四川盆地龙门山北段隐伏构造带特征及其油气勘探意义

2014年以来,四川盆地西北部(以下简称川西北)龙门山山前带双鱼石地区上古生界天然气勘探获得重大发现,先后在中二叠统栖霞组、中泥盆统观雾山组钻获多口工业气井。随着勘探工作的稳步推进,深入开展龙门山逆冲推覆构造前锋带—山前带构造特征研究,是认识该区上古生界气藏特征、拓展油气勘探领域的重要前提。为此,基于新部署的线束三维地震、重磁电资料以及ST9等井的实钻成果,解剖了龙门山逆冲推覆构造前锋带—山前带的构造特征,进而分析了该区隐伏构造带的构造特征、天然气成藏条件和勘探前景。结果表明:(1)龙门山逆冲推覆构造前锋带(1)号隐伏断裂之下发育大型隐伏构造带,其地层受挤压褶皱变形,发育下三叠统嘉陵江组以下的海相地层;(2)由南到北(1)号断裂底部滑脱层位由浅变深,从南部的寒武系滑脱,向北部逐渐转换为基底卷入,古生界及其以上地层整体抬升,越向北抬升幅度越大,断裂上下盘之间的位移距离、褶皱幅度也越大,北部相对南部上古生界构造整体抬升约1 500 m;(3)在剑阁以西—广元地区,(1)号断裂东侧整体表现出"三级台阶"的构造格局。结论认为:龙门山逆冲推覆构造前锋带(1)号断裂下盘隐伏构造带面积达1 800 km2,具有优越的天然气成藏条件,其中观雾山组和栖霞组储集条件、保存条件好,是川西北深层海相层系天然气勘探拓展寻找新突破的重要领域,具有现实的油气勘探意义。

Characteristics of buried structures in the northern Longmenshan mountains and its significance to oil and gas exploration in the Sichuan Basin

Since 2014, great gas discoveries have been achieved in the Upper Paleozoic Shuangyushi zone in the frontal belts of the Longmenshan mountains, northwestern (NW) Sichuan Basin, where multiple gas wells with industrial flows have been drilled and completed successively in the Middle Permian Qixia Fm and Middle Devonian Guanwushan Fm, etc. Along with the steady progress of exploration there, to make a further in-depth study on the characteristics of the frontal–piedmont zones in the Longmenshan thrust–nappe structures will be of great significance to both a better understanding of Upper Paleozoic gas reservoirs in this study area and the expansion of oil and gas exploration field. In view of this, based upon the 3D seismic surveys, gravity–magnetic–electronicdata, and practical drilling information from exploration wells like Well Shuangtan 9, the characteristics of the frontal–piedmont zones in the Longmenshan thrust–nappe structures are analyzed as well as the features of the buried structures, gas pooling conditions and exploration prospect there. The following findings were obtained. (1) A huge buried structure is well developed underlying the Longmenshan front fault belt No.1 (LSFFB 1) and marine-facies strata are thus formed below the Lower Triassic Jialingjiang Fm due to the roles of compressive folding, extrusion and deformation. (2) The detachment horizons from LSFFB 1 go from shallow to deep in S–W trending, turning from Cambrian slip in the south to basement-involved in the north. The Paleozoic and its overlying strata were overall uplifted with a higher degree northward and so did the strata between the up-thrown and down-thrown sides of the faults with a greater displacement distance and higher folding degree, resulting in that the Paleozoic strata in the north are 1 500 higher uplifted than those in the south. (3) In the western Jiange and Guangyuan areas, the eastern LSFFB 1 presents a pattern of "three steps" as a whole. In conclusion, there are good gas pooling conditions in LSFFB 1, the down-thrown sides of which cover an area of 1 800 km2, where the Guanwushan and Qixia strata provide good accumulation and preserving spaces for gas reservoirs, which will become favorable gas exploration targets in the deep marine-facies strata in NW Sichuan Basin.