The Genesis and Mechanism of Polygonal Fault Systems in Jinghai Sag of The Pearl River Mouth Basin

Abstract

The Jinghai sag is located along the continent slope of the Pearl River Mouth Basin, a typical Mesozoic-Cenozoic superimposed basin in Continent-Ocean Transitional Zone(COT). Previous studies on the fault systems, mainly focused on the faults of the continent shelf which controlled the depositional evolution and the hydrocarbon migration, very little research has been conducted on the polygonal fault systems(PFS) of continent slope, which characterized by polygonal planform geometry, small fault throw and extensive distribution. The deposition rate of continent slope reached summit two times during 17.5Ma~15.5Ma and 13.8Ma~12.5Ma. The seismic velocity of mid-Miocene formation which developed PFS has a relatively low velocity, showing the overpressure which caused by rapid deposition rate. Based on magnetic and gravity data, the Mohorovicic Discontinuity and the average heat flow of continent slope is shallower and higher than that of continent shelf area respectively. The overpressure has formed in response to the relatively high heat flow and the thick mid-Miocene mudrock of the Jinghai sag. The ODP(Ocean Drilling Program)1148 is located in the south of the Jinghai sag, which can be used to study the paleo depositional environment. Quantitative analysis indicates smectite transited to illite rapidly at 16Ma. The PFS of the Jinghai sag is related to overpressure which mainly caused by undercompaction, and partly due to hydrothermal-pressurization and smectite illitization(S/I). With the overpressure of the Miocene mudrock of the Jinghai sag reach the breakthrough pressure, the PFS of the Jinghai sag initiated to active, and then the PFS returned to inactive until the pore pressure reached the breakthrough pressure next time. The extensively distributed PFS are formed during periods of pressure accumulations and releases. The hydrocarbon cannot migrate upward when the PFS is inactive, and hydrocarbon can only migrate under Miocene deep water mudrock; when the pore pressure accumulated to breakthrough pressure, the PFS can play a role as migration path for underneath hydrocarbon. The areas where the PFS undeveloped in Miocene mudrock are interpreted as sandstone-rich reservoir in deep water environment, and the deposition facies analysis can be controlled by the distribution and characteristics of the PFS where have no prior wells drilled. The genesis and mechanism of the PFS developed in Jinghai sag have practical meaning in costly deep water exploration.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017