Sandstone Diagenesis Research Articles

Diagenesis and reservoir quality evolution of estuarine sandstones: Insights from the Cenomanian-Turonian Yolde Formation, northern Benue Trough, NE Nigeria

Estuarine sandstones often exhibit complex diagenetic pathways due to the interaction of fluvial and shallow marine processes, resulting in significant reservoir heterogeneities. The Yolde Formation is an important petroleum reservoir in the Northern Benue Trough, NE Nigeria. However, diagenetic and reservoir quality evolution of the formation are still poorly understood. This study utilizes thin-section petrography, XRD, SEM, and fluid inclusion microthermometry to analyse the composition, diagenesis, and reservoir quality of the sandstones. Results show that the sandstones are mainly arkose and subarkose in composition; they are generally well-sorted and vary in grain size from very fine-to coarse-grained. Three diagenetic stages are identified: early, middle, and late. The early stage includes feldspar dissolution, formation of kaolinite, development of clay coating, and mechanical compaction. The middle stage comprises the formation of authigenic chlorite and Fe-oxide cements. The late-stage diagenesis involves illitization of kaolinite, formation of dickite, albitization of K-feldspar, and precipitation of quartz overgrowths. The diagenetic evolution of the sandstones was strongly controlled by their compositional makeup. The feldspathic nature of the sandstones resulted in the formation of kaolinite, illite, and quartz overgrowths, through feldspar dissolution and silica supply. The inner and middle estuarine facies have inhibited the development of quartz overgrowths due to well-developed clay coatings, whereas the outer estuarine facies were well-cemented by quartz overgrowths due to lack of clay coatings. Illitic clay coatings were found to be effective in preventing quartz cementation. Fluid inclusion analysis reveals that the formation temperatures of the quartz overgrowths range from 98 °C to 135.5 °C, corresponding to burial depths ranging between 2.3 and 3.5 km. These findings would provide useful insights that would reduce the risks of exploration of the sandstones and also supplies potential input data for forward diagenetic modelling of their subsurface equivalents in similar depositional settings elsewhere.

Diverse Behaviors of Feldspar Grains during Sandstone Diagenesis: Example from the Xujiahe Formation in the Western Sichuan Basin, China

Feldspar alteration is among the most important processes in clastic rocks during diagenesis, but uncertainty remains about the factors that control feldspar diagenesis under subsurface conditions. Hence, the Upper Triassic Xujiahe formation of the Western Sichuan Basin were examined by an integrated petrographic, mineralogical and geochemical approach to unravel the causes and effects of feldspar diagenesis, with implication for mass transfer and openness of the geochemical system. The sandstones at various depths demonstrate three distinct, separate diagenetic behaviors of detrital feldspar within a single formation including (1) the complete dissolution of both plagioclase and K-feldspar in the upper member; (2) conservation of abundant detrital feldspar grains with minor albitization or overgrowths within the lower member of depths greater than 5 km; and (3) complete disappearance of K-feldspar within the uppermost horizons of the lower member, while plagioclase have survived in significant amounts. The exceptional disappearance of K-feldspar is the result of selective dissolution of K-feldspar during burial, accompanied by illite cementation and substantial K transfer at a scale of tens of meters. It is apparent that the clay diagenesis in the overlying mudstones, rather than porewater chemistry, is the major control of the reactivity of K-feldspar in adjacent deeply buried sandstones.

The interplay of detrital modes and depositional facies on diagenesis and petrophysics of deep-marine Miocene sandstones (Cilento Group), Southern Apennines, Italy

The Cilento Group (Langhian-to-Tortonian) is a thick turbiditic succession of the Southern Apennines foreland region that unconformably overlain the Lucanian oceanic terranes. The San Mauro Formation (SMF) form the uppermost portions of the Cilento Group, and consists of 1400–1600 m thick turbiditic succession including quartzolithic, volcanolithic, and quartzofeldspathic sandstones with several carbonatoclastic layers interbedded, passing upward from distal-to proximal-facies associations. Here, based on the vertical changes of texture and petrology in the succession, the SMF was divided in two main sections. The key depositional features and the main post-depositional processes, of lower- and upper-part of SMF, are compared with the most significant petrophysical properties with the purpose to elucidate the major controls on the porosity and permeability evolution during burial. With this aim, fifty-four thin sections were petrographically investigated, while mercury intrusion analysis was performed on twenty-four samples. Compaction, cementation, dissolution and fracturing occurred in the SMF, altering porosity and permeability. The main authigenic minerals are: 1) carbonates, mainly calcite and less dolomite, dominantly occurring as replacement of framework grains; 2) Phyllosilicates, mainly developing in the upper part of SMF as pore-filling cement or as small and incomplete grain coatings; 3) Fe-oxides occurring as coatings or localized crystals. The relationship between the compactional porosity loss (COPL) and the cementational porosity loss (CEPL) testifies the primary role of compaction in decreasing porosity, reducing the intergranular volume (IGV) to 11.1% in the lower SMF and to 16.7 % in the upper SMF. The coarse upper SMF samples exhibit lower mean porosity and permeability (6 % and 215 mD, respectively) than the fine lower SMF samples (9.2 % and 774.2 mD, respectively). In the upper section of SMF, a comparatively more widespread pore-filling cementation and the abundant detrital matrix affect depositional intergranular porosity, reducing the pore size and interconnectivity. Moreover, the higher amount of rigid grains (petrofacies dependent) and their brittle deformation produce an intricate fragmentation and micro-fracturing system, altering pores geometry and affecting permeability. On the contrary in the lower SMF, ductile components suffer compaction more than rigid ones, leading the lowest IGV. In addition, a higher volume of authigenic carbonates occur as replacement on framework components, hence playing a minor occluding role and less affecting the pore system. Composition and tectonics rule the effects of diagenetic processes in the San Mauro turbiditic succession, following the vertical evolution of depositional facies association and petrology.

How does the burial rate control the diagenesis of sandstone? Insights from a diagenetic physical simulation experiment

The study conducted physical simulation experiments on sandstone samples from the Junggar Basin to investigate how burial rates influence sandstone diagenesis and reservoir quality. Results show that the mechanical compaction under a negative burial rate (tectonic uplift) almost stops to destroy the sandstone reservoir space, the capacity of fluid seepage is the strongest and the sandstone tends to develop ‘weak compaction–strong dissolution’ diagenetic facies. For positive burial rates, sandstones with a low burial rate tend to develop ‘medium compaction–medium dissolution’ diagenetic facies; sandstones at a medium burial rate easily form ‘strong compaction–weak dissolution’ diagenetic facies, and sandstones at a high burial rate tend to develop ‘weak compaction–weak dissolution’ diagenetic facies. Experimental results indicate that the compaction strength and damage to sandstone reservoirs may not consistently rise with the burial rate. Faster burial rates do not always intensify compaction; the degree of compaction depends on fluid overpressure. If the increase in burial rate does not induce the fluid overpressure in sandstones, the burial rate is higher and the destruction degree of primary pores caused by mechanical compaction is greater; mechanical compaction also simultaneously causes the diagenetic system to be more closed and the dissolution to be weaker. If the increase in burial rate can induce the fluid overpressure in sandstones, the burial rate is higher, the inhibition of mechanical compaction by fluid overpressure is more pronounced. However, fluid overpressure also strengthens the closure of the diagenetic system, hindering the injection of external acidic fluids into the sandstone, which is not conducive to dissolution. Overall, low burial rates with normal pressure favour secondary pore development, high burial rates with overpressure preserve primary pores, while medium burial rates with normal pressure are unfavourable for primary and secondary pores.

The impact of tectonic inversion on diagenesis and reservoir quality of the Oligocene fluvial sandstones: The upper Huagang formation, Xihu Depression, East China Sea Shelf Basin

Inversion tectonics are of paramount importance for forming a range of hydrocarbon traps and reservoirs for petroleum exploration. Also, the uplift and exhumation of strata, the activation and creation of faults and leakage or breaching of hydrocarbon traps are often attributed to multiple inversion. Changes in fluid activity and the geochemical system are frequently observed during such tectonic inversions. However, the impacts of tectonic inversion on fluid flow and potential diagenetic alterations within deeply buried sandstone reservoirs are not fully understood, posing challenges to accurately assessing reservoir quality and furthering exploration efforts. The Oligocene fluvial sandstones in the Yuquan anticline of the Xihu Depression were investigated by XRD, optical microscope and SEM-EDS-CL along with and petrophysical data to illustrate the impact of tectonic inversion on the evolution of diagenesis and reservoir quality. The two representative exploration wells, C-1 and C-2, originating from the same provenance and sedimentary background in Yuquan anticline, exhibit significant diagenetic alterations attributed to varying degrees of tectonic inversion. Notably, well C-1 has experienced more intense alterations and dissolution of detrital grains compared to well C-2 (average secondary pore, well C-1: 4.8%, well C-2: 2.9%), particularly the complete dissolution of plagioclase grains. Moreover, the content of authigenic kaolinite and quartz cement of well C-1 is higher than that of well C-2, averaging 7.2%, 4.5% and 5%, 1.7% respectively. This diagenetic difference is attributed to the fact that the activity of early NE-NNE oriented faults and the development of late nearly E-W oriented faults are stronger in the northern well C-1 field than in the central well C-2 field. The structural characteristics resulting from tectonic inversion led to enhanced efficiency in both organic acid supply from underlying source rocks in well C-1 field and export of dissolution products within the diagenetic system when compared with Well C-2 field. Regional tectonic inversion plays a predominant role in improving reservoir quality than sedimentary factors in the study area. While the well C-1 has a higher porosity than well C-1 (well C-1: average 18%, well C-2: average 13.1%), the permeability of well C-1 is worse than that of well C-2, with an average of 14.7 mD and 27.9 mD respectively. The alteration and dissolution associated with tectonic inversion significantly enhance the porosity of sandstone reservoirs. However, the formation of secondary pores and dissolution products can complicate the pore system of reservoirs, thereby reducing the reservoir permeability. Our research aims to provide a better understanding of fluid-rock interactions and genetic mechanisms in gas-bearing sandstone reservoirs under varying degrees of tectonic inversion. This will support pre-drill evaluations of reservoir quality in deeply buried formations as well as other analogous targets.

Coevolutionary Diagenesis in Tight Sandstone and Shale Reservoirs within Lacustrine-Delta Systems: A Case Study from the Lianggaoshan Formation in the Eastern Sichuan Basin, Southwest China

Tight sandstone and shale oil and gas are the key targets of unconventional oil and gas exploration in the lake-delta sedimentary systems of China. Understanding the coevolutionary diagenesis of sandstone and shale reservoirs is crucial for the prediction of reservoir quality, ahead of drilling, in such systems. Thin-section description, scanning electron microscopy (SEM), X-ray diffraction (XRD), fluid inclusion analysis, porosity and permeability tests, high-pressure mercury intrusion (HPMI) measurements and nuclear magnetic resonance tests (NMR) were used to reveal the coevolutionary diagenetic mechanisms of a sandstone and shale reservoir in the Lianggaoshan Formation of the Eastern Sichuan Basin, China. The thermally mature, organic-matter-rich, dark shale of layer3 is the most important source rock within the Lianggaoshan Formation. It started to generate abundant organic acids at the early stage of mesodiagenesis and produced abundant hydrocarbons in the early Cretaceous. Porewater with high concentrations of Ca2+ and CO32− entered the sandstone reservoir from dark shale as the shale was compacted during burial. Potassium feldspar dissolution at the boundary of the sandstone was more pervasive than at the center of the sandstone. The K+ released by potassium feldspar dissolution migrated from the sandstone into mudstone. Grain-rimming chlorite coats occurred mainly in the center of the sandstone. Some silica exported from the shale was imported by the sandstone boundary and precipitated close to the shale/sandstone boundary. Some intergranular dissolution pores and intercrystal pores were formed in the shale due to dissolution during the early stages of mesodiagenesis. Chlorite coats, which precipitated during eodiagenesis, were beneficial to the protection of primary pore space in the sandstone. Calcite cement, which preferentially precipitated at the boundary of sandstone, was not conducive to reservoir development. Dissolution mainly occurred at the early stage of mesodiagenesis due to organic acids derived from the dark shale. Calcite cement could also protect some primary pores from compaction and release pore space following dissolution. The porosity of sandstone and shale was mainly controlled by the thickness of sandstone and dark shale.

Diagenesis, facies and palaeocurrent analysis of Upper Rewa Sandstone around Sagar, Central India

The stratigraphic surface represented by a major contact between the Archean Bundelkhand Granite and the extensive Proterozoic Vindhyan sediments is a regional basement cover unconformity. This crystalline–sedimentary interface reflects an intense weathering of continental crust during marine transgression. Three time-transgressive sand deposition events viz. Kaimur, Rewa and Bhander are mainly witnessed during the entire Vindhyan sedimentation. Stratigraphically, the Upper Rewa Sandstone comprises one of these events in the Vindhyan Basin. Considerable progress has been made in interpreting these sandstones as a function of entirely marine process to a combination of tidal–fluvio–eolian activities. All the results have so far been attained on the basis of sedimentary facies, provenance, palaeocurrent analysis, and some of petrography. A distinct differentiation between marine and fluvial components of the Upper Rewa Sandstone still remains uncertain. Here, we use diagenesis as a parameter for the first time along with facies and palaeocurrent analysis to acquire a clear comprehension of marine- and fluvial-dominated processes. The present study spans 27 square kilometer area covering 15 stratigraphic sections with a collection of 571 directional data from the facies specific sedimentary structures, and 28 samples obtained for the thin section analysis.The lower unit of the Upper Rewa Sandstone shows facies association typical of tidal environment, along with polymodal palaeocurrent. These rocks are well sorted and dominantly lithified by authigenic cement. Profuse development of a complex of syntaxial, passive pore fills, and grain replacive cements reflect normally pressured nature of the lower unit sandstones. The upper unit of the Upper Rewa Sandstone however, reveals a facies pattern resembling fluvial processes and unimodal palaeocurrent plot. They exhibit poor sorting, siliceous, and ferruginous matrix, and development of intense stylolites. Since, pressure solution is a dominant mode of lithification; the upper unit rocks therefore, are overpressured. Sand deposition event pertaining to the Upper Rewa Sandstone is found to comprise both marine and fluvial processes. Such a diagenesis based approach can be applied for marking a possible facies based unconformity between genetically different depositional units.

Diagenesis and petrophysics of Miocene sandstones within southern Apennines foreland, Italy

Deep-marine clastic turbidite beds commonly occur in the Southern Apennines and record the main tectonic phases associated with the structural evolution of the area. Based on variations of detrital compositions through time, the sandstone suites are traditionally subdivided into four petrofacies: (I) quartzolithic (Burdigalian-Langhian), (II) quartzofeldspatholithic (Langhian-Tortonian), (III) quartzofeldspathic (Serravallian-Tortonian), and (IV) arkose (Tortonian-Messinian) sandstones. Since the complexity and diversity of these sandstone petrofacies, we aim to investigate their major diagenetic events and relate them with their detrital modes and physical properties in order to obtain key basic information at a regional scale, to better understand the main controls on petrophysical properties in this and in similar contexts. The petrophysical analysis reveals that the quartzolithic sandstones have the lowest porosity values, with a median of 4.56%. In contrast, porosity increases to median values of 16.75% in the quartzofeldspathic and arkosic petrofacies. The median permeability values range between 4.85 and 236.38 mD, and its distribution is generally poorly correlated with any other petrophysical parameter. The most widespread diagenetic minerals observed are carbonates, clay minerals, Fe-oxides and hydroxides, and less quartz. These sandstones are characterized by high compaction, which is the primary factor controlling porosity loss since the early diagenetic phases. In contrast, the early precipitation of pore-filling carbonate cement is assumed to have partly reduced the effect of compaction in quartzolithic and arkosic sandstones. Nonetheless, variations in petrophysical features among petrofacies can be traced back to the detrital composition, with the highest but less effective porosity associated with the presence of ductile grains, siliciclastic matrix and feldspars. Accordingly, this study reveals how linking detrital modes to the key petrophysical parameters may help obtain significant information about the regional control on the evolution of porosity and permeability in clastic turbiditic wedges of foreland domain settings as a baseline useful to further local and detailed studies.

Facies-related diagenesis of Jurassic sandstones, central Junggar Basin, NW China: implications for reservoir quality evolution

Abstract Hydrocarbon exploration in China has recently expanded to deep tight oil and gas from the Lower Jurassic sandstone reservoirs in the central Junggar Basin. These deeply buried sandstones (>4.5 km) have generally been overpressured due to hydrocarbon saturation, but have not been intensively investigated in terms of diagenesis and reservoir quality. The Lower Jurassic sandstones are feldspathic litharenites (average Q 28 F 29 R 43 ) and were deposited in the lacustrine–deltaic environments of the high-palaeolatitude continental basin under warm, humid climatic conditions. Progressive burial with only slight uplift of the strata led to a relatively stable burial–thermal regime, resulting in minor diagenetic changes consisting of compaction, cementation by carbonates, quartz and clay minerals, and partial dissolution of feldspars and rock fragments. Despite generally similar diagenetic histories, the studied sandstones reveal considerable variations in the development of pore systems, porosity and permeability. The best reservoirs are represented by medium- to coarse-grained depositional facies with higher porosity (average 8.43%) and permeability (average 0.59 mD). All the evidence suggests that integrated consideration of the roles of depositional settings and diagenesis, linked with burial–thermal history and overpressure evolution conditions, provides a helpful means of predicting reservoir quality.

Volcanoclastic and epiclastic diagenesis of sandstones associated with volcano-sedimentary deposits from the upper Jurassic, Lower cretaceous, Paraná Basin, southern Brazil

The opening of South Atlantic Ocean produced an immense volume of lava that covered an active aeolian system during Lower Cretaceous. The contact of Botucatu Formation sandstones with the Serra Geral volcanic flows generated a variety of volcano-sedimentary features and deposits. During volcanism, the temperature contrast between lava and wet or water-saturated sediments formed volcanoclastic features and related sedimentary deposits. In moments of magmatic quiescence, epiclastic deposits were generated due to the return of sedimentation. Volcanoclastic deposits include volcanic breccias with sandy matrix, and epiclastic deposits include conglomerates and conglomeratic sandstones with intraclasts. Quantitative petrography of the sandy matrix samples of these deposits was analyzed, in order to discuss the paragenesis of volcano-sedimentary interactions. Regular burial diagenetic processes were discussed for epiclastic sandstones and contact diagenesis processes for volcanoclastic sandstones. The original composition of volcanoclastic and epiclastic sandstones is heterogeneous. However, the main difference between them is the presence of textural fluidization indicators found in the volcanoclastic sandstones, along the lava-sediment interface. These textural characteristics indicate that the unconsolidated sediments were probably wet or saturated in water during the interaction with the volcanic flow. Epiclastic sandstones have infiltrated clays and neoformed smectites as their main diagenetic constituent due to alteration of available volcanic lithoclasts. For volcanoclastic sandstones, the formation of opal, chalcedony and megaquartz cements through burial is unlikely, due to the intergranular volume occupied by these cements, indicating near-surface early diagenetic conditions. Therefore, the mechanisms of dissolution and repreciptation of the siliceous fluids, incorporated in the system, formed mainly the chalcedony cementation. Furthermore, zeolite cementation, which occurs along the lava-sediment contact, is another important indicator of contact diagenesis for volcanoclastic sandstones. During telodiagenesis, the formation of secondary porosity by dissolution of volcanic lithoclasts and detrital K-feldspar grains was of great importance in volcanoclastic and epiclastic sandstones.

Impact of coal evolution on formation of tight sandstone reservoirs: A case study in the Daniudi gas field, Ordos Basin, China

Tight sandstone gas in coal-bearing strata has become an important unconventional gas worldwide. This research aims at understanding the impacts of coal evolution on diagenesis and reservoir quality of sandstones by means of thin section, cathode luminescence (CL), scanning electron microscope (SEM), stable isotope analysis, and fluid inclusions. The results show that carbonate cement was the main type of cement developed in tight sandstone reservoirs, of which the carbonate ions were jointly provided by coal layers and reaction in adjacent mudstones. For sandstones close to coal layers, their carbonate ions were mainly controlled by coal evolution and there was various carbonate cement developed due to cations supplied by seawater and the intense dissolution of volcanic fragments and feldspars. Combined with coal thermal simulation and burial history, it implies that at different maturity stages of coal evolution, its impact on sandstone diagenesis was different. In the early stage, the main product was CO2, which caused the dissolution of early calcite and promoted intense compaction. In the mature stage, CO2 and other hydrocarbon gases were released, resulting in more dissolution and silica cementation. In the over-mature stage, the main product was methane, and CO2 that did not spill out preserved by forming carbonate cement. The coal evolution had a strong impact on the reservoir quality of sandstone with large grain sizes, but little effect on medium- and fine-grained sandstones. High porosity and low permeability reservoirs were usually formed in small conglomerates and coarse-grained sandstones away from coal layers. While low porosity and high permeability reservoirs were distributed close to coal layers. This study aids in understanding the formation mechanism of tight sandstone reservoirs in coal measures and provides theoretic support for further exploration.

Petrography and diagenesis of Neoproterozoic Bhander Sandstones, Vindhyan Supergroup, Southeastern Rajasthan, India: Implications for provenance and reservoir characteristics

Petrography and diagenesis of Neoproterozoic Bhander Sandstones, Vindhyan Supergroup, Southeastern Rajasthan, India: Implications for provenance and reservoir characteristics

Impact of magmatic intrusion on diagenesis of shallow marine sandstones: An example from Qasim Formation, Northwest Saudi Arabia

Igneous intrusions are common in sedimentary basins, and their occurrence can significantly affect the diagenesis and reservoir quality evolution of sandstones, thereby strongly impacting their hydrocarbons-, geothermal-, and CO2-storage potentials. The Qasim sandstones in the Tabuk region (NW Saudi Arabia) experienced shallow burial diagenesis (<2 km) when the Tertiary magma intruded to form basaltic sills (0.4–4 m thick). The sedimentology, tectono-stratigraphic framework, provenance, and chemostratigraphy of the Qasim Formation have been extensively covered in the literature. However, the impact of the magmatic intrusion on diagenesis and reservoir quality evolution of the sandstones remains enigmatic. This study employed thin-section petrography, QEMSCAN, XRD, SEM, and energy-dispersive spectrometer analyses to investigate the role of magmatic intrusion on diagenesis and reservoir quality of the Qasim sandstones. The results of the study indicate that reservoir porosity is principally influenced by primary depositional characteristics (grain size and sorting), diagenetic alterations, and magmatic intrusions. Sandstones with coarser grain size and better sorting have the best intergranular porosity and vice versa. The “normal” diagenetic processes that have significantly affected the reservoir porosity of the sandstones occurred during both shallow burial (eodiagenesis) and uplift (telodiagenesis). The eogenetic alterations include mechanical compaction, early diagenetic cementation by calcite, pyrite, and kaolinite, whereas the telogenetic alterations include the formation of kaolinite, goethite, hematite. Overall, mechanical compaction is the main driver for porosity loss in the sandstones. The intrusion-related diagenetic processes include the dissolution of quartz grains, rounded quartz overgrowths, and calcite cement, and the transformation of kaolinite into dickite and chlorite. Detrital quartz and rounded quartz overgrowths have undergone dissolution due to acidic pore fluids from magma and high temperature. The transformation of kaolinite into dickite occurred in a dissolution-recrystallization fashion, and the amounts of kaolinite and dickite increase in fine-grained sediments away from sill contact due to hydrodynamic processes that deposited muscovite (which form kaolinite) in low energy environments. The chloritization of kaolinite was localized, and the magma-induced dissolution of goethite likely supplied the requisite high Fe content. Additionally, the intrusion has resulted in the dissolution of the early calcite and increase in porosity towards the sill contact. However, values for compactional porosity loss have relatively remained similar both at and away from the sill contact, as the sill is too thin to exert significant vertical loading. This study has relevance to understanding hydrocarbon exploration and exploitation in sediment-lava sequences, and to understanding the development of sediment-lava systems.

Diagenesis, Diagenetic Facies and Their Relationship with Reservoir Sweet Spot in Low-Permeability and Tight Sandstone: Jiaxing Area of the Xihu Sag, East China Sea Basin

The optimization of reservoir sweet spots is the key to the efficient exploration and development of low-permeability and tight sandstone gas reservoirs. However, offshore deep, low-permeability and tight sandstone has the characteristics of large burial depth, large diagenesis heterogeneity and prominent importance of diagenetic facies, which make it difficult to predict reservoir sweet spots. This work comprehensively used logging data, core observation, conventional core analysis, thin section, powder particle size analysis, clay X-ray diffraction analysis, cathode luminescence analysis, scanning electron microscopy and energy spectrum analysis and carried out the study of diagenesis, diagenetic facies and reservoir sweet spots of low-permeability and tight sandstone of H3 and H4 (the third and fourth members of Huagang Formation) members in the Jiaxing area of the Xihu Sag. The results show that the H3 and H4 sandstones were divided into five diagenetic facies types, and chlorite-coated facies and dissolution facies were favorable diagenetic facies belts. The H3 member mainly develops chlorite-coated facies, dissolution facies and quartz-cemented facies, whereas the H4 member primarily develops quartz-cemented facies and chlorite-coated facies. The percentages of type I sweet spot, type II1 sweet spot and type II2 sweet spot in the H3 reservoir are approximately 21%, 23% and 26%, respectively, whereas the percentages of type I sweet spot, type II1 sweet spot and type II2 sweet spot in the H4 reservoir are about 16%, 15% and 16%, respectively. The distribution rules of reservoir sweet spots were investigated. Type I sweet spot was mainly developed in the areas of chlorite-coated facies and dissolution facies of medium sandstone and coarse sandstone in the channel bar and braided channel sedimentary microfacies. Type II sweet spot was primarily distributed in the areas of quartz-cemented facies, chlorite-coated facies and minor dissolution facies of medium sandstone, fine sandstone and sandy conglomerate in the braided channel, subaqueous distributary channel and channel bar sedimentary microfacies. Type III sweet spot was chiefly developed in the areas of tightly compacted facies, calcite-cemented facies and quartz-cemented facies of fine sandstone, siltstone and a small amount of sandy conglomerate in the subaqueous distributary channel sedimentary microfacies.

Depositional and diagenetic controls on porosity evolution in sandstone reservoirs of the Stuttgart Formation (North German Basin)

The Mesozoic succession of the North German Basin comprises permeable sandstones operated by ‘conventional’ open doublet systems for geothermal heat production. The high permeability, exceeding 6 Darcy at individual localities, is the result of undercompacted grain fabrics, low abundance of authigenic minerals and large secondary porosity volumes. The discrepancy of undercompacted grain fabrics and porosity volumes of >20% at depth of up to 2500 m gave rise to a basin-scale study, in which classical petrographic methods were employed to reconstruct the diagenesis of sandstones on the example of the Stuttgart Formation (Schilfsandstein, Late Triassic). Associated to the deposition of sandstones in distinct fluvio-deltaic environments, four diagenetic pathways are indicated by systematic variations of detrital and authigenic assemblages. These four pathways are herein introduced as Delta Channel, Delta Plain, Fluvial Channel, and Floodplain Diagenesis Types. In response to the history of the North German Basin, high permeabilities of sandstones of the Delta Channel and Fluvial Channel Diagenesis Types are related to the subsequent control of depositional and diagenetic regimes on porosity evolution. Pervasive eogenetic calcite cementations contributed to preservation of pre-burial intergranular volumes and effectively prevented the mechanical compaction of grain fabrics during late Triassic–late Jurassic burial diagenesis. The uplift from deep to shallow burial depth, associated to latest Jurassic–Cenozoic structural differentiation and inversion of the North German Basin, triggered substantial dissolution processes that mainly affected carbonate cementations. This opened pores and contributed to the evolution of secondary porosity, in particular in sandstones of the Delta Channel and Fluvial Channel Diagenesis Types. Moderate post-inversion reburial rates resulted only in negligible mechanical compaction and reduction of secondary porosity. The improved knowledge of diagenetic pathways, herein exemplified by sandstones of the Stuttgart Formation, will significantly contribute to improved predictions of Mesozoic hydrothermal reservoir in the North German Basin.

Diagenesis and the effects of cataclastic deformation on the Permo-Triassic New Red Sandstone, Isle of Arran, Scotland

Diagenesis in the Permo-Triassic New Red Sandstone, Isle of Arran is characterized by early cementation of hematite, clay, and calcite minerals, followed by burial compaction, quartz, feldspar, and pyrite cementation. Cataclasis post-dated the quartz and feldspar cementation and reduced the grain and pore aperture size in deformed samples. Samples with cataclastic bands typically have 18% porosity and 8.81 mD permeability on average. Whereas, undeformed samples have an average porosity of 22% and an average permeability of 381 mD. Cataclasis was not as important as diagenesis in controlling sandstone porosity and permeability. However, cataclasis resulted in lower porosity and very poor to medium permeability in deformed samples. Cataclastic bands compartmentalize reservoir sands and cause a high heterogeneity in undeformed porous sandstones. Poikilotopic and blocky calcite cement postdates early clay and hematite cement. In addition, burial quartz and feldspar overgrowths also postdate the early clay and hematite. However, the poikilotopic calcite fills in framework grains that have larger void volumes than the grain/grain contacts where quartz overgrowths are present. Cataclasis resulted in fracturing of quartz and feldspar overgrowths. Therefore, the cataclasis occurred after the development of quartz and feldspar cementation. Dissolution postdated the formation of authigenic feldspar and pyrite formation resulted from hematite reduction. The distributions of grain and pore sizes against cumulative mercury volumes in studied samples shows a high level of reduction of grain and pore aperture sizes for deformed samples from single-cataclastic and multi-cataclastic bands. The distribution of apex volumes illustrates that the effective mercury porosity of the multi-cataclastic band sample may be reduced up to > 2 times in comparison to undeformed samples. However, the sample of a thin single cataclastic band has only a slightly lower apex volume in comparison to the host sample.

Effect of CO2-Brine-Rock Interactions on the Pore Structure of the Tight Sandstone during CO2 Flooding: A Case Study of Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin, China.

CO2 flooding is an effective technique to enhance oil recovery from tight sandstone reservoirs. The CO2-rich fluid reacts with the in situ minerals and results in the corrosion and precipitation of minerals in the sandstone, affecting the connectivity and morphological features of pores. However, the diagenesis of the tight sandstone is complex, and the structural modification behavior and mechanism in different lithofacies of tight sandstone during CO2 flooding are still unclear. This study combined CO2 flooding, thin-section casting, scanning electron microscopy, nuclear magnetic resonance, X-ray diffraction, and fractal analysis methods to investigate the changes in the pore structure of tight sandstone after CO2 flooding. The results show that the cement of the tight sandstone is complex and diverse. The tight sandstone can be divided into two types of lithofacies: clay cementation (CL) and ferrocalcite cementation (CA). The mineralogical alterations occur differently in each lithofacies of tight sandstone. Alterations in the cement minerals affect the pores morphology of tight sandstone depending on their mineralogical structure and texture, and the CO2 flooding mainly changes the micromorphology and heterogeneity of large pores. Clay minerals dominate the cement in the CL lithofacies of tight sandstone. The dissolution mainly occurs in small pores because the precipitation of new minerals and exfoliation of skeleton particles partially block large pores and transform them into small pores. Thus, the number of small pores of CL lithofacies increases while the number of large pores decreases. Such a phenomenon hinders the increase in porosity and permeability. On the other side, in the CA lithofacies of tight sandstone, the cement is dominated by ferrocalcite, and the dissolution of ferrocalcite is the primary mechanism of mineralogical alteration. As the ferrocalcite dissolution expands, it creates new flow paths, improving the connectivity of pores. The petrophysical properties of CA lithofacies improve significantly after CO2 flooding. There is a crucial need to study the changes in diagenetic characteristics of tight sandstone due to CO2 flooding. Such type of study provides insights related to the improvement and evaluation of the development of tight sandstone reservoirs during CO2 flooding.

Impact of meteoric water flushing on diagenesis of deep-marine turbidite sandstones: A case study from the Tertiary sandstones of Frigg and Grane fields, northern North sea

Meteoric water flushing into deep-marine environments is a topic of debate since the actual transport mechanism remains elusive. In order to evaluate the potential impact of meteoric water on early diagenesis of deep-marine sandstones, two submarine fan sandstone reservoirs, the Eocene Frigg (Frigg oilfield) and the Palaeocene Heimdal (Grane field) that have the same source area, were compared and contrasted. Petrographic studies reveal that both sandstone reservoirs are at the eodiagenesis stage (<70 °C) at the present day. However, the Eocene Frigg sandstones are characterized by geological features suggestive of potential meteoric water diagenesis, such as dissolution and kaolinization of the silicate grains, whereas the Palaeocene Heimdal sandstones show negligible alteration. The δ13CV-PDB and δ18OV-PDB values of siderites in the Eocene Frigg sandstones range from +5.2‰ to +16.7‰, and from −8.2‰ to −6.6‰, respectively, indicating siderite formation during methanogenesis in meteoric pore water. Results from generic hydrogeochemical modelling scenarios lend further support to the hypothesis of meteoric water flushing. Thus, we suggest that massive meteoric water might have been brought into the Frigg turbidite sands by basinward migration of the meteoric water. This occurred because the East Shetland Platform experienced two stages of relative sea-level fall (during and at the end of the Eocene, Priabonian), and flushing might have taken place via connected incised canyons. The location of the Frigg sands deposited immediately below the Shetland escarpment and the narrow palaeo-shelf area may have facilitated such meteoric water incursions. Our study suggests meteoric water intrusions as a possible diagenetic control of deep-marine sandstones, and, therefore, meteoric water diagenesis should be considered in the prediction of properties of deep-marine sandstone reservoirs.

Diagenesis of the Paleogene Sandstones in the DN2 Gas Field, Kuqa Foreland Basin and Its Link with Tectonics

AbstractWe investigated the diagenesis of sandstones from DN2 Gas Field of Kuqa Foreland Basin (KFB) to infer timing of fluid migration and discuss link between fluids and tectonics. Textures and chemical composition of authigenic minerals, fluid evidence from fluid inclusions and formation water measurements were used to fulfill the aim. Eodiagenesis occurred with the participation of meteoric waters and connate waters. Mesodiagenesis are related to high salinity fluids, which were attributed to originate from overlying Neogene Jidike Formation evaporite (main minerals including halite, anhydrite, glauberite, carnallite and thenardite). The onset of high salinity fluid migration was inferred to occur during the late Miocene (12.4–9.2 Ma) by using integration of Th measured in the present study and K‐Ar dating of authigenetic illites from previous study. The period is consistent with the crucial phase (13–10 Ma) witnessing the rapid uplift of southern Tianshan Mts and the stage when calcite and anhydrite veins formed in the studied strata. We thus argue that diagenesis related to high salinity fluids occurred as a response to Tianshan Mts's rapid uplift and related tectonic processes. The flow of high salinity fluids was probably driven by density gradient and channeled and focused by fractures formed contemporaneously.

Diagenesis of tight sandstone and its influence on reservoir properties: A case study of Fuyu reservoirs in Songliao Basin, China

Diagenesis of tight sandstone and its influence on reservoir properties: A case study of Fuyu reservoirs in Songliao Basin, China