柴达木盆地西南区古近系浅水三角洲形成条件及砂体特征

Shallow delta forming condition and sand body characteristics in Paleogene of Southwestern Qaidam Basin

柴达木盆地西南区古近纪沉积时期古构造平缓,具备"盆浅湖阔"的古地貌特征;古气候以干旱到半干旱为主,母源区物理风化强烈,为大型浅水三角洲的沉积提供了充足的物质基础;同时,古水深约在10 m以内,相对湖平面变化相当频繁,水体来回动荡,说明柴西南区在古近纪具有发育浅水三角洲沉积的古地理环境。总体岩性以中—细砂岩、粉砂岩、砾岩和含砾砂岩夹棕红色泥岩为主,粒度概率曲线多呈现两段式的特征,具有丰富的反映强水动力成因的沉积构造和生物遗迹化石。依据气候变化、相对湖平面变化和物源供给等方面的耦合关系,可以将研究区古近纪浅水三角洲砂体形态分为树枝状、坨状和席状3种,大致对应于下干柴沟组下段、下干柴沟上段和上干柴沟组的砂体形态特征。砂体的分布主要受七个泉、红柳泉、阿拉尔、昆北和XI号共5条规模较大的断裂坡折带及其派生的挠曲坡折带的影响,并将断裂坡折带分为逆源型和顺源型2种,在此基础上建立柴西南区古近纪构造坡折带控砂模式。

The paleostructure of southwestern Qaidam Basin was extremely gentle during Paleogene and had palaeogemorphic characteristics with a shallow basin as well as a wide lake. Its geologic climate was mainly arid and semi-arid, which could result in strong weathering of provenance and provide abundant clastics for large-scale shallow water delta deposits. The paleo-water depth was within 10 m and the relative lacustrine level chang was quite frequent. All those mean that it has the paleogeography for the study area to develop the sediments of shallow water delta. The overall lithology of shallow water delta is characterized by middle-fine sandstone, siltstone, conglomerate or pebbly sandstone interbeded with brownish red mudstone. The particle size probability curve of sandstone mainly displays as two-section. The sedimentary structures of strong water dynamics as well as bioturbations are plentiful in the sandstones. The sand bodies morphology of shaollow delta in study area can be divided into arborization, cuspate and sheet shapes according to the coupling relation of climatic change, material source and so on, which roughly correspond to the shapes of E31, E32 and N1. The distribution of Paleogene sand bodies in the study area is controlled by the five large-scale fracture slope break zones and the derived flexure slope break zones, such as Qigequan, Hongliuquan, Alaer, Kunbei and XI, which can be divided into inverse source type and following source type. On that basis, the sandbodies controlling model by the structure slope zones of Paleogene formation in Southwestern Qaidam Basin can be established.