Reservoir Characterization of the Giant Sulige Gas Field, Ordos Basin, China
Dongbo He1, Ailin Jia1, and Tianguang Xu2
1 Research Institute of Petroleum Exploration and Development, PetroChina, Beijing, China
2 IHS Energy, Houston, TX
The Sulige Gas Field, located in the central part of the Ordos Basin, was discovered in 2000 and was put on stream in 2003. Total in-place proven reserves for the field are estimated at 18.85 Tcfg, making it the largest gas field in China. The field is characterized by large stratigraphic closure, low gas richness, low reservoir pressure, low permeability, and significant reservoir heterogeneity. The unique interplay between sedimentary facies and diagenesis has resulted in complex reservoir heterogeneities.
Reservoirs are dominated by coarse-grained sandstones of the Lower Permian Shihezi and Shanxi formations, especially those of the Shihezi 8 and Shanxi1 units. These sands, with an average burial depth of 3,200-3,500 m, were deposited in a braided-river environment. Coarse-grained bar and basal channel sandstone facies provide effective reservoirs with porosity ranging from 5% to 12% and permeability in the range of 0.02-2 md. Due to deep burial depths, primary porosity was reduced significantly; however secondary dissolution pores developed, which account for 80% of the total pore spaces. Coarse-grained sandstones with rich quartzite rock fragments experienced less compaction due to abundant rigid grains, allowing the flow of fluid. As a result, secondary dissolution pores developed in these sandstones.
Traps in the field are predominately stratigraphic variations associated with facies changes. Due to the resolution limit of seismic data, a two-row 800-m spacing exploratory well pattern was used to delineate reservoirs. Data from 16 densely drilled wells indicate that net pays are typically less than 8 m in thickness with lateral continuity of several hundred meters. The significant heterogeneity in the field suggests a dense production well pattern.