曲靖盆地古近—新近纪构造演化与生物气成藏的关系

通过对地震、地质和钻井等资料进行综合分析,对曲靖盆地的构造特征、盆地形成和演化进行了较为系统的研究,分析了构造演化与生物气晚期成藏的关系。早-中渐新世的断陷阶段为主力气源岩大套湖相暗色泥岩蔡家冲组形成时期;渐新世晚期—上新世早期为盆地的整体抬升萎缩阶段,使蔡家冲组沉积有机质免遭大量消耗,保障了第四纪以来生物气成藏的有机物质;晚上新世为盆地拗陷阶段,是茨营组次要煤系气源岩、储集层、盖层和岩性圈闭形成的主要时期;上新世末的压扭抬升萎缩阶段为断背斜和断鼻等构造圈闭和构造-岩性复合圈闭发育时期;第四纪为盆地稳定沉降阶段,是生物气聚集成藏时期。气源岩的生物气产率、断裂的垂向输导作用和圈闭的保存条件以及他们之间的动态配置共同控制了生物气晚期成藏。

Tertiary Tectonic Evolution and Its Relation with Biogas Accumulation in Qujing Basin

The Paleogene-Neogene sedimentary basins in northeastern Yunnan province have good exploration prospects of biogenic gas, among which the representative one is Qujing basin. Through tectonic and sedimentary interpretations by 2D seismic lines, combining with geologic data and well logs, this paper discusses the tectonic/structural features and the basinal formation and evolution, and analyzes the relation between the tectonic/structural evolution and biogas accumulation. The study shows that during the fault-depression stage of Early-Middle Oligocene, the Caijiachong formation with mass lacustrine dark mudstone was deposited, which was as a major gas source rock. From Late Oligocene to Early Pliocene, as a result of cooling due to the entire uplift and erosion of this basin, the over-consumption of the organic matters in this formation was avoided, providing the organic materials for biogas accumulation since Quaternary. In the depression stage of Late Pliocene, the Ciying formation was deposited, which was the main period of forming its secondary coal-measure gas source rock, reservoir rock, cap rock and lithologic traps. In the end of Pliocene, the basin underwent the stage of compressionshearing uplift and shrinkage, characterized by the development of structural traps like fault-anticline and fault nose and structural- lithologic traps. In the Quaternary, the basin was in steady subsidence stage, which was a period of hiogas accumulation. The rate of biogas generation from gas source rocks, the vertical conductivity of faults, the preserving conditions of traps and their dynamic assemblages could be the key factors for controlling the late-stage biogas accumulation in this basin.