The central paleo-uplift belt (north) of the Songliao Basin, as one major region of risk exploration of natural gas, has favorable conditions for natural gas accumulation. However, no breakthrough has been made in this region for decades, and the hydrocarbon accumulation conditions and exploration perspective have not been thoroughly studied. This paper analyzes natural gas accumulation conditions and exploration targets in the central paleo-uplift belt (north) using core, mud logging, well logging, 3D seismic, gravity and magnetic data as well as a great deal of statistics, analysis and geology studies, and proposes new four cognitions:(1) the central paleo-uplift belt has features of buried hill and was formed on the thrust folded base; (2) the source rock has a high organic matter abundance, huge gas generating potentials and strong lateral gas supplies in the Shahezi Formation of the Xujiaweizi rift in the east of the central paleo-uplift belt (north); (3) the central paleo-uplift belt (north)contains four types of reservoirs:granite, metamorphite, conglomerate and sandstone; (4) the central paleo-uplift belt (north)contains three types of traps, two sets of conductive systems and three types of gas pools. Based on the results, the exploration targets of the basement buried hill gas pools and the gully lithologic gas pools have been selected, 10 class I and 6 class Ⅱ exploration regions have been evaluated and optimized. This provides the scientific base for risk exploration of deep gas reservoirs in the central paleo-uplift belt (north), the Songliao Basin.

Eastern Yanchang oilfields, production using conventional wells(which have already been more)caused problems of low productivity and low efficiency in shallow,low-pressure,low-permeability and ultra-low permeability reservoirs. However, it is difficult to drill horizontal wells in such reservoirs (at low temperature, low pressure and shallow depth). To solve these problems, supporting techniques for drilling and completion of extended-reach horizontal wells were studied and applied in shallow and ultra-shallow layers. The innovative combination of "arcual" horizontal well with shallow fracture network in shallow and low-pressure reservoirs where horizontal fractures are easily induced by fracturing stimulation increased more producing reserves by at least 30% than conventional drilling method in a horizontal section, obtaining effective production of reservoirs with low vertical stress. The collaborative optimization design of well trajectory and well profile, low-cost pressuring, casing optimization, increase of the axial pressure and transmission efficiency of drilling string were applied and created successful construction of many horizontal wells in very complicated conditions. This study provides solid technical support for improving the development of shallow and ultra-low permeability reservoirs in eastern Yanchang oilfields with more economic and social benefits.

In-depth analysis of remaining resources and exploration degree in the Raoyang sag indicated sustainable exploration potential, and made clear future key exploration domains and directionsin the low-lying zone, slope zone and subtle buried hill, and pointed out potential issues in further exploration, such as scattered seismic data, unclear structural relationship and distribution of effective reservoirs, unidentified controlling factors and models of hydrocarbon accumulation. In order to discover more resources and high-quality large-scale reserves, and further increase resource conversion ratio, principles focusing on "overall understanding, comprehensive evaluation and integral deployment" for secondary exploration in the Raoyang sag were put forward, namely,to establish an overall 3D seismic data platform of the whole sag, to rebuild structure model, sedimentary model and reservoir geological model of the sag, to quantify the spatial distribution of hydrocarbon resources, to build multi-field hydrocarbon accumulation models, and to carry out integral and fine exploration in optimized key areas and targets. Following these principles, since 2005, a series of important discoveries have been made and constructed 200 million tons of large-scale economic reserves area in the Raoyang sag. This paper analyzes and summarizes the methods, techniques and experiences of the secondary exploration in the Raoyang sag, which may provide reference to and enlightenment on further exploration in other similar sags.

The deep coalbed methane (CBM) is a new research subject in current CBM exploration and development in China. Based on the isothermal adsorption experiments, the adsorption effect of simulated deep coal beds under varying temperature and pressure conditions was analyzed. Then, the critical depth for gas-bearing of the Carboniferous-Permian coal beds in the Jiyang depression was determined, and the adsorption capacity of deep coal beds in this depression was predicted through regression analysis. It is revealed that the gas-bearing property of the coal beds are controlled jointly by temperature, pressure and coal rank in deep environment (>2000 m). The study results indicate that the critical depth for gas-bearing of the deep coal beds in the Jiyang depression ranges from 800 m to 1200 m, below which the adsorption capacity decreases as the depth increases, due to the covariant effect between temperature and pressure. According to the predicted results of the adsorption capacity of deep coal beds by the Langmuir model, the high-rank coals at depth of 4000 m still have higher adsorption capacity (12.29 cm³/g as predicted), while the low-rank coals only have adsorption capacity of 1.83 cm³/g. Due to thermal effect at high temperatures in deep environment, coal rank increases with increasing burial depth. The adsorption capacity of deep coal beds occurs transition at depth of 3000-3500 m due to three effects of temperature, pressure and coal rank, larger than which the adsorption capacity increases with increasing burial depth.

Based on 319 measured analysis data of formation water in Chang 6 pay zone of Yanchang Formation in the Jiyuan area of the Ordos Basin, the features (e.g. type, chemical composition, ion ratio, and salinity) and planar distribution rule of formation water were investigated, and the implications of such features for hydrocarbon accumulation were revealed. Salinity of formation water of Chang 6 pay zone is overall higher in Mesozoic strata, where the formation water shows higher salinity, recording as CaCl₂-V and IV type with major ions of Cl^- and K⁺+Na⁺, and the average values of Na⁺/Cl^- coefficient, desulphidation coefficient and metamorphism coefficient range in 0.49-0.53, 5.8-7.2, and 40-51, respectively. The overall stratigraphic enclosed environment is fine. The current formation water in Chang 6 pay zone is in a closed fluid dynamic environment, with the features varying vertically and horizontally. In Chang 6₁ at the top of Chang 6 pay zone, the salinity and metamorphism coefficient of formation water in the western and eastern parts are generally higher than that in the central part, and the Na⁺/Cl^- coefficient and desulphidation coefficient of formation water in the western and eastern parts are generally lower than that in the central part. This indicates that the sealing capacity of the fluid dynamic environment in the western and eastern parts are better than that in the central part. It coincides with the fact that there are more oil reservoirs discovered in the western and eastern parts than that in the central part. The present fluid dynamic environment reflected by the formation water can effectively represent the accumulation and preservation degree of oil reservoirs, and the regions with weaker present fluid dynamic environment are more favorable for oil reservoir preservation.

Due to the poor quality of seismic data, traditional seismic data interpretation methods cannot provide complete understanding on the distribution thickness and plane characteristics of Jurassic at the northern margin of the Qaidam Basin. In this paper, a new structure interpretation model for Jurassic faulted depression + depression was established after the structural evolution of the northern margin in the Early and Middle Jurassic was analyzed. By virtue of this model, a series of new Jurassic sags have been discovered, including Hongsanhan, Jiandingshan and Changweiliang sags. On the basis of section morphology and structure characteristics, the Jurassic sags strongly reworked are classified into 5 types, i.e., incompetent bed dominated strongly deformed sag, competent bed dominated strongly deformed sag, basement fracture dominated strongly deformed sag, compaction-flexture weakly deformed sag, and vertically uplifted weakly deformed sag. The strata of faulted depression stage are mainly controlled by the discordogenic fault in the basement at the northern margin of the Qaidam Basin, and the strata of depression stage are distributed in the whole area. They both present obvious inheritance. The relationship between the stress field and the trending of present trunk fault in the basement was analyzed. It is indicated that the morphologic characteristics of present Jurassic sags are controlled by the barrier of Qilian Mountain and the left-lateral strike slip of Altun Mountain. The source rocks are more superior where the Early Jurassic faulted depression is located. Parallel faulted depression is favorable for the long-distance migration of oil and gas generated by the source rocks, resulting in great probability of formation of outside-source oil and gas reservoirs.

It is important to identify the stress sensitivity of tight oil reservoir and its influence factors, when the tight oil reservoir is developed by quasi-natural energies. Stress sensitivity experiment was conducted on the tight oil reservoir in Chang 7 Member of Yanchang Formation in the Ordos Basin under the reservoir pressure. It is indicated that the permeability of the Chang 7 tight oil reservoir is of medium to weak stress-sensitive. The petrology and micro-pore structure of the Chang 7 tight oil reservoirs with different types of stress sensitivity were analyzed and compared by the techniques of cast thin section, constant pressure mercury intrusion, constant-rate mercury intrusion, field emission scanning electron microscopy and Micro-CT, etc. The results suggest several influence factors on stress sensitivity, including the content of plastic components, types and contents of interstitial materials, and pore throat size. Through comprehensive analysis of the practices in the pilot development zone and the reservoir characteristics of Chang 7 tight oil reservoirs, it is found that the stress sensitivity affects greatly the oil production rate and the decline rate of horizontal wells due to sharp decrease of formation pressure in the stimulated intervals of Chang 7 reservoirs. Moreover, different development plans are prepared for the Chang 7 tight oil reservoirs depending on provenances, origins and physical properties, so as to mitigate the influence of stress sensitivity on productivity.

Four third-order sequence boundaries are identified and three third-order sequences(SQ7, SQ8, SQ9) are divided by high-resolution sequence stratigraphy theory and method, new seismic data processing and interpretation combined with lithological-electrical response characteristics. Stratigraphic units of twelve fourth-order sequences or systems tracts are divided by therelatively stable transgressive surfaces and abrupt changes of lithology in the third-order sequences as the boundaries. Core facies and electrofacies are systematically analyzed and two sedimentary systems (fan delta and lacustrine) are identified based on provenance research in this study area. Moreover, the planar distribution law of each sedimentary system in high-resolution sequence stratigraphy framework is studied. Hereinto,the fan delta plain range is smaller near northern provenance entrance of the study area, but the fan delta front subfacies range is wider in the central part of the study area.

This paper studies the development characteristics and control factors of trachyte fractures developed in the underwater environment in the Oulituozi region, the Liaohe Basin, using cores, thin sections and imaging logging data etc., with the intent to analyze the formation mechanism of trachyte reservoirs for supporting oil and gas exploration and development. The study results show:(1) four types of fractures including blasting fractures, shrinkage fractures, tectonic fractures and dissolution fractures are developed in the trachyte; (2) blasting fractures, shrinkage fractures and dissolution fracturesare highly filled, making most of them ineffective; and tectonic fractures play the major role to reform the trachyte to reservoir; (3) tectonic location is an important factor that controls the development of fractures which almost occur in a well-developed fault zone, the frontal belt of the upthrown side of a reverse fault and near major faults; (4) the single-layer thickness is positively related to the fracture development, that is, the thicker the single layer is, the more developed fractures are in the trachyte.

In the past few years, new discoveries (Zhangjiaduo and Qu1 Block oil pools etc.) have been continuously obtained in exploring the Fu3 Member of the Qutang sub-sag in Haian sag, Subei Basin. which further presents favorable exploration prospect within this formation. By dissecting typical oil pools and analyzing hydrocarbon distribution features in the Fu3 Member of the Haian sag, it is realized that hydrocarbon accumulation in the Fu3 Member is mainly controlled by fine tectonic background, source-reservoir-caprock assemblage and abnormal pressure. Further studies indicate that:(1) the evolution difference of source rocks is crucial for hydrocarbon enrichment difference of the Fu3 Member in various sub-sags; (2)the oil-source faults with continuous long-term activities played important role for hydrocarbon accumulation in the Fu3 Member; (3) reasonable configuration between sand body distribution and geologic structure space was favorable for forming subtle traps (such as fault-lithology and sand updippinchout traps) in southern sub-sags. Based on the above, it has concluded that except for the Qutang sub-sag, the Fu3 Member in the regions near the Sunjiawa sub-sag, Fengbei sub-sag and Fuan sub-sag in the north have relatively favorable hydrocarbon accumulation conditions, and the major exploration target is tectonic oil pools; the Xinjie sub-sag and Haibei sub-sag in the south have lower sand ratios, being favorable regions for exploring lithological oil pools.

Petroleum exploration in sub-salt layers has always been the focus all over the world. By comprehensive study on the geology of the sub-salt layers of the Gabon Basin, and analogy with the basins at the passive continental margin in South Atlantic in terms of the dynamic deformation mechanism of the lithosphere during rifting period, it indicates that the Gabon Basin is a wide rift basin with large rift development scale and petroleum exploration potential. Analysis of typical reservoir discoveries in the Gabon Basin indicates that (1) the sub-salt layers have superior petroleum geology conditions because there were abundant lacustrine source rocks during rifting period, with big hydrocarbon source potential and sufficient hydrocarbon supply; (2) the sub-salt layers have two sets (upper and lower) of reservoir-caprock assemblages, and hydrocarbons are mainly accumulated in the sandstone reservoirs in the Gamba Formation and the Dentale Formation in fluvial-delta facies in the upper assemblage, with fine reservoir properties; (3) the development of sub-salt lacustrine source rocks and the formation of traps are key factors controlling hydrocarbon accumulation in sub-salt layers in deep water regions. The comprehensive study reaches the conclusion that the sub-salt layers of the Gabon Basin have the potential of developing large-scale oil and gas fields, thus they are the target area for oversea petroleum exploration.

The coherence technique can objectively describe the plane distribution of the fault system developed in a special period. This paper introduces how the coherence technique is used to analyze the faulting characteristics within the 3D study area at the south margin of the Baiyun sag, the Pearl River Mouth Basin, in order to further understand the structural features of the Baiyun sag. The analysis results indicate that the Lower Oligocene Enping Formation top (T₇⁰) is a regional unconformity, below which NE faulting activity was dominant and stopped in the Early Oligocene, and above which NW fault activity was dominant and kept active from the Eocene to the Miocene. The faulting stage of the Baiyun sag ended in the early Oligocene, and then the depression characteristics was kept for a long term. The faulting and structural evolution within the study area reveals that the Lower Oligocene Enping Formation top (T₇⁰) is the fault-depression transition interface in the Baiyun sag, which is significant for sedimentary evolution and hydrocarbon accumulation within the deepwater area in the sag.

Natural gas hydrate (NGH), with huge reserves, is a type of highly efficient and clean energy producing hardly any burning waste. In order to work out a practical NGH exploitation scheme, the accumulation features of NGH were identified, and the exploitation methods tested in laboratory were compared. Finally, a well-factory NGH exploitation scheme was proposed on the basis of the experiment of CH₄ displacement by CO₂ emulsion and the ICP shale oil extraction technology, which takes possible formation damage into account. The study results reveal that the frozen wall and heating well pattern arrangement, as core processes of ICP technology, can effectively prevent heat loss and provide heat consistently for NGH decomposition. The CO₂ emulsion replacement technology can generate CO₂ hydrate while effectively producing CH₄, so that the formation can be protected from damage due to NGH decomposition. The injection and production monitoring system can monitor the production process in a real-time manner, with high operability and precision.

Deliquification is the primary technique for stabilizing gas production and improving gas recovery in the gas field with water produced, and foam deliquification is the key subject of research for purpose of enhancing natural gas production and reducing cost. However, there is no systematic method to evaluate and compare the effects of foam deliquification in gas wells with different working conditions. This paper introduces a fuzzy quantitative evaluation method which provides evaluation results based on four indicators (i.e. daily gas production, daily water production, tubing-casing pressure difference, and daily cost of foam injection) and using the linear analysis method, the analytic hierarchy process and a fuzzy synthetic operator. Applied to a total of 30 gas wells producing by foam deliquificaton in Sulige Gas Field and Chongqing Division of PetroChina Southwest Oil and Gas Field Company, the method delivered comprehensive evaluation indicators for measuring the effect of foam deliquification in gas wells. Since this method incorporates all technical and economic factors that influence the results and benefits of foam deliquification, and considers the membership relation between evaluation indicators and the weight coefficient of every indicator, it can be used to comprehensively and quantitatively evaluate the effect of foam deliquificaton in gas wells with different conditions, and it is helpful to the selection of target wells and the best foam agent before deliquificaton.