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With continued progress of exploration degree on the western slope of Xihu Sag of East China Sea Shelf Basin, exploration strategy has gradually changed from structural reservoirs to structural-lithological composite ones. Through the systematic evaluation of the hydrocarbon-generating potential of coal-measure source rocks, it was concluded that coal seams and carbonaceous mudstone are the main source rocks in this area. According to the study of sedimentary system based on the "Source-to-Sink" system, 8 types of structural-lithological composite trap were proposed. Furthermore, the accumulation conditions and distribution characteristics of oil and gas were studied, it is then confirmed that effectiveness of trap is the main controlling factor for oil-and-gas accumulation in the upper and middle members of Pinghu Formation, while the hydrocarbon-supplying capacity and physical properties of reservoir are the main controlling factor in the lower member of Pinghu Formation. Consequently, a comprehensive evaluation system was established. By the means of the comprehensive evaluation of structural-lithological composite reservoirs, the Eocene-Paleocene delta in Yingcuixuan area of the northern segment of the western slope belt, the Eocene tidal flat-delta in Pingnan, Chuyang and Baoshi areas, and the inner zone of Pinghu slope are large-scale reserve areas, which is valuable and favorable for further exploration. more...

The Xihu Sag has huge exploration potential and is the main target for oil and gas exploration in the East China Sea Shelf Basin. At present, it has been known that the Xihu Sag has a transitional facies with sedimentary system mainly composed of tidal flat deposits, and the provenance of which is dominated by western and northern uplift area. However, the transport process of sands from source area to deposit area, namely "source-channel-sink" system, has not been well studied. In this study, we take the Pinghu Formation of Baochu slope belt in the Xihu Sag as research object, and use borehole and seismic data to preform high-precision restoration of palaeo-geomorphology. Moreover, on the basis of palaeo-geomorphology restoration, we make qualitative and quantitative analyses of palaeo-water-system and palaeo-break. Results show that the location of sink zone is controlled by palaeo-geomorphology, the distribution of sand transporting channel is controlled by palaeo-water-system, and the development of sedimentary facies is controlled by palaeo-break. Through comprehensive analysis, we suppose that the combination of "palaeo-geomorphology, palaeo-water-system and palaeo-break" restricts the process of sand body deposition from provenance to convergence area through river transportation, and controls the distribution of the sedimentary system of Baochu slope belt. more...

A low exploration success ratio on the Baochu Slope of Xihu Sag of East China Sea has been inferred, and the ratio in the Central Anticline Belt varies significantly in different sub structures. To solve this issue, the petroleum geologic conditions of the sag were systematically reviewed in light of new basin analysis methodologies, and new understanding have been achieved. Due to the differences in tectonic features and evolution histories, the Baochu Slope and Central Anticline Belt developed different petroleum systems and play types. Only those areas with overlapping favorable fairways of all petroleum system elements promise the formation of large oil and gas fields. The Baochu Slope is featured by tectonic rifting, with weak later compression and early structure formation. Its favorable conditions for hydrocarbon accumulation include: short distance to the main source kitchen; multiple, overlapping, large-scale reservoirs; multiple structural-stratigraphic trap types; good vertical sealing zones; and effective lattice pattern migration pathways. The large to medium structural-stratigraphic fields are concentrated in fairways with a large scale source kitchen and updip pinchout of tide-dominated delta sands controlled by paleo slope break which in turn were induced by either faulting or paleo morphology. The Central Anticline Belt has experienced strong inversion and compression during the late depression stage, and developed inverted compressional slip faults and large scale anticlines. Hydrocarbon accumulation is controlled by four major factors: preexisting structural highs before the compressional event, large scale structural-lithological traps, excellent timing of hydrocarbon charging during deformation, and highly effective reservoir-seal combinations. The areas with good congruence of these contro-lling factors promise the occurrence of large-scale deposits such Longjing, Guzhenzhu, Huxinting fields. more...