Abstract: Slope is a major structural unit of fault lake basin in eastern China, and also an important zone in the Bohai Bay Basin for increasing oil and gas reserves. Through years of comprehensive studies and exploration practices in slopes, it is newly proposed that the slopes (or large gentle slopes) show a high-moderate-low differentiation. To be specifc, the geologic differentiation of high, moderate and low slopes in subsidence rate, sedimentary sequence, reservoir physical property, hydrocarbon generation and evolution, formation pressure, fluid property and reservoir type results in the theory of hydrocarbon enrichment in dominant facies belts in slope of fault lake basin. Essentially, in high slope, channel/trough controls sand, and oil and gas enrich in dominant migration and preservation facies. In moderate slope, slope break controls sand, and oil and gas enrich in dominant structural-lithologic facies. In low slope, distal fan controls sand, and oil and gas enrich in dominant source-reservoir coupling facies. According to this theory, fne exploration and drilling operations have been carried out in the slopes. As a result, remarkable achievements were made in stratigraphic-lithologic reservoirs in the slope zone. During the past fve years, additional 3P oil reserves exceeded 3×10⁸t, and four 10 million-ton class proftable productivity construction blocks and three 100 million-ton class blocks with the potential of signifcant reserves increased. Thus, it is demonstrated that the theory of hydrocarbon enrichment in dominant facies belts in slope plays an important role in directing the large-scale proftable reserves increase and high effcient productivity construction.

Key words: fault lake basin, gentle slope, high-moderate-low differentiation, dominant facies enrichment, exploration practice