| After inheriting the sedimentary pattern of the Upper Permian Changxing Formation, the Lower Triassic Feixianguan Formation undergone evolutions from carbonate shelf and slope facies to platform facies in Yuanba area in the northeast of Sichuan Basin. According to the drilling and seismic data, Feixianguan Formation can be divided into 2 sequences (SQ1 and SQ2) and 8 subsequences (from C1 to C8). In the early phase of SQ1 sedimentation (i.e. Subsequences C1 and C2), influenced by the topographic difference at the end of Changxing sedimentation, open platform facies developed accompanying with quick filling in the northwestern part of Yuanba. In the late phase of SQ1 (i.e. C3 and C4), two episodes of oolitic shoal subfacies developed, the first episode developed in the early open platform and the second one widely distributed in this area except locally distribution in platform foreslope. During SQ2 sedimentation (i.e. Subsequences C5 to C8), the sedimentary facies was relatively simple only except some difference in the C5 sedimentation, in which open platform oolitic shoal and inter-shoal subfacies developed in relatively low positions. In late phase of SQ2, the open platform evolved into restricted platform and platform evaporite facies in the whole area. Although the distribution of oolitic shoal facies is controlled by stratigraphical sequence and sedimentary facies, the reservoirs in oolitic shoal facies are controlled by the coupling of high-frequence sequence and microtopography. |
