China offshore oil and gas-bearing basins are opened since 1980's only except Xihu Trough in East China Sea Basin. Up to now, 129 cooperation contracts and agreements have been signed during the four rounds biddings and bilateral-negotiations with foreign companies. All the blocks in each basin have had one to four times of joint study and exploration. Most of these blocks were failed in exploration, only few blocks were succeeded. Some partners relinquished the cooperation blocks because of the high exploration risk. Just in the relinquished blocks CNOOC selected some blocks as the self-financed exploration areas. In the recent years (especially from 1994 - 1997), there are 27 discoveries found among to 45 wildcat wells. Success ratio is 60%. We discovered QHD32-6 oil-bearing area and CFD18-1 gas-bearing area in Bohai Bay Basin, WC13-1 oil and gas-bearing area in Pearl River Mouth Basin, Ledong gas-bearing area in Yinggehai Basin, WZ12-1 oil-bearing area in Beibu Gulf Basin, etc.. These oil and gas-bearing areas not only present favorable commercial prospects, but also give us some new knowledge on the potential resources in China offshore oil and gas-bearing basins.

1. In a hydrocarbon potential depression, we should believe in the law of “Material Stability”. Even if the first stage of exploration program was not succeeded. Do not lost your mind, and believe that the generated hydrocarbon must be reserved in some certain of traps. If it is not reserved in inside of the depression, it could be find in the edge of the depression; not reserved in the shallow part of the formation, could be found in the deeper part of the formation; not in the structural trap, must in the stratigraphic or the lithologic trap. What we only have to do is just make up our mind to do the exploration survey in the potential area. We will find oil and gas field in the end if we continued our exploration work. For example, Wenchang Depression in Pearl River Mouth Basin, it is believed as a favorable block by foreign oil companies since 1984. There are 14 wildcat wells have been drilled by three oil companies. It is believed to be a favorable hydrocarbon potential area from the drilling data. But in fact, only Wenchang 19-1 and Wenchang 92 two oil and gas bearing structures have been found in the depression. Foreign partners believe it is a non commercial block and relinquished it. Since 1995, CNOOC began its self-financed exploration work. 4 recoveries have been found in 5 drilled structures which were Wenchang 8-3, Wenchang 9-1, Wenchang 13-1, Wenchang 13-2, and Wenchang 8-2. In Wenchang 13-1-1 well, the oil pay is more than 40 m, DST test production is about 14,000 BOPD. The estimated reserves in place is over 140 MMBO. Then we drilled Wenchang 13-2 structure, 50 m of oil pay has been found, DST test production is over 14,000 BOPD, the estimated reserves in place over 140 MMBO too. All these discoveries make Wenchang Depression to be becoming a new oil and gas developing area with favorable commercial value (See Figure 1, Table 1).

2. Bohai Bay Basin is a series of uplifts and pitch out belts surrounded by a hydrocarbon generating depression. Bohai Bay Basin is the main area for finding large-medium oil and gas field. Bohai Bay Basin is multiple uplifts and multiple depressions basin, it is alternates with uplift and depression. Most of the uplifts are surrounded by the hydrocarbon generating depression. According to our experience in exploration work, we believed that series of large scale of drape anticlines and rollover anticlines developed along main fault could be found in these uplifts and the plunging end. It is also called the goal for large medium oil and gas field. In Bohai Bay Basin, Suizhong 36-1 and Chengdao have been found in Liaoxi Uplift Belt and Chengbei Uplift Belt. In these giant oil fields, oil in place is around 1400-2100 MMBO. Shijiutuo Uplift Belt is relinquished by four foreign oil companies with four rounds of cooperation. CNOOC restudied this uplift belt when it was relinquished at last in 1995. Then we drilled QHD 32-6 structure. According to appraisal drilling, oil in place is proved to be 1190 MMBO. In 1996, another oil field called NB35-2 oil field have been found in this uplift belt. The evaluating work is not finished yet, but from the data we obtained now, the oil reserve in place about with 700 MMBO scale. In 1997, BZ 3-1 oil field have been found in the South of QHD 32-6, it is a favorable potential area too. So we can say that there are around 2100 MMBO of oil in place have been discovered in Shijiutuo Uplift Belt in nearly three years.

Because of the succeeded in these uplifts, we believed that it is possible to find giant oil and gas fields with 700 MMBO of oil in place in Shaleitian Uplift Belt, Bongdong Uplift Belt, Bonan Uplift Belt, Laibei Uplift Belt, and Miaoxi Uplift Belt which are located in Bohai Bay Basin (See Figure 2). These uplifts are the main exploration areas in the near future. We hope o improve the reserves in the future though further exploration work.

3. A new phase of oil and gas discovery period could be happened in the Eastern Pearl River Mouth Basin probably. Huizhou oil field, Lufeng oil field, Xijiang oil field, and Liuhua oil field have been found in this area via sino-foreign cooperation exploration. Oil production is up to 270,000 BOPD at present. From our restudied, we believed that there are three other new areas are worthy of further exploration.

 Xijiang Sag and Enping Sag are the favorable hydrocarbon generating sags. A series of favorable targets for drilling could be found in the two sags and adjacent areas.

 A series of low amplitude structural traps have been found though 3-D seismic and seismic velocity research in adjacent area of the oil fields discovered in Huizhou Sag.

In the Eastern Pearl River Mouth Basin, the objective formation is self-generation and self-reservoir in Paleogene. It is a new field for looking for traps with good reservoir. A series of new oil and gas fields will be found in this area after continued exploration work.

4. Lishui Sag and Jiaojiang Sag in East China Sea Basin are also called great potential areas. There are 14 dry holes have been drilled by foreign oil companies in Taibei Depression, East China Sea Basin after the fourth rounds bidding. These terrible facts make the foreign oil companies lost their sheet anchor in this area But just though these dry holes we believed that the real hydrocarbon potential area must in Lishui Sag and Jiaojiang Sag, the west of the cooperation block. These two half-graben depressions in early Paleogene are the favorable hydrocarbon generating sags, and worthy of further exploration work.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas