Carbonate Shoals Research Articles

The early–middle Cambrian siliciclastic tide‐dominated succession in eastern Korea

AbstractThe basal Taebaek Group (Myeonsan and Myobong formations) illustrates second‐order transgression on the eastern Sino‐Korean Block during the early–middle Cambrian (Stage 3?–Wuliuan). The irregular palaeotopography of Precambrian basement led to the development of localised alluvial fans and fan deltas. As transgression continued, tidally influenced estuarine and associated shallow subtidal flats developed. Continued transgression resulted in a relatively deep subtidal environment, with postulated carbonate shoals serving as barriers. Stacking of facies indicates a general deepening‐upward trend, transitioning from terrestrial–nearshore to marine environments dominated by tidal effects. The study area subsequently experienced shallowing, leading to the emergence of tidal inlet and dune environments. Siliciclastic input was attenuated, facilitating the development of carbonate shoal complexes. The overall transgressive succession of the basal Taebaek Group aligns with similar sequences observed in the Sino‐Korean Block and western Laurentia, where the lowermost Cambrian strata which formed in various depositional environments on the uneven basement rocks were followed by extensive tide‐dominated siliciclastic successions. These successions were then overlain by epeiric carbonate platforms that developed synchronously across the area as siliciclastic input diminished with the continued transgression. This comparison demonstrates that the transgressive sequences in the Sino‐Korean Block and western Laurentia can be understood within a framework that explains how extensive Cambrian carbonate platforms initially developed due to a second‐order eustatic sea‐level rise during the Cambrian greenhouse period.

Effects of basin tectonic evolution on multi-phase dolomitization: Insights from the Middle Permian Qixia Formation of the NW Sichuan Basin, SW China

The platform-marginal shoal carbonates of the Middle Permian Qixia Formation in the NW Sichuan Basin, as important ultra-deep (>5000 m) hydrocarbon reservoirs, experienced multi-phase dolomitization and recrystallization during the long-term and complex basin tectonic evolution; however, there is no consensus on the dolomitization mechanism. Four types of dolomites have been identified: medium- and coarsely-crystalline dolomites within patchy dolomitized limestone (LD) at the top of the carbonate strata, massive medium to coarsely-crystalline dolomites categorized as porous dolomites (MD1) and tight dolomites (MD2) at the bottom of the carbonate strata, and cement coarsely saddle dolomites (SD) in the vugs and fractures. All dolomites exhibit similar REESN patterns to the host limestone with an obviously positively Ce anomaly, and show Sr isotopes falling in the range of Permian seawater. However, the LD dolomites display a higher Sr but lower Mn concentration, and more positive δ13C and δ18O than other types of dolomites. The petrographic and geochemical results suggest the dolomites of the Qixia Formation replaced precursor grainstone by the seawater-derived fluids. A comprehensive dolomitization model is proposed, incorporating seawater-reflux, intermediate-burial and tectonic-squeegee, and hydrothermal dolomitization for forming different types of dolomites. LD dolomites are early diagenetic products after selective dissolution under a near-surface or shallow burial environment, in which the weak-evaporated Permian seawater mixed with minor meteoric water accounting for dolomitizing fluids. MD1 and MD2 dolomites are likely to form due to the residually buried seawater for widely dolomitization driven by the Late Triassic tectonic compression. Subsequently, the dolomitic strata underwent hydrothermal modification due to rejuvenation of the thrust belt to enable saddle dolomites filling pre-forming porosity. This study integrates outcrop, petrographic, geochemical and dating analysis with paleogeography and tectonic events, providing a new perspective to establish a conceptual model for multi-phase dolomitization as an analog in comparative tectonic settings worldwide.

Porosity Modification, A Key To The Carbonate Diagenetic Environments. Case Study: Reefal Limestone, Marah Formation, Miau Baru, East Borneo, Indonesia

Abstract Through the Eocene-Oligocene transition, localised carbonate shoal formed the northern part of Kutai basin. Miau Baru is a small village located in Kongbeng sub-district, East Kutai, NE Borneo which has a relatively narrow area with complex geological features. This relatively thick the carbonate successions of Marah Formation previously have less attention from diagenetic perspective. To achieve the objective in this diagenetic study of the reefal limestone of Priabonian-Rupelian Marah Formation, limestone samples were collected and sectioned. A total of 35 samples were selected, 25 from Goa Indah section and 10 from Kongbeng section. This paper compiles current knowledge on the sedimentology aspects of this Kongbeng complex region and includes the first attempt to synthesise petrographic data onto carbonate diagenetic environment and history in Miau Baru Area. Detailed identification of porosity modification helped to reconstruct carbonate diagenetic environment and its application in expecting a petroleum reservoir for the region, thereby highlighting areas where further geological research is required.

Reservoir identification of marine facies carbonate shoals in the Middle Permian in the Sichuan Basin

Controlled by the base level change cycle, the carbonates exposed from the periodically shallowed water body at the top of the high-frequency sequence develops multiple sets of thin reservoirs. Such reservoir identification has always been tricky in the prediction of carbonate reservoirs. Based on the Middle Permian strata from the Sichuan Basin, this paper established a forecasting approach of high-frequency-sequence-controlled carbonate thin reservoir. The steps are as follows: (1) Combine the measured results of samples from the typical outcrops with changes of carbon and oxygen isotopes, thus to construct a division program for the 3rd-order sequence of the Middle Permian strata. (2) Compare the carbon and oxygen isotope changes and the global sea level change trend, determine the controlling factors of the 3rd-order sequences. (3) Carry out spectrum analysis based on the natural gamma energy spectrum logging (ln (Th/K)) curves of Wells Long-17 and Anping-1 to establish a high-frequency sequence framework division program, and then apply the logging curve characteristics to correct the top interface positions of the high-frequency sequences, which is the potential location of carbonate reservoir. The research results suggest that the Middle Permian in the Sichuan Basin can be divided into three 3rd-order sequences, which are PSQ1, PSQ2 and PSQ3 from bottom to top. Among which, PSQ1 is mainly affected by global sea level (GSL) changes, and the changes originated from its tuning curve can be directly applied to the division of high-frequency sequences and reservoir prediction. PSQ2 and PSQ3 are jointly controlled by global sea level changes and regional tectonics. In cope with the influence of regional tectonics, the positions of the four-level sequence interface established according to the tuning curve need to be fully considered, and good results were achieved through systematical correction. This method can effectively reduce the risk of reservoir prediction caused by marine carbonate heterogeneity, and provide scientific foundation for the prediction of high-frequency cyclic reservoirs of platform facies and platform margin subfacies in other marine carbonate sedimentary basins.

Quantitative 3-D Model of Carbonate Reef and Shoal Facies Based on UAV Oblique Photogrammetry Data: A Case Study of the Jiantanba Outcrop in West Hubei, China

Aiming at the problem of insufficient data acquisition for steep carbonate outcrops, we used unmanned aerial vehicle oblique photogrammetry to quantitatively evaluate Jiantianba outcrops from a global perspective. This method can quickly and flexibly acquire, process and interpret outcrop data, establish the three-dimensional digital outcrop model and quantitative reservoir geological knowledge database. Through the fine analysis and comparative study of multiple outcrop sections, we use lithofacies combination, hierarchical interface, sequential indicator stochastic simulation and multiple-point geostatistical simulation methods to establish a three-dimensional multi-point statistical model of the outcrop area. The results show that the model of the Jiantianba carbonate outcrop has three-dimensional coordinates that correspond to oblique photograph image data, allowing for both the accurate location of any point and the measurement of the rock body, and thus providing a base for studying the stratigraphic architecture of the outcrop. Through fine-scale geological and statistical analyses of the geological parameters of three geologic sections, we established a database and model of a typical outcrop, including the types, continuity and scale of lithology, as well as different lithological combinations and the geological evolution of the region. And we established a geological model and quantitative geological database via sequential indicator simulation, using virtual well location, lithological combination, stratigraphic correlation, three-dimensional stratigraphic and lithological models of a typical outcrop. Subsequently, we analyzed the training images of different microfacies based on these models combined with the outcrop geometry. Finally, we established a three-dimensional geological model of reef–shoal facies that is more in line with our current understanding of the geology of the outcrop area by multi-point geostatistical simulations.

Factors that controlled deposition of lacustrine, mixed siliciclastic-carbonate sediments in the upper fourth member of the Eocene Shahejie Formation in the Zhanhua Sag, East China

The Zhanhua Sag is located in the east-central Jiyang Depression, southern Bohai Bay, northeast China, and is a half-graben, lacustrine basin. Mixed siliciclastic-carbonate sediments are quite common in Eocene deposits of the Zhanhua Sag. We established characteristics of the lithofacies, facies associations, and depositional environments of the upper fourth member of the Eocene Shahejie Formation in the Zhanhua Sag, using core descriptions, thin sections and well logging data. Mixed grainstones and mixed wackestones are two lithofacies of mixed sediments identified in the study area. Paleogeomorphology, sediment supply and gravity flows jointly controlled development of the mixed sediments in this study. Subaqueous highs, formed by terrigenous clastic sediments, provided substrates for carbonate production. The Ordovician carbonate strata exposed on both sides of the rise provided the material for carbonate production on the north and south sides. The clastic sediment supply, however, developed mainly on the south side. Less siliciclastic influx in the north was probably associated with weakening of tectonic activity and less topographic difference. Massive mixed sediments were the product of gravity flows developed under low-energy hydrodynamic conditions between the carbonate shoals and a deltaic system. Most of the siliciclastic material in the mixed sediments was transported by gravity flows from the fan delta to lows in a shallow lake.

Permian mixed carbonate–siliciclastic lagoon coastal system in West-Central Gondwana

Although in recent years mixed carbonate–siliciclastic platforms have been increasingly recognized in the Permian of Western Gondwana, there is no detailed description of these mixed systems in West-Central Gondwana. This study focuses on the Permian of the Sergipe–Alagoas Basin (NE Brazil), with the aim of filling this knowledge gap. Based on sedimentological data and microfacies analysis, six facies associations (FA) were identified: FA1 is an eolian–fluvial system, composed of stratified sandstones, FA2 and FA3 represent a mixed tidal flat–lagoon system composed of microbialites, pelitic/micritic sediments and intraclastic packstones, FA4 represents shoreface wave deposits composed of laminated sandstones and silicified grainstones, and FA5 and FA6 are interpreted as a barrier–tidal inlet system, composed of oolitic/intraclastic grainstones, subtidal microbialites, and mixed rudstones. Facies associations reflect the paleoenvironmental conditions of a coastal environment, with dunes and small rivers adjacent to a mixed carbonate–siliciclastic ramp. This is defined as a lagoon system associated with tidal flats protected from the high-energy region by carbonate shoals and microbial constructions. These deposits represent the eastern portion of a shallow epicontinental sea that covered lowlands in West-Central Gondwana within the arid–semiarid belt, whose climate characteristics are reflected in eolian deposits, desiccation structures, mixed carbonate–siliciclastic facies, and fossil records. The main diagenetic feature observed in petrographic analyses was an early diagenetic silicification. Although the absence of spiculites and shelf configuration did not allow a correlation with the Permian glass ramp model, similarities were observed with Eocene glass ramps of Australia, and a relationship with the Permian Chert Event is suggested.

Facies, sequence framework, and reservoir quality along a Triassic carbonate ramp: Kangan Formation, South Pars Field, Persian Gulf Superbasin

The Lower Triassic Kangan Formation of the Persian Gulf Superbasin forms one of the largest reservoirs in the South Pars/North Field, the world's largest gas field. The upper Khuff-equivalent, gas-hosting carbonate-evaporite succession was formed on a gently-sloping, homoclinal carbonate ramp in a warm, tropical aragonite sea under a hot-and-arid greenhouse climate. This study explores the impact of vertical variations in depositional facies and diagenetic features on the reservoir quality to improve the prediction of the subsurface facies and hydraulic flow unit distribution.Detailed core logging and petrographic analysis resulted in the recognition of twelve distinct carbonate-evaporite facies that are grouped into four major facies associations, from shallowest to deepest: supratidal to restricted subtidal, lagoon, carbonate shoal, and shallow mid-ramp. The facies associations are stacked into two long-term sequences KS2 (58 m) and KS1 (77 m) bounded by anhydrites and/or unconformities; three higher frequency sequences (KS1a, KS1b, and KS1c) can be identified within the KS1. Based on the petrophysical properties, sedimentary facies characteristics and their diagenetic modifications, five hydraulic flow units (HFUs) are identified, with the best porosity-permeability values and flow- and storage capacities in the late transgressive (TSTs) and the earliest highstand systems tracts (HSTs) of the two long-term sequences. The reservoir quality and the distribution of hydraulic flow units are a product of interactions between primary mineralogy, depositional facies and their stacking, and the early- and late-diagenetic alterations. The reservoir quality decreases in landward direction, being the lowest in the mud-supported textures of the lagoon and supratidal to restricted subtidal facies associations. The seaward improvement in reservoir quality reflects a change to grain-supported textures that formed along the tide-and wave-agitated ooid-bioclastic shoals, whereas the best reservoir quality characterizes the shallow mid-ramp open-marine facies association, where secondary porosity was improved by dissolution and dolomitization. Both the early and late diagenetic dolomitization overall improved the reservoir quality, with the dolomitization pathways controlled by the porosity and permeability heterogeneity in the original depositional facies. The results may be useful in reservoir modeling, recognition of the productive zones, and the further development of the South Pars/North Dome Field.

Создание концептуальной геологической модели, основанной на литолого-петрографических исследованиях, на примере пермокарбоновой залежи Усинского месторождения

The work intends to develop a conceptual model of the Permo-Carboniferous deposit of the Usinskoye field. In the furtherance of this goal, the authors have addressed the tasks of designing a specialised table format to describe thin sections of carbonate core samples and create a database based on the developed format for the subsequent analysis. The paper deals with studying the Permo-Carboniferous deposit of the Usinskoye field, located in the Komi Republic. A unique database of 1,710 described thin sections from 12 wells was created based on the developed format. The classification of carbonate rocks by R.J. Dunham supplemented by Embry and Kloven (based on the dominating fabric in limestone, type of cementing agent and their relationships in the rock) was used as a basis. According to rock material compositions and textural properties, the following nine rock lithotypes were identified: Mudstone, Wackestone, Packstone, Grainstone, Boundstone, Floatstone, Rudstone, Crystalline Carbonate (Dolomite), and Clayey Carbonate Silicite. By following the rock lithology type identification results and the seismic data, three main facies zones were determined in the section of the mid- Carboniferous and Lower Permian sediments: carbonate shoal (inner ramp zone), organogenic buildup (middle ramp zone) and shallow marine shelf plain (mid-ramp zone, partially outer ramp zone). A moderately deep marine shelf plain facies (outer ramp zone) was provisionally defined. Following the study results, two profiles of the Mid-Carboniferous and Lower Permian sediments of the Usinskoye field were plotted along two well lines. The presented sections support the conceptual model developed. Based on the core sample findings, the zone of organogenic build-ups stands out in the eastern part of the field, dating back predominantly to the Late Carboniferous and Early Permian periods. An inner ramp with the carbonate shoal facies supposedly exists in the field north-west. The conducted work has resulted in the development of a conceptual model of the Permo-Carboniferous deposit of the Usinskoye field, which can be used for further development of more reliable 3D facies models, commercial-scale estimations of reserves and field development designs.

Sedimentary characteristics and formation mechanisms of the Ordovician carbonate shoals in the Tarim Basin, NW China

The Ordovician carbonate shoals are extensively developed in the Tarim Basin, NW China, and are taken as important petroleum reservoirs. The sedimentary characteristics and controlling factors of these shoals are poorly studied. Multiple means, including field geological investigations, core observations, thin section identifications by scanning electron microscope and cathodoluminescence microscope, granulometric analyses, and geochemical analyses, are conducted. The classification of limestones and shoals in the Ordovician of the Tarim Basin is established. Four types of shoals are developed, namely, intraclastic shoals, bioclastic shoals, oolitic shoals, and algal shoals. It is demonstrated that these four shoal types are deposited in progradation sequences, among which, the low energy bioclastic shoals are in the sequence of retrogradation. Different sedimentary environments develop various sedimentary combination patterns: the platform margin facies mainly develop reef–shoal combinations followed by shoal–shoal combinations; the open platform facies mainly develop shoal–shoal combinations and then reef–shoal combinations; the restricted platform facies develops restricted shoal combinations. Intraclastic shoals are the most developed shoals, which are directly controlled by hydrodynamic force that caused by sea level changes and paleogeography that dominated by tectonic movement. In the platform margin facies, shoals are well-developed with large thickness and limited horizontal extent, and the main shoal type is bioclastic shoal, followed by algae shoal. In the open platform facies, Shoals usually present thin-layered with large horizontal extent. The main shoal type is intraclastic shoal that shows a gently moundy or sill-like structure, the followings are bioclastic shoals and oolitic shoals that developed in the highland near the platform margin facies. In the restricted platform facies, a small number of shoals are developed, which distributed in striped shape paralleling to the shoreline. This study provides great guidance for predicting shoals in different sedimentary facies and build good foundation for future shoal-related reservoirs assessment and prediction.

Conductivity of hydraulic fracturing in tight carbonate intra-platform shoal reservoirs

The carbonate intra-platform shoal is a typically tight oil and gas reservoir with low porosity and low permeability. A common way to stimulate this type of reservoir is through acid fracturing. Hydraulic fracturing, however, is another relatively newer technique for the stimulation of tight carbonate reservoirs, which form self-propped and propped fractures. In order to explore the applicability and efficacy of hydraulic fracturing in such reservoirs, a series of experiments were conducted on intra-platform shoal carbonates from the Ordovician Yijianfang and Yingshan Formation in the Tarim Basin of China. The results show that under low closure pressures, self-propped fractures provide a relatively high conductivity for fluid flowing as long as the closure (confining) pressure is relatively low, with almost 50,000 mD-ft, five times greater than propped fractures. Thus, the generation of self-propping fractures is a viable stimulation mechanism for tight carbonate reservoirs. However, with increasing closure pressure, the conductivity of self-propped fractures declines rapidly, decreasing to 15 mD-ft when the closure pressure increased to 4000 psi. At higher pressures (e.g. higher than 4000 psi), fracture conductivity is greater for propped fractures. The roughness of the fracture surfaces has an apparent controlling effect on fracture conductivity under various degrees of displacement. However, under high closure pressures the fracture conductivity is not significantly related to the degree of displacement. The conductivity of propped fractures is positively correlated with the size and concentration of the proppants. The inhomogeneous distribution of proppants on the surface of fractures intensifies the damage of self-propped fractures under high closure pressures. Although fracture conductivity declines sharply with increasing confining pressure, a series of additional fractures are formed in the surrounding rocks due to widening of pre-existing micro-fractures and stylolites. These types of fractures can intercommunicate with the newly created fractures, thereby enhancing the stimulation effect. In general, self-propped fractures are not suitable for high closure pressures. However, proppants compensate for this shortcoming.

Salt anomalies in potash beds of the Esterhazy Member, Devonian Prairie Evaporite Formation, Saskatchewan, Canada

The Esterhazy Member of the western Canada Prairie Evaporite has been mined underground for sylvite (KCl) since the early 1960s. Although the geology of the Esterhazy Member ore body is largely considered a regional flat lying continuous series of thin potash hosting beds, there are numerous occurrences where the ore has been either replaced or removed leaving behind uneconomical halite-rich sections. An explanation of the underlying controls on the formation of these salt anomalies has been somewhat elusive although the overwhelming assumption remains that these features developed in lows on a salina. This paper proposes that salt anomalies formed because of two processes, early compaction of carbonate shoals of the Winnipegosis Formation and tectonics that resulted in multiple stages of block movement during the deposition of the upper Prairie Evaporite. Since these two processes can result in a significantly different size to a salt anomaly, encountering one or the other type can have a significant effect on the economics of the ore body. This paper looks at some of the geological methods that might provide geologists with means to predicting salt anomalies.

Mixed siliciclastic–carbonate systems and their impact for the development of deep-water turbidites in continental margins: A case study from the Late Jurassic to Early Cretaceous Shelburne subbasin in offshore Nova Scotia

Progradation of deltaic systems that reach the shelf edge is considered a primary mechanism to deliver sand to continental margins. Sediment bypass processes can dominate the outer shelf to upper slope transition, causing poor preservation of reservoir quality sandstones. Turbidites can carry the bulk of the coarse fraction downdip where a thicker sand pile can be deposited once the channel-to-lobe transition is reached. Predicting this lateral and vertical variability along the slope is challenging. This configuration can be further complicated when mixed siliciclastic–carbonate systems are present. The Roseway–Missisauga case study from Nova Scotia is used here to explore potential implications associated with the development of deep-water turbidites that are time equivalent to outer-shelf mixed siliciclastic–carbonate units. Two scenarios are possible: (1) The carbonate factory is dominant, and the development of carbonate reefs and pinnacles on the outer shelf prevents the passage of siliciclastic systems beyond the shelf break; in this case, the siliciclastic component is sequestered within the inner-outer shelf. (2) Favorable conditions for carbonate production gradually deteriorate by the activation of fluviodeltaic systems that prograde outboard reaching the slope region. In this last scenario, low relief and lateral discontinuous carbonate shoals are ubiquitous in the outer shelf representing the last outboard remnants of the carbonate factory. Shelf-edge deltas circumvent or breach these carbonate shoals, establishing sedimentary pathways on the shelf-break region that connect with deep-water turbidites. Observations suggest that this last scenario is the most likely in this part of the Scotian margin during the Late Jurassic to Early Cretaceous.

Diagenesis and reservoir quality of carbonates rocks and mixed siliciclastic as response of the Late Carboniferous glacio-eustatic fluctuation: A case study of Xiaohaizi Formation in western Tarim Basin

Abstract Late Carboniferous-Permian was a longest-lived and most widespread ice age. The depositional environment, diagenesis and reservoir quality of carbonate rocks as response to the glacio-eustatic changes was analyzed in this study. The Upper Carboniferous Xiaohaizi Formation in the western Tarim Basin is an important oil reservoir. The dolostones from the Xiaohaizi Formation have very good physical properties, with maximum porosity and permeability of 16.6% and 214mD, respectively. The sedimentary facies of the Xiaohaizi Formation consist of carbonate shoals and lagoon on a mixed siliciclastic-carbonate platform. There was a strong heterogeneity of carbonate rocks deposited on shoals. Porous dolostones always alternate with tight sparry allochemical limestones. Based on the petrographic observation and stable isotope measurement, the controlling factors for the heterogeneity and reservoir quality were revealed. The palaeosol, vadose silt, karst breccia, detrital kaolinite, darkening of dolostones and enrichment of 18O indicate the short-lasting subaerial emergence during the glacial period. During the glacial sea level fall, the mixed siliciclastic and carbonate allochems deposited on bioclastic and intraclastic shoals, while dolomitization of calcic grains occurred under a relatively low-temperature. Through the correlation analysis of physical property, lithology, rock components, it's suggested that the reservoir quality of the Xiaohaizi Formation was controlled by mixed sedimentation and dolomitization associated with glacio-eustatic fluctuation. At the burial diagenesis, the effect of hydrothermal fluid on physical properties of dolostones with relative high primary porosity was positive, whereas resulted in the poikilotopic calcite cementation. The assembly of fluorite, dickite and authigenic pyrite were indicators for hydrothermal fluid accumulation. The control of glacio-eustatic fluctuations on sedimentation and diagenesis in carbonates could be used to forecast the reservoir quality.

What matters for flow and recovery in carbonate gas reservoirs: Insights from the mature Central Luconia Province, offshore Sarawak, Malaysia

Miocene carbonate reservoirs in Central Luconia, offshore Sarawak, Malaysia, have been delivering gas for over 30 yr. In this paper, learnings from that period of production are used to understand the key drivers affecting flow during production and recovery optimization in existing fields as well as development decisions for new discoveries. The large data set, generated over more than 40 yr, was analyzed in a consistent manner through a holistic database, constrained by a stratigraphic framework, to allow reservoir units to be compared like-for-like (“integrated knowledge base” [IKB] concept). Carbonate reservoir heterogeneities impacting flow are grouped into “horizontal–heterogeneities”—argillaceous flooding layers and exposure-related karst—and “vertical–heterogeneities”—large-scale architectural elements, found especially along platform margins. Both types of heterogeneities control water ingress during production and influence the recovery mechanism. Argillaceous flooding layers can act as baffles, holding back water rise during production, or can form pressure compartments. Long-lived, fault-bounded reef margins, carbonate shoals, islands, and karsts can be vertical conduits for aquifer inflow. Platform shape and architecture impact column height and hence recovery efficiency. Additional drivers impacting recovery were found to be gas-column height, aquifer size and permeability, pressure connection to neighboring fields, and field development concepts. All drivers identified impact decisions throughout the field life, e.g., well count and design, intervention capabilities, evaluation and mitigation of early-water breakthrough, reservoir management, selecting enhanced recovery methods, and abandonment pressure. The IKB allowed to derive “big rules” on what matters for flow, which were used to decide on development strategies for greenfields in Central Luconia. The presented outcomes can be extrapolated to comparable carbonate systems, whereas the IKB approach can be adapted and applied to other mature basins and reservoir types where equally vast and historic data sets are awaiting to be used in the current era of digitalization.

Modern carbonate ooids preserve ambient aqueous REE signatures

Skeletal and non-skeletal components of marine sedimentary rocks have been analyzed for the purpose of reconstructing the rare earth element (REE) and yttrium (Y) compositions of paleo-seawater, but skeletal carbonates frequently have proven to be unreliable recorders of seawater chemistry. Here, we present a systematic multi-technique assessment of rare earth and other trace elements in ooid sands from the modern Great Bahama Bank (GBB–marine) and Great Salt Lake (GSL–continental) based on strong-acid (hydrofluoric and nitric) and weak-acid (acetic) digestions, as well as laser ablation (LA) of ooid cortices and nuclei. The results show that Bahamian ooid cortices possess shale-normalized REE + Y features nearly identical to those of shallow seawater, including limited contamination from siliciclastic REEs. An admixture of even 0.2% of detritally sourced material can modify the primary marine REE + Y patterns by, for example, increasing the light REE (LREE) content. Mean values of LA data for Bahamian ooid cortices exhibit similar REE + Y signatures to those produced by acetic acid digestion, but LA data are generally noisier, primarily as a result of low REE concentrations and the small volume of carbonate ablated in analysis. Screening out samples with ΣREE <0.9 ppm reveals more uniform, seawater-like REE + Y patterns in individual ooid cortices as well as high, uniform REE distribution coefficients (116 ± 21, from La to Lu) with respect to ambient seawater. Ooid laminae show no distinct alternation between oxic and anoxic pore fluid characteristics, suggesting that growing ooids primarily formed under oxic conditions in carbonate shoal settings, even when buried during their resting phase. Unlike Bahamian ooids, shale-normalized REE + Y patterns of GSL ooids digested in weak acid show slightly depleted LREE compositions that may deviate from ambient fluids owing to release of REEs from an included clay fraction based on comparison between digestion and LA methods. Multiple ablation spots within the same cortex allowed direct comparison of more and less contaminated portions of a single ooid. Nearly uniform positive Ce anomalies may be related to strongly alkaline water conditions. Individual ablation spots that are partially overprinted by siliciclastic-derived REEs contain variable Zr contents (from 0.096 to ~4 ppm) and flattened shale-normalized LREE patterns. The least-contaminated GSL ooid cortex yields an LREE-depleted REE + Y distribution. Normalization of the REE + Y distributions of least-contaminated GSL ooids using reliable GBB ooid distributions (i.e., with ΣREE > 0.9 ppm) returns a flat pattern, suggesting similar degrees of LREE depletion controlled by carbonate complexation under similar aqueous alkalinity conditions in Great Salt Lake and Bahamian waters. In summary, ooids can be a reliable proxy for REE + Y characteristics of ambient surficial waters when adopting suitable analytical methods, including laser ablation, that allow the identification and isolation of a contamination signal from siliciclastic detritus.

Pore characteristics of the carbonate shoal from fractal perspective

The carbonate shoal is a notable reservoir for the hydrocarbon enrichment. Fractal theory has been proven to be a robust analytical tool to indicate the pore irregularity and surface roughness for permeable medium. With the aim to study the pore structure of carbonate shoal from fractal perspective, a series of parallel experiments including nuclear magnetic resonance (NMR), high pressure mercury intrusion (HPMI) and scanning electron microscopy (SEM) were carried out in this paper to calculate the fractal dimensions and the results given by various techniques were compared. The relationships between the fractal dimensions along with the petrophysical properties including the pore structure parameters were analyzed. Based on the fractal dimensions, the tested samples and the different pore types could effectively be distinguished. Classification methods for pore system including macropores (dp＞1000 nm), mesopores (1000 nm＞dp＞100 nm), Ⅱ-micropores (100 nm＞dp＞25 nm) and Ⅰ-micropores (dp＜25 nm) in the tight carbonate shoal reservoir were verified by the HPMI-based fractal dimensions. Fractal curves obtained from the NMR data could be divided into two different segments, which respectively corresponds to the seepage pores and adsorption pores. The SEM-based fractal curves were presented as a straight line. Results showed that micropores in the carbonate shoal made greater contribution to the total fractal dimension, although macropores and mesopores tended to have a relatively higher fractal dimension. Porosity and permeability of the tested samples were mainly governed by the meso- and macropores. Negative linear correlations were identified on the one hand between the HPMI-based total fractal dimensions (DHPMI) and the maximum mercury saturation (SHgmax), and the other hand the fractal dimensions of the macropores (DHPMI-macropores) and the average pore size (rave), respectively. Due to the different sensitivities, no clear correlations were obtained between the fractal dimensions obtained from the HPMI, NMR and SEM data in the whole range of pore size distribution, but DHPMI-macropores showed a positive correlation with DNMR-seepage pores.

Echinoids from the upper Qishn Formation (lower Aptian) in the Haushi-Huqf area, Oman Interior Basin, Arabic Peninsula

The Haushi-Huqf region (central Oman) is a structural high on which the Qishn and Nahr Umr formations represent the upper Barremian–lower Aptian and Albian intervals, respectively, which record some of the transgressive events that covered large portions of the Arabian Peninsula during the mid-Cretaceous. With a complex post-depositional evolution through processes of burial, uplift, tilting and intense denudation, the current landscape allows a glimpse of a virtually exhumed ocean floor, as it would have occurred about 120 myr ago. A NW–SE hiking of about 1200 m in Wadi Bauw, in the southeasterly portion of Jabal Dumur, has allowed the uninterrupted monitoring of the transition of depositional palaeoenvironments from a restricted lagoon to an open-marine platform, below storm wave base. The echinoids Tetragramma deshayesi (Cotteau, 1864), Coenholectypus neocomensis (Gras, 1848) and Pliotoxaster cf. comanchei (Clark, in Clark and Twitchell, 1915) come from the upper part of the Qishn Formation and are dated as early Aptian, based on the presence of the foraminifer Voloshinoides murgensis Luperto Sinni and Masse, 1993 and the rudists Glossomyophorus costatus Masse, Skelton and Slišković, 1984, Offneria nicolinae (Mainelli, 1983) and O. murgensis Masse, 1992. All specimens were collected from marly facies in which orbitolinid foraminifera predominate, in what could have been the external portion of an open platform, immediately after the occurrence of carbonate shoals consisting of oncolitic/peloidal grainstones/packstones and rudists that occur in association with bioconstructions of the Lithocodium/Bacinella consortium. Morphological characteristics of the echinoids suggest that, although Tetragramma was epifaunal and Coenholectypus endofaunal, both would have inhabited shallow and protected environments, composed of coarse sandy sediments in an internal lagoon. In contrast, the genus Pliotoxaster would have been better adapted to deeper environments or to settings with lower oxygen levels, i.e., being more closely compatible with the environment it was found in. Thus, it is likely that representatives of Coenholectypus and Tetragramma had been transported from the lagoonal regions and/or near the carbonate shoals to deeper regions with fore-bank characteristics.

Genetic types of carbonate shoal reservoirs in the Middle Triassic of the Sichuan Basin (SW China)

Abstract Carbonate shoal reservoirs of the Middle Triassic Leikoupo Formation in the Sichuan Basin were investigated for potential gas resources. The carbonate shoal reservoirs can be divided on the basis of their formation mechanism into residual intergranular pore, karst (further divided into syndepositional-, burial-, and epigenetic-karstification types), dolomitized, and composite reservoirs. Residual intergranular pores and dissolved pores are the main reservoir spaces in the residual intergranular pore reservoirs. Isolated intragranular corroded pores and moldic pores are the main reservoir spaces in syndepositional-karstification reservoirs, the main spaces in burial-karstification reservoirs are dissolved pores and holes, and pores and caves on various scales are the main reservoir spaces of epigenetic-karstification reservoirs. Intercrystal pores and intercrystal solution pores are the main reservoir spaces of dolomitized reservoirs. Composite reservoirs are characterized by a variety of reservoir spaces with pores, holes, and fissures. High-quality shoal reservoirs were formed by superposition of carbonate shoals and constructive diagenetic modification. Preservation of residual intergranular pores was crucial for development of residual intergranular pore reservoirs. Platform-interior micro-highlands, relative declines in sea level, and rainfall controlled the development of syndepositional-karstification reservoirs. Dissolution by unsaturated acidic fluids, pre-existing pores and pore throats, and the direction of corrosive fluid flow were important for burial-karstification reservoirs. Epigenetic-karstification reservoirs formed as a result of significant sea-level drops or tectonic movement. Dolomitized reservoirs formed in shoals affected by dolomitization caused by evaporation of seawater or that occurred during burial. Composite shoal reservoirs were formed by two or more processes.

The disappearance of a Late Jurassic remnant sea in the southern Qiangtang Block (Shamuluo Formation, Najiangco area): Implications for the tectonic uplift of central Tibet

Located between the Bangong-Nujiang suture zone and the Qiangtang Block in central Tibet, the Najiangco area (~5 km to the north of Nima-Selinco) contains an Upper Jurassic-Lower Cretaceous sedimentary succession deposited during a period of marine regression. The youngest marine sedimentary unit in the Najiangco area is the Upper Jurassic Shamuluo Formation, which consists of sandstone, limestone, siltstone, and shale. Sedimentary facies analysis shows that tidal flat and subtidal lagoonal facies characterized the northern margin of the basin, while delta front and prodelta facies dominated the middle part, and carbonate shoal and patch reef facies prevailed along the southern margin. Provenance analysis, including petrographic modal analysis of sandstones and U-Pb dating of detrital zircons, shows that a recycled orogen in the central Qiangtang to the north of Najiangco area was the source of the sandstones in the Shamuluo Formation. Biostratigraphy and U-Pb zircon dating of a porphyritic granitoid dike (151 ± 2 Ma) indicate that the Shamuluo Formation was deposited during the Late Jurassic (Oxfordian to Kimmeridgian). During Middle Jurassic time, the southern Qiangtang Basin was dominated by shallow-marine environments. Later, during the Late Jurassic (Oxfordian to Kimmeridgian), the shallow-marine facies retreated to the southern margin of the basin. Combined with regional paleogeographic data from central Tibet, two stages of southward retreat of the Qiangtang remnant sea, and three stages of topographic uplift of the Qiangtang Block can be recognized during late Middle Jurassic to Early Cretaceous time.