吐鲁番坳陷二叠系烃源岩有机质沉积特征

DEPOSITIONAL CHARACTERS OF ORGANIC MATTERS FROM THE PERMIAN SOURCE ROCKS IN THE TURPAN DEPRESSION

二叠系烃源岩是我国西部吐鲁番坳陷中重要的潜在生烃层系。以地表剖面暗色泥岩的分析资料为主,结合部分井下烃源岩样品分析数据,探讨了吐鲁番坳陷二叠系烃源岩沉积时水体的Eh值、pH值及沉积特征。利用烃源岩分布、有机碳含量及有机质显微组份的赋存状态等特征对烃源岩的有机质沉积特征进行了研究。通过对烃源岩有机质分布的影响因素分析,发现烃源岩有机质的赋存大体上有3种不同的状态:顺层富集型、局部富集型及分散型。泥质碳酸盐岩类和泥岩类烃源岩主要沉积在半深湖—深湖相带,沉积物中有机碳含量高;灰色、深灰色泥岩及黑灰色碳酸盐岩主要沉积在浅湖相带,有机碳含量偏低;绿灰色、灰色泥岩主要沉积在河流—三角洲平原相带,沉积物中有机碳含量低。烃源岩沉积时具还原程度强的沉积相带是原始有机质沉积的良好场所,沉积有机质能及时沉积并保存,烃源岩大多有机质丰度高,有机质类型好;反之,则明显变差。

The Permian source rocks were important and potential hydrocarbon forming system in the Turpan Depression, Turpan-Hami Basin, northwest China. Based on analyses of the dark mudstones' outcrops along the surface sections, combining with geochemical data from the core samples, the paper studied the palaeosalinity of the water body in which the source rocks were deposited as well as the depositional features. The Eh and pH values of the water body were deduced from the mineral assemblages, organic carbon contents and other sedimentary factors. The sedimentary characteristics of organic matters of the source rocks were analyzed by the distribution of the source rocks and organic carbon contents as well as hosting state of the organic macerals in the main. It was discussed that the hosting state of organic matter in the source rocks could be classified into three types, namely, bedding-enrichment type, part-enrichment type and dispersion type. The source rocks of argillaceous carbonates and mudstones deposited mainly in an environment of semi-depth and deep lacustrine facies, with a high content of organic carbon, and the grey or dark grey mudstones deposited mostly in an environment of relatively shallow lacustrine facies, with less rich in organic carbon content. The green-grey or light-colored mudstones deposited mostly in an environment of fluvial or delta-plane facies, so poor in organic carbon content. It was concluded that the sedimentary zones of strong reducing environment were better positions for deposits of primary organic matter, where the organic matters could be well buried and preserved, and the source rocks were generally higher in organic contents and better in organic types.