北部湾盆地主要凹陷流沙港组二段主力烃源岩再评价

Main source rock reevaluation of Member 2 of Liushagang Formation in the sags of Beibuwan basin

运用"湖相烃源岩非均质性"的思维方式,对北部湾盆地主要凹陷古近系流沙港组流二段湖相烃源岩进行了对比研究和再评价。基于大量地球化学和沉积地质资料的统计和分析,将烃源岩的有机碳含量、生烃潜量、氢指数和4-甲基C_(30)甾烷指数等作为评价和识别本区湖相烃源岩质量的有效指标,首次建立了北部湾盆地湖相烃源岩评价新标准,据此将流二段湖相烃源岩划分出优质、好、一般和差4类烃源岩。在此基础上,开展了优质烃源岩的形成条件和油源对比研究,认为受烃源岩形成条件不同的影响,北部湾盆地各凹陷流二段湖相主力烃源岩的质量存在较大的差异,其中涠西南凹陷最优,其主力烃源岩为流二段底部厚约50~100 m的湖相优质烃源岩,乌石凹陷其次,优质湖相主力烃源岩仅分布在乌石凹陷的东北洼,且优质烃源岩对这两个凹陷主力油田的形成和分布有明显的控制作用;而海中凹陷和迈陈凹陷主力烃源岩为流二段湖相一般烃源岩,可能不发育较大面积优质湖相烃源岩。本文研究成果对北部湾盆地,特别是对尚未突破的海中凹陷和迈陈凹陷的勘探方向选择有重要启示。

With the thought of lacustrine source rock heterogeneity, lacustrine source rocks analogue of Member 2 of Liushagang Formation （El 2） in the main sags of Beibuwan basin is studied and reevaluated. Based on statistics and analysis of geochemistry and sedimentary data, the total organic carbon content, potential hydrocarbon genera-tion amount, hydrogen index and 4-methyl-C30 sterane index of source rocks are chosen as effective indicators to i-dentify and evaluate the lacustrine source rocks. An evaluation criteria on lacustrine source rocks is firstly estab-lished and such 4 types as high quality, good, common and poor source rocks are characterized for source rocks of El 2. High quality source rock occurrence condition and oil-source correlation study show that the quality of lacus-trine source rock of El 2 in different sags is variable due to different occurrence conditions. Among these sags, Weixinan sag has the most favorable occurrence condition of high quality lacustrine source rock and the main source rock is the lower part of E4 with thickness of 50-100 nx Wushi sag is the second with high quality source rock in the northeast. The formation and distribution of major oil fields in Weixinan and Wushi sags are significantly controlled by high quality source rock. The main source rocks in Haizhong and Maichen sags are common quality, while high quality lacustrine source rocks in these two sags are believed to be very limited. The research result is significant for explora-tion directions in Beibuwan basin, especially in Haizhong and Maichen sags without breakthrough yet.