Geological Interpretation and Hydrocarbon Exploration Potential of Three Types of Mound‐shaped Reflectors in the Meishan Formation, Southern Qiongdongnan Basin

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Abstract
Many mound‐shaped reflectors with different features and shapes are evident in the Upper Member of the Meishan Formation from seismic profiles taken in the deep‐water area of the Southern Qiongdongnan Basin. Based on the drilling, 2D and 3D seismic data from the study area, descriptions of the seismic reflection characteristics as well as the geometric shape, wave impedance inversion, analogy and comparative analyses are carried out. Taken in conjunction with research on the paleostructure and paleosedimentary background, we consider that the mound‐shaped seismic reflectors are distributed in the Southern Slop belt and the Southern High‐rise of the Qiongdongnan Basin, which can be subdivided into three types: reefs, contourite mounds and magmatic diapirs. The first type, reefs, includes patch reefs, platform marginal reefs and pinnacle reefs. Patch reefs present mound‐shaped seismic facies with medium frequency and a moderately strong amplitude, being distributed at the uplift of the fault control platform on the Southern Slop belt. The platform marginal reefs have flat mound‐shaped seismic facies with strong amplitude and medium frequency, developing at the margin of the carbonate platform in the Southern High‐rise. The pinnacle reefs have mound‐shaped seismic facies with strong amplitude and medium frequency and are developed on an isolated volcanic cone. The boundaries between individual reefs are clear on the seismic section, with reef ridge and reef ditch developed, the phenomenon of ‘front product’ being visible within, two‐way superelevation between wings is developed and they exist visibly as mounds in any viewed direction of the cross‐section. They are slightly asymmetrical in the direction perpendicular to the paleodepth, the reef body being steep near the deep‐water side, while being gentle near the shallow water side. The wave impedance of a patch reef is about 7 kg/m3×m/s, while the wave impedance of a platform marginal reef is about 7.5 kg/m3×m/s. The second type ‐ contourite mounds ‐ are mainly developed under the slope break of the southern slope fault control platform's edge. They are subdivided into two types: conical and flat. The former has mound‐shaped seismic facies with medium‐strong amplitude and low frequency, the latter having mound‐shaped seismic facies with medium amplitude and low frequency. The internal texture of the mounds is not clear on the seismic section, with the boundaries between contourite mounds being blurred. They are mound‐shaped only in cross‐section, being banded in the extending direction. The upper surface of a single contourite mound is relatively gentle near the deep‐water side, while being steep near the shallow water area. The wave impedance of contourite mounds is about 5.8 kg/m3×m/s, which is speculated to represent a marly to calcareous clastic deposit. The third type is the magmatic diapir, which has ‘roots’. They have a dome‐shaped upper boundary, are bottomless, with a chaotic interior. They penetrated multiple formations, opening towards the base. There are two major accumulation assemblages of reefs, one is the platform margin reef accumulation assemblage with distant source rocks and long‐distance migration through an unconformity surface in the Southern High‐rise, the other is the patch reef accumulation assemblage with twin sources and short distance migration through faults in the Southern Slop belt near the Central Depression zone. The latter is the main exploration targets at present. There are contourite mound accumulation assemblages with nearby source rocks and short distance vertical migration through faults, which are potentially important targets. The magmatic diapirs pierce the overlying strata and form good hydrocarbon traps and migration pathways, thus representing potential prospecting targets.

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