Study On Sequence Stratigraphy Of Damoguaihe Group In The Northwestern Beier Depression

The northwestern Beier depression is one of the most important petroleum exploration regions in Hailaer basin. Guided by the theories of sequence stratigraphy, sedimentology, helped by seismic data, borehole log, logging curves and cores, detailed research on the sequence stratigraphical framework, depositional system of the Damoguaihe group in the northwestern Beier depression have been carried out in the paper. The formation of Damoguaihe group in the northwestern Beier depression is a super sequence which can be divided into five stages. They are the lowstand system tracts (LST) period, the transgressive system tracts (TST) period, the early stage of highstand system trarcts (HST) period, the middle stage of highstand system trarcts (HST) period and the late stage of highstand system trarcts (HST) period. The five stages can be subdivided into twelve fourth-order sequences. LST includes one fourth-order sequence(DⅠ1); TST includes two fourth-order sequences(DⅠ2 and DⅠ3); the early stage of HST includes two fourth-order sequences(DⅠ4 and DⅠ5); the middle stage of HST includes four fourth-order sequences(DⅡ1-DⅡ4); the late stage of HST includes three fourth-order sequences(DⅡ5-DⅡ7). Four types of facies, fifteen types of sub facies and twenty-six types of micro facies have been distinguished in the area. During the LST deposition stage, the water was relatively shallow. Fan delta was developed near growth faults, while braided river delta was developed in the northern part. During the TST deposition stage, the lake water sharply became deep and braided river delta was developed in the eastern, western and northern parts of the study area. The water of the early stage of HST was relatively steady, and braided river delta was still developed in the western slope, and meandering river delta was developed in the northern part. The water of the middle stage of HST became shallower and only meandering river delta can be found in the stage. The water of the late stage of HST became much shallower than the middle stage and the anastomosed stream delta was developed in this stage. The sand body is remarkablely controlled by sedimentary facies, and sequence stratigraphy provides a powerful weapon for the prediction of lithologic traps. Combined with sequence evolution regulation, planner distributional characteristics of the advantaged micro facies and sand body thickness characteristics of every fourth–order sequence, four kinds lithologic traps is predicted in this area. They are the up-dip pinch-out sandstone traps of the front of the fan delta, the up-dip pinch-out sandstone traps of delta, sand lens traps of the lay out belt of belt and the turbidite traps respectively. Based on the characteristics of source rock, reservoir and cap rock, the formation of Damoguaihe group can be divided into three sets of reservoir-cap combination. There are three favorable zone in the first one, two in the second one and two in the third zone respectively.