鄂尔多斯盆地盐池地区中生界油藏分布规律及成藏主控因素

Distribution and main controlling factors of Mesozoic reservoir in Yanchi area of Ordos Basin

基于井震结合,精细刻画低级序断层特征,分区带分析油藏类型及分布,研究中生界油藏的成藏主控因素。研究结果表明,研究区自西向东依次发育西缘冲断带、过渡带及天环拗陷带3个区带。西缘冲断带大断层发育,构造起伏大,主要发育断块油藏,油层分布层位分散,油藏规模较小,孤立分布。过渡带低幅度构造、低级序断层发育,低级序断层主要为早白垩世末之前形成的逆断层(Ⅰ期),少数为古新世(Ⅱ期)形成; Ⅰ期断层早于中生界油藏的主成藏期,断层的封闭与否关乎油藏的形成及流体分布,油藏类型主要为构造-岩性油藏,局部形成连片。天环拗陷带油藏以岩性油藏为主,大面积连片分布。过渡带、天环拗陷带油藏主要分布在延长组中下组及侏罗系。研究认为,该区构造控藏作用明显,具备Ⅰ期低级序断层沟通油源是基础,发育低幅度构造及相对高渗储集区是核心,油藏是否遭受Ⅱ期断层作用调整改造是关键。早白垩世末期,原油在异常压力驱动下,沿Ⅰ期开启断层及叠置砂体输导运移,受围岩遮挡或Ⅰ期封闭断层封堵,在西缘冲断带的构造圈闭、过渡带及天环拗陷的低幅度构造、相对高渗储集区聚集成藏,形成断块油藏、构造-岩性油藏及岩性油藏;受Ⅱ期断层影响,部分油藏遭受调整破坏。

Because of the complicated structural condition,different grade and period faults,it is great significance to define the Mesozoic reservoir characteristics and the main controlling factors for petroleum exploration. Based on the combination of wells and seismic,the authors depict the feature of lower order faults,analyse the reservoir characteristics,and focus on the main controlling factors. According to the structural distribution and fault grade,there are three zones from west to east,including western margin thrust belt,transition zone and Tianhuan depression. The first zone has large faults,large tectonic amplitude,where the fault block reservoir develops,with the feature of dispersed oil layer,small reservoir size and isolated distribution. Low amplitude structure and low-grade fault develop in the transition zone,the low-grade fault is mainly reverse fault forming before the end of early Cretaceous( phase i),few is Paleocene( phase ii). Owing to that the fault phase i is earlier than the Mesozoic reservoir accumulation period,the sealing of which is crucial to the reservoir accumulation and fluid distribution,and the reservoir widely distributed is mainly structural-lithologic reservoir. The lithologic reservoir wide-ranging distributes in the Tianhuan depression. The reservoirs of the later two zones locate in the middle and lower Yanchang Formation and the whole Jurassic. It is obvious to the reservoirs controlled by the tectonic,the fault phase i communicating the oil source is the basis,the low amplitude structure and relative high permeability reservoir is the core,and whether the reservoirs damaged by the fault phase ii is the key. At the end of early Cretaceous,the crude oil migrates through the opening faults and stacked sand-body under the abnormal pressure,accumulates in the structural traps,related high permeability reservoir and lithologic traps blocked by the surrounding rocks and the sealing faults,and then forms the related reservoirs in the three zones. Affected by the fault phase ii,some reservoirs may suffer adjustment and damage.