塔里木盆地志留纪地层、沉积特征与岩相古地理

Stratigraphy,sedimentary characteristics and lithofacies palaeogeography of the Silurian in Tarim Basin

近年来塔里木盆地志留系油气勘探的发现,使志留系研究重新受到重视,但仍存在一些制约勘探的基础地质问题需要深入研究。本文在塔里木盆地志留系野外露头、井下地层岩性段划分对比的基础上,详细研究了不同岩性段的沉积体系、沉积相和微相特征,恢复了塔里木盆地志留纪不同时期的岩相古地理格局。根据志留纪地层发育和沉积特征,塔里木盆地志留系可分为柯坪—塔中和塔东地层分区,并进一步划分为柯坪、巴楚、塔中、塔北、满东和英吉苏6个地层小区。塔里木盆地志留系主要发育碎屑潮坪(夹风暴)沉积体系、无障壁滨岸海滩—陆棚沉积体系、碎屑潮坪—滨岸砂坝复合沉积体系和辫状河三角洲沉积体系4种类型,经历了海域范围逐渐缩小、古气候由温暖潮湿向炎热干旱过渡的过程。盆地整体地势呈南高北低,物源主体来自塔东和塔东南地区。平面上西部水体较浅而开阔,东部较闭塞,海侵方向来自北部和西北部。塔里木盆地志留纪古环境和古地理特征反映了在沿海岸带附近发育潮下潮道与砂坝砂体,尽管单砂体厚度较薄,但多套单砂体纵向上叠置可以形成砂体相对富集的砂层组,具有形成岩性—地层圈闭的地质条件。

A great attention has been repaid to the study of the Silurian in the Tarim Basin due to the discovery of petroleum in the interval, but there still exists some basic geologic problems which hinder the exploration. Based on the division and correlation of different lithologic members between the field outcrops and the wells, the depositional systems and sedimentary facies and microfacies of these members have been studied in detail. Then, the lithofacies palaeogeographic evolution of the Silurian in the Tarim Basin has been reconstructed. According to the stratigraphic and sedimentary developing features, the Silurian in the Tarim Basin can be divided into Keping-Tazhong and Tadong stratigraphic regions, and be subdivided into six stratigraphic sub regions including Keping, Bachu, Tazhong, Tabei, Mandong and Yingjisu. Mainly four types of depositional systems were developed in the Silurian in the Tarim Basin.They were the shore-neritic shelf system, detrital tidal flat （intercalated tempestites）, detrital tidal flat-shore composite and braided delta systems. Sedimentary facies are dominated by detrital tidal flat, shore beach, offshore subaqueous sand bar, neritic shelf and braided delta. The Tarim Basin experienced a process of marine domain gradually decreasing and paleoclimate changing from warm and humid to hot and dry during the Silurian. The topography of the Silurian basin was higher in the south and lower in the north and the sediments mainly came from Tadong and Tadongnan areas. There were a relatively shallow and extensive waterbody in western area, and a relatively euxinic body in eastern area. Marine transgression was from the north and northwest.The palaeoenvironmental and palaeogeographic characters show that subtidal channel and bar sandbodys were developed along the shore-line.Although the single sandbody was thin, relatively abundant sandbody assemblages can be formed by the vertical stacking of many sets of single sandbodys. Thus the geologic conditions for forming litho-stratigraphic traps are favorable.