Ancient hydrological conditions of the Ordovician transformation zones in the Halahatang area, northern Tarim

From north to south, the Halahatang area of northern Tarim can be divided buried hill in karst, interbed karst bedding reform, interbed karst platform-margin overlap, and karst slope zones. Among these, the inter-bed karst-bedding reform zone is the transition area between the buried hill karst and bedding karst zones. This paper presents a detailed analysis on the ancient karst hydrological conditions for sediment discontinuity in the Pre-Silurian and Lianglitage formation karst period. The results suggest that the surface drainages are characterized overall by runoff from north to south and deeply incised valleys that decided the depth of the local discharge datum. There are various kinds of underground rivers, which are of flow in and flow out types as pipeline systems, mostly in single inflows and outflows. On seismic reflection profiles, these rivers can be recognized by continuous or intermittent beaded strong reflections, of which the karst subsurface river of the Lianglitage formation is particularly obvious. Most of beheaded rivers of the Pre-Silurian have hidden outlets. In the vicinity of these underground rivers, there exist big fissure-cavity bodies which are the important targets in current exploration. Thus accurate reconstruction and description of the hydrology of these ancient karst drainages would be help to search for favorable reservoirs of hydrocarbon.