西藏南羌塘坳陷泥质烃源岩评价及有利生烃区预测

Characteristics of the Mesozoic marine argillaceous source rocks and prediction of favorable hydrocarbon generation area in South Qiangtang Depression,Xizang(Tibet)

羌塘盆地是中国特提斯域面积最大、最具有含油气远景的沉积盆地,盆地内沉积有多套泥质烃源岩层,其中上三叠统土门格拉组、中—下侏罗统曲色—色哇组以及上侏罗统安多组三个层段生烃潜力巨大。本文在充分收集前人资料基础上,通过有机地球化学分析对南羌塘坳陷中生界上述三套泥质烃源岩层进行了综合评价研究,并对它们的生烃有利区进行了预测,探讨了优质烃源岩形成的主控因素。结果显示土门格拉组烃源岩有机质丰度中等偏低,有机质类型主为Ⅱ型,热演化程度较高,整体为差—中等生油岩,主力生烃坳陷位于分布在双湖多普勒乃—查郎拉和安多达卓玛—土门一带;曲色—色哇组有机质类型较好(Ⅱ1型),但有机质丰度偏低,且处于高成熟—过成熟阶段,属差生油岩,有利生烃区主要位于西部昂达尔错、中部扎曲乡和东部达玛尔地区,同时在双湖毕洛错地区发育一个相对局限的优质烃源岩区;安多组有机质丰度高,且有机质类型好(Ⅱ1型),处于生油高峰的成熟阶段,为好生油岩,但分布局限,主要位于安多114道班地区。沉积环境是南羌塘坳陷优质泥质烃源岩形成的主控因素:土门格拉组时期,温暖、潮湿的气候条件以及海—陆过渡环境相对较高的沉积速率,为该套优质泥质烃源岩的形成提供了条件;对于曲色—色哇组,广海边缘外陆棚体系不利于有机碳的沉积和埋藏,而相对局限的障壁—潟湖沉积环境则是优质烃源岩的有利形成区;上侏罗统安多组深水台沟相低能、静水的还原环境,以及浅水区稳定的有机质供给促进了该区富有机质黑色岩系的形成。这些研究对进一步明确羌塘盆地主力烃源岩特征与分布及下一步油气勘探具有重要参考价值。

Objectives:The Qiangtang Basin is the largest and most potential petroliferous basins in eastern Tethys,China.It contains a series of marine argillaceous source rocks,in which the Upper Triassic Tumengela Formation,Lower—Middle Jurassic Quse and Sewa Formations and Upper Jurassic Amdo Formation have great potential for hydrocarbon generating.Methods:Based on the previous data,we evaluated the hydrocarbon-bearing potential of the three Mesozoic argillaceous source rocks in the South Qiangtang Depression through a comprehensive organic geochemical analysis,predicted the favorable hydrocarbon generation area and discussed the main factors in controlling high-quality source rocks.Results:The organic matter of the Tumengela Formation is mainly type II and has medium to low organic abundance and relative high maturity,overall belong to the medium-to good-grade source rock.And its favorable hydrocarbon generation areas are located in the Duopuleinai—Chalangla area of Shuanghu and Dazhuoma—Tumen area of Amdo.The Quse and Sewa Formations is poor source rock with mainly II1-type organic matter,low organic material abundance and highly to past maturity,and its favorable source rock kitchens contain the Angdaercuo area in the West of the South Qiangtang Depression,Zhaqu area in the Central and Damaer area in the East.In addition,a comparatively restricted favorable hydrocarbon generation area is existed in the Bilong Co area,Shuanghu.The Amdo Formation is generally classified as Type II1 and in the mature thermal evolution phase and has high abundance of organic matter,forming a set of superior source rock with a restricted favorable hydrocarbongenerating area in the 114 Station of Amdo.Conclusions:Sedimentary environment is the main factor in controlling the deposition of superior argillaceous source rocks in the South Qiangtang Depression.Both of the warm humid climate and relative higher accumulation rate provide a favorable condition for source rock deposition of the Tumengela Formation.The Quse—Sewa Formations deposited in the epicontinental—shelf settings are poor source rocks,whereas those deposited in a barrier—lagoon environment are very good source rocks.Sluggish and reducing environment in trench and stable organic matter supply from shallow water facilitate the deposition of organic carbon-rich sediments in the Amdo Formation.