Diagenetic Assemblages Research Articles

Characteristics and Genetic Mechanism of Pore Throat Structure of Shale Oil Reservoir in Saline Lake—A Case Study of Shale Oil of the Lucaogou Formation in Jimsar Sag, Junggar Basin

The shale oil reservoir of the Lucaogou Formation in the Jimsar Sag has undergone tectonic movement, regional deposition and complex diagenesis processes. Therefore, various reservoir space types and complex combination patterns of pores have developed, resulting in an intricate pore throat structure. The complex pore throat structure brings great challenges to the classification and evaluation of reservoirs and the efficient development of shale oil. The methods of scanning electron microscopy, high-pressure mercury injection, low-temperature adsorption experiments and thin-slice analysis were used in this study. Mineral, petrology, pore throat structure and evolution process characteristics of the shale oil reservoir were analyzed and discussed qualitatively and quantitatively. Based on these studies, the evolution characteristics and formation mechanisms of different pore throat structures were revealed, and four progressions were made. The reservoir space of the Lucaogou Formation is mainly composed of residual intergranular pores, dissolved pores, intercrystalline pores and fractures. Four types of pore throat structures in the shale oil reservoir of the Lucaogou Formation were quantitatively characterized. Furthermore, the primary pore throat structure was controlled by a sedimentary environment. The pores and throats were reduced and blocked by compaction and cementation, which deteriorates the physical properties of the reservoirs. However, the dissolution of early carbonate, feldspar and tuffaceous minerals and a small amount of carbonate cements by organic acids are the key factors to improve the pore throat structure of the reservoirs. The genetic evolution model of pore throat structures in the shale oil reservoir of the Lucaogou Formation are divided into two types. The large-pore medium-fine throat and medium-pore medium-throat reservoirs are mainly located in the delta front-shallow lake facies and are characterized by the diagenetic assemblage types of weak compaction–weak carbonate cementation–strong dissolution, early medium compaction–medium calcite and dolomite cementation–weak dissolution. The medium-pore fine throats and fine-pore fine throats are mainly developed in shallow lakes and semi-deep lakes. They are characterized by the diagenetic assemblage type of strong compaction–strong calcite cementation–weak dissolution diagenesis. This study provides a comprehensive understanding of the pore throat structure and the genetic mechanism of a complex shale oil reservoir and benefits the exploration and development of shale oil.

Diagenesis and genesis of clay minerals in the Triassic sandstones of the Panchet and Parsora formations, Damodar–Son Basin, India

The Triassic sandstones of the Panchet and Parsora formations are deposited in the Damodar and Son basins of the Peninsular India. Though both the formations have similarity in depositional and lithification processes, their diagenetic history and authigenic clay minerals formation have little differences. To improve understanding of diagenesis and associated clay minerals, the present study examined in Triassic sandstones. The Lower Triassic Panchet sandstones are poorly sorted arkose and originate from nearby granitoid terrain. The early diagenetic assemblages of the sandstones are kaolinite–iron oxide, montmorillonite, quartz and cement in the sandstones formed in oxy-acidic environment under shallow burial condition. Authigenic chlorite and recrystallised interstitial components such as orthomatrix are the main late diagenetic products formed, respectively, at oxy-acidic and reducing alkaline conditions where carbonate and allogenic iron cements were emplaced as the latest events. The Panchet and the Parsora formations are the riverine Triassic deposits and have almost identical early diagenetic assemblages. The kaolinisation in sandstone is loss of feldspar and is more pronounced in the Parsora Formation. The late diagenetic products in both the formations essentially similar and differ only in their relative chlorite content, which was formed under elevated temperature as an effect of tectonic activity. Allogenic iron and carbonate cements are post-tectonic.

Diagenetic history and provenance of Devonian terrestrial sandstones at the margin of Gondwana: Padeha Formation, Eastern Alborz, Iran

• The northern margin of Gondwana in Iran rifted in the Devonian, forming a passive continental margin to the Paleotethys Ocean. • The Padeha sandstones were sourced by the late Neoproterozoic granitic gneisses of Iran. • Further mesodiagenesis was terminated by the Eocimmerian collision. The northern margin of Gondwana in Iran rifted in the Devonian, forming a passive continental margin to the Paleotethys Ocean, the details of which are obscured by later collision. A record of the early phase of opening of Paleotethys is preserved in the 600-m thick Padeha Formation, a Middle Devonian terrestrial rift basin succession. The sandstones are quartz arenite, sub-arkose and arkose and contain minimal detrital matrix (~1%). Bulk-rock geochemistry, according to major elements, suggests a source to the sandstones that was dominated by I-type granite, but also, based on trace elements, included small amounts of intermediate or mafic rocks. Low Zr and HREE contents suggest little or no sandstone in the source area and thus low abundance of fine-grained heavy minerals (<1%). The sandstones were sourced by the widespread granitic gneisses of central and northern Iran that formed at the northern margin of Gondwana in the late Neoproterozoic–Ediacarian. Diagenesis of the sandstones is dominated by compaction and fracturing of framework quartz without widespread stylolites in the sandstones, and cementation by silica, carbonates and Fe-oxides. Fractures within quartz grains are filled mostly by silica, but locally by carbonate and authigenic K-feldspar. Albitization of plagioclase, K-feldspar overgrowths and partial dissolution of detrital K-feldspar post-date silica cements and suggest temperatures between 70 and 120 °C. The particular diagenetic assemblage of altered K-feldspar and limited quartz stylolites was a consequence of a high geothermal gradient of ~70 °C/km at a burial depth of <1.5 km. Further mesodiagenesis was terminated by the Eocimmerian collision.

Geology, alteration system, and uranium metallogenic potential of Pasha-Ladoga Basin, Russia

The Mesoproterozoic Pasha-Ladoga Basin is located at the margin of the Karelian Craton and Svecofennian Belt in northwest Russia. It is the only basin in Russia that is currently recognized to host unconformity-type uranium mineralization (e.g., Karku deposit). Mineralization is associated with the Priozersk Formation which comprises a basal sequence of immature arkosic sandstones and conglomerates interpreted as high-energy braided stream channel and channel bar facies within an alluvial fan depositional environment. Diagenetic and hydrothermal alteration of the Priozersk sandstones is similar to that observed in unconformity-related uranium deposits of the Athabasca Basin (Canada) and includes three main stages: (1) an early diagenetic assemblage comprising illite-smectite (K-rectorite), calcite, minor chlorite, and kaolinite precipitated at 100–150 °C; (2) a peak diagenetic-hydrothermal assemblage composed of kaolinite-dickite, Fe- and Fe-Mg chlorite, calcite, and sulfides formed at 150–250 °C; (3) late diagenetic-hydrothermal event involving fracture-controlled kaolinite and late hematite. Similar to the Athabasca Basin type uranium mineralization, the Karku deposit exhibits anomalous concentrations of LREE, Nb, Pb, and positive correlations between U and Cu, Zn, Ag, and Pb that is a geochemical signature of unconformity-type uranium mineralization. Based on oxygen and hydrogen stable isotope study, it was found that fluids in equilibrium with early and peak diagenetic clays were consistent with oxidized basinal brines and reduced basement fluids. In contrast to the Athabasca Basin, mineralizing fluids at the Karku area were more reduced and potassic. A new geological model for the Karku deposit is proposed based on petrographic observations and sandstones geochemistry. The proposed model envisages the permeable rocks of paleoregolith and the coarse-grained basal part of the Priozersk Formation as the main pathway for migration of uraniferous fluids responsible for the formation of Karku uranium deposit. From the perspective of the geological setting, the Karku deposit differs from deposits of the Athabasca Basin in that it occurs in a rapidly deposited rift-related basin with associated mafic magmatism, the host sandstone sequence is relatively thin (e.g., Priozersk Formation) and exhibits abrupt lateral facies variations, and the widespread presence of early diagenetic interstitial swelling clays. Based on these sedimentological characteristics, it is considered that the Karku deposit setting precludes optimum conditions to create high tonnage high-grade unconformity-type U deposits as found in the Athabasca basin.

Sandstone Diagenesis in Sediment–lava Sequences: Exceptional Examples of Volcanically Driven Diagenetic Compartmentalization in Dune Valley, Huab Outliers, Nw Namibia

At the base of many flood basalt sequences and along volcanic rifted margins, volcanism can compete with the existing sedimentary environments, resulting in interbedded sequences of volcanic rocks and sediments. Here we report on sediment interlayers that are found in the lowermost volcanic units of the Etendeka flood basalts in NW Namibia (Twyfelfontein and Awahab formations), part of the much larger Parana–Etendeka Igneous Province. The sandstone bodies, predominantly eolian dunes, are isolated in a sequence of Lower Cretaceous (∼ 134 Ma) lava flows. The uppermost part or where sediment deposition and lava emplacement is observed to interact is characterized by barchanoid dunes, which were actively migrating during the emplacement of the lava flows. The fossil (isolated by lava) barchan dunes studied in Dune Valley show three characteristically different diagenetic styles. In Dune Valley, each dune body is completely encapsulated by lava, with additional igneous intrusions cutting through some bodies. We recognize three distinct styles of diagenesis: Type 1: fossilised dunes that are red in color and lack major authigenic mineralization, with grain compaction and subsequent porosity loss being the dominant diagenetic process. Type 2: dunes that have been bleached white, which have undergone a more complex diagenetic pathway. Type 2 dunes have abundant calcite, kaolinite, and bohmite as authigenic phases and lack hematite grain coatings. Detrital plagioclase is absent in white dunes (XRD analysis), with pseudomorphs of kaolinite common. This diagenetic assemblage results in the white dunes having lower porosity and permeability compared to the red dunes. The observations are probably due to a flux of carbon dioxide (CO2), hydrogen sulfide (H2S) and/or hydrogen (H2)–rich hydrothermal groundwater derived from igneous intrusions below. Type 3: “hot contact” effects at lava-flow contacts, where the unconsolidated dunes were rapidly indurated during lava emplacement (volcano-eogenesis). Type 3 diagenesis is restricted to << 1 m depth below lava contacts and common in dunes displaying both Type 1 and Type 2 diagenesis. The distribution of diagenetic Types 1 and 2 is dune specific, and throughout Dune Valley approximately half of the dunes have been bleached (e.g., Type 2 diagenesis), whereas diagenetic Type 3 is a hot contact phenomenon and is therefore found along all basal lava and dike contacts. This work has relevance to understanding the development of sediment–lava systems, to hydrocarbon exploration and development in preserved sediment–lava sequences, and the hydrothermal process described provides an example of natural CO2 sequestration.

Regional metamorphism and the alteration response of selected Silurian to Devonian mineral systems in the Nymagee area, Central Lachlan Orogen, New South Wales: a HyLogger™ case study

ABSTRACTThe southern Cobar Superbasin includes the Mount Hope, Cobar and Rast troughs and adjacent Winduck, Mouramba and Kopyje shelves. These stratotectonic units host a range of mineral systems within deformed upper Silurian to Lower Devonian rocks. Despite the metallogenic importance of the terrane the regional metamorphic- and alteration-related mineral assemblages have not been systematically described until now. Here, we present a study using the HyLogger™ spectral scanner to systematically map changes in both background and alteration-related mineralogy for 14 mineral systems across the Nymagee area. The study found that, with the exception of the late diagenetic zone assemblages associated with the Winduck Shelf, the mineral assemblages for zones away from mineralisation are consistent with uniform sub- to lowest-greenschist facies metamorphic conditions, with no evidence for a regional, thermal field gradient across the basin. By contrast, proximal to mineralisation, there are significant changes in alteration-related mineral abundances and species reflecting elevated fluid temperatures. For several zones, including Browns Reef, Great Central, Hera, May Day, Nymagee and Shuttleton, there is a systematic change in chlorite composition from Fe- and/or Fe–Mg-chlorites to more Mg-rich varieties towards mineralisation. In addition, talc was noted for May Day, Mineral Hill and Shuttleton. The change in chlorite composition suggests that these ore-forming fluids were undersaturated with respect to iron. Furthermore, although present in discrete zones, carbonate alteration is not a dominant alteration type. However, at Manuka and Blue Mountain, the mineralisation is associated with carbonate-rich (calcite, ankerite, dolomite) units that have been dolomitised (Mg alteration) but not all dolomite-rich zones are mineralised. For Mineral Hill, there are systematic changes to mineral species/chemistry with observed data consistent with Parkers Hill being proximal to the centre of a mineralising system. The G and H Lode area (Southern Ore Zone) is slightly more distal whereas the Pearse deposit may represent a zone more distal again.

Chemistry of diagenetic features analyzed by ChemCam at Pahrump Hills, Gale crater, Mars

The Curiosity rover's campaign at Pahrump Hills provides the first analyses of lower Mount Sharp strata. Here we report ChemCam elemental composition of a diverse assemblage of post-depositional features embedded in, or cross-cutting, the host rock. ChemCam results demonstrate their compositional diversity, especially compared to the surrounding host rock: (i) Dendritic aggregates and relief enhanced features, characterized by a magnesium enhancement and sulfur detection, and interpreted as Mg-sulfates; (ii) A localized observation that displays iron enrichment associated with sulfur, interpreted as Fe-sulfate; (iii) Dark raised ridges with varying Mg- and Ca-enriched compositions compared to host rock; (iv) Several dark-toned veins with calcium enhancement associated with fluorine detection, interpreted as fluorite veins. (v) Light-toned veins with enhanced calcium associated with sulfur detection, and interpreted as Ca-sulfates. The diversity of the Pahrump Hills diagenetic assemblage suggests a complex post-depositional history for fine-grained sediments for which the origin has been interpreted as fluvial and lacustrine. Assessment of the spatial and relative temporal distribution of these features shows that the Mg-sulfate features are predominant in the lower part of the section, suggesting local modification of the sediments by early diagenetic fluids. In contrast, light-toned Ca-sulfate veins occur in the whole section and cross-cut all other features. A relatively late stage shift in geochemical conditions could explain this observation. The Pahrump Hills diagenetic features have no equivalent compared to targets analyzed in other locations at Gale crater. Only the light-toned Ca-sulfate veins are present elsewhere, along Curiosity's path, suggesting they formed through a common late-stage process that occurred at over a broad area.

How sandstone porosity and permeability vary with diagenetic minerals in the Scotian Basin, offshore eastern Canada: Implications for reservoir quality

The Scotian Basin is an under-explored offshore basin that has produced gas and minor oil from Jurassic–Cretaceous deltaic sandstones. Reservoir quality is an important exploration risk in the basin. This study evaluates whether there are systematic stratigraphic or geographic variations in diagenetic mineral assemblage control of reservoir porosity and permeability. Three data sets are used: a compilation of all available core-plug porosity and permeability measurements (3271 in total), a subset of 577 corresponding to thin sections with lithofacies assignments, and a subset of 35 thin sections principally from reservoir sandstones, in which modal abundance of diagenetic minerals has been determined by image analysis of scanning electron microscope backscattered electron images. Abundances above a 4% threshold of diagenetic kaolinite, calcite and ankerite correlate inversely with permeability, but chlorite and possibly siderite correlate positively. Diagenetic assemblages show no systematic variation with stratigraphy or geography, both of which may involve variable detrital supply. Permeability shows the well-known variation with grain size and depth of burial, but after correcting for these effects it is greatest in estuarine channel and river-mouth turbidite sandstones. This is because these facies preferentially develop chlorite rims on detrital quartz grains and are also the sites of enhanced flow of late basinal fluids resulting in secondary porosity. Local effects such as facies distribution, architecture of channel sandstones, fluid pathways related to complex salt tectonics, and the effects of overpressure are more important than predictable basin-wide effects in controlling sandstone reservoir quality.

Diagenetic evolution of the volcaniclastic deposits: an example from Neoproterozoic Dokhan Volcanics in Wadi Queih basin, central Eastern Desert, Egypt

Wadi Queih basin hosts a ∼2,500-m thick Neoproterozoic volcanoclastic successions that unconformably lie over the oldest Precambrian basement. These successions were deposited in alluvial fan, fluviatile, lacustrine, and aeolian depositional environments. Diagenetic minerals from these volcaniclastic successions were studied by X-ray diffractometry, scanning electron microscopy, and analytical electron microscopy. The diagenetic processes recognized include mechanical compaction, cementation, and dissolution. Based on the framework grain–cement relationships, precipitation of the early calcite cement was either accompanied or followed by the development of part of the pore-lining and pore-filling clay cements. Secondary porosity development occurred due to partial to complete dissolution of early calcite cement and feldspar grains. In addition to calcite, several different clay minerals including kaolinite, illite, and chlorite with minor smectite occur as pore-filling and pore-lining cements. Chlorite coating grains helps to retain primary porosity by retarding the envelopment of quartz overgrowths. Clay minerals and their diagenetic assemblages has been distinguished between primary volcaniclastics directly produced by pyroclastic eruptions and epiclastic volcaniclastics derived from erosion of the pre-existing volcanic rocks. Phyllosilicates of the epiclastic rocks display wider compositional variations owing to wide variations in the mineralogical and chemical compositions of the parent material. Most of the phyllosilicates (kaolinite, illite, chlorite, mica, and smectite) are inherited minerals derived from the erosion of the volcanic basement complex, which had undergone hydrothermal alteration. Smectites of the epiclastic rocks are beidellite–montmorillonite derived from the altered volcanic materials of the sedimentary environment. Conversely, phyllosilicate minerals of the pyroclastic rocks are dominated by kaolinite, illite, and mica, which were formed by pedogenetic processes through the hydrothermal influence.

Clinoptilolite as a new proxy of enhanced biogenic silica productivity in lower Miocene carbonate sediments of the Bahamas platform: Isotopic and thermodynamic evidence

Massive clinoptilolite authigenesis was observed at about 1105 meters below sea floor (mbsf) in lower Miocene well-compacted carbonate periplatform sediments from the Great Bahama Bank [Ocean Drilling Program, ODP Leg 166, Site 1007]. The diagenetic assemblage comprises abundant zeolite crystallized within foraminifer tests and sedimentary matrix, as well as Mg smectites. In carbonate-rich deposits, the formation of the zeolite requires a supply of silica. Thus, the objective of the study is to determine the origin of the silica supply, its diagenetic evolution, and consequently the related implications on interpretation of the sedimentary record, in terms of local or global paleoceanographic change. For lack of evidence for any volcaniclastic input or traces of Si-enriched deep fluids circulation, an in situ biogenic source of silica is validated by isotopic data and chemical modeling for the formation of such secondary minerals in shallow-water carbonate sequences. Geochemical and strontium isotopic data clearly establish the marine signature of the diagenetic zeolite, as well as its contemporaneous formation with the carbonate deposition (Sr model ages of 19.6–23.2 Ma). The test of saturation for the pore fluids specifies the equilibrium state of the present mineralogical assemblage. Seawater–rock modeling specifies that clinoptilolite precipitates from the dissolution of biogenic silica, which reacts with clay minerals. The amount of silica (opal-A) involved in the reaction has to be significant enough, at least 10 wt.%, to account for the observed content of clinoptilolite occurring at the most zeolite-rich level. Modeling also shows that the observed amount of clinoptilolite (∼ 19%) reflects an in situ and short-term reaction due to the high reactivity of primary biogenic silica (opal-A) until its complete depletion. The episodic occurrence of these well-lithified zeolite-rich levels is consistent with the occurrence of seismic reflectors, particularly the P2 seismic sequence boundary located at 1115 mbsf depth and dated as 23.2 Ma. The age range of most zeolitic sedimentary levels (biostratigraphic ages of 21.5–22 Ma) correlates well with that of the early Miocene glaciation Mi-1 and Mi-1a global events. Thus, the clinoptilolite occurrence in the shallow carbonate platform environment far from volcanogenic supply, or in other sensitive marine areas, is potentially a significant new proxy for paleoproductivity and oceanic global events, such as the Miocene events, which are usually recognized in deep-sea pelagic sediments and high latitude deposits.

Diagenetic signatures of stratal surfaces in the Upper Jurassic Fulmar Formation, Central North Sea, UKCS

AbstractSedimentation rate and changes in relative sea level affect early diagenetic cementation along key stratal surfaces within the deeply buried Upper Jurassic Fulmar Formation, South Central Graben, UKCS. As a result of the bioturbated nature of the shallow marine strata, variations in ichnofabrics and cross‐cutting relationships between trace fossils are important in identifying and correlating stratal surfaces. Two transgressive and two regressive surfaces/sequence boundaries were studied, using petrographical, mineralogical and geochemical techniques. Cements, including early diagenetic pyrite framboids, grain‐rimming apatite, microcrystalline dolomite and ankerite, occur at, above and immediately below both transgressive and forced regressive surfaces/sequence boundaries. Breaks or subdued rates of sedimentation associated with both types of surfaces meant that the sediment package resided within early diagenetic zones for prolonged periods of time, enhancing diagenetic reactions at and below the surfaces. The distribution of ankerite, despite being a deep‐burial cement, was primarily controlled by concentrations of bioclasts that are particularly abundant at transgressive surfaces. The diagenetic character of the forced regressive surfaces/sequence boundaries is more complex than that of the transgressive surfaces, both in terms of mineralogy and paragenesis. This is attributed to the superimposition of diagenetic assemblages as a result of erosion, re‐sedimentation and sediment by‐passing. Although the diagenetic signature of the surfaces has been modified by dissolution/replacement reactions during deep burial, early diagenetic signatures can still be distinguished.

Diagenetic to low-grade metamorphic evolution of clay mineral assemblages in Palaeozoic to early Mesozoic rocks of the Eastern Taurides, Turkey

AbstractPalaeozoic to early Mesozoic rocks of both autochthonous and allochthonous units of the Eastern Tauride Belt have different textural and mineralogical features related to their varying deposition regimes and thermal histories. The Devonian and Carboniferous formations in the allocthonous units have anchizonal to epizonal grades of metamorphism, whereas those of the autochthon experienced only early diagenetic conditions. A well-developed penetrative slaty cleavage is also found in Devonian formations of the allocthonous units, which is not observed in equivalent rocks of the autochthon. Phyllosilicates of the uncleaved diagenetic assemblages consist mainly of illite, chlorite, kaolinite, mixed-layer illite-smectite, chlorite-vermiculite and chloritesmectite, whereas in addition to these, metamorphic allochthonous units contain some index minerals such as dickite, paragonite, Na and K mica, pyrophyllite, stilpnomelane and chloritoid. These latter assemblages show a higher degree of textural and mineralogical maturation and lower b cell dimensions than the autochthonous rocks, suggesting that they originated in a relatively high heat flow basin such as an extensional setting.

Tectonic–palaeoenvironmental forcing of clay-mineral assemblages in nonmarine settings: the Oligocene–Miocene As Pontes Basin (Spain)

Two small, alluvial-lacustrine subbasins developed during the early restraining overstep stages of the Oligocene–Miocene As Pontes strike-slip Basin (NW Spain). Later, the basin evolved into a restraining bend stage and an alluvial-swamp-dominated depositional framework developed. The palaeobiological record demonstrates that the Oligocene–Miocene palaeoclimate in NW Spain was subtropical, warm and humid to subhumid. The metamorphic and igneous basin catchment yielded clay assemblages made up by kaolinite, illite and Al-smectite. Illite occurred as an original mineral in the source rock area, whereas kaolinite and Al-smectite resulted mainly from weathering of feldspar and clinochlore, respectively. This detrital primary clay assemblage remained preserved in the colluvial, alluvial fan and shallow lacustrine facies, whose early diagenesis was influenced by diluted, poorly evolved pore waters with neutral to slightly alkaline pH. The original clay assemblage was mildly to strongly transformed under early diagenetic conditions in the lacustrine and swampy environments where significant hydrochemical and Eh–pH changes took place. A fibrous magnesium-rich clay mineral-dominated assemblage (palygorskite and sepiolite) formed in shallow, saline lakes and palustrine zones under the influence of magnesium-rich, alkaline waters. Moreover, kaolinite-enriched assemblages formed in deep lacustrine, swamp and swamp-related alluvial zones under the influence of slightly to highly acidic pore waters. Pore water acidic conditions, characterising environments with organic matter accumulation, led to early diagenetic transformation of Al-smectite into kaolinite. This process was relatively limited in some environments such as organic matter-rich bottoms in meromictic lacustrine zones, whereas it was pervasive in peat-forming swamp zones. The stratigraphic relationships between the diverse clay mineral assemblages in the As Pontes Basin fill demonstrate the coeval development of rather diverse clay compositions. This clearly reflects the influence on clay assemblages of palaeoenvironmental changes forced by the morphological and tectonic evolution of the catchment–basin system. The interplay between climate and tectonic processes in the source areas did not result in major variations of the clay minerals fed into the basin. Conversely, this tectonic-sedimentation interplay influenced the evolution of the drainage and the water balance in the depositional zones, causing a complex environmental–hydrochemical evolution to occur. As a consequence, drastic early diagenetic changes affected the original clay mineral assemblages and resulted in a variety of early diagenetic assemblages. The As Pontes case study emphasises the major influence of palaeogeographical and tectonosedimentary evolution on the clay mineral record in nonmarine depositional systems. The depositional record in As Pontes Basin demonstrates that morphological and tectonically forced environmental and hydrochemical changes can result in variations similar to those forced by low-order climatic changes and that, in some cases, the role of climate can be negligible.

The Role of Iron in Mudstone Diagenesis: Comparison of Kimmeridge Clay Formation Mudstones from Onshore and Offshore (UKCS) Localities

ABSTRACT Siliciclastic muds and mudstones commonly contain 5% iron. At deposition, most of this iron is in the oxidized form, FeIII, whereas in ancient mudstones it is predominantly in the reduced form, FeII. In most fine-grained siliciclastic sediments iron reduction is an important process during burial diagenesis. A combination of geochemical and petrologic techniques has been applied to organic-rich mudstones of the Kimmeridge Clay Formation to investigate this valence change. These sediments were collected from a variety of depositional (shallow shelf to deep graben) environments and diagenetic (0.5-4.5 km burial) settings, and our analyses have shown that significant FeIII survives burial to depths of the order of 4 km. At these depths FeIII is located (together with Al) in dioctahedral micaceous clays (notably smectite). In such sites, it is apparent that FeIII is very difficult to reduce. Our data suggest that diagenetic redox reactions are unlikely to be significant in the burial interval 0.5-4.0 km. It is also apparent that substantial iron reduction took place early during burial diagenesis with precipitation of pyrite, siderite, and ferroan carbonates in the sulfate-reduction, methanogenic, and decarboxylation zones, respectively. All these minerals are found in the studied mudstones from the shelf localities. In contrast, within the graben mudstones, pyrite is the dominant FeII-rich mineral species present. We propose that the different style of diagenesis in the two settings was produced by a fraction of the reactive iron (i.e., that contained within soil sesquioxides) being converted to the pyrite prior to sediment remobilization. Resedimentaton then allowed a second phase of sulfate reduction with replenished pore-water sulfate from the anoxic bottom graben water . The remaining reactive iron was converted to pyrite, thereby preventing precipitation of Fe-rich carbonates in the deeper diagenetic zones. Hence, the diagenetic iron-mineral assemblages in the different facies are quite distinct. Diagenetic assemblages present in ancient siliciclastic sediments thus offer valuable insights into both sedimentary and diagenetic processes, but the links are complex and must be interpreted with care.

Fluid Evolution Interpreted from Diagenetic Assemblages and Salinity Data in Permo-triassic Sandstone, Northern Perth Basin, Western Australia

ABSTRACT Shallow-marine Permo-Triassic sandstone of the northern Perth Basin, Western Australia, shows a progressive range down-dip in pore-water salinities, and in the mineralogy of diagenetic assemblages. These features have been interpreted as resulting from a history involving three hydrocarbon injections, and the introduction of meteoric waters. The sandstone units, some containing oil and gas, dip and thicken southward toward the basin depocenter. Salinity in both irreducible water of the hydrocarbon column and pore water of the water leg increases southward. Isohalines in the hydrocarbon column are broadly parallel to those in the water leg, but water salinity in the oil column is higher than in the water leg. In the north, typical diagenetic mineral assemblages include kaolinite, siderite, pyrite, Ti-oxides, and quartz, but farther south they include chlorite, pyrite, albite, Ti-oxides, and quartz. Detrital feldspar and mica in the north are partly dissolved and altered to kaolinite, whereas to the south, they show chlorite alteration. The authigenic phosphate xenotime increases from north to south. The contrast in diagenetic assemblages from north to south, together with the increase southward in water salinity, point to southward flow of introduced water, and siderite zonation indicates that the introduced water ranged from meteoric to saline. The water probably originated on basement near the northern margin of the basin, which, although now exposed, may have been repeatedly submerged and exposed since the Permo-Triassic. It flowed down dip, generally diluting saline connate waters of the marine sequence as it penetrated the basin. Flow began during early diagenesis. There were at least two episodes of oil emplacement. The presence of the first oil halted most mineral diagenesis in the reservoir, and solid bitumen envelopes developed around radioactive monazite grains. As the oil leaked out, mineral diagenesis recommenced, and traces of residual oil were trapped in the growing quartz. Emplacement of the second (present) oil again halted mineral diagenesis, and a little of the new oil adhered to envelope surfaces or penetrated envelope cracks. A late gas inflow displaced the oil-water contacts. The fresh-water influx continued below the hydrocarbon column, lowering salinity in the water leg and promoting quartz cementation. The influx may still be going on.

Sedimentological and geochemical controls on ooidal ironstone and ‘bone-bed’ formation and some comments on their sequence-stratigraphical significance

Abstract Ironstones and ‘bone-beds’ are both formed in shallow marine settings where the prevailing sediment accumulation rates are very low. A review of published work shows that both sediment types cap coarsening upward successions and are commonly developed in mudstonedominated settings. Both can be shown to contain similar sedimentary structures (e.g. rip-up clasts, disarticulated shell debris) and a diverse assemblage of ichnogenera. However, the early diagenetic assemblages within each sediment type are both different from each other and from the mudstones within which they are enclosed. In ooidal ironstones, the cement assemblage comprises glaucony, berthierine and siderite; in ‘bone-beds’ it comprises early apatite, glaucony and later calcite and pyrite; in the mudstones it is predominantly pyrite. It is argued that the physical environment in which the ironstones and ‘bone-beds’ formed was similar, whereas their diagenetic environments were rather different. In particular, the characteristic cement assemblage of ironstones was produced in suboxic pore waters which were subjected to intense Fe reduction, whereas the characteristic cement assemblage of ‘bone-beds’ was produced in suboxic pore waters where there was limited Fe reduction. As a consequence of these chemical and stratigraphical constraints it is believed that ‘bone-beds’ formed on marine flooding surfaces and ironstones formed either at sequence boundaries, major flooding surfaces or maximum flooding surfaces.

Meteoric-like fabrics forming in marine waters: Implications for the use of petrography to identify diagenetic environments

Petrographic fabrics have long been used to identify meteoric diagenesis of carbonate sediments. However, on the basis of oxygen isotopic data, we document similar fabrics forming in marine pore fluids in the shallow subsurface of Great Bahama Bank. Therefore, petrographic fabrics alone are not reliable indicators of diagenetic environments, even for shallow-water sediments. In our study, skeletal grainstones show two distinct diagenetic assemblages: either dissolution of aragonitic grains and minimal cementation (high-permeability intervals) or abundant blocky spar cement and neomorphism of aragonitic skeletal grains (low-permeability intervals). These marine-burial fabrics are present as shallow as 110 m below sea level, well above the aragonite compensation depth, a feature that must be considered for models of diagenesis in ancient carbonate sediments. Marine-burial diagenesis may be important in ancient carbonate sediments deposited in moderate water depths or in shallow water during rising sea level where meteoric diagenesis is suppressed.

Illite/Smectite Geothermometry of the Proterozoic Oronto Group, Midcontinent Rift System

AbstractCharacterization of the Nonesuch Formation, middle unit of the Proterozoic Oronto Group, as a potential hydrocarbon source for the Lake Superior basin portion of the Midcontinent Rift system requires an understanding of the thermal maturity of the region and its relationship to the thermal history. Illite/smectite (I/S) expandability data were collected from the Nonesuch Formation and the overlying Freda Sandstone and compared with organic thermal maturity data; both data sets coupled with a thermal and burial history for the White Pine area of Michigan allow regional interpretation of maximum formation temperatures of the Nonesuch Formation and the Freda Sandstone with respect to time. Samples collected from drill holes in northeastern Wisconsin display nearly pure smectite within the lower Freda Sandstone trending abruptly to ordered I/S within the Nonesuch Formation. Regular trends of decreasing expandability with depth occur in four other drill holes to the northeast. Comparison of I/S expandability between similar stratigraphic intervals reveals a significant trend of increasing thermal maturity to the northeast, with the lowest thermal maturities observed in the Iron River Syncline area just west of White Pine, Michigan.I/S geothermometry suggests maximum temperatures in the Nonesuch Formation of 140°C in Wisconsin, 115°C in the Iron River Syncline area, 160°C at White Pine, and 190°C near the southern portions of the Keweenaw Copper District. The geographic pattern of temperatures determined from I/S geothermometry is identical to that determined from organic thermal maturity indicators in the Nonesuch Formation (Imbus et al., 1988, 1990; Hieshima and Pratt, 1991; Pratt et al, 1991; Mauk and Hieshima, 1992).Regular variations in I/S expandability with depth occur in the Freda Sandstone and the Nonesuch Formation near the southern limits of the Keweenaw Copper District. These variations suggest a fossil geothermal gradient of 55°C/km and limit the thickness of sediment above the Nonesuch Formation to approximately 3 km. In comparison, 3.6 km of Freda Sandstone are presently exposed near the Wisconsin border, and numerical modeling suggests a range of 4–6 km of sediment overlying the Nonesuch Formation. None of the data indicate the presence of the Bayfield Group sediments above the Nonesuch Formation at the time of clay diagenesis. Samples from White Pine suggest a two-stage burial history: 1) clay reaction, possible hydrocarbon maturation, and copper-sulfide mineralization at maximum temperatures above 100°C during the main rifting and burial event, followed by 2) fracturing, reverse faulting, and fluid circulation during a rift-terminating compressional event that may have allowed petroleum migration and native copper mineralization at temperatures below 100°C. Abrupt changes in I/S expandability with depth and the presence of poorly crystalline I/S (greater than 80% expandable) and kaolinite in the Freda Sandstone in Wisconsin appear to represent later overprinting of the diagenetic assemblage by fluids that were probably cooler and of differing composition than earlier diagenetic fluids. However, the authigenic assemblage from the vicinity of White Pine, Michigan, which includes up to 25% expandable I/S, appears to represent a diagenetic profile formed during the main rifting and burial event. Therefore, these expandable I/S-type clays are essentially 1.0 billion years old.

Analcime: A characteristic authigenic phase of andean alluvium, northern chile

AbstractAnalcime (NaAlSi2O6.H2O) forms a characteristic authigenic phase in Andean alluvium of northern Chile. Three stratigraphic units ranging from the lower Cretaceous to the Miocene were studied in order to investigate the presence of analcime in diagenetic assemblages of different ages. The units were deposited in forearc, foreland/back‐arc, and intra‐arc basinal settings.Characteristics of other sedimentary analcime occurrences were examined in order to define conditions favourable for analcime precipitation. Zeolite zonation and evidence for the original presence of volcanic glass, common to many sedimentary analcime occurrences, are absent in the studied units. Analcime precipitation requires a high Na+/H+ ratio, relatively low Si4+/Al3+ activity and low H2O activity.Diagenetic examination of the three stratigraphic units revealed early quartz and feldspar overgrowths, mechanical clay infiltration, and dissolution of labile rock and mineral fragments, particularly intermediate plagioclase (An30–80). Conditions for these processes are favoured by oxidizing groundwater at near neutral pH. Later diagenesis involved the precipitation of analcime, non‐ferroan calcite, quartz, and authigenic clays as the main cementing phases. These formed due to the concentration of oxidizing groundwater and resulted from its continued reaction with detrital feldspars and rock fragments of andesitic and dacitic composition.Analcime precipitation in Andean alluvium resulted from the evolution of saline, alkaline brines under an arid, evaporative climate in closed hydrographic basins with circulating groundwater systems.

Active Fault Participation in the Diagenetic Modification of Sandstone Reservoir Properties

In sedimentary basins undergoing regional strain, faults have a potential for influencing subsurface fluid flow by providing some of the driving energy for fluid movement. Variable displacement on faults in the slip direction results in systematic volume changes in the surrounding sedimentary rocks. Compressed and dilated volumes are distributed according to position relative to the fault. Intermittent seismic slip produces rapid pressure changes and high hydraulic gradients capable of causing movement of large fluid volumes or of maintaining pressure differentials if pore fluid migration is obstructed. As a result of the hydraulic gradients generated by individual faults, subsurface fluids may either be transferred between formations juxtaposed across the fault or vertically transported along the fault. Faults may thus provide the means of mixing of subsurface fluids, and are potentially zones of intense diagenetic modification. An appreciation of fault-influenced diagenetic modification is particularly pertinent to an understanding of the heterogeneity of sandstone reservoirs where pore water and hydrocarbon migration from source to reservoir rocks are an integral part of the hydrocarbon accumulation process. Mineralogical and fabric modifications to rock components may result in either significant enhancement of porosity or extensive cementation and compaction that overprints regional burial diagenetic assemblages. Inactive ormore » cemented faults may behave as hydrocarbon seals. The potential contribution of faults to sandstone reservoir heterogeneity should always be considered in models of burial diagenesis and hydrocarbon migration.« less