东海某凹陷断陷期重要不整合面特征及其对沉积演化的控制

Characteristics of major unconformities and their control over sedimentation in rifting stage:A case from the East China Sea

以东海某凹陷二维地震、三维地震及钻测井等资料为基础,开展控凹断层活动定量分析,并且探讨了断陷期重要不整合面特征及其对沉积演化的控制作用。研究表明,东海某凹陷断陷沉积时期,断裂活动从剧烈到平静,地形高差从大到小,形成了3个重要不整合面:断陷幕次界面T4^0和T3^4,断陷―断坳转换面T3^2。由此,将断陷期划分为3期断陷幕和1期断坳幕:断陷Ⅰ幕(古新统)为构造启动期,其受控于三潭断裂和白堤断裂,分布范围局限,发育近源沉积体系;断陷Ⅱ幕(始新统宝石组)和断陷Ⅲ幕(始新统平湖组下段)为强烈断陷期,地层被断层错断严重,水深较大,西部缓坡带以受潮汐影响的三角洲、潮坪沉积为主,东部陡坡带以扇三角洲沉积为主,受断陷格局限制,扇三角洲以快速沉积、堆积厚度大、沉积范围小、相变快为主要特征;断坳幕(始新统平湖组中上段)为断陷萎缩期,地层开始向坳陷沉积转换,盆地与南部海域沟通变强且水深变浅,凹陷内部以平行岸线的潮流作用为主,发育大规模的非对称性三角洲和陆架潮汐砂脊等,储层富集,可作为有利勘探目标。此外,断陷幕发育扇三角洲等近源沉积,是岩性油气藏的有利发育区,也为深层油气勘探提供了方向。

Based on 2D and 3D seismic data and well drilling data from a Sag in the East China Sea, through quantitative analysis of active sagcontrolling faults, the characteristics of the major unconformities and their control over sedimentation in rifting stage are discussed in this paper.The study shows that during the rifting stage, fault activities changed from violent to calm, and the topographic differences changed from large to small, that resulted in three major unconformities: second-order unconformities between rifting episodes T4^0 and T3^4, second-order unconformity between rifting episode and rift-depression episode T3^2. Upon the facts, the rifting stage of the sag may be subdivided into 3 rifting episodes and 1 rift-depression episode. The rifting episode I of Palaeocene is the initiation time of tectonic movement, controlled by the Santan fault and the Baidi fault with limited distribution and near-source depositional systems. The rifting episodeⅡ of Baoshi Formation and the rifting episode Ⅲ of Lower Pinghu Formation are the period of intense tectonic movement. During the period, the strata were strongly dislocated by faults, the region was covered by deep water. Tidal delta and tidal flat deposits dominated the gentle slope belt in the west, and fan delta developed in the steep slope belt in the east of the basin. Restricted by rifting depression, the fan delta is characterized by high sedimentation rate, large thickness, limited depositional area and dramatic facies change. The rift-depression episode in the middle and upper stages of the Pinghu Formation represents a shrinking period, the region began to convert to a depression. Water exchange between the basin and the seas in the south was intensified, water became shallower, large-scale asymmetric deltas and tidal sand ridges were well developed. The rift-depression episode and rift episode are favorable for the formation of lithological reservoirs, and the excellent targets for deep oil and gas exploration.