Diagenetic Modification Research Articles

The dolomite problem: evidence from 3D modeling, XRD and geochemical data of Zechstein reefs (Upper Permian, Germany)

Three dimensional modeling of the limestone and dolomite distribution in an Upper Permian (Zechstein) stromatolite-bryozoan reef, ca. 500m in diameter and 35m thick (77 drill cores, 172 point data), shows that dolomite occurs as laterally and vertically discontinuous intervals. The prevailing mineral phases are near stoichiometric dolomite and Mg free calcite, respectively (370 XRD and 341 XRF analyses). Both ∂ 13 C and ∂ 18 O (526 analyses) show a spread of ca. 10‰ and covary with the mineralogy, and the heaviest dolomite and calcite ∂ 13 C differ by ca. 1,5‰. Diagenetic modifications caused by flowing meteoric fluids could account for the observed “inverted J” trend of stable and the radiogenic signature of 8 7 Sr/ 86 Sr (23 analyses), but neither vertical nor horizontal gradients occur in the modeled reef. Because dolomite geometries are incompatible with those predicted by fluid flow models, and the limestone-dolomite difference in ∂ 13 C overlaps estimates of isotope fractionation associated with Mg content, the studied dolomite was a depositional Very High Mg Calcite recrystallized to dolomite in a semi-closed diagenetic system rather than a Low Mg Calcite transformed by a dolomitization process. The isotope pattern suggests biogenic fractionation and/or loss of heavy stable and light Sr isotopes during diagenesis.

Early diagenesis of organic-rich marls under shifting suboxic to euxinic conditions: The lower Toarcian of the Bächental basin

This study applied a multianalytical approach to identify active early diagenetic processes during deposition of the Lower Jurassic Bächental bituminous marls (Western Tethys realm). Results point to a major redox control on organic matter (OM) preservation and early diagenetic modifications. Whereas Mn reduction was the main OM oxidation process causing a maximum OM loss of ~1.5% and a lowering of hydrogen index values by up to 400 mgHC/gTOC under suboxic conditions, sulfate reduction was the dominant degradation process under anoxic-euxinic conditions. OM oxidation processes caused an increase of pH values (up to >9) and alkalinity in pore waters. These changes initiated carbonate precipitation in all samples, with high Mn-contents and predominant kutnohorite mineralization in suboxic layers, and presence of Mn-bearing calcite in anoxic units. The actual occurrence of the distinct carbonate phases (dolomite, kutnohorite, Mn-bearing calcite and siderite) reflects the redox pattern established by previous studies. Dissolution of biogenic silica and incipient alteration of pyrite in response to changing pore water pH and alkalinity conditions was found limited to the anoxic-euxinic intervals. In summary, this study provides detailed insights into the early diagenesis of organic-rich marls in a semi-restricted basin setting under varying oxygen depletion.

In Situ Geochemical Analysis of Organics in Growth Lines of Antarctic Scallop Shells: Implications for Sclerochronology

Bivalve shells are extensively used as bioarchives for paleoclimate and paleoenvironmental reconstructions. Proxy calibrations in recent shells are the basis for sclerochronology and the applications of geochemistry data to fossils. Shell geochemical information, however, could be altered with the disappearance of intercrystalline organic matrix components, including those linked to shell growth increments, during early diagenesis. Thus, an evaluation of the chemistry of such organics is needed for the correct use of sclerochronological records in fossil shells. Here, we use atom probe tomography (APT) for in situ geochemical characterization of the insoluble organic matrix in shell growth increments in the Antarctic scallop, Adamussium colbecki. We confirm the presence of carboxylated S-rich proteoglycans, possibly involved in calcite nucleation and growth in these scallops, with significant concentrations of magnesium and calcium. Diagenetic modification of these organic components could impact proxy data based on Mg/Ca ratios, but more importantly the use of the δ15N proxy, since most of the shell nitrogen is likely bound to the amide groups of proteins. Overall, our findings reinforce the idea that shell organics need to be accounted for in the understanding of geochemical proxies.

Pore characteristic responses to categories of depositional microfacies of delta-lacustrine tight reservoirs in the Upper Triassic Yanchang Formation, Ordos Basin, NW China

Primary depositional variations interact with diagenetic modifications to exert strong control on pore networks in tight hydrocarbon reservoirs. However, the linkage between pore characteristics and depositional microfacies remains enigmatic, which restricts the prediction of high-quality reservoir distributions. The continental tight reservoirs of the Yanchang Formation in the Ordos Basin are targeted to clarify and estimate the sensitivity of pore characteristics to depositional microfacies. A multi-methodical approach utilizing petrographic assessments, logging evaluations, digital imaging analyses and high-pressure mercury intrusion experiments enables us to reveal the high microheterogeneity of pore types, pore-size distribution and pore fractal features. The results identified the microfacies including distributary channels (DCs), mouth bars (MBs), interdistributary bays (IBs), sheet sands (SSs), turbidite fans (TFs), shallow lacustrine mudstones (SLMs) and semi-deep lacustrine mudstones (SDLMs) from the Chang 8 to Chang 6 members. Dominated by residual intergranular pores and microfractures, the main storage spaces of the DC and MB microfacies are within mesopores, which provide 62%–92% and 81% of the total pore volume, respectively. However, the micropores that are mainly composed of clay intercrystalline pores and feldspar dissolved pores provide 55%–70%, 52%–62% and 53%–80% of the total pore volume for the IB, SLM and SDLM microfacies, respectively. In addition, the pore fractal dimension results indicate that the mesoporous structure of the argillaceous microfacies (IB, SLM and SDLM) is more heterogeneous than that of the arenaceous microfacies (DC, MB and TF). Therefore, the dominant reservoir spaces of the different depositional microfacies are distributed over diverse pore-radius ranges, and the varying degrees of mesoporous development are a crucial factor affecting the reservoir capacity. Moreover, the fractal dimensions have a significant negative correlation with pore volume in the meso-micropore range. Characterized by lower pore sorting coefficients, broader skewness and more abundant rigid siliceous grains, the arenaceous microfacies with lower fractal dimensions and higher pore volume have better reservoir capacity than argillaceous microfacies that are rich in ductile minerals. Microscopic observations show that the muddier reservoirs of interdistributary bays and lacustrine mudstones are more susceptible to pore-filling chlorite and illite, which can deteriorate the reservoir quality. These results indicate that the controlling effect of depositional microfacies on pore characteristics is fundamentally caused by differences in mineralogical composition and depositional texture, and these results could provide new insights into the mechanisms of microfacies controls on reservoir quality.

Geochemical and mineralogical studies of ca.12 Ma hydrothermal manganese-rich rocks in the Hokuroku district in Japan

Geological and geochemical studies were performed on ca.12 Ma Mn-rich rocks in the Hokuroku district in Japan to constrain the origin of Mn-rich hydrothermal fluids and precipitation pathways of Mn oxides. Mn-rich rocks occur in hyaloclastites of rhyolite lavas in the studied areas. Colloform textures of Mn oxides were common in those Mn-rich rocks. Carbonaceous layers uniquely alternate with Mn oxides in the colloform textures. The Mn oxides layers in the colloform textures were composed of aggregates of nano-scale todorokite, birnessite, and pyrolusite. The carbonaceous layers also host apatite nano-crystals. These findings imply that the carbonaceous layers most likely represent fossilized microbial mats of Mn-oxidizing bacteria. The close association of nano-scale Mn oxides with organic matter, clay, and silica minerals suggests that Mn oxides initially precipitated near venting areas of low-temperature hydrothermal fluids.The early Mn oxides were trapped in the hyaloclastites and experienced diagenetic modifications combined with hydrothermal alteration, seawater penetration, and microbial activities in the hyaloclastites. Such secondary processes further enriched Mn in the hyaloclastites, forming various Mn silicates (e.g., pennantite and shirozulite) and veins made of Mn (II and III) silicate and oxide/silicate (e.g., rhodonite and braunite).Later hydrothermal fluids also brecciated the massive Mn-rich rocks. Rhodochrosite and rhodonite occurred in such brecciated Mn-rich rocks or sheared hyaloclastites preferentially. The carbon isotope compositions of rhodochrosite ranged from −26.7‰ to −20.8‰ (VPDB). The mudstone and early colloform textures in the hyaloclastites contained appreciable amounts of organic matter. Such organic matter was degraded by microbial processes under the influence of the later hydrothermal activities, followed by the formation of rhodochrosite. These findings contribute not only to understanding the formation mechanisms of hydrothermal Mn-rich rocks, but also to future exploration of Mn deposits.

The origin, differential diagenesis and microporosity characteristics of carbonate mud across a late Paleogene ramp (Iraqi Kurdistan region)

Microporosity is common in many Middle Eastern carbonate reservoirs and various interpretations have been offered to explain its development, especially in Cretaceous reservoirs. In this study outcrops of late Paleogene carbonates of the north-eastern Iraq have been studied to assess the variation of porosity across a carbonate ramp and the factors that control its distribution. The depositional system of this late Paleogene succession was a homoclinal ramp dipping SW. Inner-mid ramp carbonate facies are massive to thick bedded units, mostly composed of packstone to grainstone, characterized by high diversity of benthic foraminifers, red algae, corals, gastropods and bivalves. Outer ramp carbonate facies are thin bedded wackestone to calcimudstone. In terms of matrix-dominated facies two distinct rock fabrics and pore systems have been identified: low porosity inner-mid ramp facies and higher porosity outer ramp facies. The inner-mid ramp carbonates show evidence of neomorphism and recrystallization of matrix mud which forms larger crystal size (average 3–4 μm) with smaller pore size (0.5 μm or less) and average porosity (8.7%). The outer ramp mud matrices exhibit the major component of planktic foraminifera and coccolith debris have smaller crystal size (average 1 μm) with larger pore size (1–2 μm), the average porosity (13.1%). For the inner-mid ramp matrices carbon and oxygen (C & O) stable isotopic values are negative indicating likely meteoric diagenesis. The relatively low strontium (Sr) and magnesium (Mg) values and high manganese (Mn) implies likely recrystallization and replacement of the marine precursor, probably high Mg-calcite and aragonite. In the outer ramp facies, the presence of coccolith and planktic foraminiferal debris indicates less diagenetic modification; the C & O stable isotopic values show less altered range with very low Mn and relatively high Sr and Mg also supporting less alteration. The textural and geochemical evidence supports an origin from a more stable, likely low Mg-calcite precursor.

DIAGENETIC CONTROLS ON THE RESERVOIR QUALITY OF TIGHT OIL‐BEARING SANDSTONES IN THE UPPER TRIASSIC YANCHANG FORMATION, ORDOS BASIN, NORTH‐CENTRAL CHINA

Tight oil‐bearing sandstones in the Chang 4+5, 6 and 7 Members of the Upper Triassic Yanchang Formation in the Ordos Basin, north‐central China, in general consist of fine‐grained, moderately‐ to poorly‐sorted lithic arkoses (average Q53F30R17) deposited in a fluvial‐dominated lacustrine‐deltaic environment. Diagenetic modifications to the sandstones include compaction and cementation by calcite, dolomite, ankerite, quartz, chlorite, kaolinite and illite, as well as partial dissolution of feldspars and minor rock fragments. Porosity ranges up to ~7% of the rock volume and was reduced more by cementation than by compaction. Fractures (tectonic macrofractures and diagenetic microfractures) provide important oil migration pathways and enhance the sandstones' storage potential. The pore network is heterogeneous due to processes related to deposition and diagenesis, and there are considerable spatial variations in porosity and pore connectivity. The pore system includes both macropores and micropores, and pore network variations depend on the type and distribution of authigenic cements.An analysis of the diagenetic and porosity characteristics of core samples of the Yanchang Formation sandstones from wells in the Youfangzhuang oilfield resulted in the recognition of six petrofacies (A‐F) whose characteristics allow reservoir quality to be predicted. Fluid performance analysis for selected sandstone samples using nuclear magnetic resonance combined with helium porosity and air permeability shows that high permeability and large pore throats together result in high movable fluid saturation potential, and that effective pore spaces and throats are beneficial for hydrocarbon storage and flow. Relatively higher porosity and permeability tend to occur in petrofacies B sandstones containing abundant pore‐lining chlorite with lesser kaolinite and minor carbonate cements, and in petrofacies C sandstones with abundant pore‐filling kaolinite cement but little chlorite and carbonate cements. These petrofacies represent the best reservoir‐quality intervals.A reservoir quality prediction model is proposed combined with the petrofacies classification framework. This model will assist future development of tight sandstone reservoirs both in the Upper Triassic Yanchang Formation in the Ordos Basin and elsewhere.

Diagenetic differentiation in the Ordovician Majiagou Formation, Ordos Basin, China: Facies, geochemical and reservoir heterogeneity constraints

Diagenesis is a key process that affects carbonate reservoir formation and evolution. This paper presents a detailed study of a carbonate shoal reservoir in the Ordovician Majiagou Formation, Ordos Basin, China. This reservoir records different diagenetic features that are related to reservoir heterogeneity. Petrographic observations and geochemical data reveal that the two facies present in a single shoal exhibit differences in rock and pore textures, spatial distributions, and reservoir properties. The shoal margins consist of grain dolomite with few intergranular pores and pervasive cement infilling, whereas the shoal core is dominated by crystalline dolomite with abundant intercrystalline pores. These two types of reservoir experienced multi-stage and different diagenetic pathways. The shoal deposits were pervasively dolomitized during the very early diagenesis with the dolomitizing fluid of highly saline seawater due to evaporation. The shoal margins were then cemented due to the inflow of hypersaline seawater and gypsum precipitation during penecontemporaneous to shallow burial, which resulted in significant reservoir degradation. However, this densification and relatively poor reservoir properties of the shoal margins protected the shoal core from cementation and alteration by hypersaline seawater. During deep burial, continued diagenetic modification and recrystallization occurred preferentially in the shoal core as the diagenetic fluid flowed and migrated into the layers with relatively high porosity–permeability. This resulted in intense lithological and porosity transformation within the shoal core. Multiple stages of diagenetic differentiation affected the formation and preservation of dolomite reservoir, which provide new insight into the carbonate reservoir heterogeneity. The results have implications for future oil–gas exploration in high-quality reservoirs developed in old superimposed basins, such as the Ordos Basin and those in a similar geological setting.

Impact of facies and diagenetic variability on permeability and fluid flow in an oolitic grainstone—Pleistocene Miami Oolite

AbstractThe Miami Oolite of South Florida is representative of a grainstone‐rich carbonate unit that has been surficially karsted, and therefore may be considered as an analogue for subsurface reservoirs/aquifers with ‘high’ permeability extremes. The deposit can potentially serve to improve a conceptual understanding of heterogeneity as imparted by shallow‐marine facies changes and early meteoric diagenetic modification. Reviewed here are recent studies of the Miami Oolite with the intent to emphasize those key aspects of the facies and early diagenesis that most impact permeability and fluid flow. The Miami Oolite displays the preserved morphology of a fossilized ooid sandbody, even though it has been subaerially exposed in a tropical climate since its deposition approximately 120 kyr bp during the last interglacial highstand. The depositional motif is one of a dip‐oriented, tidal bar belt of shoals and shallow channels fronted by a strike‐oriented barrier bar. The barrier bar comprises cross‐stratified grainstones and locally burrowed grain/packstones, while the tidal shoals and channels are more commonly burrowed pack/grainstones. Surficial karst features (dolines and stratiform caves) have been added during the ca 120 kyr of subaerial exposure, but of more significance is the associated solution‐enhancement of the widespread burrowed facies. Since the Miami Oolite is the uppermost portion of the Biscayne Aquifer, there is also an understanding of fluid flow through the deposit that sheds valuable insight on the larger scale, shallow subsurface plumbing. The pore system comprises matrix pores (interparticle and separate vugs) and touching‐vug macropores that are commonly associated with burrowed [Ophiomorpha] intervals. Ground‐penetrating radar, well and flow test data indicate that matrix porosity provides most of the groundwater storage, whereas the touching vug macropores account for the majority of flow. The dolines and shallow caves seem to be sufficiently spaced as to generally not be in direct connection, with the result that they are less important in terms of regional flow than the prevailing pore system.

Controls on Reservoirs Quality of the Upper Jurassic Mengyin Formation Sandstones in Dongying Depression, Bohai Bay Basin, Eastern China

The Upper Jurassic Mengyin Formation sandstones are important targets for petroleum exploration in Dongying Depression of Bohai Bay Basin, Eastern China. Although the current burial depth of the Upper Jurassic Mengyin Formation sandstones is shallow (900–2500 m), the reservoir rocks are characterized by low porosity and low permeability due to the complex diagenetic modifications after deposition. Experimental tests and statistical methods, such as thin section, scanning electron microscopy (SEM), cathodoluminescence (CL), high pressure mercury injection (HPMI) and fluid inclusion analysis are conducted to delineate the mineralogical, petrographic and petro-physical characteristics. Results show that physical and chemical processes, including burial depth, burial and thermal history and pore fluid evolution, are both important for the diagenetic modifications that result in a variety changes in pore system and reservoir quality. According to numerical simulation of porosity evolution during lengthy burial and thermal history, porosity loss due to the early deep burial process under the high paleo-geothermal gradient can reach about 20%. Moreover, the burial history (effective stress and temperature) has a better guidance to reservoir quality prediction compared with current burial depth. The extensive compaction in sandstones also resulted in extremely low pore fluid flow during subsequent diagenetic processes, thus, the reaction products of dissolution cannot be removed, which would be precipitated as carbonate cements during stable reburial phase. Dissolution resulted from uncomformity-related meteoric flushing have been the most important porosity-enhancing factor in Mengyin Formation sandstones in spite of low thin section porosity averaged out to 3.22%. Secondary pores derived from dissolution of unstable silicates are more likely to develop in sandstones near the regional unconformity. The oil source fault activities may enhance the heterogeneity of reservoir rocks and control the reservoir quality by inducing micro-fractures and providing the main pathways for hydrocarbon migration.

Diagenetic formation of bedded chert: Implications from a rock magnetic study of siliceous precursor sediments

Bedded chert is a distinctive rock type characterized by a rhythmic alternation of quartz-rich chert beds and clay-rich interbeds. The origin of the lithologic variations is not well understood. A rock magnetic study of a sequence of alternating bands of radiolarian-rich and clay-rich bands recovered by ODP leg 129, which is considered to be the best-known analog for the bedded chert precursor sediments, shows that the relative abundances and characteristics of magnetic phases are similar in both sediment types, with only a minor (1.5 times on average) increase in bulk concentration in clay-rich bands. Comparison with magnetic properties of the diagenetically mature chert-shale couplets suggests that siliceous precursor sediments undergo strong differential diagenetic modification. A simple silica transfer model, with dissolution of silica in proto-shale donor beds and deposition in proto-chert receiver beds, indicate that the known features of the bedded chert sequences, such as the difference in thickness, cosmic dust content between chert and shale, can be reproduced in case of donor beds having initially lower clay (higher biogenic silica) content compared to receiver beds. While contrary to the current depositional model explaining chert beds formation by episodic radiolarian bloom on a background of slow accumulation of clays, an association of proto-shale beds with high productivity episodes is supported by an independent paleoproductivity proxy – their higher Al/Ti values, and is in full agreement with the known silica diagenesis trends, particularly, with the retarding influence of detrital minerals on silica transformations. The impetus for diagenetic silica segregation is provided by initial variability in clay content within a siliceous sedimentary succession. Silica dissolution starts in clay-poor intervals. When diffusing silica encounters clay-rich beds, adsorption to clay lowers dissolved silica content below the saturation level of the higher solubility phases and leads to deposition of low solubility quartz. Early deposition of quartz cement prevents significant compaction of chert layers, while the loss of silica in donor beds leads to their compaction and concomitant increase in non-reactive clay and cosmic dust particles. Severe distortion of primary environmental signals during differential diagenesis questions the use of bedded chert sequences for cyclostratigraphic studies.

Spectrum of pore types and networks in the deep Cambrian to Lower Ordovician dolostones in Tarim Basin, China

Abstract The complex tectonic activities and multi-stage diagenetic modifications lead to the formation of a heterogeneous pore network spectrum ranging from macropores (centimeter-size) to nanometer-size pores ( Three types of matrix dolomites (dolomicrite, microbial dolomite and crystalline dolomites) and two types of dolomite cements (saddle dolomite or crystalline dolomites) are recognized in the Cambrian to Lower Ordovician dolostones. The pore systems include open vugs (enlarged dissolution pores), fabric dissolution pores, intercrystal (dissolution) pores, micro-fractures, as well as intracrystal pores, forming dual or multiple pore networks. The NMR T2 spectra are mainly characterized by bi-modal distributions and mostly right-skewed. The very low T2 components ( 1000 ms) represent the vuggy pores with pore sizes spanning from hundreds of μm to thousands of microns. The transition zones (100 ms

Reservoir rock typing and zonation in sequence stratigraphic framework of the Cretaceous Dariyan Formation, Persian Gulf

This study delineates reservoir and non-reservoir zones in the Lower Cretaceous Dariyan Formation as units that are characteristic of the stratigraphic section representative of portions of the Persian Gulf offshore area. Reservoir rock types are categorized by textural and diagenetic properties. Static flow zones were delineated by porosity and permeability measurements of cored intervals. Electrofacies were prepared from clusters of petrophysical data to define reservoir zones for areas lacking wells with cored intervals. The attributes of these reservoirs are integrated into a sequence stratigraphic framework. This research indicates that rock types and reservoir zones of the Dariyan Formation differ in the studied fields located to the west and to the east in the Persian Gulf. These differences are interpreted to have resulted from a differing tectono-stratigraphic framework that controlled depositional facies and subsequent diagenesis. For example, reservoirs associated with the lower and upper carbonate units of the Dariyan Formation have different lithofacies and diagenetic modifications that resulted from deposition at two intrashelf basins at areas to the northwest and to the southeast in the Persian Gulf, and subsequent exposure to meteoric water flows during subaerial exposure.

Rare Earth Elements and Yttrium (REY) Geochemistry of Reefal Limestones in the Ordovician, Tarim Basin, NW China and their Paleoenvironment Implications

This study examines the rare earth elements and yttrium (REY) concentrations of twenty‐five samples from the reef outcrop exposed along the Lianglitage Mountain in the Ordovician, Tarim Basin in China. The concentration analysis provides constraints on the paleoenvironment during reef deposition. Based on the detailed sedimentology and petrographic work, we divide the reef facies into four sub‐facies: the base facies, reef‐core facies, reef‐flank facies, and sealing facies. The geochemical data (such as major and trace elements, carbon and oxygen isotopes, and REYs) are further used to study the coeval seawater characteristics as well as potential diagenesis overprints. The result indicated that the diagenesis has little effect on the REY patterns of the reefal limestones. The REY concentrations of the reefal limestones are overall low (ranging from 3.69 to 19.60 ppm, arithmetic mean=10.22 ppm, SD=5.4). The PAAS‐normalized REY patterns are consistently flat compared to the typical well‐oxidized, shallow marine water patterns. However, the light REE (LREE) depletions, positive La anomalies, negative Ce anomalies and positive Y anomalies, suggest that these reefal limestones are likely an indicative of contemporaneous seawater REY signals. The seawater‐like Y/Ho ratios (average at 37.51) further support that REY signals in these limestones are likely a reflection of seawater with little diagenetic modifications. The low Y/Ho ratios presented only in the reef‐flank facies and sealing facies are likely a suggestion of detrital contamination. Hence, this study confirms that REY patterns of the limestones at the base facies and reef‐core facies can record ancient seawater information, and reefs can be used as a potential geochemical proxy for paleoenvironment studies throughout the Earth's history.

Significance of fluid chemistry throughout diagenesis of aragonitic Porites corals – An experimental approach

AbstractMarine carbonates are among the most important archives of environmental information in both modern and past environments. Widely used but particularly sensitive archives are the aragonitic skeletons of scleractinian corals. However, due to the metastable nature of aragonite, a multitude of chemical, mineralogical, and (micro)biological processes can lead to diagenetic alteration of these archives and their proxy information can be altered or lost. Here, hydrothermal alteration experiments were performed to create a better understanding of the diagenesis of an often‐utilized genus, Porites. Same‐specimen subsamples were heated in two fluid types and at two temperatures and durations, and their resulting alteration features were assessed to allow insight to the mechanisms and drivers of diagenetic modification. Experiments with fluid temperatures of 130°C induced remobilization and darkening of organic matrices, with no other evidence for alteration observed. In contrast, specimens exposed to a temperature of 160°C underwent significant diagenetic alteration dependent on fluid type. Fluid chemistry, particularly Mg/Ca ratios, was found to regulate the type of alteration (e.g. neomorphism or reprecipitation). Reaction with meteoric water resulted in almost complete neomorphism of the aragonite skeleton to blocky calcite, as well as significant exchange with the experimental fluid. In comparison, samples altered in the experimental burial fluids record no mineralogic or significant isotopic change, but instead, small‐scale dissolution–reprecipitation of the primary skeleton, along with the precipitation of pore‐filling aragonitic needle cements was observed. The δ18Ocarbonate data indicate transformation via dissolution–reprecipitation mechanisms depending on the degree and mechanism of diagenesis, and are strongly dependent on fluid chemistry. The main outcome of this work is that a multi‐proxy approach has the best potential to shed light on the interpretation of processes and pathways of aragonitic coral alteration. This study has implications for early alteration of carbonate archives within varying diagenetic fluids, with results aiding in the identification of past burial conditions.

Diagenetic evolution of lower Jurassic platform carbonates flanking the Tazoult salt wall (Central High Atlas, Morocco)

AbstractPlatform carbonates diagenesis in salt basins could be complex due to potential alterations of fluids related and non‐related to diapirism. This paper presents the diagenetic history of the Hettangian to Pliensbachian platform carbonates from the Tazoult salt wall area (central High Atlas, Morocco). Low structural relief and outcrop conditions allowed to define the entire diagenetic evolution occurred in the High Atlas diapiric basins since early stages of the diapiric activity up to their tectonic inversion. Precipitation of dolomite and calcite from both warmed marine‐derived and meteoric fluids characterised diagenetic stages during Pliensbachian, when the carbonate platforms were exposed and karstified. Burial diagenesis occurred from Toarcian to Middle Jurassic, due to changes of salt‐induced dynamic related to increase in siliciclastic input, fast diapir rise and rapid burial of Pliensbachian platforms. During this stage, the diapir acted as a physical barrier for fluid circulation between the core and the flanking sediments. In the carbonates and breccias flanking the structures, dolomite and calcite precipitated from basinal brines, whereas carbonate slivers located in the core of the structure, were affected by the circulation of Mn‐rich fluids. The final diagenetic event is characterised by the income of meteoric fluids into the system during uplift caused by Alpine orogeny. These results highlight the relevant influence of diapirism on the diagenetic modifications in salt‐related basins in terms of diagenetic events and involved fluids.

Pore‐Throat Combination Types and Gas‐Water Relative Permeability Responses of Tight Gas Sandstone Reservoirs in the Zizhou Area of East Ordos Basin, China

With the aim of better understanding the tight gas reservoirs in the Zizhou area of east Ordos Basin, a total of 222 samples were collected from 50 wells for a series of experiments. In this study, three pore‐throat combination types in sandstones were revealed and confirmed to play a controlling role in the distribution of throat size and the characteristics of gas‐water relative permeability. The type‐I sandstones are dominated by intercrystalline micropores connected by cluster throats, of which the distribution curves of throat size are narrow and have a strong single peak (peak ratio >30%). The pores in the type‐II sandstones dominantly consist of secondary dissolution pores and intercrystalline micropores, and throats mainly occur as slice‐shaped throats along cleavages between rigid grain margins and cluster throats in clay cement. The distribution curves of throat size for the type‐II sandstones show a bimodal distribution with a substantial low‐value region between the peaks (peak ratio <15%). Primary intergranular pores and secondary intergranular pores are mainly found in type‐III samples, which are connected by various throats. The throat size distribution curves of type‐III sandstones show a nearly normal distribution with low kurtosis (peak ratio <10%), and the micro‐scale throat radii (>0.5 (μm) constitute a large proportion. From type‐I to type‐III sandstones, the irreducible water saturation (Swo) decreased; furthermore, the slope of the curves of Krw/Krg in two‐phase saturation zone decreased and the two‐phase saturation zone increased, indicating that the gas relative flow ability increased. Variations of the permeability exist in sandstones with different pore‐throat combination types, which indicate the type‐III sandstones are better reservoirs, followed by type‐II sandstones and type‐I sandstones. As an important factor affecting the reservoir quality, the pore‐throat combination type in sandstones is the cumulative expression of lithology and diagenetic modifications with strong heterogeneity.

Lateral and vertical variations in sedimentology and geochemistry of sub-horizontal laminated travertines (Çakmak quarry, Denizli Basin, Turkey)

The laterally extensive sub-horizontal laminated travertine deposits outcropping in the Çakmak quarry (Denizli Basin, Turkey) allows reconstructing their three-dimensional geobody architecture. Based on field observations, detailed petrographic and geochemical analyses (stable carbon and oxygen isotopes, major and trace elements) the most important controlling porosity-permeability parameters in this lacustrine-dominated facies, in relation to microfacies and diagenetic modifications were constrained. Sedimentological analyses and the observed architectural geometries allowed to subdivide the sub-horizontal carbonate succession into three main depositional units. These units in turn, based on macro- and microscopic observations were subdivided into nine dominant lithotypes reflecting an overall shrub flat and a marsh pool depositional setting. The shrub flat facies mainly comprises dendritic shrub crusts boundstone, pustular grainstone and clotted micrite mudstone to boundstone. In contrast, the marsh pool environment is dominated by coated reed rudstone to boundstone, cryptalgal silty bioclast-rich bioturbated mudstone and peloidal packstones.The diagenetic study revealed that the sediments were affected by dissolution, cementation, sparmicritization, recrystallization, and local formation of Fe-oxi/hydroxides and chalcedony. Among the diagenetic products, in particular, the calcite cements were investigated, applying micro-analysis of stable carbon and oxygen isotopes. Accordingly, the co-variation between the obtained isotopic values and the elemental concentrations (Sr, Mg, Na, and S) point to a thermogene fluid system that likely circulated through the subsurface limestones, dolomites and evaporites of the Lycian Nappes. These findings ensure an in-depth understanding of continental carbonate deposition, i.e. sub-aqueous travertines, in the context of a lacustrine depositional system.

Collagen sequence analysis of fossil camels, Camelops and c.f. Paracamelus, from the Arctic and sub-Arctic of Plio-Pleistocene North America

Proteomic analyses of ancient remains are increasing in number and offer great potential to recover phylogenetic information on extinct animals beyond the reach of ancient DNA, but limitations in proteomic techniques remain unclear. Here we carry out LC-MS/MS sequence analysis of a ~3.5 million year old giant camel specimen from Nunavut along with the younger Pleistocene remains of the Yukon giant camel (c.f. Paracamelus) and the western camel (Camelops hesternus) for comparison with complete sequences to both extant camels (Bactrian and Dromedary) and the alpaca. Although not complete (~75–80% sequence coverage), no amino acid sequence differences were confidently observed between the giant camels and the extant Dromedary, indicative of a closer relationship than that of the extant Bactrian lineage. However, multiple amino acid changes were observed for the western camel (Camelops) collagen sequence, placing it as a sister group to these members of the Camelini tribe consistent recent ancient DNA analyses. Although this supports a role for the sequencing of ancient collagen in the understanding of vertebrate evolution, these analyses highlight the limitations in phylogenetic reconstructions based on partial sequence data retrieved from proteomic analyses, particularly, the impact of omitting even only a single peptide on the resulting tree topology. The presence of other non-collagenous proteins, such as biglycan and PEDF, indicates a further resource for phylogenetic information, but none more promising than the degraded camel albumin seemingly observed in the Pliocene specimen. SignificanceAs proteomics is becoming more frequently used in the study of ancient proteins, an emerging field known as ‘palaeoproteomics’ (or ‘paleoproteomics’), understanding the limitations of the technique is essential.Here, through the study of the oldest undisputed collagen sequences obtained from proteomics, we confirm that some peptides following diagenetic modifications of tryptic sites are no longer matched with standard searches, but can be matched with Error Tolerant searches.We also demonstrate the ability to retrieve phylogenetic information consistent with that of ancient DNA methods, but that with the omission of only one or more key peptides, the inferred evolutionary relationships change. This is a significant finding for the field of palaeoproteomics implying a need for better understanding the particular composition of the partial sequences retrieved from proteomic analyses.

Mineralogy and geochemistry of silicate, sulfide, and oxide iron formations in Norway: evidence for fluctuating redox states of early Paleozoic marine basins

Laterally extensive silicate and sulfide iron formation associated with jasper (hematitic chert) beds and volcanogenic massive sulfide (VMS) deposits in Norway provide evidence of early mineral assemblages and redox conditions within coeval early Paleozoic seawater. Calculated detrital-free compositions record mixed hydrothermal (e.g., Fe, Cu) and seawater ± biogenic (e.g., Si, Ni, S, REE, P) components. Rare earth element (REE) patterns are characterized by small to large negative Ce anomalies and insignificant to locally large positive Eu anomalies, reflecting seawater REE carried to the seafloor by Fe–P-rich particles later modified by diagenetic processes. Protoliths of silicate iron formation precipitated in anoxic and intermittently euxinic deep waters by the diagenetic modification of amorphous Si–Fe oxyhydroxides and/or Si–Fe–OOH gels, based on possible modern analogues in the Red Sea. Diagenetic minerals include nontronite, greenalite, stilpnomelane, magnetite, manganosiderite, apatite, and iron sulfides. In sulfide iron formation, a local predominance of pyrrhotite over pyrite records highly reducing conditions caused by organic material. The geochemical data provide evidence for Mn–Fe–P shuttle and redox processes in a stratified basin with oxic or suboxic shallow waters and silica concentrations much higher than those of modern seawater. Hydrothermal plume-derived Fe present within the anoxic layer and near the chemocline formed mixed-valence oxyhydroxides and silicates and, intermittently, sulfides by reaction with aqueous Si and H2S, respectively, the latter derived from bacterial reduction of seawater sulfate at the chemocline. Major sustained fluxes of hydrothermally derived reductants (Fe2+, Mn2+, H2S, H2) produced from large seafloor systems such as Lokken may have changed the redox state of seawater in local, and possibly regional, basins from weakly or moderately oxic to intermittently anoxic or euxinic conditions.