Large-scale exploration for oil and gas started in north China during the 1960s, with Tertiary formations considered the main exploratory objective. In 1975, oil finds in Sinian dolomite beneath the Tertiary unconformity led to the discovery of the prolific Renqiu oil field, a buried-hill type. Since then, there has been intensified exploration for buried-hill oil pools. To date, more than 40 buried-hill oil and gas pools, and nine suites of oil-bearing formations, have been discovered. Reserves from these oil fields constitute 22% of the regional total, and current production accounts for 30% of the total. The success ratio in the exploration for hydrocarbons in the buried hills is 10 to 30%.

Based on the distribution of source rocks, and the pattern of oil migration, the buried-hill oil fields are categorized as follows: (1) hydrocarbons migrated into the reservoir through the unconformity surface and the fault planes, (2) through the unconformity surface, (3) through the fault planes, and (4) through the fault planes and then along the unconformity surface.

Two main types of the buried hills are classified according to the characteristics of their development during the Tertiary: (1) subsidence type, and (2) elevation and denuded type. The two types of reservoirs form a composite oil and gas accumulation zone with distinctive reservoir sequences. These factors provide a theoretical basis for finding various types of oil and gas pools on different parts of the buried hills.

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