“源热共控”琼东南盆地的天然气勘探潜力

古近系渐新统崖城组烃源岩是目前琼东南盆地被钻井证实的最主要的1套烃源岩,其有效烃源岩是油气勘探能否获得突破的关键因素。为此,从"源""热"共控的角度对该区各凹陷的生烃潜力进行了详细分析,结果表明:①崖城组存在海陆过渡相与浅海相泥岩2种类型烃源岩,两者的生烃母质均为腐殖型干酪根,少量为偏腐殖混合型,有机质丰度的高低均受到陆源输入物多少的控制,前者的有机质丰度普遍较高,而后者丰度则偏低;②海陆过渡相煤系烃源岩的主要形成环境为三角洲平原、海岸平原、障壁海岸的潮坪沼泽等;③受新生代岩石圈伸展拉张程度控制和新构造运动的影响,深水区地温梯度、大地热流值、烃源岩热演化程度普遍较高,高热背景不仅决定了深水区凹陷以生气为主,也使浅水区埋深较浅的烃源岩处于生油窗而生成石油,同时,高地温与高热流除了促进烃源岩的生烃作用外,还能加速烃源岩中残留烃的裂解,提高生烃效率和生烃能力。最后,根据"源""热"共同作用控制油气生成的原理,对琼东南盆地主要凹陷进行了油气勘探潜力综合评价与初步排队。结论认为可将11个凹陷分为3种类型,其中崖南、陵水、宝岛、乐东、华光为Ⅰ类凹陷,勘探潜力最大。

An analysis of natural gas exploration potential in the Qiongdongnan Basin by use of the theory of joint control of source rock and geothermal heat

The Oligocene Yacheng Fm contains the most important source rocks that have been confirmed by exploratory wells in the Qiongdongnan Basin. The efficiency of these source rocks is the key to the breakthrough in natural gas exploration in the study area. This paper analyzes the hydrocarbon potential of each sag in this basin from the perspective of the joint control of both source rock and geothermal heat. Two types of source rocks occur in the Yacheng Fm, namely mudstone of transitional facies and mudstone of neritic facies. Both of them are dominated by kerogen of type Ⅲ, followed by type Ⅱ2. Their organic matter abundances are controlled by the amount of continental clastic input. The mudstone of transitional facies is commonly higher in organic matter abundance, while that of neritic facies is lower. The coal measure source rocks of transitional facies were mainly formed in such environments as delta plains, coastal plains and barrier tidal flat marsh. Due to the control of Cenozoic lithosphere extension and influences of neotectonism, the geothermal gradient, terrestrial heat flow value and level of thermal evolution are generally high in deep water. The hot setting not only determines the predominance of gas generation in the deep water sags, but can promote the shallow buried source rocks in shallow water into oil window to generate oil. In addition to promoting the hydrocarbon generation of source rocks, the high geotemperature and high heat flow value can also speed up the cracking of residual hydrocarbons, thus enhancing hydrocarbon generation efficiency and capacity. According to the theory of joint control of source quality and geothermal heat on hydrocarbon generation, we comprehensively evaluate and rank the exploration potentials of major sags in the Qiongdongnan Basin. The sags are divided into 3 types, of which the type Ⅰ sags including Yanan, Lingshui, Baodao, Ledong and Huaguang are the highest in hydrocarbon exploration potential.