Focused Fluid Flow System Modeling In The Baiyun Deep-water Area, Pearl River Mouth Basin, Northern South China Sea

The Pearl River Mouth Basin is located on the northern shelf of South China Sea.It is a Cenozoic quasi-passive continental marginal basin developed on the complex basement of Late Cretaceous.The Baiyun Sag lies in the deep-water area of the Pearl River Mouth Basin.It is the largest and deepest sag in the northern South China Sea with the highest hydrocarbon potential.The pressure distribution of the Baiyun Sag has been explained by combining analysis of measured pressure data and prediction of overpressure using the seismic velocity volume,and the pressure evolution history has been modeling via PetroMod software based on the collection of various simulation parameters and the selection of reasonable mathematical models.At the same time,the focused fluid flow system is used as the research object,and the main characteristics of focused fluid flow system in this study area were systematically analyzed by combining macroscopic and microscopic observations from the geophysical analysis for high-resolution 3D seismic data and the thin section,clay minerals and isotope geochemical analysis.Finally,the main mode of focused fluid flow in this study area was established based on the analysis of typical reservoirs.The main results obtained in this research are as follows:?1?Toady,shallow formations are mainly normally pressured but the deep Enping and Wenchang Formations are over-pressured in the Baiyun Sag;The overpressure systems is distributed in the Enping and Wenchang Formations in the western,main and eastern sub-sags,and the over-pressured system in the main sub-sag has the wider spatial distribution and higher pressure coefficients than in the other sub-sags.?2?Multiple over-pressured systems were developed during the overpressure evolution history each with unique pressure increase–decrease cycles.The main sub-sag has two and a half cycles,23 Ma and 10 Ma are episodes of overpressure release.The western and eastern sub-sags have one and a half cycles,10 Ma is when overpressure release occurred.The southern sub-sag has experienced one cycle of pressure increase–decrease but has always remained in the normal pressure range.Fault activity controlled by episodic tectonic events is the major factor for overpressure release,rapid deposition of overlying mudstone formations plays a role in the preservation of overpressure.?3?Compaction disequilibrium caused by the high sedimentation rates is believed to be the main mechanism for overpressure generation in the Baiyun Sag,overpressures linked to hydrocarbon generation in source rocks is a secondary mechanism and the contribution does not exceed 20%.?4?The hydrocarbon migration and accumulation processes are closely related to the overpressure evolution.Following this evolution,the normally pressured zones in the shallow sandstone formations are the most prospective hydrocarbon accumulation areas,the rapid release of overpressure due to the Baiyun and Dongsha Events are important periods for hydrocarbon accumulation,the directions in which pressure reduces fastest is the priority direction for hydrocarbon migration.?5?A variety of vertical focused fluid flow systems including active faults,mud diapirs,mud volcanoes,gas chimneys,etc.were developed in the Baiyun Sag,and the Cenozoic volcanic activity provides a pathway for CO₂ migration.The fluid flow system in the study area is divided into fault-related fluid flow systems,cylindrical fluid flow systems and volcanic complex systems according to its genetic mechanism and characteristics in 3D seismic.?6?The material migrates from the deep overpressure layer to the upper layer during the fluid flow process causing abnormal mineral transformation and a series of diagenetic responses in the shallow reservoir rock.The diagenetic response of fluid flow in the study area is mainly characterized by the dissolution of minerals such as feldspar and calcite,and the precipitation of minerals such as dawsonite and kaolinite.In addition,the intense flow of hot fluid causes the homogenization temperature of fluid inclusions above the highest temperature experienced by the formation,i.e.,the current formation temperature.?7?The rapid release of overpressure in the Baiyun Sag at 10 Ma related to the Dongsha Event,coincided with the petroleum charging period,is a very favorable period for oil and gas accumulation.Another episode of overpressure release linked to the Baiyun Event at 23 Ma,is also a favorable period for hydrocarbon accumulation.Overpressure fluid flow is characterized by fluid accumulation over time and focusing in space,and the combined result not only provides sufficient energy for the formation of focused fluid flow system,but also causes the fluid,driven by pressure gradient,episodic charging in vertical along the faults and cylindrical fluid flow systems such as mud diapirs,gas chimneys and mud volcanoes and rapid accumulation of nature gas.?8?The rapid flow of hot fluids,when causing the vertical focusing of hydrocarbon fluids,it also causes vertical migration of CO₂,which is generated in the mantle and degassing of the Cenozoic volcanic complexes,along the fault system linking the basement and cap layer.Three gas reservoir types including high CO₂ reservoirs,hydrocarbon reservoirs and mixed reservoirs were formed in the Baiyun Sag,under the common control of factors including the charging sequence of hydrocarbon gas and CO₂,the difference in fluid potential between hydrocarbon and CO₂,and the location of focused fluid flow systems.In conclusion,the determination of paleo-pressure field,focused pressure release and fluid flow process has important theoretical significance and production guidance value for revealing the accumulation rules and distribution characteristics of petroleum,predicting favorable exploration zones and avoiding high CO₂ risks.