Paleogene Sag Structure,Magmatic Transformation And Source Rock Distribution Of Xijiang Main Sag

Magmatic activity developed in the Xijiang main sag in Paleogene.Drilling disclosure and seismic identification show that it mainly developed in Wenchang period and Enping period,and it is dominated by volcanic eruption in Wenchang period.There are both volcanic eruption and magmatic intrusion in Enping period.Magmatic activity has the migration characteristics of Wenchang period mainly in the east of the basin and Enping period in the west of the basin.After Zhuhai period,there was no magmatic activity in Xijiang main sag,but the geochemical classification of volcanic rock samples shows that the magma sources from Wenchang period to Enping period are different.Comparing the temporal and spatial distribution of magma with other regions in the basin,it is show that the magmatic activity in Xijiang main sag was concentrated in Paleogene,which is related to the near continental margin background of passive continent during the early development of the basin,Magmatic activity is very important to the development of extensional fault sag structure and low angle detachment fault in the Rift-Period,which affects the evolution of sag in this period.Magmatic activity is no longer developed in and after Zhuhai period,and the basic structure is no longer changed.The tectonic evolution of the basin during the Rift-Period is mainly controlled by nee trending low angle boundary faults in the north.The main Xijiang sag developed and evolved under the extension detachment system of the Pearl River Mouth Basin in the Paleogene.The evolution of the basin during the Paleogene period is mainly controlled by NEE trending faults.The sag controlling faults are the result of the superposition and development of faults in different periods for a long time.The evolution analysis of NEE low angle sag controlling fault shows that it cuts the preexisting imbricate thrust plane of Mesozoic subduction period.Therefore,it is considered that the low angle fault of early Wenchang period inherits the pre-existing low angle extension normal fault of Shenhu period,and the upwelling of deep magma in Paleogene is an important condition for the development of low angle fault.NWW-EW trending faults are mainly active in Enping period,but there are early Wenchang activity in the east of the basin,which is the result of the development of similar polarity regulation zone of NEE trending faults in the early stage,and E Paleo-Rise is developed.After the faults in late Wenchang period are connected with each other,the activity is weak until Enping period is active again.Under the continuous transformation of deep magmatic heating,the basin developed two types of structural styles: the West synclinal extensional structure and the east oblique oblique extensional structure.In the west of the basin,the same parallel displacement transfer zone developed on the low angle fault slip surface,and A Paleo-Rise continued to move until the end of Enping.The rise activity and magmatic activity show correlation,indicating the connection between each other.The Wenchang period in the Xijiang main sag has the characteristics of high subsidence rate,which is an important pre-requisite for the formation of deep lacustrine source rocks.Through drilling sedimentary facies analysis,provenance analysis and seismic facies identification,the sedimentary development model of Wenchang period in Xijiang main sag is established,and the sedimentary facies distribution of each member of Wenchang Formation in Xijiang main sag is restored on the basis of seismic facies distribution.The development of deep lacustrine source rocks in the basin is affected by the differential evolution between the East and the west of the basin in the early and late Wenchang period.The F1 fault in the early Wenchang period controls the sag at a low angle,extensive coarse clastic marginal facies deposits are developed in the basin,which is located in the subsidence center of Xijiang 33 East subsag.At the same time,it is controlled by NWW-EW trending F3 fault,and thick lacustrine source rocks are developed;In this period,the stratum thickness in the west of the basin was small,mainly marginal facies deposition,and the distribution of lacustrine source rocks was relatively limited;In the late Wenchang period,the sag controlling fault gradually became steeper and the basin expanded.The NEE trending controlling fault in the west was more active,and the subsidence center of the basin migrated to west.At the same time,under the control of NEE trending F7 fault,A Paleo-Rise was formed to separate Xijiang 33 Xiwa and Xijiang 32 Xiwa.A relatively closed lake environment was formed in the west of the basin,which was conducive to the formation of stable and developed thick lacustrine source rocks.The igneous rock of magmatic activities in the Rift-Period occupy the sedimentary space or destroy the formed source rocks to varying degrees.