Abstract. The Bohai Bay Basin which has been exploited for nearly 40 years is a composite basin consisting of the early Proterozoic-Paleozoic and Meso-Cenozoic formations. Since the discovery of the first oilfield the exploration course can be divided into 4 stages. In each stage there was its specific major exploration frontier, new sequence and oil reservoir type under the guidance of the relevant theory and by using new techniques. As a result, each stage had an increased peak of discovered oilfield number and reserves. At present, the Bohai Bay Basin has become the largest oil production area in China. In the long term future, there will be many frontiers and more potential resources for further exploration. 1. PRESENT SITUATION OF PETROLEUM EXPLORATION AND DEVELOPMENT IN BOHAI BAY BASIN The Bohai Bay Basin covering an area of about 200 000 km2 (about 74 O00 km2 offshore) is located in the eastern part of North China. It was formed and evolved on a crystalline basement solidified from Archeozoic to Early-Middle Proterozoic. Afterwards it had undergone Middle-Late Proterozoic, Paleozoic, Mesozoic and Cenozoic development stages. At the end of Tertiary it became the present basin (Fig. 1). Its major oil pay horizons are the Tertiary formations and secondly the pre-Tertiary formations3*'.
The regional reconnaissance survey began in 1956, including regional geophysical prospecting and regional exploratory wells. Since the first discovery well, the Bohai Bay Basin has become one of the richest petroliferous basins in China. Up to now, the length of finished seismic lines is 630000 km, the number of exploratory wells is 11 600. The Bohai Bay Basin is composed of 47 faulted-depressions, in which 170 oil and gas fields have been found.
Each faulted-depression is characterized by extensive block-faulting activities, more faults, great lithofacies variation, more stratigraphic-unconformable accumulation processes. As a result, they make up a group of oil and gas pools characterized with one type being predominant. Vertically, they are superimposed one over another and horizontally, they are composed of different types of oil and gas pools in different layers, and connected with each other, finally forming an oil-bearing zone (a play) which is called a complex petroleum accumulation zone (CPAZ)'*6. Each CPAZ possesses a larger oil pool and many other small-medium oil pools. Oil and gas fields have been discovered successively and periodically, so that the reserves and annual production have been increasing continuously. The annual oil production has been increasing year by year for more than 30 years and has reached up to 64 million tons of oil and 7 billion cubic meters of gas. From 1963 to 1993, it can be divided into 4 explora