Pathway System of Large-Scale Petroleum System and Its Controls on Hydrocarbon Accumulation in the Bozhong Sub-Basin, Bohai Bay Basin

The geological evolution of the Bozhong sub-basin, characterized by rapid subsidence and strong fault activities since the initiation of the Dongying Formation, exerts important effect on the characteristics of hydrocarbon accumulation in this area, which is distinguished from those in other areas in the Bohai Bay Basin. Uplifts around depressions serve as predominant sites for hydrocarbon accumulation, and the distribution and evolution of pathway systems have obviously been controlling hydrocarbon migration and accumulation. Based on clarification of the distribution of hydrocarbon source rocks and petroleum resources in the large-scale Bozhong petroleum system, growing characteristics of all kinds of potential pathways and their spatial relevance are analyzed by means of multiple data, and then three pathway system types controlling hydrocarbon migration and accumulation: fault-dominant, carrier bed-dominant and unconformity-dominant types are identified. Long-term growth faults adjoining uplifts act as the predominant pathways for vertical fluid release and hydrocarbon migration, and control hydrocarbon accumulation in the Neogene reservoirs on uplifts. Especially in the active stage of tectonic movement, the open long-term growth faults must be preferential pathways for the rapid vertical hydrocarbon migration. The interconnected sandbodies in the Paleogene Shahejie and Dongying Formations, directly contacting with active hydrocarbon source rocks, serve as the primary sites and starting stations for the early and late hydrocarbon migration and accumulation, respectively. Therefore, they not only control hydrocarbon accumulation in the Paleogene traps, but also directly impact hydrocarbon supply to the Neogene trap on uplifts to some extent. Unconformity T8, resulted from regionally tectonic transformation in last Mesozoic, serve as pathway for deep lateral hydrocarbon migration and control hydrocarbon accumulation in the buried hill type reservoirs due to its improved permeability attributed to long-term weathering process. Three types of pathway systems all impact even control the times, locations, velocities and scales of hydrocarbon accumulation in various degrees. However, the fault-dominant type pathway system shows the most outstanding effect among them.