Effects of Diagenetic Alterations on Hydrocarbon Reservoirs and Water Aquifers

Diagenesis includes all the biological, physical, chemical, biochemical, and physicochemical alterations that occur immediately after deposition and prior to low-grade metamorphism. These processes exert direct control on the hydrocarbon reservoir quality and on the hydraulic properties of groundwater aquifers [1]. As stated by Ahr (2008), “Reservoirs and aquifers differ only in the fluids they contain” [2]. Thus, methods to assess the geological storage and transmission capacity of hydrocarbon reservoirs are applicable to groundwater aquifers [3]. Overall, diagenetic alterations are, directly or indirectly, driven and mediated by fluid flows in sedimentary basins that determine the porosity and permeability evolution of carbonate and sandstone reservoirs/aquifers. The pervasiveness and extent of cementation in any reservoir/aquifer are determined by the rate and volume of fluids percolating through these sediments, temperature and geochemical conditions, and the time available for cementation [4]. Thus, publishing papers dealing with diagenesis and reservoir quality evolution in MDPI’s Water is highly relevant and a welcome approach.

Moreover, reservoir/aquifer layers (carbonates and sandstones) experience several phases of diagenesis, which are related to the tectonic history of the basin. Episodes of subsidence and uplift may induce dramatic changes in the pressure–temperature realms and in formation water chemistry that are reflected in various phases of mechanical and chemical compactions as well as mineral dissolution, transformation, and cementation [1,5]. Diagenesis may result in deterioration through mechanical and chemical compaction, preservation through the prevention of mechanical compaction and cementation, or enhancement due to the dissolution of allochems (framework grains) and intergranular cements [2,6,7,8,9].

This Special Issue, “Effects of Diagenetic Alterations on Hydrocarbon Reservoirs and Water Aquifers”, is a collection of five papers that document new geological, geochemical, and sedimentological data from basins in the Middle East, Canada, and China.

The paper by Salih et al. [10], entitled “Hydrothermal Fluids and Cold Meteoric Waters along Tectonic-Controlled Open Spaces in Upper Cretaceous Carbonate Rocks, NE-Iraq: Scanning Data from In Situ U-Pb Geochronology and Microthermometry”, provides petrographic and geochemical data on Upper Cretaceous carbonate rocks in NE Iraq. These authors documented the role of high-temperature and hydrothermal brines in the precipitation of saddle dolomite cement within the fractures. The in situ U-Pb dating method was used to date the age of the saddle dolomites, and it linked their dating to the tectonic evolution of the Zagros Foreland Basin.

The second paper by Salih et al. [11], entitled “Tracking the Origin and Evolution of Diagenetic Fluids of Upper Jurassic Carbonate Rocks in the Zagros Thrust Fold Belt, NE-Iraq”, studied the evolution of diagenetic fluids from the Upper Jurassic Barsarin Formation in the Zagros Foreland Basin in Iraqi Kurdistan. The study was based on fieldwork, petrography, carbon and oxygen isotopes, and in situ strontium isotope ratios using high-resolution laser ablation ICP-MS. The study provides a detailed account of the evolution of different fluids during the diagenesis of the Upper Jurassic Barsarin Formation and how the fluids shifted from the original chemistry and affected the host carbonate rocks. The study also provides insights into the dynamics of the diagenetic fluids during the Zagros orogeny within the formation of the Zagros Foreland Basin.

The paper by Al-Aasm et al. [12], entitled “Dolomitization of Paleozoic Successions, Huron Domain of Southern Ontario, Canada: Fluid Flow and Dolomite Evolution”, integrated petrographic, isotopic, fluid inclusion microthermometry, and geochemical analyses of Paleozoic carbonate successions from multiple boreholes within the Huron Domain, Southern Ontario, in order to shed light on the nature of the diagenetic alterations and the chemical composition of the fluids responsible for causing dolomitization on a regional scale. This study also evaluated the nature and origin of dolomitized beds in the successions studied. Detailed geochemical analyses of, for example, major (Ca, Mg), minor, trace (Sr, Na, Mn, and Fe), and rare-earth elements (REEs), along with the use of an environmental scanning electron microscope (ESEM) coupled with an EDAX detector, were utilized to analyze selected carbon-coated thin-section samples to investigate the nature of the dolomitization and subsequent recrystallization of the precursor dolomite matrix.

The paper by Mansurbeg et al. [13] addresses the controversies regarding the nature and origin of drusy mosaic dolomite cement, which commonly occludes pores in carbonate reservoirs. The study sheds light on the nature and origin of the drusy dolomite in terms of whether it has been directly precipitated as cement or formed by the replacement of precursor drusy calcite. The study unravels the role of diagenetic paleofluids in forming drusy mosaic dolomite cement and has implications for accurate construction of paragenetic sequences and related reservoir quality features.

The most important diagenetic alteration in quartz-rich sandstones is the precipitation of syntaxial quartz overgrowths around detrital quartz grains. Deeply buried sandstone reservoirs lose primary porosity through the process of quartz cementation. In the voluminous literature on quartz cementation in sandstone reservoirs, the source of silica needed for quartz cementation is uncertain, and the issue is strongly debated among researchers. The paper by Ren et al. [14] used high-precision secondary ion mass spectrometry (SIMS) for micrometer-sized quartz cement in order to obtain oxygen isotopes. The obtained δ¹⁸O values coupled with petrographic, cathodoluminescence (CL), and fluid inclusion data were used to constrain the origin of the silica in quartz cement in the deep-buried Upper Triassic tight sandstones in the western Sichuan Basin, China.

There is mounting evidence of the presence of unextracted conventional and unconventional hydrocarbon reserves and untapped groundwater aquifers in deeply buried and complex diagenetic regimes and tectonic settings. The current predictive tools and concepts to predict reservoir/aquifer quality in petroleum and groundwater exploration have limitations. Further studies are necessary for the development of multidisciplinary diagenetic models at the basin scale coupled with geological processes operating on the reservoirs after deposition. These studies might contribute to a better understanding of the parameters controlling reservoir/aquifer quality evolution and help in developing more accurate predictive tools.