Ooid Grainstone Research Articles

New advances on the carbon isotope and rare earth elements chemostratigraphy of the late Ediacaran Tamengo Formation (Corumbá Group, Brazil)

The Tamengo Formation (Corumbá Group, midwest Brazil) is a carbonate-dominated succession of major importance to unravel the environmental and biological changes during the Ediacaran–Cambrian transition in Gondwana. Although it has been extensively studied in terms of sedimentology, isotope geochemistry, biostratigraphy, and geochronology, these studies are constrained to the Corumbá region. This work presents new sedimentological, C and O isotope chemostratigraphy, and rare earth elements (REE) plus yttrium (REY) data of four sections of the Tamengo Formation in the Serra da Bodoquena region, approximately 200 km south of Corumbá, discussing how these new data are related to the type sections. The sections present ooid grainstones, with hummocky, swaley, and wavy structures, interbedded with mudstones and shales, indicating deposition of fine particles by suspension fallout in-between periods of high-energy, with reworking by storm waves. The δ13Ccarb curves show positive plateaus with minor, short-lived negative excursions linked to facies variations, which are related to a large δ13Ccarb depth gradient in a redox-stratified water column. The REY profiles present middle REE (MREE)-bulge patterns and absent to slightly positive Ce anomalies, consistent with MREE adsorption onto Fe–Mn oxyhydroxides in the water column followed by release in pore waters. Subsequently, REY remobilization during anoxic diagenetic stages resulted in the MREE-bulge pattern and overprinted some of the original seawater REY features, including the Ce anomalies. The Tamengo Formation in the Serra da Bodoquena region represents a storm-dominated carbonate ramp, as previously interpreted for this unit in the Corumbá region. Nevertheless, there is a significant shift of 2 ‰ in the peaks of δ13Ccarb data between both localities, which may be related to differences in the overall sector of the carbonate ramp. There are also substantial variations in the δ13Ccarb record of the Tamengo Formation and other coeval Gondwana basins, such as the Nama and Itapucumi groups, with different peak values, which may be related to latitudinal differences on the inorganic carbon isotope composition or to the degree of basin restriction.

Importance of arsenic bioaccessibility in health risk assessment based on iron “Minette” rocks and related soils

Total element concentrations by themselves are not always good predictors of toxicity and are therefore not suitable for eco- and/or human toxicological risk determination. In addition, despite the growing call for harmonization, countries show significant variation in risk assessment tools, screening/background values, protocols and legal management of soils. By incorporating mobility and bioaccessibility/availability into soil risk assessments, location-specific physico-chemical and geological conditions can be considered in routinely applied general risk assessment methodologies. Minette soils and rocks are a great case in point since they often are associated with high geogenic As concentrations and consequently potential risks. Minette iron ores form the world largest Fe ore deposits since the “great oxidation”. For the first time, oral bioaccessibility during direct ingestion was assessed on Minettes from Luxembourg by applying the in vitro Solubility/Bioavailability Research Consortium (SBRC) method. Out of > 180 samples, 25 representative samples were selected providing a unique dataset which showed an average gastric bioaccessibility of ∼10% (7.8 ± 4.0 mg/kg) of the total As-concentration, with a maximum of 45% (17.9 mg/kg). Of importance is that bioaccessibility of As in Minette rocks and soils are controlled by, and can be estimated from, lithology, mineralogy and total Ca content. Soils and ooid grainstones with an iron oxide or clayey matrix, are characterized by average gastric bioaccessible As concentrations < 6 mg/kg. Gastric As bioaccessibility is highest in Fe-bearing calcite-cemented bioclastic grainstones (∼12 mg/kg). Importantly, for all samples the maximal bioaccessible As concentrations remain below the threshold from which significant adverse non-carcinogenic and/or carcinogenic health effects are expected. These new results are in strong contrast with what total As concentrations might suggest. Considering bioaccessibilities, consequently, can help to avoid disproportionate, costly and environmentally impacting risk management strategies. Furthermore, this study illustrates the importance of cross-disciplinary collaboration between geo- and health scientists.

Fossils, pseudofossils and problematica from the Ura Formation: Ediacaran of the Patom Basin, Siberia

The pseudofossils, problematica and actual macrofossils from the Middle Ediacaran Ura Formation of the Patom Basin of Siberia are critically reviewed. Both the pseudofossils and problematica are flat-clasts of ooid grainstones that are interpreted as a product of submarine gravitational or storm-induced erosional episodic events. Some extremely flat batten-shaped ooid and cementstones 2D-clasts closely resemble shreds of ribbon-like macrophyte phytoleims randomly scattered on a bedding plane. They can potentially be considered as a cement-incrustation along the algae thalli buried within ooid sands or shales, however, there is no certainty in their taxonomic identification. At the same time, other abiotic or microbially induced formation mechanisms of these fragments cannot be excluded. The Ura macrofossils, similar in size to the 2D-clasts, are described from the same outcrop, approximately 60 m downsection. They are represented mainly by the ribbon-shaped carbonaceous compressions of Liulingjitaenia alloplecta that demonstrate the stratigraphically lowest appearance of the Miaohe-type fossils in mid-Ediacaran (pre-Shuram, 580–570 Ma) strata of Ura Formation in close association with Doushantuo-type acanthomorphic acritarchs. These fossils are present as a detrital component or as in situ buried large (decimeter scale) colonial clusters. The colonies covered significant surface areas of the soft muddy sea-floor beneath wave-base. L. alloplecta themselves, commonly attributed to green macroalgae, could also be considered as composite colonies of filamentous microalgae or cyanobacteria.

SEDIMENTOLOGY, PALAEOGEOGRAPHY AND DIAGENESIS OF THE UPPER PERMIAN (Z2) HAUPTDOLOMIT FORMATION ON THE SOUTHERN MARGIN OF THE MID NORTH SEA HIGH AND IMPLICATIONS FOR RESERVOIR PROSPECTIVITY

This paper provides an updated understanding of the reservoir stratigraphy, sedimentology, palaeogeography and diagenesis of the Upper Permian Hauptdolomit Formation of the Zechstein Supergroup (“Hauptdolomit”) in a study area on the southern margin of the Mid North Sea High. The paper is based on the examination and description of core and cuttings data from 25 wells which were integrated with observations based on existing and new 3D seismic.Based on thin‐section petrography of cuttings and core from the wells studied, it is evident that Hauptdolomit microfacies are distributed in a relatively predictable way, and well‐defined platform interior, platform margin, slope and basin settings can be distinguished. Platform margins are typically characterised by the development of ooid shoals and, to a lesser‐extent, by microbial build‐ups. High‐energy back‐shoal settings are characterised by a more complex combination of peloid grainstones, thrombolitic and microbial build‐ups, and fine crystalline dolomites. Lower energy lagoons which developed further behind the platform margin are characterised by a variety of microfacies types; fine crystalline dolomites are common in this setting as well as peloidal facies and local microbial build‐ups. Intertidal and supratidal settings are typified by increased proportions of anhydrite and the development of laminated microbial bindstones (stromatolites). Platform margins are in general relatively steep and pass into slope and basinal settings. Only a few wells have penetrated Hauptdolomit successions deposited in a slope setting, and these successions are characterised by a range of resedimented shallow‐water facies together with low‐energy laminated dolomicrites and fine crystalline dolomites. Slope zones in the study area are interpreted from seismic data to be typically 1‐1.5 km in width. Basinal Hauptdolomit deposits have been strongly affected by post‐depositional diagenesis and are dedolomitised to variable degrees. The original depositional facies are rarely preserved.Diagenetic studies show that dolomitisation has affected almost the entire Hauptdolomit Formation throughout the study area in both basinal and platform settings. The dolomite is considered to result from seepage‐reflux processes and is an early diagenetic phase. Mouldic porosity is present in many facies types as a result of dissolution, especially in ooid grainstones, thrombolitic build‐ups and peloidal facies. The dissolution cannot be associated with any one diagenetic phase but was most likely a result of the dolomitisation process itself. Stable isotope analyses indicate that all dolomites were precipitated from Permian marine‐derived pore fluids. Fluid inclusion analyses of dolomite cements indicate that cementation continued into the burial realm. Anhydrite cementation occurs in two phases: early anhydrite precipitation was associated with dolomitisation, and can be distinguished from a later, pore‐filling cement which is highly detrimental to reservoir quality.The Hauptdolomit succession in basinal wells (and in some slope wells) in the study area has undergone significant dedolomitisation. Dedolomitisation was a shallow burial process which affected precursor dolomites, whereby excess calcium from the transition of gypsum to anhydrite during burial combined with CO2 and organic acids derived from basinal sediments. The process was triggered by excess calcium reacting with excess carbonate ions from dissolution.3D seismic volumes supplemented by numerous 2D lines were available in the study area and allowed an interpretation to be made of Hauptdolomit gross depositional settings; platform margins and base‐of‐slope polygons were mapped, with the greatest confidence in areas of 3D seismic. The basin, slope and platform settings were distinguished using seismic data integrated with the results of micro‐facies analysis and incorporating seismic‐to‐well ties. The data shows that large parts of the study area are characterised by the presence of polyhalites within the overlying (Z2) Stassfurt Halite Formation, which may create particular seismic geometries at the Hauptdolomit slope. These are interpreted to be intra‐Stassfurt Halite features, providing an alternative model to the thickened, prograded Hauptdolomit which has been suggested in previous publications.Because few wells drilled in the study area had the Hauptdolomit as the primary target, cores were limited but significant data was obtained from cuttings analyses. More than 400 thin sections were evaluated, allowing depositional models based on microfacies observations to be developed, verifying the seismic‐scale observations.

Sedimentological, Diagenetic, and Sequence Stratigraphic Controls on the Shallow to Marginal Marine Carbonates of the Middle Jurassic Samana Suk Formation, North Pakistan

This study presents a thorough analysis of the sedimentology, diagenesis, and sequence stratigraphy of the Middle Jurassic Samana Suk Formation in the Hazara Basin of northern Pakistan. Focusing on two sections, namely Mera Rehmat and Por Miana, the research aims to unravel the complex geological processes within the formation. The examination of microfacies reveals nine distinct depositional textures, ranging from mudstone to wackestone, packstone, and grainstone, indicating various inner ramp environments such as open marine, lagoon, and coastal settings. Petrographic investigations shed light on diagenetic processes, including micritization, cementation, dissolution, compaction, neomorphism, and dolomitization. Six cementation types are identified, and the dolomitization patterns vary, providing insights into lagoonal environments and mudstone replacement. Sequence stratigraphic analysis uncovers intriguing patterns within the Samana Suk Formation. The high-stand system tract is characterized by mudstones, pelloidal grainstones, and dolomitized mudstones, indicating periods of high sea level. In contrast, the transgressive system tract displays ooidal grainstones, pelloidal packstones, and pel-bioclastic grainstones, representing transgression and inundation of previously exposed areas. A significant finding is the impact of diagenesis on reservoir quality parameters, specifically porosity, and permeability. Diagenetic processes, cementation types, and dolomitization patterns have significantly altered the pore network, highlighting the importance of considering diagenesis in assessing the Samana Suk Formation as a hydrocarbon reservoir. This research provides a comprehensive understanding of the sedimentology, diagenesis, and sequence stratigraphy of the Middle Jurassic Samana Suk Formation. The findings contribute to our knowledge of similar carbonate reservoirs globally, enhancing the exploration and development of hydrocarbon resources in comparable depositional environments.

Cambrian Series 2 (Stage 4) calcimicrobial reefs in Shandong Province, North China: Calcimicrobial diversity and contribution to reef construction

Calcimicrobes together with archaeocyaths became significant in reef construction during the early Cambrian; however, purely calcimicrobial reefs have not been as well documented as archaeocyath-bearing reefs. Here we describe the age-specific features of calcimicrobial reefs of the Zhushadong Formation (Cambrian Series 2, Stage 4) at the Sunmayu and Jinhe sections in Shandong Province, North China. The calcimicrobial reefs of the Sunmayu section are constructed mainly by Epiphyton, Kordephyton, and tubiform microbes. The reefs are in some cases dominated by Amgaina, passing from a framework of Kordephyton, Epiphyton, and tubiform microbes to dendritic growth of Epiphyton, and finally to aggregations of Amgaina. Calcimicrobial reefs in the Jinhe section are likewise constructed mainly by Epiphyton, Kordephyton, and tubiform microbes, but with subordinate Tarthinia and Bija. Epiphyton and Kordephyton as well as Amgaina were thus the main framework builders in the Zhushadong calcimicrobial reefs. Tubiform microbes contributed as encrusters, enhancing the framework rigidity, whereas Tarthinia and Bija played subordinate roles as encrusters and consolidators. Interreef deposits of the Sunmayu reefs are of peloidal or ooidal grainstone; these reefs are inferred to have formed in a high-energy shoal setting. In contrast, interreef deposits of the Jinhe reefs are of peloidal and intraclastic wackestone or grainstone with bioclasts such as trilobites and hyoliths. Tubular cavities, possibly representing burrows, are common in the spaces among frameworks. These calcimicrobial reefs are interpreted to have formed in a subtidal setting, where the water energy was lower than that in the Sunmayu section. In North China, reefs of Cambrian Epoch 2 are limited, as development of the carbonate platform began only toward the end of that epoch. The reefs of the Zhushadong Formation are therefore among the earliest on the North China Platform. The calcimicrobial reefs contain various kinds of calcimicrobes, and the calcimicrobial assemblages are similar to or more diverse than those of coeval archaeocyath-bearing reefs during this period of archaeocyath decline. Calcimicrobes thus flourished at that time and contributed greatly to the construction of reefs. These purely calcimicrobial reefs provide further information on the spatial and temporal changes in calcimicrobial diversity and reef construction during the period before and after the extinction of archaeocyaths.

Hydrothermal silicification and its impact on Lower–Middle Ordovician carbonates in Shunnan area, Tarim Basin, NW China

The Shunnan area is a newly discovered hydrocarbon field in Shuntuoguole lower uplift, in which the host rocks are Lower–Middle Ordovician carbonates. The carbonate reservoirs are well affected by hydrothermal silicification with well‐preserved intercrystalline pore of replacement quartz and megaquartz cements. Based on the petrography, pore types, and diagenetic processes, this research evaluated the reservoir quality and re‐constructed the porosity evolution in different diagenetic stages. Samples are collected from 7‐cored wells distributed along NNE‐ or ENE‐trending sinistral strike‐slip faults. Nine types of lithofacies were identified in the study area, including mudstone, bioclast wackestone, peloidal bioclast packstone, boundstone, peloidal grainstone, ooid grainstone, slightly silicified limestone (silica less than 25%), strongly silicified limestone (silica around 50%), and chert. Chert has better porosity (average 21.3%) and permeability (average 44.7 mD) than other lithofacies. The main reservoir pores are mainly developed in forms of fractures, intercrystalline replacement quartz and dissolved vugs. The porosity evolution shows that the primary porosity was limited due to cementation and compaction, and the earlier secondary porosity was occlusive as a result of coarse calcite cementation. Therefore, the precursor limestones may be tight, and the reservoir quality of the Lower–Middle Ordovician carbonates was controlled by hydrothermal silicification. During the hydrothermal silicification event, the burial depth of the Lower–Middle Ordovician carbonates was more than 3,000 m and their stratum temperature was higher than 120°C, representing a deep burial condition. The statistical data of porosity‐depth/temperature trends for carbonates suggest that the early secondary pores would be significantly reduced when the burial temperature reaches 100°C (or depth of strata reaches 3,000 m). Therefore, the dissolved vugs in strongly silicified limestone or hydrothermal chert are caused by hydrothermal silicification and not inherited from precursor carbonate rocks.

MICROFACIES STUDIES OF THE JURASSIC TAKATU FORMATION, WESTERN SULAIMAN FOLD-THRUST BELT, PAKISTAN

Takatu Formation is well exposed in the western Sulaiman Fold- Thrust Belt, Pakistan. It has an unconformable upper contact with Sembar Formation which is marked by oxidized surface and lower contact with Triassic Wulgai Formation. The Takatu Formation contains a wide variety of limestones, which are fine to coarse grained, palatal, lumpy, skeletal, micritic, nodular, oolitic, and intraclastic. The Takatu Formation is mainly carbonate-litho package with minor siliciclastic content interbedded as shale and marls. The petrography of limestone allowed the differentiation and demonstrated of four major and sub microfacies types. These includes, Mudstone, Wackestone, Packstone and Grainstone microfacies, which are further sub-divided into five microfacies such as, Bioclastic wackstone, Calcispheric packstone, Peloidal packstone, Ooidal grainstone, and Lithoclastic grainstone. These microfacies were compared with standard microfacies and standard zones for their possible depositional environments. On the bases of our studies, it is interpreted that Takatu Formation was deposited in diverse environment ranging from the marginal shallow shelf, upper slope and in deeper parts of the shelf.

3D modelling of the upper Tortonian-lower Messinian shallow ramp carbonates of the Hyblean domain (Central Mediterranean, Faro Santa Croce, Sicily)

This work illustrates a combined modelling approach using digital photogrammetry and geological modelling to create a high detailed 3D facies model of the inner environment of the Monte Carrubba carbonate ramp outcropping in the South Sicily. The Monte Carrubba Formation (Tortonian-lower Messinian) is the youngest marine Miocene carbonate deposit of the Hyblean region, prior to the Messinian crisis of the Mediterranean. In particular, Faro Santa Croce outcrop, which is in vertical thickness up to 8 m, shows the most proximal sector of this ramp. The Faro Santa Croce outcrop offers the opportunity to investigate and reproduce a highly-detailed facies heterogeneity 3D model for a very narrow and limited area of few squared kilometers (0.1 Km2), developed in a tectonically stable area. Within this small areal, five facies have been recognized and modelled revealing a high level of facies heterogeneity. In this area marine ooidal shoals (ooidal grainstone to packstone) interfingered with shallow water seagrass environment (green-algal-floatstone facies and bioclastic grainstone-to-packstone facies) with abundant mollusc fauna. The distal part of this vegetated environment (fine-grained mollusc-packstone facies) passed basinward into coral mounds (coral boundstone). The combination of digital photogrammetry and 3D geological modelling software has allowed to obtain a very high-resolution model of facies heterogeneity, evidencing the complexity of facies associations and, in particular, the development of a facies mosaic that can be underestimated by a classical 1D or 2D field analysis, especially in limitedly exposed outcrops.

Deeper-water deposition in intrashelf basins: Example from the Lower Cretaceous (Albian) upper Glen Rose Formation in the Houston trough, eastern Texas

In easternTexas, a thick (150-200 ft) interval of Lower Cretaceous upper Glen Rose D strata was deposited in the Houston trough intrashelf basin landward of the Stuart City reef margin and seaward of the main eastern Texas intrashelf basin. The stratigraphic section in this intrashelf basin forms a primarily oil-bearing hydrocarbon system where porous grainstones in the Alabama Ferry and Fort Trinidad fields were deposited coevally with organic-rich calcareous argillaceous mudstones. Here, we present evidence to suggest that the depositional setting within the Houston trough and the contained ooid grainstone beds were of deeper-water origin (400-500 ft), below storm wave base. Further, the water column was stratified, allowing dysaerobic to anaerobic conditions to develop, fostering accumulation and preservation of organic matter. Grain-rich gravity flows consisting of ooids, intraclasts, and thin-walled mollusks, derived from the shallower shelf to the northwest, were deposited in the basin, forming the Alabama Ferry and Fort Trinidad fields. The upper Glen Rose interval is composed of lithofacies that vary laterally within the Houston trough. Five lithofacies are recognized on the basis of sedimentary features, texture, fabric, biota, and organic-matter content: (1) laminated to nonlaminated argillaceous lime mudstone to lime wackestone; (2) laminated to nonlaminated calcareous siliciclastic mudstone to siltstone; (3) intraclast–ooid skeletal lime packstone; (4) intraclast–bivalve–ooid lime grainstone; and (5) oyster lime floatstone and rudstone. Grain-rich lithofacies laterally adjacent to and interbedded with organic-rich argillaceous lime mudstones and calcareous siliciclastic mudstones were previously interpreted as being deposited in a tidally influenced, high-energy, shallow-water shoal environment; however, reconstruction of the depositional setting and interpretation of the vertical and lateral distribution of lithofacies suggest that these lithofacies were deposited as grain-rich gravity flows into the deeper-water Houston trough. This interpretation suggests a new conceptual framework integrating facies models from submarine channel and fan complexes to explore for grainstone reservoirs similar to Alabama Ferry and Fort Trinidad fields.

A lost carbonate platform deciphered from clasts embedded in flysch: Štramberk-type limestones, Polish Outer Carpathians

Limestones designated the Štramberk-type are the most common carbonate exotic clasts (exotics) embedded in the uppermost Jurassic–Miocene flysch deposits of the Polish Outer Carpathians. About 80% of stratigraphically determinable carbonate exotics from the Silesian, Sub-Silesian and Skole units (nappes) are of Tithonian (mostly)–Berriasian (sporadically Valanginian) age. A study of these exotics revealed eight main facies types: coral-microbial boundstones (FT 1), microencruster-microbial-cement boundstones (FT 2), microbial and microbial-sponge boundstones (FT 3), detrital limestones (FT 4), foraminiferal-algal limestones (FT 5), peloidalbioclastic limestones (FT 6), ooid grainstones (FT 7), and mudstones-wackestones with calpionellids (FT 8). Štramberk-type limestones in Poland and the better known Štramberk Limestone in the Czech Republic are remnants of lost carbonate platforms, collectively designated the Štramberk Carbonate Platform. Narrow platforms were developed on intra-basinal, structural highs (some of them are generalized as the Silesian Ridge), with their morphology determined by Late Jurassic synsedimentary tectonics. An attempt was made to reconstruct the facies distribution on the Tithonian–earliest Cretaceous carbonate platform. In the inner platform, coral-microbial patch-reefs (FT 1) grew, while the upper slope of the platform was the depositional setting for the microencruster-microbial-cement boundstones (FT 2). Microbial and microbial-sponge boundstones (FT 3), analogous to the Oxfordian–Kimmeridgian boundstones of the northern Tethyan shelf (also present among exotics), were developed in a deeper setting. In the inner, open part of the platform, foraminiferal-algal limestones (FT 5) and peloidal-bioclastic limestones (FT 6) were deposited. Poorly sorted, detrital limestones (FT 4), including clastsupported breccias, were formed mainly in a peri-reefal environment and on the margin of the platform, in a high-energy setting. Ooid grainstones (FT 7), rarely represented in the exotics, were formed on the platform margin. Mudstones-wackestones with calpionellids (FT 8) were deposited in a deeper part of the platform slope and/or in a basinal setting. In tectonic grabens, between ridges with attached carbonate platforms, sedimentation of the pelagic (analogous to FT 8) and allodapic (“pre-flysch”) Cieszyn Limestone Formation took place. The most common facies are FT 4 and FT 1. Sedimentation on the Štramberk Carbonate Platform terminated in the earliest Cretaceous, when the platform was destroyed and drowned. It is recorded in a few exotics as thin, neptunian dykes (and large dykes in the Štramberk Limestone), filled with dark, deep-water limestones. Reefal facies of the Štramberk Carbonate Platform share similarities in several respects (e.g., the presence of the microencrustermicrobial-cement boundstones) with reefs of other intra-Tethyan carbonate platforms, but clearly differ from palaeogeographically close reefs and coral-bearing facies of the epicontinental Tethyan shelf (e.g., coeval limestones from the subsurface of the Carpathian Foredeep and the Lublin Upland in Poland; the Ernstbrunn Limestone in Austria and Czech Republic). Corals in the Štramberk Limestone and Štramberk-type limestones are the world’s most diverse coral assemblages of the Jurassic–Cretaceous transition. The intra-basinal ridge (ridges), traditionally called the Silesian Cordillera, which evolved through time from an emerged part of the Upper Silesian Massif to an accretionary prism, formed the most important provenance area for carbonate exotic clasts in the flysch of the Silesian Series. They are especially common in the Lower Cretaceous Hradiště Formation and the Upper Cretaceous–Paleocene Istebna Formation. The Baška-Inwałd Ridge and the Sub-Silesian Ridge were the source areas for clasts from the Silesian and Sub-Silesian units (e.g., in the Hradiště Formation), while the Northern (Marginal) Ridge was the source for clasts from the Skole Unit (e.g., in the Maastrichtian–Paleocene Ropianka Formation).

Hydrothermal silicification confined to stratigraphic layers: Implications for carbonate reservoirs

Hydrothermal silicification generates secondary porosity and permeability and could play an important role in carbonate reservoirs. We investigated the Cristal Cave, which is hosted in carbonate units of the Caboclo Formation, São Francisco Craton, Brazil, to assess the role of sedimentary facies and stratigraphy in hydrothermal silicification. Our results indicate that in the cave area, the carbonate units are composed of (1) ooidal grainstones, (2) intraclastic grainstones and rudstones, (3) heterolites, (4) marls, (5) stromatolites, (6) oncolithic-intraclastic grainstones and rudstones, and (7) hydraulic breccias. QEMSCAN and energy dispersive X-ray spectroscopy (EDS) analyses show that hydrothermal silicification is the most common diagenetic process in these rocks and reveal a mineral paragenesis composed of quartz, chalcedony, K-feldspar, barite, hyalophane (Ba-feldspar), talc, and chlorite. The marls (Unit 4) are impermeable rocks that represent a maximum flooding surface and divide the sedimentary succession into a bottom and an upper interval. They behaved as a sealing unit that prevented the upward flow of silica, channeling hydrothermal silicification in the bottom stratigraphic interval. Within the silicified interval, units 1, 2, and 3 show great differences in the degree of silicification since the ooidal grainstones (Unit 1) concentrated the hydrothermal fluids, reaching the highest degree of silicification in the stratigraphic column. We conclude that the stratigraphic framework can control the vertical distribution of hydrothermal fluids and the development of layer-parallel fluid flow conduits. The Cristal cave serves as an analog, which could contribute to the understanding of silicified carbonate reservoirs.

Porosity distribution in cyclic dolomites of the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin, China

ABSTRACT Increasing interests in hydrocarbon resources at depths have drawn greater attentions to the deeply-buried carbonate reservoirs in the Tarim Basin in China. In this study, the cyclic dolomite rocks of Upper Cambrian Lower Qiulitag Group from four outcrop sections in northwestern Tarim Basin were selected to investigate and evaluate the petrophysical properties in relation to depositional facies and cyclicity. The Lower Qiulitag Group includes ten lithofacies, which were deposited in intermediate to shallow subtidal, restricted shallow subtidal, intertidal, and supratidal environments on a carbonate ramp system. These lithofacies are vertically stacked into repeated shallowing-upward, meter-scale cycles which are further grouped into six third-order depositional sequences (Sq1 to Sq6). There are variable types of pore spaces in the Lower Qiulitag Group dolomite rocks, including interparticle, intraparticle, and fenestral pores of primary origin, inter crystal, and vuggy pores of late diagenetic modification. The porosity in the dolomites is generally facies-selective as that the microbially-originated thrombolites and stromatolites generally yield a relatively high porosity. In contrast, the high-energy ooidal grainstones generally have very low porosity. In this case, the microbialite-based peritidal cycles and peritidal cycle-dominated highstand (or regressive) successions have relatively high volumes of pore spaces, although highly fluctuating (or vertical inhomogeneous). Accordingly, the grainstone-based subtidal cycles and subtidal cycle-dominated transgressive successions generally yield extremely low porosity. This scenario indicates that porosity development and preservation in the thick dolomite successions are primarily controlled by depositional facies which were influenced by sea-level fluctuations of different orders and later diagenetic overprinting.

Sedimentological attributes of the Middle Jurassic peloids-dominated carbonates of eastern Tethys, lesser Himalayas, Pakistan

The carbonate factories of the tropical to sub-tropical regions characterize a number of facies, attributable to biological and oceanographic variables. For this purpose, the Middle Jurassic carbonate rock unit (Samana Suk Formation) of the Upper Indus Basin of Pakistan was studied to understand the various Tethyan platform attributes. Four sections from Hazara area were chosen which were also compared with a section from the Nizampur Basin. The resultant microfacies were placed in their paleoenvironmental settings to reveal the facies distribution and geometry of the platform. The platform represents a gently dipping ramp, dominated by peloidal–bioclastic–ooidal grainstones and peloidal lime mudstones. The allochems and fauna show some contrasting signatures regarding paleoclimate. These microfacies infer about existence of a relatively extended tropical climatic belt in the paleo Tethys during the Middle Jurassic. The notable presence of planktonic lime mudstone facies corresponds with global marine transgression, occurring in the middle of rock unit. The nature of ooids and types of fauna of sub-tropics to tropics advocate an extended warm climatic zone in comparison with today. The dominance of gently dipping inner-ramp shoals accompanied by lagoons to steep dipping outer-ramp planktonic lime mudstones restricted the development of least encountered middle-ramp facies. The ramp platform was struck by waves and tides of different strengths including low-middle amplitude storms and the overall absence of evaporites and primary dolostones designate a humid region as manifested by euryhaline conditions. The study has drawn paleoenvironmental and palaeoceanographic similarities with sections in Tanzania, western India, NW Iran, NW Afghanistan and Western Australia. The oceanic parameters compare with present day Trucial Coast of Persian Gulf, reflecting an almost similar nature of paleoceanography in the paleo Tethys.

Microfacies and diagenetic overprints in the limestones of Middle Jurassic Fort Member (Jaisalmer Formation), Western Rajasthan, India: Implications for the depositional environment, cyclicity, and reservoir quality

The present study investigates the microfacies and the effects of diagenetic processes on the reservoir quality of limestones of the Fort Member, Jaisalmer Formation, Western Rajasthan. The study interpolates an integrated approach including field studies, petrography, staining, and X‐ray diffraction techniques. The Fort Member is composed of medium‐ to thick‐bedded, nodular highly fossiliferous limestones, which are fine‐ to medium‐grained, poorly‐ to very well‐sorted, and comprise angular‐ to well‐rounded grains of low to high sphericity. These limestones contain abundant skeletal (bivalves, brachiopods, echinoderms, foraminifera, algae, gastropods, sponge, coral, and calcispheres) and non‐skeletal grains (intraclasts, ooids, and peloids). On the basis of detailed petrographic investigations, eight microfacies have been recognized including: bioclastic wackestone, whole‐fossil wackestone, bioclastic lithoclastic packstone, aggregate grainstone, peloidal grainstone, ooidal grainstone, bioclastic grainstone, and coated bioclastic grainstone. Based on the microfacies analysis, the limestones of the Fort Member are inferred to have been deposited in shoal, lagoon, and open marine environments. The diagenetic overprints of the Fort Member limestones is characterized by the several diagenetic features such as micritization, cementation, dolomitization, compaction, dissolution, silicification, neomorphism (aragonite to calcite transformation and development of microspar), and secondary porosity. Such main diagenetic features affecting the carbonates are developed in marine phreatic, meteoric phreatic, mixed meteoric phreatic and burial diagenetic environments. Micritization of allochems, dolomitization of lime muds, and cementation by isopachous calcite, granular calcite cement, neomorphism, and recrystallization of bioclasts are developed in meteoric‐phreatic diagenesis. The meteoric‐vadose diagenesis led to prevalence of dissolution process and sparry calcite infilling the fractures. Dolomitization, blocky calcite cement, and silicification processes were developed during the burial diagenesis. These diagenetic processes have direct effects on the modification of both intergranular and intragranular/crystalline porosity. In this manner, it is found that the calcite cementation, micritization, and compaction were responsible for the reduction of the pore spaces, whereas dolomitization, dissolution, formation of moldic pores, and fracturing increased the pore volumes. Therefore, the enhanced pore volumes improved the carbonate reservoir characteristics, suggesting the limestones of the Fort Member to be a better carbonate reservoir for hydrocarbon accumulation.

Ooid Cortical Stratigraphy Reveals Common Histories of Individual Co‐occurring Sedimentary Grains

AbstractOoids are a common type of carbonate sand grain that form through a combination of constructive and destructive mechanisms: growth via precipitation and diminution via physical abrasion. Because growth and abrasion obey distinct morphometric rules, we developed an approach to quantitatively constrain the history of growth and abrasion of individual ooid grains using the record of evolving particle shape preserved by their cortical layers. We designed a model to simulate &gt;106 possible growth‐abrasion histories for each pair of cortical layer bounding surfaces in an individual ooid. Estimates for the durations of growth and abrasion of each cortical layer were obtained by identifying the simulated history that best fit the observed particle shape. We applied this approach to thin sections of “modern” lacustrine ooids collected from several locations in the Great Salt Lake (GSL), UT, to assess the spatial and temporal variability of environmental conditions from the perspective of individual grains within a single deposit. We found that GSL ooids do not all share the same histories: Clustering ooid histories by a Fréchet distance metric revealed commonalities between grains found together locally within a deposit but distinct differences between subpopulations shared among localities across the GSL. These results support the tacit view that carbonate sedimentary grains found together in the environment do reflect a common history of sediment transport. This general approach to invert ooid cortical stratigraphy can be applied to characterize environmental variability over &lt;1,000 year timescales in both marine and lacustrine ooid grainstones of any geologic age.

Stratigraphic relationships between shallow-water carbonates and primary gypsum: insights from the Messinian succession of the Sorbas Basin (Betic Cordillera, Southern Spain)

The lack of modern analogues for the thick primary selenite gypsum deposits formed in the Mediterranean area limits the understanding of the Messinian (Miocene) evaporitic systems and particularly of their lateral relationships, their evolution through time and their related ecosystems. Some complex stratigraphic relationships of these extreme environments are revealed in the Messinian Caños Formation along the northern flank of the Sierra Alhamilla at the southern boundary of the Sorbas Basin (southern Spain). Here, the bottom-grown selenite gypsum beds pinch-out against graded clastic carbonate beds (ooid grainstones, rudstones and mudstones) emplaced by gravity flows. We infer that these tabular bodies formed along a low gradient ramp, derived from the periodic erosion of coeval shallow-water to coastal carbonate factories located on top of the Sierra Alhamilla ridge. The carbonate deposits bear many similarities with the Terminal Carbonate Complex described in the northern, steeper margin of the basin; both deposits show a cyclical pattern given by the repetition of couplets consisting of coarser- and finer-grained deposits. This rhythmic facies organization recalls the precessional-controlled cyclic pattern of the gypsum unit. Clastic carbonates and gypsum were deposited during the same precessional hemicycle but appear to be mutually exclusive. In fact, the clastic carbonates represent the lateral equivalent of the gypsum beds, or are occasionally found at their base or at both their base and top, but never interstratified with them. Such an arrangement suggests that during the peak of the evaporitic phases a strong pycnocline acted as the top boundary for gypsum and the lower boundary for carbonate-bearing gravity flows, with the exception of debrites, high-density gravity flows and slumps. These facies architectures appear to be mainly controlled by the interplay between the variable depth of the pycnocline and the density of the brine within the evaporitic cycle.

Iron-rich carbonate tidal deposits, Angepena Formation, South Australia: A redox-stratified Cryogenian basin

The Cryogenian Angepena Formation (ca. 650 Ma) of South Australia records deposition in a peritidal environment equivalent to, and landward of the Balcanoona reef complex, and offers valuable insights into shallow marine chemistry during the nonglacial interlude between two global ice ages. The sedimentary facies comprise an iron-rich marine red bed succession and include red mudcracked dolomitic shales, tepee dolostone, crossbedded intraclastic ooid grainstone, and tepee-related sheet cavities with marine cements. These facies are interpreted to have been deposited in a Cryogenian tidal flat to subtidal setting that is laterally equivalent to the reefal and back reef environments of the Balcanoona Formation. Marine cements from this environment are characterized by well-preserved, non- and bright-cathodoluminescent zoning, and strong negative to slightly positive cerium (Ce) anomalies (Ce/Ce*). We suggest that the Angepena Formation is indicative of intermittent synsedimentary iron oxide precipitation in an oxidized peritidal environment in an otherwise strongly ferruginous and anoxic oceanic setting. Large negative Ce anomalies develop over a narrower depth range when compared to modern oxic marine settings, highlighting the importance of dissolved Mn and Fe concentrations in controlling the magnitude of the negative Ce anomalies. Overall, coupled sedimentological and geochemical evidence suggest a Neoproterozoic shallow marine system at the interface of an oxidizing atmosphere and an anoxic ferruginous marine system.

Facies control on selective dolomitization and its impact on reservoir heterogeneities in the Samana Suk Formation (middle Jurassic), Southern Hazara Basin (NW Himalaya, Pakistan): an outcrop analogue

This studied rocks form part of the southern Hazara basin in the NW Himalayan Fold and Thrust belt, Pakistan. Thick inner to mid ramp carbonate platform deposits of Samana Suk Formation (Bajocian-Callovian) mostly composed of medium to thick bedded limestone with occasional dolostone beds are exposed in various localities. In the present studies, attempt has been made to understand degree of diagenetic alterations (i.e., dolomitization) within various limestone facies due to fluid-rock interaction phenomenon, and its impact on the reservoir behavior of altered rocks based on field investigations, petrographic studies, stable isotope signatures and porosity/permeability analysis. Field observations revealed various limestone types (oolitic, burrowed, fossiliferous, micritic and sandy), whereas dolomite occurs as completely replacive phase (Dol-I), and partially replacive patchy bodies (Dol-II) respectively. Petrographic studies showed various limestone and dolomite facies, which include: (i) Grainstone facies (bioclastic grainstone, peloidal grainstone, ooidal grainstone and pel-bioclastic grainstone), (ii) Packstone facies (bioclastic packstone and peloidal packstone), (iii) Wackestone facies (bioclastic wackestone), (iv) Mudstone facies (lime mudstone, and (v) Dolomite facies (coarse crystalline strata-bound Dol-I, and fine crystalline patchy Dol-II) respectively. O/C isotope analysis revealed that Dol-I shows signatures of δ18O (−5.84 to −3.91‰ V-PDB), and δ13C (+0.6 to +2.37‰ V-PDB) are within the limit of the carbonate marine seawater signatures, hence originated from sea-water or modified sea-water, whereas Dol-II exhibited depleted δ18O values (−6.88 to −5.87‰ V-PDB) and slightly depleted δ13C signatures (+0.968 to +1.85‰ V-PDB), indicating high temperature dolomitizing fluids. During early stage of marine diagenesis, which resulted in the cementation of pores within the grainstone-packstone facies, whereas mudstone-wackestone facies remained unaltered due to low porosity and permeability. Late stage dolomitization caused partial to complete alteration of mudstone-wackestone facies, in contrast grainstone-packstone facies are not affected by dolomitizing fluids due to the fact that the pore network of these coarser facies had already been occluded by the preceding marine cementation event. Porosity and permeability analyses revealed relatively high porosity values (4–8%) and permeability values (11.5 mD) in the dolomitized facies, whereas unaltered limestone facies showed considerably low porosity/permeability values (> 1%). In conclusion, less porous/permeable fine-grained facies evolved into more porous and permeable units due to the interaction of dolomitizing fluids of hydrothermal origin, which confirms that the original sedimentary texture of rock has implications on the selectivity of any diagenetic alteration.

Geological setting of the Moorowie Formation, lower Cambrian Hawker Group, Mt Chambers Gorge, eastern Flinders Ranges, South Australia

Geological mapping in 1970 of lower Cambrian outcrop in the eastern Flinders Ranges of South Australia included the description and naming of the Moorowie Formation, representing the uppermost Hawker Group. The mapping is supported by 885 m of measured sections. An early Cambrian regressive-marine shelf-margin succession is described, from the massive Wilkawillina Limestone (base), the grey laminated limestones, syndepositional slump-induced intraformational folds and breccias, collapse talus, and graded sediment gravity flow deposits of the Mernmerna Formation, terrace-edge attrition megabreccias and reefs of the Moorowie Formation, passing up to the red beds of the Billy Creek Formation (top). Rapid changes in sedimentary facies are attributed to basement block movements and diapiric influence on sedimentation with abrupt vertical relief and paleoslope indicated by syndepositional slumping and platform margin collapse. Tuffs in the Mernmerna Formation record contemporaneous volcanism. Massive archaeocyathan limestones, ooid grainstones, peloid limestones, reef-sourced megabreccias and red shales define five members in the overlying Moorowie Formation, signalling shallow-marine regressive conditions and the development of a biologically diverse carbonate platform seaward of evaporitic lagoons and supratidal sabkhas. The megabreccias of the Moorowie Formation formed as thin semi-autochthonous debris aprons or shallow tidal-channel infills resulting from gradual and persistent wave attrition and repeated collapse of a carbonate terrace that was vigorously reworked by tidal currents. Shale interbeds within the Moorowie Formation represent lightly channelised shallow-marine ramp deposits, with adjacent mud flats, as easterly equivalents of the Oraparinna Shale, the earliest of which formed the substrate to the attrition megabreccias. Emergent evaporite diapirs near Mt John and Mt Frome are the probable sources of coarse siliciclastics within the carbonates. Some of the siliciclastics were likely transported onto the carbonate platform by sandstorms or migrating dunes. Late in the Cambro-Ordovician Delamerian Orogeny, earlier salt diapirs at depth were compressed and reactivated as highly mobile evaporite-rubble breccias and intruded as small plugs and dykes into fissures in a lithified and folded cover.The intrusive breccias include metasediment and metabasic xenoclasts attributable to the Callanna Group, while diapir-related faults also host minor copper, lead and barite mineralisation. Documentation of this unique record contributes to wider investigations of the Ediacaran-to-early-Cambrian succession of the Flinders Ranges sector of the Arrowie Basin, adding to its global heritage values, effective management and appreciation by the wider community.