通过对廊固凹陷古近系沙河街组四段-沙河街组三段下亚段烃源岩热解数据、碳氧同位素和分子地球化学参数的深入分析，探讨了不同构造气候条件下低丰度湖相烃源岩的发育机理。岩石热解和干酪根显微组分分析表明沙河街组四段-沙河街组三段下亚段有机质类型主要为Ⅱ型，有机质丰度存在明显差异。结合区域构造-气候背景以及碳氧同位素和分子地球化学参数，建立了沙河街组四段-沙河街组三段下亚段的烃源岩发育模式。沉积于裂陷初始阶段的沙河街组四段烃源岩，其湖盆宽/深比小，气候干热，水体为咸水-半咸水，发育稳定的盐度分层。有机-无机碳同位素的负偏移与较低的Pr/Ph值反映了相对低的古生产力和较好的保存条件。沙河街组三段下亚段烃源岩沉积于强烈裂陷阶段，湖盆宽/深大，气候温暖湿润，水体为淡水，上、下水体交换相对频繁，发育不稳定的温度分层，碳同位素正偏移以及C27/C29甾烷的变化反映了生产力增高和陆源有机质输入增多。%Rock-Eval data,carbon and oxygen isotopes and molecular geochemical parameters of the source rocks in E s4 (the 4th member of Paleogene Shahejie Fm )-the lower Es3 ( the 3rd member of Paleogene Shahejie Fm ) in Langgu Sag were integrated to study the formation mechanism of lacustrine source rocks with low abundance under different tectonic and climatic conditions .Rock pyrolysis and kerogens′maceral composition analysis revealed that the E s4 and the lower Es3 are the similar in the major kerogen type of Ⅱ,but are quite different in organic matter abundance .Based on analyses of carbon and oxygen isotopes in the context of regional tectonic and palaeo-climate settings,we established depositional models for the source rocks in Es4-lower Es3.The source rock in the Es4 was deposited in a lake basin at the initial stage of rifting,with low width/depth ratio,arid climate,and brackish water or saline water with stable salinity stratification .Its 13 C-depleted of the inorganic and organic matter and lower Pr /Ph might reflect relatively low primary productivity and better organic preservation .In contrast,the source rock in the lower E s3 was deposited in a lake basin in the intense rifting stage,with a high width/depth ratio,warm and humid climate,fresh water,and frequent water-column overturning with unstable temperature stratification .Its 13 C enrichment of the inorganic and organic matter and C 27/C29 sterane ratios were interpreted to be caused by higher primary productivity and more terrigenous organic matter input .
展开▼
