Abstract: Changchang-Heshan sag is a deepwater sedimentary basin and located on the continental slope in the northern South China Sea. It is an important strategic exploration block after oil and gas exploration breakthrough in Baiyun and Liwan sags in adjacent deep-water regions. Guided by the theory of classic sequence stratigraphy and based on the characteristics of the seismic reflection and stratigraphic cycles,the Oligocene Zhuhai Formation could be divided into six third-order sequences(namely ZHSQ1,ZHSQ2,ZHSQ3,ZHSQ4,ZHSQ5 and ZHSQ6 from bottom to top). Six types of seismic facies,including high continuity,weak-medium amplitude sheet seismic facies of shelf,S-shaped and oblique progradational seismic facies of shelf-edge,variable amplitude,medium continuity,mound-shaped bidirectional overlap seismic facies of continental slope and so on,are identified within the Zhuhai Formation. S-shaped and oblique progradational seismic facies represents shelf-edge delta,while mound-shaped bidirectional overlap seismic facies represents deepwater fan. The shelf-edge delta and deepwater fan systems have been changed with the formation condition of tectonic and sea level variation. The results show that during the deposition of ZHSQ1 and ZHSQ2,the boundary faults of the sag were active and the shelf slope-break belt was controlled by these faults. The depositional system was limited by the assemblage of shallow marine and proximal deltas. The basin stepped into depression stage and the boundary faults were inactive during the depositional period from ZHSQ3 to ZHSQ6;meanwhile,the shelf slope-break belt was characterized by the sedimentary slope break belts,migrated forward to the central sea direction and became larger all the time under the control of relative sea level falling with stable material source supply from the ancient Pearl River. At the same time,the shelf-edge delta prograded with distance nearly 20,km,and the slope got increasingly steep. Therefore,the occurrence frequency and scale of deep-water fans in front of the shelf-edge delta increased from early to late,and the position became closer to the shelf-edge delta.

Key words: shelf-edge delta, deep-water fan, seismic facies, Zhuhai Formation, Changchang-Heshan sag, northern South China Sea

摘要： 位于现今大陆斜坡之上的长昌—鹤山凹陷是继中国南海北部白云和荔湾深水区油气勘探突破之后的又一个深水战略性勘探区块。根据地震反射上超、下超和顶超等典型反射终止关系、地震相组合以及旋回特征,将渐新统珠海组自下向上划分为ZHSQ1—ZHSQ6共6套三级层序,并在层序格架中,识别出陆架弱—中振幅高连续席状、陆架斜坡变振幅中连续S型—斜交前积楔形、盆地斜坡变振幅中连续丘形双向上超等6种地震相类型,其中大型斜交S型前积和陆坡丘形双向超覆地震反射分别代表陆架边缘三角洲和深水扇沉积。研究表明,随着构造活动、海平面等地质条件的变化,陆架边缘三角洲及深水扇发育特征发生了相应变化。在珠海组ZHSQ1和ZHSQ2沉积时期,断层继承性活动,陆架坡折类型为断控型,沉积体系以局限浅海和近源的三角洲沉积组合为特征。在珠海组ZHSQ3—ZHSQ6沉积时期,盆地进入拗陷阶段,陆架坡折类型为沉积型坡折;在古珠江稳定充裕的供源作用下,陆架坡折带不断向海迁移且范围逐渐变大;伴随着相对海平面下降,大套的前积体不断向前推进,最大前积距离近20,km,且前积角度不断增大;同期深水扇自下而上出现频率增高,且位置更靠近陆架边缘三角洲,规模变大。

关键词: 陆架边缘三角洲, 深水扇, 地震相, 珠海组, 长昌—, 鹤山凹陷, 南海北部

CLC Number: 

• P539.2