东濮凹陷柳屯洼陷盐湖盆地超压成因

Overpressure genesis in the Liutun salt-lake sag,Dongpu Depression,Bohai Bay Basin

渤海湾盆地东濮凹陷柳屯洼陷盐湖盆地普遍发育超压,其对盐湖盆地内泥岩裂缝的形成和泥岩裂缝油藏成藏效应等具有明显的控制作用,因此查清盐湖盆地超压成因显得尤为关键。运用地球物理和地球化学资料,综合超压成因机理分析的各种方法,从膏盐层与超压发育的关系进行解剖。研究结果表明,东濮凹陷盐湖盆地在沉积速率相对较低的情况下,膏盐层封闭作用是使欠压实成为主要超压成因机制的根本地质因素。进一步开展欠压实作用对超压的定量评价,结果表明其贡献为53.068%～71.416%,平均为62.758%。因此在盐湖盆地,膏盐层强封闭作用造成的欠压实是形成超压的决定性因素,尤其是强超压的发育受到膏盐层的控制。在盐间泥岩段由于膏盐层的影响,生烃作用﹑粘土矿物脱水作用对超压的形成有一定的贡献,但是其作用能力有限。综合研究认为,盐湖盆地膏盐层的性质是影响和控制超压成因的关键因素。

Overpressure is common in the Liutun salt-lake sag in the Dongpu depression and obviously control the formation of mudstone fractures and affects the hydrocarbon accumulation in these fractured mudstone reservoirs.Therefore,it is important to look into the genesis of overpressure in the salt-lake sag.We analyzed the relationship between the gypsum and overpressure based on the geophysical and geochemical data and combined with the analysis of overpressure genesis mechanisms.The results show that the undercompaction caused by sealing process of the gypsum cap rocks was the major mechanism for the formation of overpressure in salt-lake sags of the Dongpu depression where the deposition rate was low.Further quantitative evaluation of the undercompaction indicates its contribution to the overpressure is 53.068%-71.416%（62.758% in average）.This leads to a conclusion that the extensive undercompaction caused by the strong sealing capacity of gypsum cap rock in the salt-lake sag is the determinative factor of the overpressure,especially the ultra-strong overpressure.Hydrocarbon generation and clay-mineral dehydration of inter-salt mudstone also can contribute to a certain limited extent to the formation of overpressure.Based on the above analysis,we propose that the properties of the gypsum cap rocks are the pivotal factors affecting and controlling the formation of overpressure in salt-lake sags.