琼东南盆地砂岩侵入体成因及其对天然气水合物成藏的影响

Genesis of sandstone intrusion in Qiongdongnan Basin and their influence on gas hydrate accumulation

目前对有关砂岩侵入体在天然气水合物成藏系统中的作用的研究较少。中国南海海域同时具备形成砂岩侵入体和天然气水合物藏的地质条件,但侵入体对天然气运移及天然气水合物成藏的作用值得进一步探讨。为此,基于研究区地震资料并结合前人研究成果,精细刻画了琼东南盆地砂岩侵入体的地质特征,结合地质背景确定了砂岩侵入体的成因机制,从而确立砂岩侵入体的演化模式,并总结砂岩侵入体对天然气运移和水合物成藏的作用。结果表明:①琼东南盆地陵南低凸起发育多个大型砂岩侵入体,它们主要位于黄流组顶部,两翼斜向上伸入莺歌海组,个别规模较小的侵入体整体处于莺歌海组底部地层。砂岩侵入体在地震剖面上呈U型或宽V型强振幅反射,横向展布范围为3~5 km,竖直高度约为350~750 m,平面形态为圆形或椭圆形,具有环形特征。②中新世中期沉积供源砂体,中新世晚期沉积泥质低渗盖层,同时为梅山组砂体超压提供条件,上新世时期形成多边形断层,梅山组砂体由于欠压实未固结,液化后受火山活动扰动发生流化侵入。砂岩侵入体的各触发机制在时空上相互匹配,具备良好的砂岩侵入体形成条件。③砂岩侵入体可作为甲烷气由深部到浅层的接力运移通道,大量深部热解气经过深部的底辟和裂缝之后,可以通过砂岩侵入体运移到浅层稳定域中形成水合物藏,对水合物勘探、开发具有重要意义。

At present,there is less research on the role of sandstone intrusion in gas hydrate accumulation systems.There are geological conditions for sandstone intrusion and gas hydrate accumulation in the South China Sea,but the effect of sandstone intrusion on gas migration and gas hydrate accumulation is worth further study.Therefore,based on the seismic data and previous research results,the geological characteristics of the sandstone intrusion in Qiongdongnan Basin are finely characterized,and the genetic mechanism of the sandstone intrusion is determined in combination with the geological background,so as to establish the evolution model of the sandstone intrusion and summarize the role of the sandstone intrusion on gas migration and gas hydrate accumulation.The results show that:①Large sandstone intrusions are developed in the Lingnan low bulge of Qiongdongnan Basin,mainly located at the top of Huangliu Formation,with two wings extending diagonally upward into Yinggehai Formation,and a few small sandstone intrusions are located at the bottom of Yinggehai Formation.The sandstone intrusions present a U‑shaped or wide V‑shaped strong amplitude reflection in the seismic section.The transverse distribution range is 3-5 km,and the vertical height is about 350‑750 m.The plane shape is circular or oval,with ring characteristics.②The sedimentary source sand body in the middle Miocene and the sedimentary low permeability mud cover in the late Miocene provide the conditions for the overpressure of the sand body in Meishan Formation.Polygonal faults were formed in the Pliocene period,and the sand body in Meishan Formation is not consolidated due to undercompaction and is disturbed by volcanic activities after liquefaction.The trigger mechanisms of sandstone intrusions match each other in time and space and have good conditions for forming sandstone intrusions.③Sandstone intrusions can be used as a relay migration channel for methane gas from deep to shallow layer,and a large number of deep thermal decomposition gas can migrate through sandstone intrusions to shallow stable regions to form hydrate reservoirs after deep diapirs and fractures,which is of great significance for hydrate exploration and development.