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The Late Cambrian (Furongian) Chaomidian Formation accumulated in an epeiric sea that covered a large part of the North China Plate and extended from China to Korea. The depositional environment of the formation is commonly considered to have been affected by storms that broke up numerous limestone layers, but that was tectonically quiet. It is here argued, however, that some features of the formation – more in particular the many dozens of breccia layers and the occurrence of slid-down limestone blocks that are embedded in autochthonous oolites – can be explained satisfactorily only if some significant fault activity took place during its accumulation. The faulting may have been due to differential loading and subsidence, but an endogenic origin seems more probable. This implies that the structural history of the study area in Shandong Province may need re-consideration.

The Wangjiazhuang porphyry copper (-molybdenum) deposit is located at Zouping volcanic basin in Shandong Province, East China and hosted in the Wangjiazhuang intrusive complex emplaced along a late volcanic conduit. There are two types of ores in this deposit: early disseminated and stockwork ores in the ore-bearing intrusion, and late massive sulfide-quartz veins above brecciated quartz monzonite. The ore minerals are mainly pyrite, chalcopyrite, and subordinately magnetite, tennantite, molybdenite with minor bornite, enargite, galena and sphalerite, etc., and gangue minerals including K-feldspar, biotite, quartz, muscovite-sericite, chlorite and calcite. Combined with fluid inclusion study, the scanning electron microscope-cathodoluminescence (SEM-CL) study of quartz in the deposit and wall rocks shows significant differences between the two types of quartz in the ores. In addition, four types of primary-pseudosecondary fluid inclusions in the quartz have been recognized. They are one- or two-phase aqueous inclusions with vapor/liquid ratios less than 30% to 40% (type I); gas-rich inclusions with vapor/ liquid ratios more than 50% (type II), some of which contain some small opaque minerals, probably chalcopyrite; multiphase fluid inclusions with daughter minerals of halite±anhydrite±opaque (chalcopyrite)± sylvite±hematite±unknown crystal (type III); and mica-bearing fluid inclusions (type IV). Quartz containing abundant muscovite-bearing and halite-bearing fluid inclusions in the mineralized quartz monzonite with potassic-silicic alteration, have better oscillatory growth zoning with CL-colors from bright in the core to darker in the rim, indicating variations of element concentrations in the fluid media from which quartz grew during the later period of magmatic-hydrothermal process. In contrast, the quartz in the sulfide-quartz veins contains mainly fluid inclusions of low-to-medium salinities and does not show oscillatory zoning, indicating that there was less fluctuation in composition and element concentrations of the hydrothermal fluids. However, the quartz containing halite-bearing fluid inclusions and being associated with copper-molybdenum mineralization in the sulfide-quartz veins shows zoning in its rims, indicating variations in composition and element concentrations of the hydrothermal fluids.

Tight sandstone gas reservoirs constitute an important type of unconventional gas resources and may be of crucial significance as environment-friendly source of energy in the world. However, the dynamics of densification and evolution of the low-porosity and low-permeability sandstone reservoirs are intensively debated in literature. Recently, the question of sequence of reservoir densification and hydrocarbon accumulation has arisen. A typical tight sandstone reservoir (the first member of the Shanxi Formation and the eighth member of Shihezi Formation) in the Permian Sulige gas field in Central China provides an excellent case study to probe these questions. By using a variety of different methodologies such as scanning electron microscopy (SEM), X-ray diffraction (XRD), cathodoluminescence, fluid inclusion analysis and confocal Raman spectrometer analysis, we identified different authigenic minerals such as illite, kaolinite, dickite, chlorite and smectite illitization coexisting with siliceous and calcitic cements. These authigenic minerals precipitated at different diagenetic stages, related to the changing conditions during compaction and lithification, and therefore, changed the porosities of the host rocks during each of these stages. The types of pores in the Sulige gas field are dominated by illite and kaolinite intercrystalline pores, matrix solution pores, and primary intergranular pores. The results of this study suggest that compaction and cementation are the reasons for the most distinct loss in porosity. On the other hand, kaolinite and illite preserve their intercrystalline pores, which could be identified as the main reserve space of the tight gas reservoirs. Therefore, the secondary porosity of the sandstone reservoirs forms the most important contributor to the gas reservoir potential in the Sulige gas field. In addition, it could be inferred that the tight gas accumulation took place during the densification of the reservoirs. The implications of the presented results may also help to understand the formation of other tight reservoirs world-wide.

A large, typical, marine, organic-rich shale is developed in the Ordovician Wufeng Formation-Silurian Longmaxi Formation in the Sichuan Basin, China, which has rich shale gas resources. This paper uses this type of shale as an example to analyze the influence of the depositional environment on the enrichment mechanisms and sedimentary processes of its organic matter (OM). Based on thin sections and field emission scanning electron microscope observations from well core samples, along with X-ray diffraction and geochemical analysis, the mineralogy, organic and inorganic geochemistry characteristics, as well as the lithofacies, are analyzed. The results show that the shale is mainly composed of quartz and clay minerals but also contains calcite, feldspar, pyrite, etc. The OM includes solid bitumen, algae, graptolite, spongy spicule, radiolarians, etc. The total organic carbon (TOC) content ranges between 0.5% and 6.17%, with an average value of 2.5%. Based on the mineral compositions, the TOC content and sedimentary structure, five kinds of lithofacies are identified. According to the primary productivity, redox conditions, paleosalinity and sedimentation rate characteristics, the formation's sedimentary environment is divided into five intervals. During the OM enrichment processes, the primary productivity, sedimentation rate and redox conditions affect the generation, accumulation and preservation of OM. There is a good positive correlation between primary productivity and TOC content, as well as the redox condition. Sedimentation rate is inversely proportional to TOC content. The change and evolution of the sedimentary environment over these five intervals form different OM occurrence characteristics and associations with lithofacies. Based on the characteristics of OM, lithofacies and the sedimentary environment, as well as the geological background of the study area, the mode of OM occurrence and enrichment was proposed.

Interpretation of the continental or marine character of sediments on the basis of the paleosalinity is increasingly based on the concentration of some trace elements. The paleosalinity can be affected, however, also in deep-marine environments, by discharge peaks of major sediment-laden rivers. These induce hyperpycnal flows that run down the basin slope. The huge volume of fresh water gradually mixes with the ambient saline seawater, diminishing the paleosalinity. The temporary lowered paleosalinity may be preserved in the sedimentary record in the form of ‘diverging’ concentrations of trace elements and trace-element ratios. This is exemplified for the Early Cretaceous Lingshandao Formation on Lingshan Island (western Yellow Sea). It was found during field work and geochemical analysis that hyperpycnal currents may not only lower the salinity of a deep-marine environmental setting, but may also carry along continental plant fragments and other biological remains, which may obscure the marine character of the sediments. It is deduced that detailed facies analysis in the field is consequently required to reveal the true character of a marine environment.

The ongoing exploration for shale oil and gas has focused sedimentological research on the transport and deposition mechanisms of fine-grained sediments, and more specifically on fine-grained mass-flow deposits. It appears, however, that no easily applicable classification scheme for gravity-flow deposits exists, and that such classifications almost exclusively deal with sandy and coarser sediments. Since the lack of a good classification system for fine-grained gravity flow deposits hampers scientific communication and understanding, we propose a classification scheme on the basis of the mud content in combination with the presumed transport mechanism. This results in twelve types of gravity-flow deposits. In order to show the practical applicability of this classification system, we apply it to the Triassic lacustrine Yanchang Formation in the southern Ordos Basin (China), which contains numerous slumps, debris–flows deposits, turbidites and hyperpycnites. The slumps and debrites occur mostly close to a delta front, and the turbidites and hyperpycnites extend over large areas from the delta slopes into the basin plain. The case study shows that (1) mud cannot only be transported but also deposited under active hydrodynamic conditions; (2) fine-grained gravity-flow constitute a significant part of the lacustrine mudstones and shales; (3) muddy gravity flows are important for the transport and deposition of clastic particles, clay minerals and organic matter, and thus are important mechanisms involved in the generation of hydrocarbons, also largely determining the reservoir capability for unconventional petroleum.

The National Natural Science Foundation of China (grant No. 41672120 and No. 41402120), and Shandong University of Science and Technology Research Fund (grant No. 2015TDJH101).

Fine-grained sediments from the Late Triassic Yanchang Fm. in the Ordos Basin (central China) were studied by core analysis and geophysical logging. Part of the mudstones in this formation are stratified, part of them are unstratified; the various mudstones can be subdivided into eight types on the basis of their structures and textures. They represent a variety of environments, ranging from delta fronts and subaqueous fans to deep-water environments. Part of the sediments were reworked and became eventually deposited from subaqueous gravity flows, such as mud flows, turbidity currents and hyperpycnal flows that easily developed on the clay-rich deltaic slopes. The sediments deposited by such gravity flows show abundant soft-sediment deformation structures. Understanding of such structures and recognition of fine-grained sediments as gravity-flow deposits is significant for the exploration of potential hydrocarbon occurrences. Because fine-grained deposits become increasingly important for hydrocarbon exploration, and because the sediments in the lacustrine Yanchang Formation were deposited by exactly the same processes that play a role in the accumulation of deltaic and prodeltaic fine-grained sediments, the sedimentological analysis provided here is not only important for the understanding of deep lacustrine sediments like the Yanchang Formation, but also for a better insight into the accumulation of fine-grained prodeltaic deep-marine sediments and their potential as hydrocarbon source rocks and reservoir rocks.

Pebbles up to 10 cm in diameter, as the main clastic component of gas reservoirs in the Permian Sulige Gas Field, deposited more than 150 km away from their northward provenance (Yinshan Mountains) in the northern Ordos Basin. The transportation and depositional mechanisms of these pebbles have been unclear for decades. Based on core examinations, facies analysis, and mathematical modeling (stress field calculation of pebbles), it is thought that (1) these coarse-grained deposits were transported and deposited in braided rivers; (2) the size of coarse-grained sediments and pebbles in the Sulige Gas Field is in accordance with the size of flash flood; (3) flash flood is regarded as a crucial important transporting mechanism of pebbles; and (4) the “flash flood genesis” braided river-braided delta sedimentary model provides a reasonable understanding for prediction of coarse-grained reservoirs for gas exploration not only in the Permian Sulige Gas Field but also in similar coarse-grained sediments elsewhere.

During the Upper Paleozoic large-scale tight sandstone oil and gas reservoirs developed in the southern Ordos Basin of China. Due to the heterogeneity of sedimentary facies and diagenesis, the reservoir quality of these deposits is extremely uneven. Based on field profile observations, conventional thin section, cast thin section and scanning electron microscope (SEM) analysis, combined with general physical analysis, the sedimentary characteristics, physical properties and main controlling factors of the reservoirs of the Upper Paleozoic Shanxi Formation of the southern Ordos Basin were determined. The results of this contribution show that the Shanxi Formation is dominated by delta facies, with small-scale lacustrine facies. The delta facies can be further divided into distributary channel, natural levee, interdistributary swamp, underwater distributary channel, underwater distributary bay, mouth bar and sand sheet microfacies. Diagenesis within the study area has generally entered the mesodiagenetic B phase. Compaction and cementation were the main destructive stages of diagenesis. While cementation blocked the reservoir pores and reduced the physical properties, dissolution effectively improved the physical properties of the reservoir. The physical reservoir properties of the studied sandstone reservoirs are controlled by different sedimentary microfacies that are mainly, represented by distributary channel, underwater distributary channel and mouth bar microfacies that also show the best properties. These microfacies are followed by sand sheets. The lowest properties can be observed in the natural levees.

The occurrence and enrichment of organic matter (OM) are influenced by the deposition of minerals in shale reservoirs. However, inadequate research has been published on the origin, spatial relationships, and significance of organic matter–mineral associations (OMMA) for predicting “sweet spots” in shale gas reservoirs. We describe OMMA in the Wufeng Formation and the Longmaxi Formation in the Sichuan Basin, which is the largest commercial shale gas accumulation outside of North America. We characterize the presence of solid bitumen (SB) and graptolite fragments (the most important kinds of OM in these reservoirs) with four important minerals: i.e., quartz, clay minerals, carbonates, and pyrite. The occurrence of OMMA is influenced by the physical (e.g., morphology, texture, and density) and chemical properties of the associated minerals, which influences the accumulation of OM and the development of porosity. By carefully studying the type and relative amount of OM, we have determined how the surface porosity, pore diameters, and amount of porosity in each kind of OMMA influence the quality of shale gas reservoirs: e.g., diagenetic quartz or clay minerals encased in SB is the most favorable kind of OMMA. The quality of OM–carbonate associations is lower because they contain a relatively low amount of OM. Similarly, the relatively low abundance and dispersed distribution of OM–pyrite associations do not improve the quality of shale gas reservoirs. We identified four important lithofacies using the color, composition, and sedimentary structures of samples. The heterogeneous distribution of OMMA has important implications for shale gas reservoirs by influencing hydrocarbon generation and gas storage conditions in each lithofacies: i.e., siliceous shale > argillaceous siliceous shale > calcareous shale > silty shale.

Some sections on Lingshan Island, in the Yellow Sea, close to the city of Qingdao, show the Early Cretaceous Lingshandao Formation. The exposed succession consists mainly of turbidites, some of which show uncommon soft-sediment deformation structures, and contains well-developed, large-scale slumps that consist of sets of turbidites. The sediments were deposited as part of lobes of turbidites and hyperpycnites (deposits of flood-generated sediment-laden bottom currents derived from an estuary) in a deep-sea environment that formed part of an inter-arc basin with a tensional tectonic regime. Some of the slumps have dimensions of the order of metres to tens of metres; they consist of the turbidites that build most of the succession. The soft-sediment deformation structures in the turbidites represent a wide variety of types, most of which are common in subaqueous environments with a high accumulation rate. The most interesting are, however, structures that indicate active tectonic tension. These structures are boudins, which are present with two subtypes. The most common are pinch-and-swell structures, sometimes leading to isolated parts of the affected sandy layer, enveloped by shales of the under- and overlying shale layers. The second type consists of domino boudins, which have, as far as we are aware of, not earlier been described from sediments that were still unlithified during their formation. The presence of the boudins and associated structures, which indicate synsedimentary stretching related to an extensional tectonic setting, form an additional argument in the long-lasting debate about the question of whether a compressional or an extensional tectonic regime dominated in the area during the Early Cretaceous.

The fine-grained autochthonous sedimentation in the deep part of a Late Triassic lake was frequently interrupted by gravity-induced mass flows. Some of these mass flows were so rich in water that they must have represented slurries. This can be deduced from the soft-sediment deformation structures that abound in cores from these lacustrine deposits which constitute the Yanchang Fm., which is present in the Ordos Basin (central China). The flows and the resulting SSDS were probably triggered by earthquakes, volcanic eruptions, shear stress of gravity flows, and/or the sudden release of overburden-induced excess pore-fluid pressure. The tectonically active setting, the depositional slope and the high sedimentation rate facilitated the development of soft-sediment deformations, which consist mainly of load casts and associated structures such as pseudonodules and flame structures. Sediments with such deformations were occasionally eroded by slurries and became embedded in their deposits.

It is becoming an important controlling factor of gas exploration and exploitation in the east part of Sulige gas field in the Ordos Basin where the reservoir of main gas formations is tight sandstones. Employing experimental methods of slice identification, casting slice, scan electron microscope, and X-ray diffractions, we studied the characteristics of petrology and diagenesis on reservoirs in Shan1 section of Shanxi formation and He8 section of Shihezi formation of the Permian system in the East II part of Sulige gas field. The results include: (1) the main sandstones in these areas are dominated by lithic sandstone and lithic silicarenite with low grade of maturity; (2) the diagenesis of sandstone in these areas mainly include compaction, cementation, corrosion and alteration. Conclusions are as follows: (1) the diagenetic stage reached period B of the middle diagenetic stage; (2) the early diagenetic compaction is one of the main factors to decreasing porosity; (3) the secondary pores formed by corrosion in acidity medium conditions in period A of the middle diagenetic stage can distinctly ameliorate the poor reservoir capability of sandstone and; (4) cementation in period B of the middle diagenetic stage is the most important factor leading to poor physical property of sandstone reservoirs. Keywords: Ordos Basin, Permian, Sandstone reservoirs, Diagenesis, Porosity evolution

Given sustaining exploration, the eastern zone of the Sulige gas field may soon become a key area of exploitation. In order to explore its genesis, types and distribution of the reservoir sandstones in the eastern zone of this gas field, we focused in our study on the provenance and detailed sedimentary facies of sandstone of the He8 (the eighth part of the Shihezi formation, Permian system) and Shanxi1 (the first part of the Shanxi formation, Permian system) members, based on core observations, analyses in petrography, granularity and logging. The results show that: 1) the sandstone provenance of Shanxi1 and He8 in the eastern zone of the Sulige gas field is from the north of the Ordos Basin, characterized by dual directions from the north and northeast. 2) The He8 and Shanxi1 members were deposited in a fluvial-delta sedimentary system. The He8 was mainly deposited in braided rivers, including braided channels, channel bars, levee and floodplain sub-environments, whereas the Shanxi1 Member was deposited in braided rivers and deltas, including braided channels, channel bars, floodplains, tributaries and inter-tributary sub-environments. 3) Sedimentary facies bands migrated in drastic fashion towards the basin from the Shanxi1 to the He8 Member. Base levels of sedimentation generally present a trend of small increases in-amplitude, large decreasing amplitudes and slow and gradual Increases. 4) The continuity of the reservoir sandbodies along the source direction is better than that perpendicular to the direction. Compared with Shanxi1, both dimensions and continuity of the sandbodies in He8 are better from which we conclude that it is the most favorable part of the reservoir.

Ribbon rocks are characterized by an alternation of millimeter- to centimeter-thick limestone and argillaceous deposits (marlstone or shale). The sedimentary processes and diagenetic characteristics of ribbon rocks might be critical to the formation of limestone conglomerates. According to detailed field measurement and laboratory analyses (thin section observation and XRD analysis), four types of ribbon rocks are classified, i.e., limestone and marlstone couplet (L-M), limestone and shale couplet (L-S), thin-bedded lime mudstone (Ltb), and laminated limestone and marlstone couplet (Cl). These ribbon rocks were mostly deposited in low-energy subtidal environments (below fair-weather wave base). Ribbon rocks exhibit various subtle deformation structures such as intrastratal cracks and “boudin” structures. Differential cementation of carbonate and argillaceous layers during early diagenesis is a prerequisite condition for the deformation of ribbon rock under compaction. Ribbon rocks would be deformed into limestone conglomerates under differential compaction that might be triggered by external forces such as storm and earthquake.

The new recently demonstrated reserves of oil and gas in the Ordos basin are found at the top of petroliferous basins in China. Gas pools discovered in recent years in the Permian system have become the main natural gas resource in the basin. Therefore, synthetic research on fluid inclusions should be done in order to ascertain the pool-forming stage, the maturity of organic matters and the properties of Paleo-fluids. The main types of fluid inclusions in the Permian system in the basin include brine inclusions, carbon dioxide inclusions and organic inclusions. Homogenization temperatures (HT) of brine inclusions can be divided into four intervals: 66-83 °C, 86-108 °C, 112-138 °C and 142-153 °C. The fluid inclusions in the interval of 112-138 °C are much more than that of other intervals, indicating that the second stage of hydrocarbon migration associated with the third temperature interval is the most important stage of gas pool formations. The fluid inclusion has extensive applications in the study of gas geology, not only in ascertaining the formation stage of gas pools, but also in estimating the maturity of organic matter and restoring Paleo-fluids. The result of testing the HT of brine inclusions shows that there are two stages of gas pool formations in the Permian system occurring ±150 Ma and ±100 Ma. The maturity of organic matter is moderate to high, a conclusion based on the color of fluid inclusions (radiated by fluorescence). The high salinity of Paleo-fluids of the NaCl-H 2 O and CaCl 2 -H 2 O systems shows good preservation conditions of the Paleo-fluids. Two stages of reservoir filling, high maturity of organic matter and good preservation conditions are factors favorable for the formation and preservation of large-scale gas pools in the Permian system in the Ordos basin.

The advantage of high-resolution sequence stratigraphy, which takes base-levels as reference, is that it can be applied to continental depositional basins controlled by multiple factors and can effectively improve the accuracy and resolution of sequential stratigraphic analysis. Moreover, the principles of base-level cycles are also suitable for analyzing sequential stratigraphy in continental coal-bearing basins because of their accuracy in forecasting distribution of coal measures. By taking the Dongsheng coalfield in the Ordos basin as an example, the extensive application of base-level cycles in exploration and exploitation of coal is analyzed. The result shows that the Yan'an formation in the Dongsheng area is a long-term base-level cycle which is bordered by nonconformities and made up of five mid-term cycles and 13 short-term cycles. The long-term cycle and the mid-term cycles are obvious in comparison with a transverse profile. The episodic coal accumulation in the Mesozoic Ordos basin means that the deposition of primary matter (peat bogs) of coalification is discontinuous, periodical and cyclical in the evolution of the basin. The episodic accumulation of coal measures in the Yan'an stage is controlled by ascending-descending changes of a long-term cycle and middle-term cycles. Coal measures formed during the early and late periods of the long-term cycle are characterized by multiple layers, big cumulative thickness and poor continuity. Coal measures formed in the mid-term of the long cycle are dominated by good continuity, fewer layers and a small additive thickness, which is favorable for the accumulation of thick and continuous coal measures in the transition stage of mid term base-level cycles.