台湾中西部沉积盆地三维密度结构及其重力异常

3-D DENSITY STRUCTURE AND GRAVITY ANOMALY OFTHE WEST-CENTRAL SEDIMENTARY BASIN OF TAIWAN

台湾中西部沉积盆地是台湾省海陆油气勘探的重点区域。本文利用石油钻井资料和重力观测数据,依据Kriging插值及台湾中国石油公司深度 密度经验公式,计算了该地区沉积地层三维密度及剩余密度结构,并依据三维有限元解泊松方程方法,通过求解重力位计算出三维剩余密度体产生的剩余重力异常。密度结构显示,在0～6km深度范围,自更新统到先中新统的沉积地层大致分为2 0～2 30g cm3、2 30～2 45g cm3、2 45～2 60g cm3、2 60～2 70g cm3等4个密度层。其中,台中盆地陆缘区中下部2～4km深度存在较大沉积凹陷,凹陷区密度比周围低0 05～0 10g cm3。重力计算结果表明,本区沉积地层产生的剩余重力异常在-20～+15mGal之间,深部凹陷对应负异常低值圈闭,其形态和量值与实测重力异常一致。

The West-central sedimentary basin is one of the important areas for oil-gas exploration in Taiwan.Based on the data of oil wells and gravity surveys,the Kriging insertion equations and the depth-density empirical formula,the authors calculated the 3-D density structure and residual density structure of the sedimentary strata of the study area.Moreover,according to the Poisson equation and the FEM,the authors also computed the residual gravity anomaly caused by the 3-D residual density structure.The 3-D density structure shows that the Pleistocene to pre-Miocene sedimentary strata are broadly divided into four density layers at 0 to 6 km depths,i.e.the 2.0-2.30 g/cm^3,2.30-2.45 g/cm^3,2.45-2.60 g/cm^3 and 2.60-2.70 g/cm^3 layers.There is a relatively large sedimentary depression at 2-4 m km depths in the middle and lower parts of the continental margins of the Taichung basin.The density there is 0.05-0.10 g/cm^3 lower than that in the surroundings.The gravity calculation indicates that the residual gravity anomaly produced in the sedimentary strata in the area ranges from (-20) to (+15) mGal,and that the deep depression corresponds with a trap with a low negative anomaly,whose shape and value are consistent with those and the measured gravity anomaly.