The Silurian play in the Tazhong Uplift, Tarim Basin, northwestern China

Silurian sandstone reservoirs in the Tazhong Uplift in the Tarim Basin, northwestern China are rich in bitumen and movable oil. Analysis of the biological markers indicates that the bitumen and movable oil came from two source rocks in the Cambrian and Middle/Upper Ordovician system, both of which are belong to marine facies. The Silurian system experienced three stages of oil filling, namely Late Caledonian period (400 Ma), Late Hercynian period (240 Ma) and Yanshan-Himalayan (40 Ma) period. Most of the oil emplaced in the Late Caledonian was subsequently oxidized to bitumen. The movable oil found at the present time was mainly generated from the Middle/Upper Ordovician source rocks, with only a minor contribution from the Cambrian source rocks generating in the two later periods. Bitumen and heavy crude were largely trapped below a red mudstone unit while movable oil was concentrated below an underlying gray mudstone unit (the middle sub-section of bitumen sandstone section). Lateral migration into the Silurian reservoirs took place from northwest to southeast in each stage and was controlled by an inherited palaeohigh. While individual relatively limited in extent, the tidal-flat sandstones of the main reservoir have stacking patterns that generate extensive sand sheets. Faulting can both enhance connectivity and create traps.