Evolution of the Oligocene-Miocene shelf slope-break zone in the Baiyun deep-water area of the Pearl River Mouth Basin and its significance in oil-gas exploration

Abstract

The exploration practice and integrated sequence-stratigraphic studies for years demonstrated that the Oligocene-Miocene boundary was an important transfer interface of sedimentary-tectonic-climate conditions in the Pearl River Mouth Basin and corresponded to the Baiyun movement(in about 23.8Ma),during which the seafloor spreading ridge of the South China Sea jumped,was deflected and accompanied with material source changes of the paleo-Pearl River.This event resulted in the shift of the shelf slope-break zone from the neighborhood of the southern uplift area of the Baiyun Depression in late Oligocene to the northern slope in Miocene.Moreover,the shelf slope-break zone has basically stood there since about 21Ma.In OligoceneMiocene,two shelf slope-break zones with different development types and locations controlled the development of the two different sequence units in the Baiyun Depression,i.e.a depositional shelf slope-break zone extending in NNE orientation shifted from NW to SE until 23.8Ma and developed a regressive progradation sequence unit in the Zhuhai Formation,and subsequently,another shelf slope-break zone with NE strike has stood steadily around the north slope of the Baiyun Depression since 21Ma and formed a transgressive aggradation sequence unit.Moreover,the migration of shelf slope-break zones also controlled the development of prospective reservoirs and hydrocarbon accumulation belts(including shallow marine delta with relatively abundant sand,lowstand shelf-edge delta and deepwater fan sandstones in the continental slope of the Pearl River) within various sequences of the Baiyun deepwater area.It was confirmed that both SQ23.8 and SQ21.0 third-grade sequences were the product of sedimentation corresponding to the Baiyun Movement,and they were of conditions to develop lowstand deepwater fans with most abundant sand in the Baiyun deepwater area.Integrated investigations basically demonstrated that reservoir-seal assemblages of the Oligocene Pearl River coast-delta-deepwater fan depositional system and Neogene deepwater deposits were major objective strata for further exploration.Up to now,natural gas discoveries in the Baiyun deepwater area and Panyu gas province have confirmed the control effect of the shelf slope-break zones on gas reservoirs,indicating a great potential of petroleum exploration in the Baiyun deep-water slope area of the northern South China Sea.