咸化湖盆地层超压特征及成因机制——以柴达木盆地英西地区渐新统为例

Characteristics and origin mechanism of formation overpressure in the saline lacustrine basin:A case study from the Oligocene in the west of Yingxiong ridge,Qaidam Basin

为了探索咸化湖盆地层超压分布特征及成因机制,以处于咸化沉积环境且普遍发育地层超压的柴达木盆地英西地区渐新统为例,在对膏盐岩和地层超压空间分布特征描述的基础上,采用测井曲线组合分析法和声波速度-密度交会图法判别地层超压成因类型,结合地质条件分析地层超压形成、分布及保存的控制因素,并探究了地层超压对油气成藏的贡献作用.认为英西地区地层超压与膏盐岩有良好的对应关系,英西地区渐新统的地层超压是多种因素叠加的结果,盐间、盐下地层的构造背景、沉积环境不同导致超压成因机制具有差异性.研究结果表明:英西地区纵向上膏盐岩发育顶部深度与地层压力系数突变深度(2 000~2 500 m)大致相同,洼陷中心的地层压力随埋深增加呈常压—超压—强超压的变化特征;横向上膏盐岩集中分布区地层超压发育范围达到最大,甚至发育强超压(压力系数>1.7);盐间主要的超压成因机制是地层快速沉积和膏盐岩封闭引起的不均衡压实增压作用、盐类矿物充填孔缝、石膏转化脱水等,压力系数一般为1.5~1.8;盐下主要的超压成因机制为咸化源岩高效生烃造成孔隙流体膨胀增压作用以及地层沉积不均衡压实增压作用等,发育压力系数为1.7~2.0的强超压;深层膏盐岩的强塑性和流动性有利于超压形成后的保存;地层超压在储层储集性能、油气运移及压力封闭等方面对油气成藏具有积极意义,进一步的勘探开发可集中在厚层盐下超压裂缝有利区.

To explore the distribution characteristics and origin mechanism of formation overpressure in the saline lacustrine basin, the Oligocene in the west of Yingxiong ridge of Qaidam Basin was taken as a study. Based on the description of spatial distribution characteristics of gypsum-salt rock and formation overpressure, the origin types of formation overpressure were identified using logging curve combination analysis method and acoustic velocity-density crossplot method. The controlling factors of overpressure formation, distribution and preservation were analyzed combined with geological conditions analysis method, and the contribution of formation overpressure to hydrocarbon accumulation was explored. It indicates that there is a good corresponding relationship between overpressure and gypsum-salt rock in the west of Yingxiong ridge, and the formation overpressure of the Oligocene in the west of Yingxiong ridge is the result of superposition of many factors. The origin mechanism of overpressure is different due to the different tectonic background and sedimentary environment of the inter-salt and subsalt strata. The results showed that the top depth of gypsum-salt rock is about the same as the abrupt change depth of pressure coefficient in the west of Yingxiong ridge(2 000—2 500 m). And the formation pressure in the center of the depression changes in three stages with the increase of buried depth: ordinary pressure section, overpressure section and strong overpressure section. In the area where the gypsum-salt rock is concentrated, the overpressure develops to the largest extent and even develops to strong overpressure(pressure coefficient >1.7). The main overpressure origin mechanism of inter-salt strata can be summarized as disequilibrium compaction caused by rapid deposition, sealing of gypsum-salt rock, and conversion and dehydration of gypsum, etc. The pressure coefficient of the inter-salt strata is generally 1.5—1.8. The main overpressure origin mechanism of subsalt strata can be summarized as pore fluid expansion caused by efficient hydrocarbon generation of salinized source rock, and disequilibrium compaction of formation sedimentation. Strong overpressure with pressure coefficient of 1.7—2.0 is developed in subsalt formation. The strong plasticity and fluidity of deep gypsum-salt rock are conducive to the preservation of overpressure. Formation overpressure has positive significance for reservoir performance, oil and gas migration and pressure sealing. Further exploration and development can be concentrated in the field of subsalt overpressure fracture favorable areas.