Abstract:

A gasfield with reserves exceeding 100 billion cubic meters has been discovered in the Central Canyon on the southern slope of the Lingshui Sag in Qiongdongnan Basin. However, the northern slope shows poor oil and gas enrichment, with gas detected but no fields found. One of the key reasons is the absence of large-scale high-quality reservoirs encountered during drilling. To clarify the sedimentary evolution model and distribution patterns of high-quality sand bodies on the northern slope of the Lingshui Sag, this study integrated drilling, logging, mud logging, testing, and seismic data, using techniques such as thin section observation, grain size analysis, and physical property testing. Core facies, logging facies, and seismic facies analyses were carried out for the key strata to establish the sedimentary evolution model of Meishan Formation. Combined with reservoir microscopic characteristics and fault-sand matching, the oil-gas geological significance was clarified. The results showed that during the Meishan Formation period, sediment sources were provided by Hainan Island, and a shelf delta-submarine fan sedimentary system was developed. In the study area, the microfacies sand bodies of channels and channel-lobe complexes were relatively coarse and thick, with box-shaped or bell-shaped logging curves, and stratification and bioturbation were observed in the cores. Seismic data showed U-shaped or V-shaped low-frequency continuous parallel reflections, which served as the main exploration targets in the study area. The development of submarine fans and the differentiation of their internal sand bodies were mainly controlled by fluctuations in relative sea level, paleogeomorphic features, and the intensity of sediment supply. During the second member of the Meishan Formation (hereinafter referred to as Meishan 2) period, the relative sea level dropped, the sediment supply was abundant, and the relative accommodation space was relatively small, with A/S ≤ 1 (A representing relative accommodation space and S representing sediment supply). Sediments were transported over long distances to the continental slope, forming multiple phases of submarine fan progradation. Laterally, the development of submarine fans and the differences within their internal sand bodies were controlled by paleogeomorphology and distance from the sediment source, mainly developing in the proximal slope break zones and fault-controlled slope break zones formed by synsedimentary faults. The Meishan 2 reservoirs in the study area had porosity ranging from 8.40% to 26.24%, and permeability ranging from 0.05×10^-3 µm² to 26.49×10^-3 µm², mainly characterized by medium porosity and ultra-low to low permeability. High-quality reservoirs were controlled by late-stage reworking. Contour currents could wash, transport, and redeposit gravity flow sediments formed earlier, significantly improving reservoir physical properties. Under the general background of sand deficiency in the study area, the coupling between faults and sand bodies constrained the degree of oil and gas enrichment. Drilling results showed that oil and gas were highly active near the No.2 fault zone. The sand body enrichment zone of the No.2 fault zone was an important oil and gas target for future exploration.

Key words: Qiongdongnan Basin, Lingshui Sag, Meishan Formation, submarine fan, sedimentary evolution, fault-sand transport