琼东南盆地崖城区泄压带流体活动特征及成岩响应

The characteristic of fluid activities and diagenetic responses of the pressure discharging zone in Yacheng area,Qiongdongnan Basin,South China Sea

基于琼东南盆地西部崖城地区10余口钻井的实测钻井资料,以及流体包裹体、激光拉曼探针、古压力热动力学模拟、有机地球化学、阴极发光、铸体薄片和碳酸盐胶结物碳氧同位素组成等多项测试分析,剖析了超压顶界面分布以及附近的泄压带流体活动特征,同时揭示了流体活动造成泄压带岩场响应。研究区泄压带可能位于超压顶面附近地层;地层测试、泥浆密度、测井曲线和速度谱资料确定了崖城区现今超压顶面深度主要分布在3 000-4 000 m,而且由构造高部位向低部位加深;各井超压顶面附近的泄压影响范围不同,具体范围可由镜质体反射率Ro得到。现今超压顶面与古超压顶面（泄压流体排放期）深度变化较小。泄压带流体具有相对高温高压、含有酸性和烃类等特征,流体活动使得泄压带成岩场的温压条件和孔隙流体介质发生变化从而影响了水-岩作用;主要表现为：①泄压带地层的Ro和黏土矿物出现提前转变趋势;②有机质Tmax异常小和S1/（S1＋S2）异常大;③泄压带储层发生的热流体酸性溶蚀导致次生孔隙带发育;④长石颗粒钠长石化,自生石英和碳酸盐胶结物异常发育,其为深部超压有机流体排放的产物。总之,研究区超压顶面附近的泄压带可能为天然气与优质储层及盖层耦合有利聚集带,是今后琼东南盆地天然气勘探的现实区域。

Based on the multiple tests and analysis results of more than ten wells in Yacheng,West Qiongdongnan Basin,including drilling and logging data,Laser Raman microprobe,thermodynamic modeling of paleopressures,organic geochemistry,cathodoluminescence,casting thin-section,the carbon-oxygen isotope composition of carbonate cements,etc.,this paper analyzes in detail the distribution of the top of overpressure system and characteristic of fluid activity in the nearby discharging zone,and also reveals the diagenetic responses of pressure discharging zone caused by fluid activity.In the present study area,the pressure discharging zone is possibly located in formations around the top of overpressure system.Using the data of formation testing,mud density,well logging and velocity spectrum,it has been confirmed that the top of overpressure system is mainly located within the depth of 3 000-4 000 m beneath the earth and its depth increases from structural high to low position;the influential range of pressure discharging in each well around the top of overpressure zone is different,which can be obtained using vitrinite reflectance（Ro）.It is indicated that the current top of overpressure system（in the pressure discharging and fluid draining period）has little difference in depth as compared with the top of paleo-overpressure system.The fluid in the discharging zone is characterized by relatively high temperature and pressure as well as the accumulation of acidic materials and hydrocarbons.The fluid activities cause changes in the temperature,pressure and pore fluid medium of diagenetic field in pressure discharging zone,thus exerting influences on water rock interaction.This phenomenon is specifically described as below：（1）The Roand clay mineral show a transformation in advance;（2）The organic matter Tmaxis extremely small and S1/（S1＋S2）is extremely large;（3）The development of secondary porosity is resulted from hot fluid acidic dissolution in reservoirs of the discharging zone;（4）The albitization of feldspar particles as well as the abnormal development of authigenic quartz and calcite cement are caused by the discharging of deep overpressure organic fluid.In conclusion,the pressure dischargingzone around the top of overpressure system may be favorable for gas accumulation,the formation of high-quality reservoirs and the coupling of cap rocks,which is the target zone for natural gas exploration in Qiongdongnan Basin from now on.