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ABSTRACT: Petroleum Geology of the Zhu-1 Depression, Pearl River Mouth Basin, People's Republic of China

C. L. Aguilera, B. J. Huizinga, A. J. Lomando

The Pearl River Mouth basin, located in the South China Sea between Hainan Island and Taiwan, has been the focus of an intense exploration effort during the 1980s. In 1979 the international oil industry, in cooperation with the China National Oil and Gas Exploration and Development Company (CNOGEDC), jointly acquired over 60,000 km of seismic, gravity, and magnetic data covering an area of approximately 240,000 km2. Three major subbasins, Zhu-1, Zhu-2, and Zhu-3 were defined. Chevron, in partnership with Texaco and AGIP (ACT group), concentrated their effort on the Zhu-1 depression, which was interpreted to contain as much as 7800 m of sedimentary section. This subbasin, bounded by the Wansha and Donsha massifs to the north and south, is the most inboard of the three depressions, thereby possibly prolonging anoxic lacustrine conditions prior to the Neogene marine incursion. Additionally, the Zhu-1 depression should have directly received Miocene sediment potentially supplying the subbasin with high-quality reservoirs.

Within the Zhu-1 depression, the ACT group focused in on Block 16/08, which covered the deepest part of the Zhu-1 depression. The block was awarded to the consortium in January 1983. Structuring within the block ranges from Paleogene tensional block faulting created during the early formation of the overall Pearl River Mouth basin, to draping over basement highs and carbonate buildups during the Neogene. The Pearl River Mouth basin exhibits classic rift basin geometry with early nonmarine continental fluvial/lacustrine deposition (Zhuhai Formation) during the Oligocene and capped by a lower Miocene marine incursion (Zhu Jiang Formation).

Integrated interpretations, exploration drilling, and constant refinement of the geological model led to the discovery of two oil fields, Huizhou/21-1 and Huizhou/26-1, both of which are currently under development and will represent the first commercial oil production from the entire Pearl River Mouth basin.

Unlike the HZ/21-1 field that sits directly above the deepest portion of the Zhu-1 depression, the low-relief HZ/26-1 field is 40 km from the postulated source kitchen. High-risk with moderate reserve potential was assigned to HZ/26-1 during early prospect ranking. The discovery and subsequent trade for Amoco's LH/11-1-1 well lead to the upgrading and eventual drilling of the HZ/26-1 prospect. Biomarker work from ACT's HZ/21-1-1 and HZ/33-1-1, and Amoco's LH/11-1-1 wells indicated that the oils from all three discoveries were most likely generated from similar source rocks. With Amoco's LH/11-1 field located 75 km south of the Zhu-1 kitchen, long-distance migration of high-paraffin lacustrine oil seemed likely. Further petrophysical and petrological correlations identified the possibi ity that the reworked upper Oligocene Zhuhai sandstones with 5000 md of permeability acted as the migration pipeline for the Amoco discovery, and directly along its path was the HZ/26-1 prospect. The discovery of the HZ/26-1 field proves the importance of integrating and constantly updating the geologic model.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990