Research On Sequence Stratigraphy And Oil-Gas Pools Forming Conditions Of Damoguaihe Group In Wuerxun Depression, Hailaer Basin

Guided by the theories of sequence stratigraphy, structural geology, sedimentology and petroleum system and oil and gas pool forming dynamics, using seismic data, borehole log, logging curves and cores, based on the deeply study of structural framework, structural style and fault system in Wuerxun depression, detailed research and analysis on the sequence framework, sedimentary facies, structural trap, stratigraphic and lithologic trap, reservoir, hydrocarbon distribution and its controlling factors in the Damoguaihe group in Hailaer Basin have been carried out in the paper.The stratum in Wuerxun depression can be divided into 3 structural layers from bottom to top: fault depression structural layer (Tongbomiao group and Nantun group), fault-depression structural layer (Damoguaihe group and Yimin group) and depression structural layer (Qingyuangang group). There are 3 categories and 8 sub-categories of structural styles: stretch structural pattern (steep slope listric fan, rolling anticline, fault slope break on gentle slope, synthetic fault bench and antithetic fault bench), shear structural pattern and inversed structural pattern (fault inversion, folding inversion and mixed inversion), where steep slope listric fan, fault slope break belt on gentle slope and shear summed inversed structural belt controls hydrocarbon concentration in Damoguaihe group.Taking Surennuoer fault zone, the uplift zone beside Well Wu 21 and transform zone in Bayantala as boundary, Wuerxun depression can be divided into four structural active areas on plane and different areas develop different half grabens or grabes due to faults with different strikes in each area. Wuerxun depression develops two fault systems, which are lower fault system (in Tongbomiao and Nantun Group) and upper fault system(in Damoguaihe and Yimin group ) The lower fault system is formed in Tongbomiao and Nantun group deposition period when depression is controlled by NW-SE direction tension stress. The upper fault system is formed in Yimin group deposition period when depression is controlled by EW direction tension stress. After that stage depression is controlled by EW direction feebleness compress stress and forms some fault inversed structure, folding inversed structure and mixed inversed structure from the end of Yimin group to Qingyuangang group deposition period.Combined seismic data, well logging data, cores with chemical examination data, we divided Damoguaihe group into fifteen fourth-order sequences, which form an entire super sequence with five deposition stages, the first sequence (D I 1) is the stage of braided river delta, comparable shallow lake and fan delta deposition of lowstand system tracts(LST) period; The second and third (D I 2, D I 3)sequences are the stages of braided river comparable deep lake and fan delta deposition of transgressive system tracts(TST) period. The fourth and fifth ((D I 4, D I 5) sequences are the stages of meandering river delta, comparable deep lake braided river delta facies of early stage of highstand system trarcts(HST) period. The sixth-tenth (D II 1-D II 4) sequences are the stages of meandering river delta, comparable shallow lake braided river delta facies of middle stage of HST period. The eleventh -fifteenth (D II 5-D II 10) sequences are the stages of anastomosed stream delta, shallow lake braided river delta of the end stages of HST period. Sandstone is well developed with a higher sand-land ratio in the stages of LST, TST and middle stage of HST, while sand is not well developed in the rest of the two stages.According geologic data combined with reservoir seismic inversion technique, the research on hydrocarbon accumulation in the formation of interest and analysis of its control factor has been finished. It is shown that there are 17 main fault zones which have the mechanism of strike-slip, slanting stretch and horizon decoupling. The long term inherit developed faults (oil source faults) and their upper layer fault system forms dense fault zones, which control hydrocarbon accumulation in Damoguaihe group. From bottom to top Damoguaihe group develops four reservoir and capping sets. Hydrocarbon mainly distributes in set II and III in Wunan area while distributes in set I in Wubei area. Oil reservoir at here is mainly fault-lithologic reservior and fault blocks reservoir.The quality of hydrocarbon source rocks in Damoguaihe group is mainly bad-middle class, lack of hydrocarbon generate potential. Oil mainly comes from the dark shale affected by segment one of Nantun Group. The source and accumulation period of oil in Damoguaihe Group in Wubei area are the same as oil in Nantun Group. Crucial period of crude oil accumulation is the end of Yimin Group. On the other hand, the situation in Wunan area is different. Variation of GOI and crude oil maturity show that there is paleo-oil pool in Nantun group. Oil in Damoguaihe group is probably secondary oil pool due to the adjustment of oil in Nantun formation. The crucial period of crude oil pool accumulation is from the end of Yimin group to Qingyuangang group.The preferred oil accumulation sandstone in Damoguaihe group has four characteristics: the accumulative thickness is more than 10 meters and sand percentage is higher than 20% in stratum with more than 10% porosity and permeability higher than 0.2md. The main control factors of oil accumulation in Damoguaihe group are oil source faults and sand body orthogonal matched in hydrocarbon kitchen of Nantun group, structural and lithofacies belt control oil accumulation in Damoguaihe group. Thus there are three types of oil accumulation modes:â‘ oil in fan delta in abrupt slope fault step zone, after vertical movement, migrates laterally towards abrupt slope then shielded by fault and finally forms accumulation in reservoir;â‘¡oil in delta front in gradient break fault zone, after vertical movement, migrates laterally in a short distance then shielded by the border of sand body and finally forms accumulation in reservoir;â‘¢oil in delta front in strike slip inversed structure belt, after vertical movement, migrates laterally in a short distance then shielded by late period fault plane and sand body border and finally forms accumulation in reservoir mode.