浅水湖盆河控三角洲前缘砂体分布特征与沉积模式探讨——以松辽盆地北部永乐地区葡萄花油层为例

Sandbody Distribution and Sedimentary Model in Shallow Lacustrine Fluvial-dominated Delta Front: A case study from Putaohua oil layer of Yongle area in Songliao Basin

在我国古代的松辽盆地和现代的鄱阳湖等大型坳陷湖盆内发现的浅水湖盆河控三角洲的骨架砂体类型、垂向沉积层序及相带分异等方面与正常三角洲存在很大的差异,需要进一步深入研究。利用野外露头、现代沉积和地下岩芯、测井、地震等资料精细解剖及分析表明,浅水湖盆河控三角洲前缘发育大量、密集、窄的水下分流河道砂体,砂体连续且水下延伸较远,直至消失变成薄的水下薄层砂(河口坝或席状砂)。每支水下分流河道构成了由中心向两侧及前方:水下分流河道→薄层砂核部→薄层砂边缘→水下分流间湾的平面微(能量)相序列,形成三角洲前缘"河控带状体"。浅水湖盆河控三角洲前缘不同的相位空间位置具有不同的沉积模式,由岸向湖依次发育高低水位间过渡带"近岸沉积"模式、近岸浅水带"河控带状体"模式、中岸中等水深带"水下分流河道末端河控薄层砂"模式和远岸深水带"浪控席状砂"模式。

In recent years,a special kind of delta was found in ancient Songliao Basin and modern Poyang Lake and other large depression lacustrine in China. It is shallow lacustrine fluvial-dominated delta. It is with great differences between normal deltas,including the skeleton sandbody types,vertical sedimentary sequence and facies differentiation etc. Especially,the characteristics of sandbody in delta front is thin thickness and small scale and diversification,which is compared to quite different with sedimentary model of normal fluvial-dominated delta that it include to large scale and thick debouch bar in delta front. In this context,sandbody distribution and depositional model in shallow lacustrine fluvial-dominated delta front,deserves further study. Through analysis of outcrop,modern sedimentary and cores,logging and seismic data,it is realized that underwater distributary channel sandbody as the framework of shallow lacustrine fluvial-dominated delta front is abundant,closely and narrow. It is continual than original understanding and it extends to the far underwater,until disappearing into thin debouch bar and sheet sand. Debouch bar sandbody is not well preserved and is fluvial-dominated. Debouch bar is located mainly in the underwater distributary channel on both sides and the front. It is consistent with the source direction,and has narrow banded morphology. Characteristics of debouch bar is more similar to thin sand under water. In each of the underwater distributary channel is formed in the center of the sequence of sedimentary microfacies. This sequence is underwater distributary channel→ the core of thin sand→ the edge of thin sand→underwater distributary bay from the center to the edge. The study also found that distribution of sandbody is controlled by the position of sedimentary facies in the front of shallow lacustrine fluvial dominated delta. It can be divided into transition zone between high level and low level,shallow water zone of the nearshore,medium depth zone of the shore and deepwater zone of the offshore. Sedimentary model of the nearshore is formed in the transition zone between high level and low level. Its main sedimentary features show that continuous and narrow fluvial-dominated banded deposition is the main sedimentary,with its restrictive and straight distributary channel sandbody as the center. And small scale discontinuous thin sand underwater is more. Sedimentary model of fluvialdominated banded deposition is formed in the shallow water zone of the nearshore. Its main sedimentary features show that the sequence of sedimentary microfacies is formed with its less restrictive and dendritic underwater distributary channel sandbody as the center. This sequence is underwater distributary channel→ underwater thin sand( residual debouch bar) → underwater distributary bay from the center to the edge. Sedimentary model of terminal distributary channel and fluvial-dominated sheet sand is formed in the medium depth zone of the shore. Its main sedimentary features show that distribution of sheet sand is large area,and is fluvial-dominated. The sequence of sedimentary microfacies is formed with terminal underwater distributary channel sandbody as the center. This sequence is terminal underwater distributary channel→ the core of sheet sand → the edge of sheet sand→ underwater distributary bay from the center to the edge. Sedimentary model of wave-dominated sheet sand is formed in the deepwater zone of the offshore.Its main sedimentary features show that the distribution of single sheet sand is perpendicular to the source direction,and a plurality of sheet sand is arranged parallel to each other. An obvious strip and annulus distribution in the inner sheet sand is formed. The sequence of sedimentary microfacies is the core of sheet sand → the edge of sheet sand→underwater distributary bay from the center to the edge. Wave-dominated sheet sand is relatively rare in the shallow lacustrine fluvial-dominated delta. Because the river is main effect,and the wave is relatively small in the shallow lacustrine.