摘要
本文以北部湾盆地涠西区块为例,针对断陷盆地勘探早期资料不足、认识局限、手段单一等特征,提出了以地震相-沉积相演化为主体,以油气成藏组合为重点,围绕沉积体系发育的动力学机制和油气有机成藏的动力学过程,依托盆地模拟手段,开展重点构造的综合评价。
在三级层序识别与划分的基础上,根据地震波阻的外部几何形态+内部反射特征+界面接触关系+地球物理属性,在工区内确定出5大类13种地震相类型。地震相与沉积相并非一一对应,但各类前积相多表现为三角洲、扇三角洲及水下扇的快速堆积;平行相、充填相与滨浅湖、半深湖的水动力条件相关性较好;杂乱相则多为断层改造作用下连续沉积体扭曲、张断的反映。
就沉积演化而言,古新世时整个北部湾盆地处于初始断陷期,以杂乱反射为代表的近源冲积扇与红层泛滥平原大量发育。始新世是盆地断陷扩张的时期,在湖相背景下广泛发育三角洲、扇三角洲沉积。进入渐新世,整个北部湾盆地处于断陷期的尾声,盆地性质逐渐向坳陷转换,海中凹陷部分构造到渐新世末甚至发生强烈反转。结合各种沉积参数(砂岩厚度、泥岩厚度、含砂率等)建立了古近系流沙港组的沉积模式。最后探讨了盆地中沉积体系形成和发育的动力学机制,认为主要是构造动力、充填动力和气候动力共同作用的结果。
根据油气成藏组合五大要素(源岩、储层、盖层、运移、圈闭),考虑剩余地层压力分布特征与流体势梯度趋势,在研究区内可识别出三大类成藏组合类型:完整成藏组合、运移成藏组合和不完整成藏组合。
充分运用盆地模拟手段和道积分反演技术,针对六个重点构造进行地质风险评价。这些构造形成时期较为集中,主要形成于始新世—渐新世的喜山二幕构造运动,到渐新世末基本定型。其中,海2构造成藏条件相对较好,为Ⅰ类圈闭构造;但此构造高点偏移较大,具有一定的勘探风险;其次是海1、涠9南和涠10-10构造,多为Ⅱ类圈闭构造;封盖能力和油源问题是其制约因素;涠10-5和涠10-4构造成藏组合条件较差,较多III类圈闭构造;部分构造破碎,勘探和开发的地质经济风险很大。
Aiming at scarce information, inadequate acquaintance and simple method of faulted basins, this paper takes the evolvement of seimic facies and depositional facies as primary research contents, choosing the play for petroleum and gas as research emphases. Meanwhile, based on dynamic principles of depositional system and dynamic processes of the accumulation of hydrocarbon and supported by the analysis and simulation for basins, the paper selects six structures as significant evaluating targets: an example from Weixi survey in Beibuwan basin.
Based on establishment of third-order sequences, in term of external geometric shape, internal reflection configuration, contact relationships of interface and characteristic properties of geophysics of such seismic impedance, there are five seimic facies and thirteen seimic sub-facies in the survey. Though seimic facies is not always accordant with depositional facies, Foreset facies usually indicates delta, fan delta and submarine fan, Parallel and Filling facies often involves in lacustrine hydrodynamic condition, Disorder facies reflects the distortion and disconnection of continuous deposits frequently by fault reconstruction.
According to the viepoint of depositional evolvement, the whole basin experienced initial faulted phase in Palaeocene Epoch. Alluvial fans which were near provenance and showed disorder reflection in seismic profiles and flood plains with red layers developed well at that time. During Eocene Epoch, Beibuwan basin began expanding. Delta and fan delta could be found at higher position of the lacustrine area. In Oligocene Epoch, Beibuwan basin came into the evening of faulted phase. The tectonic evolutive phase of this basin transformed from faulted phase to depression phase gradually. Some stuctures in Haizhong depression even reversed at the end of Oligocene Epoch. Combined many depositional parameters(thickness of sandstone, thickness of mudstone, sandstone percent content), the depositional pattern of Liushagang formation of Palaeocene System is established. Eventually, the paper researches the dynamic principle on formation and development of depositional systems, which is co-effected by tectonic power, filling power and climatic power.
In the light of five elements of play for petroleum and gas(source rock, reservoir, caprock, migration, trap), the distrubution of residual stratic pressure and the trend of fluid potenial, three plays can be identified in the survey which are complete play($MRTS), migrating play(MRTS) and incomplete play(RTS, TS).
In the end, the separate geological risk assessments of six structures are evaluated by the analysis and simulation for basins and reservoir inversion. These structures nearly form in the same period, which begin in Eocene and Oligocene Epoch with Xishan Movement of EpisodeⅡand finish in the end of Oligocene Epoch. Among these traps of structures, the trap of H2 belongs to typeⅠwith fine conditions of reservoir better than others. However, that the summit of such structure deviates much hazards the exploration. The second fine structures are H1, W9s and W10-10 respectively. A great number of traps of those structures belong to typeⅡwith restrained factors of source rocks and the capibility on sealing. The traps of W10-5 and W10-4 are not beneficial to reservoirs and many of them belong to typeⅢ. They risk much on exploration and development with structures broken in part.
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