Studies On The Deep Fault Depression Structure Characteristic And Oil&gas Accumulation In Southern Part Of Songliao Basin

The Songliao Basin, located in Northeast China (NE China) with an area of about 260000 km2, is the most important oil and gas concentrating region in china. It is traditionally recognized that the Songliao Basin is divided into northern and southern sectors along the boundary between the Heilongjiang and Jilin provinces, and these two sectors cover 120000 and 140000 km2 respectively. Since the breakthrough of crude oil exploited from Well SongJi3 in Songliao Basin in 1959, Songliao Basin has entered the stage of highly containing water now and the yield presents the downward trend, after 40 years of exploration and development. So it is necessary that the exploration target would turn to deep oil and gas layers from middle and shallow layers. This paper mostly studies structural characteristics of the south of Songliao Basin, analyzes the characteristics of the migration and accumulation of natural gas, meanwhile, it discusses the potentiality and direction of deep exploration in the future. Firstly, the paper synthetically analyzes that structural framework and tectonic evolution of rift basins in the south of Songliao Basin, by used of four long seismic profiles and past geophysical and geological data. After describing the framework of the rift basins in the south of Songliao Basin in detail, it is thought that the deep rift area of Songliao Basin is mainly central overlapping rift area, which is in NNE direction. In the south, it branches a fault zone of NE direction, namely Shiwu rift, Dehui rift and Yushu rift in turn from south to north; and near Songhua River, there is another branch of NE direction, namely Sheli rift, Halahai rift and Xujiaweizi rift in the north of Songliao Basin etc. The central overlapping rift area is divided into Changling deep Depression and Qianan deep Depression by Daqingzijing lobe of NW direction. Changling deep Depression is in NW direction. Basement faults are mainly in NNE direction and that in NW direction take second place. There are mostly two NW faults, one is located alone Songhua River and another is the dividing line of Central deep Depression Area, Southeast Uplifted Area and Southwest Uplifted Area. The later faults'trends are mainly NNW and approximate NS direction and it is proved the change in the direction of stress field of Songliao Basin. The tectonic movement in the south of Songliao Basin is mainly divided into four stages: the end of Yingchen Formation, the end of Denglouku Formation, the end of Nenjiang Formation and the end of Mingshi Formation. In the end of Yingcheng Formation and Denglouku Formation, the manifestation of the tectonic movement is mainly that the strata rise in small vertical relief, with the phenomena of denudation occurred in partial structure. Toward the end of Nenjiang Formation deposition, the spreading Japan Sea applied westward compression to the basin. This resulted in the Nenjiang movement wrenching, rotation and transpression, creating folding and uplift, particularly in the eastern part of the basin. As a result of folding, uplift and inversion in the eastern part of the basin, the Sifangtai Formation and Mingshui Formation depocenters shifted westward, largely into the Central Depression Area. During movement of the end of Nenjiang Formation, and little or no deposition was recorded in Southeast uplifted portion of the basin. The tectonic movement of the end of Nenjiang Formation enabled some reversal structure further formed, and rose and denuded. The thickness of denudation reached nearly one thousand meters. Using the 3D seismic data of 700 km2 of the Shi Wu rift basin for full 3D seismic interpretation, and using technique of variance cube to research the fault system, it is considered that Shi Wu rift basin is a NNE half graben basin, which is controlled by Sangshutai Fault zones. There are three NNE basement faults with structural feature of counter-clockwise strike-slip in the Basin. The partial structures distributed in pinch-and-swell form in the depression, and its trend is the same as the three NE faults. They are Xidingjia-Pijia Structural Belt, Houwujiahu-Siwujiazi-Taipingzhuang-Shuanglong Structural Belt (Central Structural Belt) and Aijia-Qinjiatun Belt respectively. These three structural belts are also main petroleum accumulation area of Shiwu rift. The petroleum reservoirs of Shiwu Fault Depression have the following basic laws. Plenty petroleum layers are distributed in vertical, which crossed well interval is long. The distribution of oil-gas reservoirs is discovered from Member 4 Formation Quantou to base rock. The feature of zonal distribution of discovered oil-gas reservoirs is obvious. From exploration results, the discovered oil and gas reservoirs concentrated and distributed over the three NNE fault belts in Shiwu Fault Depression. The types of petroleum reservoir are various, vertical type is a combination of petroleum series, which mainly formed overlapping accumulation area. The law of petroleum distribution is that oil and gas distributed in a circlearound oil generating center, in the oil generating center there is natural gas reservoir, outward there are oil and gas reservoir and pure oil reservoir respectively. To the features and difficult points of glutenite and thin interbed in the rift strata of the south of Songliao Basin, the reservoir prediction is studied by means of coherence analysis technique, inversion of seismic impedance and inversion of reservoir features with characteristic curve reconstructing. The reservoir predicting technique was found, which is combined with various technique fitting for practical geological features of the area. The distributional mode of deep petroleum of the south of Songliao Basin may be divided into following modes: fault block lithology and fault lithology forming reservoir mode(spanning mode), the lower generating and the upper accumulating mode, which is communicated with deep major fault fracture; lithology structure forming reservoir mode, the lower generating and the upper accumulating mode, which is communicated with fault fracture; primary oil-gas forming reservoir mode, generation of itself, accumulation of itself and capping formation of itself mode; stratigraphic overlapping forming reservoir mode, the lower generating and the upper accumulating mode; the burial hill of the base rock forming reservoir mode, the upper generating and the lower accumulating mode; inorganic CO2 forming reservoir mode. The controlling factors of petroleum forming reservoir can be divided into the following aspects: deep rift basin controls petroleum distribution on the whole and each rift has a respective petroleum-bearing system. It is necessary for forming reservoir of petroleum accumulation that deep rift communicates the relation between hydrocarbon rocks and shallow reservoir layers. The distribution of area cap formation and direct cap formation decides the vertical distribution of petroleum, and the physical properties of reservoir is the key factor of restricting deep productive capacity. In a word, there is enormous potentiality in deep exploration of Songliao Basin. To the shallow secondary reservoir, certain achievement has been made, and some of reserves have been proved. However, being effected with complicated depositional environment and multiple-stage tectonic movement, and faults and fractures are developed, the type of structure is complicated, the physical properties are out of condition to reservoir, and anisotropism is strong, it is difficult to search large-scale and self contained oil and gas fields in the shallow. The result of exploration was also proved that the proved petroleum reservoirs are in small scale at present and rely mainly on the fact that the secondary oil-gas reservoirs, whose productive capacity is rather lower. It is even difficult to break through the aspects of structure and reservoirs with the existing prospecting technique. After recent years'prospecting progress, it is considered that the thought of deep prospecting in the south of Songliao Basin in the future should be turned firstly from searching secondary oil-gas reservoirs to the searching of primary oil-gas reservoir near the area of the...