南海北部神狐海域甲烷水合物BHSZ与BSR的比较研究

The disagreement between BSRs and the base of methane hydrate stability zones in the Shenhu Area north of the South China Sea

天然气水合物（主要是甲烷水合物）因其重要的资源、环境意义越来越受人们关注.其在海底沉积物中的稳定存在及分布受温度、压力、甲烷供应量等因素的控制.勘探工作中,经常把似海底反射层（BSR）对应于甲烷水合物的稳定带底界（BHSZ）.通过对南海北部地区甲烷水合物BHSZ与BSR的对比研究,我们发现在南海北部部分地区二者并不一致,二者之间的误差较大且呈一定的规律性分布.在神狐地区北部,水深较浅、沉积速率较快,BHSZ与BSR的误差为负,绝对值达192%;而在水深较深、基底为隆起的、沉积速率相对慢的神狐东南部,BHSZ与BSR的误差逐渐过渡到为正值,误差约为45%.我们综合分析了由速度-深度关系、BSR深度处反射时间、海底温度、平均热导率、静水/静岩压力模型、水合物稳定P-T方程等参数、流体活动性等计算参数可导致的的误差范围,最后认为导致BHSZ与BSR之间误差的主要因素可能是对BSR的理论认识上,在南海北部地区地震反射识别的部分BSR对应的可能是游离气带顶界（TFGZ）或古BSR或仅仅是由近水平地层或不整合面封存的含气层,而非传统意义上对应于BHSZ的BSR.而造成BHSZ与BSR规律性分布的基础地质因素则可能为在张裂基底上不同构造部位发育的不同的沉降、沉积过程及其热响应,进而造成不同的甲烷生成量、聚集量以及不同的水合物系统相对沉积物的迁移速率,最后产生不同深度的游离气顶界或不同深度的残留异常“古BSR”或含气层.

Through comparing the BSR（bottom simulating reflector） depth with the base of methane hydrate stability zone（BHSZ）,or the BSR induced heat flow with the probe-measured heat flow in the northern South China Sea,we found there are some regional and regular differences between them for the most parts of area,although at some places they have good agreement relationship.In the north of the Shenhu area,where the water depth is shallow and the sedimentation rate is fast,it appears great negative difference（as large as-192%）.However,to the southeast of the Shenhu area,where the water depth is deeper,sedimentation rate is relatively slow and an uplift basement topograph exists,it changes to positive difference（as large as ＋45%） gradually.The differences change so great,that haven＇t be observed in the other places of the world.After analyzing the errors from the process of heat flow measurement,the BSR depth,the relationship of thermal conductivity with the sediments depth,and the fluid flow activities,we concluded that the difference should be not caused by these errors.Such big disagreement may be due to the misunderstanding of BSR.Some of the ＇BSR＇ could represent the paleo-BSR or the top of free gas zone.Thus,we call these differences as theoretic errors.And there may be a self-consistent process： fast but different tectonic subsidence and sedimentation rate causes different thermal response,as well as methane gas producing and accumulation.Further more,it induces the different velocity of BSR upward migration and consequently produces the anomalous ＇BSRs＇.