Mesozoic hydrocarbon accumulation model in the Northern South China sea

Abstract

The ChaoShan Depression is the largest Mesozoic depression covering an area of 3.7 x 104 km2 where the relict Mesozoic strata are up to 5000 m thick. It has experienced disruption since the late Mesozoic. In early survey, the oil-gas migration condition and reservoiring mechanism are poorly expounded from the Meso-Cenozoic tectonic superposition due to poor seismic imaging to the deep Mesozoic layers. New seismic surveys using long streamers and quasi three-dimensional layouts has improved obviously the deep images of the Mesozoic formations, enabling analysis of tectono-stratigraphic features and petroleum geology. Correlating to the regional Mesozoic stratigraphic and facies characteristics from the well LF35-1-1 and onshore outcrops, two sets of source layer tested with high organic carbon content are interpreted within the upper Triassic to mid-Jurassic semi-closed gulf sequences which become thicker toward the east side of Dongsha Island. Three sets of potential reservoir beds are also interpreted, which are a basin-floor fan sandstone layer of the Triassic-Jurassic boundary, a limestone layer atop the mid Jurassic, and a sandy layer in the Upper Jurassic. Over the low bulge of the ChaoShan Depression, a big anticline is found bounded by two sets of faults which played as migration passage for hydrocarbon from the deep Mesozoic source to the reservoir layers. Likely, one set of the fault act reverse barrier to block the petroleum escape, thus form the fault-bounded trap, favorable for future exploration.