南海北部北部湾盆地迈陈凹陷中南部X井油气成藏地质条件

根据X井钻遇的古近系和新近系有机质镜质体反射率分析,确定X井生烃门限深度在2 600m以下,那么该井钻遇始新统流沙港组泥岩有机质丰度较高的井段（2 100~2 285m）尚未达成熟门槛,而生烃门限以下的烃源岩有机质丰度较低,生烃潜力差,不能提供充足的烃源供给,故导致X井勘探失利。X井位于北部湾盆地迈陈凹陷东3洼南部边缘,钻遇流沙港组厚度达568.5m,且泥岩厚度占该组地层的62.3%,其中单层泥岩最大厚度高达115m,表明流沙港组沉积时期水体环境偏深,沉积可容空间大,可见X井钻探结果与前人＂迈5号同生断裂始新世活动弱,沉积可容空间小,导致东3洼缺少油气形成的物质基础＂的结论存在较大偏差。由于X井处在东3洼陷边缘,流沙港组顶部剥蚀强度较大,推测靠近东3洼沉降中心的始新统流沙港组湖相泥岩沉积厚,埋深亦加大,完全能够达到有机质成熟生烃门槛,故应具有较好的油气成藏地质条件和勘探潜力。

GEOLOGICAL CONDITIONS FOR HYDROCARBON ACCUMULATION: A CASE FROM WELL X IN SOUTHERN CENTRAL PART OF MAICHEN SAG OF BEIBUWAN BASIN

The vitrinite reflectance analysis of the samples from the Well X indicates that the threshold depth for oil generation is deeper than 2 600 meters, and the Eocene Liushagang mudstone with higher abundance of organic matter ranging in the depth of 2 100~2 285 m is above the threshold, while the organic matter of Liushagang mudstone below the threshold is too low to provide sufficient hydrocar- bon supply for oil accumulation. As the result, the exploration of the Well X is failed. However, the Well X is located in southern edge of the East-3 Depression, where the Liushagang Formation is 568.5 m in thickness, of which 62.3 % are mudstone and the maximum thickness of a single lager of mud- stone is over 115 m. It means that water was rather deep during the deposition of the Liushagang For- mation and the accommodation space was large. It is obviously different with the conclusion from pre- vious studies, which believed that the Mai 5 growth faulting was weak then, that resulted in small ac- commodation space, so the East-3 Depression is lack of organic matters for oil and gas generation. Furthermore, as the Well X is located in the southern edge of the East-3 Depression, denudation of the Liushagang Formation might be quite intensive, so we speculated that the thickness of the Liush- agang Formation must be larger and might exceed the threshold for hydrocarbon generation. The ex- ploration potential in East-3 Depression remains lange and should not be ignored.