Grain-size characteristics of fine-grained sediments and association with gas hydrate saturation in Shenhu Area, northern South China Sea

Abstract

Fine-grained gas hydrate (GH) reservoirs are extensively studied worldwide, among which the Shenhu Area (located on the northern slope of the South China Sea) is a world-class GH exploration area. However, the lithology, physical properties, and depositional origins of the fine-grained GH reservoirs are not well known. Understanding how sediment grain-size parameters affect the fine-grained GH reservoir quality could provide and important breakthrough for reservoir evaluation. Eight cores, recovered from various expeditions of the Guangzhou Marine Geological Survey, can be combined with 2D/3D seismic data to provide a rare opportunity to systematically investigate the grain-size characteristics of the GH reservoir, as well as the surrounding sediments.A combination of lithology, grain size characteristics (mean size, sorting, skewness, kurtosis), high-resolution seismic features, and associated bivariate and cluster analysis results support the identification of two distinct intervals of fine-grained sediments that were deposited by different sedimentary processes. There is a relatively higher content of coarser silt in the lower interval than in the upper interval, and their boundary depths are highly consistent with those of the GH-bearing layer and the overlying non-GH-bearing layer. With respect to the unconsolidated GH-bearing sediments from Well G, both the porosity (52%–64%) and sorting coefficient (1.68–2.2) have limited variation, while high GH saturation (>30%) occurs at the top layer. The positive correlation between saturation and the coarsest one-percentile grain size (R = 0.55) reveals that an increase in the coarse fraction/particle size favours the development of a larger pore throat diameter and improves the initial permeability and reservoir properties.The seismic features and cross-plots of the coarsest one-percentile and median values indicate that the lower thin-bedded fine-grained sediments with hydrate may be fine-grained turbidite complexes, including channels/levees/lobes and mass transport deposits. If this is the case, then it may be inferred that turbidite sediments provide good reservoir physical properties, favourable for GH formation and accumulation. These insights into the relationship between the Quaternary fine-grained turbidites and GH saturation may promote a clearer understanding of the characteristics and development of fine-grained GH reservoirs globally, including in the Shenhu Area of the South China Sea.