Hydrocarbon enriched depressions in Bohai Bay Basin is rich in oil and gas resources. Lithological stratigraphic oil and gas reservoirs and subtle buried hill oil and gas reservoirs are the important exploration targets for discovery of large-scale reserves in the future. Guided by the new knowledge about hydrocarbon accumulation theory and based on advanced exploration technology, Huabei Oilfied Company has developed a new mindset of òsecondary explorationófor hydrocarbon enriched depressions. It also comes up with the secondary exploration method for hydrocarbon enriched depression, which is focused onòcomprehensive knowledge, comprehensive assessment and comprehensive deployment.óThe method includes construction of 3D seismic data platform for the whole depression, re-construction of geological models for the structure, sedimentation and reservoir of depression, re-description of quantitative characteristics of hydrocarbon distribution, creation of multi-area hydrocarbon accumulation models, and unfolding comprehensive preliminary exploration of multi-area targets. This method is of great realistic significance to exploration of hydrocarbon enriched depressions in Bohai Bay Basin and increase of large-scale reserve through sustainable exploration.

Langgu Depression is an old area with an exploration history of more than five decades. A number of large-scale research projects have been launched, which only found a batch of relatively small oil and gas reservoirs, unable to achieve the expected results. The conversion rate of resources remains at a low level and there is still a great potential for exploration. On the basis of the second 3D seismic acquisition and joint processing of the whole depression, a new round of research has been launched including analysis of the whole structure, study of the whole sedimentary sand body, dynamic simulation of the whole oil and gas accumulations and comprehensive assessment of the whole belt. The study defines six belts and four fields as favorable exploration orientations. Daliuquan nose structural belt is selected for multi-area exploration, such as complicated faulted blocks, shallow-layer natural gas and deep conglomerate. With important results achieved, the exploration efforts led to discovery of large-scale reserves. The exploration experience and methods are of certain help and guiding significance to exploration of other old areas.

Daliuquan structure is a nose structural belt re-defined on the secondary joint 3D seismic integrated platform of Langgu Depression. It has a number of structures and exploration fields, with favorable conditions for containing oil and gas. In recent two years, five exploration wells have been successfully drilled in Hupoying area thanks to a number of measures, such as detailed interpretation of structures, detailed projection of reservoirs, comprehensive detailed assessment, detailed verification of targets, and determination of accurate well locations as well as re-understanding of structural origins, sand body microfacies, accumulation model and exploration potential. A new mindset for exploration of Gu?an region has been created to turn from middle and deep layers to shallow layers, from partial area to the whole area, from oil reservoirs to gas reservoirs, and from determination of properties to determination of quantum for AVO. As a result, highyield natural gas reservoirs have been found in the shallow layers. A total of 13 wells were drilled in Daliuquan structural belt in accordance with the overall exploration mindset. Of those, 11 wells acquired industrial oil and gas flow, with the drilling success rate reaching 85 percent. A number of new discoveries have been made in exploration. The overall reserves exceeded 5000 × 104t, making exploration results remarkable. The success of detailed exploration in Daliuquan nose structural belt has certain guidance on some other old exploration areas.

Vertical continuous high-resolution logging information is used in logging assessment of hydrocarbon source rock to evaluate the content of organic carbon in mudstone of the formation. This will solve the bottleneck of identifying space distribution of regional hydrocarbon source rock owing to inadequate geochemical data and provide the basis for calculation of reserves and making decisions on oil and gas exploration. The study adopts Δ logR method and is combined with geological statistical analysis and logging information corrected under different geological conditions to assess hydrocarbon source rock. This method used to appraise 191 wells in Langgu Depression. Based on regional and stratigraphic sediment data, it make detailed description of space distribution of high-quality, medium-quality and poor hydrocarbon rock in different stratum series. In Langgu Depression, hydrocarbon source rock developed mainly in Es3 and Es4 with the thickness mainly reaching 700 meters to 3000 meters at the medium and low quality. High-quality hydrocarbon source rock is distributed in small area with short thickness, only 30 meters to 70 meters. High-quality source rock developed mainly in the lake invasion system. With the center of source rock changing, high-quality source rock of Es4 is mainly distributed in the Caojiawu-Yongqing zone while high-quality source rock of Es3 is distributed in Jiuzhou-Gu?an zone.

Langgu Sag is a gas enriched depression in the northern part of Jizhong Depression. No substantial progress has been made in the long-term exploration of this area. Currently, the discovered natural gas is mainly concentrated in the middle and shallow layers. To compare the geological conditions for natural gas accumulations in Baxian Sag with those in Langgu Sag, it is assumed that Langgu Sag also has an early lake-forming period with high-quality hydrocarbon source rock existing in Es4-Ek Formation. The desirable reservoirs developed in deep layers, indicating that this sag has the conditions for large-scale natural gas accumulations. The areas with most potential for natural gas exploration should be deep buried hill gas reservoirs and deep natural gas reservoirs surrounding the deep sag. Four orientations for exploration are revealed-shallow-layer structure, deep buried hill, deep conglomerate body and deep compact sandstone. The article elaborates that deep compact oil and gas is the orientation for exploration and study. It also makes a macro assessment of natural gas exploration potential in Langgu Sag.

Hupoying North anticline is located in the northeastern wing of Daliuquan Structure of Langgu Depression. It is at the junction of two formations with positive faults, which have the same direction but opposite dips. With faults developing in this region, the formation is extremely broken with interpretation of structure and level diversified. Exploration of this area has come to a standstill for many years. High-quality 3D seismic data has been acquired in recent years. Detailed analysis and interpretation have been made on the structure - a key geological problem in exploration of this area, with the structural stress effect model established. Based on in-depth analysis of structural origin mechanism, it is held that this area is controlled by the near east-west directional Jiuzhou fault and Hupoying fault and developed under the effect of stress fromthese two structures. The structural shape is a squeezed anticline, which is cut into pieces by a number of branch faults of Jiuzhou fault and Hupoying fault. Based on the analysis, this area belongs to òreverse stress transfer beltówith the core of the anticline at the junction of two formations that have the same direction but opposite dips. Based on such knowledge, the structure is interpreted in details and the fault systems are assembled reasonably to locate the shape of trap and provide the basis for drilling of exploration wells.

With exploration degree rising, lithological stratigraphic trape have become the main exploration targets in the Mesozoic and Cenozoic high-maturity basins. The study of sequential stratigraphy and sedimentary development of Langgu Depression is of great significance to further exploration of lithological stratigraphic traps in the depression. Based on joint 3D seismic data about Langgu Depression and guided by sequential stratigraphic theories, the Paleogene system of this depression is divided into 12 three-level sequences with each further divided into three area series of low level, high level and lake invasion. Influenced by structural movement, physical sources and climatic changes, a variety of infill patterns developed within the depression, forming various facies belts, such as fan delta, braided river delta, alluvial fan. The study points out that Daliuquan nose structural belt and the western belt of Hexiwu are favorable belts developed in the lithological stratigraphic traps in the depression. Es3 and the upper subsection of Es4 are favorable levels of the sand bodies in the depression, with development of braided river delta sand body of SQ3 low level system, near-bank underwater fan sand body of SQ6 low level system and fan delta sand body of SQ7 high level system, providing favorable reservoir space for lithological stratigraphic oil reservoirs

Steep slope belt of Langgu Depression has a large set of conglomerate body deposit developed along the declining disc of Daxing fault. From the south to the north, the deposit is divided Gu?an conglomerate group, Jiuzhou conglomerate group, and Caiyu-Fengheying conglomerate group. Drilling of Daxing conglomerate body has an active oil and gas show. It is an important exploration field. However, the physical properties of most conglomerate body reservoirs are poor with undesirable exploration results. Based on the differential analysis of Daxing fault in the areas of times of activity, style of structural activity, conditions of physical sources and oil sourcing window of conglomerate bodies, it is assumed that difference in times of activity for Daxing fault not only influences the scale and physical sources of conglomerate bodies but also the quality of conglomerate body reservoirs. The difference of Daxing fault tectonic style reflects conglomerates body deposits of different types. The lithology of mother rock and the difference in late diagenetic compactness determine distribution of high-quality conglomerate reservoirs. Conglomerate bodies and their contact surface with the surrounding source rock determine the differential in oil sourcing window in space and control the scale of conglomerate body reservoirs at the same time.

Langgu Depression is a Cenozoic remaining basin with strong uplift and denudation in the late period. Numerous natural gas reservoirs of different types have been found in the middle and shallow layers (with the buried depth less than 3000 meters). Based on the source rock conditions, natural gas is divided into four types of origin. Taking into consideration the geological conditions of different structural positions, it is assumed that the origins of gas reservoirs in the middle and shallow layers are mainly Es3 biological gas reservoirs developed under degradation from methane into bacteria as well as Es3 and Es4 low-maturity gas reservoirs. Based on analysis of development process of the reservoirs of these two types, it is considered that biological gas reservoirs are controlled mainly by distribution of gas source rock and most abundant in the area close to the trap of gas source rock. Low-maturity gas reservoirs are controlled by hydrocarbon source rock and distributed in the surrounding area, migrating along the transportation formations composed of faults and sandstone. Gas is abundant mainly in the reverse faults

Buried hill of Langgu Depression has a unique geological structural background. Three apparent structural belts are distributed in space-Daxing fault buried hill structural belt, Hexiwu fault buried hill structural belt and Fengheying-Houshangcun structural belt. The static elements of oil and gas accumulations in those deep buried hill are quite different in space allocation. Top reservoirs of Daxing fault buried hill structural belt are likely to be dolomite of Wumishan Formation of the Middle Proterozic Jixian System. Source rock and its cap formation are high-quality Es3-Es4 mudstone. The cross section and unconformity surface are their important carrier systems. The tops of Hexiwu fault buried hill structural belt and Fengheying-Houshangcun structural belt are Carboniferous- Permian sandstone reservoirs or Ordovician limestone reservoirs, with development of inner curtain carbonate rock reservoirs. Source rock of the former is mainly Ek formation with some Carboniferous-Permian and Es3 and Es4 source rock. The unconformity surface and cross section are the main oil and gas transportation channels. Source rock of the latter is quality Es3 and Es4 source rock. Tongbozhen cross section is the main vertical and lateral oil and gas transportation channels. Unconformity surface in the buried hill is of vital importance to oil and gas accumulation in the buried hill. The results of accumulation model indicate that the exploration prospect of those buried hill structural belts are quite different one another. Daxing buried hill structural belt is the top exploration target

Fengheying buried hill of Langgu Depression has an exploration history of more than four decades. It saw development of two types of hydrocarbon reservoirs - buried hill top and buried hill inner curtain. Exploration degree is high for buried hill top but low for buried hill inner curtain. Based on the study of buried hill?s development characteristics, oil source conditions, reservoir-cap assemblage, oil and gas transportation systems and types of trap, the results indicate that Fengheying buried hill is characterized in early development and shaped in a late period at the peak of hydrocarbon sourcing and expulsion. However, the trap of buried hill inner curtain has the characteristic that it was developed and shaped earlier than the peak of hydrocarbon sourcing and expulsion. A number of sets of oil formation series developed in the buried hill inner curtain that is filled and injected many times and migrated continually. Based on the analysis, Fengheying buried hill has a better potential for exploration. Of that, Jing-101 buried hill has three sets of oil formation series which belong to the two types of buried hill top and inner curtain (Fujunshan Formation and Wumishan Formation). They are the top targets for making breakthrough in exploration of Fengheying buried hill and its inner curtain

Located in the eastern part of Langgu Depression with a high degree of exploration. Hexiwu Structure has a certain potential for resources and a background for exploration. A series of targets have been found through the efforts for 3D seismic interpretation, analysis of the whole structure, study of overall sedimentary sand body, simulation of the whole oil and gas reservoir, integrated assessment of the whole zone and belt and detailed research. The study indicates that this region saw development of three sets of main source rock formation, which are dark mudstone of Es3 and Es4 as well as mudstone of Carboniferous- Permian deposited under deep lake environment. The structure is relatively rich in oil and gas resources with development of three major sedimentary systems-fan delta, delta and river, providing eight sets of good reservoir-cap assemblages from buried hill to the Neocene System. Hexiwu Structure saw development of five types of structural, stratigraphic, lithological, compound and buried hill oil and gas reservoirs. The result of hydrocarbon accumulation model shows that oil and gas accumulations in Hexiwu Structure are mainly controlled by hydrocarbon sourcing faults, unfolding of Paleogene sand body and development degree of fractural reservoirs. The comprehensive study indicates that the future exploration orientation is the western part of Hexiwu area and buried hill.

Exploration degree is high in Jizhong Depression. Currently, most of the structural targets in the middle and shallow layers have been located and brought under progressive development in the medium and later stages. The oil and gas reserves and production are inevitably on the decline, making exploration of the structures in the middle and shallow layers more and more difficult and affecting a healthy development of the company. Comparably speaking, exploration degree is relatively low in deep buried hill and its inner curtain, where there is a great potential for exploration. They are the main exploration fields at the present time. However, most of the current seismic data about this field have the problems of low signal-noise ratio, poor imaging and inaccuracy. Therefore, the study is focused on the target zone of Fengheying buried hill and the seismic geological conditions in the target zone. Good results have been achieved in study of the seismic acquisition method, such as times of fold, receiving line interval, channel density and geometry of uniformity. With the quality of seismic data remarkably improved, a set of seismic acquisition method has been summarized to facilitate exploration of deep buried hill and its inner curtain in Jizhong Depression so as to provide favorable basis for seismic data acquisition in the similar areas.

Gu?an area is an old exploration area of Huabei Oilfield, where exploration becomes more and more difficult. However, the success rate of exploration well has been remarkably improved in recent years thanks to introduction of various new technologies. AVO technology is an important technological method for direct inspection of gas reservoirs. The result is more obvious when used to inspect shallow high-porosity sandstone. The article makes an detailed elaboration about successful application of AVO technology for shallow natural gas exploration in Gu?an area. Based on analysis of seismic attributes and AVO forward and inversion, it is confirmed that shallow gas reservoirs in Gu?an area have bright points and AVO characteristics of III type. Various technological methods, such as amplitude attributes, AVO inversion, and AVO elastic wave inversion, are used to analyze gas-sensitive attributes and predict the distribution characteristics of shallow gas in Gu?an area. The actual drilling results confirmed effectiveness of the above-stated methods, leading to a set of technology and mindset appropriate for exploration of shallow gas in Gu?an area

Daxing conglomerate bodies developed in the declining disc of Daxing fault in the western part of Langgu Depression. Good results of exploration and development have been achieved in Xing-9 Well area, but no fresh breakthroughs have been made in exploration of conglomerate bodies. In recent years, application of Wefox imaging and processing system has significantly improved the imaging results of seismic data. Daxing conglomerate bodies are effectively identified thanks to application of sequential stratigraphic analysis technology, seismic attribute acquisition technology and seismic facies analysis technology. Based on those efforts, the law for distribution of high-quality conglomerate body reservoirs is found with the help of analysis of various parameters, the prediction of reservoir and the study of the sedimentary microfacies of Daxing conglomerate bodies. The predicted favorable area of conglomerate bodies is 161 square kilometers in aggregation while the predicted natural gas and oil reserves are 710 × 108m3 and 1000 × 104t respectively, indicating that Daxing conglomerate bodies have a great potential for exploration. The results of drilled wells have confirmed that this set of method is applicable and effective.

Langgu Depression is an old oil and gas exploration area with industrial oil and gas flow confirmed in oil testing. Large-scale reserves are found in the area. Those results help accumulate the rich experience for discovery of gas formations, assessment of gas formations and emancipation of gas formations, namely looking for gas and testing gas. Based on the practices, it is realized that various data can be comprehensively used to interpret gas formation and substantially improve the conformity rate of interpretation. Organic combination of various methods, such as conventional well log, gas-inspecting well log and logging, is the most effective method to find and identify gas formations. Neutron gamma time lapse logging remains as the effective applicable technology for identification of gas formations. APR-TCP combination and point method to calculate AOF are now the applicable technologies for testing of gas wells. The factor of natural gas volume is big and more seriously damaged by filtration of drilling fluid as compared to oil formations. Use of high-quality and light drilling fluid is an important precondition for looking for gas and testing gas. Meanwhile, a lot of problems still need to be brought under study and consideration

Huabei Oilfield Company has stepped up integration of management on preliminary geological study in recent years in the light of high exploration degree of oil-enriched depressions, difficulty for discovery and confirmation of preliminary exploration targets, and potential of extensive resources. Based on the research topics both inside and outside the oilfield, the company pays attention to multi-disciplinary research, highlights creation approaches, gives priority to first discovery rights, takes advantage of technological characteristics of universities and promotes understanding of geological knowledge. While making research in parallel, it uses the technological advantages of different research institutions, focuses on settlement of key geological bottlenecks and confirmation of traps, fully exposes geological contradictions and effectively avoid geological risks. The company also improves the overall plan for research of depressions, consolidates the fundamental geological research and practices three-level examination of well location to reduce the risks for exploration of complicated regions. Meanwhile, the company has developed an effective pattern for integrated research and management, which improves the whole exploration results and plays an important role in discovery of large-scale reserves in the highly-mature exploration areas of North China. In addition, this pattern has also laid a foundation for Huabei Oilfield Company to become a regional energy company in terms of reserves