Fine-grained Sedimentary Rocks Research Articles

Lithofacies Characteristics and Controlling Factors of Fine-Grained Sedimentary Rocks in the Lower First Member of the Shahejie Formation in the Northern Lixian Slope of Raoyang Sag, China

In order to further explore the potential of unconventional oil and gas in the northern Lixian Slope of Raoyang Sag, the lithofacies types and their controlling factors of the lower first member of the Shahejie Formation (Es1L) in the Xiliu area were analyzed through the comprehensive application of core log studies, micro-sections and their magnified analyses, mineral suits recognitions with XRD, and major and organic geochemistry. The results show that the lacustrine sedimentary rocks of Es1L in the Xiliu area comprise five lithofacies types: medium organic silty mixed fine-grained rock, medium organic massive clay rock, high organic shale calcareous mixed fine-grained rock, high organic matter shale calcareous clay rock, and high organic matter shale limestone. The vertical variation of the characteristic element reflects that the deposition in Es1L has different stages; that is, the bottom of Es1L is formed with low source influence, high water salinity, and strong reduction conditions, while the upper fine-grained rocks are formed in a sedimentary environment with high source influence, low water salinity, and weak reduction conditions. The change of the sedimentary environment controls the development of lithofacies and the occurrence of organic matter. These results advocate for the basic research of fine-grained sedimentary rocks in the faulted lacustrine basin aid for further unconventional oil and gas exploration.

Linking South China Plate to Arabian margin of Gondwana: Significance for Cambrian global plate reconstruction

Large-scale plate convergence processes led to the assembly of the Gondwana supercontinent during the Ediacaran–Cambrian. Herein, we present the results from petrological and geochemical studies on fine-grained sedimentary rocks belonging to the Lower Cambrian strata from the western part of South China Plate (SCP) in the Yangtze terrane, to reveal the location of SCP in the Gondwana supercontinent. Our results show that these rocks are of high compositional and textural maturity and belong to the same tectono-sedimentary cycle. The rare earth element (REE) distribution curve standardized by Post-Archean Australian Shale is similar to that for typical post-Archean shale. Characteristics of major and trace elements indicate that these fine-grained sedimentary rocks might mainly be derived from moderately felsic magmatic provenance with only a minor supply from mafic–ultramafic sources. Trace element ternary discriminant plots reveal the geotectonic setting as a continental island arc, although some passive margin sediments might have also been involved; R-type cluster analysis based on trace elements and REEs also shows that the tectonic setting of the sample provenance area is most likely to be that of continental island arcs and active continental margins. Combined with the published detrital zircon geochronological data of SCP and global tectonic-magmatic activity during the Cambrian, we propose that the Lower Cambrian clastic sediments in SCP were mainly supplied by the Cadomian island arc along the northern margin of Gondwana during the Ediacaran–Early Cambrian. Based on the above analysis, we believe that SCP might have been located closer to Arabian Plate than to Australian Plate.

Sedimentology and sequence stratigraphy of lacustrine deep-water fine-grained sedimentary rocks: The Lower Jurassic Dongyuemiao Formation in the Sichuan Basin, Western China

Despite the sedimentology of deep-water fine-grained sedimentary rocks from marine environments having received highlighted attention in recent years, their counterparts from the lacustrine environments are rarely reported. Based on a complete sedimentological, petrographic, and geochemical analysis of three continuous cored wells of the Lower Jurrasic Dongyuemiao Formation, Sichuan Basin, West China, this study illustrates the range of lithofacies, cyclicities, and transportation and deposition mechanisms of this lacustrine deep-water succession. The aim of this research is to establish a synthesis depositional model for lacustrine deep-water fine-grained sedimentary systems in the Sichuan Basin. The results indicate that seven lithofacies were developed in the Dongyuemiao Formation and they were stacked in a repeated pattern to constitute ca 10–20 m thick composite cycle sets. A typical composite cycle set comprises an upper and lower unit. Specifically, the lower unit is characterized by basal relatively coarse-grained detrital-rich lithofacies interpreted to record gravity flows that grade upward into relatively fine-grained detrital-lean lithofacies interpreted as suspension deposits from the water column. The upper unit is stacked in a reverse pattern and characterized by basal relatively fine-grained detrital-lean lithofacies that grade upward into relatively coarse-grained detrital-rich lithofacies. Such cyclicities in the Dongyuemiao Formation are associated with third-order lake level variations. Specifically, the relatively coarse-grained detrital-rich lithofacies is interpreted to be deposited from low-density turbidity currents, likely hyperpycnal turbidity flows, during maximum lake level regression when terrigenous fluvial and delta systems developed in the basin margin. The relatively fine-grained detrital-lean lithofacies is interpreted to be deposited from suspension fallout during maximum lake level transgression. This cyclicity pattern and frequent occurrence of sediment gravity flow deposits in the Dongyuemiao Formation suggests a dynamic hydrographic condition in the deep-water lacustrine environment of the Sichuan Basin.

Unravelling the fabrics preserved inside early diagenetic concretions: Insights for the distribution, accumulation and preservation of organic-rich mud in the interior of epicontinental basins

Fine-grained sedimentary rocks generally undergo severe mechanical compaction during burial, which complicates the recognition of primary mudstone fabrics and associated sedimentary features. Early diagenetic concretions, however, provide a rare glimpse of primary fabrics because cement filling the pore space prevents the collapse of original grain arrangements. Hand specimens of concretions collected from the basal condensed section of the Late Jurassic-Early Cretaceous Vaca Muerta Formation (Neuquén Basin, Argentina), allow for analysis of sedimentary processes responsible for the dispersal, accumulation and burial of organic carbon-rich sediment in an epicontinental sea. Representative samples from central basin depositional localities were examined by optical, scanning electron microscopy and energy dispersive X-ray spectroscopy. Petrographic observations were complemented with palynological and organic geochemical analyses. Close examination of uncompacted fabrics reveals a significantly more complex and dynamic depositional scenario than previously assumed (suspension settling). Although many of the component grains in the studied samples were originally delivered to the sediment–water interface by suspension settling processes (i.e., marine snow, hypopycnal plumes, pumice rafts), there is substantial evidence of episodic sedimentation controlled by punctuated events of seafloor disturbance and erosion. The common presence of muddy intraclasts indicate that the seafloor was frequently reworked by bottom currents that caused the widespread distribution of organic carbon-rich sediment across distal basin depositional environments. Bottom current circulation supplied oxygen to the sediment–water interface and created suitable conditions for benthic life, contravening the assumption of bottom water anoxia as a prerequisite for organic carbon preservation. The excellent preservation state of freshwater algae (Pediastrum complex) suggests that organic matter contained inside mud composite particles can travel long distances before being deposited in distal depositional settings. Encapsulation protects organic components from mechanical/biogenic degradation and provides an anoxic microenvironment for preventing the oxidation of the organic matter contained inside of mud composite grains. The study shows that organic carbon encapsulation may be an important mechanism for organic carbon preservation in relatively energetic and non-anoxic settings, calling for a critical reappraisal of the processes responsible for the sequestration of organic carbon from the biosphere and its long-term storage in organic-rich mudstone successions.

Lithofacies types, reservoir characteristics and silica origin of marine shales: A case study of the Wufeng formation–Longmaxi Formation in the Luzhou area, southern Sichuan Basin

Shale, which is a fine-grained sedimentary rock, differs in reservoir quality (RQ) and completion quality (CQ) as the result of variations in its mineral and organic-matter compositions. The RQ and CQ directly affect shale gas exploitation. Therefore, classification of the lithofacies types of shale, analysis of the reservoir differences of different lithofacies, and identification of the superior shale lithofacies with the best RQ and CQ are important for decisions on well placement, targeting the “sweet spot” zone and optimizing the expected ultimate recovery of a shale gas well. In this study, several methods including microscopic observation of thin sections, X-ray diffraction analysis, core analysis and elemental geochemistry were used to investigate the lithofacies types, reservoir characteristics and silica origin of the marine shales of the Wufeng Formation–Longmaxi Formation (WF Fm.–LMX Fm.) in the Luzhou area, southern Sichuan Basin, China. The lithofacies characteristics exhibit marked differences through the WF–LMX shale succession. Specifically, the lower gas-bearing zone (LMX-1) is mainly composed of siliceous shales and siliceous–argillaceous mixed shales, whereas the upper gas-bearing zone (LMX-4) mostly contains argillaceous shales and argillaceous–siliceous mixed shales. The lower shale gas reservoirs have a higher silica content and lower clay and carbonate contents than the upper zones, and thus superior petrophysical qualities. The upper part of the WF Fm. (WF-2) and the lower part of the LMX Fm (LMX-1, LMX-2) are rich in bio-chemical authigenic silica, whereas the silica minerals of other intervals are mainly derived from allochthonous terrigenous material. The siliceous shales and siliceous–argillaceous mixed shales with a biogenic quartz content greater than 40% have the best RQ and CQ and are the most promising lithofacies for shale gas in the Luzhou area. Although the upper gas-bearing zones represented by LMX-4 are also good shale gas reservoirs, the RQ, CQ and gas content are generally worse than those of the lower gas-bearing zones because of the silica origin and lithofacies types; thus, extracting shale gas from the upper zones would be a greater challenge.

Sm-Nd isotopic compositions of deep-marine mudstones, Xigaze forearc basin, southern Tibet: Implications for drainage evolution and expansion

Provenance analysis of fine-grained sedimentary rocks has great importance to understanding the tectonic and paleogeographic evolutions of convergent margin systems. This study conducted Sm-Nd isotopic analysis of mudstone samples from Cretaceous deep-marine strata in the Xigaze forearc basin to evaluate the provenance, paleogeography, and regional drainage patterns in the arc-forearc region. Ten mudstone samples yield εNd(t) ranging from −7.44 to + 0.58 and Nd model age from 0.862 to 1.50 Ga. The mudstones were sourced from a mixture of the Gangdese arc and the Central to Northern Lhasa terrane. The Gangdese arc was the dominant source area, and the relative contribution of the Central to Northern Lhasa terrane sources increased up-section. Our interpretation is consistent with studies on coarser components. We interpret these results as a long-term signal of drainage capture of drainage networks that originated in the southern end of the Gangdese arc and brought Gangdese sources into the forearc basin during its early stages. Fostered by the uplift of the Lhasa terrane, drainage capture eventually reached the Gangdese mountains and regions to the north, resulting in the erosion and supply of more materials with negative εNd(t) from the Central and/ or Northern Lhasa to the basin. Similar patterns of Sm-Nd provenance evolution and drainage expansion have been observed in other well-studied forearc basins (e.g., the Great Valley forearc basin of California) and may reflect an important exhumation trend in convergent margin systems.

Eocene Paleoclimate Evolution under the Background of Warmhouse-Hothouse Conditions in the Continental Fushun Basin: Implications from Magnetic Susceptibility and Color Reflectance.

Paleocene–Eocene hyperthermal events are a current research focus in the fields of sedimentology and paleoclimatology. The Fushun Basin in northeast China contains continuous continental Eocene fine-grained rocks, and a series of Eocene hyperthermal events in the Fushun Basin have been identified. Because of the high cost of high-precision isotope data testing, it is necessary to find new and alternative paleoclimate parameters. In this study, Eocene coal and oil shale-bearing layers in the Fushun Basin are used as research objects. The high-precision data of magnetic susceptibility, color reflectance, rock composition, and cluster analyses are used to conduct a vertical comparison in the same category and compare that analysis with the identified Eocene hyperthermal events in the Fushun Basin. The preliminary results show that high-frequency-dependent susceptibility, high color reflectance a* (redness)/L* (lightness) values, and high kaolinite content in the study area have good correspondence with global hyperthermal events and can be used as effective parameters for the identification of continental basin hyperthermal events. The detailed magnetic susceptibility and color reflectance data also reveal that the Eocene strata in the Fushun Basin recorded the Late Lutetian Thermal Maximum (LLTM) and 13 short-term hyperthermal events during the Early Eocene Climatic Optimum (EECO). These results indicate that the parameters of rock physical properties can be used to study the evolution of the paleoclimate in geological history, and it has universal practicability in continental and marine fine-grained sedimentary rocks.

Whole-rock geochemistry and zircon O-Hf isotope compositions of ca. 2.35 Ga strongly peraluminous granites: Implications for increase in zircon δ18O values during the Paleoproterozoic

Zircon oxygen isotope ratios have been used to trace the incorporation of sedimentary rocks into magmas. The dramatic increase in maximum zircon δ18O values in the Paleoproterozoic observed in global databases coincides with changes in surface environments (e.g., the rise of subaerial and oxidative weathering), implying a connection between elevated zircon δ18O and these changes. Zircon δ18O between 2.5 and 2.2 Ga, however, is relatively under-constrained owing to limited available data in this age range. To augment data from this critical time period and understand potential causes for the elevated zircon δ18O values, we report U-Pb zircon ages and δ18O values of zircon, as well as, whole-rock major and trace element geochemistry of Paleoproterozoic strongly peraluminous granites (SPGs) from the southwestern margin of the Yangtze Block (China). Our geochronological data demonstrate that these SPGs crystallized at ∼2.35 Ga and that inherited zircon with ages of 2428–2721 Ma are present in these granites, indicating the source rocks of these granites were deposited, subsequently metamorphosed, and partially melted between 2.43 and 2.35 Ga. Synmagmatic zircon from samples dated in this study have εHf(t) values of −6.4 to −0.9 and high δ18O values of 7.6–9.9‰, elevated above the maximum value observed in Archean zircon (∼7‰). These granites can be divided into two groups based on whole-rock geochemistry. Both Group 1 and Group 2 granites were derived from a similar high δ18O, metapelitic source, but were generated by dehydration melting and hydrous melting, respectively. Our results demonstrate that the fine-grained sedimentary rocks from which the SPGs were derived had relatively high δ18O (as compared to older sedimentary rocks) by 2.43–2.35 Ga. The depositional time interval of the high-δ18O sedimentary sources for SPGs studied here coincides with the emergence of continental crust above sea level and the Great Oxidation Event. Supporting the findings of previous studies, the contemporaneity of our dataset with these changes in Earth’s surface environments suggests that subaerial and potentially oxidative weathering contributed (at least partially) to the elevation of δ18O of fine-grained sedimentary rocks. Recycling of these high-δ18O sedimentary rocks into magmas contributed to the dramatic change in δ18O of magmatic zircon in the earliest Paleoproterozoic. In addition, although this study is focused on a single locality, our results suggest that the abrupt shift observed in global zircon δ18O data sets likely occurred by 2.35 Ga. Last, a literature compilation of zircon δ18O data from SPGs suggested that zircon δ18O values may have also experienced a stepwise increase in the Neoproterozoic to Phanerozoic from 12 to 14‰. The coincidence of these increases in zircon δ18O values with global oxygenation events suggests that atmospheric oxygenation may have contributed to the increase in δ18O of sedimentary rocks.

Origin and significance of authigenic quartz and albite in lacustrine calcareous fine-grained sedimentary rocks

Generally, researchers believe that authigenic quartz precipitates in acidic medium and authigenic albite precipitates in alkaline medium. However, the lacustrine carbonate-rich fine-grained sedimentary rocks in the upper fourth sub-member and the lower third sub-member (Es4s–Es3x) of the Shahejie Formation in the Jiyang Depression were observed and it was found that authigenic microcrystalline quartz and albite generally develop together. Moreover, authigenic quartz and albite also develop with microcrystalline Fe-rich dolomite and calcite in clay laminae, while in carbonate laminae, they primarily develop with recrystallized calcite. In this study, thin-section, cathodoluminescence, field-emission scanning electron microscopy (FESEM), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and inclusion analyses were conducted to study authigenic minerals. The results show that authigenic quartz and albite are characterized by rich Ge in mudstone, and the material components mainly originate from the transformation of clay minerals and the dissolution of terrigenous feldspar debris. In limestone, they are characterized by poor Ge, and the material components primarily originate from the transformation of clay minerals. The paragrowth and intergrowth of authigenic quartz and albite primarily depend on the degree of supersaturation of the corresponding ion concentration under high temperature, high or ultrahigh pressure. During the early diagenetic stage, authigenic quartz and albite initially formed in a weakly alkaline environment; however, on entering the middle diagenetic period, authigenic quartz, albite, and sparry calcite formed in a weakly acidic environment, and the formation rate reached its peak. Our results elucidate the migration direction and enrichment law of shale oil in calcareous fine-grained sedimentary rocks through the genetic mechanism of authigenic quartzs and albites. Shale oil is generated from organic matter in the clay lamina, following which it migrates, accumulates, and preserves to the adjacent sparry calcite lamina under the joint driving effect of organic matter hydrocarbon generation overpressure and clay mineral dehydration overpressure.

The retention of precursor biotic signatures in the organonitrogen and organooxygen compounds of immature fine-grained sedimentary rocks

To reveal the impact of biomass input (marine algae, terrestrial plants and lacustrine Botryococcus braunii) on the composition of high molecular weight organooxygen and organonitrogen compounds (Ox, Ny and NyOx), solvent extracts of immature–early mature rock and coal samples were systematically characterized by Fourier transform ion cyclotron resonance mass spectrometry using different ionization modes. In total, 16 samples from the marine Schöneck, Dynow and Posidonia formations, the lacustrine Wealden Formation and the terrestrial New Zealand coals were investigated.Coals as the in-situ deposits of terrestrial plants consist to a great degree of aromatic polyoxygenated Ox and N1Ox compounds, representing the degradation products of lignin like phenolic ketones and carboxylic acids as well as their condensation products with the degradation intermediates of labile organonitrogen compounds. Aliphatic Ox moieties derived from plant protective constituents principally waxes and cutan show a pronounced even or odd carbon number predominance of the C23–C33 species with C26, 28, 30 or C27, 29, 31 as the major homologs. In contrast, marine and lacustrine microbial communities contribute plentiful middle-chain C22, C24 or C23, C25 Ox compounds. Marine rock extracts are characterized by abundant organonitrogen compounds, especially N2 and N2Ox classes, interpreted as signatures of protein-rich marine algae that were successfully preserved via a degradation-recondensation pathway, whereas the low protein content of lacustrine Botryococcus braunii is reflected by a low amount of N1Ox and N1 classes. Highly aliphatic algaenan of Botryococcus braunii sterically protected its oxygen-bearing groups from degradation, leading to a great abundance of Ox compounds, furthermore, it characterizes the Botryococcus braunii source by substantial heteroatomic species containing more than 40 carbon atoms.

Side Resistance of Drilled Shafts in Weak Fine Grained Sedimentary Rock

Load transfer mechanism in side resistance of rock socketed drilled shafts has been studied for the past four decades using results of axial load tests and theoretical methods. Various models for prediction of side resistance have been proposed. Only few studies (e.g., Horvath et al. 1983; Rowe and Armitage 1987; Hassan et al. 1997, Miller 2003, Abu-Hejleh et al. 2003) have been completed on socket side resistance of drilled shafts in weathered and fractured fine-grained rock. These studies, however, were based on only a limited number of load test data. A survey of current predictive models has been conducted. This survey shows most of the current models include strong and intact rocks in their databases. Almost all of the current models use a power function to correlate side resistance of rock socket to rock unconfined compressive strength. A database of side resistance of large diameter drilled shafts in only weak fine-grained rocks, such as, weak shales, mudstones, and siltstones (i.e., Intermediate Geomaterial first introduced by O’Neil et al. 1996, Hassan et al. 1997, and O’Neill and Reese 1999) has been complied in this study. The range of weak rocks considered herein corresponds to an unconfined compressive strength of 0.48 to 4.8 MPa. Analysis of this database shows that a linear model best predicts the side resistance of drilled shafts in weak fine-grained sedimentary rocks.

Coarse calcite grains in hydrocarbon-rich tuffaceous mudstone in a Permian volcanic lake—An indicator for subaqueous volcanic deposition and related hydrocarbon generation

Sediments originated from volcanic-hydrothermal activities show clear relationship with hydrocarbon generation and accumulation in fine-grained sedimentary rocks at a regional scale. However, the fundamental processes and mechanisms of this relationship are still poorly understood at a microscopic scale to hamper efficient hydrocarbon exploration. In addition, coarse grains common in volcanic lacustrine fine-grained sediments have been traditionally interpreted as diagenetic products during early and late burial. Our study, however, suggests that coarse calcite grains are not diagenetic, but formed by subaqueous explosion. The calcite grains occur in lacustrine fine-grained sedimentary rocks in the middle Permian Lucaogou Formation in the Santanghu intracontinental rift basin, NW China. They are sand sized, angular, mono- and polycrystalline, and discretely scatter in silt to clay-sized non-calcite pyroclastic matrix. The calcite grains and the mud matrix are interpreted as a pyroclastic debris flow deposit. In-situ 87Sr/86Sr ratios (0.704932–0.705840, and 0.705137 on average) of the calcite grains are similar to those of mantle rocks, indicating an origin closely related to mantle-derived fluids. The in-situ δ13C values (4.24‰ to 5.06‰, and 4.57‰ on average) and extremly negative δ18O values (−21.70‰ to −21.18‰, and − 21.49‰ on average) indicate that extensive hydrothermal alteration and microbial activities influenced the formation of the calcite. Element composition and homogenizaiton temperature of fluid incusions of calcite and biomarkers of the mudstone all suggest a relatively low-temperature hydrothermal condition in a saline and reducing environment. Furthermore, hydrocarbon-rich inclusions are rich in the calcite grains and have two modes of occurrence and maturity. They indicate that the organic matter was generated from algae bloom, which was caused by volcanic-hydrothermal activities. Alternatively, the hydrocarbon may have an inorganic origin originated from a mantle source. Our findings provide an alternative interpretation on the origin of coarse calcite grains in fine-grained sedimentary rocks and demonstrate a microscopic relationship between volcanic-hydrothermal activities and hydrocarbon generation and accumulation in a tectonic active rift basin.

Coupling Relationship Between Densification Characteristics and Tight Oil Accumulation of the FGSR Reservoir: A Case Study of the Chang 7 Member of the Triassic Yanchang Formation in the Zhenyuan Area of the Southwest Ordos Basin, Central China

Fine-grained sedimentary rock (FGSR) reservoirs in a deep-water sedimentary environment from the Late Triassic Yanchang Formation in the Ordos Basin in central China have huge potential for tight oil production. According to the comprehensive research of fluid inclusion experiment, scanning electron microscopy, cathodoluminescence, thin section identification, and whole rock analysis, the petrographic characteristics, fluorescence spectrum parameters, and the homogenization temperature of oil inclusions and associated brine inclusions in the FGSR reservoir were analyzed to explore the coupling relationship between densification characteristics and tight oil accumulation of the FGSR reservoir. The results show the FGSR reservoir diagenesis has experienced compaction, cementation, and multiple stages of dissolution. A comprehensive analysis of burial history shows that the Chang 7 Member FGSR reservoir in the study area has three phases of oil charging, corresponding to the geologic time range of 132–124, 120–102, and 98–97 Ma. The organic acid produced by the first-stage oil charging accelerated the dissolution of feldspar and cuttings, and the dissolution pores increased significantly. Then, the organic acid in the second-stage oil charging accelerated the dissolution, and the abnormally high hydrocarbon expulsion pressure inhibited the damage of compaction on the pores of FGSR reservoir. Finally, the thermal evolution degree of source rocks reached the highest in the third-stage oil charging. The constructive effect of large-scale organic acids on pores and the protective effect of the chlorite membrane on pores inhibited the densification process of the FGSR reservoir. In short, the FGSR reservoir has experienced a cyclical reservoir densification process of “reducing porosity–increasing porosity–preserving porosity.” The coupling of pore evolution and oil charging of the FGSR reservoir is the key to the extensive accumulation and large-scale reservoir formation of tight oil in the Chang 7 Member of the Yanchang Formation in the Zhenyuan area, southwest Ordos Basin. The significance of the study may help to understand the process of the tight oil accumulation and predict the tight oil of the FGSR reservoir distribution.

Application of μXRF analysis on the Upper Cretaceous Mancos Shale: A comparison with ICP-OES/MS

This study investigates the elemental signature of fine-grained sedimentary rock (mudstone) obtained using energy-dispersive micro-X-ray fluorescence (μXRF). A short section (28.5 cm) of Upper Cretaceous Mancos Shale was analysed with an Itrax μXRF core scanner at 0.2 mm intervals for comparison to elemental and petrographic composition. Fifteen subsamples were collected at 2 cm intervals for ICP-OES/MS analysis to obtain absolute concentrations of elements and oxides, and thin sections were used for petrographic and microfacies analysis. The data obtained from the Itrax core scanner show a high level of reproducibility between separate scans and moderate to strong Spearman-Rho correlation (R2 > 0.6) to ICP-derived concentrations for several oxides: Fe2O3, SiO2, Al2O3, K2O, TiO2, and MnO. Common elemental ratios used in palaeoenvironmental studies (e.g., Zr/Rb, Ca/Ti, Mn/Ti) also showed good correlation to ICP-derived data when the trends were compared. The elemental signature obtained from the Itrax also showed good relationships to the petrography and lithology, with the Si/Ti ratio accurately identifying quartz-rich layers, and the Ca/Ti and Mn/Ti ratios corresponding to increased carbonate content, which are of importance in the planning of hydraulic fracturing for unconventional resources. At core scale, the elemental trends in Zr/Rb, Ti, and Si were useful for detecting short-term, rapid patterns of sedimentary events, likely resulting from storm activity (e.g., tempestites, wave-enhanced sediment gravity flows). The high-sampling resolution data from μXRF core scanners can provide advantages over traditional geochemical techniques since data aliasing is minimized. Analysing these elements and ratios in longer core sections from the Mancos Shale may serve as a proxy for determining high-frequency climatic changes in the Cretaceous Interior Seaway and allow for improved correlations of intra- and inter-basinal events in other Cretaceous basins.

Bedding parallel fractures in fine-grained sedimentary rocks: Recognition, formation mechanisms, and prediction using well log

Core, thin section, conventional and image logs are used to provide insights into distribution of fractures in fine grained sedimentary rocks of Permian Lucaogou Formation in Jimusar Sag. Bedding parallel fractures are common in fine grained sedimentary rocks which are characterized by layered structures. Core and thin section analysis reveal that fractures in Lucaogou Formation include tectonic inclined fracture, bedding parallel fracture, and abnormal high pressure fracture. Bedding parallel fractures are abundant, but only minor amounts of them remain open, and most of them are partly to fully sealed by carbonate minerals (calcite) and bitumen. Bedding parallel fractures result in a rapid decrease in resistivity, and they are recognized on image logs to extend along bedding planes and have discontinuous surfaces due to partly-fully filled resistive carbonate minerals as well as late stage dissolution. A comprehensive interpretation of distribution of bedding parallel fractures is performed with green line, red line, yellow line and blue line representing bedding planes, induced fractures, resistive fractures, and open (bedding and inclined) fractures, respectively. The strike of bedding parallel fractures is coinciding with bedding planes. Bedding parallel fractures are closely associated with the amounts of bedding planes, and high density of bedding planes favor the formation of bedding parallel fractures. Alternating dark and bright layers have the most abundant bedding parallel fractures on the image logs, and the bedding parallel fractures are always associated with low resistivity zones. The results above may help optimize sweet spots in fine grained sedimentary rocks, and improve future fracturing design and optimize well spacing.

Characterization, Classification, and Evaluation of the Reservoir Pore Structure Features of Lacustrine Fine-Grained Sedimentary Rocks. A Case Study of the Fourth Member of the Shahejie Formation in the Chenguanzhuang Area of the Southern Gently Sloping Zone of the Dongying Depression, Bohai Bay Basin

With the development of unconventional oil and gas exploration “from sea to land,” lacustrine fine-grained sedimentary rocks (FSR) have gradually attracted the attention of scholars and become an important topic in the field of unconventional oil and gas, but the research is still in its initial stage. In this study, lacustrine FSR in the Dongying Depression of the Bohai Bay Basin are used as the research object, and nuclear magnetic resonance (NMR) and quantitative image characterization are used to characterize the pore structure of the reservoir in the study area on multiple scales, analyze the reservoir characteristics control factors, and classify and evaluate the reservoir. The results show that: 1) the favorable petrographic phases of the FSR reservoir can be classified into six types of organic-rich lime mudstone, organic-rich laminoid lime clay rock, organic-rich laminoid clay micritic limestone, organic-bearing banding clay micritic limestone, organic-rich banding lime clay rock, and organic-bearing lumpy clay micritic limestone. With an average porosity of 12.3% and an average permeability of 10.58 mD, the overall reservoir is a typical low-porosity-low-permeability type; 2) the reservoir space types are diverse, with strong microscopic inhomogeneity; pores with a pore size of less than 2 nm almost have no contribution to the reservoir space; the pore volume and pore area are mainly provided by organic matter pores at the 100 nm level, mineral intergranular pores, and clay mineral shrinkage pores/slits. The FSR reservoirs in the study area are classified into three categories, and the pore structure of the reservoirs from categories I to III deteriorates in turn. This study provides a basis for the microscopic characterization, classification, and evaluation of lacustrine FSR reservoirs and their exploration.

The influence of Early Cretaceous paleoclimate warming event on sedimentary environment evolution and organic matter sources in Yin'e Basin: Evidence from petrology and molecular geochemistry

The lacustrine organic-rich fine-grained sedimentary rocks of the Early Cretaceous Bayingebi Formation are widely distributed in the Yin'e Basin and they have become important source rocks for oil and gas exploration. To reveal the influence of paleoclimate warming event on sedimentary environment evolution and organic matter sources, the petrological and molecular geochemical analyses of the upper member of the Bayingebi Formation (K1b2) were carried out using optical microscopy, X-ray diffraction, scanning electron microscopy, Rock-Eval pyrolysis and gas chromatography–mass spectrometry, in order to establish the paleoecological models of the Early Cretaceous freshwater and saline lakes. The oil shale and dark mudstone of K1b2 are both in the immature-early mature stage and have good potential for hydrocarbon generation, with a dominance of type I and type II1 kerogen. Two types of primary dolomite were found in this formation: microcrystalline dolomite with uniform structure, and the microbially originated spherical dolomite aggregates. The K1b2 can be divided into four units based on its lithology and logging curve characteristics. Unit II was deposited in the peak of lake development, under the warm-humid paleoclimate with abundant annual precipitation. There was greater influx of freshwater than other units, which resulted in the unstable stratification of the water column and less reducing condition of the lake bottom water body. Lower aquatic organisms and vascular plants were unprecedentedly prosperous, which were indicated by short-term thriving phenomena of telalginite, and the large input of terrestrial OM and highest abundance of the OM compared to other units. A lake water salinization event occurred during the sedimentation period of Unit III under the cooling climate, causing a transition from a freshwater lake to a high-salinity lake with reducing condition and stable stratification of the water column. Salinity of lake water increased the competitiveness of salt-tolerant algae and bacteria. The salty environment is beneficial for the thriving of single to halophilic genus of microorganism and lead to significantly lower abundance of telalginite. The biological sources of OM were mainly bacteria, algae and emergent plants, with limited contribution of terrestrial higher plants. The anoxic alkaline environment and high salinity condition at the lake bottom were beneficial for the formation of carbonate minerals and the preservation of OM. This were characterised by medium abundance of OM and high hydrocarbon generation potential. The study on the petrological and molecular geochemical characteristics of these organic-rich fine-grained sedimentary rocks is of great significance for investigating the genetic mechanisms and resource potential of oil and gas.

Occurrence, Genesis, and Significance of Analcime in Fine-Grained Sedimentary Rocks

Natural analcime, an aluminosilicate mineral with multiple genetic mechanisms, widely occurs in fine-grained sedimentary reservoirs rich in oil and gas. Researchers have discussed the source and formation mechanism of reservoirs and the influence of morphology formation, occurrence characteristics associated with minerals, geochemical data, and Si/Al ratio on reservoir properties. The occurrence location, particle size, automorphism, purity, and fracture development can indicate the source of analcime macroscopically. The correlation between the enrichment of associated minerals and the content of analcime indicates that the associated mineral assemblage or correlation provides a material source for the formation of analcime or effectively improves the formation environment. Geochemical data are often used to identify analcime related to primary magmatic crystallization and hydrothermal processes. The genetic source grouping scheme based on the Si/Al ratio, which is a traditional means to identify the source of analcime, has been widely used in the research on analcime. After more than 200 years of study, research has shown that analcime distributed in fine-grained sedimentary rocks was mainly formed by burial alteration of volcanic materials (V-type analcime), conversion of nontuffaceous materials (N-type analcime), hydrothermal deposition mineralization (H-type analcime), and precipitation directly from an alkaline lake or pore water (P-type analcime). Based on reservoir properties, analcime that formed before the organic acid release stage of source rocks can effectively improve the porosity through precipitation–dissolution mechanisms after the release of massive organic acid, whereas the cementation formed by the transition of the fluid from acid to alkaline during the intermediate diagenetic stage would reduce porosity to some extent.

Petrological and organic geochemical characteristics of the Permian Lucaogou Formation in the Jimsar Sag, Junggar Basin, NW China: Implications on the relationship between hydrocarbon accumulation and volcanic-hydrothermal activities

The coupling relationship between volcanic-hydrothermal activities and hydrocarbon generation has been a common understanding in recent years. However, there is a lack of theoretical and practical guidance in the application hydrocarbon production. The fine-grained sedimentary rocks of the middle Permian Lucaogou Formation in the Jimsar Sag in the Junggar Basin, NW China, are critical source rocks and have been simplify regarded as a mudrocks for target for tight oil exploration. Recently studies show that rocks were deposited in an intracontinental lacustrine environment with multiple volcanic-hydrothermal activities. Their complex micro-scale lithological characteristics restrict the understanding and further exploration and development of tight oil. Hence, this study focuses on the detailed lithological and their organic geochemical characteristics of the Lucaogou Formation using microscopic thin-section observation, scanning electron microscopy, X-ray diffraction, Rock-Eval, saturated hydrocarbon chromatography and mass spectrometry. The following key conclusions were obtained. First, the massive mudrocks are sub-divided into lithofacies for hydrocarbon generation, including tuffaceous shale, calcareous shale and dolomitic shale, and, microinterval reservoirs, including dolostone and sedimentary tuff. Second, the origins of organic matter in Lucaogou Formation were mainly plankton, bacteria with minor of land plants. The depositional environment is high salinity and anoxic conditions. Materials derived from volcanic-hydrothermal activities, lacustrine precipitation, and terrestrial transportation mixed with various proportions to form this formation. Third, a mixture of carbonate minerals, felsic minerals and clay minerals is the best source rock, and the higher the mixture, the better. This research not only provides insight into the influence of volcanic-hydrothermal activities on lithology and organic matter enrichment, but also providing a potential scheme for the tight oil exploration and development in similar rifted geological settings. • A mixture lithology of carbonate minerals, felsic minerals and clay minerals is the best source rock. • The better the mixture of carbonate minerals, feldspar minerals and clay minerals, the better the quality of hydrocarbon source rock. • Volcanic-hydrothermal activities are favorable to the enrichment and preservation of organic matter. • The Lucaogou formation was formed in a high salinity and anoxic condition, and the biological source was mainly plankton.

Diagenesis and Its Impact on the Reservoir Quality of Continental Shales: A Case Study of the Lower Jurassic Da’anzhai Member of the Ziliujing Formation in the Sichuan Basin, China

The Jurassic continental shale oil (gas) is a favorable unconventional resource in the Sichuan Basin of China. In this paper, analysis methods such as core and outcrops observation, thin section identification, scanning electron microscopy (SEM), and X-ray diffraction (XRD) are used to describe the characteristics of lacustrine fine-grained rock reservoir in Da’anzhai Member of the Ziliujing Formation in Sichuan Basin, and it discussed the influence and control of lacustrine fine-grained rock diagenesis on the quality of the reservoir, in order to determine the shale reservoir control factors. The results show that there are three types of rocks in the Da’anzhai Member, which are mudstone, siltstone, and limestone, and the rock combination of shell shale intercalated with shell limestone is developed in the Da 2 submember. Fine-grained sedimentary rocks in this section have undergone compaction, cementation, dissolution, metasomatism, transformation of clay minerals, and hydrocarbon generation of organic matter, which are currently in the middle diagenetic stage A or B substage. Compaction and cementation are the main factors that control the physical use of shale and limestone, and strong cementation is the main reason for the tightness of limestone reservoirs in the Da’anzhai Member in the northern and eastern parts of Sichuan Basin. The difference between dissolution and cementation is the main control factor for the formation of limestone or argillaceous limestone reservoirs in the same section in central Sichuan Basin. The organic-rich shale and shell shale of the Yuanba and Fuling area are the most favorable reservoirs of the Ziliujing Formation in this region. The shell limestone that experienced favorable dissolution in the Da’anzhai Member in the central of the Sichuan Basin has become a limestone reservoir. Discussing the impact of continental fine-grained rock diagenesis on the reservoir can better explore and develop similar intervals, enrich unconventional shale oil and gas accumulation and storage theories, and provide basic theoretical support for finding favorable unconventional shale oil and gas reservoirs.