Xujiahe Formation Petroleum System in Central Sichuan Basin, China

The Golden Autumn Tight Gas Block is located in the northwest of the Central Sichuan Basin and covers an area of 4,067 km2. Shell, in partnership with PetroChina, has been appraising the gas potential of the area since 2010. The primary target interval, the Late Triassic Xujiahe Formation, is about 1000m thick, and thickens towards the northwest. It is dominated by alternating sand and shale rich members. From bottom to top, the Xujiahe Formation is separated into six, thick members labeled the Xu1 (T3x1) to Xu5-6 (T3x5-6) respectively. The Xu1, Xu3(T3x3) and Xu5-6 members dominantly comprise coaly and calcareous shales versus the sand dominated Xu2 (T3x2) and Xu4 (T3x4) members. The Xujiahe Formation is considered a self-charged, over-pressured petroleum system with complicated migration history and complex play types. The major source rocks are dark shales and coals within the Xu1, Xu3 and Xu5-6 members. Reservoirs are tight, discontinuous channelized calcarenite sands in shaley formations (e.g. Xu5-6) and regionally amalgamated, largely continuous, litharenite sands in the Xu2 and Xu4 members. The Jurassic overburden is about 3000m thick and mainly comprised of red lacustrian mudstones and siltstones. Gas and condensate are present and productive from the Xujiahe formation across the entire block. However, zones with higher WGR and even water satured zones occur in the Xu2 and Xu4 members and are interspersed with drier gas zones. In contrast, minimal water production is observed from reservoir sands in the Xu3 and Xu5-6. This indicates various play types within different zones. Key Words: Xujiahe Formation, Sichuan Basin, tight gas, basin centered gas (BCG), inefficient and efficient BCG, petroleum system, calcarenite sand, over-pressured, play concept model.