High-precision sequence stratigraphy and sandbody architecture of the Lower Xiaganchaigou Formation in the slope area of Western Qaidam Basin

Characterizing the internal architecture of the reservoir of the Lower Xiaganchaigou Formation in the slope area of Western Qaidam Basin can provide some reference for the exploration and development of lithologic reservoirs in this area. Guided by high-precision sequence stratigraphy, sandbody architecture theory and method, the authors studied the characteristics of sequence stratigraphy and sandbody architecture in the Lower Xiaganchaigou Formation on the basis of well-seismic combined calibration and comprehensive analysis of seismic, well logging, drilling and core data. The results show that the Lower Xiaganchaigou Formation in the slope area of Western Qaidam Basin is a large regional lacustrine invasion sequence, which can be divided into 2 third-order sequences and 4 fourth-order sequences (EMSQ1~4). From early to late, A/S (the ratio of accommodating space and sediment supply) continues to increase, whereas the sequence structure of the fourth sequence gradually changes from deepening-upward semi-cycle to non-symmetric deepening-upward full-cycle. Meanwhile, the sedimentary facies changes from braided river delta plain to delta front, and finally changes to shore-shallow lacustrine facies. The stacking patterns and distribution regularity of different types of sandbodies are closely related to the fourth-order sequence structure. The distributary channel sandbodies in delta plain facies are mainly developed in EMSQ1, and cut each other and overlap vertically so that their connectivity is very good. The subaqueous distributary channel sandbodies in delta front are mostly developed in EMSQ2~3, and are isolated or overlapped occasionally so that their connectivity is not very good. The beach-bar sandbodies of the shoreshallow lacustrine facies are chiefly developed in EMSQ4, and are isolated from each other so that their connectivity is bad. It is concluded that the further exploration of lithologic reservoirs should focus on shore-shallow lacustrine beach-bar sandbodies in EMSQ4 sequence.