Deep-water Sedimentology Of The Miocene Zhujiang Formation In Baiyun Sag, Pearl River Mouth Basin

The deposition of deep-water sediment gravity flows is one of new directions of thecurrent oil and gas exploration because of its huge potential hydrocarbon significance. Thedeep-water oil and gas exploration in China started late. The Miocene submarine fan system inBaiyun sag, Pearl River Mouth Basin, northern South China Sea is a key strategic area and oneof the goals. The important discovery of Liwan3-1gas field in April2006determined the hugepotential for oil and gas resources in Pearl River Mouth Basin. Based on the detailedobservation and description of precious drilling cores, the seismic data, logging data and theformer research, a systematic study of deep-water depositional systems of the MioceneZhujiang Formation in the Pearl River Mouth basin was made, and the following results wereobtained:⑴Based on petrographic analysis of drilling cores, fourteen types of lithofacies wereidentified in the depositional system of the Miocene Zhujiang Formation, Pearl River MouthBasin, including mega-thick and thick-bedded sandstone facies, thick-bedded normal gradedsandstone facies, thick-bedded inverse grading and inverse-normal graded sandstone facies,parallel bedding sandstone facies, tabular and trough cross-bedding sandstone facies, wavy,vein and lenticular bedding fine sandstone facies, two-way cross-bedding sandstone facies,slumped and deformed sandstone facies, thin sandstone facies, thin-bedded interbedded (silt)sandstone and mudstone facies, thick-bedded siltstone facies, bedded mudstone facies, wavy,vein and lenticular globigerina limestone facies, and heterologous bioclastic limestone facies.Six kinds of lithofacies associations were identified, including sandstone association,mudstone association, sandstone and mudstone interbedded association, siltstone association,deep-water in-situ limestone association and deep-water off-site limestone association. Thencombined with logging data, seismic facies and the former research, the submarine fan facies,basin facies and deep-water traction flow were identified in Zhujiang Formation. Thesubmarine fan can be further divided into main channels of the inner fan, distributary channelsof the middle fan, levee-overbank microfacies and outside fans. The lower ZhujiangFormation were basin-floor fan and slope fan dominated by sandstone lithofacies association.The main channels of the inner fan and distributary channels of the middle fan are the mostfavorable microfacies for reservoir development. The middle to upper Zhujiang Formation were outer fan and basin facies consisting mainly of the mudstone association, siltstoneassociation and deep-water in-situ limestone association. The model of “Shelf edge delta-Submarine fan-Deep-water traction flow" was established.⑵An abundance of tractive structures was developed in Zhujiang Formation, BaiyunSag, Pearl River Mouth basin. The tractive structures included doubling mud layers, tabularcross-bedding and trough cross-bedding sandstone facies, wavy-bedding, flaser-bedding andlenticular-bedding siltstone facies, bidirectional cross-bedding siltstone facies, wavy-beddingand flaser-bedding globigerina limestone facies. They were often rich in bioturbation. Theywere totally different from pelagic low-density turbidite and hemipelagite-pelagite. Combiningwith the depositional setting, the rich tractive structures were believed as deep-water deposition.The discovery of the deep-water tractive deposition in Miocene Zhujiang Formation would beof great importance. It could be inferred that deep-water current had been existing since theformation of the shelf-break during late Oligocene (23.8Ma) in Baiyun sag, which influencedand controlled the resources and depositional processes in Zhujiang Formation. The tractivecurrent may influence and change the composition and their distribution. And this would have agreat enlightenment to the reconstruction of the palaeo-ocean circulation in the South China sea.Further studies will be needed.⑶The sandstones of Zhujiang Formation in Baiyun Sag mainly were lithic arkoses andfeldspathic lithic sandstones, charactered by high content of feldspars and fragments. Thefragments were mainly metamorphic and volcanic, and a few plutonic. The quartz undercathodoluminescence mainly show violet-blue and purple-brown light. The feldspar mainlyshow bright blue light, and only a small amount show yellow green light. REE results indicatethat average ΣREE of mudstones was closer to that of Mesozoic intermediate-felsic volcanicrocks in the Yanshan fold belt. δEu had obviously negative anomaly. LREE was rich and HREEwas defective. The distribution model was consistent with the upper crust, and close to thecurve of acid volcanic rocks and granites in the Yanshan fold belt. Although active tractioncurrent existed during the Zhujiang Period, the main source area was still the ancientsedimentary rocks in the inland areas of South China and the acid volcanic rocks and granites inYanshan Fold Belt, which are carried by the ancient Pearl River. In the latest Oligocene, theSouth China Sea movement made the second sea-floor spreading of South China Sea basinand the jumping of the shelf break from the south side of the sag to the north side in BaiyunSag. This tectonic event occurred in the adjacent basins around the Tibetan Plateau, such asQaidam Basin, Tarim Basin, Linxia basin, Xunhua basin, Kumkol basin, Hoh Xil Basin, andalso occurred in the eastern basins, such as Yingge Sea Basin and Bohai Bay Basin, reflectingthe Qinghai-Tibet Plateau uplift event may occur in the latest Oligocene, so that theprovenance during the Zhujiang period expanded westward and the source area became larger.⑷According to the relative sea-level changes of the Pearl River Mouth Basin, theseismic profiles, logging and core data, three sequence boundaries of class Ⅲ correspondingto episodic tectonics and one maximum flooding surface were identified in ZhujiangFormation. The Zhujiang Formation could be divided into a long-term sequence, three mid-term sequences and five short-term sequences. The reservoirs were MSC1and MSC2sequences.⑸The Baiyun sag in Pearl River Mouth basin has high geothermal gradients and heatflow values, and three cycles of overpressure. According to the impregnating thin sections,scanning electron microscopy, vitrinite reflectance aand X-ray diffraction, the sandstonediagenesis of the submarine fan of the Miocene Zhujiang Formation in Baiyun Sag wasstudied in detail. The destructive diagenesis on the reservoirs included the main compactionand cementation. The constructive diagenesis included dissolution, ferrocalcite cementationand overpressure. Integrating paleo-temperatures, the diagenetic ways, the diagenetic typesand evolutional sequences, we believed that the Zhujiang Formation sandstones were mainly atB stage of Middle diagenesis.⑹The deep-water limestones in Zhujiang Formation were mainly non-gravity flowcarbonate rocks deposited by the deep-water traction current (such as contour current, internalwaves and tides). The limestones were mainly formed by the complete globigerina and is apotential oil and gas reservoir in Pearl River Mouth Basin. The diagenesis of the limestoneswere studied in detail. It was believed that all the significant diagenesis are destructive andoccluding pores in burial. The process of mechanical compaction, pressure solution, the fillingand cementation of secondary minerals played a major role in the reduction of primaryporosity, they were subject to the clay content and compression resistance of framework grains.The deep-water limestones in Zhujiang Formation had experienced mechanical compactionand pressure solution for about2.5~3km burial depth (including1.0~3km water depth),however, they entered the overpressure cycles at an early stage. Then high pore fluid pressureresisted mechanical compaction and pressure solution in deeper burial, and the limestones weredominated by the filling and cementation of secondary minerals. At the same time devoid oflater tectonic fracturing, the deep-water limestones in Zhujiang Formation ultimately becameporous reservoirs with high porosity and low–very low permeability.