川西北地区构造地质结构与深层勘探层系分区

Structural geology and favorable exploration prospect belts in northwestern Sichuan Basin, SW China

川西北地区经历了震旦纪至中三叠世的海相盆地和中—新生代陆相盆地构造演化过程,期间遭受多个阶段性区域构造活动影响,深层发育复杂的地层分布及构造变形,在关键构造期形成了包括震旦系灯影组阶梯状台地、志留纪末西部古隆起、晚古生代—中三叠世被动陆缘等特征性的构造和沉积结构。晚三叠世以来的中—新生代多期不同方向的陆内挤压构造作用则形成了盆地周缘及内部的复杂冲断褶皱构造。龙门山北段冲断带前缘发育深层原地冲断构造,表现为受下三叠统和下寒武统中的两套滑脱层控制而成的多层次变形结构,包括浅层褶皱构造、中层薄皮冲断构造和深层基底卷入构造。米仓山冲断带前缘主要表现为受下三叠统盐滑脱层分隔的双层变形结构,包括浅层单斜构造和深层叠瓦逆冲构造。盆地内部则主要表现为晚期形成的多排北东东向基底卷入褶皱变形。川西北地区深层有利勘探层系和区带具有明显的分区特征,震旦系灯影组有利勘探层系主要分布于川西北东部和北部地区,以九龙山构造带、梓潼向斜带、盐亭斜坡带最为有利;下古生界受加里东古隆起和晚期褶皱改造,盆地中西部深层最为有利,以梓潼向斜带为典型;上古生界有利区则主要分布于龙门山北段及其前缘地区,中构造层隐伏薄皮冲断构造带是重点勘探领域。

The northwestern Sichuan region has experienced multi-stage tectonic evolution including marine cratonic basin from the Sinian to the Middle Triassic and intra-continental basin from the Late Triassic to the Cenozoic. Several regional tectonic activities caused complicated stratigraphic distribution and structural deformations in the deep-buried layers. During the key tectonic periods, some characteristic sedimentary and deformation structures were formed, including the platform margin of Dengying Formation, the western paleohigh at the end of Silurian, and the passive continental margin of the Late Paleozoic–Middle Triassic. The Meso-Cenozoic intra-continental compressional tectonic processes since the Late Triassic controlled the formation of complex thrusting structures surrounding and inside the northwestern basin. The northern Longmenshan fold-thrust belt has a footwall in-situ thrust structure, which is controlled by two sets of detachments in the lower Triassic and lower Cambrian and presents as a multi-level deformation structure with the shallow folds, the middle thin-skin thrusts and the deeper basement-involved folds. The thrust belt in front of the Micangshan Mountain shows a double-layer deformation controlled by the lower Triassic salt detachment, which is composed by the upper monocline and deep-buried imbricate thrust structures. The interior of the basin is characterized by several rows of large-scale basement-involved folds with NEE strike direction. From the perspective of structural geology, the favorable exploration reservoirs and belts in northwestern Sichuan have obvious zoning characteristics. The favorable exploration layers of Dengying Formation of Upper Sinian are mainly distributed in the eastern and northern areas of the northwestern Sichuan Basin, in which the Jiulongshan structural belt, Zitong syncline and Yanting slope are the most favorable. The Lower Paleozoic was transformed by Caledonian paleo-uplift and late Cenozoic folding, and the midwest area such as the Zitong syncline is a potential area for hydrocarbon exploration. The favorable part of the Upper Paleozoic is mainly distributed in the northern Longmenshan belt and its frontal area, where the deep-buried thin-skin thrust structures in the footwall are the key exploration targets.