川东南地区飞仙关组层序岩相古地理特征

在层序地层学和沉积学理论指导下,以油气勘探为目标.针对川东南地区飞仙关组,取三级层序体系域作为成图单元,采用体系域压缩法和瞬时法进行层序岩相古地理编图,详细探讨了川东南地区早三叠世飞仙关组古地理特征及演化.研究表明:川东南飞仙关期发育海相地层,可识别出2个三级层序:SQ1和SQ2,二者皆为Ⅱ型层序.SQ1TST期,由于海平面的上升,研究区发育较深水环境的混积陆棚;进入SQ1HST期由于前期的沉积填充,盆地水体逐渐变浅盆地进入了开阔台地沉积环境,开始了鲕滩发育演化阶段.SQ2TST和SQ2EHST期研究区沉积格局基本上延续了SQ1HST期沉积特征,其中鲕滩在SQ2EHST进入了发展最高峰期;SQ2LHST由于相对海平面下降和沉积物的继续填充,研究区转入局限台地沉积环境,沉积-套区域上分布广泛的紫红色泥岩、白云岩、膏岩组合.SQ2EHST为研究区鲡粒滩最发育层位,具有较大的勘探价值.

Sequence-based lithofacies and paleogeography of Lower Triassic Feixianguan Formation in southeastern Sichuan Basin

Guided by the sequence stratigraphy and sedimentology and aimed at oil-gas exploration, the authors recognized two third-order sequences (SQ1, SQ2) in the Feixianguan Formation, both of which belong to the type II sequence. Sequence-based lithofacies and paleogeographic maps of Feixianguan Formation were compiled by means of condensation and instantaneity with mapping units of sedimentary system tracts. The result shows that, from the phase of SQ1TST to that of SQ2LHST, the deposition evolution changed from the mixing shelf facies to the platform fades. That is to say, during the phase of SQ1TST, the mixing shelf constituted the setting of the study area due to the rising of the sea level. During the phase of SQ1HST, based on previous deposition, the depth of the basin water gradually turned shallow and the environment changed form the mixing shelf to the open platform; at that time, the oolite beach was developed. During the phases of SQ2TST and SQ2EHST, the setting of the study area didn't change, but the oolite beaches experienced something unusual. During the phase of SQ2EHST, the number and the sizes of the oolite beaches became optimal. During the phase of SQ2LHST, however, the relative falling of the sea level and the deposition of the sediments resulted in the change of the environment of the study area from the open platform to the restricted platform, with the sedimentary rocks being mainly limestone, dolomite and gypsum rocks. It is thus concluded that the oolite beaches were mainly developed during the phase of SQ2EHST, and these beaches hence deserve much attention in future exploration.