The marine shale source rock system has the high-temperature pyrolysis gas-sourcing ability in the later period. Take for examples the Cambrian Qiongzhusi Formation and Silurian Longmaxi Formation in the Yangtze area and the Changcheng-Qingbaikou Xiamaling Formation in North China area. The gas-sourcing potential of marine mud shale source rock in the later period is brought under study on the basis of the closed system and the open system to determine whether marine mud shale has the high-temperature pyrolysis gas sourcing ability in the later period. Two parameters – LGP and LGT – are used to evaluate the high-temperature pyrolysis gas-sourcing ability. The thermal development process of gas from source rock is a very complicated process. When thermal pyrolysis hydrocarbon sourcing takes place, a certain quantity of melting-resistant molecule – NSOs – is generated from condensation polymerization. NSOs is cracked into hydrocarbon gas under the high-temperature development stage. The rock pyrolysis experiment shows that the samples of source rock from Qiongzhusi Formation and Longmaxi Formation can produce heat-cracked hydrocarbons under the high-temperature development stage. For example, SP-1 can still produce 0.092mg/g (HC/TOC) after R_o =2.42%, indicating that source rock can still keep the sourcing ability under the hightemperature development stage, but the sourcing efficiency is low. The LGP - LGT standard is used to evaluate the gas-sourcing ability of marine mud shale in the later period. LGT of the inspected marine mud shale is >1, indicating the ability to produce high-temperature cracked gas of B type in the later period. LGP is distributed between 0.51 and 0.55, indicating the medium-level potential for high-temperature cracked gas. The average potential of inspected samples for high-temperature cracked gas is 2.6 mg/g (HC/TOC), belonging to the relatively low natural gas productivity. Based on calculation, it is about 0.2 m³/t (HC/rock). Take shale from the Silurian Longmaxi Formation in China^’s Shunan area for instance. The actually inspected gas content is about 3m³/t (HC/rock). The high-temperature cracked gas will account for 6.7 percent.

The Changxing Formation and Feixianguan Formation in Sichuan Basin are enriched in reef oil and gas resources, which are distributed in the areas of the platform and the platform margin slope in northeast Sichuan Basin. This region is an important area for exploration. The reef-bank gas reservoirs in Longgang and Yuanba areas of northeastern Sichuan Basin have good conditions for accumulations. Distribution of the gas reservoirs are controlled by a number of factors, such as the sedimentary facies, reservoir development characteristics, dolomitization and tectonic action. The important controlling factors for abundance and high production also include the variety of gas reservoirs, fractural development degree and structural activity. At present, the exploration results in Yuanba area are better than those in Longgang area, because the reef-bank scale is larger in Yuanba area with a weaker heterogeneity. The area is less affected by the tidal channel systems. Therefore, Yuanba area has better conditions for accumulation and abundance. Longgang area is apparently affected by the structural effect and tidal channel systems. The reservoirs are strong in heterogeneity and experience strong adjustment and transformation in later periods. The area is characterized as a number of isolated reef beach reservoirs. The study indicates that the efforts should be focused on seismic data quality, further research on the sedimentary subfacies, reservoir prediction and oil and gas testing to improve exploration of reefbank gas reservoirs in Longgang and Yuanba areas. The reef-bank bodies in the platform peripheral area, with a large thickness and a large area, should be the important targets for exploration.

In addition to the original deposition, replacement dolomite is the main origin of dolomite. The study of crystal structure of replacement dolomite with different burial stages can help analyze the development process and main influencing factor of replacement. Three kinds of replacement dolomite were chosen for study – the Miocene Cayman Formation in Cayman Island of the Caribbean Sea, the Miocene Xuande Formation in South China Sea and Sinian Dengying Formation in Sichuan Basin. They were replaced from original calcite and not affected by terrestrial materials, with the original grain structures kept. The X-ray diffraction is used to analyze the mineral contents, ordering degrees and crystal cell parameters and the TEM is used to observe the microtopograph of dolomite crystals. With the burial depths ranging from several meters to hundreds of meters and even thousands of meters, there are two typical changes. One is that the crystal forms change from euhedral-subhedral-anhedral, reflecting the restrain of the burial diagenesis on development of replacement dolomite. The other is that a gradually higher ordering degree with crystal cell parameters closer to ideal values reflects that stable burial diagenesis is propitious to the transformation of dolomite from disorder to order. There is no inevitable relation between crystal form of dolomite and the ordering degree. With the X-ray diffraction used, the high-magnesium calcite, high-calcium dolomite and low-calsium dolomite were tested in Cayman Formation samples. The petrographic characters and crystal structures reflected the replacement dolomite experienced the changes from highmagnesium calcite to high-calcium dolomite until low-calcium dolomite, which can be considered as a gradual process that magnesium ions were involved into the crystal lattice of calcite. After a stable development for a long period of time, these three kinds of replacement dolomite gradually became ideal dolomite. The magnesium ion sources of the three kinds of replacement dolomites were nothing but seawater. The paleokarstification catalyzed the replacement dolomitization to a certain extent so that the original porosity is not easy to be destroyed after dolomitization, forming a good reservoir space for oil and gas migration in the later periods.

Structural sedimentation is an important factor for changes in accommodation space of a basin, exerting an obvious controlling effect on sedimentation rate and sedimentation system. Structural sedimentation gradient is the characterization value of horizontal variation for basin structural sedimentation in a certain geological period, indicating the variation characteristics of this basin^’s accommodation space during this period. Based on the analysis of structural sedimentation gradients of the second member and third member of the Triassic Xujiahe Formation in Sichuan Basin, it is found that structural sedimentation gradient has an apparent controlling effect on sedimentation system and distribution of sedimentation facies. In the zone with a high structural sedimentation gradient, accommodation space changes quickly with development of water systems while the sedimentation facies changes significantly. In the zone with a low structural sedimentation gradient, accommodation space changes slowly with dissipation of water systems and the sedimentation facies is distributed steadily. Based on the analysis of the fourth member of the Xujiahe Formation in Guang’an region, it is found that structural sedimentation gradient also influences distribution of reservoirs. In the regional zone with a high structural sedimentation gradient, channel hydraulic power is strong owing to a high slope of landform. The physical properties of sandstone reservoirs are relatively desirable. As compared to the basins of other types, the rift basins represented by Nanpu Depression have a high structural sedimentation gradient, basically at 1 to 7 (m/Ma)/km, causing the sedimentation facies to become narrow and change quickly. The length of the subfacies is only 6 to 13 kilometers, with the lithological characters and physical properties changing quickly. The developed favorable reservoirs are small in scale and strong in heterogeneity. In the foreland basins and sag basins represented by the Xujiahe Formation of Sichuan Basin and the Shanxi Formation of Ordos Basin structural sedimentation gradient changes slowly, basically at 0 to 1.5 (m/Ma)/km, leading to large scale of sedimentation system and extensive distribution of sedimentation facies. The length of the subfacies reaches 50 to 150 kilometers, forming large-scale channel delta and enabling favorable reservoirs to be distributed steadily in large areas. These basins are favorable zones for development of large-scale lithological oil and gas areas.

Microbial oxidation of hydrocarbon gases can be divided into aerobic type and anaerobic type. Both of them can result in the enrichment of δ¹³C values of alkane gases. Some of the gaseous hydrocarbon components show priority during microbial oxidation. In general, propane and n-alkanes are usually preferentially oxidized as compared to ethane and iso-alkanes, respectively. And this has been confirmed by thermodynamics and culture experiments in anaerobic environment. Microbial oxidation can lead to partially reversed δ¹³C order, while component order does not reverse necessarily. But in some oxidation environments, ethane can be oxidized prior to propane. This means that the oxidation mechanism of aerobic oxidation is different from that of anaerobic oxidation or different type of microorganisms may lead to the change of priority during microbial oxidation.

Most of the tight sandstone and carbonate rock gas reservoirs are mainly low-porosity and low-permeability reservoirs with a variety of complex porous genesis and types. The porosity and permeability have an important influence on the seismic elastic modulus and velocity, making gas detection very difficult. To focus on the low porosity and low permeability complex pore media, the integrated evaluation of porous structure and seismic rock physical analysis can be used to evaluate how the complexity of porous structure and heterogeneity of porous fluid distribution influence seismic wave velocity, thus unveiling the law that seismic wave velocity changes with the physical properties and fluid properties. The White model is modified to represents the relationship between compressional wave velocity and gas saturation and establish the quantitative prediction template of gas saturation of low-porosity and low-permeability reservoir in order to promote seismic quantitative interpretation technological research and development. Good results were achieved when the research results were used to make seismic quantitative prediction of gas saturation in the tight sandstone gas reservoir of Xujiahe Formation in Sichuan Basin, SW China.

The shale reservoir is an important unconventional oil and gas reserve, while well logging evaluation techniques plays a significant role in oil and gas exploration and development. As for the non-structural fracture shale reservoir, the current existing conventional well logging evaluation technology both at home and abroad cannot meet the demand of exploration and development for such a reservoir. This article takes shale reservoir of the Qingshankou Formation in Gulong Sag of Songliao Basin for instance, aiming to meet the need for exploration and engineering transformation. Guided by the unconventional ideas, the logging quantitative evaluation method is studied on the basis of the quality of source rock, mineral components, formation pressure, rock brittleness and fractural identification. Then, three characterization parameters – organic carbon content of shale, formation pressure factor and rock brittleness – are selected and optimized to establish the effective logging identification and quantitative evaluation method for shale reservoirs, finally creating the systematic logging evaluation technology for shale reservoirs. This technology fills a domestic blank in logging evaluation of unconventional shale reservoirs and provides an important technological support for China’s exploration and development of shale reservoirs.

At present the precision of migration velocity model is the key problem for the application effect of reverse time migration (RTM). Based on this, we present RTM-based migration velocity analysis method in the angle-domain in this paper. First, we develop the research of wavefront polarization method to calculate RTM-based angle domain common-image gathers. This method has definite physical meaning and high computational efficiency. Compared with conventional offset domain and shot domain common-image gathers, angle domain commonimage gathers could effectively solve multi-path problem, reduce migration artifacts and improve imaging quality of gathers. Second, we derive new angle-domain migration velocity update equation and have formed RTM-based migration velocity analysis method in the angledomain. The numerical test results of model data show the correctness of the method in this paper.

Interval multiple is one of noise which is quite difficult to suppress in seismic data processing due to its irregular reflection. It appears as artifact in migration which affects the seismic processing quality. In marine seismic data processing, sea bottom related interval multiples exist widely because the strong impedance difference at sea bottom leads to sea bottom related interval multiple especially under the sea bottom where high velocity layer or scattering body exists. In order to solve this problem, a method for prediction of sea bottom related interval multiple prediction is proposed, which is based on wave equation. Thanks to the clear sea bottom reflection, sea bottom related interval multiple was predicted by convolution and cross-correlation with sea bottom reflection and those wave-field reflected under sea bottom. This is a data-driven multiple prediction method which overcomes the disadvantage of velocity dependence and has higher computation efficiency. Simple model test shows its validity. Finally, based on Sigsbee2B model test, sea bottom related interval multiple is reasonably predicted, confirming the adaptability of this method to complex substructure.

Nano-scale and micro-scale pore throats dominate the unconventional tight oil and gas reservoirs. Pore-throat microscopic structure is a key factor to lead to the low-permeability and low-porosity reservoir features. It is also the foundation to establish the criteria for evaluation of unconventional oil and gas reservoirs and interpret the basic geologic features, such as hydrocarbon occurrence and accumulation. Currently, microscopic pore-throat characterization techniques for unconventional tight oil and gas reservoir are based on nanoscale material science, physical chemistry and analytical chemistry. Initial progress has been made in the study of pore size, morphology, distribution and 3D connection by means of direct image observation under field-emission scanning microscope, indirect numerical value measurement such as gas absorption, and 3D value reconstruction modeling pore structure by X-CT, improving the characterization accuracy of nano-scale microscopic pore-throat structure. However, more efforts are required to improve pore-throat structural characterization techniques in the areas of principle testing, integration of multi-data, combination of multi-techniques and pre-experiment disposition, and multi-scale feature characterization. This study will provide the data for precise evaluation of favorable tight reservoir, tight oil and gas zones and other unconventional hydrocarbon sweet spots.

Based on the key accumulation conditions, such as hydrocarbon source rock, the palaeo-temporary structures and gas migration, as well as the spatial-temporal relationship, there are four important favorable accumulation conditions for coal-type gas in Qaidam Basin. The Jurassic source rock is widely distributed with co-existence of a multiple of depressions. The potential reserve is re-evaluated at 13100×10⁸m³, providing a material basis for large-scale accumulation of coal-type gas. The secondary structures developed in the basin during the key accumulation period, such as ^“basin peripheral palaeo-slope, nose uplift and depression belt^”. The basin peripheral zone is the directive zone for long-term hydrocarbon migration, forming a good background for accumulation. The basement weathering crust, whose reservoir space is based on dual media – fractures an pores, is extensively distributed, free from of influence of buried depth. The hydrocarbon migration and accumulation are in the pattern of ^“vertical migration and accumulation from source-controlled faults and lateral transportation on unconformity surface^”.helpful to a large-scale accumulation of natural gas. The study of hydrocarbon accumulation conditions and abundance law provides a theoretical basis for coal-type gas exploration and develops the new theories on the reservoirs controlled by large-scale basin peripheral palaeo-slope structures. It is proposed that the new important exploration areas should be located above and below the basin peripheral unconformity surface. The Altyn Tagh foothill palaeo-nose uplift and slope should be highlighted as the new belt and zone for exploration. This theory leads to remarkable results in oil and gas exploration. The newly additional controlled and proven natural gas reserves have reached 1104×10⁸m³ in recent years, making new breakthroughs in coal-type gas exploration in Qaidam Basin.

CBM Adsorption/Desorption data are the key parameters to appraise CBM resources and productivity. The current conventional Isotherm adsorption experiment can only provide the relationship between adsorption volume and pressure at a fixed temperature set at the beginning of experiment. How to get the real adsorption/desorption data in the in-situ condition become a challenge for the CBM laboratory. To overcome the bottlenecks existing in isothermal CBM adsorption testing which currently adopts the US5058442/4528550 patent and IS-300 insitu absorption instrument, such as small testing scope, damage of sample^’s original structure, and free from restraint by surrounding pressure, Langfang CBM laboratory created a new CBM adsorption/desorption simulation device that overcomes the shortcoming that the current in-situ adsorption testing does not take into account the influence of stress and cannot reflect the real adsorption status of the formation. Particularly, the method for in-situ adsorption testing of non-adsorptive gas provides the parameters for calculation of variable volume free space. Successful development of this instrument localizes the isothermal adsorption apparatus and creates the in-situ CBM adsorption simulation under the high-temperature condition. The research indicates that the effect of high temperature on CBM adsorption capability is higher than that of pressure under the conditions of high temperature and pressure (>85℃ , >30MPa), thus confirming that a ^“critical depth^”exists in the relationship between CBM content and buried depth. Namely the CBM content in the shallow layer increases with the rise of buried depth and reaches the maximum value at a certain buried depth. Then the CBM content decreases with the further rise of buried depth after exceeding the certain depth.