莺歌海盆地东方区中深层强超压环境压力-应力耦合下流体保存与排放的动力学机制

Many petroleum basins in the world contains overpressure, a phenomenon that the formation pore pressure is significantly higher than the hydrostatic pressure. It has been proved that the overpressure transition to normal pressure are enrichment zones of petroleum. However, a large of hydrocarbons have also been found in the strong overpressured situation recently. Research on the accumulation and distribution of petroleum is relatively weak in the strong overpressured environment in mid-deep of sedimentary basin. The dynamic mechanism of fluid preservation and leakage controlled by pressure-stress coupling is the key of understanding the hydrocarbon accumulation for the strong overpressured environment in the mid-deep sedimentary basins. The two gas fields, the DF13-1and DF13-2, located in the Dongfang area of Yinggehai Basin, South China Sea, which is famous for the strong overpressure and diapir, are the perfect nature laboratory for this research. In this project, we try: (1) to reveals the dynamic mechanism of hydraulic fracture formation in cap rock, fluid preservation and episodic leakage under pore pressure-stress coupling by carrying out the researches on paleo-pressure evolution, the paleo-tectonic stress field characteristics, cap rock mechanics properties, fluid flow conduct and process based on the data of geology, drilling, logging, geophysics and geochemistry, (2) to clarify the different mechanism of authigenic mineral transformation, dissolution and chemical precipitation according to the analysis on the differences of diagenetic sequence, diagenetic mineral assemblages controlled by the processes of fluid preservation and episodic leakage. Through these work, we will evaluates the sealing capacity of traps, predict the advantage migration pathway and direction of overpressure fluid, and illustrate the distribution of differential gas pools(hydrocarbon gas-rich pools, hydrocarbon gas-bearing pools and CO2-rich pools). It would provide important scientific bases for gas exploration in Yinggehai Basin, South China Sea.

沉积盆地中深层强超压环境油气成藏机理的研究相对薄弱，压力-应力耦合下流体保存和排放机制是强超压环境油气成藏研究的关键。莺歌海盆地东方区中深层强超压环境发现的天然气田为该项研究提供了理想的天然实验室。基于地质、钻井、测井和地震等资料，通过对莺歌海盆地东方区压力发育演化、构造应力场特征、盖层岩石力学性质、流体排放通道和排放过程等方面的研究，阐明东方区中深层强超压环境压力、应力、岩石力学性质共同控制下的圈闭盖层保存和破裂过程，揭示含烃（CO2）流体保存与幕式排放的动力学机制及其差异成岩响应，评价超压圈闭油气封闭能力，预测超压流体优势运移通道和运移方向，探讨不同类型天然气藏的分布规律，为莺歌海盆地强超压环境的油气勘探提供科学依据。

压力—应力耦合下流体保存和排放机制是强超压环境油气成藏研究的关键。通过对莺歌海盆地中央坳陷带超压发育演化、东方区古构造应力场特征、流体排放通道的识别和流体排放过程等方面的研究，主要取得以下成果和认识：（1）阐明了莺歌海盆地超压发育演化过程及超压成因机制。莺歌海盆地强超压体系位于中央坳陷带内部，超压中心与底辟发育带吻合较好；超压主要发育于17.5Ma以来，3Ma左右增压幅度最大。中央坳陷带古近系超压主要为欠压实与生烃增压特别是生气作用形成的，为自源型超压，而新近系主要为深部超压向中浅层垂向传递形成的。（2）揭示了莺歌海盆地东方区中深层强超压环境压力—应力耦合下盖层破裂机制。东方区中深层最小水平主应力均未超过相应深度的垂向主应力，为伸展或走滑应力机制，压力—应力耦合系数约为0.7左右，超压封存能力较高；中新世—上新世走滑挤压应力场可能是东方区超压流体垂向释放的重要触发机制。（3）东方区底辟伴生断裂及水力破裂是主要的超压流体释放通道，超压流体释放引起地震剖面上的管柱体和气烟窗、储层流体非均质性和轻烃运移分异，流体活动的瞬态温度响应和储层成岩矿物组合类型的差异是超压流体释放的重要识别标识。（4）超压封存条件、流体类型及充注过程的差异是DF13-2和DF13-1气田流体非均质性和储层成岩矿物组合类型差异的关键。DF13-2区超压保存条件较好，主要为烃类气体充注，而东方13-1区超压幅度强，水力破裂发育，油气保存能力较弱，晚期高温无机CO2流体充注导致黄流组储层长石、碳酸盐岩矿物溶解和粘土矿物成岩转化增强是储层成岩矿物组合类型差异性的主要因素。