| The Xihu Sag is a fault depression basin of which the east is a rift and the west is an overlapping. The east of Xihu Sag is cliffy and the west is gentle .it is situated in the east of the East China Sea Shelf Basin. It closes to the Diaoyu Island folded doming-up region. The direction of strike in it is NNE. Its area is about 4.6×104km2. The Huagang Formation in this sag is a suit of river facies and lake facies strata, having thickness from 600 - 1000m. It is one of the formations of interest in exploration activity in the Xihu Sag.Based on multidisciplinary analyses of many well drilling, drill core,well logging, palaeophyte,seismic and some other geologic information, we recognized Sb4,Sb5,Sb6,the interface of the peak and the bottom in theⅢlevel sequences. They are relevant to the seismic reflection boundaries T30,T21 and T20. we classified the sequence strata through the researches of the lithology characters, electric natures and features of the palaephyte, and compared with other log. We established the sequence framework and model of Huagang Formation in Xihu Sag through repeated comparision an fitting between well drilling sequence and seismic sequence. Huagang Formation could be devided into 2Ⅲlevel sequences. They have better horizontal trace comparison in well drilling and seismic. But for the reason of data, theⅣlevel sequences are hard to recognized.Through multidisciplinary analyses of the data of the drilling core , well drilling and well logging, we were certain that there are 7 sedimentary subfacies and 9 sedimentary microfacies occurred. The sedimentary subfacies are braided predelta, braided delta front, shore-shallow lake, shallow-half deep lake, river channel and river flood, while the microfacieses are subaqueous distributary channel, interdistributary bay, delta front sheet sand, distributary bar, braided prodelta mud, distributary channel, flood plain and shore-shallow lake sand bar. These subfacies and microfacies are belonging to 3 types of sedimentary facies, braided stream, braided delta and lake. Which compose 3 types of sedimentary systems.The Huagang Formation is a layer that deposited at the time that the Xihu Sag went into downwarping after the uplifted in the late time of the Pinghu Formation rift faulting. It is mainly lake deposit. It reflects the sedimentary progress that water ingresses. It mainly happens 2 episodic uplifts under the falling back ground.There are 3 suits regional assembles of reservoir strata and capping formations. The first suit is upward half-cycle of sequence SⅢ4. The regional capping formation is the thick mud which develops near the maximum transgression of sequence SⅢ4. It is composed of predelta mud and shallow lake mud. The reservoir strata is delta front sand body that deposits in upward half-cycle of sequence SⅢ4. The second suit is the assemble of descending half-cycle of sequence SⅢ4 and upward half-cycle of sequence SⅢ5. The regional capping formation is the thick mud which develops near the maximum transgression of sequence SⅢ5. It is composed of predelta mud and shallow lake mud. The reservoir strata is delta front sand body and mouth bar sand body that deposit in descending half-cycle of sequence SⅢ4 and the upward half-cycle of sequence SⅢ5. The third suit is descending half-cycle of sequence SⅢ5. Its regional capping formation is the thick mud which develops at the bottom of the descending half-cycle of sequence SⅢ5. It is mainly composed of predelta mud and shallow lake mud. The reservoir strata is delta front sand body that developed in the early descending half-cycle of sequence SⅢ5. In addition, there are also many suits of local assembles of reservoir strata and capping formations that developed in Huagang Formation.The reservoirs in the formation are mainly controlled by the deposition condition, such as sediments provenance, diagenesis and some other elements. The depositional environment and hydrodynamic force affect thickness, grain size, shale content and carbonatie content of the sandstone directly. The provenance determines the rock type of the depositional reservoir. It also affects compositional maturity, texture maturity and diagenetic process of the rock. Diagenesis could affect original porosity, secondary porosity and cement content.Finnally, we analyzed the beneficial reservoir belt in this area. The beneficial reservoir belt of sequence SⅢ4 in Huagang Formation distributes at the Pinghu Structural Belt in the middle of the West Slope Zone, Xileng Structural Belt in the middle of the Center Anticlinal Zone and Sudi Structural Belt at the south of the Center Anticlinal Zone. The beneficial reservoir belt of sequence SⅢ5 in Huagang Formation distributes at Pinghu Structural Belt and Xileng Structural Belt.
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