# Fault Controls on Hydrocarbon Migration--An Example from the Southwestern Pearl River Mouth Basin
## 2 Geological Setting  
The PRMB is an extensional rift located on the continental shelf and slope of the northern South China Sea [54; 55]. The western part of the PRMB, called the Zhu III subbasin, consists of a series of SW-NE striking half-graben and graben structures separated by host areas (Figure 1). The study area is located in the Wenchang A basin, which is one of these half-graben structures, with the main boundary faults in the south and east and onlapping sediments occurring in the north and west of the basin (Figure 1) [56; 57]. The Cenozoic sedimentary succession of the basin comprises a good source-reservoir-seal vertical sequence (Figure 2), with Eocene and Oligocene lacustrine source rocks overlain by Oligocene to Miocene marine sandstone reservoirs, which are intercalated with marine mudstones, forming excellent seals (Figure 2) [53]. During its Cenozoic evolution, the basin experienced multiple tectonic phases, including a Paleocene-to-Early-Oligocene rifting stage, a Late-Oligocene-to-Early-Miocene post-rift subsidence stage and a Middle-Miocene-to-the-present-tectonic reactivation stage [58]. Under the influence of the clockwise-rotating stress field during the Cenozoic, the fault systems in the study area exhibit a complex pattern consisting of multiple stages and directions. During the Paleocene to Early Oligocene, the stress field was characterized by NW-SE extension, resulting in the formation of a series of NE-trending faults primarily within the Late Eocene and Early Oligocene Wenchang and Enping Formations (Figure 2) [53]. Subsequently, from the Late Oligocene to Middle Miocene, the stress field shifted towards near N-S extension, leading to the development of a series of nearly E-W-trending faults, predominantly distributed in the Zhuhai and Zhujiang Formations of the same age (Figure 2). Lastly, since the Late Miocene to the present, the stress field exhibited NNE-SSW extension, giving rise to a series of WNW-trending faults primarily developed within the Miocene Hanjiang and Yuehai Formations (Figure 2). Hydrocarbon charging was initiated in the basin center during post-rift subsidence and has led to oil and gas filled reservoir intervals in the tilted fault blocks associated with the half graben structures at the southern edge of the basin, where several hydrocarbon fields have been discovered (Figure 3). All of these fields are fault-bound, and fault rock seals often control the column heights in the interbedded reservoir-seal intervals. Discoveries have been made in various strata, including the early Miocene (N\({}_{1}\)z\({}_{2}\)), late Oligocene (E\({}_{3}\)z), and even the early Oligocene (E\({}_{3}\)e) (Figure 3d). However, the main hydrocarbon accumulations are observed in the Mid-to-Late Oligocene E\({}_{3}\)z\({}_{2}\) and E\({}_{3}\)z\({}_{3}\) sequences as in the fault-bound fields Field A, Field B, and Field C (Figure 3). The hydrocarbon distribution pattern, both in the plan view and cross-section, indicates that faults F1 and F2 act as the major sealing faults for Field A (Figure 3a). In contrast, the accumulation of Field B primarily relies on the fault sealing provided by F3 (Figure 3b). Similarly, F5 controls the major hydrocarbon accumulation in Field C, acting as the primary sealing fault, while F6 serves as a secondary sealing fault (Figure 3c). There is an evident and noteworthy pattern in the distribution of hydrocarbon fill within the basin. Specifically, the fields situated in the central region of the basin predominantly contain natural gas, whereas the accumulations towards the periphery of the basin contain crude oil. It is noteworthy that the oil accumulations near the basin's edges are also positioned at higher elevations, prompting the inquiry of whether the oil has migrated away from the reservoirs, which are now gas-filled, and what role fault seals may have played during that process.