The pre-Cenozoic basement in Bohai Sea area is basically covered by Mesozoic, which are mainly consists of volcanic rocks except from several uplift areas. However, due to few exploration for volcanic rocks as the target layer, only 428, Qinhuangdao 30, Kenli 16-1 and other small-medium-sized oil and gas fields are found accidentally. In particular, research on seismic identification and reservoir prediction of volcanic edifice has not been carried out yet. Prospecting in volcanic rock in Songliao and Junggar Basin shows that Oil and gas accumulation is not only related to the lithology and lithofacies of single volcano, but also controlled by volcanic edifice macroscopically, which is more concentrated in the center of volcanic edifice. There is a good correlation between the volcanic vent and strike-slip and extensional structural belt of Yanshan period showing that the middle Yanshan strike slip extensional fault controlled the main volcanic activity in central of Bohai Bay. There are four main volcanic eruptions has been recorded and the U-pb-zircon dating data showed that the age of volcanic rocks ranged from 94 to 130 Ma which corresponds to the middle Yanshanian period and became younger eastward which is related to the retreat of the paleo Pacific plate and the eastward movement of the hot spot. According to the geochemical analysis, the volcanic rocks are mainly from collision island arc with some shallow continental crust.
In Bohai Sea area, Yanshanian extension and Himalayan extension strike slip jointly control the formation of effective fractures. In addition to the formation of new fractures in Himalayan period, early filling fractures can be reactivated and become effective reservoirs again. Firstly, tectonic fragmentation is the key to connect multi-scale isolated pores and improve seepage capacity. At the same time, fracture is the premise of matrix dissolution to form fracture network reservoir. At last, the tectonic fragmentation is more likely to occur in the zone near the crater where the primary fractures are developed. Domestic volcanic exploration has proved that the near Crater facies zone with primary fractures such as blast fractures and joint fractures is more likely to respond to structural fragmentation. In these areas, the structure is high, the fault density is high and the activity rate is high, so the exploration potential of volcanic rocks is huge.
Based on the drilling data of 24 wells that has encountered volcanic rocks, this paper establishes the geological and seismic identification marks for the division of volcanic eruption periods. By using high-precision three-dimensional seismic data, we trace and depict hundreds of volcanic edifice in the central and western areas of the Bohai Sea. Various geophysical techniques such as 3-D visualization, wave class, coherence slice as well as fault-likelihood attribute were used to characterize craters, volcanic necks and volcanic edifices. Meanwhile combined with fracture prediction we find out the high quality reservoir distribution and the prospecting well has turned to be a success and the highest yield raise to more than 6600 barrels per day which also confirmed the accuracy and operability values of this method.
Note: This paper was accepted into the Technical Program but was not presented at IMAGE 2021 in Denver, Colorado.