分形在南海北部陆坡断裂与天然气水合物关系研究中的应用

Application of Fractal to the Relations between Faults and Gas Hydrate on the Northern Continental Slope of the South China Sea

根据分形几何学的原理,利用数盒子法计算了南海北部陆坡断裂的平面分布分维值,并以此探讨了断裂与天然气水合物的关系,结果表明:属被动陆缘的北部陆坡断裂越发育水合物异常越少,而属主动陆缘的台西南则断裂越发育水合物异常就越多。这一结果也反映了:被动陆缘环境的张性断层因缺乏相伴生的褶皱而不利于水合物的发育,并可能作为天然气的泄漏通道而破坏水合物成藏;主动陆缘环境的断裂以挤压逆冲断层为主,并伴生强烈的褶皱,其中断裂是深部烃源向上运移的通道,褶皱则是浅部天然气聚集成藏的圈闭。

Fractal dimensions of spatial fault distribution on the northern continental slope of South China Sea were calculated with box-counting method. Overlay of the dimensions with evidence and proxies of potential gas hydrate shows that the relations of faults and gas hydrate are completely different between the northern slope and the offshore area of southwestern Taiwan. Potential gas hydrate on the northern slope, a passive continental margin, seems to concentrate in the area where faults are thin, while the situation is opposite in the offshore area of southwestern Taiwan which belongs to a part of the active aceretionary wedge. This result indicates that the dominating extensional faults on the passive continental margin, lacking of accompanying folds, mainly act as vents for gas leaking to the seafloor instead of efficient migration pathways for thermogenic gas to form hydrates. In contrast, the imbricate fold-and-thrust structures, commonly dominating on the active continental margin, are of benefit to the formation of gas hydrate, with thrusts acting as gas migration pathways and folds as storages.