Large Reservoirs Development Condition and Characteristics of the Second Member of Liushagang Formation in the Southern Steep Slope Zone, Wushi Depression

Key words: 
• steep slope zone /
• Wushi depression /
• reservoir /
• Liushagang Formation /
• fracture accumulated sand

Abstract: Comprehensive analysis of drilling, palaeontology, seismic, sections and other data, the large reservoirs development conditions, types, characteristics and the favorable exploration directions of the southern steep slope zone in Wushi depression was analyze. It proposed that the lake level, provenance, fracture and paleogeomorphology coupling controlled the development of large reservoirs in the steep slope of fault-depressed lacustrine basin. Among them, the strong lake retreat was the basic condition of the Palaeo-lake evolution for developing large reservoirs, the enough supply of provenance was the material guarantee for forming large reservoirs, the significant fracture accumulated sand effect and suitable topography were the important catchment mechanism for constructing large reservoirs.The main types of large reservoirs were fan delta and braided river delta.The fan delta was mainly controlled by the fracture surface gentle slope zone and valley topography, the lithology was relatively coarse, the sedimentary microfacies were mainly underwater distributary channel and subaqueous inter-distributary bay.The braided river delta was mainly controlled by transition fault zone, broom fault zone as well as relatively wide and gentle topography, the lithology was relative fine, the sedimentary microfacies were mainly distributary mouth bar and distal bar, besides underwater distributary channel and subaqueous inter-distributary bay.On this basis and combined with the characteristics of large reservoirs, it suggested that the favorable exploration layer for the eastern part of the southern steep slope zone was the highstand system tracts of lower sequence and for the western part of the southern steep slope zone was the lowstand and transgression systems tracts of upper sequence in the second member of Liushagang, Wushi depression.Due to the front of the large reservoirs lithologic traps, lithological-structural traps and structural traps were developed, and were shallow buried, closed to the hydrocarbon supply center, at the same time located in the dominant direction of oil and gas accumulation, therefore, was the favorable exploration area.