珠江口盆地阳江凹陷始新统古湖泊环境与优质烃源岩发育模式

Paleolacustrine environment and development pattern of high-quality source rock of Eocene in Yangjiang sag,Pearl River Mouth Basin

阳江凹陷是珠江口盆地近年获得重要油气勘探突破的凹陷,但其优质烃源岩形成条件尚不明确。综合应用钻井有机、无机地球化学和地震资料,分析了珠江口盆地阳江凹陷始新统文昌组烃源岩特征,探讨了其古湖泊环境和湖盆充填类型,建立了优质烃源岩发育模式。研究发现:(1)阳江凹陷文昌组发育厚层半深湖相烃源岩,有机质丰度高,有机质类型以Ⅰ-Ⅱ1型为主,文昌组下段横向非均质性强,TOC平均值为2.2%~3.5%,文昌组上段均质性强,TOC平均值为2.3%~2.6%,以浮游藻类成因有机质为主。(2)阳江凹陷中、小型物源输入与裂陷活动耦合创造了有利于半深湖发育的可容空间,温暖湿润的气候条件是水生浮游藻类勃发的前提,火山喷发提升湖盆局部初始生产力,强还原性的水体环境有利于有机质的保存。(3)依据不同裂陷活动、物源供给、古气候和古环境差异耦合作用结果,划分出了3种湖盆充填类型,其中平衡充填型湖盆在洼陷带发育厚层泥岩与薄层砂岩交互的波动深湖岩相组合,欠充填型湖盆具有满盆广覆厚层均质泥岩的特点,过充填型湖盆基本不发育半深湖亚相烃源岩。(4)平衡充填型湖盆具有以“生产力+保存条件”双因素控制的优质烃源岩发育模式,湖泊初始生产力高,总体处于中等还原环境,形成有机质类型好、丰度高的非均质烃源岩,欠充填型湖盆具有以“保存条件”主控的优质烃源岩发育模式,湖泊生产力中等,但水体还原性强,形成有机质丰度中等的均质烃源岩。综合分析认为恩平21洼、恩平20洼和恩平27洼的文昌组叠合了平衡充填型与欠充填型湖盆优质烃源岩,具有较高的生烃潜力。研究结果对阳江凹陷以及珠江口盆地其他新区洼陷烃源岩预测具有一定指导意义。

In Pearl River Mouth Basin,Yangjiang sag is the one that has achieved an important breakthrough in oil-gas exploration in recent years,but the formation conditions of high-quality source rocks in the sag are still unclear.Comprehensively using drilling organic and inorganic geochemistry and seismic data,the paper analyzes the characteristics of source rocks of Eocene Wenchang Formation in Yangjiang sag,Pearl River Mouth Basin,explores the paleolacustrine environment and lake-basin filling type in the study area,and establishes the development pattern of high-quality source rocks.The findings shows that:(1)Wenchang Formation in Yangjiang sag develops thick layer semi-deep lacustrine source rocks with high organic matter abundance,dominated by the type I-II1 organic matters.The lower Member of Wenchang Formation has strong lateral heterogeneity and an average TOC of 2.2%-3.5%.The upper Member of Wenchang Formation has strong homogeneity and an average TOC of 2.3%-2.6%,which is mainly composed of organic matters originated from phytoplankton.(2)In Yangjiang sag,the coupling of small-and medium-sized provenance inputs and rifting activities has created an accommodation space for the development of semi-deep lacustrine facies,warm and humid climate conditions are the prerequisite for the blooming of aquatic phytoplankton,volcanic eruptions can improve the initial productivity locally in the lake basin,and the strongly reducing water environment is conducive to the preservation of organic matters.(3)Based on the coupling effects of different rifting activities,sedimentary input,paleoclimate,and paleoenvironment,three lake-basin filling types are classified.In particular,the balanced filling type develops a fluctuating deep lacustrine lithofacies combination of thick layer mudstone and thin layer sandstone in the center sag,the lake-basin underfill type is characterized by a whole basin covered with thick layer homogeneous mudstone,and the lake-basin overfill type basically does not develop semi-deep lacustrine source rocks.(4)The balanced lake-basin filling type has a high-quality source rock development pattern controlled by both productivity and preservation conditions.The lake has high initial productivity and is generally in a medium reducing environment,thus forming good heterogeneous source rocks with high organic matter abundance.The lake-basin underfill type has a high-quality source rock development pat tern mainly controlled by preservation conditions,of which the lake has medium productivity but strong reducibility of water body,thus forming homogeneous source rocks with medium organic matter abundance.The comprehensive analysis shows that the superimposed highquality source rocks of balanced lake-basin filling type and lake-basin underfill type are developed in the Wenchang Formation of Enping 21 subsag,Enping 20 subsag and Enping 27 subsag,which has high hydrocarbon generation potential.These findings have a certain guiding significance for the prediction of source rocks in Yangjiang sag and sags in other new areas in Pearl River Mouth Basin.