A Discussion On Origin Of Seismic Mound Reflections In Miocene,Liwan Sag, Pearl River Mouth Basin

Since the large-scale reef oil-gas pools were found in Miocene in the Dongsha massif and Shenhu massif, Pearl River Mouth Basin, northern South China Sea, researchers have done a lot of work to the recognition of reef reservoirs in Pearl River Mouth Basin. Seismic mound reflection is one of the main identifying characteristics of reefs, therefore, the areas of seismic mound reflections are often regarded as the key regions of reefs. There are a clear set of seismic mound reflections in Zhujiang formation in Lower Miocene, Liwan sag, southern Pearl River Mouth Basin, for its origin and relationship with reefs, there are still unknown. This paper focuses on the geometric features, geophysical parameter characteristics and the Paleotectonic and Paleogeography of the seismic mound reflections and tries to reveal the origin and relationship between seismic mound reflections and reefs, and promote the further study on the idengtification of reef researviors and the characteristics of marine sedimentary in this area.Based on previous work and the available data to the pearl river mouth basin, through the analysis of seismic reflection characteristics, forward model, seismic inversion and seismic attribute, we learned the geometric features, the formation background and the differences between the seismic mound reflections and common mounds, furthermore, we investigated the type, the forming mechanism and the sediment forms of the bottom currents, and obtained the following conclusions:(1) Seismic mound reflections in Liwan sag are formed in Early Miocene, approximately corresponded to the geological time between23.8Ma and21Ma, and located between seismic horizon T6and Tmt, its thickness is about between60m and140m and the width is about between500m and2000m and the density is from1to3per2km2.covering an area of2400km2. The shape of single mound is irregular ellipse or oval, with the overall distribution of sheets and locally of moniliform, near arranged in NE-SW, it changed drastically in northwest and tardily in southeast. On the section, there is a trend of migrating to northwest high structural parts, and with the form changes as smaller lens, bigger lens and box from southeast to northwest.(2) The amplitude of the top of the seismic mound reflections is strong, the inner is weak to blank, and the bottom is weak. There are drape structures on the top of them and parallel reflections on the bottom of them. Seismic inversion shows that the maximum value of wave impedance is6.7e+06kg/m3*m/s, far less than the wave impedance value of reef limestone. Compared with the same interface amplitude value of mudstone/limestone, mudstone/sandstone and mudstone/gas-bearing sandstone, its amplitude value is also obviously low. Forward model analysis shows that the morphological characteristics of seismic mound reflections are similar to the unbalanced clamp of the overlap thick mudstone with thin sandstone.(3) Analysis of Paleotectonic, Paleogeography and sedimentary facies shows that the seismic mound reflections distributed in secondary depression to slope area, the sedimentary setting was deep shelf to bathyal environment.(4) There are not clear volcano and diapir pierce paths on the bottom of the seismic mound reflections, therefore, they are not volcanic domes and mud diapirs, and there are differences with reefs in geophysical parameters characteristics and background. And they are also incompatible with carbonate-mud-mounds in high-speed characteristics.(5) Seismic mound reflections and the overlying strata have the similarity with sediment waves caused by contour currents and internal-wave in both morphology and triggering mechanism, so they may be the sediment waves under the interaction of contour current and internal-wave.