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.