重力流沉积砂岩的成因、改造及油气勘探意义

Genetic Mechanism and Rework of Deep-Water Sedimentary Sand and Its Significance for Petroleum Exploration

在前人研究基础上,结合最新研究成果,以深水砂岩形成为线索,讨论重力流沉积砂岩成因机制以及后期底流改造作用,分析争议问题,探讨解决方法,并揭示油气勘探意义。重力流沉积砂岩成因机制复杂,将浊流严格定义为紊流支撑,悬浮沉降的重力流,有利于砂岩成因机制解释及砂体分布预测。具有正粒序构造的砂岩为浊流成因,块状砂岩和逆粒序砂岩为砂质碎屑流成因。浊流砂岩沉积在平面上向四周散开,形成朵叶状砂体,碎屑流砂岩沉积沿流体搬运方向展布,形成舌状砂体。重力流沉积受后期底流改造而形成含牵引流构造的砂岩,但牵引流构造不是底流改造砂体所特有的标志,需结合砂体分布特征及沉积环境加以鉴别。水道内重力流沉积受底流影响,天然堤不对称发育,水道发生单向迁移,砂体平面展布范围变大,但砂体垂向厚度变小。同时,受底流改造的天然堤之间会形成局部负向地形,为后期水道、朵体有利沉积场所,易形成连续垂向叠置砂体。深水砂岩预测需将重力流沉积和底流改造结合起来。

Based on previous studies and the latest researches, we discussed the genetic mechanism of gravity flow sedimentary sandstone and subsequent bottom current reworking based on the formation of the deep-water sandstone. It is conducive to answering the formation mechanism of sandstone and predicting sandstone distribution if turbidity currents are defined as the turbulent supporting flows that deposited by suspension. Normally-graded sandstone is deposited by turbidity currents. Massive sandstone and reversely-graded sandstone are deposited by sandy debris flows. Sandy lobes deposited by turbidity currents scatter around, however sandy tongue-like deposits deposited by sandy debris flows are distributed along flow direction. Gravity flow sedimentary sandstone may develop traction flow structures for bottom current reworking, hut traction flow structures in deep-water environment are not the unique mark of bottom current reworking, which should be recognized by combining regional sand distribution and sedimentary environments. The influence of bottom current to gravity flows in channels leads to asymmetrical levees and unidirectionally migrating channels. The sand distribution is therefore extensive with the decrease of vertical thickness. Meanwhile, the concave landform forming between the higher levees formed by bottom current reworking is preferential position for subsequent channels and lobes deposition, which conduces to the formation of successive stacking sands. It is necessary to combine the study of gravity flows and bottom current reworking in the prediction of deep-water sandstone.