The Palaeotopography characteristics of peripheral areas of the prolific oil gas depressions in North China Offshore basins are studied with the mass balancing reconstruction method.

Those sags along the detachment faults are prolific oil gas depressions.

Despite their long history of development, some oil-rich sags still have a large quantity of residual resource in the forms of five potential play targets: undeveloped sags in the developed depression, hidden reservoir, complex fault structure, and deeply buried and small hills and some special reservoirs.

Based on the research and exploration of lithostratigraphic reservoir in the Jizhong depression of the Bohai Bay basin and Erlian basin, the hydrocarbon distribution in a continental oil-rich sag has "complementarity" feature, viz.

Based on the comparison of oil-rich and gas-rich sags in the Bohai Bay basin,two main controlling factors of gas reservoir-forming are summarized: gas-rich sags have source rock of partial humic or high maturation sapropel type;thick abnormal pressure mudstone in lower Tertiary is the cap,or "quilt".