浊流和碎屑流等粗粒沉积是深海油气和天然气水合物良好的储层,它的发现和识别对于深海油气和天然气水合物的勘探具有重要意义。在南海北部陆坡神狐海域水深1805 m海底采集的长5.5 m柱状沉积物由2层富有孔虫层和2层深灰色泥质粉砂层相间组成,其中深灰色泥质粉砂层(A_1和A3_)主要为黏土矿物、石英和长石组成,粒径中值在6~15μm之间,Ti/Al、K/Al和Fe/Al比值类似于南海神狐海域表层半深海沉积特征,而富有孔虫砂层(A_2和A_4)的粒度具有向上递减的特征,形成由粗到细的正粒序构造,粒径中值高,Ti/Al、K/Al和Fe/Al比值较高,与A_1和A_3灰色泥质粉砂层相比,含有更多的粗粒陆源物,推测可能是浊流经底流改造的碎屑流沉积。浮游有孔虫^(14)C定年表明下部的A_4层可能晚于42.46~43.53 ka B P,A_2层晚于15.1~15.3ka B P。浊流从浅海搬运来的大量有孔虫在陆坡经碎屑流改造形成富集Ca CO_3的有孔虫富集层。
Prof.Feng Dong’s laboratory at the South China Sea Institute of Oceanology,CAS,reported time integrated variation of sources of fluids and seepage dynamics archived in authigenic carbonate from cold seeps.The results were published in Chemical Geology(2014,385:129—139).A careful examination of the authigenic carbonates from the Gulf of Mexico Gas Hydrate Seafloor Ob-
Temperature and pressure on seafloor of the northern slope in the South China Sea are suitable for gas hydrate formation, but bottom simulation reflector (BSR), an indication of gas hydrate occurrence, only occurred in limited areas of the slope. Drillings in the BSR-distributed area (the District S) on the northern slope of the South China Sea suggested that gas hydrate only occurred at Sites SH2, SH3, and SH7 with high saturation (up to 20%-40%), and there is no hydrate at Sites SH1 and SH5 although the distance between SH1 to SH3 is only 500m. In this paper, we investigated seafloor gradient, fault development, temperature, and pressure in the District S on the northern slope of the South China Sea to understand the possible factors con- trolling BSR distribution and gas hydrate occurrence. The District S is a structurally fractured continental slope zone and its seafloor gradient varied greatly. The BSR-occurred areas have an average gradient of 19.89×10 2 whereas the BSR-free zone has the average gradient of 10.57×10 2 . The calculated relative structural intensities from fault densities and displacements show that the BSR-distributed areas tend to occur in the areas with a moderately high structural intensity, where faults frequently developed close to the seafloor that are possibly favored for lateral migration of gases. On the basis of temperatures and pressures at drilling sites, hydrate-occurred Sites SH2, SH3, and SH7 are located within the thermodynamically stable area for methane hydrate, and hydrate-absent Sites SH1 and SH5 are out of the thermodynamically stable area for methane hydrate formation, suggesting that both BSR and the thermodynamic stability are necessary for hydrate occurrence in the subsurface.
WANG HongBinYANG ShengXiongWU NengYouZHANG GuangXueLIANG JinQiangCHEN DuoFu
