Secondary exploration is a new petroleum exploration idea and method for oil-rich sags. Essentially it is the reappraisal of the distribution law of undiscovered petroleum resources in oil-rich sags. The core is to cognize, evaluate and deploy on the whole. Oil-rich sags have favorable conditions for carrying out secondary exploration, which include abundant residual petroleum resources, and unbalanced exploration in different zones and domains. New exploration theories and recognitions provide theoretical guides for secondary exploration. High accuracy acquisition techniques and large area processing techniques of 3D seismic data have witnessed a rapid development and have been widely applied, providing strong technical support for secondary exploration.

Lixian slope in Raoyang sag is an old exploration area that has been explored for more than 40 years. Because the attitude of stratum is gentle and no structural traps developed in the slope, no progress has been made in exploration since the structural reservoirs were discovered at the early stage of exploration. Through secondary precise 3D data acquisition and processing, meticulous minor-fault micro-range structure depiction and interpretation, precise and comprehensive prediction and description of small sands, selection of precise fracturing technologies, integrated implementation of precise exploration and development in the past three years, newly added proved, probable and possible reserves have reached 100 million tons, important discoveries of stratigraphic and lithologic reservoirs have been regained, so new breakthroughs have been made in the precise exploration of the old exploration area.

The pericline area of Yanling buried hill has superior oil source conditions and it has favorable geological conditions for the formation of lithologic reservoirs. The ancient landform in the area fluctuates, structural and sedimentary slope breaks developed here, and two groups of fault systems developed, the major of which is NE and the other is NW. Because the area lies at the depositional system edge intersections, the ratio of the total thickness of sandstone to the stratigraphic thickness is moderate. The sand body of the SW and NW provenance matches the NE fault system, so it is easy to develop structural-lithologic complex traps. Controlled by particular sedimentary and structural setting, three types of traps developed in the pericline area, i.e. the fault-nose and fault-block trap with the characteristics of the area, the structural-lithologic trap taking faults as main controlling factors, and the lithologic-structural trap controlled by lithologic updip pinchout. Besides, three reservoir accumulation models developed including the accumulation model of sedimentary slope breaks, the accumulation model of fault slope breaks, and the stratigraphic accumulation model.

After 30 years of exploration in Raoyang sag, the buried hill reservoir featuring in large scale, shallow depth, and simple structure has been discovered. Buried hill study and analysis of reservoir accumulation conditions in the past few years indicate that Raoyang sag has huge exploration potential and a lot of small and deep buried hill traps, internal traps of buried hills and other types of buried hill traps need to be further studied and evaluated. Changyangdian buried hill structural belt and Suning buried hill structural belt have low exploration degree and complicated reservoir accumulation conditions, so the exploration work has been stagnant at one time. In 2006, Huabei Oilfield Company laid emphasis on the exploration of deep buried hills, listing Changyangdian and Suning buried hill structural belts as key exploration zone, fully using newly acquired 3D seismic data to find buried hill traps. Beginning with the old data, through finding out new hints to develop new recognitions, re-examining old wells to solve difficulties, identifying buried hill horizons through strata contrast and tracking, focusing on the key of reservoir accumulation, reinforcing evaluation on oil sources, creating new geological cognitions, building buried hill reservoir accumulation models, the company discovered buried hill traps in Chang 3, Chang 6 and Ninggu 8x wells. After drilling, high-production commercial oil and gas flow is gained, and new breakthroughs in buried hill exploration are fulfilled.

The exploration potential research of Jizhong Depression shows that Wen'an and Lixian are favorable lithostratigraphic reservoir exploration zones. The five advantages in the formation of lithostratigraphic reservoirs in gentle slope zones of petroleum-rich sags are systematically analyzed, seven reservoir models are set up including the up-dip pinch-out sandstone reservoir, the sand lens reservoir, the river sand lithologic reservoir, the stratigraphic overlap reservoir, the stratigraphic unconformity reservoir, structural-lithologic reservoir and structural-stratigraphic reservoir in gentle slope zones, and three lithostratigraphic reservoir accumulation series in the inner, central and outer slope zones are established. The main research route, the control factors of reservoirs and the major methods for the discovery of the two reservoirs, the mid-low channel sandstone reservoir in Wen'an slope outer zone and the lithostratigraphic reservoir in the northern-central Lixian slope zone, are summed up. It could have some guiding effect on the exploration in gentle slope zones of petroleum-rich sags

In recent years, great breakthrough has been made in the exploration of internal reservoir of buried hill, buried hill exploration targets have been shifted from simple and obvious reservoirs to relatively more complex ones. But the internal reservoirs of buried hill that have been discovered mostly lie in the negative structural unit of Jizhong Depression. The accumulation condition of internal reservoirs of buried hill located in sags is usually superior to that of uplifts. By the study on the buried hill and internal strata, structural features, reservoir-capping-assemblages, condition of hydrocarbon supply and preservation in Niutuohzhen uplift, it is regarded that the uplift located between two oil-rich subsags is abundant of resources, open channels for oil migration, and there are three internal source-reservoir-assemblages consisting of lower Palaeozoic, upper and intermediate Proterozoic. Because the Cambrian system of Palaeozoic (except for Fujunshan Formation) and Hongshuizhuang Formation of Jixian system of middle Proterozoic are capping formations, and the Wumishan Formation of Jixian system is reservoir bed, which consists of reservoir-capping-assemblages and it's partly distributed, whether the Yangzhuang Formation of Jixian system and the Chuanlinggou Formation of Changcheng system can serve as capping formations is the key to develop internal reservoir of buried hills.

Based on study on structure, sedimentary characteristics and reservoir forming conditions of Baxian southern subsag, the sequence stratigraphic synchrone frame of the region is first set up, where the formation from lower submember of Es1 to Es4 is divided into seven third-order sequences and the space distribution as well as the feature of depositional systems and sedimentary facies are confirmed, and three favorable zones of developing lithologic stratigraphic trap are given. The lithostratigraphic traps are effectively identified by comprehensively using such approaches as analysis of seismic facies and seismic properties, inversion. Lithostratigraphic reservoir models in slope area, the central anticline belt and deep depression area are established, and then the favorable exploration targets are exactly selected and confirmed. Wenan 11 and Wenan 1 wells were successfully drilled, which proves that the area is favorable for lithostratigraphic reservoir exploration and is of guiding significance for further exploration in other petroleum-rich subsags of Jizhong Depression

Aer sag in Erlian Basin is one with reversal anticline structures, which is formed through multi-stage activities. Study of the hydrocarbon accumulation conditions of the sag shows that the main hydrocarbon source rocks are distributed in the K1ba and the upper section of K1bt1.Main oil layers include the upper section of K1bt1, the lower section of K1bt1, K1ba and Pz. Fan-delta is the main sedimentary type. Reservoir properties are generally poor, but high-quality reservoir develop in the poor reservoir, the best one is in the lower section of K1bt1. Four sets of reservoir developed in Aer sag. The main reservoir types include inverted anticline, broken nose, structural-lithologic reservoir, and tuff buried-hill reservoir in west-slope. Aer sag is another oil-rich sag in Erlian Basin

The collapse column damages the continuity of coal seams, and connects the adjacent water-rich layers, it has been considered as one of the most important factors restricting the effective CBM exploration and development. To improve the exploration efficiency and avoid risks, authors in this paper systematically analyze and study the origin mechanism and conditions, the profile shape and seism reflection features, distribution regularity and effect factors of karst collapse in coal measure strata in Zhengzhuang Block in Qinshui Basin. With a large-scale structure overthrew from southwest area to northeast, the collapse columns present ribbon distribution from north to east, and crowd in the east and west. This is caused by inequality of Ordovician limestone deposits, strong water runoff direction underground of the area, and regional structural background; and it has nothing to do with the trend of local drape

Horizontal multi-branch well technology is a newly developed one for CBM development. It has been applied to more than 60 horizontal wells in southern Qinshui Basin, which shows that single well production varies a lot. Based on horizontal well development in Fanzhuang Block, gas content, drilling footage, branch spread and drainage continuity are considered to be major factors influencing production of horizontal wells, through comprehensively analyzing geological conditions, drill project and drainage control of individual wells. It is critical to increase CBM production of horizontal wells by optimizing well location and branch trajectory, and protecting reservoirs during drilling.

Continental rift has such sedimentary features as multi-provenance, proximal provenance, narrow facies belt and frequent change, so it is ready to deposit multiple sand bodies. Multivariant sedimentation types provide favorable conditions for the formation of lithostratigraphic hydrocarbon reservoirs.However, complex geologic characteristics and geophysical response bring difficulties to the application of seismic data to reservoir predication, and thereby influence exploration results. Based on the geological and geophysical characteristics of four main sedimentary sand bodies in continental rifts, i.e. the delta front sand body,the fan-delta sand-conglomerate, the lake floor fan glutenite, and the fluvial sand body, the seismic reservoir prediction methods and technical route are studied and are proved to have higher accuracy and coincidence after being verified by practical application and data.

Es1 and Es2 are the main drilling target strata of Lixian slope of Raoyang sag. They are dominated by deposits of sand and shale interbedded layers on the delta front and have great potential in lithologic reservoir exploration. However, because themonoclinal stratum of sandstone is thin and fine, and its geophysical response is close with the nearby mudstone stratum, it gets hard to distinguish them, to vertically recognize thin sands and horizontally describe sand body shape only using conventional seismic data. Through comprehensive analysis and practical application, on the basis of accurate well-seismic correlation and delicate seismic interpretation, in accordance with the predicting flow of point correlation, thread restriction, cube calculation and segmental description, the spectrumwhitening and frequency-broadening processing technology, the pseudo-acoustic model inversion technology, the spectral decomposition technology and other technologies are used for reservoir prediction, which enhances the precision of prediction and provides important technological support for exploration of lithologic reservoirs and discovery of hundred-million-ton scale reserves.

Lixian slope zone is thought as the main direction of hydrocarbon migration in Raoyang sag. Traps are mostly structural-lithologic and lithologic, and the reservoir accumulation conditions are more complicated. The lithologic reservoir in Lixian slope zone is less explored than the central structural high in Raoyang sag, and it is the main potential area of exploration in the sag. Based on the study on accumulation conditions of lithologic reservoir in the slope zone, four types of reservoir accumulation patterns have been established exploration, i.e. the forward fault type, the side-dip pinch-out type controlled by reversed faultnose, the complex fault-block type, and the up-dip pinch-out type controlled by slope-break zone. Target exploration research methods have been adopted, such as the use of seismic facies features to describe sand body geometry and the fine reservoir prediction to confirmlithologic trap boundary. And finally new breakthroughs were made in exploration, and 50 million tons of reserves were discovered in Lixian slope zone.

The pyrolith reservoir of Honghaoershute sag in Erlian Basin is an important exploration area, where the andesite widely distributed in the third member of Aershan Formation is the main target strata, and it is one of the exploration difficulties to predict the distribution of the pyrolith reservoir effectively. Aiming at the andesite reservoir characteristics and prediction difficulties, the authors predict the spatial distribution patterns and the thickness change of andesite with some measures such as natural gamma analog acoustic wave inversion, natural gamma reservoir parameter inversion, seismic attributes and so on. According to the characteristics of andesite lithofacies, synthesizing the fracture structure, seismic facies, seismic energy, frequency attenuation attributes, the result of andesite inversion prediction and the strata thickness gotten from drilling, the authors predict the andesite facies. It is believed that the development areas of the fracture and the volcano slopes at a middle distance from the volcano center are favorable reservoirs.

Sulige gas field is characterized by low pressure, low permeability, and low abundance. It is a lithologic reservoir dominated by fluvial sand bodies, with widely pool distribution, flat structure (the dip angle is less than 1°), but no development of faults. So it is suitable for the application of broadband excitation techniques. Surface structure of the northern area of the gas field varies a lot horizontally. Test shows that single shot energy, frequency, signal-to-noise ratio of the northern area is directly related to whether they are excited below the free surface. Based on reinforcing study on the correlation between the surface structure and free surface of the area in recent years, to design the excitation depth no longer relies on the high-speed layer top, but on the correlation between the free surface and the spin-down layer top and high-speed layer top, and the surface speed. Raw data featuring in high consistency of energy with frequency and broad band are obtained, and better seismic and geologic results are acquired

Xing-9 conglomerate reservoir, a lithologic condensate reservoir, lies in the central and northern of downthrow side of Daxing fault west of Langgu sag. Five wells have been drilled in the conglomerate terrain, two of which have stable commercial oil and gas flow. Through the origin analysis of conglomerate, study on lithologic oil and gas accumulation conditions, target processing of high-fidelity seismic data, fine horizon calibration, and multiple seismic attribute analysis in the past years, the oil and gas reservoir accumulation patterns of "slip-phase conglomerate rich in oil and gas as a whole" and "gas reservoir below gas reservoir" are established. Through adopting the strategy of "evaluating on the whole, implementing step by step, combining rolling evaluation and capacity construction", the rolling evaluation has progressed a lot, and the conglomerate reserves have exceeded 10 billion cubic meters.