Seismic Characteristics of Large-Scale Sand Injectites in Baiyun Sag, Pear River Mouth Basin, South China Sea

Abstract

Three-dimensional seismic data suggest the presence of six large-scale mounded structures at approximately 250m above the 18.5Ma sequence boundary surface of Neogene Baiyun sag, Pearl River Mouth Basin, South China Sea. These structures are most distinctive for their asymmetric mounded shape in cross section and irregular oval shape in plan view. They have bases raging from 826 to 3542m in diameters, with the maximum height of 403m and the largest volume of approximately 0.53km³. The mounded structures have similar seismic reflection attributes as the sand injectites discovered at Nowegian North Sea and other places around the world. The top and base of these mounds mainly respond to continuous, medium- to high-amplitude, positive and negative reflection events; the cores of mounds are dominated by incoherent and chaotic reflections that are greatly different from the parallel sheet-like reflections of the surrounding strata. The mounded structures are interpreted as sand injectites that were generated by soft sediments deformation within Neogene Baiyun sag. The mounded structures can be classified as five types of sand injectite by their seismic reflection, which are: 1) wing-like;2) conical;3) mounded;4) crestal, and 5) sheet-like. Wing-like injectites extend and tilt like wings towards outside along the edge of parent sandstones bodies. Conical injectites are characterized by cone-shape sand bodies opening upwardly, and showing V-shape amplitude anomalies on the seismic profile. Mounded injectites are those dome structures formed by liquefied sand injecting upward, and seen as irregular ellipse in plan view. Crestal injectites consist of complicated networks of intrusion formed by unconsolidated sand interfingering. Sheet-like injectites are concordant intrusion sill, with height ranging from tens to hundreds of meters. Wing-like and crestal sand injectites are distributed as isolated structures. Conical and mounded sand injectites are characterized by linearly distributing large structure groups, which extend along a preferable direction. Subsurface sand injectites are potential reservoirs with high porosity and permeability, and are usually confined by low-permeability mudstone and shale, making them good lithologic trap with advantageous reservoir-seal combination conditions. Many commercial reservoirs of sandstone intrusions have been discovered, indicating that sandstone intrusions have favorable prospects in hydrocarbon exploration.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016