz content is high in the upper, unconfined turbidite formation (Algyő), whereas feldspars and rock fragments are more widespread in the lower formations (Szolnok and Endrőd). Eogenetic minerals are framboidal pyrite, calcite, and clay minerals. Mesogenetic minerals are ankerite, ferroan calcite, albite, quartz, illite, chlorite, and solid bituminous organic matter. Eogenetic finely crystalline calcite yielded δ13 C V − PDB values from 1.4 to 0.7‰ and δ18 O V − PDB values from –6.0 to –7.4‰, respectively. Mesogenetic ferroan calcite yielded δ13 C V − PDB values from 2.6 to –1.2‰ and δ18 O V − PDB values from –8.3 to –14.0‰, respectively. In the upper part of the turbidite systems, remnants of the migrated organic matter are preserved along pressure dissolution surfaces. All these features indicate that compaction and mineral precipitations resulted in tightly cemented sandstones prior to hydrocarbon migration. Interconnected, secondary, open porosity is associated with pyrite, kaolinite/dickite, and postdates of the late-stage calcite cement. This indicates that dissolution processes took place in the deep burial realm in an extraformational fluid-dominated diagenetic system. The findings of this study add a unique insight to the previously proposed hydrological model of the Pannonian Basin and describe the complex interactions between the basinal deposits and the basement blocks.

Early silicification of the Cyrenaican chert, Libya: The importance of moganite as a transitional silicon dioxide phase

AbstractThe Messinian lagoonal carbonate–evaporite sequence of Cyrenaica, north‐east Libya, hosts: (i) opaline nodules in gypsiferous microbial‐rich mudstone; (ii) nodules replacing poikilotopic gypsum cementing bioclastic carbonates; and (iii) bedded porcelanite with large lenticular gypsum pseudomorphs intercalated with recrystallized microbial mats. Optical microscopy, scanning electron microscopy, backscattered electron images – secondary electron images, X‐ray fluorescence, X‐ray diffraction and cathodoluminescence techniques were employed to elucidate on the early stages of chert diagenesis and demonstrate the role of moganite in the formation of varieties of chalcedony during silicification. The opaline nodules composed of opal‐A, exhibit shrinkage cracks lined by isopachous fibrous quartzine underlain by a thin birefringent zone. This suggests that quartzine may develop by crystallization and conversion of opal‐A through a transitional moganite. The quartzine is followed by amorphous silica gel that converts into length‐fast chalcedony. By contrast, lutecite evolved by silica replacement of gypsum through a transitional moganite phase, which forms an amorphous zone in apparent optical continuity with the fibrous lutecite; as moganite advances by pseudo‐crystalline terminations controlled by the gypsum cleavage. In gypsified oyster shells, intercrystalline organic matter enveloping pseudomorphed shell microstructures enabled moganite nucleation that merges syntaxially around the gypsum crystal rim. The associated lutecite occurs as radial‐fibrous beekite aggregates that are consistent with early shell silicification. The bedded porcelanite consists of opal‐CT groundmass undergoing recrystallization and enclosing micro‐cavities lined by quartzine. The associated pseudomorphs display phases of gypsum replacement by moganite and lutecite at the outer margin, followed by dissolution and cavity filling by quartz varieties in response to fluid flushing. In all cases, organic matter and microbes played a vital role in the silicification process.

Effects of Diagenesis on the Reservoir Quality in the Upper Sands of Lower Goru Formation, Badin Block, Lower Indus Basin, Pakistan

The core samples of B member of upper sand reservoir rocks of the lower Goru Formation from three wellsof the Badin block were studied using thin section, XRD, and SEM techniques to investigate the diagenetic trends andtheir effect on reservoir quality. Microscopic study indicates that the B sand unit is mature with quartz as thepredominant mineral constituent with variable amount of feldspar and lithic minerals. The QFL plot indicates that mostof the samples are plotted in the field of quartz arenite, sub-litharenite and sub-arkose respectively. A few samplesbelong to the category of litharenite and feldspathic litharenite. The feldspars were partially to completely altered tokaolinite and other clay minerals. Coarse-crystalline or micro-crystalline calcite is the predominant cementing material.Bulk rock XRD analysis also confirms that the main mineral constituents of there samples are quartz and calcite invariable proportions. The undulose extinction and fracturing of quartz grains indicate that the area remained understress. Moreover, such fracturing is post-depositional and therefore is the product of late diagenesis. Scanning ElectronMicroscopic (SEM) images at 50 micrometer (μm) size show irregular type of fracturing within the quartz grains. Thislate stage fracturing of quartz has also generated various types of channels which may serve as secondary porosity. Thequartz overgrowth was observed in some samples due to late stage diagenesis. The micro-crystalline cement in the formof calcite is mostly present within the pores in fractured quartz. The results indicate that the diagenesis played asignificant role in improving the reservoir characteristics of B sand by increasing the porosity due to quartz fracturing,feldspar dissolution, alteration and quartz corrosion. Hence, better understanding of reservoir heterogeneities inporosity through diagenetic studies can be helpful in evaluating potential reservoir horizons for hydrocarbonaccumulation on spatial and temporal scales.

THEORETICAL ASPECTS OF THE STUDY OF THE SEDIMENTARY STRATA BEDDING

The bedding of sedimentary strata is the most noticeable and widespread sedimentological feature. In a general geological literature, often there is only a statement of its presence and distribution globality, but often there is no explanation for its occurrence, or it differs significantly. An analysis of previous studies has established that a large number of factors are involved in its formation. It is shown that such physical factors as gravity and the dynamics of the sediment transport environment are more global in nature and manifest themselves in all sedimentation environments, in contrast to geological factors. The last factors also play a significant role in the formation of bedding, as well as sedimentogenesis in general, but they have a subordinate significance, since they do not always appear. In the study of lithogenesis is observed some inconsistency, when mainly geological factors are used in the description of sedimentogenesis, and physical and chemical factors are used at different stages of diagenesis. Another inconsistency is much more attention given, in theoretical and practical geological studies, to a quantitative study of the thickness of sediment compared to the area of their distribution. Almost no attention is paid to the study of dependencies between these geometrical quantities, quantitatively characterizes the sedimentary bed. However, such dependencies are present at least in the sedimentary strata, which covers the northern part of the Ukrainian shield. The study of such dependencies in any territories can contribute to a quantitative assessment of factors influencing sedimentogenesis, and also have significant practical significance.

Depth of Diagenetic Processes and Lower Boundary of the Biosphere in the South Caspian Basin

An assessment of the depth of the diagenetic zone and biosphere in the South Caspian Basin (SCB) is given based on a set of more than 400 measurements (lithological composition and reservoir properties of rocks, temperature, and pressure) on 41 onshore and offshore hydrocarbon fields; it is also based on about 200 isotope-geochemical analyses of gases and oils and hydrochemical analyses of formation waters. It is established that an extended diagenetic zone is formed in the SCB under conditions of an avalanche sedimentation rate in the Pliocene–Quaternary period and low temperature (temperature gradients were within 15–21°C/km). This is confirmed by the nature of the depth variations in the physical and mechanical parameters of the rock-fluid system in the SCB (water and gas saturation, porosity and permeability of sediments, temperature and pressure). From the point of view of oil and gas generation processes, the boundary between the end of the stage of late diagenesis and the beginning of the early catagenesis stage in the SCB is located at a depth of about 4.5 km, which corresponds of a vitrinite reflectance value of 0.5% and formation temperature of about 80°C. According to the isotope-geochemical parameters, microbiological activity, and biochemical processes, the lower boundary of the biosphere is outlined at a depth of about 2–2.5 km, which corresponds to a temperature of 45–50°C.

Structural and Crystal-Chemical Features and Rb–Sr Age of Globular Glauconite in the Ust-Il’ya Formation (Lower Riphean, Anabar Uplift)

For the first time, mineralogical, structural, crystal-chemical and isotope-geochronological data were obtained for a glauconite sample from the Lower Riphean Ust-Il’ya Formation on the Magan River (western slope of the Anabar Massif, northern Siberia). It has been established that glauconite from the lenticular layer of friable glauconitite (glauconite grains ≥80%) contains two populations of mixed-layer crystals, in which mica layers (93%) alternate with either smectite-type (7%) or vermiculite-type layers (7%) with the short-range order factor R = 3. The ratio KAl = (VIAl / (VIFe3+ + VIAl) equal to 0.45 allows identifying the micaceous mineral as glauconite with the unit-cell parameter b = 9.065 A and K2O content of 8.29%. Comparison of new data with the previously obtained mineralogical and crystal-chemical characteristics of Ust-Il’ya glauconites from the Kotuikan River section (2.5 km above the Il’ya River mouth), located 65 km south of the Magan River section, showed both similarities and differences. Analysis of the crystal-chemical heterogeneity of Kotuikan glauconite samples from rocks of different lithological types revealed peculiarities in the distribution of individual mica varieties composing the glauconite globules. Isotopic dating of glauconite from the Magan River section was performed in combination with simulation of the distribution of octahedral cations and comparison of the results obtained with Mossbauer and infrared (IR) spectroscopy data. Such an approach combined with the mineralogical and crystal-chemical analyses contributes to correct interpretation of the stratigraphic significance of isotope data. The results obtained provide grounds for the conclusion that isotopic dates of glauconite from the Magan section (1474 ± 21 Ma) coincides within the error limit with the earlier Rb–Sr and K–Ar dating of the Ust-Il’ya Formation (1485 ± 13 and 1459 ± 20 Ma, respectively) based on the glauconite in the Kotuikan section (Zaitseva et al., 2016). The former value marks the stage of early diagenesis of sediments and is suitable for estimating the age of this formation.

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

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

Significance of 2-methylhopane and 22,29,30Trisnorhop17(21)-ene biomarkers in holocene sediments from the Gulf of Tunis - Southern Mediterranean Sea

The investigation of lipid biomarkers in sediments deposited during the Holocene in the Lagoon of Tunis, located at the southern edge of the Mediterranean Sea, revealed two important hopanoid biomarkers: 2-methylhopaneand 22,29,30 trisnorhop-17(21)-ene. These compounds are considered of special interest because they reveal information about climate conditions during the Late Holocene sediment deposition. These hopanoids, or bacteriohopanepolyols (BHPs), indicate a considerable bacterial contribution to the organic matter preserved in these sediments.The occurrence of 2-methylhopanein substantial amounts indicates that cyanobacteria, Rhodopseudomonas palustris bacteria, and genus Desulfovibriobacteria communities made large contributions to the sediment's organic content. In addition, the presence of these bacteria communities have been interpreted as reliable indicators of hydrologic modifications associated with the development of wetland systems during the Late Holocene.The occurrence of 22,29,30trisnorhop17(21)-ene also suggests a considerable bacterial contribution to the organic matter and indicates an immature diagenesis stage. It also reflects the development of favourable conditions to the preservation and accumulation of organic matter within an anoxic depositional environment.

Diagenesis of Hunton Group Carbonates (Silurian) West Carney Field, Logan and Lincoln Counties, Oklahoma, U.S.A.

The West Carney Hunton Field (WCHF) is an important oil field in central Oklahoma. Deposited during a series of sea-level rises and falls on a shallow shelf, the Cochrane and Clarita Formations (Hunton Group) have undergone a complex series of diagenetic events. The Hunton section of the WCHF comprises dolomitized crinoidal packstones, brachiopod “reefs” and grainstones, thin intervals of fine-grained crinoidal wackestones, and infrequent mudstones that were diagenetically affected by repeated sea-level change. Widespread karst is evidenced by multiple generations of solution-enlarged fractures, vugs, and breccias, which extend through the entire thickness of the Hunton. Karst development likely occurred during sea-level lowstands. Partial to complete dolomitization of Hunton limestones is interpreted to have occurred as a result of convective circulation of normal seawater during sea-level highstands. Open-space-filling calcite cements postdate dolomitization and predate deposition of the overlying siliciclastic section, which comprises the Misener Sandstone and Woodford Shale. Petrographic evaluation and carbon and oxygen isotope values of the calcite cements suggest precipitation by Silurian seawater and mixed seawater and meteoric water. Carbon and oxygen isotopic signatures of dolomite may have been partially reset by dedolomitization that was concurrent with calcite cementation. Fluid inclusions in late diagenetic celestite crystals observed in the Clarita Formation indicate that the WCHF was invaded by saline basinal fluids and petroleum after burial, during later stages of diagenesis. The timing of late diagenetic fluid flow and petroleum generation likely was during the Ouachita orogeny, which was occurring to the south. There is no evidence that late diagenetic fluids significantly altered the dolomite reservoir that formed earlier. The WCHF provides an ancient example of early diagenetic dolomitization by seawater that remains relatively unaltered by later diagenetic events.