陆丰凹陷东部缓坡带层序地层样式及控制因素

Sequence architecture and controlling factors of the Wenchang formation in the gentle slope of eastern Lufeng depression,Pearl River Mouth basin

综合运用钻测井和三维地震资料,对陆丰凹陷东部缓坡带文昌组层序地层格架、坡折带类型及层序样式进行了详细探讨,认为古近系文昌组自下向上可划分为5个三级层序,代表了一个由弱到强再变弱的完整裂陷旋回.陆丰凹陷东部缓坡带文昌组层序构成样式划分为3类6种:斜坡型(平缓斜坡型和翘倾斜坡型)、挠曲型(先存潜山挠曲型和沉积地形挠曲型)和断阶型(二级断阶型和多级断阶型),并分别建立了相应的层序地层模式.研究结果表明:初始断陷时期,断裂活动弱,陆丰凹陷缓坡带总体表现为平缓的斜坡地貌特征,坡度约0.8°~2.0°;断陷强烈期,岩浆作用活跃,同沉积断层差异性活动,形成二级和多级断裂型以及岩浆底辟改造斜坡型坡折带,在深湖背景下发育众多低位砂及高位滑塌砂体,地震反射以丘形为特征,钻测井显示厚层泥岩夹薄层细砂岩;弱断陷期,受控于断层活动减弱以及前期的岩浆底辟活动的影响,整体在缓坡带发育差异明显的多种层序样式,缓坡带北段发育先存潜山型挠曲坡折带(坡度约3.8°~6.0°),控制发育厚层S型地震前积的低位扇体,是研究区有利的勘探目标;断陷衰弱期,断裂活动近乎停止,大部分同沉积断层停止活动并转化为地形挠曲坡折带,辫状河三角洲推进距离远,整体为一套富砂沉积,可见红色泥岩夹层.

Based on drilling,logging and 3Dseismic data,the sequence framework,slope break type and sequence architecture in the eastern Lufeng depression,Pearl River Mouth basin were discussed.The Eocene Wenchang formation can be divided into five 3-order sequences,representing a complete syn-rift evolution composed of initial rift,climax rift,weak rift and late rift stage.The sequence architecture of the eastern Lufeng depression can be classified into six types,namely,gentle slope break type,upwarping slope break type,pre-buried hill flexureslope break type,depositional topography flexure slope break type,double-stage fault terrace slope type,and multi-stage fault terrace slope type.The corresponding sequence architecture models were also established.Results show that at the initial rift stage,the faults are less active.The geomorphic features of eastern Lufeng depression are generally characterized by gentle slopes,with a slope gradient between 0.8°to 2.0°.During the rift-climax stage,the sequence architectures of double-stage fault terrace slope type,multi-stage fault terrace slope type and upwarping slope break type are developed due to various syn-depositional faulting and magma activities.A number of low-stand fans and slump gravity sandbodies are deposited in a deep lake environment.These fans deposits show typical mounded configurations on seismic sections and defined by fine-grained sandstones interbedded with thick deep lacustrine mudstones from boreholes data.Affected by the decreasing faulting and pre-magma diaprism in the weak rift stage,various sequence architectures can be observed in the study area.Distributed along the northern margin of the eastern Lufeng depression,the pre-buried hill flexure slope break type is developed and the slope gradient ranges from 3.8°to 6.0°.The thick S-shaped progradational low-stand fan deposits controlled by the pre-buried hill flexure slope break type are formed,which are favorable targets of petroleum exploration in future.During the late rift stage,the faulting becomes nearly stopped and most of the syn-sedimentary faults translate into the flexure slope break belts.Extended in a relatively long distance,lager-scaled braided delta deposits are generally abundant of thick bedded sandstones interbedded with some red mudstones.