Differential Petroleum Migration Behaviors in Faults and Carrier Beds: A Case Study From the Northeastern Part of Baiyun Depression, South China Sea

Abstract

The Baiyun Depression is the most advantage deep-water petroleum exploration area in South China Sea. In recent years, lots of hydrocarbon reserves are found in the northeastern part of Baiyun Depression. In this study, an integration of geological and geochemical analyses is employed to investigate the petroleum origin and migration behaviors in the northeastern part of Baiyun Depression. Petroleum in study area is characterized by high Pr/Ph (pristane/phytane), medium oleanane and “V-shaped” steranes, suggesting it is mainly originated from source rock of Enping Formation in the eastern sag. Generally, buoyancy and excessive pressure are the major driving forces for secondary hydrocarbon migration. However, the main driving force may differ in the different part (or period) of secondary migration. Active faults and sandstones in the Lower Zhujiang Member, which are mainly deltaic deposit, are petroleum migration pathways in study area. Vertical migration is along the faults and lateral through the sandstones. During vertical, faults activities are episodic. The opening faults provide as effective and rapid conduits for vertical migration, and the driving force is deep-buried excessive pressure. The best evidence for episodic petroleum migration is homogenization temperatures of fluids inclusions near the faults are higher than their background burial temperatures. In contrast, petroleum, which is far away from the faults, is differential accumulated. That is, natural gas reservoir is closer than oil. Quantitative grain fluorescence indicates the gas reservoir is paleo-oil reservoir. So we may reasonably conclude that the oil is driven by buoyancy to migrate via sandstones to the closer trap in the early time. Afterwards, the gas arriving at the closer trap displaces the pre-existing oil, and the oil spills out and migrates to the farther trap. During this process, migration pathways are mainly controlled by structural morphology of the sandstones, and the “petroleum mainstreams” are called as the preferential migration pathway (PMP). Secondary hydrocarbon migration in study area is obviously divided into two processes: vertically episodic migration along the faults and laterally smooth migration through the PMP. Obvious differences of migration behaviors exist between the vertical and lateral migration. This study provides an excellent case to understand secondary migration.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018