Diagenesis and Reservoir-Quality Evolution of Deep-Water Turbidites: Links to Basin Setting, Depositional Facies, and Sequence Stratigraphy

A study of the distribution of diagenetic alterations and their impact on reservoir-quality evolution in four deep-water turbidite successions (Cretaceous to Eocene) from basins in active (foreland) and passive margins revealed the impact of tectonic setting, depositional facies, and changes in the relative sea level. Diagenetic modifications encountered in the turbiditic sandstones from the passive margin basins include dissolution and kaolinitization (kaolin has δ18OV-SMOW = +13.3‰ to +15.2‰; δDV-SMOW = -96.6‰ to -79.6‰) of framework silicates, formation of grain coating chloritic and illitic clays, cementation by carbonates and quartz, as well as the mechanical and chemical compaction of detrital quartz. Kaolinitization, which is most extensive in the lowstand systems tracts, is attributed to meteoric-water flux during major fall in the relative sea level. Preservation of porosity and permeability in sandstones from the passive margin basins (up to 30% and 1 Darcy, respectively) is attributed to the presence of abundant rigid quartz and feldspar grains and to dissolution of carbonate cement as well as mica and feldspars. Diagenetic modifications in turbidites from the foreland basins include carbonate cementation and mechanical compaction of the abundant ductile rock fragments, which were derived from fold-thrust belts. These diagenetic alterations resulted in nearly total elimination of depositional porosity and permeability. The wide range of δ13CV−PDB values of these cements (about -18‰ to +22‰) in passive margin basins is attributed to input of dissolved carbon from various processes of organic matter alterations, including microbial methanogenesis and thermal decarboxylation of kerogen. The narrower range of δ13CV−PDB values of these cements (about -2‰ to +7‰) in the foreland basins suggests the importance of carbon derivation from the dissolution of carbonate grains. The generally wide range of δ18O values (about -17‰ to -1‰) of the carbonate cements reflect the impact of oxygen isotopic composition of the various fluid involved (including marine depositional waters, fluxed meteoric waters, evolved formation waters) and the wide ranges of precipitation temperatures. Results of this study are anticipated to have important implication for hydrocarbon exploration in deep-water turbidites from passive and active margin basins and for pre-drilling assessment of the spatial and temporal distribution of reservoir quality in such deposits

Diagenesis and Reservoir-Quality Evolution of Deep-Water Turbidites: Links to Basin Setting, Depositional Facies, and Sequence Stratigraphy

A study of the distribution of diagenetic alterations and their impact on reservoir-quality evolution in four deep-water turbidite successions (Cretaceous to Eocene) from basins in active (foreland) and passive margins revealed the impact of tectonic setting, depositional facies, and changes in the relative sea level. Diagenetic modifications encountered in the turbiditic sandstones from the passive margin basins include dissolution and kaolinitization (kaolin has δ18OV-SMOW = +13.3‰ to +15.2‰; δDV-SMOW = -96.6‰ to -79.6‰) of framework silicates, formation of grain coating chloritic and illitic clays, cementation by carbonates and quartz, as well as the mechanical and chemical compaction of detrital quartz. Kaolinitization, which is most extensive in the lowstand systems tracts, is attributed to meteoric-water flux during major fall in the relative sea level. Preservation of porosity and permeability in sandstones from the passive margin basins (up to 30% and 1 Darcy, respectively) is attributed to the presence of abundant rigid quartz and feldspar grains and to dissolution of carbonate cement as well as mica and feldspars. Diagenetic modifications in turbidites from the foreland basins include carbonate cementation and mechanical compaction of the abundant ductile rock fragments, which were derived from fold-thrust belts. These diagenetic alterations resulted in nearly total elimination of depositional porosity and permeability. The wide range of δ13CV−PDB values of these cements (about -18‰ to +22‰) in passive margin basins is attributed to input of dissolved carbon from various processes of organic matter alterations, including microbial methanogenesis and thermal decarboxylation of kerogen. The narrower range of δ13CV−PDB values of these cements (about -2‰ to +7‰) in the foreland basins suggests the importance of carbon derivation from the dissolution of carbonate grains. The generally wide range of δ18O values (about -17‰ to -1‰) of the carbonate cements reflect the impact of oxygen isotopic composition of the various fluid involved (including marine depositional waters, fluxed meteoric waters, evolved formation waters) and the wide ranges of precipitation temperatures. Results of this study are anticipated to have important implication for hydrocarbon exploration in deep-water turbidites from passive and active margin basins and for pre-drilling assessment of the spatial and temporal distribution of reservoir quality in such deposits

Diagenesis and Reservoir-Quality Evolution of Deep-Water Turbidites: Links to Basin Setting, Depositional Facies, and Sequence Stratigraphy

A study of the distribution of diagenetic alterations and their impact on reservoir-quality evolution in four deep-water turbidite successions (Cretaceous to Eocene) from basins in active (foreland) and passive margins revealed the impact of tectonic setting, depositional facies, and changes in the relative sea level. Diagenetic modifications encountered in the turbiditic sandstones from the passive margin basins include dissolution and kaolinitization (kaolin has δ18OV-SMOW = +13.3‰ to +15.2‰; δDV-SMOW = -96.6‰ to -79.6‰) of framework silicates, formation of grain coating chloritic and illitic clays, cementation by carbonates and quartz, as well as the mechanical and chemical compaction of detrital quartz. Kaolinitization, which is most extensive in the lowstand systems tracts, is attributed to meteoric-water flux during major fall in the relative sea level. Preservation of porosity and permeability in sandstones from the passive margin basins (up to 30% and 1 Darcy, respectively) is attributed to the presence of abundant rigid quartz and feldspar grains and to dissolution of carbonate cement as well as mica and feldspars. Diagenetic modifications in turbidites from the foreland basins include carbonate cementation and mechanical compaction of the abundant ductile rock fragments, which were derived from fold-thrust belts. These diagenetic alterations resulted in nearly total elimination of depositional porosity and permeability. The wide range of δ13CV−PDB values of these cements (about -18‰ to +22‰) in passive margin basins is attributed to input of dissolved carbon from various processes of organic matter alterations, including microbial methanogenesis and thermal decarboxylation of kerogen. The narrower range of δ13CV−PDB values of these cements (about -2‰ to +7‰) in the foreland basins suggests the importance of carbon derivation from the dissolution of carbonate grains. The generally wide range of δ18O values (about -17‰ to -1‰) of the carbonate cements reflect the impact of oxygen isotopic composition of the various fluid involved (including marine depositional waters, fluxed meteoric waters, evolved formation waters) and the wide ranges of precipitation temperatures. Results of this study are anticipated to have important implication for hydrocarbon exploration in deep-water turbidites from passive and active margin basins and for pre-drilling assessment of the spatial and temporal distribution of reservoir quality in such deposits

Modelling of reservoir quality in quartz-rich sandstones of the Lower Cretaceous Bentheim sandstones, Lower Saxony Basin, NW Germany

The Lower Cretaceous Bentheim sandstones of the Lower Saxony Basin in Northwest Germany (one of the main onshore oil fields of Western Europe) are mature quartz arenites in terms of texture and mineralogy. This study shows that the mineralogical maturity to a large degree is the result of diagenetic processes. Hence, the present day detrital composition and texture is not what they were at the time of deposition. The sandstones are highly porous and permeable and show presence of quartz cement as overgrowths which cement the sandstone, partly dissolved detrital feldspar grains, oversized pores caused by complete feldspar dissolution and authigenic kaolinite-dickite booklets. The porosity and permeability have been measured in the laboratory. The results were combined with the observations made by conventional optical microscopy and quantification of detrital and diagenetic components-textures by point counting, cathodoluminescense petrography, and back scattered electron imaging. These techniques have been undertaken in order to assess the impact of the presence of quartz overgrowths and feldspar dissolution on porosity and permeability. XRD analyses were done for identifying the mineralogy of the main detrital and diagenetic components, in particular for detrital and authigenic clay minerals. Quartz cement in the form of syntaxial overgrowths on detrital quartz grains played an important role in determining the petrophysical properties in the Bentheim sandstones. This kind of cement usually causes harmful reductions in porosity and permeability. However, in the Bentheim sandstones quartz cementation possibly had a positive impact in terms of porosity preservation. Cement precipitated in limited amounts and probably helped the reservoir framework to withstand the destructive effect of overburden pressure and consequent mechanical compaction. The study shows that detrital quartz grains were important, and that possibly the different types of quartz grains had different susceptibility in terms of acting as host grains for precipitation of authigenic quartz. The presence of detrital feldspar and its composition was another major parameter during diagenesis and consequent porosity-permeability modification. Dissolution of feldspars may have been an important internal (local) source of silica and aluminium for kaolinite-dickite authigenesis and possibly also for quartz cementation, and created oversized secondary pores.Popular summary: Hydrocarbons, ground water and economic minerals occur in pores (voids) between sediment grains, which later may form more solid and less porous sedimentary rocks. Thus porosity is a term that describes the abundance of these pores and can be defined as the quantity of oil, gas, water and economic minerals a rock can hold and it is expressed as percentages (%). The other important parameters in any oil, gas or water reservoir rock (mostly sedimentary rocks such as sandstones and limestone) is permeability. Permeability is the ability of a rock to transmit oil, gas and water and can depends on the interconnectedness of the pores. The more interconnected pores will lead to more efficient production of oil in oil wells and water in ground water wells. Not all sedimentary rocks are characterised by sufficient porosity and permeability. Accordingly not all rocks can be considered as good reservoirs. Each well (especially oil wells) costs huge amounts of money. When oil companies invest they naturally want to minimise the risk of not striking oil. In other words the ratio of success in striking (finding) oil is directly dependent on the amount of porosity and permeability. Exploration geologists always try to predict porosity and permeability ahead of drilling, by examining adjacent exposed bedrock. They play a profound roll in estimating the reserves (the amount of oil or gas which can be produced) and production rates in suspected oil fields. Porosity and permeability can be accounted for and evaluated through the direct and indirect parameters which control them. One of the many factors controlling porosity and permeability is cementation, i.e., the process of forming new minerals inside the pores, which leads to diminishing porosity and permeability (negative effect). In many quartz grain rich sandstones, such as the Cretaceous Bentheim sandstones and the Jurassic North Sea reservoir sandstones, the cement mineral is quartz minerals which precipitate on already-existing quartz grains. Sandstones consist of different kinds of quartz grains such as monocrystalline quartz (which consists of one single crystal) and polycrystalline quartz (which consists of more than one crystals). Different quartz types have different susceptibilities for hosting quartz cements. Hence, if we know which type of detrital quartz grain is abundant in a certain reservoir rock, so we can presuppose the degree of cementation which in its turn the degree of porosity and permeability reduction. It is believed that monocrystalline quartz grains are more susceptible for quartz cements (secondary quartz) than polycrystalline quartz. This suggestion is tested in this study. In some cases the formation of quartz cement is strengthening the framework of the reservoir rocks and thus help the entire reservoir to withstand the destruction effect of mechanical compaction (mechanical compaction is resulted from the pressure of overlying bed rocks which compresses the pores, thus leading to less pore ratios within the rocks). In this project, it is concluded that the quartz cements were precipitated in limited amounts, supporting the reservoir sand grains against negative effects of mechanical compaction. It was proved in this study that some minerals dissolve and produce new pores and largely enhance the total amount of porosity. This project stresses that multitude factors control porosity and permeability, and that each reservoir is unique and must be studied and examined separately. Hence, conclusions should not necessarily hold for all reservoirs because of the multitude of geological factors influencing porosity and permeability

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 [...

Editorial for Special Issue “Chemical, Mineralogical and Isotopic Studies of Diagenesis of Carbonate and Clastic Sediments”

Diagenesis of carbonates and clastic sediments encompasses the biochemical, mechanical and chemical changes that occur in sediments after deposition and prior to low-grade metamorphism [...

A new approach to predict carbonate lithology from well logs: A case study of the Kometan formation in northern Iraq

Understanding the spatial variation in lithology is crucial for characterizing reservoirs, as it governs the distribution of petrophysical characteristics. This study focuses on predicting the lithology of carbonate rocks (limestone, argillaceous limestone, marly limestone, and marl) within the Kometan Formation, Khabbaz Oil Field, Northern Iraq, using well logs. Precise lithology prediction was achieved by applying multivariate regression method on neutron, sonic, and density logs. Gamma-ray and elemental concentrations from bulk-rock X-ray fluorescence spectroscopy were employed to identify clay minerals, paleoenvironments, and quantify the shale content. The results indicate that the Kometan Formation predominantly comprises limestone, marl, marly limestone, and argillaceous limestone in the middle section. The middle part exhibits a higher shale content compared to the lower and upper parts. A statistically significant correlation (R2 = 0.83–0.85) between described and predicted lithology was established. The model with a higher coefficient of determination (0.85) was tested for further predictions in other wells in the Kirkuk Oil Field. This research can be valuable for lithological and petrophysical characterization of carbonate reservoirs and electrofacies analysis, particularly in situations where core data is unavailable

Geochemical and dynamic model of repeated hydrothermal injections in two mesozoic successions, provençal domain, maritime alps, se-France

A field, petrographic and geochemical study of two Triassic–Jurassic carbonate successions from the Maritime Alps, SE France, indicates that dolomitization is related to episodic fracturing and the flow of hydrothermal fluids. The mechanism governing hydrothermal fluids has been documented with the best possible spatio-temporal resolutions specifying the migration and trapping of hydrothermal fluids as a function of depth. This is rarely reported in the literature, as it requires a very wide range of disciplines from facies analysis (petrography) to very diverse and advanced chemical methods (elemental analysis, isotope geochemistry, microthermometry). In most cases, our different recognized diagenetic phases were mechanically separated on a centimetric scale and analyzed separately. The wide range of the δ18 OVPDB and87Sr/86Sr values of diagenetic carbonates reflect three main diagenetic realms, including: (1) the formation of replacive dolomites (Type I) in the eogenetic realm, (2) formation of coarse to very coarse crystalline saddle dolomites (Types II and Type III) in the shallow to deep burial mesogenetic realm, respectively, and (3) telogenetic formation of a late calcite cement (C1) in the telogenetic realm due to the uplift incursion of meteoric waters. The Triassic dolomites show a lower87 Sr/86Sr ratio (mean = 0.709125) compared to the Jurassic dolomites (mean = 0.710065). The Jurassic calcite (C1J) shows lower Sr isotopic ratios than the Triassic C1T calcite. These are probably linked to the pulses of the seafloor’s hydrothermal activity and to an increase in the continental riverine input during Late Cretaceous and Early Cenozoic times. This study adds a new insight into the burial diagenetic conditions during multiple hydrothermal flow events.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Hydrothermal carbonate mineralization, calcretization, and microbial diagenesis associated with multiple sedimentary phases in the upper cretaceous bekhme formation, Kurdistan region Iraq

Hydrothermal diagenesis during the Zagros Orogeny produced three phases of saddle dolomites (SD1, SD2, and SD3) and two phases of blocky calcites (CI and CII) in the studied sections of Bekhme Formation (Fm) (Campanian–Maastrichtian). Field observations, as well as petrographic, cathodoluminescence (CL), Scanning Elecron Microscope (SEM), and oxygen–carbon isotope analyses, indicated that the unit went through multiple submergence–emergence phases after or during hydrothermal diagenesis. These phases resulted in a characteristic calcretized 2–6-m-thick layer within the Bekhme Fm. Several pedogenic textures (e.g. alveolar, pisolite, and laminar fabric microfeatures) were observed. Strong evidence of microbial alteration and diagenesis in this formation brings new insights into its depositional history. The microbial activities developed on the original mineral surface were associated with a great variety of processes including dissolution, re-precipitation, replacement, open-space fillings, microporosity development, grain bridging, and micritization. Probable oxalate pseudomorphs embedded in these fabrics and regular filaments preserved along crystal boundaries suggest the activity of fungi, while frequent coccoidal, rod-like, and chain-like forms attached to the surfaces of dolomitic and calcitic crystals point to bacterial colonization. Extracellular polymeric substance (EPS) was often visible with fungal and bacterial forms. These features, together with stable isotope data, invoke that near-surface conditions occurred sporadically in the Bekhme Fm after the first generation of hydrothermal dolomitization. These new findings allow recognition of unreported sedimentological phases based on new evidence in the Spelek–Sulauk area during the Upper Cretaceous.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Origin of Drusy Dolomite Cement in Permo-Triassic Dolostones, Northern United Arab Emirates

While the characteristics and origin of drusy calcite cement in carbonate deposits is well constrained in the literature, little attention is paid to drusy dolomite cement. Petrographic observations, stable isotopes, and fluid-inclusion microthermometry suggest that drusy dolomite cement in Permo-Triassic conglomerate/breccia dolostone beds in northern United Arab Emirates has precipitated as cement and not by dolomitization of drusy calcite cement. The low δ18OVPDB (−9.4‰ to −6.2‰) and high homogenization temperatures of fluid inclusions in drusy dolomite (Th = 73–233 °C) suggest that dolomitization was caused by hot basinal brines (salinity = 23.4 wt% NaCl eq.). The δ13CVPDB values (+0.18‰ to +1.6‰) and 87Sr/86Sr ratio (0.708106 to 0.708147) indicate that carbon and strontium were derived from the host marine Permo-Triassic carbonates. Following this dolomitization event, blocky calcite (Th = 148 °C; salinity = 20.8 wt% NaCl eq.) precipitated from the hot basinal brines. Unravelling the origin of drusy dolomite cement has important implications for accurate construction of paragenetic sequences in carbonate rocks and decipher the origin and chemistry of diagenetic waters in sedimentary basins