Oil-rich sags in fault basins in East China, have favorable conditions for hydrocarbon accumulation. Over 40 years of extensive exploration, these sags can be classified as matured areas with high degree of exploration. The subtler exploration targets become more and more difficult to be identified and discovered. However, since these sags still contain large quantities of remaining resources, they are still promising zones for revealing large-scale oil and gas reserves. Based on existing resources, a series of delicate and innovative exploration practices including detailed studies, fine description, intensive searching, meticulous operations, careful selection and innovative management have been developed to enhance geologic understanding, to verify exploration targets, to forecast the hydrocarbon potential, and to achieve high-efficiency management over relevant exploration operations in matured areas. In this paper, these exploration practices are presented with the Niudong ultra-deep and ultra-high temperature buried hill oil reservoir in the Baxian sag, the stratigraphic lithologic oil reservoir in weak structural belt of the Lixian slope in the Raoyang sag, the Daliuquan complex structural reservoir in the Langgu sag, and the reservoir in the Aer sag (a newly discovered sag in the Erlian Basin) as examples. It is concluded that these delicate and innovative exploration practices provide valuable guidances for further exploration and continuous reserve enhancement within matured areas in East China.

Seismic technology is the core for petroleum exploration, and it is gradually extended to petroleum appraisal and development stages. In the petroleum exploration, appraisal and development stages, when the geologic tasks, understandings and available data are different, the requirements for seismic technology are varying. Accordingly, it is necessary to clarify key issues in application of seismic technology during different stages, so as to ensure the appropriateness of the technology. During the exploration stage with limited data available for the concerned area, prospects, specific target and geologic conditions, seismic technology is indispensible. During the appraisal stage with clarified constraints, seismic technology may be deployed to determine favorable lithologies, reservoir properties and oil/gas-bearing potential, with much higher accuracy required than that in the exploration stage. During the development stage, seismic technology is mainly used to highlight the detailed features of specific reservoirs and oil pools; though geologists are less dependent on seismic technology, the technology can still be extensively deployed.

Reefs are greatly developed in Cenozoic petroliferous basins in the South China Sea and its surrounding area. According to the collected data on regional geology and reef reservoirs, a comprehensive analysis was made on the development regularity of Cenozoic reefs. The results indicate that the Cenozoic reefs in the South China Sea were built from coralgals, and they are dominantly infralittoral reefs. In the northern and southern parts of South China Sea, there are diverse reefs, including tower reef, platform-edge reef and massive reef. In the western part, platform-edge reef is typical. In the eastern part, tower reef is dominant. Reefs in the South China Sea were developed earlier in the north than in the south across the region, and earlier in the east than the west within the same tectonic belt. All reefs reside on the secondary positive tectonic belts in basins. The reefs were mainly formed during the Miocene, consisting of three main periods, i.e. Early Miocene, Middle Miocene and Late Miocene. They grew with the sea level rising, and exposed after a transitory regression. Hydrocarbon-rich sags spread widely in the South China Sea, reef reservoirs provide excellent preservation conditions, and thick marine mudstone serves as regional effective seal. Moreover, fault surfaces and unconformities act as main pathways for vertical hydrocarbon migration, and overpressure offers a driving force for hydrocarbon migration. All these conditions suggest a bright exploration prospect for the Cenozoic reef reservoirs in the South China Sea.

In the Northwest Sichuan Basin, there is a great variety of structural patterns with complicated structural characteristics formed as a result of structural movements during multiple stages, which are significant for petroleum exploration. In this paper, the seismic, logging and outcrop data of the Northwest Sichuan Basin were analyzed comprehensively. The results show that the major structural patterns include superposed basement thrust and sliding-nappe structure, imbricate fan thrust fault, fault-related fold, back-thrust fault and anticline. Considering the time that the structural patterns were formed, three structural deformation stages can be identified, i.e. the thrust-nappe stage during the Indosinian period, the thrust-nappe stage during the Yanshanian period, and the thrust-nappe–sliding-nappe stage during the Himalayan period. Among these structural patterns, the fault-related fold and high & steep anticline may form structural and lithologic traps, and they are targets in current exploration. The structural triangle belt composed by back-thrust faults may also form structural and lithologic traps, serving as potential favorable exploration targets.

There are different understandings on the hydrocarbon accumulation patterns of the Ordovician, Devonian and Carboniferous reservoirs in the Maigaiti slope, the Southwest Tarim depression. Based on the analysis of structure and evolution features of the Southwest Tarim depression and the Bachu uplift, and considering the reservoir development and distribution characteristics of source rocks, it is found that oil and gas are mainly distributed in the fault zone, and oil and gas properties are different in the structural traps formed during different periods. For example, the Bashituopu fault zone and the Yubei fault zone were formed in the late Hercynian contain oil while the Selibuya fault zone and the Mazhatage fault zone were formed in the Himalayan are rich with gas, suggesting the distribution feature as oil in the south and gas in the north. According to the study results, the hydrocarbon accumulation pattern of the Carboniferous and Devonian systems in the western Maigaiti slope is the same as that of the Carboniferous and Ordovician systems in the eastern Maigaiti slope. Moreover, hydrocarbons came from the Cambrian source rocks. Hydrocarbons accumulated in large scale in the Cambrian–Ordovician during the early Hercynian. During the late Himalayan, the Bachu uplift rose, and the oil pools formed earlier reached the conditions for pyrolysis, and oil and gas vertically migrated, forming the Yasongdi and Hetianhe gas fields. The Cambrian pre-salt dolomite formation is the major target for future exploration.

Architecture of multiphase narrow-channel sand bodies has always been one of the subjects to be urgently defined in development of the Bohai oilfield. Taking the BZ25-1S oilfield as an example, guided by the theory of sand body architecture, available data (e.g. drilling, well logging, seismic and outcrop) were used to ascertain the geologic model of shallow water deltaic sand bodies, and the fine stratigraphic framework was established. By applying three types of single well correlation models (i.e. discontinuous distributary interchannel sandbody, elevation difference of the distributary channel sandbody tops, and difference in scale of distributary channel sandbody deposits) on drilling profiles, together with some seismic response features (e.g. wave amplitude variation and complex wave occurrence), well data and seismic data were combined to analyze the reservoir architecture of single-phase channel sand bodies, and the single-phase channel sand bodies were classified. The study results indicate that the thicknesses of single-phase narrow-channel sand bodies have logarithmic correlation with the widths of these sand bodies, with average width/thickness ratio of 35. The widths of single-phase narrow-channel sand bodies are 200-400 m, and they can extend across the whole study area. These study results have been adopted as effective guidance for oilfield water injection optimization, liquid producing structure adjustment and well pattern arrangement, laying a solid geologic foundation for oilfield development.

Brittleness of tight sandstone is a key element for exploration and development of tight reservoirs. Taking the Longmenshan Foreland Basin as an example, microscopic identification of sandstone composition and mineral content analysis with X-ray diffraction were carried out with the geological data, e.g. outcrop rock samples, drilling cores, rock slices, and geochemical characteristics. It was demonstrated that the sandstone has higher debris content. Accordingly, a rock brittleness formula corresponding to the regional setting was established. The research results show that the tight sandstone reservoirs in the Longmenshan Foreland Basin are more brittle. According to the analysis of rock mechanics features, the reservoir rocks in the study area are characterized by high Young's modulus and low Poisson's ratio, with better brittle behavior and fracturing potential. Finally, the correlation between geochemical characteristic index and brittleness of sandstones was analyzed, and the relationship between index of compositional variability (ICV) and chemical index of alteration (CIA) of tight sandstones were discussed.

The pebbled sandstone section is the major oil-producing interval of Carboniferous in the Central Tarim 16 reservoir. Due to small thickness and uncertain superposition pattern, sub-layer division and correlation there cannot come to an agreement after production, which has seriously affected the development programming and remaining oil potential tapping. In order to dissect the stratum superposition pattern of the pebbled sandstone section and the sub-layer distribution characteristics thereof, sub-layer division and correlation were conducted for the interval by integrating the geology and well logging data and newly processed 3D seismic data, and using the method of model guidance, cycle contrast, dynamic verification and regional closure. The results indicate that the pebbled sandstone section has the stratum superposition pattern of denudation after overlapping deposition. On the basis of comprehensive analysis of lithologic, cyclic and electrical properties of the sub-layers, and combining the marker beds for stratum correlation, the pebbled sandstone section was divided into five sub-layers in two sand groups from lower to upper. By analyzing the stratum correlation methods for different stratum superposition patterns, the “oblique correlation” method suitable for the study area was selected for correlation of sub-layers in the study area. The reliability of the division result was further validated by the existing dynamic production data. Moreover, the plane distribution scope of all five sub-layers were analyzed. It is found that the distribution scope decreases from lower to upper.

The Oulituozi area, an important resource replacement area for an annual light oil production of 500?04 t in the Liaohe oilfield, contains original oil in place (OOIP) of 3000?04 t. By the interpretation of newly acquired and processed 3D seismic data and drilling/logging data and the study of structural development history, the structural characteristics and genesis of the Cenozoic Paleogene formations in the Oulituozi area were analyzed, and their controlling effects on oil and gas migration and accumulation were investigated, in order to identify the favorable targets for future exploration. Following conclusions are drawn based on the study. First, the structural framework of the Oulituozi area was shaped in the Paleogene, when it sequentially evolved through four stages, i.e. arching, rifting, attenuating, and re-rifting, and underwent two major structural movements, i.e. extensional movement in the third member of Shahejie Formation (Es3) and strike-slip movement in the Dongying Formation (Ed). Second, the present structural framework in the study area is the superposition of the extensional system and the strike-slip system, and the structural evolution difference between the north and the south led to the unique flower structure with alternate normal and reverse faults and also the conversion of fault property. Third, major fault demonstrated a sealing performance during the oil and gas migration, and its barrier effect was unfavorable to the secondary migration of oil and gas. Fourth, intense structural movements significantly improved the reservoir physical properties. Fifth, the structural high in the flower structure and the fault-nose attached to the major fault are the most favorable zones for oil and gas accumulation, and stratigraphic oil reservoirs are developed within the overlap zones in the west.

Based on core, wireline logging, mud logging, organic geochemical and production testing data, the main controlling factors of hydrocarbon accumulaiton in the Chang 81 reservoir of Yanchang Formation in the Honghe Oilfield were analyzed from the aspects of source rock, reservoir, cap rock and source-reservoir assemblage. The results show that lithologic oil reservoirs are dominant in the study area. The principal source rock is acted by the Zhangjiatan shale at the bottom of Chang 7 formation, where Type I organic matter is predominant with TOC of about 15%. This widely distributed source rock provides both material basis and migration dynamics for the formation of oil reservoirs. The Chang 8₁ reservoir in the study area is of low porosity and ultra-low permeability, and its physical properties are controlled jointly by sedimentation, diagenesis and micro-fracture reworking, which affect its oil-bearing potential accordingly. The thick mudstones in the mid-upper part of Chang 7 formation act as the physical and overpressure sealing cap rocks, providing good preservation conditions. The Zhangjiatan shale at the bottom of Chang 7 formation is in extensively direct contact with the Chang 8₁ reservoir, thus hydrocarbons are supplied in the shape of plane, which is the key to the hydrocarbon accumulation in the Chang 81 reservoir.

The western Hunan Province has experienced two periods of large-scale formation and evolution of foreland basin, i.e. the Nanhua-late Silurian when the marine foreland basin was formed, and the Devonian-Early Cretaceous when the continent foreland basin was formed. As for the sedimentary environment evolution model, the marine foreland basin was formed and evolved through the stages of continental rift clastic rocks, passive continental margin clastic rocks continental shelf–carbonate continental shelf–carbonate platform, early marine flysch in foreland basin, and late marine molasse in foreland basin, with the stage of late continental molasse in foreland basin missing; the stage of late marine molasse in foreland basin is transient, while the stage of early marine flysch in foreland basin is long. The continental foreland basin was formed and evolved through the stages of clastic rock of continental rift extension, carbonate platform–platform basin in passive continental margin, early marine flysch in foreland basin, and late continental molasse in foreland basin, with the stage of late marine molasse in foreland basin missing; the stage of early marine molasse in foreland basin is transient, while the stage of late continental molasse in foreland basin is long. The differences between the two periods are subject to the palaeo-geographical location during tectonic movements. During the former period, the formation of marine foreland basin was controlled by the formation, development and extinction of the Huanan Ocean at the southeast margin, presenting as a marine foreland basin near the passive continental margin. During the later period, the formation of continental foreland basin was controlled by to the formation, development and extinction of the Mianlue Ocean in the north, presenting as a continental foreland basin distant to the passive continental margin. In western Hunan Province, the formation and evolution of the complex superimposed foreland basin reflects the tectonic and sedimentary evolution history of the entire Yangtze platform margin, corresponding to two Wilson cycles.

Based on the previous studies of plate evolution and the analysis of regional geological characteristics in the North Seram Basin, Indonesia, the basin evolution was divided into four stages, i.e. the Early Triassic initial rifting, the Middle Triassic–Middle Jurassic rifting, the Late Jurassic–Middle Miocene passive continental margin, and the Late Miocene–Quaternary thrusting. Then, the control of tectonic evolution during different stages over hydrocarbon accumulation conditions was discussed in order to provide references for the exploration in the Banda Arc area. Affected by the Mesozoic tectonic activities, the North Seram Basin experienced the sedimentary evolution from carbonate ramp to platform during the rifting stage, and major source rock in the Upper Triassic–Middle Jurassic Saman Saman Formation was developed under the control of the paleotectonic framework to distribute in the restricted platform during the rifting stage. The favorable Manusela Formation carbonate reservoirs developed during this stage on the highs in the south of the basin are the primary exploration targets. The tectonic activities during the thrusting stage led to the differences in thermal evolution of source rocks, and influenced the trap types and their distribution in tectonic belts. Moreover, the reactivated faults and micro-fractures improved the physical properties of the carbonate reservoirs, and constituted the main hydrocarbon migration pathways together with the Pliocene unconformity.

In the Kuqa foreland basin, the buried anticlines are the primary exploration targets, and they are usually discovered and ascertained with seismic exploration technologies. Throughout the oil and gas exploration history of the Kuqa foreland basin, the development of seismic exploration technologies under went four stages: 1) seismic crooked line survey – poststack time migration; 2) seismic straight line survey – poststack/prestack time migration; 3) seismic wide line survey – prestack time/depth migration; and 4) 3D seismic wide azimuth survey – anisotropic prestack depth migration. Thanks to new seismic exploration technologies and methods, the signal to noise ratio and migration imaging accuracy of the seismic data have been improved gradually, and numerous buried subsalt anticline traps in the Cretaceous system were discovered and ascertained. Most of these traps are distributed in the Dabei-Keshen area, and they are mainly large and medium gas fields, forming gas field clusters in the buried anticlines. It is shown that the seismic exploration technologies play a positive role in solving the four problems constraining the seismic exploration in the Kuqa foreland basin, namely, low S/N ratio in diluvial fans, serious static correction problem derived from the rugged topography, poor imaging quality of subsalt structures, and velocity pitfall caused by gravel layers. As the development of buried anticlines in the Kuqa foreland basin proceeds into the middle and late stages, the development is becoming more and more difficult, thus finer seismic exploration technologies are required. Especially, the single point, high density and wide azimuth 3D seismic exploration technology and anisotropic prestack depth migration technique will be the top concerns.

The Chang-6 reservoir in the Jiyuan area, the Ordos Basin, is a typical ultra-low permeability reservoir, for which the fluid identification is a challenging task due to its strong heterogeneity and less oil-water differentiation. In this paper, based on the coincidence of three porosity curves, i.e. compensated neutron log (CNL), compensated density log (CDL) and acoustic time (AC), the porosity curve indexes are constructed to reflect the lithologic purity. By combining the deep inductive resistivity (DIR) log reflecting the fluid properties and the AC log describing the reservoir properties, the fluid identification indexes are constructed to determine the oil-bearing potential of the reservoir. Then, oil-water layer and water layer are further identified in accordance with the relationship between water productivity and water saturation. Eventually, the oil/water identification chart is prepared by integrating the fluid identification indexes and the water productivity. The logging data of 58 wells in the study area were interpreted and compared with the well testing data. The results show that the proposed method can achieve an accuracy of 91.4% in fluid identification, suggesting an enhanced accuracy of fluid identification in reservoirs.

The HJB 3D area is characterized by large surface relief and significant changes in velocities and thicknesses of low-velocity zones. The 3D seismic data acquired in the area have low signal-to-noise ratio (SNR) and severe static correction problems. Consequently, the images derived from such data are poor in quality, making the wave group features of the target formation impossibly be tracked continuously. For static correction of the 3D seismic data, the conventional residual static correction techniques cannot effectively resolve the above-mentioned problems. After researches and tests, this paper presents an improved residual static correction technique, which covers the deficiencies of some algorithms for conventional residual static correction techniques. The improved technique has been applied in the processing of HJB 3D seismic data, with outstanding imaging performance.

As an important parameter as well as difficult problem in shale exploration, gas content of the reservoir determines whether the reservoir is economical for shale gas development. Field desorption is a direct and common method. Enhancement of absorbed gas measurement accuracy is critical for accurate evaluation of shale gas content. Conventional field desorption devices are less accurate and less automatic. Under such circumstance, a rock desorbed gas determinator has been independently developed. In this paper, the principles, measurement process and calculation of shale gas content of the determinator were presented, and it was verified that the determinator can produce reliable analysis results. The rock desorbed gas determinator was successfully deployed in Well CY1 in 2014, thereby providing valuable reference for identification of “low-risk zones”.