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.

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.

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.