When comparing four prototype basins, they not only have different tectonic and depositional backgrounds, but they also have different petroleum accumulative (elements) conditions. Therefore, these prototype basins contain noticeable distinctions in their correlations of accumulative elements and in their accumulative processes; in addition, they also display their own accumulative features and patterns. In the rifted basin, because the petroleum rich depression contains high quality hydrocarbon source rock of large thickness and broad distribution, it thus has the characteristics of a “petroliferous rich depression”. In the depressed basin, the lithostratigraphic traps were not only favorably developed on the delta front, but they also perfectly correlated with the hydrocarbon source rock that was located in the central area of the lake, which displays the characteristics of accumulating oil on the delta front. In the foreland basin, the development of a lithostratigraphic reservoir in the foreland thrust belt was controlled by faults and fans; this characteristic is represented by the northwestern margin in the Zhungeer basin. In the marine craton basin, oil and gas accumulations were controlled by the reef flat on the platform margin and other high energy facies. Understanding these characteristics and patterns will benefit oil and gas exploration in the future.
7.1 Terrigenous Rifted Basin with the Characteristics of a “Petroliferous Rich Depression” In eastern China, there are many Mesozoic and Cenozoic rifted basins that include the Bohai bay basin, the Southern North China basin, the Erlian basin, the Hailaer basin and the Yilan–Yitong basin. Because of the extensional and rifted movements, some relatively isolated depressions were formed, which contained diversified measures for natural resources and different outcomes for petroleum exploration (Gao and Zhao, 2001; Yuan and Qiao, 2002). Some depressions contain plentiful oil and gas and they yield good exploration results. Other depressions have the preconditions for oil generation. However, only a few, small size oil and gas fields were discovered. In addition, some depressions lack the preconditions for oil generation; therefore, they are worthless for petroleum exploration. Exploration practices indicated that petroleum rich depressions are primary targets for upcoming oil and gas exploration and for increasing the petroleum reserve. Inside a petroleum rich depression, high quality hydrocarbon source rock provided sufficient oil and gas supplies. Vertically, oil and gas could be accumulated in various stratigraphic sequences and in all kinds of reservoir bodies. Horizontally, multiple sequences and different types of trap overlapped with each other to form a connected unity. Together, they demonstrate the characteristics of a “petroliferous rich depression”. Here we highlight the geological setting and petroleum accumulative pattern for the characteristics of a “petroliferous rich depression” in a terrigenous rifted basin.
7.1.1 Geological Settings for a “Petroliferous Rich Depression” Compared with a low abundance petroleum depression, a petroleum rich depression has perfect hydrocarbon generating conditions, excellent reservoir settings, broadly distributed traps, a highly efficient migrating passage and adequate space–time configuration. (1) In a petroleum rich depression, a perfect hydrocarbon generating condition is the first requirement in order to achieve a “petroliferous rich depression”. According to the statistical analysis of the Bohai bay basin, petroleum rich depressions have ideal conditions for hydrocarbon generation. These conditions are listed as follows: 1) The types of kerogen were generally good, types I – II1 kerogen. 2) The source rock contained a high abundance of organic substances; commonly, the content of organic carbon was higher than 1%; the content of organic carbon could reach 2% – 4% in a major section of the depression. 3) The intensity of hydrocarbon generation was high; on average, the intensity of oil generation was larger than 50×104 t/km2 . 4) The superb hydrocarbon source rocks were broadly distributed; in general, they occupied 50% – 80% of the depressed region (Table 7.1). 5) The resource abundance was high; the average abundance of the resource was larger than 15×104 – 20×104 t/km2 ; in major sections of the depression, the resource abundance could be more than 40×104 t/km2 (Zhao et al., 2004).