柴达木盆地南八仙地区渐新世浅水三角洲物源及沉积演化

以古近系渐新统下干柴沟组上段(E23)为研究对象,在精细岩芯观察和钻(测)井资料综合分析的基础上,结合重矿物组合、ZTR指数、砂岩碎屑组分、粒度累积概率图和砂地比等参数综合分析结果,对柴达木盆地南八仙地区E23湖盆扩张期弱退积型浅水三角洲物源方向、砂体展布规律和沉积微相发育演化进行了深入研究。结果表明：① 南八仙地区E23沉积时期物源主要来自南祁连山中酸性岩浆岩和中高级变质岩发育区,物源方向为近北东—南西向,稳定重矿物(磁铁矿、石榴子石、白钛矿和锆石)含量高(质量分数介于81. 0%~99. 9%)、不稳定重矿物(绿帘石和角闪石)含量低。② 南八仙地区E23沉积时期为浅水三角洲—滨浅湖沉积环境,主要发育水下分流河道砂体,岩性主要为岩屑长石砂岩,其次为长石岩屑砂岩,砂体的成分成熟度一般中等—较差、分选磨圆度中等—较差,粒度概率图以反映河流作用的一跳跃一悬浮式为主;沉积构造类型多样,主要发育块状层理、正粒序层理、平行层理和槽状交错层理。③ E23沉积时期,南八仙地区继承性发育3条北东—南西向呈条带状展布的砂体富集带(砂地比值介于35%~65%之间),早期到中期砂体发育程度增强,呈进积式;中期到晚期砂体发育程度有所减弱,呈弱退积式。④ E23沉积时期,南八仙地区处于浅水三角洲前缘亚相沉积环境,持续发育3个呈北东—南西向展布的沉积朵叶体,主要发育水下分流河道微相,其次为水下分流间湾微相,早期到中期浅水三角洲前缘发育规模增大,中期到晚期发育规模减小。

Provenance analysis and sedimentary evolution of Oligocene shallow water delta in the Nanbaxian area, northern Qaidam basin

This paper focuses on studying the upper member of the Xiaganchaigou Formation (E23) in the Late Oligocene of the Paleogene. The research objective is to deeply examine the provenance direction, sand body distribution law, and sedimentary microfacies development and evolution of the weak regressive shallow water delta in the expansion period of the E23 in the Nanbaxian area. To achieve this, a comprehensive analysis was conducted using fine core observation, drilling (logging) data, heavy mineral assemblage, ZTR index, sandstone clastic composition, grain size accumulation probability map, and sand- land ratio. The results show that: ① The provenance of the E23 sedimentary period in the Nanbaxian area mainly comes from the southern Qilian Mountains, which consist of intermediate- acid magmatic rocks and intermediate- high- grade metamorphic rocks. The provenance direction is almost northeast- southwest. The stable heavy minerals (magnetite, garnet, ilmenite, and zircon) have a high content (ranging from 81. 0% to 99. 9%), while the unstable heavy minerals (epidote and amphibole) show low content. ② The E23 sedimentary period in the Nanbaxian area is characterized by the development of a shallow delta- shallow lake sedimentary environment, primarily featuring underwater distributary channel sand bodies. The predominant lithology is lithic feldspathic sandstone, followed by feldspathic lithic sandstone. The composition maturity and sorting roundness are categorized as medium- poor. The particle size probability curve is mainly one jump and one suspension type, reflecting the influence of river action. Various sedimentary structures are present, including block bedding, normal sequence bedding, parallel bedding, and trough cross- bedding. ③ During the E23 sedimentary period, three NE- SW trending sand body enrichment zones were inherited in the Nanbaxian area (sand to ground ratio ranging from 35% to 65%). The degree of sand body development increased from the early to the middle stage, showing a progradation trend. However, the development of sand bodies weakened from the middle to the late stage, showing signs of weak retrogradation. ④ The shallow water delta front sedimentary subfacies were inherited in the Nanbaxian areaduring the E23 sedimentary period, with the continuous development of three sedimentary lobes. The underwater distributary channel microfacies are mainly developed, followed by the underwater interdistributary bay microfacies. The development scale of the shallow water delta front increases from the early to middle stage and decreases from the middle to late stage.