银根—额济纳旗盆地天草凹陷下白垩统巴音戈壁组有效烃源岩地球化学特征及其形成环境

对于低勘探程度的小型断陷湖盆，寻找有效烃源岩对于勘探方向的选择具有非常重要的意义。本文基于天草凹陷新钻井的大量分析测试资料，对主要勘探层系下白垩统巴音戈壁组的纵横向烃源岩地球化学特征及形成环境进行了系统分析，并阐述了沉积环境变化对有效烃源岩发育的控制作用。研究结果表明：巴音戈壁组烃源岩有机质丰度达到了中等—好的级别，以巴音戈壁组二段有机质丰度最高（w（TOC）平均为1.1%）；有机质类型为Ⅱ₁-Ⅱ₂型，且巴音戈壁组二段的Ⅱ₁型有机质丰度最高，属于腐殖腐泥型；烃源岩热演化程度整体达到了低熟-成熟阶段，巴音戈壁组二段为成熟的烃源岩，具备较高的生烃能力；巴音戈壁组有效烃源岩下限为w（TOC）=1.0%，有机质来自高等植物和水生生物的混合源，且低等水生生物贡献比例稍大；巴音戈壁组沉积期为高盐度和还原的沉积环境，盆内藻类勃发和盆外适当的陆源有机质输入是湖盆水体较高古生产力和较好有机质类型的必要条件，持续稳定的缺氧环境为有机质的保存提供了良好条件，这是巴音戈壁组有效烃源岩形成的两个关键条件。天草凹陷南次洼比北次洼更适合有效烃源岩的发育，且发育的有效烃源岩具有更高的有机质丰度和成熟度，是下一步油气勘探的有利区域。

Geochemical Characteristics and Formation Environment of Effective Hydrocarbon Source Rock of the Lower Cretaceous Bayingebi Formation in Tiancao Sag,Yingen-Ejinaqi Basin

For small-scaled rift lake basins with a low exploration degree, it is of great significance to search for effective source rocks for the exploration purpose. Based on a large number of analysis test data of new drillingwells in Tiancao sag,the authorsstudiedthe geochemical characteristics and sedimentary environment of the hydrocarbon source rocks in the vertical and horizontal directions of the Lower Cretaceous Bayingebi Formation, and described the effect of sedimentary environment changes on the development of effective source rocks.The results reveal that the organic matter abundance of the source rocks of the Bayingebi Formation reachesa medium to good level, and the second member of the Bayingebi Formation has the highest organic matter abundance(average w(TOC)=1.1%). The organic matter is of type Ⅱ₁-Ⅱ₂, and the proportion of type Ⅱ1in the second member of the Bayingebi Formation is higher, belonging to humic-sapropelic.The thermal evolution degree of the source rocks isat low-mature to mature stage. The second member of the Bayingebi Formation belongs to mature source rocks with high potential of hydrocarbon generation.The lower limit of the effective source rock of the Bayingebi Formation is defined as w(TOC)=1.0%, the organic matter wasderived from a mixed source of higher plants and aquatic organisms, and the contribution proportion of lower aquatic organisms is slightly higher. The Bayingebi Formation was deposited in areducingenvironment with high salinity.The algal bloom within the basin and the appropriate terrestrial organic matter input outside the basin are necessary conditions for the increase of water paleo-productivity in the lacustrine basin, and the continuous and stable anoxic environment provides good conditions for the preservation of organic matter. These are the two key conditions to form effective source rocks.The southern sub-sag is more suitable for the development of the effective source rocks than the northern sub-sag in Tiancao sag, and the effective source rocks have higher organic matter abundance and maturity. So the southern sub-sag is the favorable area for oil and gas exploration.