Lithological characteristics and hydrocarbon gas sources of gas hydrate-bearing sediments in the Shenhu area, South China Sea: Implications from the W01B and W02B sites

摘要

The Shenhu area, located in the Pearl River Mouth Basin, South China Sea (SCS), is currently one of the most promising exploration areas for gas hydrates. In this study, the lithological characteristics, as well as the molecular and isotopic composition of hydrocarbon gases are systematically reported for the first time for core sediments obtained from two new drilling sites (referred to as W01B and W02B) in the southeast Shenhu area. Both gas hydrate-bearing and gas hydrate-free samples are characterized by fine-grained sediments (dominantly coarse silt and to a lesser extent, fine and medium silt) and similar median grain-sizes (5.94 Phi to 6.49 Phi), sorting (2.32 to 2.59), kurtosis (0.82 to 0.97), skewness (- 0.12 to 0.03) and mineral compositions. Abundant authi-genic pyrites (including euhedral and framboidal pyrites) also occur in these sediments. All these characteristics collectively indicate that the core sediments from the southeast Shenhu area formed in a relatively stable, low-energy and anoxic sedimentary environment. Nevertheless, the hydrate-bearing layers in this study are characterized by lower sand contents and more foraminifera than gas hydrate-free layers. The molecular and isotopic composition of hydrocarbon gases hosted by secondary minerals in the studied sediments indicates that they are of thermogenic origin. Combined with previously published data, we suggest that the abundant foraminifera within the sediments of the Shenhu area play a significant role in controlling the formation of gas hydrates by increasing the porosities of sediments, and that the thermogenic gas is an important source for the hydrocarbon gases of hydrates in the SCS.