ABSTRACT: Gas exploration potential in offshore Bohai, North China

Zhu, Weilin and Jiandang Ge , CNOOC Ltd, Beijing, China

Bohai Bay basin is divided into onshore and offshore parts. Exploration of onshore Bohai indicated that it is a typical oil basin. Only few small gas fields have been found so far onshore, mainly with associated gas.

Compared with the onshore, offshore Bohai has better geological conditions for forming gas pools. In recent years, several relative larger gas pools have been found. Two of these have reservoirs in shallow depth (~1000m). Gas in offshore Bohai is generated by over-matured Eocene mudstone, which is the main source rock in Bohai Bay basin, and thick matured Oligocene mudstone. According to a recent resource assessment, total gas resource in offshore Bohai is 1.23 TCM (>40 TCF). Larger gas fields are expected to be found with further exploration.

Four factors are responsible for the gas richness in offshore Bohai: (1) The deposition center of Bohai Bay basin in Oligocene was transformed into offshore gradually, resulting in thicker sediments which are 3-4 times as thick as that for onshore Bohai. The thick sediments made the lower source rock reach overmature to generate gas. Oligocene sediments also include coal-measure strata, another important gas source rock. (2) Offshore Bohai lies on the center of a mantle plume with high heat-flow. (3) Tertiary tectonic movement is relative weak in offshore Bohai. (4) Seal quality of offshore Bohai is much better than that of onshore Bohai.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia