Comparison of source rocks from the Lower Silurian Longmaxi Formation in the Yangzi Platform and the Upper Devonian Semiluksk Formation in East European Platform

Due to the depletion of conventional hydrocarbon resources,both China and Russia are giving more attention to the exploration and production of unconventional oil and gas resources,especially those generated and accumulated within source rocks.In an attempt to further understand the mechanisms of these resources,detailed mineralogical,lithological,and geochemical studies were performed to compare source rock samples from i)the Longmaxi Formation of the Lower Silurian in the Sichuan Basin(China),and ii)the Semiluksk Formation of the Frasnian Stage of the Upper Devonian in the Volga-Ural region of the East European Platform(Russia).The results showed that the main mineral of the source rocks of both formations is chalcedony that formed during an outpouring of deep fluids onto the sea bed.Simultaneous thermal analyses of this mineral indicated similar thermophysical properties to those of the hydrothermal-sedimentary chalcedony from jasper.Moreover,a direct relationship between the chalcedony content and the total organic matter content in the samples from the two formations was established.The presence of biophilic chemical elements in the siliceous component of the source rock samples indicate that high total organic content values were likely due to the presence of biophilic chemical elements in deep fluids,which led to the large-scale development of biota and subsequent accumulation of organic matter during sedimentation.The findings also revealed that the organic matter in the source rocks of the two regions was at different stages of maturity.The organic matter in samples from the Volga-Ural region was less mature and only at the early stage of oil generation,whereas the organic matter in samples from the Sichuan Basin reached both oil and gas generation stages.The Longmaxi Formation is already in the shale gas exploration and development stage,and the Semiluksk Formation could also be regarded as an exploration target for shale oil reservoirs.

Waxes in asphaltenes of crude oils and wax deposits

Composition and molecular mass distribution of n-alkanes in asphaltenes of crude oils of different ages and in wax deposits formed in the borehole equipment were studied. In asphaltenes, n-alkanes from C12 to C60 were detected. The high molecular weight paraffins in asphaltenes would form a crystalline phase with a melting point of 80–90 ℃. The peculiarities of the redistribution of high molecular paraffin hydrocarbons between oil and the corresponding wax deposit were detected. In the oils, the high molecular weight paraffinic hydrocarbons C50–C60were found, which were not practically detected in the corresponding wax deposits.

Dolomitization of the lower cretaceous carbonate reservoir in the Euphrates Graben,Syria

Results of this study are based on core materials description,thin sections,Cathodoluminescence(CL),and Scanning Electron Microscope(SEM)examinations.The Lower Cretaceous over the Euphrates Graben area was characterized by carbonate sedimentation in shallow marine environments.The low energy lagoonal to inner shelf sediments of the Judea Formation includes micritic mudstone to wackestone texture,dolomitic limestones and dolostones.Two types of dolomites recognized in the carbonates of the Judea Formation,the preserving microcrystalline dolomites which commonly founded in the partially dolomitized micritic limestones,and the destructive coarsely crystalline dolomites which commonly founded in the dolostones and dolomitic limestones.Petrographic examinations indicate that the preserving microcrystalline dolomites represent subtidal cycles developed in a shoal to open marine depositional environments,they probably formed under conditions of the shallow burial diagenesis.The destructive coarsely crystalline dolomites may develop in more basinward,open marine environments under conditions of the deep burial diagenesis that accompanied by rising in temperature,pressure,and burial depth.It is believed that evolution of the diagenetic history of the Judean Formation sediments occurred in two diagenetic stages;the shallow burial diagenesis,and the deep burial diagenesis.Compaction processes,early fracturing,micritization,early calcite,and the early phase of dolomitization were part of the multiple diagenetic alterations during the shallow burial diagenesis.The deep burial diagenesis was marked by dissolution,late stage of dolomitization and replacement,mechanical and chemical compaction,and the late calcite precipitation.It is believed that dolomitization of the Judea Formation carbonates in the Euphrates Graben has contributed to improving the reservoir properties by increasing the porosity and thus the permeability.

Impact of microfacies and diagenesis on the reservoir quality of Upper Devonian carbonates in Southeast Tatarstan,Volga-Ural Basin,Russia

The results of integrated sedimentology,petrography,and petrophysical study of the Upper Devonian(Middle Famennian)Dankovo-Lebedyansky carbonates from Southeast Tatarstan of the Volga-Ural Basin revealed a variety of microfacies and diagenetic events that impacted the reservoir quality.Although our earlier study documented microfacies analysis and depositional environments,none of the studies focused on diagenesis,microfacies interaction,and their controls on the studied sediment's reservoir quality.Based on petrographic and microfacies analyses,the seven identified microfacies types are peloidal grainstone MF 1,cemented bioclastic peloidal grainstone MF 2,echinoderm-concentrated packstone MF 3,algae packstone MF 4,bioclastic wackestone MF 5,whole-fossil wackestone MF 6,and dolomite MF 7.For the investigated sediments,a gently deepening carbonate ramp depositional model with an inner,middle,and outer ramp setting is proposed.The observed diagenetic events in this study include micritization,calcite cementation(six cement types),dolomitization(six dolomite types),dissolution(fabric and non-fabric-selective dissolution),compaction,and microfracturing.The identified microfacies were classified into three distinct classes based on their petrophysical characteristics.MF 1 and MF 7 are microfacies types with the best reservoir quality.MF 3 and MF 4 are microfacies types of moderate reservoir quality.MF 2,MF 5,and MF 6 are microfacies types with poor or non-reservoir quality.Calcite cementation,micritization,and compaction are the primary diagenetic modifications responsible for porosity reduction.Moldic pores created by dissolution are a significant porosityimproving process.Porosity is locally enhanced by stylolite and microfractures.Dolomitization improved reservoir quality by creating intercrystalline and vuggy porosity.Understanding the impact of microfacies and diagenesis on reservoir quality is crucial for understanding reservoir properties in nearby fields with similar settings.

Microfacies analysis and depositional environment of the Upper Devonian Dankovo-Lebedyansky sediments,Tatarstan,Volga-Ural Basin,Russia

The Upper Devonian,Middle Famennian Dankovo-Lebedyansky sediments in Southeast Tatarstan territory are a carbonate sequence composed of limestone and dolomite,which is important hydrocarbon reservoir units,therefore,it is necessary to conduct in-depth study on its microfacies and depositional environment.In this study,a multidisciplinary approach that combines core observation with thin section examination is used.The limestone contains abundant skeletal grains(echinoderms,foraminifera,algae,gastropods,and calcispheres),as well as non-skeletal grains(intraclasts and peloids).On the basis of detailed petrographic investigations,six sedimentary microfacies can be identified,including(i)peloidal grainstone(MF 1),(ii)cemented bioclastic peloidal grainstone(MF 2),(iii)echinodermconcentrated packstone(MF 3),(iv)algae packstone(MF 4),(v)bioclastic wackestone(MF 5),(vi)whole-fossil wackestone(MF 6),as well as dolomite or dolostone as diagenetic facies(MF 7).Based on microfacies analysis,the Dankovo-Lebedyansky sediments were deposited in three distinct sedimentary facies belts(shoal,lagoon and open marine environment).In order to reflect dispositional energy condition,the microfacies were grouped into facies associations:(i)low-energy microfacies associations including MF 5 and MF 6,(ii)moderate energy microfacies associations including MF 1,MF 2,MF 3,and MF 4.The dolomite,or diagenetic facies(MF 7),is the result of slightly to extensively dolomitization of limestone in the Dankovo-Lebedyansky sediments.Most of frequent dolostone types are euhedral planar-e and subhedral planar-s.According to petrographic characteristic of the dolostone,a seepage reflux model can be used to explain the dolomitization process of Dankovo-Lebedyansky sediments.

Sedimentary diagenesis and pore characteristics for the reservoir evaluation of Domanik formations(Semiluksk and Mendymsk)in the central part of Volga-Ural petroleum province

The Upper Devonian Semiluksk(Domanik)and Mendymsk carbonate successions have been identified as the most promising unconventional oil and gas deposits in Russia's Volga-Ural petroleum province.Three lithofacies were identified:limestones,organic matter-rich siliceous carbonate,and dolomitic limestones/dolomite.Their sediments are thought to have accumulated in the relatively deep-sea shelf depositional environment.According to X-ray analysis,calcite is the most common mineral phase in the limestones.The siliceous carbonate rocks'major mineral phases were identified to be quartz,followed by calcite.The rocks have relatively poor reservoir qualities,with porosity ranging from 0.1 to 5%and permeability ranging from 0.01 to 10 mD.Petrographic examination of thin sections reveals four major types of pores:1.interparticle pores,which are abundant in all of the examined lithofacies;2.intercrystalline pores formed between dolomite crystals and the very fine components of the siliceous carbonate rocks.3.vugs as a result of leaching,and 4.fracture as a result of tectonic and/or diagenetic events.Plotting the obtained data from the petrographically examined thin sections on the ternary pore plot diagram resulted in the identification of six pore facies and their related diagenetic patterns.Pore facies 1 depicts the pores that formed during the depositional process.Pore facies 4 displays moldic pores.The vugs are represented by pore facies 6.Pore facies 2 is a mixture between pore facies 1 and pore facies 4.Pore facies 3 is a mixture between pore facies 1 and pore facies 6.Pore facies 5 is a mixture between pore facies 4 and pore facies 6.Comparing the petrographic results and the identified pore facies show that pore facies 6 have touching pores and they played the most important role in improving the reservoir quality.The pore facies 4 which have separate pores have the most influence in the reduction of the reservoir quality.The diagenetic origin secondary pores are the most pores identified in the studied carbonate section.This reflects the strong modification of the pores by the diagenetic overprinting.The massive dissolution that forms the touching vugs,and the dolomitization process are the most diagenetic processes that affect the pore system within the studied rocks.

Microfracture Characterization in Sandstone Reservoirs:A Case Study from the Upper Triassic of Syria's Euphrates Graben

The Euphrates Graben is located in eastern Syria.The Upper Triassic Mulussa F Formation sandstones serve as the primary reservoir intervals in the majority of the graben fields.The study’s findings were based on core studies:petrographic examination of thin sections,scanning electron microscope(SEM),imaging of backscatter scanning electron microscope(BSE),X-ray microprobe examinations,and carbon-oxygen stable isotope analysis of microfracture-filling cements.Three of the most common types of microfracture found in the investigated sandstones are intragranular or intracrystalline microfractures,grain boundary or grain-edge microfractures,and transgranular(crossing grains)microfractures.Sandstone microfractures that are open and free of secondary mineralization improve sandstone storage and permeability.However,microfractures that are cemented and filled with secondary mineralization reduce storage and permeability.Common siderite and pyrite cements were identified within the microfractures and the nearby sandstone matrix.Larger anhedral or euhedral siderites are thought to form during shallow burial diagenesis,whereas poikilotopic siderites are thought to form during deep burial diagenesis.Poikilotopic pyrite is believed to be a diagenetic cement,which is attributed to the reduction of iron oxides present in the sediments in the presence of hydrocarbons.Microfractures reflect tectonic,overpressure,and diagenetic origins.Microfractures of tectonic origin are associated with folding and thrust activities over the Euphrates Graben area,and they were formed at the beginning of the Upper Triassic with siderite and pyrite cement equilibration temperatures of approximately 100–105℃,and they continued forming from the middle to the end of the Upper Triassic with cement equilibration temperatures of approximately 90–100℃in conjunction with the first phase of the Euphrates Graben.Microfractures related to diagenetic and overpressure processes are tension microfractures and were formed in compression settings during the Upper Triassic.

Cementation Characteristics and Their Effect on Quality of the Upper Triassic, the Lower Cretaceous, and the Upper Cretaceous Sandstone Reservoirs, Euphrates Graben, Syria

This article presents the results of cementation characteristics and their effect on sandstone reservoir quality of the Upper Triassic Mulussa F, the Lower Cretaceous Lower Rutbah, and the Upper Cretaceous Post Judea Sandstone formations in selected fields in the Euphrates Graben area, Syria. This study emphasises the role of cementation in the evaluation of the diagenetic history of the sediments, developing effective porosity, as well as evaluation of reservoirs stimulation procedures and potential for formation damage of the sandstone reservoirs. Quartz cement is present as well developed tabular or pyramidal syntaxial overgrowths. Kaolinite cement is present as vermicular aggregates which are most abundant within sandstones of the Mulussa F Formation. Carbonate cements include siderite and dolomite. Four lithofacies were identified within the studied formations;lithofacies-1 and 2 correspond to fluvial depositional environments, lithofacies-3 and 4 correspond to fluvial to estuarine channel environments. The Post Judea Sandstone and the Lower Rutbah reservoir units are typically lithofacies-3 sequences in which quartz overgrowths are the dominant cement. Because the total cement is more extensive in the Post Judea Sandstone Formation than in the Lower Rutbah Formation, resulting in high porosity(up to 26%) and permeability(6 000 mD), the reservoir quality is predicted to be best in the Post Judea Sandstone Formation. The reservoir units in the Mulussa F Formation contain the highest cement volumes comprised of early siderite and kaolinite, which, with the development of compaction-resisting quartz overgrowths and resultant compactional pore volume loss, has resulted in typically lower porosity being preserved than in the Lower Rutbah and Post Judea Sandstone formations.

Porosity enhancement potential through dolomitization of carbonate reservoirs, a case of study from the Euphrates Graben fields, East Syria

The dolostones and dolomitic limestones of the lower Cretaceous Judea formation are a key target ofhydrocarbons in most of the Euphrates Graben fields. Core materials investigation, thin sections petrographicallyexaminations, and petrophysical evaluations were obtained to determine enhancement ofthe porosity through dolomitization. Results showed that the lagoon-shallow marine carbonates of theJudea formation are subdivided into two main zones;the upper “limestone zone” is micritic limestonesdominated with no reservoir potential, and the lower “dolomite zone” is dolomitic limestones anddolostones dominated with good to very good reservoir potential. Dolomitization of the mudstones andwackestones of the micritic limestones resulted in formation of microcrystalline dolomicrite and earlyfabric destructive dolomites. Conversely, dolomitization of the packstones and grainstones resulted information of the fabric destructive and saddle dolomites. Based on petrography data, dolomitization ofthe “limestone zone” is interpreted by the seawater dolomitization mechanism at low-temperatures,while dolomitization of the “dolomite zone” is interpreted by the burial dolomitization mechanismunder high temperature and pressure conditions. The “limestone zone” is characterized by the poorestreservoir quality, while the “dolomite zone” is characterized by the best reservoir quality. The seawaterdolomitization did not significantly enhance the porosity, while the burial dolomitization contributes toenhancing the preserved secondary porosity. Stylolites microfractures and dissolution seams associatedwith dolomitization played as major factors in porosity enhancement of the dolostones and dolomiticlimestones and serving as pathways for hydrocarbon migration. Dissolution processes increased theporosity and more permeability unless they are filled with the precipitated dolomite and/or calcite.Calcification had significant effects on the porosity by blocking the cavities and channels and decreasedthe effective pore volume.

Euxinia as a dominant process during OAEla(Early Aptian)on the Eastern Russian Platform and during OAElb(Early Albian)in the Middle Caspian

A detailed study of the mineral composition and microstructure of the black shales associated with OAEla(Eastern Russian Platform),OAElb(Middle Caspian),and the host rocks has been carried out using X-ray diffraction,scanning electron microscopic,and microprobe analyses.The results provide important constraints for depositional environments in the sedimentary basins.Black shales with pyrite framboids imply euxinic(sulfidic)conditions with increased organic matter preservation.Disintegrating framboids suggest partial or complete dissolution of the organic matter inside the framboids due to increasing water oxygenation.OAEla on the Eastern Russian Platform is heterogeneous as it includes thin interbeds of concretionary coccolith limestones within the interval of bituminous shales,and correlates with the Lower Aptian Rhagodiscus angustus nannofossil zone.The coccolith limestones indicate short intermittent episodes of interrupted stagnation,rapid oxygenation,and restoration of normal marine conditions.The presence of montmorillonite,albite,microcline,and diopside in the bituminous siltstones and in the host siltstones of OAEla on the Eastern Russian Platform,as well as a high content of titanium in the black siltstones correlated with OAElb in the Middle Caspian allow proposing significant input of pyroclastic material into the extant sea.