南海北缘陆相断陷盆地优质烃源岩发育机制:以珠江口盆地顺德凹陷始新统文昌组为例

Formative mechanisms of high-quality source rocks in continental fault basin of northern South China Sea:a case study from the Eocene Wenchang Formation in Shunde sag,Pearl River Mouth Basin

珠江口盆地顺德凹陷始新统文昌组发育3套有利烃源岩,但对其层系归属、优质烃源岩特征和发育模式认识不清晰。基于高分辨率三维地震、钻井和地球化学资料,开展文昌组层序地层划分、梳理烃源岩发育特征、明确有机质富集主控因素并建立优质烃源岩发育模式。结果表明:顺德凹陷始新统文昌组可划分为3个三级层序,其中层序SQ2对应文昌组二段,烃源岩岩相类型以褐黑色、灰黑色纹层状页岩为主,有机质丰度较高,为优质烃源岩发育的主要时期。层序SQ1和SQ3分别对应文昌组三段和一段,烃源岩岩相以深灰色块状泥岩夹粉砂质泥岩为主,有机质丰度相对较低,主要发育一般烃源岩。有机质母源类型以陆源高等植物和水生浮游生物混源为主,其中层序SQ2时期藻类贡献较大。文昌组优质烃源岩发育受构造作用—古气候耦合约束,有机质富集主控因素为古生产力和古氧化还原条件。层序SQ2时期强断陷作用加剧火山活动,火山灰尘稳定沉降驱动古生产力较高;强烈的构造沉降及温暖—湿润的古气候促使湖平面上升,温度控制下的水体分层导致湖泊底水缺氧,于半深湖环境沉积厚层富有机质页岩。

Three sets of favorable source rocks developed in the Eocene Wenchang Formation of the Shunde sag,Pearl River Mouth Basin.However,their stratigraphic attribution,organic matter characteristics and development patterns are unclear.Based on high-resolution 3D seismic,drilling,and geochemical data,this study documents sequence stratigraphic divisions of the Wenchang Formation,and sorted out the characteristics of hydrocarbon source rocks.The current study not only identifies main controlling factors for organic matter enrichment,but also establishes development patterns of high-quality hydrocarbon source rocks of the Wenchang Formation in Shunde sag.Results and observations from the current study suggest that the Eocene Wenchang Formation can be divided into three third-order sequences,among which the sequence SQ2 corresponds to the Member 2 of the Wenchang Formation.The lithofacies of source rocks developed during the development of SQ2 are mainly composed of brown-black and gray-black laminated shale with high organic matter abundance,and are considered as high-quality source rocks.The SQ1 and SQ3 sequences correspond,respectively,to the Members 3 and 1 of the Wenchang Formation.The lithofacies of source rocks developed during the development of SQ1 and SQ3 are mainly composed of dark-gray massive mudstone with silty mudstone with relatively low organic matter abundance.Also,the parent-source types of them were dominated by terrestrial higher plants and aquatic plankton,and the contribution of algae in the SQ2 sequence was more developed.The development of high-quality source rocks in the Wenchang Formation was constrained by tectonism-palaeoclimate coupling,which suggests that palaeoproductivity and palaeoredox conditions are the main controlling factors of organic matter enrichment.In addition,during the development of SQ2 sequence,the strong fault depression intensified volcanic activity which drived higher palaeoproductivity by large amount of volcanic dust subsiding stably.Also,the strong tectonic subsidence and the warm-humid palaeoclimate promoted the rise of lake levels,and the stratification of the water body under temperature control led to the oxygen depletion of the lake bottom water,giving rise to the thick organic-rich shales in the semi-deep lake environment.