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Organic matter is the material basis for shales to generate hydrocarbon, as well as the main reservoir space and seepage channel for shale gas. When the thermal evolution degree is consistent, the organic carbon content in present shales is subject to the abundance of primitive sedimentary organic matter. Deep geofluids significantly influence the sedimentary organic matter’s enrichment, but the mechanism remains unclear. This paper is aimed at determining how hydrothermal and volcanic activities affected the enrichment of sedimentary organic matter by studying lower Cambrian shales in the lower Yangtze region and upper Ordovician-lower Silurian shales. Oxidation-reduction and biological productivity are used as indicators in the study. The result shows that hydrothermal or volcanic activities affected the enrichment of sedimentary organic matter by influencing climate changes and the nutrients’ sources on the waterbody’s surface and reducing water at the bottom. In the lower Cambrian shales of the Wangyinpu Formation in the lower Yangtze region, hydrothermal origin caused excess silicon. During the sedimentary period of the lower and middle-upper Wangyinpu Formation, vigorous hydrothermal activities increased the biological productivity on the waterbody’s surface and intensified the reducibility at the bottom of the waterbody, which enabled the rich sedimentary organic matter to be well preserved. During the sedimentary period of the lower upper Ordovician Wufeng Formation and the lower Silurian Longmaxi Formation in the upper Yangtze region, frequent volcanic activities caused high biological productivity on the waterbody surface and strong reducibility at the bottom of the waterbody. As a result, the abundant organic matter deposited from the water surface can be well preserved. During the sedimentary period of the upper Longmaxi Formation, volcanic activities died down gradually then disappeared, causing the biological productivity on the water surface to decrease. Besides, the small amount of organic matter deposited from the water surface was destroyed due to oxidation.

The abundance of organic matter in shales, which has a direct effect on the hydrocarbon generation potential of shales, is an important organic geochemical parameter for evaluating shale gas reservoirs. The total organic carbon content (TOC content) in shale is controlled by the abundance of original sedimentary organic matter. Therefore, it is very important to study the mechanism of organic matter enrichment in shale. In this paper, the Lower Cambrian marine shales from the Lower Yangtze region are selected as the research subject, most of which originate from a typical area well called Well JXY1. The degree of pyritization (DOP) is used to characterize the redox environment of the water body, while the P/Al ratio is used to analyze the biological productivity of paleoseawater. The paleosalinity of seawater is calculated via carbon and oxygen isotope analysis. In addition, the early Cambrian hydrothermal activities were studied by using core description; Si, Al, Fe, and Mn elemental analysis; and oxygen isotope calculations. The results show that during the early Cambrian Wangyinpu sedimentary period, the seawater was an anaerobic water body with H2S, and the oxygen concentration was approximately 0 mL/L. In the middle stages of the Wangyinpu sedimentary period, the water body had the strongest reducibility and the highest biological productivity. Moreover, the paleoocean in this period between the Yangtze plate and the Cathaysian plate was greatly affected by hydrothermal activities, with temperatures ranging from 90°C to 120°C. Active hydrothermal activities promoted high biological productivity and an anaerobic environment, both of which were conducive to the preservation and enrichment of organic matter, resulting in extremely high TOC content in the Wangyinpu shales (from 6.5% to approximately 16%).

Using the fractal geometry method, the microscopic pore structures of tight sandstone reservoirs in Kalpintag Formation of Shuntuoguole area in Tarim Basin were conducted fractal characterization on the base of test analysis data such as physical property, cast thin section, scanning electron microscope and mercury injection, and the genetic mechanism of pore structure heterogeneity was investigated. The storage spaces are dominated by intergranular dissolved pore, intragranular dissolved pore and residual intergranular pore, and the throat type consists of the necking throat, lamellar throat, curved lamellar throat and tube-shaped throat. The microscopic structure type includes Type I (fractal dimension≤2.350), Type II (2.350

Over the past 2 years, a great progress in petroleum and gas exploration has been made in tight lacustrine carbonate rocks of E 3 2 in the Yingxi area. Many high-yield commercial oil flow wells with more than 1000 tons of oil production per day have been found, but their reservoir space type and storage capacity are the key issues highly concerned and disputed by many explorationists. According to the study on systemic petrology and mineralogy, analysis of physical property, study on capillary pressure curve, analysis of ultra-micro crystal morphology, analysis of micro-area geochemical composition, and other comprehensive analysis, and pore types of the E 3 2 reservoir are mainly intercrystalline pores of dolomite, and a small number of dissolution phenomena have been found. Therefore, large-scale oil and gas reservoirs can be formed. The analysis involves these key results and evidences: (1) under the microscope and scanning electron microscope, many intercrystalline pores are developed in dolomite rocks, presenting a “high-wide platform” phenomenon on the capillary pressure curves to imply “small bore—large quantity”. (2) The electron probe shows that the molecular formula of dolomite is Ca(Mg0.96Fe0.04)(CO3)2. In the penecontemporaneous dolomitization process, intercrystalline pores come from volume shrinkage when Mg2+ ions replace Ca2+ ions, and a small number of Fe2+ ions are involved in this chemical reaction. The 13.36% plane porosity can be formed in ideal conditions. (3) According to the statistical data, intercrystalline pores are enlarged by organic acids in several samples, no evidence is available for leaching dissolution of atmospheric water because of exposure, and many types of feldspar do not dissolve. The reservoir rocks are formed due to a high deposition rate. (4) According to the measured pore sizes and the values based on the chemical theory, the pore size is in the range of 0.4–1.07 μm. These pores can store all oil, gas, and symbiosis substances, which can flow or migrate in the pores because of the pore sizes. The pictures of fluorescence thin sections and the backscattered electron images of SEM show that these micro-pores are full of hydrocarbons, creating the oil and gas accumulation pattern of “containing oil for the whole sag” and “producing oil for each well” in the Yingxi area. Based on the oil and gas accumulation model, the Yingxi area is an integrated oil and gas field with the most potential and industrial value in Qaidam Basin. The research results are crucial to the petroleum and gas exploration of carbonates in a saline lacustrine basin.

Carbonate dissolution during the process of burial and evolution by percolating acid fluid was simulated using core plugs to analyze the characteristics and controlling factors of Cambrian carbonate rock dissolution in the Tarim Basin. The results showed that mineral composition and reservoir space type control selective dissolution. In the carbonate rock strata with high calcite content, the calcite is likely to dissolve first to form secondary dissolution pores; gypsum and anhydrite in the carbonate rock can be dissolved to form mold pores in contemporaneous and penecontemporaneous stages. Porous carbonate has mainly enlargement of matrix pores, with porosity and permeability increasing correspondingly, but not obviously. In comparison, dominant channels for fluid are likely to occur in fractured carbonate or porous carbonate forming cracks under high pressure, resulting in a relative reduction in the dissolution volume, but great increase of permeability. With the rise of temperature and pressure, corrosion ability of acid fluid to carbonate rock increases first and then decreases, there exists an optimum range of temperature and pressure for dissolution, which corresponds to the buried depth of 2 250−3 750 m of the Cambrian. Considering reservoir characteristics of the study area, it is concluded that calcite in the penecontemporaneous period is the material basis for the development of dissolution pore, and carbonate rock were mainly dissolved by early atmospheric fresh water, superimposed and reformed to form high quality reservoirs by multiple acid fluids including deep hydrothermal fluid and acid fluid generated during the process of organic thermal evolution under burial condition. Key words: Tarim Basin, Cambrian, carbonate rock, dissolution, dissolution mechanism, simulation experiment

The Chang 6 reservoir of Yanchang formation in Jingan oilfield has relatively poor quality: the porosity ranges mainly from 8 to 14%, and permeability ranges from 0.018 to 10.48 md, with a mean of 1.10 md, categorized as low porosity and low permeability reservoir type with strong heterogeneity. Petrographic and geochemical analyses of various components in these sandstones have provided clues of diagenesis. The sandstones at the early stage of diagenesis are characterized by non-ferroan calcite cementation, grain-coating, pore-lining clay minerals, and initial dissolution of detrital grains. Authigenic quartz, pore-filling and grain-replacive laumontites, albitized detrital plagioclase, authigenic K-feldspar, illite and late ferroan calcite cement dominate the late diagenesis. Reservoir quality is influenced by products of late diagenesis stage, the cement (calcite, albite, quartz, illite, and laumontite) occluded primary and secondary porosity. The organic acidic fluid derived from organic matter in the source rocks during the late diagenesis, which results in the dissolution of detrital plagioclase and laumontite cement and was the main reason for the enhancement of reservoir-quality. As a result of that, the residual primary pore and dissolution pore comprised the main accumulation space for oil and gas, and thus become important targets for hydrocarbon exploration. Keywords: Ordos basin, Chang 6 member, Diagenesis, Reservoir-quality, Dissolution

The tight sandstone reservoir of the Donghetang Formation underwent complex diagenetic processes for which the controlling factors remain unknown. Here, we present a case study of the lower member of this formation and show that this part of the reservoir consists mostly of lithic quartz sandstone with poor physical characteristics. Using core observation, thin section identification, electronic microprobe, scanning electronic microscopy, X-ray diffraction, and inclusion fluorescence microscopy, we identified five major diagenetic processes: compaction, cementation, dissolution, metasomatism, and hydrocarbon charging. Compaction and cementation have negative effects on petrophysical properties (referred to porosity and permeability of sandstone in this paper), and dissolution improves these properties. Diagenesis is affected by the depositional environment, and clastic constituents control compaction and dissolution; burial history and the geotemperature field control diagenesis at the macroscopic scale, while hydrocarbon charging affects the degree of cementation and dissolution. As oil saturation increases, the percentage of calcareous cements decreases and the development of secondary dissolved pores improves.

The characteristics of the Lower Qiulitage Group in Bachu uplift, which originated in the Upper Cambrian period, are not clear. Based on four core observations, identification of 40 thin sections, and geochemical analysis of samples, this study concludes that crystal dolomite reservoirs developed in the Lower Qiulitage Group in Bachu uplift. Intercrystalline pores and dissolved pores formed the main reservoir space, while dissolution pores and fractures were developed in the middle and bottom part of the formation. The reservoir features low porosity and low permeability, and the fine-medium crystalline dolomite reservoir has the best pore structure. According to a petrochemical analysis, the Lower Qiulitage Group reservoir experienced intense dolomitization, with slightly higher content of trace elements Mn and Sr and negative oxygen isotropy. Tectonic hydrothermal dolomitization developed locally under the influence of base faults, and vug pores were filled with saddle dolomites and siliceous filling. The presence of rare earth elements Ce and Eu presented positive anomaly characteristics. It suggested that the CO2 acid dissolution caused by hydrothermal fluids may be a new mechanism for the increase in pores in deeply buried reservoirs, which is worth further study. RESUMEN Las características del Grupo Qiulitage Inferior en el levantamiento de Bachu, que se originó en el período Cámbrico Superior, no están claras. Con base en cuatro observaciones centrales, la identificación de 40 secciones delgadas y el análisis geoquímico de muestras, este estudio concluye que los reservorios de dolomita de cristal se desarrollaron en el levantamiento del Grupo Qiulitage Inferior en Bachu. Los poros intercristalinos y los poros disueltos formaron el espacio del depósito principal, mientras que los poros de disolución y las fracturas se desarrollaron en la parte media e inferior de la formación. El reservorio presenta baja porosidad y baja permeabilidad, y el reservorio de dolomita cristalina fina-media tiene la mejor estructura de poros. Según un análisis petroquímico, el reservorio del Grupo Qiulitage inferior experimentó una intensa dolomitización, con un contenido ligeramente mayor de oligoelementos Mn y Sr e isotropía de oxígeno negativa. La dolomitización hidrotermal tectónica se desarrolló localmente bajo la influencia de fallas de base, y los poros vug se llenaron con dolomitas de silla y relleno silíceo. La presencia de elementos de tierras raras Ce y Eu presentó características de anomalías positivas. Esto sugirió que la disolución de ácido de CO2 causada por los fluidos hidrotermales puede ser un nuevo mecanismo para el aumento de poros en depósitos profundamente enterrados, lo que merece un estudio más profundo.

Marine shale gas exploration in southern China has successes and failures. Under the condition of great hydrocarbon generation material basis, shale gas wells drilled from some shale gas blocks are rich in methane, while the wells in other shale gas blocks with high nitrogen and low hydrocarbon gas, which indicates that they have different accumulation mechanisms. Therefore, the study of gas composition in shale will help us to figure out the mechanism of shale gas accumulation and loss. In this paper, the Lower Cambrian shale from Wei-201 well in Upper Yangtze Weiyuan Block and Jiangye-1 well from Lower Yangtze Xiuwu Basin are selected as research object, and shale samples are used for tests and experiments including analysis of gas composition and nitrogen isotope, test of porosity and TOC content, overburden permeability test, permeability test before and after methane adsorption under different osmotic pressure, permeability test parallel and vertical to the bedding surface, FIB-SEM (Focus Ion Beam Scanning Electron Microscope) and FIB-HIM (Focused Ion Beam Helium Ion Microscope). Finally, the reasons for the difference in the gas components of the Lower Cambrian shale gas in Weiyuan Block and Xiuwu Basin are studied by means of seismic interpretation, core description and outcrop observation besides the tests and experiments. The results show that the gas components of the Lower Cambrian shale in Weiyuan Block, the Upper Yangtze, mainly consist of methane, derived from liquid hydrocarbon cracking. The sealing capacity of roof and floor, the great self-sealing of shale and the flat anticline structure contribute to the high methane content in shale gas. The Lower Cambrian shale gas in Xiuwu Basin, the Lower Yangtze, is mostly nitrogen, which is derived both from atmosphere and deep crust-upper mantle. The detachment layer at the bottom of the Lower Cambrian, the widely developed deep faults and the Jurassic volcanic activity are the reasons for the high nitrogen and low hydrocarbon of shale gas. Based on the above analysis, the patterns are summarized for shale gas accumulation in the simple anticline background and reservoir destruction in the complex syncline background.

The Upper Triassic Xujiahe (须家河) Formation in the Sichuan (四川) Basin, Southwest China is distinctive for the basin-scale sand deposition. This relatively rare sedimentary phenomenon has not been well interpreted. Here we addressed this issue by discussing sedimentary framework and conceptual model. Analysis of sedimentary setting implied that the basin received transgression during the deposition. It had multiple provenance supplies and river networks, as being surrounded by oldlands in multiple directions including the north, east and south. Thus, the basin was generally characterized by coastal and widely open and shallow lacustrine deposition during the Late Triassic Xujiahe period. This is similar to the modern well-known Poyang (鄱阳) Lake. Therefore, we investigated the framework and conceptual model of the Sichuan Basin during the Xujiahe period with an analogue to the Poyang Lake. Results show that the conceptual model of the deposition can be divided into transgressive and regressive stages. The first, third and fifth members of the formation are in transgressive stage and the deposits are dominated by shore and shallow lacustrine mud. In contrast, the deposition is mainly of braided river channel sand deposits during the regressive stage, mainly including the second, fourth and sixth members of the formation. The sand deposited in almost the entire basin because of the lateral migration and forward moving of the cross networks of the braided rivers. The multiple alternations of short and rapid transgression and relatively long regression are beneficial to the basin-scale sand deposition. Thus, the main channel of the braided river and its extensional areas are favorable for the development of hydrocarbon reservoir. This provides practical significance to the reservoir evaluation and exploration. In addition, the results also justify the relatively distinctive sedimentary phenomenon in the study area and may also have implications for understanding the large-scale sand deposition elsewhere.

This article put forward an effective algorithm to examine outlier detection based on clustering. Clustering and outlier detection methods are analyzed well by partitioning method. The results show that: it reduced time complexity of the outlier detection greatly, and with the increase of the data, this algorithm is still useful.