Micritic Dolomite Research Articles

Characteristics and Genesis of Carbonate Weathering Crust Reservoirs: A Case from the Ma5Member of Ordovician in Gaoqiao Area, Ordos Basin, China.

The hydrocarbon reserves in carbonate rocks account for about half of the global hydrocarbon reserves and are an important reservoir type. The Ordovician Majiagou Formation in the central Ordos Basin is a representative weathering crust reservoir. The Caledonian movement uplifted the stratum as a whole, and subsequently, 120 million years of exposed weathering, denudation, and leaching created this unique karst paleomorphology. Dolomite reservoirs have developed dissolved pores and microfractures, which are the best reserved spaces for natural gas and good hydrocarbon migration channels. This paper takes the Ma5Member (hereinafter referred to as Ma51+2) carbonate reservoir in Gaoqiao Gas Field as the research target, based on the core, thin section, cathodoluminescence, logging data, etc., and systematically study the effect of karstification on the reservoir and the genesis of the dolomite reservoir. The results show that the depositional period of the Ordovician Majiagou strata is a regression cycle and the depositional environment is a limited evaporative tidal flat. The reservoir lithology of Ma51+2 is mainly gypsiferous dolomite and micrite dolomite. The reservoir space types consist of intergranular pores, gypsum mold pore, intragranular dissolution pores, intercrystalline pores, and microfractures. The porosity has values from 0.3 to 11.2% (mostly less than 5.0%) with an average of 3.3%, and the permeability ranges from 0.003 to 13.2 mD (mostly less than 1 mD) with an average of 0.36 mD. Karstification is divided into three periods, including syngenetic karst, eogenetic karst, and burial karst. The sedimentary microfacies determine the material basis of the reservoir, and multistage karstification finally modifies the physical properties. By deeply exploring the formation mechanism and influencing factors of the carbonate reservoir in the Ordovician Majiagou Formation, it provides an important theoretical basis and practical guidance for oil and gas exploration and development. At the same time, it also has important reference value for understanding and predicting the development law and distribution characteristics of carbonate reservoirs under similar geological background.

Characterization of seismic-scale petrofacies variability in the Arbuckle Group using supervised machine learning: Wellington Field, Kansas

The Arbuckle Group in southern Kansas has been investigated for carbon geosequestration-related studies. In this study, we evaluated the seismic-scale petrophysically defined facies variability of the Arbuckle Group at the Wellington Field, Kansas, using quantitative seismic interpretation and a supervised Random Forest classification approach. We first defined three petrophysics-based rock types (petrofacies) from core-derived porosity and permeability measurements using the flow-zone indicator approach. Then, using the artificial neural network, we classified these petrofacies in noncored intervals. We observed that petrofacies 1 corresponds to medium and coarse-grained dolomitic packstone, wackestone, and dolomitic breccia with up to 8% porosity and D-scale permeability values. Whereas petrofacies 2 and 3 correspond to argillaceous and fine-grained micritic dolomites and dolomitic mudstones with lower permeability values for a given porosity, with respect to petrofacies 1. Using the common reflection-point gathers, we performed prestack seismic inversion and calculated various amplitude-variation-with-offset (AVO) attribute volumes. We used these elastic properties and AVO attribute volumes as input for estimating supervised seismic-scale 3D petrofacies and petrofacies probability volumes using the Random Forest algorithm. Results reveal the complex distribution of the petrofacies in the candidate injection and baffle zones in the study area, where petrofacies 1 is mainly prevalent within the lower and upper portions of the Arbuckle group, whereas petrofacies 2 and 3 are mainly present in the middle Arbuckle interval. The workflow we present through this study provides the spatial variability of facies distribution that is reflective of the actual lithology and petrophysical properties of the Arbuckle group in the study area with limited well control.

Diagenetic alterations induced by lamina-scale mass transfer and the impacts on shale oil reservoir formation in carbonate-rich shale of the Permian Lucaogou Formation, Jimusar Sag

Diagenesis plays a crucial role in shale oil reservoir formation. However, complex organic-inorganic interactions result in pore system formation and heterogeneity changes at the centimeter, or even micrometer scale, which controls the evolution of shale oil reservoirs. In this paper, an integrated approach including core observation, thin section observation, cathodoluminescence thin section observation, scanning electron microscope, μ-XRF analysis, electron microprobe analysis, AMICS analysis, LA-ICP-MS analysis, and isotope analysis was used to clarify the laminae combination carbonate-rich shale, diagenetic alterations in different types of shale laminae and the controls on shale oil reservoir formation. According to the lamina combination, the carbonate-rich shale in the Permian Lucaogou Formation can be divided into three types. They are shale consists of dolomitic lamina and terrigenous felsic lamina (Type A shale), shale consists of calcite-rich tufaceous lamina and terrigenous felsic lamina (Type B shale) and shale consists of calcareous lamina and tufaceous lamina (Type C shale). Dolomite cementation is the major diagenesis in terrigenous felsic lamina of Type A shale. The dolomite can be identified as the early stage forming from recrystallization of micrite dolomite, and the late stage precipitate from adjacent dolomitic lamina. In the terrigenous felsic lamina of Type B shale, however, the calcite cementation is the dominated diagenesis, which was sourced from adjacent calcite-rich tufaceous lamina. Also, some feldspar dissolutions occurred in terrigenous felsic lamina during organic matter evolution. For the Type C shale, the main diagenetic alteration is micrite calcite recrystallization in calcareous lamina. The dissolution and precipitation of carbonate minerals between different laminae of carbonate-rich shale reveal that the organic matter evolution significantly influence the diagenetic alterations of inorganic minerals. Particularly, the diagenetic mass transfer and redistribution at the micro-scale in a relatively diagenetic system were confirmed by organic-inorganic interactions in laminated shale. In this process, dissolution pores and recrystallization pores have provided considerable reservoir pores for shale oil.

Occurrence and Genesis of Cold‐Seep Authigenic Carbonates from the South‐Eastern Mediterranean Sea

AbstractMethane‐derived authigenic seep carbonates occur globally along continental margins. These carbonates are important archives to identify seep dynamics, the source of the ascending methane‐enriched fluids together with their timing, and are an important carbon sequestration mechanism. Recently, seep carbonates were discovered in the Levant Basin in the south‐eastern Mediterranean Sea. To elucidate past seepage activity and dynamics across the basin, different seep carbonate morphologies (chimneys, crusts and pavements) retrieved from the Levant Basin were mapped based on remotely operating vehicle data and analysed using standard sediment petrographic techniques, X‐ray diffraction and stable carbon and oxygen isotope analyses. Carbonate chimneys consist of micrite (δ13CVPDB of −10‰ to +5‰) with dispersed baryte and dolomite crystals, fan‐shaped aragonite (δ13CVPDB of −52‰ to −30‰) and high‐magnesium calcite cements, with the latter often growing from low‐magnesium calcite spherules. Botryoidal low‐magnesium calcite cements are forming in small cavities. Carbonate crusts consist of micrite with low‐magnesium calcite breccias, high‐magnesium calcite nodules (δ13CVPDB of −35‰ to −20‰) and cements, and partially replaced fan‐shaped aragonite cements. Carbonate pavements consist of low‐magnesium calcite microsparite, micritic dolomite and high‐magnesium calcite. Fan‐shaped aragonite is locally present as pore‐lining cement. Iron oxides are often seen coating the low‐magnesium calcite, high‐magnesium calcite and dolomite cements. Chimneys and crusts, characterised by high amounts of high‐magnesium calcite and aragonite, are interpreted to have formed through advective methane fluxes. Pavements, with high quantities of dolomite, are explained as the product of diffusive methane flux. Sediment petrographic and geochemical analysis of the different carbonate morphologies and cement phases therein witness distinct modes of ascending fluid fluxes and their mixing with marine pore water and/or sea water during precipitation of the individual phases.

Genesis of granular calcite in lacustrine fine-grained sedimentary rocks and its indication to volcanic-hydrothermal events: A case study of Permian Lucaogou Formation in Jimusar Sag, Junggar Basin, NW China

Granular calcite is an authigenic mineral in fine-grained sedimentary rocks. Core observation, thin section observation, cathodoluminescence analysis, fluid inclusion analysis, scanning electron microscope (SEM), and isotopic composition analysis were combined to clarify the genesis of granular calcite in the lacustrine fine-grained sedimentary rocks of the Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin. It is found that the granular calcite is distributed with laminated characteristics in fine-grained sedimentary rocks in tuffite zones (or the transitional zone between tuffite and micritic dolomite). Granular calcite has obvious cathodoluminesence band, and it can be divided into three stages. Stage-I calcite, with non-luminesence, high content of Sr element, inclusions containing COS, and homogenization temperature higher than 170 °C, was directly formed from the volcanic-hydrothermal deposition. Stage-II calcite, with bright yellow luminescence, high contents of Fe, Mn and Mg, enrichment of light rare earth elements (LREEs), and high homogenization temperature, was formed by recrystallization of calcareous edges from exhalative hydrothermal deposition. Stage-III calcite, with dark orange luminescence band, high contents of Mg, P, V and other elements, no obvious fractionation among LREEs, and low homogenization temperature, was originated from diagenetic transformation during burial. The granular calcite appears regularly in the vertical direction and its formation temperature decreases from the center to the margin of particles, providing direct evidences for volcanic-hydrothermal events during the deposition of the Lucaogou Formation. The volcanic-hydrothermal event was conducive to the enrichment of organic matters in fine-grained sedimentary rocks of the Lucaogrou Formation, and positive to the development of high-quality source rocks. The volcanic-hydrothermal sediments might generate intergranular pores/fractures during the evolution, creating conditions for the self-generation and self-storage of shale oil.

Characteristics and Geological Implications of Lacustrine Carbonate of Nantun Formation in Hailar Basin

Abstract Globally, lacustrine carbonate rocks are often found in rift lake basins. Because they often do not form large-scale conventional or unconventional reservoirs, they have not received enough attention in previous oil and gas exploration. Recent years, exploration examples in China show that lacustrine carbonate has close relationship with unconventional oil and gas. Mudstone with carbonate deposits always has a good exploration effect in the area. However, the genesis of lacustrine carbonate is still controversial. The sediment environment difference between carbonate and mudstone and its effects on unconventional hydrocarbon accumulation are not yet clear. This paper focuses on lacustrine carbonate found in the Nantun Formation in Hailar Basin. Through the analysis of lithofacies, geochemistry, and logging data, several findings have been obtained in the current study. (a) Two kinds of lacustrine carbonates, micrite dolomite and marlstone, which represent semideep to deep lake environments, have been identified in the research area. (b) Lacustrine dolomite (LD), whose genesis may relate to magmatic movement and deep hydrothermal processes, shows an extremely dry, hot paleoclimate and a saline, anaerobic ancient water condition. (c) Carbonate deposition in mudstone is related to the condensation section, and marlstone and micrite dolomite represent the beginning and end of the condensation section, respectively. Compared with mudstone around carbonate rocks, LD represents a drier, hotter, and saltier environment, which is of significance for sedimentary environment identification. In addition, the mudstone with lacustrine carbonate deposits has better organic matter characteristics and can form good unconventional oil and gas reservoirs that are of great significance for further exploration.

Effect of lithofacies on differential movable fluid behaviors of saline lacustrine fine-grained mixed sedimentary sequences in the Jimusar sag, Junggar Basin, NW China: Forcing mechanisms and multi-scale models

The crude oil movability, acting as one of the controlling parameters for estimating how difficult it is to exploit petroleum resources from lacustrine shale reservoirs, had still not been investigated systematically, especially for the saline lacustrine fine-grained mixed sedimentary series. How to establish an effective assessment criterion which is suitable for deciphering shale oil movability classically, and how to probe the root cause of the differential movable fluid saturation in heterogeneous lithofacies ? In order to unravel the causal linkage and confirm a coupling relationship between lithofacies heterogeneity and hydrocarbon fluid flow activity for the saline lacustrine fine-grained mixed sedimentary series, we present a set of new results from organic geochemistry analysis, FE-SEM imaging, low-pressure N2 adsorption, high-pressure mercury injection intrusion experiments (MIP), and nuclear magnetic resonance (NMR) experiments from the targeted dessert reservoir of the saline lacustrine fine-grained mixed depositional systems of the Middle Permian Lucaogou Formation in the Jimusar Sag, Southern Junggar Basin, NW China. Our results demonstrated a relatively obvious triple-level distribution of the movable fluid-bearing capacity and also reservoir quality for the dominant five identified lithofacies. The sandy dolomitic sandstones and felsic siltstones act as the first-class categories mainly characterized by a highest total pore volume range and also pore volume within pore size range of 50–300 nm, as well as a relatively lowest D2 values defined as the pore-throat fractal dimension of movable fluid. Secondly, both the dolarenites and dolomitic mudstones are considered as the medium quality reservoirs, and particularly noteworthy is a development of the characteristic microfractures in dolomitic mudstones greatly facilitates the formation and widespread distribution of pore-fracture network systems, leading to a relatively better storage capacity for movable fluid as suggested by relatively higher pore volume and lower fractal dimension values compared with those of micritic dolomites suspected as the most unfavorable category. More importantly, a multi-scale detailed model is proposed to illustrate the potential linkage spanning a total of three different scales respectively as lithofacies associations, microscopic reservoir textures and architectures, as well as fluid occurrence behaviors. This work can be considered as an analogue to better understand the spatial distribution patterns of dessert reservoir intervals and differential recoverability of saline lacustrine mixed-source shale oil resources, and to provide further conceptual basis for optimization of development engineering technique schemes.

Constraints on Diagenetic Fluid Source and Genesis in Tight Dolostone Reservoir of Submember Ma55 of Ordovician Majiagou Formation in Northwestern Ordos Basin, China: Evidence from Petrology and Geochemistry

Diagenetic fluids is one of the most important reservoir modifiers, their differences can result in various petrography, storage capacity, and geochemical characteristics, so clarifying the diagenetic fluids is vital for understanding dolostone origin. Dolomitization is an important genetic type of dolostone reservoir, the fluid properties differ in different dolomitization, which may lead to reservoir storage capacity changes. Therefore, the identification of dolomitization fluid properties is critical for deeply understanding of dolomitization process and predicting storage capacity. Two types of dolostone are developed in submember Ma55 of Majiagou Formation in northwestern Ordos Basin, with obviously different petrological characteristics and reservoir properties. On the basis of petrological studies such as core and casting thin section observation and cathodoluminescence analysis, the diagenetic fluid properties of these two dolostone are characterized by geochemical analytical methods such as major and trace element tests. The results show that Type-1 dolostone is mainly composed of micritic dolomite, showing micritic structure and algae-rich lamina structure, and accompanied by evaporite minerals and moldic pores. This type of dolostone has a various Mn content, weak to medium cathodoluminescence intensity, high contents of TiO2, Al2O3, K2O+Na2O, Li, and U, and lower content of TFe2O3. The type-2 dolostone is composed of fine-grained dolomite with obvious residual texture of primary limestone, clear brim, and cloudy center structure, accompanied by the existence of intergranular pores. Most of this kind of dolostone have medium-strong cathodoluminescence, higher TFe2O3 content and lower TiO2, Al2O3, K2O+Na2O, Li, and U contents. Moreover, both the two types of dolostone have similar Fe/Ca and Mn/Ca ratios, and a low and concentrated CaO content, whose composition is similar to that of stoichiometric dolomite. The comprehensive analysis shows that the diagenetic fluid of Type-1 dolostone is mainly a high salinity fluid existed in plaster and calcareous sediments in a near surface environment with low temperature. The diagenetic fluid of Type-2 dolostone may be a high salinity brine formed by evaporation and concentration of seawater with normal salinity. The research results will provide a significant theoretical basis for the evaluation of dolostone reservoir quality and the prediction of favorable areas of reservoir distribution.

Multiple-Stage Injection of Deep Hydrothermal Fluids in the Dolostone Reservoirs of Ordovician Majiagou Formation, Southern Ordos Basin

The Middle Ordovician Majiagou Formation in the southern Ordos Basin develops a set of sub-and peri-tidal carbonate successions that were extensively dolomitized. Diagenetic dolomitization is widely investigated because dolostones provide high-quality gas reservoirs in the southern Ordos Basin. Nonetheless, the ultimate mechanism controlling dolomitization remains enigmatic. In this study, integrated geochemical indexes including carbon and oxygen isotopes, rare earth elements, and microthermometry, coupled with U-Pb dating and lithofacies and mineralogic studies, are investigated to elucidate the periods of hydrothermal fluids involved in the formation of different types of dolomites in the studied section from Ma5 member. The results reveal that the micritic dolomite matrix was formed under the involvement of low-salinity meteoric water in the mixing zone during a shallow-burial environment. The mixing zone of paleokarst possibly provided accommodation for this dolomitization. The fine-/medium-sized dolomites were formed due to interactions between hydrothermal fluids of comparatively low temperature and matrix carbonates, which occurred during the period of the Middle Jurassic. The dolomitic microbialites were formed due to the injection of high-temperature acid fluids associated with organic matter maturing. This dolomitization occurred during the period of the Late Jurassic according to U-Pb ages. The coarse-sized dolomite cements can be furtherly classified into two types according to different U-Pb ages and carbon-oxygen isotopes, although they are very similar with respect to mineralogic features. Comprehensive geochemical evidence revealed that the dolomitizing fluids of two types of dolomite cements were derived from hydrothermal fluids associated with periodical magmatism, which occurred during the period of the Early Cretaceous. This finding provides a new insight for elucidating the diagenetic process of reservoir dolostones from the Middle Ordovician in the Ordos Basin.

Dolomite Origins of the Carbonate Successions From the Chihsia Formation, South China: A Case Study From the Pingdingshan Section

The carbonate successions from the Chihsia Formation play an important role for hydrocarbon exploration in the South China. However, the dolomites occurring in the upper section of the Chihsia Formation have been enigmatic, which confined further understanding of the depositional and/or diagenetic fluids and hydrocarbon prediction. In this study, the Pingdingshan section was investigated as a typical outcrop of Chihsian carbonates. On the basis of lithofacies/mineralogic observation of hand specimens and thin-sections, three types of dolomites were classified according to their crystal sizes, that is, micritic dolomite, sucrosic dolomite, and saddle dolomite. Comprehensive, micro-area geochemical proxies, including homogenization temperatures of fluid-inclusions, carbon and oxygen isotopes, and trace and rare-earth elements, were analyzed on different types of dolomites and matrix calcites. The results revealed that the selected samples were representative of temporal seawater and were not contaminated by terrestrial input. The temperatures and salinities of fluid-inclusions, combined with carbon and oxygen isotopic values, revealed that sucrosic dolomites originated from penecontemporaneous micritic dolomites, which had been crystallized and altered into fine- to medium-sized dolomite clusters during the later-stage burying process. In contrast, the saddle dolomites occurring in carbonate matrix or fractures were formed in a hydrothermal environment with a dolomitizing fluid temperature higher than 170°C. This inference is consistent with evidence from rare-earth elements and carbon and oxygen isotopes. Our findings provide new clues for the origin elucidation of dolomitizing fluids in the Chihsia Formation, South China.

Origin of Dolomite in Lacustrine Organic-Rich Shale: A Case Study in the Shahejie Formation of the Dongying Sag, Bohai Bay Basin

In most organic-rich shale reservoirs, dolomite is widely distributed and has different types and crystal sizes. However, the characteristics and formation mechanism of the dolomites in organic-rich shale are still poorly understood. Petrographic and geochemical analyses were performed to interpret the formation of dolomite in the lacustrine organic-rich shale of the Shahejie Formation, Dongying Sag, Bohai Bay Basin. Four types of dolomites, which represent episodic recrystallization, were classified based on crystal size and shape: 1) micritic dolomite (Dol-1), 2) sub-to euhedral (cloudy cores with clear rims) dolomite (planar-e) (Dol-2), 3) anhedral dolomite (coarse planar-s to nonplanar crystals) in phosphatic particles (Dol-3), and 4) fracture-filling anhedral dolomite (Dol-4). Dol-1 has nonplanar mosaic micritic crystals with irregular intercrystalline boundaries and dull cathodoluminescence (CL), suggesting dolomitization during the early burial stage. Dol-1 tends to occur under high paleosalinity and warm conditions. Furthermore, the syngenetic relationship, with abundant framboidal pyrite and gypsum, suggests that bacterial sulfate reduction (BSR) may influence the formation of Dol-1. The high content of Sr and low content of Mn/Sr also indicate less influence on burial. The Dol-2 crystals show cloudy cores with clear rims attributed to progressive dolomitization during burial. Dol-2 is always associated with the organic matter within the organic matter-rich lamina. The anhedral crystals and undulate (sweeping) extinction of Dol-3, which is usually encased by phosphatic particles in the organic-rich lamina, reflect the recrystallization affected by bacteria and the subsequent thermal evolution of organic matter. Dol-4 fulfills the abnormal pressure fractures crosscutting the earlier phases (Dol-1 and Dol-2) with undulate (sweeping) extinction and different rare earth element (REE) patterns. Dol-4 may be affected by hydrothermal fluids, which are influenced by the thermal evolution of organic matter. The 87Sr/86Sr values of the four types of dolomites similarly demonstrate the same dolomitizing fluids. Dol-1 to Dol-3 have similar REE patterns but are different from Dol-4, suggesting that Dol-4 likely resulted from circulation through basinal sediments instead of different fluids. Focusing on the origin of dolomite has been instrumental in understanding the diagenetic evolution, fluid flow, and organic-inorganic interactions in organic-rich shale and, hence, the reservoir formation of shale oil.

INSIGHTS INTO THE ORIGIN OF SHALLOW MARINE DOLOSTONES, LONGWANGMIAO FORMATION (LOWER CAMBRIAN), NORTHEAST SICHUAN BASIN, CHINA: PETROGRAPHIC AND GEOCHEMICAL CONSTRAINTS

For a better understanding of the origin of Early Cambrian dolomites in the Sichuan Basin, which serve as valuable oil and gas reservoirs, we present and interpret complex multiparameter petrographic and geochemical analyses of dolomites in the Cambrian Longwangmiao Formation at the Yangsiqiao section. Our data show that these dolomites mainly occur in the middle and upper parts of the Longwangmiao Formation, and we identify two types of dolomites: micrite and fine-medium crystalline dolomites. The micritic dolomites, occasionally coupled with quartz and mica minerals, show dirty crystal surfaces, nonautogenous crystal shapes, and dark cathodoluminescence and mainly occur in pure micrite and granular dolostones, which retain the original sedimentary texture. Contrarily the fine-medium crystalline dolomites, mainly developed in crystalline and grain dolostones and generally coupled with residual calcites and altered sedimentary textures, have a dark heart-bright boundary texture under a polarized microscope and occasional zonal texture under cathode luminescence. Furthermore, the micritic dolomites have CaO and MgO contents approximately similar to those of the ideal stoichiometric dolomite, relatively high Sr concentration, Σ rare-earth element (REE) +Y concentrations, and δ13C and δ18O values located in the corresponding range of dolomite deposited from Early Cambrian marine water. However, light-REE enrichment (LREE), heavy-REE (HREE) depletion patterns, and high Al2O3 and SiO2 contents distinguish these micritic dolomites from typical marine carbonate deposits. Notably, the fine-medium crystalline dolomites have higher CaO content and lower MgO, Al2O3 and SiO2 contents, lower Sr and higher Fe and Mn concentrations than those of micritic dolomites. The ΣREE+Y concentrations and REE patterns of these fine-medium crystalline dolomites belong to the typical marine carbonate deposits. These features indicate considerable differences in origins between micrite and fine-medium crystalline dolomites. The micritic dolomites show a significant genetic response to the precipitation of seawater in shallow marine with the injection of terrestrial water during the syngenetic period, while the fine-medium crystalline dolomites have a typical response to the secondary metasomatism products of original calcites at the shallow burial stage.

Characteristics and impacts on favorable reservoirs of carbonate ramp microfacies: A case study of the Middle—Lower Ordovician in Gucheng area, Tarim Basin, NW China

Based on the latest drilling core, thin section, 3D seismic, well logging data as well as exploration results, the sequence stratigraphy and sedimentary microfacies of the Middle—Lower Ordovician carbonates in Gucheng area, and their controlling effects on the development of reservoir were examined by the theories and methods of fine carbonate sedimentology. The results show that the Middle—Lower Ordovician in Gucheng area is a set of typical carbonate ramp deposits, which can be divided into 10 microfacies in 4 subfacies as follows: back ramp, inner shallow ramp, outer shallow ramp, and deep ramp. The back ramp subfacies consists of muddy-dolomitic flat and dolomitic lagoon microfacies; it is dominated by lamellar micrite dolomite tight in lithology. The inner shallow ramp subfacies includes dolomitic shoal and dolomitic flat in shoal top and dolomitic flat between shoals microfacies; it is mainly composed of crystal dolomite with metasomatic residual structure, as well as abundant karst vugs and intercrystalline pores. The crystal dolomite has an average porosity of 4.36%. The outer ramp subfacies includes medium—high and low energy grain shoal and inter-shoal microfacies; it is dominated by sandy limestone, oolitic limestone, and micrite limestone with few pores. The deep ramp subfacies is dominated by low-energy argillaceous deposits, with local presences of storm shoal microfacies. The Lower—Middle Ordovician has six third-order sequences from bottom to top, among which sequence (SQ3) represents the third member of Yingying Formation. The main reservoir has three fourth-order high-frequency sequences. Apparently, the favorable reservoir in the study area is jointly controlled by sedimentary microfacies and high frequency sequence in the carbonate ramp. The former controls the primary pore structure, and the latter controls the intensities of karstification exposure and dolomitization in the penecontemporaneous period. The dolomitic shoals and top dolomitic flats of different stages, superimposed and connected into favorable reservoirs of considerable scale like “platform margin”, are favorable exploration facies in the carbonate ramp.

Shale lithology identification using stacking model combined with SMOTE from well logs

Shale lithology identification is the basis of geological research and reservoir characterization, and is an essential task for oil exploration. Recently, several machine learning algorithms have been applied to improve the accuracy of lithology identification. However, stacking model for lithology identification has been less used in existing studies, and less consideration was given to imbalanced lithologies problem. In this study, we build a stacking model based on random forest (RF), extreme gradient boosting (XGBoost) and linear regression (LR), and use synthetic minority oversampling technique (SMOTE) to improve the imbalanced lithologies problem, and then compare the stacking model with support vector machine (SVM), RF and XGBoost models after adjusting model parameters using grid search and fivefold cross-validation. The authors prepared a dataset consisting of logging data and core data from 13 wells in a depression in Junggar basin, China, including a total of 2352 sample points marked with lithologic labels. The lithologies identified in this study are mudstone (MS), dolomitic mudstone (DM), siltstone (S), dolomitic siltstone (DS) and micritic dolomite (MD). The results show that (1) the overall identification performance of the stacking model is better than that of the SVM, RF and XGBoost models. (2) SMOTE algorithm can effectively improve the identification performance of the minority lithologies. (3) Density log is the most important factor in identifying lithologies. The stacking model combined with SMOTE proposed in this paper has high lithology identification performance, which renders it practicable for lithology identification.

Examining and applying the theory of “exploring petroleum inside source kitchens” for continental shale oil: A case study from the Kong 2 member of the Cangdong sag in the Bohai Bay Basin, China

Continental shale oil to be studied urgently is an important unconventional “source rock oil and gas” and key domain of “exploring petroleum inside source kitchens”. Based on the “source rock oil and gas” theory and the thought of “exploring petroleum inside source kitchens”, this paper established the ”six determination” exploration workflow for continental shale oil (i.e., determining the background, source rock, reservoir, play, resources, and sweet spot), which geological characteristics and exploration workflow are significantly different from conventional hydrocarbon. A case study was conducted by the workflow in the Kong 2 member (Ek2) of the Cangdong sag in the Bohai Bay Basin, China. Semi-deep to deep lacustrine facies belts and many gentle slope structures (belts) were widespread during Kongdian Formation depositing, thus providing favorable conditions for the formation and large-area accumulation of shale oil. Ek2 developed broader-distributed, and thick-bedded dark mudstone and oil shale, which exhibit high TOC, a good kerogen type, and great hydrocarbon-generating potential. Siltstone to fine sandstone and mud to micritic dolomite reservoirs were found within the low-dip slopes and central zone, which are thick, overlapped, abundant high-angle fractures, and exhibit low porosity and ultra-low permeability. Sandstone-type and dolomite-type shale oil of Ek2 characterized source–reservoir integration and five types of sources–reservoir configurations. They mainly overlapped at the front end of the braided river delta in the slope zone and in the sag center, where are superimposed vertically, and are connected horizontally with conventional structural–lithological oil pools in the up-dip slope. The median geological resources of the two classes shale oil were 5.26×108/t, with the remaining resources accounting for 90% of the total and moderate buried depths, thus the exploration potential is great. Several special exploration wells have obtained industrial oil flow. There are two types of sweet spots: low permeability sandstone layers and interbedded layers of oil shale and dolomite. A few favorable sweet spots were identified as targets for subsequent exploration. Then, three aspects of thinking and recommendations to solve the next exploration direction and technical problems were proposed, which will provide important reference and theoretical guidance for the further exploration of the same type of shale system in China and the world.

Origin and evolution of Sr-enriched hydrothermal brines: Insights from vein mineralization of Lower Triassic marine carbonates in the Eastern Sichuan Basin, South China

This paper focuses on celestite-carbonate-cemented veins that developed in the Jialingjiang Formation of the Lower Triassic carbonates from the Libixia anticline in eastern Sichuan Basin, China. Comparative petrography and geochemistry of the host rock and vein minerals provide clues to the nature, origin, and evolution of the vein-cementing fluids. The host rocks are composed mainly of micrite dolomite which yields δ18O, δ13C and 87Sr/86Sr ratios close to the values that characterize contemporaneous seawater. Two generations of vein-mineralization were recognized based on textural relationships, fluid inclusion data and isotope geochemistry. The early generation is dominated by celestite and saddle dolomite, formed from hypersaline, Sr-enriched, hydrothermal fluids. The later generation is represented by calcites, precipitated from less saline fluids at lower temperatures. The 87Sr/86Sr ratios of vein dolomite and celestite vary from 0.7076 to 0.7081, significantly lower than those of host carbonate counterparts (0.7078 to 0.7081; average 0.7080). This isotopic discrepancy suggests that the majority of Sr was sourced from deeper sequences of the lower Triassic carbonates, probably associated with thermochemical sulfate reduction for Sr accumulation. It is argued that the fracture systems associated with the Huayingshan fault provided the heat source and served as conduits for migration of hydrothermal fluids through several hundred meters, and the final mineralization occurred as a result of mixture of Sr-enriched brines with sulfate-bearing solutions. The large celestite ore bodies in the studied area are proposed to form related partially to these hydrothermal fluids.

Clumped isotope thermometry reveals diagenetic origin of the dolomite layer within late Ordovician black shale of the Guanyinqiao Bed (SW China)

The origin of widespread dolomite deposits in Paleozoic strata is a longstanding open question. Here we use a suite of geochemical methods to investigate a prominent Paleozoic dolomite occurrence in SW China. The upper Ordovician Wufeng Formation of the upper Yangtze Block (SW China) covers the time interval just before the Ordovician-Silurian Mass Extinction event, consisting of wide-spread black shale with an interlayer of bioclastic limestone, the Guanyinqiao Bed. In the western part of the Yangtze Block, in Shizhu county however, the Guanyinqiao Bed contains a particular organic-rich dolomite interlayer of which the formation mechanism is unclear. To distinguish between potential primary dolomite precipitation and late stage diagenetic/magmatic alteration, we studied this dolomite interlayer with petrological and geochemical techniques including microscopy, REE content and carbonate clumped isotope thermometry.The petrological observations show micritic dolomite with euhedral-subhedral shape and relatively low ordering (0.33 to 0.39). Conventional stable isotope analysis revealed negative carbon (δ13C of −3.17 ± 0.04‰ to −2.48 ± 0.03‰) and oxygen isotope compositions (δ18O of −11.28 ± 0.18‰ to −10.99 ± 0.20‰), and the organic carbon content ranged from 3.69% to 4.33%. The averaged clumped isotope temperature estimate revealed hot dolomite formation conditions of 87−10+11 °C, in a presumable fluid source with isotopic composition of δ18Ow = −1.8−1.5+1.5 ‰. The elevated formation temperature estimates, the slightly depleted δ18O of dolomite (~−11.14‰) with respect to contemporary deposited limestone (~−10‰) and the δ18Ow which is similar to the assumed late Ordovician seawater points at diagenetic alteration of primary limestone at a burial depth of approximately 3000 m. The analyzed dolomite samples with low contents of Fe, Zn and small Eu anomaly show further evidence for a diagenetic fluid source and exclude magmatic/hydrothermal alteration. We suggest that at this burial depth the temperature pressure conditions triggered significant Mg release from the surrounding clay to the diagenetic fluid, providing ideal conditions to transform the primary limestone into diagenetic dolomite.

Micro- and Nanoscale Pore Structure Characterization of Carbonates from the Xiaoerbulake Formation in the Tarim Basin, Northwest China

The evaluation of pore structure is an essential part in the assessment of carbonate reservoirs. The structures (geometry and connectivity) of nm to μm-scale pore networks in outcrop samples of carbonates from Xiaoerbulake Formation in Tarim Basin of China were studied by using optical microscopy, field emission-scanning electron microscopy (FE-SEM), as well as mercury intrusion porosimetry (MIP) with fractal analyses of the data, and spontaneous imbibition tests (distilled water). The results demonstrate that the lithologies are micritic dolomites, fine-to-medium-to-coarse crystalline dolomites, microbial dolomites, and dolarenite. At micro- to nanoscales in size, pore types are dominated by intergranular, intercrystalline, and intragranular (e.g., dissolution) pores. These pore networks have pore-throat diameters from 0.01 to >10 μm. Compared with a nanoscale pore network, the μm-scale pore networks are relatively well connected and serve as the most important permeability pathways. Although the pore volume accounts for most of the total porosity, the permeability of nanoscale pore networks is low. The existence of micro-nano-fractures could improve connectivity, especially for the nanoscale pore networks, by linking the intragranular (dissolution) pores which are mostly in the range of nm-scale.

Microbial carbonate sequence architecture and depositional environments of Member IV of the Late Ediacaran Dengying Formation, Gaoshiti–Moxi area, Sichuan Basin, Southwest China

The recent discovery of the Anyue gas field, with huge reserves, has great potential for ultradeep gas production in carbonate reservoirs. Previous exploration and exploitation of Member IV of the Dengying Formation in the Gaoshiti–Moxi area deposited during the Late Ediacaran have provided useful insight into palaeo‐gas reservoirs, and geological significance of the Precambrian microbial carbonate strata. In this study, the depositional sequence architecture has been established and typical environments from Member IV of the Dengying Formation in the Gaoshiti–Moxi area have been reconstructed through detailed sedimentological analysis of cores, petrophysical data, well‐logging data and seismic interpretation. One third‐order sequence was identified, which is subdivided into seven fourth‐order sequences and 18–19 fifth‐order sequences. The third‐order sequence is bounded by denudation surfaces or onlap surfaces, and composed of transgressive system tracts and highstand system tracts. The fourth‐order sequences are bounded by instantaneously exposed punctuated surfaces or lithology and lithofacies transition surfaces, featured by specific lithological cycles, such as ‘lower microcrystalline dolomite, middle intraclastic dolomite and sparry thrombolite, and upper intraclastic dolomite’, ‘middle–lower sparry thrombolite and upper microcrystalline dolomite and sparry thrombolite’, ‘lower micritic thrombolite, middle micritic dolomite, and upper sparry thrombolite and laminites’, and ‘lower microcrystalline dolomite, middle intraclastic dolomite and argillaceous or micritic dolomite, and upper intraclastic dolomite’. The sequence characteristics reveal that periodic high‐frequency sea‐level change and palaeogeomorphology are controlling factors affecting sequence development, which lay a significant foundation for reservoir development. During the depositional period, a tidal flat on a restricted carbonate platform is well‐developed, allowing the identification of 16 microfacies, 13 microfacies associations, and 6 seismic facies. Inter‐lower tidal flats are widely developed with band‐shaped mound–bank complex, microbial mound, and shoal on the platform margin. On the inner platform, patchy microbial mounds, mound–flat combinations, and inter‐shoal/microbial mounds are developed in the Gaoshiti area, whereas patchy shoals and shoal–flat combinations are developed in the Moxi area. During the early depositional period of Member IV of the Dengying Formation, the microbial mounds and shoals were developed on the platform margin. The dolomitic flat and inter‐shoal/microbial mounds are mainly developed on the inner platform with little microbial mound and shoal deposition. During the late depositional period of Member IV of the Dengying Formation, large‐scale microbial mounds and shoals are sustainably and stably developed on the platform margin; enlarged dolomitic flat, patchy microbial mounds, and patchy shoals are predominantly developed in the platform interior with inter‐shoal/microbial mounds dying out.

Mixed siliciclastic-carbonate deposition and cyclical evolution of the Upper Shahejie Formation and its impact on reservoir development in the Eocene Huanghekou Sag, Bohai Bay Basin, East China

We have systematically investigated a typical lacustrine mixed siliciclastic-carbonate sedimentary system in this study, which primarily deposited in the Eocene Upper Shahejie Formation (Es1 formation) of the Huanghekou Sag, Bohai Bay Basin. Using synthetic lithologic data sets, we summarized eight lithofacies from mixed siliciclastic-carbonate rocks and classified them into siliciclastic- and carbonate-dominated groups, indicating the variation of relative intensity between fluvial/deltaic process and carbonate productivity. Combining petrological and mineral component analysis, most lithofacies are compositionally replaced by micrite dolomite and show vertically different scales of cyclicity. The high-level cyclicity suggests the alternation between siliciclastic- and carbonate-dominated mixed rocks, whereas the lower level indicates interbedding of different dolomite-dominated lithofacies. The controlling factors on both levels are different. The higher level is probably controlled by climate changes, which finally resulted in the relative variations of siliciclastic supply, although the lower level of cyclicity changes more frequently and is considered to have been controlled by fluctuating hydrodynamic conditions. Typical mixed rock depositing patterns are summarized as the “sandwich type,” which are mainly distributed at siliciclastic-dominated sections and the “interbedded type” at carbonate-dominated sections. Besides, reservoir physical properties among lithofacies display significant differences as well, and the grain-supported framework of carbonate-dominated lithofacies suggests most constructive in keeping intergranular pores. The investigated lacustrine cyclical sedimentation in mixed siliciclastic-carbonate rocks has been less concerned in previous studies and may serve as a useful reference for lacustrine mixed rocks analysis in other rifted basins.