藏南日喀则地区白垩纪泥岩、页岩有机质丰度及其油气地质意义
Organic matter abundance and significance of oil geology of the Cretaceous mudstones and shales from Rikaze area, southern Tibet, China
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摘要: 对藏南发育完整的、厚度巨大的海相白垩纪地层中的暗色泥、页岩开展了有机质丰度研究。综合分析结果表明：该区泥、页岩中残余有机碳含量在0.05％~1.1％之间，平均值约为0.42％，其中下白垩统约为0.531％，上白垩统约为0.265％，有机碳平均含量变化特征与该区的古地理格局及海平面升降有关。氯仿沥青‘A’含量在0.0009％~0.0072％之间，平均含量约为0.0040％。有机碳与氯仿沥青‘A’最大值与最小值之比分别为22和8。有机碳与氯仿沥青‘A’这种弱负相关性与两者遭受剧烈的地表风化作用、强烈的紫外线照射和高程度有机质热演化后的损失量有关，后者远大于前者。白垩系残余有机碳含量大于0.6％的中等—好生油岩样10件，占样品总数的27％，下白垩统样品9件，占样品总数的24.3％，上白垩统样品1件，占样品总数的2.7％，表明来自北喜马拉雅地区东山组和康马—隆子地区甲不拉组的早白垩世泥、页岩为该区主要的有效生油岩。Abstract: In this paper the research of organic matter abundance of dark mudstones and black shales in the Cretaceous strata which are developed completely and large-thickly in the marine environment from southern Tibet is carried out. The results of composition analysis reveal that the contents of organic carbon vary between 0.05% to 1.1％,and its average value during the Cretaceous period is about 0.42%, approximately 0.531％and 0.265％during the early and late Cretaceous respectively. The variety of average content of organic carbon is related to the paleogeography situation of the study area and sea-level oscillation. The contents of chloroform bitumen ‘A’ range from 0.0009% to 0.0072％, and its average value is about 0.0040％. The ratio between maximum and minimum content of both organic carbon and chloroform bitumen ‘A’ is 22 and 8. The vague negative correlation of organic carbon and chloroform bitumen ‘A’ is concerned with their lost quantities suffered from heavy surface weathering, drastic ultraviolet irradiation and high degree of thermal evolution of organic matters, and the later content is drastically higher than the former. Ten pieces of moderate and good samples of oil resource have organic carbon contents over 0.6% during the Cretaceous period, which accounts for 27% of the total samples. Nine pieces of them are from late Cretaceous, which accounts for 24.3% of the total. This fact demonstrates the Cretaceous mudstones and shales from Dongshan Formation in the north Himalayas area and Gyabula Formation in the Kangma-Longzi area are the main effective oil-bearing resource in the study area.
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