Gravity–Magnetic–Seismic Identification Method for Basement Lithology in the XS Exploration Area, South China Sea

The basement lithology in the northern South China Sea remains poorly constrained because of the lack of drilling data, which creates major challenges for predicting bedrock buried-hill reservoirs. This study is based on 2D and 3D seismic data, together with gravity and magnetic data, and is constrained by previous studies on the regional tectonic framework and lithologic distribution of the northern South China Sea. We investigated the basement lithologic composition of the XS area, established an integrated gravity–magnetic–seismic interpretation framework for basement lithology, and discussed the possible geological controls on basement lithologic distribution. The results show the following: (1) The basement in the study area is mainly composed of granite intrusive bodies, metamorphic rocks, and volcanic rocks. Granite bodies are characterized by irregular to intrusive seismic geometries, strong double-peak reflections at the upper boundary, and weak-amplitude, medium- to low-frequency internal reflections, reflecting their intrusive characteristics. Metamorphic rocks show diverse external seismic geometries, single-peak or weak reflections at the upper boundary, and medium- to strong-amplitude parallel internal reflections that intersect the upper boundary at high angles. Their gravity and magnetic responses show considerable overlap with those of granitic rocks. Volcanic rocks commonly show mound-shaped or lenticular seismic geometries and medium- to strong-amplitude, moderately continuous internal reflections. Intermediate–mafic volcanic rocks are associated with relatively stronger gravity and magnetic anomaly responses, whereas felsic volcanic rocks generally exhibit weak to moderate gravity and magnetic responses. These characteristics provide an integrated seismic–gravity–magnetic constraint framework for basement lithology interpretation in the South China Sea. (2) Integrated gravity–magnetic–seismic interpretation indicates that, near the present top of the pre-Cenozoic basement in the XS area, granite intrusive bodies are the dominant lithology. Metamorphic rocks mainly occur as discontinuous basement remnants in local structural lows or erosional windows, whereas volcanic rocks are mainly distributed near fault intersections or occur as bead-like bodies along fault zones. (3) The distribution of granite intrusive bodies and metamorphic basement is interpreted to be related to Indosinian–Yanshanian magmatic activity, tectonic deformation, and subsequent denudation. Faults formed during Early Yanshanian compression were later reactivated during the Himalayan period and may have acted as magma conduits, promoting the development of volcanic edifices along fault zones.