琼东南盆地松西凹陷湖相烃源岩发育条件新认识与优质烃源岩刻画

New understanding on development conditions of lacustrine source rocks and characterization of high-quality source rocks in Songxi Sag,Qiongdongnan Basin

针对琼东南盆地松西凹陷烃源岩发育机制不清、优质烃源岩分布不明的问题,根据已钻井原油地化特征、次洼沉积特征、地震相与属性特征、地震速度谱的对比分析,对松西凹陷东、西次洼始新统中深湖相烃源岩进行了预测,明确了松西凹陷沉降速率、物源注入点、凹陷结构三因素耦合控制了湖相烃源岩的发育与展布。松西凹陷西次洼紧邻大型物源注入点,为富砂沉积,从而导致较低的古生产力水平、较高的沉积速率和开放氧化的水体环境,不具备优质烃源岩形成的基本条件;东次洼远离大型物源注入点,仅发育小规模砂体,利于发育中—深湖相烃源岩。已钻井原油地化指标表明,研究区原油来自中深湖相烃源岩,证实松西凹陷存在优质烃源岩。始新统中深湖相烃源岩具有低频、连续、强反射地震特征,且具有低速异常,主要分布于东次洼的洼中及缓坡带。基于以上认识指导在东次洼部署钻井,首次在琼东南盆地钻遇始新统油页岩,且烃源岩指标好,提升了松西凹陷及琼东南盆地原油勘探前景。

With respect to the problems of unclear development mechanisms of source rocks and distribution of high-quality source rocks in the Songxi Sag,Qiongdongnan Basin,medium-deep lacustrine source rocks in eastern and western subsags of the Songxi Sag were predicted,according to geochemical characteristics of crude oil from drilled wells,sedimentary characteristics of these subsags,the characteristics of seismic facies and attributes,and comparison and analysis of seismic velocity spectrum.In this study,it was defined that the following three factors,namely,subsidence rate of the Songxi Sag,injection points in the provenance,and sag structure,coupled and controlled the development and distribution of lacustrine source rocks.Western subsag of the Songxi Sag was adjacent to large-scale injection points in the provenance as sand-rich sediments,and then led to low paleo-productivity level,high depositional rate,and open and oxidized water body setting.It had no fundamental conditions for the formation of high-quality source rocks.Eastern subsag was far from these large-scale injection points with the development of only small sand bodies,which was favorable for the development of medium-deep lacustrine source rocks.Geochemical indexes of crude oil from drilled wells indicated that the crude oil in the study area came from medium-deep lacustrine source rocks,which confirmed the occurrence of good source rocks in the Songxi Sag.Eocene medium-deep lacustrine source rocks had seismic characteristics of low frequency,continuous and strong reflectance in addition with low velocity anomalies that were mainly distributed in the center of eastern subsag and gentle slope zone.Above-mentioned understandings could guide well allocation in eastern subsag.Eocene oil shale was firstly encountered in the wells,Qiongdongnan Basin.These source rocks had good indexes and enhanced oil exploration prospect in the Songxi Sag,Qiongdongnan Basin.