琼东南盆地深水区陵水17-2大气田成藏机理

Accumulation mechanism of LS 17-2 deep water giant gas field in Qiongdongnan Basin

陵水17-2大气田位于琼东南盆地陵水凹陷深水区。该凹陷是继承性深大凹陷,面积达5 000km2,新生界最大厚度达13km。凹陷内发育始新统、下渐新统崖城组、上渐新统陵水组3套烃源岩,其中崖城组是主力烃源岩,包括煤系和海相泥岩。陵水凹陷是高温凹陷,崖城组烃源岩底部温度可达250℃以上,烃源岩镜煤反射率可达2%~5%。开放和封闭体系的高温生烃模拟实验显示,该套烃源岩生气下限Ro值可达4.38%以上,约为4.4%,因此崖城组主体处在生气窗内,资源量近万亿立方米,是富生烃凹陷。莺歌海组—黄流组是陵水凹陷首选勘探目的层,储层主要发育于中央峡谷水道内。该峡谷长约425km、宽度3~25 km、深度270~700 m,平行于陆坡近东西走向。峡谷水道内共充填了5期次级水道,发育轴部砂、侧向加积砂、天然堤和侵蚀残余砂4类储集体,物性极好,并形成岩性或构造-岩性圈闭。陵水凹陷北部边缘和南部边缘发育两个串珠状分布的底辟带,类型为泥-流体底辟,南部底辟带沟通了深层崖城组烃源岩与浅层峡谷水道砂岩,崖城组生成的天然气垂向运移至水道砂体,再在水道储层内发生侧向运移,呈T型运移模型,在圈闭内形成天然气田。

LS17-2 giant gas field is located in the deep water area of Lingshui sag of Qiongdongnan Basin.The sag is a successive large and deep sag covering an area of 5 000km2 with the maximum Cenozoic strata thickness of 13 km.Three sets of source rocks are developed in the sag,i.e.,Eocene and lower Oligocene Yacheng Formation as well as upper Oligocene Lingshui Formation,and Yacheng Formation is the main source rock including coal measures and marine mudstone.Lingshui sag is characterized by high temperature.The bottom temperature of source rock in Yacheng Formation can exceed 250℃,and the reflectivity of source rock is up to 2%~5%.The simulation experiments of hydrocarbon generation in open and closed systems at high temperature indicate that the lower limit of gas generation for this set of source rock can reach more than4.38%（about 4.4%）,and thus the main part of Yacheng Formation is in the gas window,whose resource quantity is nearly1 trillion cubic meters,indicating that it is a prolific hydrocarbon-generation sag.Yinggehai Formation-Huangliu Formatiom is the first exploration target in Lingshui sag,and the main reservoirs are developed in the channel of the central canyon.The canyon is about 425 km long,3-25 km wide and 270-700 m deep;it is nearly EW-trending and parallel to the continental slope.Five stages of secondary channels were filled in the channel of the canyon where four kinds of reservoirs including axial sand,lateral accretion sand,natural levee as well as erosion-causing residual sand with extremely good physical properties,forming lithologic traps or structural-lithologic traps.There are two moniliform diapir zones on the northern and southern margins of Lingshui sag,and they are mud-fluid diapers.The southern diaper zone communicates source rocks in Yacheng Formation and shallow sandstones in canyon channel.Gas generated from Yacheng Formation migrates vertically to the channel sand body,and then migrates laterally in channel reservoirs showing a T-type migration model,forming agas field in the trap.