Diagenetic Overprint Research Articles

Stress states of isolated carbonate platforms: Implications for development and reactivation of natural fractures post burial

We use 2D finite element numerical geomechanical modeling to interrogate how different scenarios of far field stress regimes, rock mechanical properties and margin morphology affect stresses post burial within a generic isolated carbonate platform. We test a range of stress scenarios that include normal and strike-slip faulting conditions, under low and high differential far field stress regimes. Rock mechanical properties are representative of both young (weak) and ancient (strong) end-member reservoir rocks and distributed within two distinct domains (rimming reef and platform interior). Results show that platform geometry is a first order control on the magnitude and orientation of stresses. Characteristic features of carbonate margins like scallops or promontories tend to preferentially concentrate stresses, and can become sweet spots for fracture development. In low differential far field stress scenarios, the maximum horizontal stress rotates to mimic the margin orientation, while no rotations are observed under high differential stress scenarios. The stiffer nature of the carbonate rocks, compared to the surrounding siliciclastic basin infill, amplifies the far field differential stresses within the carbonate platform. In high differential far field stress scenarios, the local differential stresses can increase to the point that shear failure can be achieved within the platform interior, resulting in the potential development of fracture corridors. Some margin parallel pre-existing fractures show a high likelihood of reactivation. We postulate that some early margin syndepositional fractures are long lived conduits for fluid flow as they are prone to undergo multiple reactivation phases during and post burial, thus potentially controlling diagenetic overprints and present-day fluid flow in hydrocarbon reservoirs. Results from this type of modeling can be used to predict areas of increased fracture density within a carbonate reservoir (i.e. reservoir characterization), as well as to optimize trajectories of highly deviated wellbores (i.e. wellbore stability).

Calcium isotope composition ofMorozovellaover the late Paleocene–early Eocene

AbstractOcean acidification (OA) during the Paleocene-Eocene thermal maximum (PETM) likely caused a biocalcification crisis. The calcium isotope composition (δ44/40Ca) of primary carbonate producers may be sensitive to OA. To test this hypothesis, we constructed the first high-resolution, high-precision planktic foraminiferal δ44/40Ca records before and across the PETM. The records employ specimens of Morozovella spp. collected from Ocean Drilling Program Sites 1209 (Shatsky Rise, Pacific Ocean) and 1263 (Walvis Ridge, Atlantic Ocean). At Site 1209, δ44/40Ca values start at −1.33‰ during the Upper Paleocene and increase to a peak of −1.15‰ immediately before the negative carbon isotope excursion (CIE) that marks the PETM onset. Values remain elevated through the PETM interval and decrease into the earliest Eocene. A shorter-term record for Site 1263 shows a similar trend, although δ44/40Ca values are on average 0.22‰ lower and decrease shortly after the CIE onset. The trends support neither diagenetic overprinting, authigenic carbonate additions, nor changes in the δ44/40Ca value of seawater. Rather, they are consistent with a kinetic isotope effect, whereby calcite δ44/40Ca values inversely correlate with precipitation rate. Geologically rapid Ca isotope shifts appear to reflect the response of Morozovella to globally forced changes in the local carbonate geochemistry of seawater. All data combined suggest that the PETM-OA event occurred near the peak of a gradual reduction in seawater carbonate ion concentrations during a time of elevated atmospheric pCO2, potentially driven by North Atlantic igneous province emplacement.

Temperature-induced microstructural changes in shells of laboratory-grown Arctica islandica (Bivalvia).

Bivalve shells are increasingly used as archives for high-resolution paleoclimate analyses. However, there is still an urgent need for quantitative temperature proxies that work without knowledge of the water chemistry–as is required for δ18O-based paleothermometry–and can better withstand diagenetic overprint. Recently, microstructural properties have been identified as a potential candidate fulfilling these requirements. So far, only few different microstructure categories (nacreous, prismatic and crossed-lamellar) of some short-lived species have been studied in detail, and in all such studies, the size and/or shape of individual biomineral units was found to increase with water temperature. Here, we explore whether the same applies to properties of the crossed-acicular microstructure in the hinge plate of Arctica islandica, the microstructurally most uniform shell portion in this species. In order to focus solely on the effect of temperature on microstructural properties, this study uses bivalves that grew their shells under controlled temperature conditions (1, 3, 6, 9, 12 and 15°C) in the laboratory. With increasing temperature, the size of the largest individual biomineral units and the relative proportion of shell occupied by the crystalline phase increased. The size of the largest pores, a specific microstructural feature of A. islandica, whose potential role in biomineralization is discussed here, increased exponentially with culturing temperature. This study employs scanning electron microscopy in combination with automated image processing software, including an innovative machine learning–based image segmentation method. The new method greatly facilitates the recognition of microstructural entities and enables a faster and more reliable microstructural analysis than previously used techniques. Results of this study establish the new microstructural temperature proxy in the crossed-acicular microstructures of A. islandica and point to an overarching control mechanism of temperature on the micrometer-scale architecture of bivalve shells across species boundaries.

Restricted internal oxygen isotope exchange in calcite veins: Constraints from fluid inclusion and clumped isotope-derived temperatures

The distribution of oxygen isotopes between calcite and fluid inclusions has demonstrated utility for reconstructing near-surface calcite precipitation temperatures. For calcite that formed at depth, however, the resilience of this paleothermometer to diagenetic oxygen isotope alteration is poorly constrained. Clumped isotopes also document calcite precipitation temperatures and are similarly vulnerable to diagenetic alteration. Post-entrapment isotope exchange between calcite and fluid-inclusions could alter the calcite-fluid oxygen isotope distribution (αc-fi), as well as the clumped isotope composition (Δ47) of calcite, and therefore these two seemingly independent paleo-thermometers are potentially linked via the same alteration process. Using closed-system batch fractionation equations, we have modelled various scenarios of oxygen isotope exchange between water and host-rock during burial, as well as internal oxygen isotope exchange between calcite and fluid inclusions during exhumation. Assuming both paleo-thermometers record concordant temperatures at the time of vein formation, our models predict that if a fraction of calcite is available for isotopic interaction with fluid inclusions, the fluid inclusion and clumped isotope-derived paleothermometers yield discrepant temperature estimates after exhumation. We show that the fluid inclusion thermometer is more sensitive to isotopic alteration than the clumped isotope thermometer and that the mass balance of oxygen between calcite and fluid inclusions determines the sensitivity of both paleothermometers as well as the vulnerability of fluid inclusions (δ18Ofi) to diagenetic overprinting. We applied coupled clumped isotope and fluid inclusion measurements on calcite veins from the External Albanides (Albania), which were formed at depth and subsequently exhumed, in order to compare natural samples to our isotope exchange model. These veins show strongly discrepant calcite-water equilibrium temperatures and clumped isotope temperatures, suggesting the fraction of calcite available for isotope exchange with internal fluids may indeed be a key parameter of diagenetic alteration during exhumation. Even though the clumped isotope temperatures of our samples appear to be insensitive towards internal oxygen isotope exchange, our model predicts that at low burial temperatures, the carbonate clumped isotope thermometer may be susceptible to alteration by diagenetic isotope exchange with fluid inclusions under certain conditions.

A geocellular modelling workflow for partially dolomitized remobilized carbonates: An example from the Hammam Faraun Fault block, Gulf of Suez, Egypt

Constructing geocellular models of carbonate rocks using standard software is challenging since most of modelling packages are designed, first and foremost, to represent siliciclastic depositional systems, where rock properties are strongly facies-controlled. The distribution and components of carbonate depositional facies vary drastically across the geological timescale as a result of paleoclimate and its effects on carbonate-producing biota. Furthermore, reservoir architecture is less strongly controlled by depositional environment than in clastic settings, and rock physical properties, including fracture networks, are controlled by both primary components and their subsequent diagenetic alteration. This means that rock property distribution is less predictable than in siliciclastic systems, and less well represented by geocellular models that are designed to represent sedimentary architecture. In other words, in carbonate systems, the depositional and diagenetic history needs to be reconstructed in order to successfully model reservoir properties.In this study a geocellular model was created by using a well-characterised outcrop analogue obtained from the Hammam Faraun Fault (HFF) Block, located on the eastern coast of the Gulf of Suez in Sinai, Egypt. This model integrates sedimentological, petrophysical, diagenetic, and structural information into a single database. The workflow utilizes the regional tectonic history, upscaled lithological logs, and two-stage facies modelling (reflecting in and ex situ depositional facies) and resulted in the creation a realistic model of remobilized carbonates that were deposited on the slope of a carbonate platform during a period of tectonic instability. Diagenetic overprinting was achieved using probability functions to reflect the history of burial, rifting, and the spatial relationship of stratabound and non-stratabound dolostone bodies. The study demonstrates a workflow for modelling mass-transport carbonate facies and multistage fault-related diagenesis so that flow controlling facies and diagenetically altered poroperm and fracture networks are accurately represented using commercially available modelling software, and in particular demonstrates how diagenetically controlled geobodies can be captured using simple algorithms.

Preferential Loss of High Trace Element Bearing Inner Calcite in Foraminifera During Physical and Chemical Cleaning

AbstractTrace elements (TEs) in the calcite shells of foraminifera are widely used in paleoceanographic reconstructions. Nearly all TE analytical protocols include cleaning to remove contaminant phases. Understanding, how the TE composition is altered during physical and chemical cleaning is of utmost importance in ensuring the accuracy of paleoclimate reconstructions. Aggressive cleaning steps that include corrosive reagents are commonly used to clean shells with elevated Ba/Ca and Mn/Ca ratios, under the assumption that these elements are elevated due to diagenetic overprinting. We provide evidence that elevated intrashell Ba/Ca, Mn/Ca, and Zn/Ca ratios are lattice bound and a primary signal incorporated into inner calcite during calcification in the nonspinose species Neogloboquadrina dutertrei and Pulleniatina obliquiloculata. We first demonstrate, using laser ablation depth profiling analyses, that aggressive cleaning following the full cleaning protocol traditionally used to eliminate Mn and Ba contamination, preferentially removes inner (ontogenic) calcite and favors the low Ba/Ca, Mn/Ca, Zn/Ca, and Mg/Ca crust/cortex calcite. In a separate experiment, we demonstrate that mechanical fragmentation of bulk shells prepared for solution analysis also preferentially removes inner calcite and favors the low TE crust/cortex calcite. Preferential loss of the ontogenic calcite decreases Ba/Ca, Mn/Ca, and Zn/Ca ratios by 50%–90% in N. dutertrei and P. obliquiloculata shells. This biases foraminiferal TE results toward lower values and may yield data that is problematic for the interpretation of downcore records.

Shear wave velocity estimation utilizing statistical and multi-intelligent models from petrophysical data in a mixed carbonate-siliciclastic reservoir, SW Iran

The conjugation of two or more Artificial Intelligent (AI) models used to design a single model that has increased in popularity over the recent years for exploration of hydrocarbon reservoirs. In this research, we have successfully predicted shear wave velocity (Vs) with higher accuracy through the integration of statistical and AI models using petrophysical data in a mixed carbonate-siliciclastic heterogeneous reservoir. In the designed code for multi-model, first Multivariate Linear Regression (MLR) is used to select the more relevant input variables from petrophysical data using weight coefficients of a suggested function. The most influential petrophysical data (Vp, NPHI, RHOB) are passed to Ant colony optimization (ACOR) for training and establishing initial connection weights and biases of back propagation (BP) algorithm. Afterward, BP training algorithm is applied for final weights and acceptable prediction of shear wave velocity. This novel methodology is illustrated by using a case study from the mixed carbonate-siliciclastic reservoir from one of the Iranian oilfields. Results show that the proposed integrated modeling can sufficiently improve the performance of Vs estimation, and is a method applicable to mixed heterogeneous intervals with complicated diagenetic overprints. Furthermore, predicted Vs from this model is well correlated with lithology, facies and diagenesis variations in the formation. Meanwhile, the developed AI multi-model can serve as an effective approach for estimation of rock elastic properties. More accurate prediction of rock elastic properties in several wells could reduce uncertainty of exploration and save plenty of time and cost for oil industries.

Heavy-mineral provenance signatures during the infill and uplift of a foreland basin: An example from the Jaca basin (southern Pyrenees, Spain)

ABSTRACT In the Jaca foreland basin (southern Pyrenees), two main sediment routing systems merge from the late Eocene to the early Miocene, providing an excellent example of interaction of different source areas with distinct petrographic signatures. An axially drained fluvial system, with its source area located in the eastern Central Pyrenees, is progressively replaced by a transverse-drained system that leads to the recycling of the older turbiditic foredeep. Aiming to provide new insights into the source-area evolution of the Jaca foreland basin, we provide new data on heavy-mineral suites, from the turbiditic underfilled stage to the youngest alluvial-fan systems of the Jaca basin, and integrate the heavy-mineral signatures with available sandstone petrography. Our results show a dominance of the ultrastable Ap-Zrn-Tur-Rt assemblage through the entire basin evolution. However, a late alluvial sedimentation stage brings an increase in other more unstable heavy minerals, pointing to specific source areas belonging to the Axial and the North Pyrenean Zone and providing new insights into the response of the heavy-mineral suites to sediment recycling. Furthermore, we assess the degree of diagenetic overprint vs. provenance signals and infer that the loss of unstable heavy minerals due intrastratal dissolution is negligible at least in the Peña Oroel and San Juan de la Peña sections. Finally, we provide new evidence to the idea that during the late Eocene the water divide of the transverse drainage system was located in the North Pyrenean Zone, and areas constituted by the Paleozoic basement were exposed in the west-Central Pyrenees at that time. Our findings provide new insights into the heavy-mineral response in recycled foreland basins adjacent to fold-and-thrust belts.

Diagenetic controls on the elastic velocity of the early Triassic Upper Khartam Member (Khuff Formation, central Saudi Arabia)

The Permo-Triassic intervals of the Arabian Plate host the most important gas reservoirs in the world, the Khuff Formation and its equivalents. Understanding the main controls on the elastic properties of the Khuff carbonates is crucial for hydrocarbon exploration and quantitative interpretation of sonic and seismic data. In total, 88 samples were collected from the Upper Khartam Member of the Khuff Formation (central Saudi Arabia) to investigate the controls and variations in petrophysical properties and the associated acoustic velocities. Samples were analyzed using optical microscopy, XRD, and SEM to identify their texture, mineralogy and pore types. Porosity, permeability, and compressional and shear wave velocities were measured for all samples. A significant scatter is observed in the velocity-porosity relationship of the studied samples. Depositional environment, lithofacies and minerology show no clear impact on such observed scatter. Our results show that variations in pore types, predominantly of diagenetic origin, are the main factor controlling variations of velocity at any given porosity. Samples characterized by vuggy and non-cemented moldic porosity have a higher velocity when compared to samples characterized by infilled molds, interparticle and microporosity. The impact of pore types on velocity is investigated by means of velocity-porosity trajectories obtained by the differential effective medium (DEM) modeling, which incorporates variable equivalent pore aspect ratio (EPAR) values. Three main diagenetic processes, all being non-EPAR-preserving, contribute to the evolution of velocity and porosity: 1) dolomitization creating connected pore system between the euhedral dolomite rhombs which increases permeability (best reservoir unit) but decreases pore aspect ratio and thus velocity; 2) dissolution of microporous ooids and skeletal grains creating high EPAR molds and vugs at the expense of low EPAR micropores, 3) blocky calcite cementation that does not preserve the rounded shape of open oomolds reducing EPAR from more than 0.4 to less than 0.1 for extensively infilled molds. The established impedance-porosity trajectories in this study suggest that elastic properties can provide important insights about diagenetic overprints, pore types and reservoir quality in Khuff carbonates.

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.

Orientation patterns of aragonitic crossed-lamellar, fibrous prismatic and myostracal microstructures of modern Glycymeris shells

The shells of the bivalves Glycymeris glycymeris and Glycymeris nummaria are widely used for environmental studies. They consist of aragonite and comprise four different microstructures and textures from outer to inner shell surfaces: crossed-lamellar, myostracal, complex crossed-lamellar and fibrous prismatic. We characterize with SEM, EBSD, laser-confocal microscopy and AFM imaging mineral unit size, morphology and orientation of crystallites in the different microstructural arrangements and at the transition from one microstructure to the other. We also characterize the microstructure and texture of adductor and pedal retractor myostraca and address structural characteristics at the transition from crossed-lamellar to myostracal assemblies. We find that the crossed-lamellar layer has a three-dimensional crystallographic orientational order. Each set of first-order lamellae consists of twinned aragonite; the two sets of first-order lamellae are misoriented to each other by about 30 to 40° while retaining an approximately parallel a-axis; they do not show any particular twin relationship. Myostracal aragonite grows homoepitactically onto the crossed-lamellar aragonite, but is clearly a separate microstructure, with its own crystallite size and morphology. Within adductor and pedal myostraca, prisms increase in size towards inner surfaces. In contrast to the other shell layers, the myostraca form through competitive growth. The complex crossed-lamellar aragonite initially inherits the three-dimensional texture of the crossed-lamellar microstructure, but with growth develops an axial texture, which is transmitted to the underlying fibrous prismatic microstructure. With this work we provide a modern, unaltered, reference for fossil Glycymeris shells to be used for detection of diagenetic overprint in fossil Glycymeris analogs.

Reconstructing paleoceanographic conditions during the middle Ediacaran: Evidence from giant ooids in South China

The Ediacaran was a period of intense environmental change in Earth history. Transient pulses of oxygenation and shifts in ocean chemistry have been interpreted from a range of geochemical proxies, including rare earth elements and yttrium (REY), but it is still a challenge to reconstruct paleoceanographic conditions due to the scarcity of reliable recorder. In this study, we explore the possibility that a newly-discovered deposit of giant ooids in the Nanshanping outcrop in South China contains a relatively intact record of the REE composition of Ediacaran seawater, providing an important window into the redox state of the surface ocean at this time. These ooids are relatively large in size (~1–6 mm), and exhibit densely banded laminae. A regional chemostratigraphic correlation shows that the giant-ooid-bearing succession in South China is in the upper part of the second Ediacaran positive carbon isotopic shift (EP2), and thus its occurrence was not associated with the Shuram Excursion. Additionally, the occurrences of giant-ooid-bearing oolites in different blocks were not synchronous during the middle Ediacaran. In-situ laser ablation analysis of giant-ooid cortices reveals that the shale-normalized REY patterns are similar to those of modern seawater, including depleted light REEs, superchondritic Y/Ho ratios, and positive La anomalies. Yet, no Ce anomalies (1.0 ± 0.15) in numerous ooid cortices seems to indicate a suboxidized condition of surface ocean in South China at that time. The petrographic and geochemical evidence support the weak diagenetic alteration of the pure giant ooids during burial. They formed in open-ocean settings with energetic hydrodynamic environments and were affected by early dolomitization that might preserve mostly original petrographic fabrics and REY signatures from further diagenetic overprinting. The characteristics of Ce anomalies and REY compositions acquired from the giant ooids indicate that the surface ocean was suboxidized in South China during the late stage of EP2, implying the very low atmospheric O2 level at that time. Our results suggest that giant ooids may be a reliable analytical target that provide a snapshot into palaeoceanographic conditions during this crucial period of dynamic changes in early Earth history.

Variability in Sulfur Isotope Records of Phanerozoic Seawater Sulfate

AbstractThe δ34S of seawater sulfate reflects processes operating at the nexus of sulfur, carbon, and oxygen cycles. However, knowledge of past seawater sulfate δ34S values must be derived from proxy materials that are impacted differently by depositional and postdepositional processes. We produced new time series estimates for the δ34S value of seawater sulfate by combining 6,710 published data from three sedimentary archives—marine barite, evaporites, and carbonate‐associated sulfate—with updated age constraints on the deposits. Robust features in multiple records capture temporal trends in the δ34S value of seawater and its interplay with other Phanerozoic geochemical and stratigraphic trends. However, high‐frequency discordances indicate that each record is differentially prone to depositional biases and diagenetic overprints. The amount of noise, quantified from the variograms of each record, increases with age for all δ34S proxies, indicating that postdepositional processes obscure detailed knowledge of seawater sulfate's δ34S value deeper in time.

Microfacies analysis, diagenetic overprints, geochemistry, and reservoir quality of the Jurassic Samanasuk Formation at the Kahi Section, Nizampur Basin, NW Himalayas, Pakistan

In Pakistan, carbonate rocks of Jurassic age are present entirely throughout the Indus Basin. The Jurassic carbonate rocks present in the Attock-Cherat requires a more detailed investigation as of recent discovery of gas in Jurassic carbonate rocks in the lower Indus basin. The evaluation of these Jurassic carbonate units in the Nizampur Basin of Attock-Cherat ranges should integrate the aspects of the depositional sediments, diagenetic history, and geochemistry to understand the reservoir behavior. In this study, the Samanasuk Formation of 90 m was sampled and measured in detail, and samples were collected with ~ 0.3 m interval and some samples were collected where reckoned necessary. Petrographic and microfacies analysis were conducted on about 100 samples; moreover, XRD analysis were performed. The present study aims to determine the depositional facies, diagenetic processes, and geochemical elements of this carbonate succession of the in an effort to explore their effect on reservoir quality. The Samanasuk Formation comprises eight microfacies assigning three facies belts including peritidal, lagoon, and shoal of a carbonate ramp. The recorded diagenetic processes include dolomitization, compaction, micritization, neomorphism, dissolution, and cementation in which dolomitization played an important role in enhancing the reservoir quality. In relation of their impacts on reservoir properties, the grainstone facies associated with peritidal facies and dolomudstone facie associated with carbonate sand and shoal shows the greatest reservoir quality, whereas lagoonal facies has the lowest reservoir quality. This study represents an approach to use the depositional facies, diagenetic alterations, and geochemical framework of carbonate succession in the reservoir characterization.

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.

Integrating the palynostratigraphy, petrography, X-ray diffraction and scanning electron microscopy data for evaluating hydrocarbon reservoir potential of Jurassic rocks in the Kala Chitta Range, Northwest Pakistan

In this paper, we present the palynostratigraphy, petrography, scanning electron microscopy (SEM) and X-ray diffraction (XRD) investigations to evaluate hydrocarbon reservoir potential of the Jurassic clastic-carbonate mixed sequence of the Kala Chitta Range, northwest Pakistan. The chronostratigraphic sub-divisions of the diverse lithologies within the sequence were made by using palynostratigraphy. The clastic dominated sequence of Datta Formation was assigned Toarcian-Bajocian age, while the Shinawari Formation was deposited during the Bajocian-middle Bathonian. The carbonate shoal facies of the Samana Suk Formation showed late Bathonian-Tithonian time of deposition. The primary and secondary porosities augmented by the plug porosity and permeability data suggest that the sandstone of Datta Formation is an excellent reservoir. The dominance of diverse matrix within the Shinawari Formation occluded the primary porosity. However, based on dissolution and dolomitization, the Shinawari Formation is categorized as a moderate reservoir. The dominance of various types of matrix and cement with superimposed burial diagenesis has occluded the primary porosity within the Samana Suk Formation. However, the diagenetic dissolution and dolomitization during the telogenetic stage were supported by the SEM and bulk geochemical data. Such diagenetic overprinting has significantly enhanced the reservoir potential of the unit.

An integrated facies, diagenesis and geochemical analysis along with sequence stratigraphy of the Lower Triassic Aghe-Darband basin (north-east Iran)

The Sefid Kuh Formation (Lower Triassic), in Aghe-Darband tectonic window (NE of Iran) reflects a complex setting as is deducible from intercalation of carbonates with volcanic deposits. A detailed sediment petrological, sequence stratigraphical and geochemical study of this formation was carried out to develop a comprehensive knowledge of the complexity in this formation. Field observations of four stratigraphic sections complemented by sediment petrographical analysis led to the identification of microfacies that are grouped into four facies associations. The predominance of tidal flat facies along with oolitic shoal facies and absence of any large barrier/reef features supports the existence of a homoclinal-ramp in an arc basin setting during Sefid Kuh Formation deposition. Calcite cementation and dolomitization are the main diagenetic processes that affected the original strata. Stratigraphic and field studies revealed four 3rd order depositional sequences in this formation which architecture as reflects tectonic control. The measured elemental concentrations (Mn, Sr, Na, Fe, Mg and Ca) along with petrographic studies (e.g. existence of fibrous isopachous cement) suggest aragonite as a precursor mineralogy of the Sefid Kuh carbonates that were deposited in a shallow warm-water subtropical environment. Detailed petrographic and geochemical analysis reflect marine, meteoric and burial diagenetic overprint. Variations in Sr/Ca ratio suggest that diagenetic alteration occurred in an open diagenetic system, with high rock-water interaction.

Characteristics and formation mechanisms of the unconformity-related paleokarst reservoirs in the Upper Sinian, Northwestern Tarim Basin, China

In the Late Sinian, the Tarim Basin experienced regional tectonic uplift and subaerial exposure. A regional unconformity was extensively distributed during the succession from the Sinian to Cambrian. As a result, a large number of secondary pores were developed in the Upper Sinian Qegebulake Formation. An integrated study based on petrographical and petrophysical analysis, geochemistry, karstology and fluid inclusions was performed to provide insight into the characteristics and formation mechanism of the Upper Sinian carbonate reservoirs. The reservoir rocks mainly include microbial dolomite, crystalline dolomite and breccias. Reservoir matrix porosity and permeability range from 0.30% to 11.06% and from 0.00156 mD to 1.49 mD respectively.Geological and geochemical characteristics suggest that three phases of dissolution were determined to be responsible for the development of the reservoirs within about 70 m beneath the unconformity. They are meteoric dissolution during subaerial exposure, and deep-seated hydrothermal dissolution and organic acid dissolution during the burial stage. The Upper Sinian dolomite underwent strong meteoric dissolution, which is supported by the appearance of the unconformity and the associated caves. The geochemical characteristics of the cave-filling calcite further indicate the diagenetic overprint from meteoric water, including the relatively negative values of δ18O (−18.46 to −8.5‰, VPDB) and wide range of δ13C (−8.11 to −0.24‰, VPDB), low ƩREE (1.77–4.23 ppm) and Yb–La and Yb–Ca variation. The occurrence of saddle dolomite as well as analytical results such as δ13C (0.67–2.52‰, VPDB) and δ18O (−13.21 to −9.81‰, VPDB) isotope signature and high homogenization temperature of fluid inclusions in quartz (87.2 to > 220 °C) and saddle dolomite (82.3–135.4 °C) suggest that the reservoirs rocks experienced interaction with hydrothermal fluids. Only bitumen occurred in the edge of secondary pores formed by dissolution of quartz and saddle dolomite, which suggests that organic acid dissolution likely occurred during the evolution of reservoirs.A geological model for the evolution of the paleokarst reservoirs is proposed through the analysis of major reservoir controlling factors. The primary pore types and reservoir petrophysical property of the Qigebulake Formation mainly depend on the sedimentation. Supergene karstification is the most important process for reservoir formation. Deep-seated hydrothermal activity has both positive and negative attribution on the formation of reservoirs. Organic acid dissolution plays an active role in the formation and preservation of reservoirs. This study provides a useful reference for hydrocarbon exploration in the deep buried Sinian carbonate reservoirs.

Molybdenum geochemistry in salt marsh pond sediments

The concentration and isotopic composition of sedimentary molybdenum (Mo) has been used to distinguish different redox environments in modern marine settings and in the geological record. We report Mo concentrations and δ98Mo from porewaters and sediments in three anoxic East Anglian salt marsh pond environments: (1) ‘iron-rich’ sediments containing high concentrations of dissolved ferrous iron (up to 2 mM), (2) ‘sulfide-rich’ sediments containing very high concentrations of aqueous sulfide (up to 10 mM) and, (3) sediments that we consider to be intermediate between ‘iron-rich’ and ‘sulfide-rich’ conditions. In iron-rich sediments, we suggest that iron speciation and mineralogy controls the concentration and isotopic composition of Mo. Despite similar aqueous sulfide profiles, the intermediate and sulfide-rich pond sediment have different porewater Mo concentrations and δ98Mo. In the sulfide-rich pond sediment, we suggest that the concentration and isotopic composition of Mo is controlled by solubility equilibrium with an Fe-Mo-S mineral species (e.g. FeMoS4) due to similarities in sediment and porewater δ98Mo throughout the sediment column. In the intermediate pond sediment, we conclude that active breakdown of iron oxides redistributes porewater Mo, observable as a peak of dissolved Mo (>100 ppb), which diffuses within the sedimentary porewaters. The sedimentary δ98Mo is higher in sulfide-rich and intermediate pond sediment (mean = 1.66‰, range = 0.98–1.92‰) than in iron-rich pond sediment (mean = 1.10‰, range = 0.28–1.65‰) with all ponds having sedimentary δ98Mo that is lower than seawater. The maximum sedimentary δ98Mo observed in these anoxic sediments, which is 0.5–0.7‰ lower than seawater, appears to be set by Fe-Mo-S equilibration with ambient thiomolybdate species. We suggest diagenetic overprinting can cause more efficient capture of pond water Mo and causes sediment δ98Mo of originally iron-rich pond sediment to evolve to higher values at progressively higher aqueous sulfide concentrations.

UNRAVELLING THE COMPLEXITY OF THIN (SUB‐SEISMIC) HETEROGENEOUS CARBONATE RESERVOIRS: AN INTEGRATED STUDY OF THE ALBIAN MAUDDUD FORMATION IN THE GREATER BURGAN AREA, KUWAIT

The Albian Mauddud Formation is a prolific reservoir in Kuwait and nearby countries such as Iraq and Iran but has received far less attention than the under‐ and overlying units (the Aptian Shu'aiba and Cenomanian Mishrif Formations). Detailed reservoir characterization studies of the formation are required to support field development and improved / enhanced oil recovery (EOR) programmes. In this study, 26 wells penetrating the Mauddud Formation within the Greater Burgan area of Kuwait (Burgan and neighbouring fields) were investigated, integrating the logging of 910 ft of core with petrographic investigations of 113 stained and impregnated thin sections. In the Greater Burgan area, the Mauddud Formation can be divided into a lower Clastic Member and an upper Carbonate Member which is the main focus of this paper. The primary objective of the study was to present a new characterization of this thin, heterogeneous carbonate reservoir by integrating facies analysis and sequence stratigraphy with a detailed petrographic investigation. A second objective was to identify the relative importance of depositional characteristics and diagenesis on the distribution of reservoir properties.Sandstones in the Clastic Member of the Mauddud Formation were deposited on a delta which passed laterally to the north and east into a carbonate platform. During subsequent regional flooding, increased carbonate production resulted in the development of a larger‐scale carbonate platform covering the entire study area. The Burgan field area was part of the proximal zone of this carbonate platform. A number of depositional environments were identified by integrating core and thin section data. These range from outer platform to mid‐ and inner platform, the latter including both high‐ and low‐energy settings (shoal, shoreline; and lagoonal respectively). Mud‐supported textures characteristic of low‐energy inner‐platform and mid‐ to outer‐platform settings are volumetrically dominant in the Mauddud Carbonate Member.Sequence stratigraphic analysis suggests that the Mauddud Carbonate Member is part of a major regressive phase (or highstand systems tract) of a third‐order sequence, with the regional‐scale K110 MFS positioned close to the transition with the underlying Clastic Member. Two 4th order transgressive – regressive (TR) cycles or sequences, M1 and M2, were identified within the Carbonate Member. The top‐Mauddud surface corresponds to a sequence boundary with long‐lasting subaerial exposure during the latest Albian and is characterized by both micro‐ and macroscopic karst features (calcite dissolution vugs and recrystallization in thin sections; and cavities in cores). This study demonstrates that the Burgan field area experienced significant uplift, with increased differential erosion and/or non‐deposition of the upper M2 TR cycle towards the southwest.Eogenetic marine and meteoric calcite cements partially fill macropores close to the tops of the TR cycles and remaining macropores were occluded by shallow‐burial calcite cements. Cementation by ferroan dolomite cement, which resulted from the increased overburden and associated chemical compaction, has almost entirely occluded residual macropores in the lower part of the Mauddud Carbonate Member, close to the underlying siliciclastic deposits of the Clastic Member. As a consequence, porosity is mostly preserved in the middle part of the Carbonate Member. The predominance of mud‐supported textures in these intervals together with the effects of diagenesis explain the widespread occurrence of microporosity within both micritic matrix and grains. In the Greater Burgan area, meteoric diagenesis associated with subaerial exposure(s) of the top‐Mauddud preserved and/or enhanced micrite microporosity. Plug porosity may reach 35% or more, but permeability does not exceed 100 mD.The Mauddud Carbonate Member in the Greater Burgan area is a thin, microporous reservoir, with a dual porosity component which is linked to karst alteration and not to tectonic fracturing. The distribution of reservoir properties results from a combination of primary depositional characteristics and a multiphase diagenetic overprint. This study will guide the future development of the Mauddud reservoir in the Greater Burgan area. It also demonstrates the importance of an integrated approach to constrain sub‐seismic heterogeneities in carbonate reservoirs.