涠西南凹陷流二段油页岩与泥岩形成环境及生烃特征差异性

Differences of formation environment and hydrocarbon generation characteristics between oil shale and mudstone in the second Member of Liushagang Formation in Weixinan sag

始新统流沙港组二段(流二段)油页岩和泥岩是涠西南凹陷古近系的主力烃源岩,二者形成环境和生烃特征差异研究对北部湾盆地新区优质烃源岩预测和凹陷评价意义重大。本文根据有机地球化学、元素地球化学及有机岩石学的原理和方法,基于烃源岩地球化学特征综合分析和实验室模拟研究,对比了两类烃源岩形成环境和生烃特征的差异。结果表明,涠西南凹陷流二段油页岩有机质丰度高、类型好,为很好烃源岩;泥岩有机质丰度较低、类型较差,为好烃源岩。油页岩与泥岩地球化学特征及有机质富集程度的差异是由母岩性质、生源构成、古生产力和有机质保存条件等因素共同作用的结果。油页岩沉积时期湖盆古生产力高、藻类勃发,主要形成于还原环境,有利于有机质的保存;泥岩沉积时期弱还原的水体环境、较快的沉积速率和陆源物质的大量输入不利于有机质的保存。有机质类型和生源构成的差别造成两类烃源岩生烃动力学特征和生烃演化模式存在较大差异,其中油页岩活化能分布相对集中,而泥岩活化能分布相对分散;油页岩的生油高峰早于泥岩,且生油高峰期油页岩的生油率明显高于泥岩。

The oil shale and mudstone in the second Member of the Eocene Liushagang Formation(the Liu-2 Member for short)are the main Paleogene source rocks in Weixinan sag,so the study of the differences in their formation environment and hydrocarbon generation characteristics is of great significance for predicting high-quality source rocks and evaluating depressions in the new area of the Beibuwan basin.According to the principles and methods of organic geochemistry,element geochemistry and organic petrology,this paper compared the differences in formation environment and hydrocarbon generation characteristics between the two types of source rocks on the basis of geochemical characteristics comprehensive analysis of source rocks and laboratory simulation research.The results show that the Liu-2 Member oil shale corresponds to excellent source rocks with higher organic matter abundance and better kerogen types;the mudstone in this member are evaluated as good source rocks.These differences in geochemical characteristics and organic matter enrichment between oil shale and mudstone of the study area are comprehensively affected by parent rock properties,source compositions,palaeoproductivity and organic matter preservation conditions.The palaeoproductivity was higher and algae bloomed during the oil shale sedimentary period,and it is mainly deposited under reducing environment,which is conducive to the preservation of organic matter.While during the mudstone sedimentary period,the weakly reducing environment,fast sedimentation rate and extensive input of terrestrial materials are not conducive to the preservation of organic matter.The differences in kinetic characteristics and evolution patterns of hydrocarbon generation between oil shale and mudstone of the study area are affected by organic matter types and source compositions.Among them,the activation energy distribution of oil shale is relatively concentrated,while that of mudstone is relatively dispersed.The peak oil generation of oil shale occurs earlier than that of mudstone,and the oil yields of oil shale during the peak oil generation period are significantly higher than that of mudstone.