Study on sequence stratigraphy and sedimentary systems of the Wenchang Formation in the southern Xijiang depression of the Pearl River Mouth Basin
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摘要:
珠江口盆地西江凹陷南部包括两个相邻洼陷：西江36洼和番禺4洼，钻探结果却揭示了截然不同的油气勘探前景，其中，番禺4洼探明储量已过亿吨，而西江36洼却未有商业发现，导致勘探上对西江36洼烃源潜力有所顾虑。为了揭示这种差异油气地质条件产生的原因，亟待开展两个洼陷的对比研究。以主要烃源层系文昌组为切入点，通过开展精细的层序-沉积研究，进一步系统评估西江36洼烃源潜力。依据地震、钻井及分析化验等资料，在西江36洼与番禺4洼文昌组共识别了6个三级层序。西江36洼和番禺4洼文昌组发育扇三角洲、辫状河三角洲、湖底扇及湖泊沉积等沉积相类型，垂向上沉积相带具有旋回特征。伴随裂陷不同阶段和边界断层活动差异，陡坡带和缓坡带三角洲沉积体系规模呈现差异演化特征，而湖盆和半深湖-深湖相规模总体呈现出先增大后减小的规律。沉积相带在横向上也具有明显的迁移特征，表现为文六段时期先在西江36洼开始沉积，文五段开始再扩展到番禺4洼，同时文一段时期西江36洼湖盆已经消失，只在番禺4洼沉积充填。进一步对比两个洼陷各三级层序反映优质烃源岩条件的半深湖-深湖规模及物源供给量，结果显示在文六段至文四段时期两个洼陷烃源条件基本一致。鉴于该层段作为番禺4洼的主力源岩，其巨大的生烃潜力已经被勘探所证实，因此认为不能低估具有相似烃源条件的西江36洼生烃潜力。
Abstract:The Xijiang depression in the Pearl River Mouth Basin consists of two adjacent sags, the Panyu 4 sag and the Xijiang 36 sag, in which quite different oil and gas exploration prospects have been revealed by drilling. The proved reserves of Panyu 4 sag have exceeded 100 million tons, while no commercial discoveries been found in the Xijiang 36 sag so far. In order to reveal the causes of such a difference in oil and gas geological conditions, it is necessary to carry out a comparative studies of the two sags. In this paper, the Wenchang Formation, the main source series, is selected as the key point for breakthrough, and the hydrocarbon source potential of Xijiang 36 sag is systematically evaluated through detailed sequence stratigraphic studies. Based on the data of seismic, drilling and laboratory analysis, six third-order sequences have been identified in the Wenchang Formation in both of the Xijiang 36 and Panyu 4 sags. There are fan delta, braided river delta, sublacustrine fan and lacustrine sedimentary facies in the Wenchang Formation of Xijiang 36 and Panyu 4 sags, and the sedimentary facies belts also show a cyclic patterns vertically. In the different stages of rifting and/or boundary fault activities, the scale of delta depositional system in steep slope zone and gentle slope zone shows different evolutionary features, whereas the scales of Lake Basin and its deep part in the middle lake expanded in the beginning and shrank later on. The sedimentary facies belt also has obvious migration in lateral direction. In the Wen-6 Period, the deposition was initiated in the Xijiang 36 sag, and then extended to Panyu 4 sag with time. In the period of Wen-1, the lake basin occurred only in the Panyu 4 sag and disappeared in the Xijiang 36 sag. The distribution pattern of semi deep and deep lake deposits and study of sediment sources suggest that the high-quality hydrocarbon source potential values of each third-order sequence in the two sags are comparable and the source rock conditions are basically the same in the two sags in the wen-6 and wen-4 periods. As the main source rock of the Panyu 4 sag has been confirmed by exploration, therefore, it is considered that the exploration potential of Xijiang 36 sag with similar hydrocarbon source conditions should not be underestimated too early.
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表 1 西江36洼与番禺4洼烃源岩条件对比
Table 1. Comparison of source rock conditions between Xijiang 36 sag and Panyu 4 sag
西江36洼 番禺4洼 层序 中深湖规模 物源供给量 生烃
潜力值中深湖规模 物源供给量 生烃
潜力值面积
/km2平均
厚度/m面积
/km2平均
厚度/m面积
/km2平均
厚度/m面积
/km2平均
厚度/m上文昌 文一段 0 0 − − 0 0 0 − − 0 文二段 0 0 − − 0 180 400 150 250 1.92 文三段 42 450 60 125 2.52 300 600 147 200 6.12 下文昌 文四段 83 650 80 110 6.13 132 600 110 100 7.2 文五段 45 500 70 120 2.68 50 350 80 120 1.82 文六段 0 0 − − 0 0 0 − − 0 注：生烃潜力值=（中深湖面积×中深湖平均厚度）/（物源面积×物源平均厚度）。 
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