断层对盖层的动态破坏及其对油气输导的影响——以渤海湾盆地歧口凹陷港东断裂为例

断层对泥岩盖层破坏程度的静态评价在分析油气垂向输导时具有一定的局限性,采用动态评价方法可预测油气沿断层突破盖层的时间及油气成藏区域。以地层去压实校正法和断层最大断距相减法恢复的地层古厚度和断层古断距为依据,可得到沿断层各测点处不同地质时期的"盖层连接厚度"这一定量评价参数,然后结合研究区内烃源岩的生、排烃历史,可预测油气沿断层突破盖层的时间及油气成藏区域。以渤海湾盆地歧口凹陷港东断层对盖层的动态破坏为例,根据港东断层对沙河街组一段中部泥岩盖层的破坏程度,可将断层的动态破坏分为3个主要阶段:沙河街组一段沉积期,泥岩盖层遭受完全破坏的区域比较局限;东营组—馆陶组沉积期,尽管泥岩盖层被破坏的规模有所增大,但断层的活动性与烃源岩的排烃史的耦合性不佳;明化镇组沉积期至今,泥岩盖层完全破坏的区域向西侧略有延伸,且烃源岩的生、排烃能力和断层活动性均较强,深层油气可以通过港东断层突破沙河街组一段中部盖层进入浅部地层。在港东油田明化镇组中,已发现港东油田内油气的分布范围与港东断层在明化镇组沉积期对沙河街组一段中部盖层造成完全破坏的区域范围相吻合,表明渤海湾盆地港东断层的动态破坏对油气的输导和成藏具有重要的控制作用。

Dynamic damage of fault to caprock and its influence on hydrocarbon transport: a case study of Gangdong fault in Qikou sag,Bohai Bay Basin

The static evaluation of fault damage affected on the mudstone caprock has certain limitations in analyzing vertical hydrocarbon transport; dynamic evaluation method can be used to predict the time for oil and gas to break through caprock along the fault and the hydrocarbon accumulation area. According to the paleostratigraphic thickness and the ancient fault throw recovered by the decompaction correction and maximum throw subtracting method, the quantitative evaluation parameter of "caprock connecting thickness" at different geological periods and measuring points along the fault can be obtained. Then combined with the hydrocarbon generation and expulsion history of source rocks in the study area, it is able to predict the time for oil and gas to break through caprock along the fault and the hydrocarbon accumulation area. Taking the dynamic damage of Gangdong fault to the caprock in Qikou sag, Bohai Bay Basin as an example, according to the damage caused by Gangdong fault to the middle mudstone caprock in Member 1 of Shahejie Formation, the dynamic damage of fault can be divided into three main stages:in the depositional stage of Member 1 of Shahejie Formation, the damage of the mudstone caprock was relatively limited; in the depositional stage from Dongying Formation to Guantao Formation, the damage scale of the mudstone caprock increased, but the coupling between fault activity and hydrocarbon expulsion history of source rock was not ideal; From the depositional stage of Minghuazhen Formation to the present, the area with completely damaged mudstone caprock extended slightly to the west, both the hydrocarbon generation and expulsion of source rock and the fault activity presented quite strong so that the deep oil and gas could break through the middle caprock in Member 1 of Shahejie Formation through Gangdong fault to enter the shallow strata. In the Minghuazhen Formation of Gangdong oilfield, it has been found that the oil and gas distribution area in Gangdong oilfield is consistent with the area where the Gangdong fault completely damaged the middle caprock of Member 1 of Shahejie Formation during the depositional stage of Minghuazhen Formation, which indicates that the dynamic damage of the Gangdong fault plays an important role in controlling the hydrocarbon transport and accumulation in Bohai Bay Basin.