琼东南盆地北礁凹陷梅山组顶部丘形反射特征及成因分析

南海琼东南盆地北礁凹陷中中新统梅山组顶部丘形反射目前引起广泛关注，前人推测为生物礁、重力蠕动与底流叠加成因、等深积丘等。本文通过钻井资料、二维、三维地震资料精细刻画丘形反射（残丘）和丘间水道特征及其成因。残丘及水道在北礁凸起不发育，在边缘斜坡中部和高地较发育，且有向高地两边规模减小趋势，不具对称性，残丘和水道呈平行-亚平行近E-W向展布，局部有合并分叉现象，与北礁凸起走向呈一小角度；丘宽562~1 223 m，丘高29~87 m，丘长10 km左右，存在丘翼削蚀，水道底蚀现象。地震属性分析表明三维工区西南部残丘间水道由砂泥岩互层充填，形成长条形强振幅，而残丘为中-低振幅；地震、钻井资料分析表明丘形反射（残丘）由钙质泥岩和泥岩组成，属于半深海沉积，且残丘内部波阻抗为5.0×106~6.5×106kg/m3·m/s，低于火山岩、灰岩波阻抗，属于砂泥岩地层范畴；根据梅山组下段水道由西向东强振幅变弱、分叉、前积反射和海山附近底流（等深流）沉积剥蚀特征综合判定底流古流向自西向东，根据海山两翼地震反射特征推测底流可追溯至晚中新世早期（11.6 Ma BP），综合分析认为，研究区中中新统梅山组丘形反射是晚中新世早期底流切割梅山组地层形成的残丘。

The characteristics and genesis analysis of the mound at the top of Meishan Formation in the Beijiao Sag of the Qiongdongnan Basin

The mounded reflections of the mid-Miocene upper Meishan Formation in the Beijiao Sag of Qiongdongnan Basin have been arousing widely the attention of the geologists. Previous studies inferred that the mounded reflection originated from various genesises such as reef, combined result of gravity flow and bottom current, and contourite. Based on drilling, 2D and 3D seismic data, this paper researched on the mounds(remnant mounds) and channels in detail. In the Beijiao Uplift the remnant mounds and channels were not developed well, while in the mid part and high of the marginal slope they occurred well. The scale of them became smaller on the two blank of the highland and they are asymmetric. Mounds and channels with nearly E-W orientation are liner and sub-parallel with each others and locally conjunctive and bifurcate. The strikes of those have a small angle intersection with the strike of Beijiao Uplift. The mounds display mounded widths, heights, and length of 562-1 233 m, 29-87 m, and about 10 m, respectively. There are some seismic reflections with the truncation of the blank of the mounds and the incision of the channel. The 3D seismic attributes showed that long strip strong amplitude infilled by the interbedding of sandstone and mudstone mainly occurred in the S-W part of 3D survey, however, the interior remnant mounds with mid-lower amplitude. Integrated drilling and seismic data, the results of this research showed that mounds were composed of mudstone and calcareous mudstone, which belonged to the bathyal depositional environment. The wave impedance (5.0×10⁶-6.5×10⁶ kg/m³·m/s) of the mounds were lower than that of carbonate rock and igneous rock, which also belonged the scope of the wave impedance of mudstone and sandstone. According the strong amplitude of the lower Meishan Formation channel became weaker, bifurcate, progradation from W to E and the contourite depositional-erosional characteristics in the vicinity of seamount, we inferred that paleo-current of the remnant mound and channel of formation flew from W to E. And the epoch of contourite deposition and erosion could ascend to the early later Miocene(11.6 Ma BP). Integrated analysis of various mounded reflections, we inferred that in study area mid-Miocene mounded reflections originated from the remnant underlying Meishan Formation incised by bottom current in the early later-Miocene.