DIFFERENTIATED GAS DISTRIBUTION, MIGRATION AND ACCUMULATION IN THE CENTRAL DIAPIR BELT OF THE YINGGEHAI BASIN

The gas found in the Yinggehai Basin is a kind of coal-type gas from Miocene source rocks. However, the gas accumulation models based on it is hard to explain the differentiation of gas geochemistry in the Dongfang and Ledong areas. Based on the comprehensive study of gas geochemistry, source rock, tectonic evolution and diapir activity, it is concluded that the coal-type gas can be further subdivided into the Meishan type and the Shanya type. The two groups of gas distributed in the Dongfang area and the Ledong area, are respectively from Meishan source rock and Shanya source rock. The differentiation of distribution depends upon three factors, say: the thickness of Meishan formation, the generation process and the migration system. In the Ledong area, both the gas from Meishan and Shaya Formations can migrate into the shallow and middle-deep reservoirs. Same process might happen for the gas from Meishan Formation in the Dongfang area. However, the gas from the Shaya Formation is only accumulated in the cores of diapirs as well as the traps closed to the structural ridge in the Dongfang area. Therefore, the difference in gas accumulation in the Dongfang and Ledong areas are result from the different distribution of gas groups. As both of them are controlled by the diapir activity, the gas accumulations in the Dongfang area and the Ledong area are certainly similar. The similarity and difference should be careful considered in the selection of exploration targets.