Paleokarstic water tables and their control on reservoirs in Ordovician Yingshan Formation, Tazhong Area, Tarim Basin, NW China

Haizhou QU,Maoyao LIU,Yunfeng ZHANG,Zhenyu WANG,Zhenghong ZHANG,Shiyin LI,Xingliang DENG

Table 1 Statistics on horizontal underflow zone below various stages of water table in different karst paleogeomorphic units.

Stage Karst highland/karst slope/karst depression Maximum thickness/m Minimum thickness/m Mean thickness/m Mean distance to unconformity/m Typical karst fabrics Filling characteristics Mean thickness of reservoirs/m Main class of reservoirs Third stage 20/-/- 12/-/- 15/-/- 17/-/- Several caverns, or combination of caverns and dissolved fractures, with diameter reaching meter level; near horizontal dissolved fractures cut high-angle dissolved fractures, and both fractures are quite wide; cystic vugs are large in diameter/-/- Partially filled by mud/-/- 13/-/- Ⅱ-Ⅲ/-/- Second stage 62/42/- 20/20/- 38/24/- 82/64/- Near horizontal dissolved fractures cut early-stage high-angle dissolved fractures/~/-; cystic dissolved vugs are from quite large to small in diameter/-; caverns are from large diameter to small diameter too/-; and all the karst fabrics are in overlay combination/~/- Partially filled by mud, silt-sand level debris, and breccia, etc./~/- 21/15/- Ⅱ-Ⅲ/Ⅰ-Ⅱ/- First stage 1 50/56/40 32/30/16 45/41/32 154/123/42 Cystic dissolved vug layers and near horizontal dissolved fractures/~/~ Mud, burial-stage cements/~/~ 40/29/13 Ⅱ-Ⅲ/Ⅰ-Ⅱ/Ⅰ-Ⅱ