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.

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.

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.