柯克亚凝析气田中新统西河甫组季节性河流沉积特征研究

柯克亚凝析气田中新统西河甫组在沉积特征上与正常的曲流河相比较为相似，如具有较典型的二元结构、中等砂岩成熟度、两段式的概率累积曲线、以单向水流为主的牵引流占主导地位的沉积构造等，同时也具有一些特殊性：①未发现有植物根迹，也没有含煤地层出现，在河漫滩亚相中缺少沼泽微相。②存在异常水流活动以及指示洪水重力流作用存在的沉积构造；③存在能反映氧化、河流季节性活动以及干旱气候环境的丰富而独特的遗迹化石组合，如Scoyenia组合和Skolithos组合等，上述特征反映其为干旱气候下的季节性河流沉积。

SEASONAL RIVER SEDIMENTATION OF XIHEFU FORMATION IN KEKEYA GAS CONDENSATE FIELD

Kekeya gas condensate field is located on the Kekeya anticline in southwestern Tarim depression. The sandstone of Xihefu Formation is the main reservoir of the gas condensate field. Xihefu Formation is a set of subaerial depositional sequence dominated by red, aubergine and brown sandstone, siltstone and mudstone. A lot of sedimentary structures that indicate unidirectional tractive current effect are present, such as large-scale parallel beddings, tabular cross-beddings, trough cross-stratifications, typical climbing ripple-beddings and wavy beddings, etc., as well as bedding structures such as aqueous current ripple mark, truncate ripple mark and lineation. Depositional characteristics such as typical binary structure, medium maturity of sandstone, grain size probability plot of two populations that mainly formed in meandering river are also common in the studied formation.In addition, the Xihefu formation also has some sedimentological particularities. (1) Almost all sedimentary rocks in studied area are red, aubergine or brown, with no fossil of plant root and coal-bearing strata, and no marsh microfacies in overbank subfacies which is common in a typical meandering stream. (2) There exist sedimentary structures showing the activity of abnormity current and flood gravity flow, such as graded bedding, flute casts and deep mud crack, etc. And matrix-supported gravels can sometimes be found in sandstone and mudstone. (3) Trace fossils are abundant, such as Scoyenia,Skolithos,Muensteria,Cylindricum,Palaeophycus or Planolites, among which Scoyenia and Skolithos are most abundant. Scoyenia is a familiar trace fossil in red beds, which is common in sand or mud sediments deposited in shallow lake and river. The typical environment of the trace fossil is the shallowest lake and river which exists near fresh water surface with seasonal exposure and flooding.Skolithos develops in medium to high energy environment, whose sedimentary substrate is clean and well-sorted sand. In a terrestrial basin, the trace fossil mainly occurs in a higher energy area, such as pointbar in the channel. Trace fossils in studied area occur in different sediments and form two types of trace fossil assemblages, i.e. Scoyenia assemblage and Skolithos assemblage, which reflect an environment of oxidizing, arid and river seasonal activity. Scoyenia assemblage mainly includes Scoyenia, Muensteria and sometimes Skolithos. Sedimentary settomg of the assemblage is bank subfacies, over-bank subfacies and top of low energy point bar, which reflects an environment of low energy, near water surface, periodical exposure and flooding. Skolithos assemblage mainly includes Skolithos and Cylindricum, reflecting a relatively higher energy environment, such as near-shore marine subfacies and river point-bar. For studied area it mainly developed in point-bar setting. The deep burrow indicates the existence of arid season.The study shows that Xihefu Formation does not reflect features of braided river, delta and typical meandering river, but of seasonal river under arid climate. According to its sedimentary characteristics, two subfacies (channel and overbank) and six microfacies (floodplain, crevasse splay, natural levee, point bar, lag and abandoned channel) were subdivided.