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North China basin is a large Mesozoic to Cenozoic sedimentary basin in eastern China. A result of strong block-faulting activities, the inner part of the basin reveals the characteristics of multiple uplifts and depressions. Each depression is generally an independent exploration unit.
The practice of exploration in recent years has proved the following: taking each depression as an individual unit, the basic geologic framework and evolutionary history are quickly determined. This is very important in order to achieve the best effects in petroleum exploration.
Jizhong depression is located at the western part of the North China basin. This area is about 25,000 km2 (9,650 mi2). Extensive seismic surveys and several hundred exploration wells have been completed during the past few years, resulting in the discovery of Renqiu and other oil fields. Taking the Jizhong depression as an example, the writer has considered the four following problems.
1. Pre-Tertiary fault blocks and their distributional form:
There are 11 primary fault blocks in the Jizhong depression. They are all the result of Himalayan movement, but they are obviously subjected to the effects of the structure lines of Yenshan stage.
2. Some characteristics of block-faulting activities:
At present, most geologists think that the eastern part of the Eurasian plate, where the Jizhong depression lies, is subject to the effect of subduction of the Pacific plate, which was first compressed and uplifted in Late Jurassic and Early Cretaceous times and then fractured and sagged in Late Cretaceous and early Tertiary. The writer, considers Tertiary block-faulting activities with characteristics of pulsation, through analysis of fault-block development of the Jizhong depression. This analysis follows the generation, development, and extinction of the faults in the depression. The result is that pre-Tertiary blocks are known to have experienced many processes of disruption and union. Because the active period of each fault is at a different time, the main periods of disruption and nion in the different parts of the depression are also varied.
3. Block-faulting in relation to the controlling of Tertiary sediments and structures:
Block-faulting activities controlled the differences in source area and depositional center, giving rise to regional climatic differences and leaving clear and definitive time marks in the sedimentary bodies. In the Jizhong depression, each stage has its own special sedimentary formations. By analysis of the main faults and main structures, we may make a conclusion which is somewhat different from that previously accepted--that the block-faulting activities in this area may have formed either tensile structure or compressive and compressive-shear structures. Both of these structures contain petroleum.
4. The types of oil and/or gas pools:
Based on the analysis of the geoframework and evolution of the depression, there should be three main types of oil or gas pools: (a) Tertiary oil (gas) stored in Tertiary reservoirs; (b) Tertiary oil (gas) stored in older reservoirs (i.e., buried-hill oil or gas pools); and (c) sub-Sinian and early Paleozoic oil or gas stored in reservoirs of the same age.
In the future, the exploration of the buried-hill gas or oil pools will continue. This should expedite the exploration for the other two types of oil or gas pools. In addition, we should pay much attention to exploration for Tertiary stratigraphic-lithologic oil or gas pools.
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