渤海湾盆地南堡凹陷异常压力系统及其形成机理

Abnormal pressure system and its origin in the Nanpu Sag,Bohai Bay Basin

渤海湾盆地南堡凹陷异常压力现象普遍发育,但对于凹陷中不同构造带异常压力的刻画与成因机制的探讨却较为薄弱。利用357口井1 354个钻杆测试数据(DST)和重复地层压力测试数据(RFT),测井曲线资料等,详细刻画了不同构造带的压力结构特征。研究表明:南堡凹陷地层压力系统纵向上可划分为3个带,浅部常压带(<1 800 m)、中部过渡带(1 800~2 400 m)和深部异常高压带(>2 400 m)。沙三段发育大规模异常高压,压力系数最高达1.9,超压带顶界面深度约为2 400 m;老爷庙构造带中、浅层发育低幅度超压带,压力系数约为1.2;滩海地区东一段和东二段局部发育异常低压。利用数值模拟技术和垂直有效应力—声波时差判别图版等方法,并结合烃源岩生排烃过程综合分析等,深入探讨了南堡凹陷不同异常压力系统的形成机理,研究认为:(1)深部沙三段的大规模超压主要来源于东营时期的泥岩不均衡压实作用,生烃作用也有一定贡献,但相对前者贡献较小;(2)明化镇时期,生烃作用是最主要的增压机制,而欠压实作用贡献则相对有限;(3)中浅层低幅超压带来源于深部超压的"传导",开启的断裂带为其传递通道;(4)东营末期的区域抬升剥蚀作用引起岩石骨架孔隙回弹和流体收缩,是形成本区异常低压的主要原因。

Abnormal pressure in the Nanpu Sag is prevalent,but its distribution in varying tectonic zones and its genetic mechanisms are rarely studied.So we depicted the architectural features of the pressure for varying tectonic zones in detail based on the data from 1 354 drill stem tests(DST),repeated formation pressure tests(RFT),and the well logging curves of 357 wells.The results show that vertically,the formation pressure system can be classified into three zones in the Nanpu sag,namely,the upper normal pressure zone(with burial depth less than 1 800 m),the middle transitional zone(with burial depth ranging from 1800m to 2 400 m),and the lower abnormal high pressure zone(with burial depth more than 2 400 m).Large part of the Shahejie Member 3 is controlled by significant abnormal overpressure with pressure coefficient up to 1.9,and the depth from ground to the top of the overpressure zone is about 2 400 m.The middle and upper layers of the Laoyemiao Formation overpressure is less intense with pressure coefficient about 1.2.However,the Dongying Member 1 and 2 offshore,develops abnormal low pressure in some area.In order to further discuss the abnormal pressure mechanisms in the Nanpu Sag,we resorted to methods such as numerical simulation,vertical effective stress-acoustic time cross-plot,and comprehensive analysis of hydrocarbon generation and expulsion for source rocks.Consequently,we propose four conclusions:(1)the large-scale overpressure in E s 3 mainly originates from the sandstone disequilibrium compaction during the Dongying period,and contribution from hydrocarbon generation is relatively minor;(2)in Minghuazhen period,hydrocarbon generation becomes the main mechanism for generating pressure,while the influence from disequilibrium compaction is rather limited;(3)the lower-degree overpressure of the middle and upper layers is thought to be induced by the vertical fluid migration from the deep overpressured formations through active fault zone;(4)the regional uplifting and denudation during the late Dongying period result in pore rebound of rock matrix and fluid shrinkage,which are considered to be the main cause of the abnormal low pressure in the Nanpu sag.