Abundant gas reserves have been discovered in the shallow-water delta systems developed in the Upper Triassic Xujiahe Formation in Sichuan Basin. While there are obvious differences in the sequence stratigraphic frameworks and the depositional models among the different secondary tectonic units . Based on outcrop, core, drilling, logging, seismic and laboratory analysis data from Triassic Xujiahe Formation, the structural and sedimentary background is analyzed , and the filling structure of stratigraphic sequence ,stratigraphic - sedimentary framework, sedimentary characteristics and depositional model are established. The results: (1) During the deposition of Xujiahe Formation, geology conditions were favorable for the development of shallow water deltas in the center of basin, including wide area of lacustrine basin, stable tectonic subsidence, gentle paleotopography, shallow water with frequent turbulence of the lake-level, periodically alternating dry and wet climate, abundant sediments supplied by multi-provenances and open flow channels in the southwest of the basin. (2) The Xujiahe Formation can be divided into 6 members from bottom to the top. The 2nd ~ 6th members compose of 2 super-long-term cyclic sequences, 5 long-term sequences and many mid-term sequences. The filling deposition and sequence architectures are different in different secondary tectonic units. The strata are thicker at the subsidence and deposition center in the frontal depression belts , and gradually onlap and become thinner in the foreland slope and uplift units. Accordingly the sequence architecture changed from a symmetric structure composed of LST, TST, and HST , to a dissymmetric structure without the LST, and again to a symmetric structure with both TST and HST. (3) ) From the marginal area to the deposition center, the deposition pattern of Xujiahe Formation . Influenced by tectonic activities, paleotopography, provenance, and lake-level fluctuation, mainly two kinds of shallow water delta depositions are developed, including shallow water braided delta and shallow water fan delta. These two varieties of delta are superimposed with each other, forming complex sandbodies with low composition and texture maturity, cross-bedding, intermittent positive rhythm and scour sedimentary textures. The subaqueous distributary channel dominated and no mouth bar is discovered and the channels are extended far away with many branches. But frequently diverted and abandoned channels migrated laterally, grew in a prograding/ retrograding way, and cut and superimposed with each other, forming large scale. (4)The spatial architecture of superimposed shallow-water delta sandbody is mainly controlled by the accommodation/supply ratio (A/S) and their internal structure is complex, with intense heterogeneity.
Topics: Deltaic depositional systems , Topics: Shelf and shallow water sedimentation
