快速沉积与侵蚀对地温场及天然气水合物成藏演化的影响：以南海北部陆坡为例 Impacts of fast sedimentation and erosion on the evolution of geothermal field and gas hydrate system: cases of Northern South China Sea

分类： 地球科学 >> 空间物理学 分类： 地球科学 >> 海洋科学 提交时间： 2017-03-23

张毅, 何丽娟, 汪集旸

摘要： 海洋沉积物中天然气水合物的存在与分布与地温条件紧密相关。南海北部陆坡大部分区域更、全新世沉积速率较高，同时局部地区(如神狐天然气水合物钻探区)发育有大量的海底冲蚀谷。这些快速沉积或侵蚀作用可以对地温场及天然气水合物的形成与赋存产生重要影响，然而目前注意到这些问题的研究相对较少。本文中我们结合南海北部陆坡的实际情况，建立了针对地温场发生动态变化时的水合物成藏数值模型。我们给出两个成藏模拟案例，其一是快速沉积的情形，其二是地形遭受侵蚀、产生冲蚀谷时的情形。结果表明，在这两种情形下地温场将发生不同的瞬态调整，并对天然气水合物在地层中聚集及分布产状产生重要影响。在快速沉积发生时，地温梯度将变小，天然气水合物稳定带（HSZ）将增厚；然而当其底界将随着热传递而上升，天然气水合物将不断在原稳定带底界分解，而后在新的稳定带底界处重新生成水合物。这样长时间的快速沉积作用可以造成水合物主要富集在稳定带底界处。而当快速侵蚀作用发生时，地温梯度将变大，天然气水合物稳定带的厚度将变薄，原来位于冲蚀谷处的水合物藏将被侵蚀作用破坏。而后，随着热平衡的发展，逐渐形成新的、类似于似海底反射层的稳定带底界。单纯考虑甲烷扩散作用下，高丰度水合物的形成需要较长的时间，相对而言脊下的水合物成藏较好，形成矿点的概率较大。 The occurrence and distribution of gas hydrate in marine sediments is closely related to the state of geothermal field. The sedimentation rate is relatively high during the Pleistocene and Holocene in the northern margin of the South China Sea. Moreover, there are many erosional canyons at the sea bottom in some local areas. Such fast sedimentation and erosion can greatly affect the geothermal regime and the formation and occurrence of gas hydrate deposits. However, there are few studies that consider such scenarios. In this study, considering the geological reality, we build a numerical reservoir model aimed to simulate the evaluation of the gas hydrate system upon two cases: the fast sedimentation and the development of erosional canyons. The results indicate the geothermal filed and the accumulation and distribution of gas hydrate can be adjusted dynamically when the fast sedimentation or erosion occurs. Under fast sedimentation, the geothermal gradient becomes smaller and the thickness of gas hydrate stability zone (HSZ) becomes larger. However, due to the role of heat transfer, the bottom of HSZ rises upward. This process leads to the hydrate near the previous bottom of HSZ being dissociated and depositing near the bottom of new HSZ. After a long duration, the fast sedimentation will result the formation of hydrate being mainly enriched near the bottom of HSZ. By contrast, when fast erosion occurs, the geothermal gradient increases and the HSZ turn thinner. The gas hydrate deposit system can be partially destroyed by the formation of canyons. A long time is necessary for the recovery of a gas hydrate deposit with significant abundance. Relatively, the deposits under the residual ridges are more possible to be potential “sweet spots”.

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