Abstract

Over the past few decades, many tilted-fault-block/buried-hill oil and gas traps have been discovered by geophysical exploration and associated drilling in the Mesozoic-Cenozoic rift-depression basins of eastern China, the viking graben of Europe’s North Sea basin, the Red Sea graben of Egypt, the Sirte basin of Libya, the Gippsland basin of Australia and Baffin Labrador basin of Canada.

Since large volumes of oil and gas have been yielded from these “new” type of traps, they are likely to become exploration targets in the tension-type basins of China and other countries. The occurrence of these traps may be explained by the following sequence of events:

1. Swelling of the upper mantle caused tensional stress on the crust, resulting in the formation of tensional rift-subsidence basins. The heat released by the upper mantle created rather high geothermal gradients.

2. Block-faulting of the basement rock formed a series of linearly distributed tilted-fault-block/buried-hill structures.

3. Rapid subsidence and an infilling environment during the early rifting stage created favourable conditions for maturation of source materials. Further hydrocarbons might have been generated during the later depression stage from older, preserved, sub-unconformity source beds.

4. Weathering and karstification of the uplifted parts of the tilted fault blocks over long geologic periods aided development of favourable reservoir rock properties.

5. Combinations of multiple source-rock and multiple trap assemblages associated with tilted-fault-block/buried-hill occurrences could result in an oil and gas mega-structural belt in a rift-subsidence tensional basin.