Sedimentary characteristics of the fan delta of the Suhongtu Formation in Barun central structural belt of Chagan Sag, Inner Mongolia, China

Chaggan Sag, one of the most important exploration targets of Inner Mongolia, China, locates in the west of Chagandelesu Depression of Yingen-Ejinaqi Basin. The major oil-bearing formation is the Lower Cretaceous Suhongtu... [ view full abstract ]

Chaggan Sag, one of the most important exploration targets of Inner Mongolia, China, locates in the west of Chagandelesu Depression of Yingen-Ejinaqi Basin. The major oil-bearing formation is the Lower Cretaceous Suhongtu Formation. The Suhongtu Formation is dominated by terrigenous clastics such as fine sandstone, siltstone, mudstone and conglomerate in Barun central structural belt of Chagan Sag. Based on integrated subsurface database of over 30 wells and seismic data, the sedimentary characteristics of the Suhongtu Formation were have been well studied. The results show that the depositional system of Suhongtu Formation was the flat slope fan delta. The main features of the fan delta were as follows: lithology was mainly lithic sandstone with low maturity; the probability cumulative curves of grain size and the C–M diagram were mainly characterized by traction current sedimentary mechanism. Less data show the evidence of gravity flow; the main colors of the mudstone were brownish red and fuchsia; seidment structures such as scour surface, wedge-shaped cross bedding and trough cross bedding were formed in high energy environment; from boundary to center of the basin, the sand/conglomerate transits to mixed sandstone and mudstone whereas the scale of the discontinuous normal cycles become smaller; the fan deltaic body can be divided into three subfacies: fan delta plain, fan delta front and pro-fan delta. Estimation on the Suhongtu Formation thickness along depositonal-dip shows the slope gradient of accessing lake was about 8‰, which is relatively gentle. The fault depression was getting weaker and the basement sank stably relatively during the period of Suhongtu Formation. The well-developed sandstones of the fan delta front was the most favorable reservoirs. Compared to the traditional Gilbert-type fan delta plain which was supplied by ephemeral streams with coarse grain, poor sorting and poor roundness, Chagan Sag, especially along the basin margin, also contains coarse-grained sdeiments with small transport distance. But the conglomerate content was only a small proportion of Suhongtu Formation. Based on the statistical results by logging data, the major lithology were siltstone and fine sandstone with a stacking thickness percent of 80%. These features of the fan delta of Suhongtu Formation was similar to the flat slope fan delta. Therefore, based on lithological characteristics, sedimentary tectonic and sedimentary sequence of the fan delta, we proposed a fan delta with shallow water depth and gentle slope mode, which is helpful for future oil and gas production and exploration.