# Fault Controls on Hydrocarbon Migration--An Example from the Southwestern Pearl River Mouth Basin
## 5 Discussion  
### Role of Faults for Fluid Migration in the Wenchang A Subbasin  
Hydrocarbon fluid flow within the Wenchang A subbasin is strongly linked to faults acting as both fluid pathways and flow baffles, with the hydrocarbon charge of fault-sealed reservoirs occurring along faults that cut into the deeply buried source rocks (e.g., F2, F4, F6).  
The maturity of source rocks within the basin varies from post-mature (Ro 2.0-2.2%) in the central basin to mature (Ro 1.0-1.6%) close to the southern basin boundary fault, with hydrocarbon expulsion having been estimated to have primarily occurred between 26.5 and 16 Ma in the central basin and since the N\({}_{1}\)Z\({}_{2}\) post-rift sequence (23-18.3 Ma) in the studied fault blocks [57]. During early hydrocarbon (oil) expulsion in the central part of the basin, the caprocks that form an integral part of today's traps had not yet formed [67], leading to a loss of these early hydrocarbons. During later hydrocarbon expulsion, gas in the very mature central areas of the basin and oil in the areas with a lower vitrinite reflectance migrated vertically parallel to the fault and along permeable pathways in the fault damage zones of deeply seated faults, such as F2, F4, and F6. Note that the faults were seismically active during the hydrocarbon migration, which may have enhanced the permeability in the fault damage zone fracture networks as they were close to critically or critically stressed [68; 69; 70; 71; 72; 73]. Hydrocarbon flow entered permeable reservoirs, which are capped by mudstones forming the top-seals. The period of fault active and charging was followed by a long structural quiescence from the Middle Miocene onwards and without reactivation of these hydrocarbon sealing faults. Faults located up dip have fault rock seals and form lateral flow barriers, leading to hydrocarbon accumulations. While these sealing faults may have been geomechanically unstable during the initial hydrocarbon migration, they are currently in stable conditions, and there are no indications for previous leakage from the reservoirs. The traps are all presently not filled to spill, and fault seal predictions using the [PERSON] et al. [39] algorithm suggest that the fault seals can support higher column heights.  
Previous studies conducted in various rift basins located offshore China, such as the Zhu I subbasin of the PRMB [74], the Xihu subbasin of the East China Sea Shelf Basin [10], and the Qinan area [27; 46], Tangbei area [75], and Bozhong subbasin of the Bohai Bay Basin [7], have indicated similar hydrocarbon migration and accumulation patterns controlled by fault seals, albeit with some differences. Besides China, the North Sea also exhibits a pattern of hydrocarbon migration, accumulation, and distribution under the control of fault sealing [21; 23; 42]. In all of these cases, it has been demonstrated that the sealing capacity of fault rocks directly influences whether faults act as conduits or barriers for the flow of hydrocarbons, thereby impacting the distribution of hydrocarbons. However, what sets our study apart is the unique phase configuration of hydrocarbons resulting from variations in source rock maturity and distribution, with gas generated from the basin center and oil generated from the slope/border of the basin.