Ferroan Calcite Research Articles

Formation mechanism of carbonate cement in tight sandstone reservoirs in the depression zone of foreland basin and its impact on reservoir heterogeneity: The upper Triassic Xujiahe Formation in Western Sichuan foreland basin, China

The carbonate cements in tight sandstone reservoirs are diverse, complex, and important determinants of reservoir quality. This study provides a comprehensive analysis of the characteristics and formation mechanism of carbonate cement in the tight sandstone reservoirs of the Xu3 Member within the Xujiahe Formation in the depression zone of the Western Sichuan foreland basin. Core samples were examined on thin sections using cathodoluminescence and scanning electron microscopy, fluid inclusion analysis, in situ carbon and oxygen stable isotopes, and rare earth element analysis. The study reveals that the Xu3 Member primarily comprises sublitharenite and litharenite, which has three stages of carbonate cementation within the reservoirs. On average, the reservoirs exhibit a porosity of 3.82% and a permeability of 0.1mD. In the eodiagenetic stage, calcite and dolomite (δ13CPDB = 0‰ to −4.07‰; δ18OPDB = −5.64‰ to −12.69‰) formed, filling both intergranular pores and intragranular dissolution pores. This process is driven by the expulsion of organic acids through compaction from the clay and coal-bearing sections during shallow burial. The carbon sources originate from external organic acids and internal carbonate rock fragments, which combine with Ca2+ and Mg2+ ions released from the dissolution of carbonate rock fragments. During the mesodiagenetic stage A, the predominant formation of ferroan calcite and ankerite (δ13CPDB = −5.98‰ to −7.64‰; δ18OPDB = −14.47‰ to −19.87‰) occurred, filling intergranular residual pores and secondary intragranular dissolution pores. This phenomenon is caused by the expulsion of organic acids formed by the compaction of matured organic matter at greater burial depths. The carbon sources primarily originate from external organic acids and combine with Ca2+, Mg2+, and Fe2+ ions in the sandstone, alongside accompanying organic acids. During the mesodiagenetic stage B, coarse-macro crystalline calcites (δ13CPDB = −1.06‰ to −1.64‰; δ18OPDB = −17.95‰ to −20.36‰) with significant positive Eu anomaly-filled fractures were formed. These calcites are associated with hydrothermal fluids in the deep subsurface, precipitating directly in the fractures due to the presence of hot hydrothermal fluids rich in Ca2+ and CO32−. The Xu3 Member displays two sandstone superimposed types. In the mudstone overlain by sandstone type, carbonate cement is mainly present in the middle of the cycle, and reservoir quality is relatively high at both the bottom and top of the cycle. In the sandstone overlain by sandstone type, carbonate cement is predominantly found in the middle and top sections of the cycle, with higher-quality reservoirs primarily forming at the bottom of the cycle. It resulted in significant heterogeneity observed in the tight sandstone reservoirs of the depression zone in the Western Sichuan foreland basin.

Reservoir quality of Upper Cretaceous limestones (Ahlen-Fm., Beckum Member, Münsterland Cretaceous Basin): effects of cementation and compaction on the compactable depositional volume

AbstractThe Upper Cretaceous limestones unconformably overlie Upper Carboniferous coal-bearing lithologies and are studied to assess their effect on rising mine-water levels in the Ruhr mining district. Upper Cretaceous sedimentary rocks from the Münsterland Cretaceous Basin have previously been studied regarding their sedimentary structures and fossil content. However, understanding the petrophysical and petrographic heterogeneity in regard to sedimentary properties and their effect on fluid migration pathways is yet missing. Utilizing He-pycnometry, Klinkenberg-corrected air permeabilities, p-wave velocities, transmitted and reflected light analyses, point-counting and cathodoluminescence, we assess the petrophysical, geomechanical and mineralogical properties. Porosity ranges from 1.0 to 18.7% and permeability ranges from < 0.0001 to 0.2 mD, while p-wave velocity ranges between 2089 and 5843 m/s. Mechanical compaction leads to grain rearrangement, deformation of calcispheres, foraminifera and ductile clay mineral laminae. Above and below clay laminae, compaction bands of deformed calcispheres develop. Early diagenetic mineral precipitation of ferroan calcite in inter- and intragranular pores reduces porosity and permeability and influences geomechanical properties. An underestimated aspect of limestone petrography is the relationship of the original primary compactable depositional volume and the influence of compaction, deformation and cementation during early and late diagenesis on reservoir properties. The detrital dominated limestones show an originally high compactable depositional volume (CDV). Overall, reservoir qualities are poor and indicate the sealing potential of the studied lithologies. The Upper Cretaceous (Campanian) limestones thus may act as a barrier for increasing mine-water levels from dismantled, post-mining subsurface hard coal mines in the region. Graphical abstract

Diagenetic Impact on High-Pressure High-Temperature Reservoirs in Deep-Water Submarine Fan Sandstone of Qiongdongnan Basin, South China Sea

The diagenetic evolution of sandstone is very complicated under the conditions of high temperatures and pressures in deep-water, deep-buried regimes, which have great influence on reservoir quality. This study investigates the typical reservoir target of Neogene deep-water, submarine-fan sandstones under high-temperature, high-pressure regimes in the Qiongdongnan Basin, South China Sea. Utilizing a thin section, scanning electron microscope (SEM), mineral geochemistry combined with burial history evolution, complex diagenetic events, and main controlling factors of the sandstone in the Neogene Meishan Formation were determined. The results show that the evolution of sandstone reservoirs is initially controlled by depositional framework compositions and subsequently modified by eogenetic and mesogenetic alterations during progressive burial. Eogenetic alterations mainly include the following: (1) mechanical compaction; (2) dissolution of feldspar; (3) low-Fe calcite cementation. Mesogenetic events were identified as the following: (1) dissolution of feldspar; (2) ferroan calcite and ankerite formation; (3) precipitation of quartz and clay mineral. Mechanical compaction is greatly influenced by the original depositional framework composition, and sandstone samples enriched in high contents of detrital clay matrix always experienced extensive mechanical compaction. Different phases of carbonate cement during different diagenetic regimes lead to continuous destruction on reservoir porosity. The dissolution of unstable feldspar minerals during eogenetic and mesogenetic environments leads to the development of secondary porosities and would enhance the quality of the reservoir. Overpressure formation is pervasively developed owing to early disequilibrium compaction and subsequent natural gas charging. Only well-sorted sandstones with low contents of detrital clay matrix could resist early mechanical compaction, lead to ample residual original porosities, and then undergo extensive mineral dissolution to generate sufficient secondary porosities. Subsequently, these porosities would be effectively protected by overpressure formation. Poor-sorted sandstones with high contents of detrital clay matrix would experience strong mechanical compaction and extensive destruction of original porosities. Thus, these sandstones are difficult to have significant dissolution and are unable to be effectively protected by overpressure formation. Therefore, the interplay between the original framework composition and the corresponding diagenetic pathways coupled with overpressure formation would result in strong reservoir heterogeneity for the deep-buried sandstones during progressive burial.

Diagenetic imprints in the reservoirs of Agbada Formation, Niger Delta Basin: Implication for reservoir quality and appraisal of calcite cement

The main exploration targets for hydrocarbons in the Niger Delta Basin are within the Agbada sandstone reservoirs. The current study utilizes a multitechnical approach (SEM-MLA, SEM-EDS, XRD and LA-ICP-MS) to evaluate the roles of framework composition and diagenetic processes, particularly calcite cementation on reservoir quality in Miocene intervals of the Agbada Formation sandstone, Niger Delta Basin. Petrographic analysis reveals that the sandstones are feldspatho-quartzose to quartzose sandstones, with a preponderance of quartzose sandstones. The overall diagenetic imprints suggest minimal compaction and lack of pervasive cement, which have contributed significantly to the high porosity. Although the degree of mechanical compaction is generally low, it has comparatively exercised a better control on porosity than the growth of cement, excluding some intervals with considerable cementation (siderite and ferroan calcite) where compactional porosity-loss was minimal. The occurrence of ferroan calcite cement (CaCO3 = 95.9 ± 0.4, MgCO3 = 0.8 ± 0.1, FeCO3 = 2.9 ± 0.2 mol%) is generally small, probably discontinuous and unlikely to represent significant barriers to fluid flow with their localized influence, but understanding their locally significant influence may still contribute to building good reservoir models. Petrographic observations suggest that the precipitation of “poikilitic” calcite was early, but the shale normalized REY patterns do not mimic seawater patterns (Y/Ho ratios; 36.3 ± 1.6) due to possible contributions from clastic or detrital grains and/or incorporation of particulate matter that can preferentially scavenge LREEs from the overlying water column. A possible concern during reservoir production is the occurrence of clay mineral matrix (mostly kaolinitic) anticipated to cause the production of fines and formation damage, but this can be mitigated with proper reservoir management.

Diagenesis of the Marinoan cap dolostone, Southern Amazon Craton: An unconventional petroleum system in the evolution of the Araras-Alto Paraguai Basin

The post-snowball Earth world favors the development of hydrocarbon source rock intimately linked to periods of post-glacial marine transgression generating important potential petroleum plays. In the Southern Amazon Craton, hydrocarbons occur in one of the most spectacular examples of post-Marinoan cap carbonate, related to the mid-Ediacaran to Early Cambrian (c. Post 635 Ma) interval associated with the early stabilization of the Gondwana Supercontinent. The Puga cap carbonate consists of the basal portion of the Araras Group exposed in the Araras Alto-Paraguai Basin inserted as a reservoir in an unconventional petroleum system. The cap dolostone overlies diamictites and is overlaid by bituminous limestone and shales (source-rock), and dolostone in the top of a 700 m-thick depositional sequence. The cap dolostone has a diachronic relation with the limestone beds typifying as a secondary reservoir characterized by bitumen-filled fenestral and interpeloidal porosity. The upper dolostone has a closed framework and acts as sealing rocks. This study aims to establish paragenesis sequence and decipher its control on the quality of the cap dolostone as a microbialites-dominated reservoir. Drill core and outcrop samples were analyzed using microfacies, scanning electron microscopy, and cathodoluminescence to unravel the diagenetic history. Aftermath, the Marinoan events favored dolomuds precipitation in the shallow platforms, preserving primary porosity and permeability reduced by early gypsum cementation during oversaturation events. Other destructive processes, such as mechanical and chemical compaction (e.g., stylolites) and authigenesis of pyrite and low-Mg ferroan calcite cementation, occurred during burial and mesodiagenesis. The low permeability restricts hydrocarbon migration concentrated in the upper cap dolostone, and the early cementation increases the potential for preserving these carbonate deposits. Hydrocarbon maturation was triggered during the burial and uplift of the Araras-Alto Paraguai basin. The cementation phases described here enhance our understanding of the mechanisms behind syndepositional diagenesis in microbially-induced reservoirs. This study uses proxies associated with diagenetic studies to quantify the quality of microbially induced dolostone reservoir linked to the evolution of the Neoproterozoic unconventional petroleum system. In addition, this conception will promote a guide for future exploration and understanding of petroleum plays and suggests a possible Precambrian source rocks component of traditional Phanerozoic Petroleum systems in the central part of Brazil.

Fluid evolution and related fluid–rock interactions of the Oligocene Zhuhai sandstones in the Baiyun Sag, northern margin of the South China Sea

Pore fluids control the diagenetic processes and storage spaces of deep clastic rock reservoirs and have become a major area of interest within the fields of sedimentology and petroleum geology. This paper aims to relate the diagenetic processes of the Oligocene Zhuhai sandstones in the Baiyun Sag to pore fluids varying with burial depth. The types and distribution patterns of authigenic minerals are investigated through analysis of petrographic, mineralogical, and geochemical features to illustrate the origin and flow patterns of pore fluids and their influences on reservoir diagenesis. Strong cementation of eogenetic carbonate cement near the sandstone–mudstone interface was a consequence of material migration from adjacent mudstones on a large scale. The pore fluids were mainly affected by microbial methanogenesis and carbonate mineral dissolution in adjacent mudstones during eogenesis. The pore fluids were diffusively transported in a relatively open geochemical system within a local range. Support for this model is provided by the heavier stable isotopic values present in eogenetic calcite and dolomite. Feldspar dissolution during early mesogenesis was spatially accompanied by the precipitation of authigenic quartz and ferroan carbonate cement. Pore fluids in this period were rich in organic acids and CO2, and their migration mechanism was diffusive transport. The obviously lighter carbon and oxygen isotopic compositions of the ferroan calcite support this inference. During late mesogenesis, the input of deep hydrothermal fluid might have been partly responsible for the precipitation of ankerite, barite and authigenic albite. Oil charging may have inhibited carbonate cementation and compaction, accordingly preserving porosity, and together with authigenic kaolinite, might have promoted the transition of the reservoir from water wet to oil wet to the benefit of oil entrapment. The findings reported here shed new light on the evaluation and prediction of sandstone reservoirs that have experienced multiple periods of fluid flow.

Paleoenvironment and diagenetic aspects of the limestone (Gboko Formation) around Igumale, Central Benue Trough, Nigeria: Evidence from petrography

Limestone core samples from Igumale-1 and Igumale-2 wells, Central Benue Trough, Nigeria were petrographically studied in detail to determine their paleo-depositional environment and diagenetic history. Petrographic examination of the limestone shows that the limestone comprised monocrystalline quartz, ferroan calcite (3-15 %), peloids (3-20 %), ooids (7-10 %), micrite (33-48 %) and sparites (10-50 %). Fossil fragments consist of foraminifera and gastropods (17-57%), and suggest subtidal to shallow marine environment. Four sedimentary facies (oobiosparite, biosparite, biopelmicrite, and biomicrite) were recorded and characterised by moldic and vuggy porosity. This study has provided a petrographic synopsis of the limestone for further consideration and may impact both the exploration and development of the limestone deposit.

Carbon and oxygen isotope characteristics of the Clydach Valley Subgroup, Courceyan, South Wales‐Mendip shelf, UK

AbstractThe Clydach Valley Subgroup (Courceyan) records a dip section through an Early Carboniferous shallow marine, carbonate shelf and consists of three oolitic formations separated by paludal/peritidal units with abundant evidence of subaerial exposure in proximal areas. The lower part correlates with the Kinderhookian–Osagean Boundary Excursion, with allochem data indicating a minimum δ13C value for marine carbonate of +4.5‰, with associated δ18O of −4.6‰. Marine carbonate δ13C and δ18O values of +2‰ and −2‰, respectively, were estimated for the younger part. Allochem isotopic data yield a well‐defined mixing line consistent with stabilisation in meteoric water at varied water: rock ratios, the degree of stabilisation increasing up‐dip and up‐stratigraphy. Two distinct diagenetic styles closely correlate with evidence for the presence (Diagenetic Regime 1) or absence (Diagenetic Regime 2) of subaerial exposure, non‐ferroan and ferroan calcite cement dominating respectively. Five cement zones (Zones 2–6) defined by iron content occur in Diagenetic Regime 1, irrespective of formation, indicating a similar sequence of palaeohydrological changes affected repeated depositional cycles. Zones 3–5 are considered meteoric based on their isotopic composition (δ18O: −5.8 to −11.1‰ and δ13C: −3.7 to −6.2‰) and form distinct clusters dependent on zone, age and location. Pedogenic carbonates and meteoric cements record a long term increase in meteoric δ18O values: −7.8‰ during the Kinderhookian–Osagean Boundary Excursion, −6.9‰ during diagenesis of the upper part of the subgroup and −6.3‰ associated with initial deposition of the overlying Llanelly Formation. This is consistent with global sea water trends, but an element of climate change cannot be ruled out. Increases in cement δ18O values as meteoric systems become established (Zone 3–4) indicate repeated short term variation in rainwater composition probably driven by climate change, but also suggests a link between climate and the depositional cycle.

Fluid flow and water/rock interaction during the Early Triassic evolution of the western Canada sedimentary basin as revealed by carbonate diagenesis

Diagenetic evolution of sedimentary basins, which is crucial to resource exploration and development, and carbon capture, utilization, and storage (CCUS), is mainly controlled by fluid influx associated with the regional tectonic and hydrogeologic setting of these basins. Therefore, petrographic and isotope geochemical studies integrated with geodynamics of sedimentary basins, fluid flow and transport hydrodynamics are insightful tools to reconstruct the regional history of paleofluid flow and their driving mechanisms. This study investigates carbonate diagenesis in the Lower Triassic Montney Formation of the Western Canadian Sedimentary Basin (WCSB) to constrain the origin of diagenetic fluids, their major flow systems, and their association with the tectonics and hydrogeology of the WCSB. Petrographic analysis shows various generations of early-to late-stage non-ferroan to ferroan calcite (C1–C3) and dolomite (D1-D4) cement. Bulk δ13C and δ18O values of carbonate cements range from −7.1 to +0.4‰ and −9.5 to −2.8‰ (VPDB), respectively, noticeably more depleted than those estimated for Early Triassic seawater. Their 87Sr/86Sr isotope ratio, 0.7108 to 0.7128, is significantly more radiogenic than Early Triassic seawater. These isotopic values confirm the extensive interaction of the Montney Formation pore water with hot basinal brines of Precambrian metasediments and siliciclastic strata, the influx of hydrothermal fluids, and high burial temperature in the basin. The 13C-depleted signature of these cements supports the contribution of organic carbon, from the breakdown and oxidation of organic matter and hydrocarbons via microbial and thermochemical sulfate reduction processes. The results of this study suggest that Montney Formation diagenesis was influenced by evolved Montney pore water interacting with fluids sourced from metasediments of the Cordillera thrust belt and topography-driven fluids from Columbian and Laramide tectonism. This study highlights the importance of an integrated approach to better understand changes in fluid compositions during diagenesis and reconstruction of basin-scale paleofluid flow systems in sedimentary basins.

Salt dissolution tectonism and origin of lacustrine carbonate beds: Mn‐Fe‐calcite and Mn‐siderite micro‐spherulite fabrics of the Lower Cretaceous McMurray Formation, Athabasca Oil Sands deposit, western Canada

AbstractSeveral lacustrine carbonate beds, each a metre‐thick interval of densely packed Mn‐rich sideritic micro‐spherulites or Mn‐rich ferroan calcite micro‐spherulites, are recorded for the first time within strata of the Lower Cretaceous McMurray Formation of the northern Athabasca Oil Sands deposit, western Canada. A lower McMurray lacustrine carbonate deposit is characterised by a metre‐thick bed fabric of Mn‐rich siderite micro‐spherulites. The middle and upper interval McMurray beds developed fabrics of Mn2+‐rich ferroan calcite micro‐spherulites. These carbonate beds represent saline lacustrine depositional environments that resulted from the lake bottom sediments ingressed from below by Mn2+‐Fe2+‐rich carbonate‐saturated brines. These up‐section migrations of Devonian formation water were sourced from dissolution trends developed in limestone and halite‐anhydrite beds of the underlying Devonian Prairie Evaporite during Cordilleran deformation of the Alberta Basin foreland. These brines ascended to the overlying McMurray Formation sediments along dissolution‐collapse structures such as breccia pipes, sinkholes and margins of differentially subsided Upper Devonian fault blocks. The up‐section migration of a sulphate‐saturated Fe2+ and Mn2+‐rich brine resulted in the ingress of a lower McMurray lacustrine bottom sediment at a site associated with the development of a peat mire terrain. Microbial redox of the lake bottom sediment resulted in a carbonate bed of micro‐spherulitic fabrics of Mn‐rich siderite interwoven with pyrite laminae. Subsequent salt dissolution events and up‐section migrations of Devonian brine during deposition of the middle and upper McMurray intervals resulted in similar carbonate‐saturated but sulphate‐poor chemistry. These saline flows also ingressed lacustrine bottom sediments below, and resulted in limestone beds of densely packed spherulitic fabrics of Mn‐ferroan calcite, not siderite. These deposits provide insight into largely unknown dispositions of voluminous brine resulting from salt dissolution trends below the Athabasca Oil Sands and further our understanding of controversial McMurray depositional processes.

Palaeozoic stromatoporoid diagenesis: a synthesis

Palaeozoic stromatoporoids, throughout their 100-million + year history (Middle Ordovician to Late Devonian and rare Carboniferous), are better preserved than originally aragonite molluscs, but less well-preserved than low magnesium-calcite brachiopods, bryozoans, trilobites and corals. However, the original mineralogy of stromatoporoids remains unresolved, and details of their diagenesis are patchy. This study of approximately 2000 stromatoporoids and the literature recognises three diagenetic stages, applicable throughout their geological history. Timing of processes may vary in and between stages; some components are not always present. Stage 1, on or just below sediment surface, comprises the following: micrite filling of upper gallery space after death, then filling of any remaining space by non-ferroan then ferroan calcite in decreasing oxygen of pore-waters; partial lithification of associated sediment from which stromatoporoids may be exhumed and redeposited, evidence of general early lithification of middle Palaeozoic shallow-marine carbonates; microdolomite formation, with the Mg interpreted to have been derived from original high-Mg calcite (HMC) mineralogy (likely overlaps Stage 2). Stage 2, short distance below sediment surface, comprising the following: fabric-retentive recrystallisation (FRR) of stromatoporoid skeletons forming fabric-retentive irregular calcite (FRIC), mostly orientated normal to growth layers, best seen in cross-polarised light. FRIC stops at stromatoporoid margins in contact with sediment and bioclasts. FRIC geometry varies, indicating some taxonomic control. Evidence that FRIC formed early in diagenetic history includes syntaxial continuation of FRIC into some sub-stromatoporoid cavities (Type 1 cement), although others were pre-occupied by early cement fills (Type 2 cement) formed before FRR, preventing syntaxial continuation of FRIC into cavities. Likely contemporaneous with FRIC formation, stromatoporoids in argillaceous micrites drew carbonate from adjacent sediment during reorganisation of argillaceous micrite into limestone–marl rhythms that are also early diagenetic. Stage 3, largely shallow burial, comprises the following: dissolution and silicification, but these may have occurred earlier in stromatoporoid diagenetic histories (more data required); burial pressure dissolution forming stylolites.

Influence of multiphase carbonate cementations on the Eocene delta sandstones of the Bohai Bay Basin, China

This study deciphers the origin and distribution of multiphase carbonate cementations and assesses their implications for reservoir quality of the Eocene delta hydrocarbon sandstones in the Bohai Bay Basin. Petrographic observation and stable isotopic analysis identified two phases of carbonate cementations: (1) eodiagenetic phase including non-ferroan calcite cement precipitated at 25–42 °C (δ13CVPDB: 1.0‰ to +5.7‰ and δ18OVPDB: 10.7‰ to −7.5‰); and (2) mesodiagenetic phase which includes ferroan calcite cement (δ13CVPDB: 11.8‰ to −5.7‰ and δ18OVPDB: 17.0‰ to −12.6‰) and ankerite cement (δ13CVPDB: 13.7‰ to −5.9‰ and δ18OVPDB: 17.1‰ to −12.4‰) precipitated at 90–134 °C. The eodiagenetic non-ferroan calcite cement that is mainly sourced from microbial methanogenesis of organic matter in adjoining mudrocks, extensively cemented at or near sandstone-mudstone contact. Mesodiagenetic ferroan carbonate cements are derived from predominantly thermal decarboxylation of organic acids and subordinate dissolution of eodiagenetic calcite cements in sandstones. These ferroan carbonates are preferentially concentrated at distances of 0.5–1.0 m from sandstone-mudstone contacts. Kinetic modelling reproduces evolution pathways of different diagenetic minerals in the Es4s sandstones (the upper part of the fourth member of the Eocene Shahejie Formation). More importantly, results of modelling reveal that the dissolution of non-ferroan calcite cements would not enhance total reservoir quality due to the precipitation of more stable ferroan calcite and ankerite cements. Therefore, the occurrence of multi-phase carbonate cementations at different sections of sandstones leads to significant destruction of porosity and reservoir heterogeneity during progressive diagenesis.

Carbonate cementation patterns and diagenetic reservoir facies of the triassic Yanchang Formation deep-water sandstone in the huangling area of the Ordos Basin, northwest China

The Chang 6 member sandstone is one of the main oil reservoirs of the Ordos basin, China, and consists of a lacustrine gravity flow deposited in Huangling area. Carbonate cement is volumetrically the most important diagenetic parameter controlling reservoir quality in the sandstones.Three generations of carbonate cements were recognized. The massive microcrystalline calcite, poikilotopic calcite and microcrystalline dolomite from the supersaturated alkaline pore water precipitate with basement cements at an early diagenesis stage. The ferroan calcite is mainly pore-filling and replacement cementation in the middle diagenesis stage, and ferroan dolomite is pore-filling cementation in the late diagenesis stage.The four types of reservoir facies and the distribution patterns were identified in the Chang 6 member sandstones. (1) Massive carbonate cemented facies with high contents of carbonate cements are distributed along the edges of the sandstone bodies, which are in the channels of the turbidite and sandy debris flow deposits. (2) Partial carbonate cemented facies occur along the margins of the massive carbonate cemented facies. (3) The porous facies with the lowest percentages of carbonate cements in sandstone are distributed in the middle of the sand-bodies of the sandy debris flow deposits. (4) High muddy debris facies that have high percentages of carbonate-cemented and high muddy debris sandstones occur essentially in the proximal overbank and lobe of the turbidite deposits.The reservoir quality of sandstones depended upon time of precipitation of carbonate cements on and diagenetic processes. Massive carbonate cements and the high muddy debris of sandstones undergo lots of carbonate cements and strong compaction in the early diagenesis stage, respectively. There was weak water rock reaction of dissolution in the later diagenesis process. Partial carbonate cemented facies and the porous facies of sandstones have sufficient water rock reaction and more dissolution pores.

Diagenetic history and reservoir evolution of tight sandstones in the second member of the Upper Triassic Xujiahe Formation, western Sichuan Basin, China

The second member of the Upper Triassic Xujiahe Formation is an important target for tight gas exploration in the western Sichuan Basin. The diagenetic history and reservoir evolution were investigated using integrated analyses of petrography, X-ray diffraction (XRD), scanning electron microscopy (SEM), cathodoluminescence (CL), fluid inclusion microthermometry and stable carbon and oxygen isotopes. The sandstones consist mainly of litharenites, sublitharenites and feldsparthic litharenites with fine-to medium-sand grain sizes, moderate to good sorting and subangular to subrounded roundness. The reservoir properties of the sandstones are generally poor with low porosity and matrix permeability, small pore-throat radii, and high displacement pressure. Eodiagenesis is composed mainly of mechanical compaction, precipitation of siderite, stage-I quartz overgrowths, chlorite, calcite and ferroan calcite. Mesodiagenesis consists mainly of mechanical and chemical compaction, calcite and ferroan calcite precipitation, stage-II and stage-III quartz cementation, feldspar dissolution and precipitation of illite, dolomite and ankerite. Three types of sandstone lithofacies are defined according to sandstone textures and composition, diagenetic minerals, grain sizes, sorting and pore types: lithofacies I, II and III. Compaction appears to reduce the reservoir quality more significantly compared to cementation. Authigenic chlorite coats were constructive in preserving primary porosity, while carbonate cementation decreased reservoir quality. Diagenetic evolution pathways and reservoir quality prediction models for the Upper Triassic Xujiahe Formation sandstones have been proposed to shed light on the different reservoir quality evolution patterns of various sandstone lithofacies. Lithofacies I ─ medium and coarse-grained sandstones characterized by extensive chlorite coating, feldspar dissolution and a low content of clay matrix and cement, appears to have the best reservoir properties and may serve as a high-quality reservoir facies in these tight sandstone reservoirs.

Porosity Development Controlled by Deep-Burial Diagenetic Process in Lacustrine Sandstones Deposited in a Back-Arc Basin (Makó Trough, Pannonian Basin, Hungary)

Deeply buried Pannonian (Upper Miocene) siliciclastic deposits show evidence of secondary porosity development via dissolution processes at a late stage of diagenesis. This is demonstrated by detailed petrographic (optical, cathodoluminescence, fluorescence, and scanning electron microscopy) as well as elemental and stable isotope geochemical investigations of lacustrine deposits from the Makó Trough, the deepest depression within the extensional Pannonian back-arc basin. The analyses were carried out on core samples from six wells located in various positions from centre to margins of the trough. The paragenetic sequence of three formations was reconstructed with special emphasis on sandstone beds in a depth interval between ca 2700 and 5500 m. The three formations consist, from bottom to top, of (1) open-water marls of the Endrőd Formation, which is a hydrocarbon source rock with locally derived coarse clastics and (2) a confined and (3) an unconfined turbidite system (respectively, the Szolnok and the Algyő Formation). In the sandstones, detrital grains consist of quartz, feldspar, and mica, as well as sedimentary and metamorphic rock fragments. The quartz content is high in the upper, unconfined turbidite formation (Algyő), whereas feldspars and rock fragments are more widespread in the lower formations (Szolnok and Endrőd). Eogenetic minerals are framboidal pyrite, calcite, and clay minerals. Mesogenetic minerals are ankerite, ferroan calcite, albite, quartz, illite, chlorite, and solid bituminous organic matter. Eogenetic finely crystalline calcite yielded δ13 C V − PDB values from 1.4 to 0.7‰ and δ18 O V − PDB values from –6.0 to –7.4‰, respectively. Mesogenetic ferroan calcite yielded δ13 C V − PDB values from 2.6 to –1.2‰ and δ18 O V − PDB values from –8.3 to –14.0‰, respectively. In the upper part of the turbidite systems, remnants of the migrated organic matter are preserved along pressure dissolution surfaces. All these features indicate that compaction and mineral precipitations resulted in tightly cemented sandstones prior to hydrocarbon migration. Interconnected, secondary, open porosity is associated with pyrite, kaolinite/dickite, and postdates of the late-stage calcite cement. This indicates that dissolution processes took place in the deep burial realm in an extraformational fluid-dominated diagenetic system. The findings of this study add a unique insight to the previously proposed hydrological model of the Pannonian Basin and describe the complex interactions between the basinal deposits and the basement blocks.

Zn2+-Pb2+-doped calcite shrub fabrics: Abiotic morphogenesis of travertine-like dripstone encrustation at the Jersey Zinc Mine, southeastern British Columbia

A cm-thick calcite dripstone on the floor of the abandoned Jersey Zinc Mine in southeastern British Columbia, western Canada, consists of a mixture of shrub and radial fibrous elongated columnar crystal fabrics that resulted from Zn2+ and Pb2+ doping of the calcite crystallographic lattice structure. These heavy metal elements were sourced from sulfide mineralized veins in the overlying limestone beds. Zn2+ and Pb2+ substitution for Ca2+ reduced the size of the calcite crystal structure, impacting configuration of the calcite crystal fabrics. Lower concentration levels of dopants resulted in a radial fibrous elongated columnar crystal fabric with high inter-crystalline porosity. Increased levels of Zn2+ and Pb2+ dopants resulted in a crystal splitting process forming the shrub fabric. Growth of normally flat rhombohedral face on the external surface of the dripstone encrustation was disrupted by mixtures of isotropic and anisotropic growth rates. Inhibited growth rates resulted in early stage crystal splitting processes, forming parallel arrays of precursor crystallites aligned as step-down micro-terraces separated by dam-like micro-barrage partitions with normal growth rates. Shrub fabrics developed below the encrustation surface as ion-laden dripwater films infiltrated the 20% to 25% inter-crystalline porosity. This early stage of crystal splitting transitioned into a shrub fabric substrate, each consisting of fan-shaped crystal arrays distributed along a main stem, all in optical continuity. Extreme levels of Zn2+ dopant resulted in split crystal formation of micro-spherulites on the external surface of the dripstone. Spherulite neomorphism occurred, resulting in replacement of the crystallites by an individual calcite spar during envelopment by the columnar crystal domain fabric. The spheroid forms were preserved within the columnar fabric as spheroid zones of Zn-calcite and outlined by microcrystalline ferroan calcite. The shrub fabrics have morphological similarity to dendritic branching shrubs commonly associated with biotic and abiotic travertine and some biotic tufa deposits, which form under very different depositional conditions. Abiotic Zn2+-Pb2+ dopants absorbed into the calcite crystal structure of this mine floor encrustation precluded the necessity for carbonate precipitation from supersaturated carbonate water or precursor microbial induced nucleation sites, unlike similar travertine and tufa shrub fabrics elsewhere.

Sedimentological analysis and reservoir quality of the early Cenomanian transgressive sandstones, northern Western desert, Egypt

Depicting depositional models of transgressive sandstone reservoirs is challenging due to their internal complexity and limited case studies. This study presents an example for transgressive sandstone reservoirs from the Bahariya Formation at Hayat-Yasser oil field, Shushan Basin (northern Western Desert, Egypt). The upper part of the Bahariya Formation accumulated in an estuarine setting as a consequence of a successive sea level rise. Detailed reservoir characterization, employing sequence stratigraphic and sedimentological approaches, revealed that the studied reservoir consists of a mixed siliciclastic-carbonate transgressive estuarine-shelf succession. Four conventional core samples of this succession at Khalda ridge (Shushan Basin), permit the analysis of their sedimentology and internal architecture (mesoscopic heterogeneities) along correlation lines parallel or slightly oblique to depositional dip. The succession consists of six backstepping parasequence sets that record the overall shift from tidal-dominated to wave-dominated estuaries/bays. The transgressive component of the parasequences sets includes bay mudstones, flood-tidal deltas, and shoreface/barrier deposits; while the regressive component includes estuarine point bars, bay head deltas, and tidal deltas. The reservoir properties show significant variation according to the facies assemblages. The tidally-dominated estuarine channels show low net-to-gross, as it is characterized by lenticular sandstone bodies with several lateral and vertical baffles and barriers. On the other hand, the shoreface/barrier and tidal delta sandstones show high net-to-gross and consist of amalgamated sandstone bodies with little mud barriers or baffles. Petrographically, the upper Bahariya reservoir includes intergranular and secondary porosities. The microscopic heterogeneities are mainly due to the fine grain nature, poor sorting, presence of matrix, and continuous mud barriers or baffles. Destruction of porosity, affecting the reservoir quality, occurred by silica overgrowths, ferroan calcite, grain coating chlorite, and kaolinite cementation, as well as compaction. Feldspar dissolution is the main diagenetic process responsible for the formation of secondary porosity, enhancing the reservoir quality.

The origin of carbonate cements in the Hildasay reservoir, Cambo Field, Faroe-Shetland Basin; clumped isotopic analysis and implications for reservoir performance

The early Eocene paralic sandstones of the Hildasay Member of the Flett Formation form the major oil-bearing reservoir in the Cambo Field, located in the Faroe-Shetland Basin. The sandstones locally contain calcite-cemented intervals that vary in thickness from decimetre to over 1 m. A calcite-cemented interval from well 204/10a-5 has been analysed petrographically and using clumped isotopes to determine its mode of formation and potential lateral extent. Petrographic analysis shows the cemented interval to consist of ferroan calcite, with a consistent dull red cathodoluminescence, suggesting a single phase of precipitation. The centre of the cemented interval comprises a finer grained unit with detrital kerogen and early sphaerosiderite, while the rest comprises homogeneous porosity-occluding ferroan calcite. The early sphaerosiderite in the centre is replaced by ferroan calcite with a high δ13 carbon isotopic signature (δ13 CVPDB = 7.53‰) suggesting that it formed during anaerobic methanogenesis or fermentation. Samples from the intermediate zone have a lower δ13 carbon isotopic composition (δ13 CVPDB = 0.72 to −3.68‰). In comparison the outer margin of the cemented unit has an even lower δ13 carbon isotopic signature (δ13 CVPDB = −15.5 to -15.9‰) more typical of a strong aerobic oxidation source. The oxygen isotopic signature of the cements is similar (δ18 OVPDB = −10.9 to −12.2‰). Analysis of the clumped isotopes suggest that the ferroan calcite formed at ∼40–50 °C, from pore waters that were predominantly meteoric in origin (δ18 OVSMOW = −4.8 to −5.7‰). Burial history modelling would suggest that the cements formed at a depth of ∼500–1000 m. Meteoric water was likely to have been introduced during the formation of the mid-Lutetian unconformity ∼45Mya, approximately 10 Mya after the sandstones were deposited. The model proposed is that ferroan calcite precipitation was initiated in the fine lag deposits that contained the kerogen and sphaerosiderite, and then grew outwards. If the model is correct, the cemented units should be restricted to this facies and consequently of limited lateral extent. Consequently, it is likely that the cemented intervals will have a limited impact upon the reservoir performance and are unlikely to act as major barriers to fluid flow.

Origin of multiple-phase carbonate cements in the sandstones of the third member of the Shahejie Formation in the Niuzhuang Sag, Bohai Bay Basin

Authigenic carbonate cement is one of the most abundant diagenetic minerals in sandstone reservoirs. Determining its origin and distribution may provide useful information for understanding the sandstone reservoir quality. In this study, we report results from a suite of analytical techniques to investigate the origin and evolution of carbonate cements in the third member of the Shahejie Formation in the Bohai Bay Basin. Our data indicate that the carbonate cements mainly occur in three phases: the early-phase calcite, late-phase ferroan calcite, and late-phase ferroan dolomite and/or ankerite. The early-phase calcites show depleted δ18O of the early-phase calcite (−11.8% to −7.8%), suggesting an 18O-depleted fluid origin from the mixing between lacustrine and meteoric waters. They were precipitated earlier than the quartz overgrowth at 40°C–63°C based on the oxygen isotope. The late-phase calcites were precipitated at 70°C–115°C, and they originated from water-rock interaction modified pore water at the same time or later than feldspar leaching. They show a lower average δ13C value (1.27%) than the early-phase calcite (1.65%), indicating that the interbedded shales within the sandstones most likely provided the required components for the precipitation of the late-phase calcite. Also, some Fe2+ was released during the organic acid release and then precipitated the late-phase ferroan calcites. The late-phase ferroan dolomite and/or ankerite were precipitated at 90°C–137°C in a deep diagenetic condition. They show depleted δ13C values (mean [Formula: see text]), and the carbons were mainly sourced from the thermal decarboxylation of organic matter and lacustrine carbonate. The early-phase calcite inhibits compaction while filling the pores, and the dissolution of ferroan calcite cements was the main reason for the development of secondary pores in the sandstone reservoirs. The late-phase ankerite reduces the reservoir porosity.

Sources of Ca2+ in the major carbonate cements in Eocene sandstones and conglomerates: Evidence from Sr isotopes, Sr/Ca ratios, and rare-earth elements

Different sources of calcium have important effects on the formation and distribution of carbonate cements. On the basis of identifying all possible sources of calcium, comprehensive quantitative analyses of Sr, C, and O isotopic values and trace elements (including rare-earth elements) were conducted to determine the Ca2+ sources and their changes in the same type of carbonate cements at different vertical positions in the same reservoirs. Ferroan calcite and ankerite were the major types of carbonate cements. Ca2+ in ferroan carbonate cements in the sandstones near the mudstone-sandstone (M-S) interfaces was derived primarily from diagenetic fluids mixed with Ca2+ originating from the transformation of smectite to illite and from original sedimentary water in the adjacent mudstones. However, Ca2+ in the ferroan carbonate cements in the sandstones away from the M-S interfaces originated from the transformation of smectite to illite, the dissolution of carbonate minerals, and original sedimentary water in source rocks. The gradual change of Ca source of the same type of ferroan carbonate cements indicates the diversity of migration direction of diagenetic paleo-fluid in the same period. During the transformation of smectite to illite, concentration gradients were created from mudstones to sandstones. Ca2+, Mg2+, Fe2+, and other cations in the mudstones diffused to the adjacent sandstones and provided sources of ferroan carbonate cements at the edge of the sandstones. Pore water with Ca2+, Mg2+, Fe2+, CO32−, and other cations in the source rocks was collectively charged into sandstones during oil emplacement and promoted the precipitation of ferroan carbonate cements in residual pore water. Due to oil emplacement and material supplied by the adjacent mudstones, the content of ferroan carbonate cements in the sandstones was lower, and the reservoir porosity was higher at a greater distance from the M-S interfaces. However, in thick-bedded sandstones, due to lack of material supply from the adjacent mudstones, Ca2+, Mg2+, Fe2+, and CO32− in the ferroan carbonate cements were mainly derived from source rocks. As a result, no significant changes were observed in the content of ferroan carbonate cements and reservoir porosity in the vertical direction.