| YA 13-1 gas field, the best gas field, is located in the west of Yanan depression. In the southeast, there lies YA 21-1 structure, which in general has "three poor" conditions, that is, source rock, reservoir and energy field. The natural gas of YA 13-1 comes from Yachen and Lingshui formation of Yinggehai and Qiongdongnan basin. However, the source rock of YA 21-1 comes from Ledong depression, which is the first poor. The reservoir of YA 21-1 is worse than that of YA 13-1, which is the second poor. Regarding the energy fields, it is shown that the cap rocks and reservoirs have over-pressure, which is the third poor. So, the source rock, reservoirs and energy fields controlled the petroleum accumulation of YA 21-1. |