Braided-delta sandstone and its petroleum storage capacities in different systems tracts in the Upper Member of the Silurian Kalpintag Formation on the southern margin of the Manjia'er subbasin

Abstract：Detailed core description, log study and intensive interpretation of nearly 3,000 km long seismic profiles show that the southern margin of the Manjia'er subbasin was a gentle slope continental margin in the Early Silurian. An onlap line that is nearly parallel to the pinching-out line of the Silurian is recognized at the bottom of the Upper Member of the Silurian Kalpintag Formation between well Mancan 2 and Tazhong 32. The onlap line is considered the coastline of the initial transgression in sequence 1 in the Upper Member of the Silurian Kalpintag Formation in the Mannan area. Based on the aforesaid understanding, combined with the effective recognition of the sequence boundary of wells, the Upper Member of the Silurian Kalpintag Formation in the Tazhong 32, Tazhong 34, Mancan 2 and Mancan 1 well areas is divided into two third-order sequences. Braided-delta deposits occur in both sequences but the oil storage capacities of “the sandstones of the same facies in different systems tracts” have different features. The main influence factors are the grain size and sorting of detrital grains, types and contents of cements and different burial characters of the reservoirs. The early-stage braided-delta front deposits are developed in the lowstand systems tract (LST) of sequence 1 in the Mancan 2 well area in the lower part of the slope. The main reservoir sandstone is subaqueous distributary channel sandstone of the braided-delta front, where the grains are relatively fine and the compositional maturity is relatively low. As the reservoir is buried at ~5,000 m depth and the calcite cement content is high, the storage capacity is relatively poor. By contrast, the main reservoir in the Tazhong 32 and 34 wells area in the uplift is subaqueous distributary channel sandstone of the regressive braided-delta front in the transgressive systems tract of sequence 1. As such sandstone is characterized by relatively coarse grains, relatively high compositional maturity, relatively shallow burial depth and relatively low cement contents, its porosity and permeability are notably better than those of the early-stage subaqueous distributary channel sandstone of the braided-delta front in LST.