东沙隆起台缘带灰岩下构造畸变识别及成因分析

Identification and cause analysis of low relief structures distorted by overlying carbonate platform of Dongsha massif

东沙隆起惠州台缘带具有优越的成藏条件,其储盖组合集中发育在台地区特有的一套厚层灰岩之下.该套灰岩高速且非均质性强,横向岩相变化剧烈,导致研究区真实构造难以准确恢复.本次研究结合区域井、震资料,分析构造畸变原因,以高精度叠前深度偏移速度场为数据基础,提出基于地质模式校正速度场的方法,运用变速层剥法恢复灰岩下伏构造,并通过累积误差分析恢复后构造形态合理、幅度可靠,最后通过地质成因分析构造形成的主控因素为深埋造成的压实差异.本方法在少井区实际钻井中得到证实,实钻误差远小于海上勘探标准,为同类型灰岩下伏构造恢复和评价提供了较好参考和依据.

The carbonate platform-margin belt of Dongsha massif on the east of Huizhou depression has superior hydrocarbon accumulation condition,one of the best reservoir-caprock assemblage has developed below a thick limestone strata,which is of significance in carbonate platform area. The overlying limestone strata is characterized by high compressional velocity and strong heterogeneity,because horizontal lithofacies vary enormously. Due to the overlying unusual geobody,low relief structures of clastic sedimentary rocks on the carbonate platform-margin belt of Dongsha massif have been difficult for confirmation. Our study proposes a structure mapping method by variable-velocity characterization of limestone strata and above sedimentary rocks,which effectively solves the difficulty of restoration accurately underlying structures of clastic sedimentary rock. Firstly,regional well-seismic data are utilized to analyze the reasons of structure deformation in time domain,leading to an appropriate geological model. Secondly,a high-resolution velocity cube of prestack depth migration in the study area is provided,it is corrected with our geological model,which used to time-depth conversion. Thirdly,our depth structure maps are evaluated by confidence analysis. Finally,distribution and genetic mechanism are characterized. This proposed method was confirmed in the drilling of a less well area later. The depth error of target strata is smaller than 3 in a thousand of buried depth,following the technique specifications for marine exploration. It is proved that our varying-velocity mapping method has a certain value for the similar structures below limestone strata,thus guiding structure interpretation and estimation of reserves.