Generation Of Secondary Porosity Research Articles

Modelling diagenetic reactions and secondary porosity generation in sandstones controlled by the advection of low‐molecular‐weight organic acids

AbstractHigher secondary porosity was observed in the centre of a sandstone unit in the Eocene Shahejie Formation fan delta front sandstones from the Bozhong Depression, Bohai Bay Basin. This differs from past studies showing secondary porosity mainly in the marginal parts of sandstones adjacent to shales. This study utilized reactive transport models involving low‐molecular‐weight organic acids (LMWOA) to discuss potential processes resulting in the contrary distribution of secondary porosity. An interface model simulating LMWOA diffusion from adjacent shales to the sandstone resulted in secondary porosity in sandstones adjacent to shales. In contrast, an advection model simulating advective transport of LMWOA parallel to the sandstone bedding successfully generated higher secondary porosity in the central part. The central part of the sandstone exhibited better grain sorting (greater depositional porosity) and significantly less early carbonate cements compared to the marginal sandstone parts. Consequently, the central part had greater porosity prior to the dissolution through LMWOA. The initially higher porosity in the central part allowed for a higher advective flux of LMWOA‐rich water and associated lower pH, resulting in decreased oligoclase saturation, higher oligoclase dissolution rates and ultimately higher secondary porosity. This study indicates that grain sorting during sediment deposition, early carbonate cementation, LMWOA production in adjacent shales, and advection processes collectively control the diagenetic reactions and the distribution of secondary porosity in sandstones.

Diagenetic effect on reservoir quality of siliciclastic and volcaniclastic sandstones from a Paleogene volcanic rifted margin, East Greenland

ABSTRACT Siliciclastic and volcaniclastic sediments in a volcanic rifted-margin succession may experience a complex diagenetic history during burial that can have a large impact on sandstone reservoir properties. To understand such changes, variations in initial sediment composition and succeeding diagenetic changes have been studied for a Paleogene outcrop analogue in the Kangerlussuaq area, East Greenland. The nature of the mafic volcanics-bearing succession, which consists of intra-volcanic sandstones, accommodated over quartz-rich pre-volcanic fluvial sandstones, are comparable to the settings of recently discovered hydrocarbon-producing sandstones in the Faroe–Shetland Basin on the conjugate Atlantic margin. Our petrographic and provenance investigations of the pre- and intra-volcanic sandstones are supported by geochemical and X-ray diffraction analyses. The intra-volcanic sandstones were deposited in shallow marine environments with mixed siliciclastic and volcaniclastic input, the latter rich in felsic to mafic volcanic rock fragments and feldspar grains. Similar zircon age distributions of pre- and intra-volcanic sandstones support a continued supply from the same siliciclastic sediment source after the onset of volcanism. Variations in initial detrital grain and pore-fluid (fresh to marine) compositions resulted in different diagenetic changes in the pre- and intra-volcanic sandstones. However, where siliciclastic sandstones were overlain by volcaniclastic rocks rather than massive lava flows, the diagenetic changes resemble those of the intra-volcanic sandstones. The cementing phases are typically quartz, illite (probably illitized kaolinite), and rare anatase in the pre-volcanic sandstones. Chlorite, calcite, zeolite/feldspar, opal/quartz, and titanite are characteristic authigenic phases in the intra-volcanic sandstones. Precipitation of different minerals in the pre- and intra-volcanic sandstones show that the detrital composition (and to a lesser extent depositional environment) played a major role during early and late diagenesis after deep burial (up to 6–8 km). Inter-eruptive siliciclastic units may prove to form highly valuable reservoirs when they are composed of mixed siliciclastic and volcaniclastic material. In the stratigraphically youngest intra-volcanic sandstones and pre-volcanic sandstones overlain by hyaloclastite or tuff, there is a high potential for preservation of interparticle porosity during burial (< 5 km) due to early chlorite rims and the generation of secondary porosity after the dissolution of early zeolite cement.

Provenance, diagenesis and reservoir quality of sandstone facies of the Maracangalha Formation, Recôncavo Basin – Northeastern Brazil

Sedimentary composition studies on siliciclastic reservoirs and their relationship with sedimentary provenance have proved to be useful in hydrocarbon exploration, enabling the prognosis of reservoir quality and diagenetic models. However, unconventional terrigenous reservoirs in the Maracangalha Formation, Upper Cretaceous from Recôncavo Basin, have not yet been evaluated regarding sedimentary provenance and its control over reservoir quality. This study aims to define the provenance of the Maracangalha Formation sandstones and its control on the diagenetic processes affecting the reservoir quality. Based on petrography analysis and X-Ray Fluorescence data, it was possible to identify a granite-gneissic terrain under wet climate as the primary source area for quartz-feldspathic sandstones. The granite-gneissic terrains with predominantly felsic minerals, within in the surrounding Precambrian Terrains, represent a major source area for the quartz-feldspathic sandstones found in the Maracangalha Formation. These sources are characterized by significant contributions from recycled sedimentary and metasedimentary rocks, leading to the occurrence of litho and quartzose petrofacies. The high chemical-textural sediment maturity and paleoweathering dispersion, under semi-umid to semi-arid climates, may be a result of paleoclimatic changes and the variable residence times of the sediments in the dispersion routes or temporary storage. The mineral composition and primary texture of sandstones show a clear influence on diagenetic evolution, particularly in terms of mechanical compaction inhibition, by early cementation, and the generation of secondary porosity due to the dissolution of framework grains and early cements. Therefore, understanding the sedimentary provenance, composition, and diagenesis is crucial for predicting the quality of the reservoir and the compartmentalization of the Maracangalha Formation and its analogues.

Secondary porosity generation in sandstones constrained by the fault movement, fluid flow, and mudstone–sandstone interactions, offshore Bohai Basin, China

Secondary porosity generation in sandstones constrained by the fault movement, fluid flow, and mudstone–sandstone interactions, offshore Bohai Basin, China

Hierarchical Zeolite from Coal Fly Ash Using the Biosurfactant Manosyleritritol Lipids-B as a Novel Green Mesoporogen Agent

The hierarchical zeolite 13X (SiAl = 1.50) was synthesized from coal fly ash using an alternative template for the first time. The manosyleritritol lipids-B (MEL-B) > 95% purity, a biosurfactant from Pseudozyma tsukubaensis, was used as a pore directing agent in a two-step process. The hierarchical zeolite was pre-treated with oxalic acid and subsequently subjected to desilication with ammonium hydroxide aided by MEL-B using a hydrothermal reactor at 150 ºC for 5 h. The acid pretreatment was effective with 12.10% structural aluminium removal, since it increased the Si/Al ratio from 1.50 to 1.71 and also preserved the crystalline structure for the dealuminated zeolite with 20 mM oxalic acid. The zeolite, treated with 0.55 g of MEL-B, showed remarkable textural properties, with a pore diameter of 3.72 nm, a mesopore volume of 0.520 cm3/g, a high surface area of 418 m2/g and a total volume of 0.787 cm3/g. Thermal analysis showed that the biosurfactant degrades at 172 ºC, lower temperature than the synthetic surfactants. Regarding the experiments with 0.25 g of MEL-B, it was possible to increase the pore diameter by 40.8% due to the high molecular weight of the biosurfactant (893.9 g/mol), which provided a notable increase in the textural and morphological properties of the zeolites. The high performance in the generation of secondary porosity, low environmental impact, lower calcination temperature, low biosurfactant mass required to generate hierarchical pores are promising technical implementation on inorganic-bio-nanomaterials technology.

Hierarchical Zeolite Synthesis by Alkaline Treatment: Advantages and Applications

Zeolites are of great interest to the scientific and industrial communities due to their interesting catalytic properties, such as high specific area, shape selectivity, and thermal and hydrothermal stability. For this reason, zeolites have been intensively studied and applied in several reactions of great industrial interest. However, the size of zeolite micropores may hinder the diffusion of bulky molecules in the pore system, limiting the use of zeolites in some reactions/applications that use bulky molecules. One way to address this limitation is to generate secondary porosity (in the range of supermicropores, mesopores and/or macropores) in such a way that it connects with the existing micropores, creating a hierarchical pore system. There are different hierarchical approaches; however, most are not economically viable and are complicated/time-consuming. Alkaline treatment has been highlighted in recent years due to its excellent results, simplicity, speed and low cost. In this review, we highlight the importance of alkaline treatment in the generation of secondary porosity and the parameters that influence alkaline treatment in different zeolitic structures. The properties and catalytic performance of hierarchical zeolites prepared by alkaline treatment are extensively discussed. It is expected that this approach will be useful for understanding how alkaline treatment acts on different hierarchical structures and will thus open doors to achieve other hierarchical zeolites by this method.

Hierarchical TS‐1 Through Ammonium Fluoride Etching

AbstractThe impact of the NH4F etching on the textural and catalytic properties of TS‐1 was investigated. Two TS‐1 with different crystal sizes, i. e. 200 and 100 nm; were achieved, with different textural properties as inferred from nitrogen physisorption isotherms at 77 K, mercury porosimetry, powder X‐ray diffraction patterns and transmission electron microscopy. The generation of larger mesopores within the parent samples was observed together with a reduction of the coherent crystal size (CCS) upon NH4F etching. The generation of secondary porosity further did not result in an alteration of the Si/Ti ratio, allowing to compare the catalytic outcomes of the samples as a function of the textural properties. It was found that the methyl‐phenyl sulfide (MPS) conversion kinetics importantly depend on the textural properties of the catalysts. The initial velocity was found to increase linearly with the reduction of the CCS until reaching a plateau, indicating that diffusion limitation occur up to a CCS of approximately 100 nm. Likewise, mono‐ and di‐oxidation product selectivity was governed by diffusion issues.

Experimental burial diagenesis of aragonitic biocarbonates: from organic matter loss to abiogenic calcite formation

Abstract. Carbonate biological hard tissues are valuable archives of environmental information. However, this information can be blurred or even completely lost as hard tissues undergo diagenetic alteration. This is more likely to occur in aragonitic skeletons because bioaragonite often transforms into calcite during diagenesis. For reliably using aragonitic skeletons as geochemical proxies, it is necessary to understand in depth the diagenetic alteration processes that they undergo. Several works have recently investigated the hydrothermal alteration of aragonitic hard tissues during short-term experiments at high temperatures (T > 160 ∘C). In this study, we conduct long-term (4 and 6 months) hydrothermal alteration experiments at 80 ∘C using burial-like fluids. We document and evaluate the changes undergone by the outer and inner layers of the shell of the bivalve Arctica islandica, the prismatic and nacreous layers of the hard tissue of the gastropod Haliotis ovina, and the skeleton of the coral Porites sp. combining a variety of analytical tools (X-ray diffraction, thermogravimetry analysis, laser confocal microscopy, scanning electron microscopy, electron backscatter diffraction and atomic force microscopy). We demonstrate that this approach is the most adequate to trace subtle, diagenetic-alteration-related changes in aragonitic biocarbonate structural hard materials. Furthermore, we unveil that the diagenetic alteration of aragonitic biological hard tissues is a complex multi-step process where major changes occur even at the low temperature used in this study, well before any aragonite into calcite transformation takes place. Alteration starts with biopolymer decomposition and concomitant generation of secondary porosity. These processes are followed by abiogenic aragonite precipitation that partially or totally obliterates the secondary porosity. Only subsequently does the transformation of the aragonite into calcite occur. The kinetics of the alteration process is highly dependent on primary microstructural features of the aragonitic biomineral. While the skeleton of Porites sp. remains virtually unaltered for the entire duration of the conducted experiments, Haliotis ovina nacre undergoes extensive abiogenic aragonite precipitation. The outer and inner shell layers of Arctica islandica are significantly affected by aragonite transformation into calcite. This transformation is extensive for the prismatic shell layer of Haliotis ovina. Our results suggest that the majority of aragonitic fossil archives are overprinted, even those free of clear diagenetic alteration signs. This finding may have major implications for the use of these archives as geochemical proxies.

The role of the Mid-Cimmerian Unconformity on the quality of the underlying Skagerrak sandstone reservoir in the Kittiwake Field, central North Sea

The central aim of this paper is to address the role of unconformities in affecting reservoir quality. Do they facilitate diagenesis that leads to either enhanced or reduced porosity through dissolution or cementation? Or, do they have little effect? We have investigated the Late Triassic Skagerrak sandstone reservoir underlying the Mid-Cimmerian Unconformity in the Kittiwake Field, central North Sea. There is strong evidence for the development of secondary porosity through the dissolution of unstable silicate minerals, primarily feldspars. This includes the presence of oversized pores, partial dissolution of framework grains, and complete dissolution of grains leaving remnant grain margins and partially filled cores. This dissolution as a late-stage event is demonstrated by the complete lack of compaction effects on the secondary pores and diagenetic products despite present burial depths in excess of 3000 m. These observations, coupled with an absence of systematic trends linked to the unconformity surface in respects of reservoir porosity, feldspar amount and dissolution, and kaolinization, lead to the conclusion that there has been no effect of the Mid-Cimmerian Unconformity on reservoir quality in the Kittiwake Field. There is no evidence for leaching or cementation linked to meteoric water influx either shortly after deposition or following the uplift and exposure, which led to development of the Mid-Cimmerian Unconformity. Instead, we propose that the late-stage dissolution of feldspar and generation of secondary porosity are most likely related to the influx of organic acids and carbon dioxide generated either from thermogenic maturation of the source rock or from biodegradation of oil within the reservoir near the oil-water contact (OWC) transition zone.

The Teena Zn-Pb Deposit (McArthur Basin, Australia). Part II: Carbonate Replacement Sulfide Mineralization During Burial Diagenesis—Implications for Mineral Exploration

AbstractThe Teena Zn-Pb deposit is located in the Carpentaria Zn Province (Australia), which contains some of the largest clastic dominant (CD-type) massive sulfide Zn-Pb deposits in the world. The timing of the main stage of hydrothermal sulfide mineralization in the Teena subbasin is constrained to the midstage of burial diagenesis, during a period of short-lived regional extension. To distinguish hydrothermal alteration from spatially and temporally overlapping burial diagenetic alteration, and to establish the primary controls on hydrothermal mass transfer, it is necessary to evaluate the various foot- and hanging-wall alteration assemblages that formed between early- (eogenesis) and late- (mesogenesis) stage diagenesis. To achieve this, we have statistically evaluated a large lithogeochemistry dataset (n >2,500) and selected a subset (n = 65) of representative samples for detailed mineralogical (X-ray diffraction, illite crystallinity) and petrographic (scanning electron microscopy) analyses; hyperspectral core imaging data were then used to upscale key paragenetic observations.We show that sulfide mineralization was predated by multiple diagenetic alteration assemblages, including stratiform pyrite, dolomite nodules and cement, disseminated hematite and authigenic K-feldspar. These assemblages formed during eogenesis in multiple subbasins across the broader McArthur Basin and are not part of the synmineralization alteration footprint. Whereas pyrite and dolomite formed primarily from the in situ degradation of organic matter, feldspar authigenesis was the product of K metasomatism that was focused along permeable coarse-grained volcaniclastic sandstone beds within the host-rock sequence. The immature volcaniclastic input is broadly representative of the siliciclastic compositional end member in the subbasin, which formed the protolith for phyllosilicate (illite, phengite, chlorite) formation during burial diagenesis. There is no evidence of extensive phyllosilicate alteration in any of the geochemical, mineralogical (illite crystallinity), or petrographic datasets, despite some evidence of K-feldspar replacement by sphalerite in the Lower and Main mineralized lenses. Rather, the high Zn grades formed via dolomite replacement, which is resolvable from a chemical mass balance analysis and consistent with petrographic observations.There are significant exploration implications associated with carbonate-replacement sulfide mineralization during mesogenesis: (1) the capacity for secondary porosity generation in the host rock is as important as its sulfate-reducing capacity; (2) hydrothermal mineralization has a short-range cryptic lateral and vertical synmineralization alteration footprint due to acid neutralization by a carbonate-rich protolith; and (3) the distribution and chemistry of premineralization phases (e.g., pyrite, dolomite nodules) cannot be directly related to the mineralization footprint, which is localized to the 4th-order subbasin scale. Future exploration for this deposit style should therefore be focused on identifying units that contain a mixture of organic carbon and carbonate in the protolith, at favorable stratigraphic redox boundaries, and proximal to feeder growth faults.

Residual carboxylic acids (nonpyrolytic) in mudstones and their implications and constraints on sandstone diagenesis, Bozhong Depression, offshore Bohai Bay Basin, East China

Nonpyrolytic residual carboxylic acids were measured in Paleogene mudstones (0.60% < Ro < 0.88%) from the Bozhong Depression, offshore Bohai Bay Basin (East China). These residual compounds were composed of C3-C10 fatty acids and abundant naphthenic and aromatic acids. The residual water-soluble species, which were composed of formic (methanoic) acid, propionic (propanoic) acid and butyric (butanoic) acid with a predominance of propionic acid (average: 74.4%), exhibited two decreases with increasing depths: the first decrease at a depth of ∼3150 m by ∼35% of the maximum (Ro: 0.7%–0.75%; temperature: 110–115 °C) and the second at a depth of ∼3550 m by ∼12.7% of the maximum (Ro: 0.81%–0.82%; temperature: 130–135 °C). The first decrease was identified as an export-driven process, whereas the second decrease was a thermal-breakdown-driven process.The implications for sandstone diagenesis are also analysed. The predominant window of secondary porosity generation should generally be coeval with the hydrocarbon charge. Carboxylic acids transported from underlying mudstones and naphthenic acids/CO2 generated by hydrocarbon biodegradation are the main source of acids at shallow depths. Shallow depths have an injection time of carboxylic acids (including naphthenic acids/CO2) earlier than expected (∼0.30–0.35%Ro/11 Ma at shallow depths, younger than ∼0.7%Ro/38 Ma at deep depths). These factors cause rocks at shallow and deep locations to have different diagenetic evolution paths. Therefore, it is inappropriate to use diagenetic models of shallow depths to predict the evolution of deep reservoirs.

Structural and sedimentary discontinuities control the generation of karst dissolution cavities in a carbonate sequence, Potiguar Basin, Brazil

Epigenetic karstic systems in carbonate rocks commonly result from progressive dissolution by acidic meteoric waters over thousands to millions of years. The generation of secondary porosity and permeability improvement due to dissolution in carbonate reservoirs of geofluids (e.g., groundwater, hydrocarbons, and CO2) can profoundly impact reservoir storage capacity and subsurface fluid flow. This study investigates the control of structural discontinuities such as stylolites, fractures, and primary sedimentary discontinuities on the generation of multiscale karst dissolution cavities by epigenetic fluid percolation in a Late Cretaceous carbonate sequence (Jandaíra Formation) in the Potiguar Basin, Northeastern Brazil. The study relies on micro- and macroscale analyses such as stratigraphic logs, field structural investigations, rock strength data collected in the field (Schmidt hammer), microtomographic and drone images, thin section analyses, porosity and permeability laboratory measurements. The results show that bed-perpendicular stratabound and non-stratabound stylolites and fractures can be enlarged due to meteoric water percolation until they merge and form a single channel system that crosscuts all sedimentary multilayers. Bed-parallel stylolites are ubiquitous in carbonate sequences overprinting bed interfaces and layers. Where not dissolved, bed-parallel stylolites have low porosity and permeability and thus can act as barriers to vertical fluid flow. Where dissolved, such stylolites can contribute to horizontal fluid flow and form channel porosity. The results of this study led to a formulation of a conceptual model of rock dissolution along structural and sedimentary discontinuities that affects carbonate rock successions in the subsurface.

Effects of natural gas acidic components on local porosity generation in a carbonate reservoir: Insights from reactive transport modeling

To investigate the mechanism of local porosity modification in a carbonate reservoir caused by the acidic components associated with natural gas generation, a numerical model for the charging of natural gas rich in acidic components (CO2) was developed. The acidic components are originated from maturation of organic matter in the source rock. The model includes coupled gas–water–rock interactions, multiphase flow, and spatial heterogeneities of the geochemical reactions. The simulation results indicate the following. (1) When regional groundwater travels through the gas–water contact (GWC), it is acidified by the CO2(g) in the gas zone and will dissolve carbonate minerals to form local secondary porosity. The generated local secondary porosity is approximately 0.04 (volume fraction), with a maximum value of approximately 0.27, and is located mainly in the vicinity of the GWC. (2) Under reservoir conditions, the solubility of calcite is much higher than dolomite. The higher solubility and common ion effect of Ca results in calcite being the primary reactive mineral in pure carbonate reservoirs (having both calcite and dolomite). The presence of anhydrite in the system does not change this situation. (3) The groundwater flow field causes GWC evolution to be S-shaped. The inflection and gentle-sloping parts at both ends of the GWC have different angles with the groundwater stream lines, resulting in different amounts of mineral dissolution and precipitation and porosity increase. (4) The limited amount of CO2 from the source rock and groundwater flow in the subsurface seems to be sufficient to support local secondary porosity generation.

Desarrollo de Catalizadores Sólidos tipo Zeolíticos para Valorización de Derivados de Biomasa

El objetivo principal de este plan de tesis es el desarrollo de zeolitas micro/mesoporosas mediante síntesis directa y tratamientos alcalinos post síntesis para su aplicación en procesos catalíticos de valorización de biomasa. En este trabajo se propone la generación de porosidad adicional en zeolitas microporosas con estructura ZSM-5 y ZSM-11 mediante síntesis directa empleando almidón soluble y un surfactante catiónico (CTAB) como plantillas mesoporosas. Se estudió el efecto del tiempo de cristalización y la modificación del contenido de almidón y CTAB en la síntesis sobre la obtención de la estructura cristalina y la generación de mesoporosidad en los materiales. Los sólidos obtenidos fueron caracterizados por diferentes técnicas como difracción de rayos X (DRX), Isotermas de adsorción y desorción de nitrógeno, área superficial BET, Microscopía electrónica de barrido (SEM), Espectroscopía de emisión atómica con plasma inductivamente acoplado (ICP- AES). Los resultados confirmaron que las zeolitas obtenidas por ambas síntesis poseen microporos y mesoporos en su estructura.

Relationship between diagenesis and the emplacement of bitumen in the Lower Triassic Piramboia Formation sandstones, Paraná Basin, SW Brazil

Outcrops of eolian sandstones of the Lower Triassic Piramboia Formation in the eastern margin of the intracratonic Paraná Basin are known as the main occurrence of tar sandstones in Brazil because contain heavy oil (“tar”). In order to investigate the migration of oil into the sandstones petrographic analyses of dune facies were performed. Optical (transmitted, reflected and epifluorescence lights) and Field Emission Scanning Electron (FE-SEM) microscope studies together with X-Ray Diffraction (XRD) analyses showed that the Piramboia Formation sandstones have not been affected by intense diagenesis. Eogenetic features includes mainly thin, continuous inherited ferruginous clay coatings and minor mechanically infiltrated clay coatings around detrital mineral grains. The authigenesis of smectite and K-feldspar overgrowths precipitation begin to develop and during shallow mesodiagenesis, authigenic smectite was gradually transformed into mixed-layer illite-smectite; K-feldspar overgrowths, quartz overgrowths continuous to develop, and pore-filling calcite cement were formed locally. In the late mesodiagenesis the emplacement of oil occurred, filling pores and covering grains, clay coatings and calcite. During later surface exposure, water washing by meteoric fluids and biodegradation transformed the oil into a heavy oil rich in asphaltenes. The influx of acid meteoric waters caused dissolution of detrital K-feldspars as well as meteoric waters generate organic acids which also caused the dissolution of K-feldspars and the consequent generation of secondary porosity with the precipitation of inter- and intragranular vermicular kaolinite. However, kaolinite can be also a product of eodiagenetic process similar to the occur in the telodiagenesis. To evaluate the porosity of the sandstones microtomography analysis were preformed revealing high porosity volume (ca. 26%) and high volume of porous with connectivity (ca. 26%). Although not currently commercially exploited nowadays the knowledge of the Piramboia tar sandstones diagenetic evolution contributes for a better understanding of the Paraná Basin sedimentary fill during the Lower Triassic.

Microfacies, diagenesis, and stable isotope analysis of the Jurassic Jumara Dome carbonates, Kachchh, Western India: Implications for depositional environments and reservoir quality

The microfacies, detailed diagenesis, and stable isotope analysis of the carbonates, exposed at Jumara Dome, have been analysed to explore their impact on reservoir quality. The carbonates of Jumara Dome are mostly fine‐ to medium‐grained, and some are coarse‐grained, poorly to very well sorted, having angular to well‐rounded grains of low to high sphericity. Bioclasts are the most abundant constituent in these carbonates and includes brachiopods, foraminifers, ostracodes, calcispheres, algae, bryozoans, echinoderms, sponges, gastropods, and corals. Twelve microfacies were recorded in the present investigation. The temporal distribution of microfacies association reflects an ideal shallowing‐upward sequence representing a system of bioclastic bars developed on a carbonate ramp, which separates a lagoonal environment from open marine environment. In Jumara Dome carbonates, the δ13C values range from −1.89‰ to 1.44‰ and δ18O values range from −8.95‰ to −4.26‰ and suggest a marine environment of deposition for these carbonates under warmer Jurassic climate conditions. Micritization, silicification, compaction, cementation, neomorphism, geopetal structures, and porosity are the main diagenetic features affecting the carbonates and developed in marine phreatic, fresh water phreatic, mixed marine‐fresh water phreatic, and burial diagenetic environments. The diagenetic processes (mechanical compaction and cementation) has affected the carbonates reservoir characteristics, but the dissolution has resulted in generation of secondary porosity in the form of moulds, stylolites by pressure solution, and fractures (calcite veins), and enhancing the permeability resulting in improvement of carbonate reservoir characteristics, suggesting the carbonates of the Jumara Dome could be a better carbonate reservoir for petroleum accumulation.

Tertiary hydrothermal activity and its effect on reservoir properties in the Xihu Depression, East China Sea

Three large-scale episodes of volcanic activity occurred during the Tertiary in the Xihu Depression, located in the East China Sea. Intermediate-felsic magmas intruded along faults and the associated hydrothermal fluids resulted in the hydrothermal alteration of the clastic country rock. To better describe reservoir characteristics, reservoir samples were subjected to the following investigations: thin section examination, scanning electron microscope-energy dispersive spectrometer analysis (SEM–EDS), fluid inclusion homogenization temperature tests, vitrinite reflectance measurements, and X-ray diffraction. The results of this study provide evidence of the following hydrothermal alteration phenomena: brittle fracturing, clastic particle alteration, precipitation of unique hydrothermal minerals (celestite, zircon, apatite, barite, and cerous phosphate). The presence of abnormally high temperatures is indicated by fluid inclusion analysis, the precipitation of high-temperature authigenic minerals such as quartz, illite alteration, and anomalous vitrinite reflectance. Two aspects related to hydrothermal effects on reservoir properties have been investigated in this study: (1) Deep magmatic hydrothermal fluids carry large amounts of dissolved carbon dioxide and sulfur dioxide gas. These fluids percolate into the country rocks along fault zones, resulting in dissolution within the sandstone reservoirs and the development of significant secondary porosity. (2) Magma intrusions increase the temperature of the surrounding rocks and accelerate the thermal evolution of hydrocarbon source rocks. This results in the release of large amounts of organic acids and carbon dioxide, leading the dissolution of the aluminosilicate minerals and volcanic fragments in the reservoirs, and the generation of significant secondary porosity.

The conduit-seal system of hypogene karst in Neoproterozoic carbonates in northeastern Brazil

Abstract Karst processes are of primary importance for the generation of secondary porosity and permeability in carbonate reservoirs and aquifers. The topic investigated in this study are the effects of the facies and stratigraphy of different layers on the vertical distribution of solution cavities formed by hypogenic fluids in a carbonate sequence. This work uses the Toca da Boa Vista (TBV) and Toca da Barriguda (TBR) caves as analogues of carbonate reservoirs that are affected by karst processes. These are two different caves that are 400 m apart but exhibit the same geological characteristics and occur in the Neoproterozoic Salitre Formation (700-560 Ma), Sao Francisco Craton, Brazil. These caves form the longest cave system in South America, with conduits of lengths ∼107 km (TBV) and ∼34 km (TBR). Previous studies of these caves indicated their hypogene origins. These caves developed by an ascending fluid flow, mainly along fractures. The fluid then flowed laterally and was influenced by the following five stratigraphic units, from the bottom to the top: (1) cross-bedded oolitic grainstone, (2) fine grainstone with chert nodules, (3) microbial carbonate, (4) interbedded fine siliciclastics and marls, and (5) crystalline grainstone interfingered with chert layers. Units 4 and 5 formed a stratigraphic seal. Units 1, 2, and 3 below the seal supported the lateral redistribution of flow and respective conduit development. Therefore, these units exhibit a high degree of karstification. Veins related to the first fluid phases that compose a hydrothermal mineral assemblage cut across these units. We conclude that stratigraphic control is important for determining the architecture of the hypogenic cave system at the local scale.

High-temperature alteration and porosity generation in Upper Ordovician microbial reefs, Hudson Bay intracratonic Basin, Arctic Canada

Upper Ordovician biohermal mounds of the Red Head Rapids River Formation in the Hudson Bay Basin consist of microbial and sponge boundstone and cementstone. The cementstone is made up of isopachous layers and botryoids of former aragonite now calcite. Secondary dissolution porosity and small fractures are cutting through the bioherm. Secondary pore-fillings consist of drusy calcite cement and subsequent bitumen. The δ18OVPDB and δ13CVPDB values of late cements are invariably more negative than those of the marine cements. Fluid inclusion microthermometry documents the presence of two distinct populations of homogenization temperatures, one in secondary pore-filling burial calcite cement (Th average 93 ± 10 °C) and a second one in recrystallized marine calcite cements (Th average 118 ± 25 °C). The combined δ18OVPDB and Th data suggest that burial cements precipitated from a fluid having δ18OSMOW values between +0.5 and −2‰, whereas the marine cement data indicate resetting of the fluid inclusions in the presence of a high temperature, δ18OSMOW heavy brine (+2.5 to +12‰). The lower temperature fluid inclusions of the late pore-filling cement agree with recent Apatite Fission Track data suggesting early oil window temperature at maximum burial. The higher Th values recorded in the marine cement represent resetting of initial or entrapment of new fluid inclusions from fracture-controlled circulation of basement-derived fluids. The petrographic and geochemical data suggest that fracture-controlled high temperature brine circulation occurred after the inception of burial and recrystallization of original marine aragonite to calcite, which resulted in the generation of significant secondary porosity that was later filled by lower temperature burial cements and hydrocarbons.

Origin and significance of secondary porosity: A case study of upper Triassic tight sandstones of Yanchang Formation in Ordos basin, China

The Chang7 and Chang6 sandstones in the southwestern Ordos basin are the typical tight oil sandstones in China. Pore textures and their genetic classes of secondary porosity were studied in detail using a combination of optical petrography, SEM and BSE. In order to understand generation process of secondary porosity better, the diagenetic history was reconstructed by a variety of methods, including thin section observation, SEM, BSE and fluid inclusion. Three main diagenetic events is considered to be most likely related to the generation of secondary porosity, these are: 1) meteoric water flushing; 2) formation of authigenic clay minerals; 3) leaching of feldspars, rock fragments and carbonate cements in deep burial sandstones. Particular attention was paid to the generation of secondary porosity and mass transformation in these diagenetic events. This study suggests only meteoric water flushing, which occurs at shallow depths, may result in a net increase of porosity because dissolved minerals can be removed out of sandstones in such an open system. However, below the zone of freshwater flow, dissolved minerals would precipitate as new diagenetic minerals at nearby pore spaces in sandstones, suggesting secondary porosity formed by dissolution only represents a local redistribution of the solids rather than a net increase of porosity.