构造沉降梯度对盆地沉积体系发育的控制作用

Controlling Effect of Structural Sedimentation Gradient on Development of Basin Sedimentation System

构造沉降是盆地可容空间变化的重要因素，对沉积速率和沉积体系有明显的控制作用。构造沉降梯度是某一地质时期盆地构造沉降横向变化的表征值，揭示了该时期盆地可容空间变化特征。通过四川盆地上三叠统须家河组须二段和须三段构造沉降梯度分析发现，构造沉降梯度对沉积体系和沉积相带展布有明显的控制作用。构造沉降梯度高值区，可容空间变化快，水系集中发育，沉积相变化大；构造沉降梯度低值区，可容空间变化小，水系发散，沉积相稳定分布。通过对广安地区须四段解剖发现，构造沉降梯度还对储层的分布有影响，在局部构造沉降梯度高值区，由于地形坡度较大，河道水动力较强，形成砂岩储层的物性相对较好。与其他盆地的类比发现，以南堡凹陷为代表的断陷盆地构造沉降梯度变化大，基本在1～7（m／Ma）／km，导致其沉积相带窄而变化快，亚相长度只有6～13km，岩性和物性变化快，形成的有利储层规模较小，非均质性强。以四川盆地须家河组和鄂尔多斯盆地山西组为代表的前陆盆地和坳陷盆地，构造沉降梯度变化相对较小，基本在0～1．5（m／Ma）／km，导致其沉积体系规模大、沉积相分布范围广，亚相长度达50～150km，可形成大型河流三角洲和大面积有利储层的稳定分布，是发育大型岩性含油气区的有利地区。

Structural sedimentation is an important factor for changes in accommodation space of a basin, exerting an obvious controlling effect on sedimentation rate and sedimentation system. Structural sedimentation gradient is the characterization value of horizontal variation for basin structural sedimentation in a certain geological period, indicating the variation characteristics of this basin＇s accommodation space during this period. Based on the analysis of structural sedimentation gradients of the second member and third member of the Triassic Xujiahe Formation in Sichuan Basin, it is found that structural sedimentation gradient has an apparent controlling effect on sedimentation system and distribution of sedimentation facies. In the zone with a high structural sedimentation gradient, accommodation space changes quickly with development of water systems while the sedimentation facies changes significantly. In the zone with a low structural sedimentation gradient, accommodation space changes slowly with dissipation of water systems and the sedimentation facies is distributed steadily. Based on the analysis of the fourth member of the Xujiahe Formation in Guang＇an region, it is found that structural sedimentation gradient also influences distribution of reservoirs. In the regional zone with a high structural sedimentation gradient, channel hydraulic power is strong owing to a high slope of landform. The physical properties of sandstone reservoirs are relatively desirable. As compared to the basins of other types, the rift basins represented by Nanpu Depression have a high structural sedimentation gradient, basically at 1 to 7 （m/Ma）/km, causing the sedimentation facies to become narrow and change quickly. The length of the subfaeies is only 6 to 13 kilometers, with the lithological characters and physical properties changing quickly. The developed favorable reservoirs are small in scale and strong in heterogeneity. In the foreland basins and sag basins represented by the Xujiahe Formation of Sichuan Basin and the Shanxi Formation of Ordos Basin structural sedimentation gradient changes slowly, basically at 0 to 1.5 （m/Ma）/km, leading to large scale of sedimentation system and extensive distribution of sedimentation facies. The length of the subfacies reaches 50 to 150 kilometers, forming large-scale channel delta and enabling favorable reservoirs to be distributed steadily in large areas. These basins are favorable zones for development of large-scale lithological oil and gas areas.