莺-琼盆地海相烃源岩特征及高温高压环境有机质热演化

莺歌海盆地与琼东南盆地存在渐新统海岸平原-半封闭浅海相和中新统浅海相两套海相烃源岩。其中,渐新统烃源岩发育于琼东南盆地早第三纪半地堑,存在海岸平原和半封闭浅海两类烃源岩组合。海岸平原含煤烃源岩有机质丰度高,富含陆源有机质,具有很好的生气潜力;半封闭浅海相烃源岩TOC含量总体低于1.0%,但其规模大且存在TOC1.5%的较高丰度段,故亦具有较大的生气能力。中新统海相烃源岩主要分布于莺歌海盆地裂后热沉降形成的中央坳陷,其有机质丰度横向变化比较大,位于中央坳陷带的烃源岩有机质丰度较高,TOC大多在0.40%～2.97%之间,有机质以气型干酪根为主。盆地的高地温为有机质向天然气转化提供了有利的条件,尤其是热流体活动使浅层有机质超前熟化,但地层超压对有机质热演化也有一定的抑制作用。盆地模拟结果显示,莺-琼盆地主要凹陷烃源岩大量生气时间较晚,与圈闭形成期的时空配置好,有利于成藏。

Characteristics of Marine Source Rocks and Effect of High Temperature and Overpressure to Organic Matter Maturation in Yinggehai-Qiongdongnan Basins

The Oligocene Yacheng coastal plain and semi-enclosed neritic source rocks exsit in Yinggehai Basin and the Miocene Shanya-Meishan neritic source rocks in Qiongdongnan Basin.The Yacheng source rocks develop in the Paleogene half-graben and the marine Meishan-Sanya source rock in the post-rifting Miocene formation of central Yinggehai Basin.The Yacheng source rocks consist mainly of neritic mudstones and coastal plain coalbearing strata.The carbonaceous mudstones and coals in the coal-bearing strata contain variable amounts of TOC with gas-prone terraneous matter with type III and II2 kerogen and the neritic mudstones contain abundance at>1.5 %TOC though there is<1.0%TOC in a wide extent so they are of good gas generation potential.The Miocene source rocks that occur mainly in central Yinggehai Basin are composed of deltaic to neritic deposits.The TOC content in the Miocene shales from the basin margin are only 0.4 to 0.65%but increase from 0.4%to 2.97%toward the depression region.The Miocene source rocks also contain gas-prone type Ⅱ_2-Ⅲ kerogens.The high geothermal gradient and rapid heating in the basins resulted in advanced maturation of organic matter in a shallower burial depth,thermal maturation of organic matter in strongly overpressured systems was slightly retarded and the peak gas generation was delayed.The result of 1-D basin modeling indicates that the Yacheng Formation in Qiongdongnan Basin reached its peak stage of gas generation during the middle Miocene-Pliocene time.The lower-middle Miocene marine shales in central Yinggehai Basin reached their peak stage of gas generation during the PlioceneQuaternary time,which provides favorable conditions for a large amount of natural gas accumulation