琼东南盆地松南-宝岛凹陷北坡转换带特征及其对大中型气田的控制

North slope transition zone of Songnan-Baodao sag in Qiongdongnan Basin and its control on medium and large gas fields,South China Sea

基于新采集三维地震与钻井资料分析,系统解剖宝岛凹陷地质结构及其断裂体系,开展转换断阶带特征及其对天然气成藏的控制作用研究。研究结果表明:宝岛凹陷发育北部断阶带、中部凹陷带与南部斜坡带,其中,北部断阶带自西向东发育宝岛B、A、C等多个转换断阶带,控制宝岛凹陷烃源岩、圈闭、储集层、油气运移及烃类气富集。转换带主干断裂在渐新世早期崖城组沉积期的活动性控制生烃灶与生烃潜力,自西向东活动性增强且距离物源区更近,烃源岩厚度、三角洲规模呈增大趋势,发育多个生烃洼陷;主干断裂在张扭背景下发生局部挤压,形成大型鼻状构造背景下的复合型圈闭,且向东宝岛A、宝岛C圈闭规模大于宝岛B圈闭;多级断阶控制隆起区物源碎屑注入形成大型三角洲砂体,宝岛A转换带F12断裂中西段同向转换带和东段缓坡段控制两大物源碎屑注入,形成西支、东支等多个三角洲朵叶体;主干断裂控制形成的大型构造脊紧邻生烃中心,具有高效运聚特征;主干断裂组合方式、活动时间与天然气充注期次的良好匹配,控制烃类气富集。宝岛A转换带紧邻生烃洼陷宝岛27、25、21洼,发育神狐隆起物源供给的大型辫状河三角洲沉积,具有大型构造脊高效汇聚及主干断裂平行组合且停止活动时间早的特征,是烃类气优势汇聚区,实钻发现厚层优质气层,获得宝岛凹陷首个大型气田宝岛21-1的发现,证实松南—宝岛北部转换带成藏条件好,向东转换断阶带拓展勘探潜力大。

Based on analysis of newly collected 3D seismic and drilled well data,the geological structure and fault system of Baodao sag have been systematically examined to figure out characteristics of the transition fault terrace belt and its control on the formation of natural gas reservoirs.The research results show that the Baodao sag has the northern fault terrace belt,central depression belt and southern slope belt developed,among them,the northern fault terrace belt consists of multiple transition fault terrace belts such as Baodao B,A and C from west to east which control the source rocks,traps,reservoirs,oil and gas migration and hydrocarbon enrichment in the Baodao sag.The activity of the main fault of the transition belt in the sedimentary period of Yacheng Formation in the Early Oligocene controlled the hydrocarbon generation kitchen and hydrocarbon generation potential.From west to east,getting closer to the provenance,the transition belt increased in activity strength,thickness of source rock and scale of delta,and had multiple hydrocarbon generation depressions developed.The main fault had local compression under the background of tension and torsion,giving rise to composite traps under the background of large nose structure,and the Baodao A and Baodao C traps to the east are larger than Baodao B trap.Multiple fault terraces controlled the material source input from the uplift area to form large delta sand bodies,and the synthetic transition belt of the west and middle sections and the gentle slope of the east section of the F12 fault in the Baodao A transition belt controlled the input of two major material sources,giving rise to a number of delta lobes in the west and east branches.The large structural ridge formed under the control of the main fault close to the hydrocarbon generation center allows efficient migration and accumulation of oil and gas.The combination mode and active time of the main faults matched well with the natural gas charging period,resulting in the hydrocarbon gas enrichment.Baodao A transition belt is adjacent to Baodao 27,25 and 21 lows,where large braided river delta deposits supplied by Shenhu uplift provenance develop,and it is characterized by large structural ridges allowing high efficient hydrocarbon accumulation,parallel combination of main faults and early cessation of faulting activity,so it is a favorable area for hydrocarbon gas accumulation.Thick high-quality gas reservoirs have been revealed through drilling,leading to the discovery of the first large-scale gas field in Baodo 21-1 of Baodao sag.This discovery also confirms that the north transition zone of Songnan-Baodao sag has good reservoir forming conditions,and the transition fault terrace belt has great exploration potential eastward.