前陆冲断带下盘的掩伏构造假象分析与校正——以准噶尔盆地东部“火北掩伏带”为例

Analysis and calibration of false buried structures at the footwall of foreland thrust belt:A case study on "Huobei buried structure" in the eastern Junggar Basin

造山带和盆地之间的前陆冲断带及其下盘掩伏带是油气勘探的重要领域,有关其时间域构造假象及时间域剖面复杂构造的研究多集中在高速盐丘、海底峡谷等特殊地貌或地层厚度剧变对下伏构造层成像"上拉、下拽"形成的构造假象方面,而对于火山岩体厚度变化及尖灭对下盘构造成像的影响以及"假断裂、假构造"判识方法的研究则较少。为此,以准噶尔盆地东北部火烧山油田以北石炭系克拉美丽前陆冲断带下盘的掩伏背斜带(以下简称火北掩伏带)为例,通过对火北2井钻探前后验证过程的描述,基于对正演模型、叠前深度偏移剖面及倾角测井等资料的综合分析,探究了该冲断带下盘时间域"假背斜、假断裂"形成的原因。结果表明:(1)地层速度的横向变化可导致速度异常区下伏地层构造形态畸变,出现假构造和构造高点偏离的现象;(2)火北掩伏构造和火14井北断裂是因逆冲断层上盘石炭系火山岩高速层存在且横向厚度突变导致的构造假象;(3)叠前深度偏移技术是解决时间域剖面上构造假象难题的最佳手段,在不具备进行叠前深度偏移条件时可通过建立正确的三维速度场、变速时深转换来实现成图。结论认为:(1)通过建立复杂构造区的典型构造样式及地震构造解释模型,运用模型正演,结合地层倾角测井等成果,可以识别逆掩推覆断裂复杂构造带的真假;(2)探讨火山岩发育的逆掩推覆断裂带的地震解释技术,对于指导研究区勘探目标优选具有实践意义,对类似地区的地震资料解释也具有借鉴作用。

The foreland thrust belts between orogenic zones and basins and the buried structures at their footwalls are important oil gas exploration fields.The studies on structural illusions and complex structures in time domain usually focus on the structural illusions that the images of the underlying structures are pulled up or down by the violent change of stratum thickness or special landforms（e.g.highspeed salt dome and submarine canyons）.However,the effects of the thickness change and pinchout of volcanic rock matrix on the imaging of footwall structures and the ＂false fault and false structure＂ identification methods are rarely researched.In this paper,the buried anticline at the footwall of Carboniferous Kelameili foreland thrust belt to the north of Huoshaoshan Oilfield in the northeastern Junggar Basin（hereinafter ＂Huobei buried structure＂ for short） was taken as an example.Firstly,the verification process before and after the drilling of Well HB2 was described.Then,based on the comprehensive analysis of forward modeling,pre-stack depth migration section and diplog,the cause for ＂false anticline and false fault＂ at the footwall of Kelameili foreland thrust belt in time domain was discussed.And the following research results were obtained.First,the lateral change of formation velocity can lead to distorted structural forms of the underlying strata in the velocity anomaly zones,and consequently false structure and deviated structural high emerge.Second,Huobei buried structure and Well H14 north fault are the structural illusions which are resulted from the high-velocity layer of Carboniferous volcanics at the hanging wall of thrust fault and the abrupt change of lateral thickness.Third,the pre-stack depth migration technology（PSDM） is the best tool for solving the problem of structure illusion in time-domain section.If PSDM cannot work,the structural map can be obtained by establishing a correct 3D velocity field and conducting variable velocity time–depth conversion.In conclusion,whether the complex structural belt of overthrust nappe fault is true or not can be discriminated by establishing the typical structural pattern and seismic structure interpretation model of complex structure area and applying forward modeling,combined with diplog results.In addition,the discussion on the seismic interpretation technologies used for overthrust nappe faulting zones with developed volcanics is of practical significance to the selection of exploration targets in the study area,and it can be used as the reference for the seismic data interpretation of similar areas as well.