Genesis relation of surface and underground rivers and reservoir characteristics in paleokarst drainage systems：A case study of Ordovician karst in the Tahe oilfield

The Tahe oilfield lies on a slope of Ordovician karst, where the northern Tarim basin strongly uplifted in early Hercynian time,exposed Ordovician carbonate rock suffered from karstification for 56 Ma,producing complex fracture-cave reservoirs and karst landforms with numerous stone forests and canyons, as well as several karst drainage systems( surface rivers and underground rivers). The objective of this study was to reveal the genesis relationship between these surface and underground rivers and define reservoir characteristics in this area. Based on analyses to a large number of data of drilling,clogging, seismic surveys and measurement, the karst landform at the end period of karst development was restored by the method of drilling and seismic correction. And the genesis types and reservoir characteristics of surface rivers and underground rivers were analyzed by rock-minerals analysis and three-dimensional seismic carving technology. The results show that the geomorphologic high zone (watershed) formed by strike-slip faults developed in the east and west sides of the study area, the dustpan karst depression with southward opening lies between them, and a karst basin is present in the south. The northern depression is a steep slope (gradient 2.9 °) and the southern is relatively gentle (1.5 °).There are two main karst drainage formed by many tributaries developed in the depression, one is in the west that is dominated by surface rivers in the north and turns into underground rivers in the south; the other is on the east side with alternating surface (3 sections) and underground (2 sections) rivers. There are five kinds of surface rivers with different scales and fillings formed by strong erosion and dissolution, and there are no sedimentary sandstone in most surface rivers that are completely filled by the karst lacustrine marl with breccia, but no karstification, and compacted and cemented in the later period, and has almost no reservoir space. However, the underground river caves and sinkholes were filled a large amount of sandstone and mudstone, which were protected by the cave supporting, and the compaction effect was weak, among them, the porosity of sandstone reach 20%. Research suggests that the underground rivers are actually the extension of surface rivers below the ground. Originally there were sandstone deposits in surface rivers. Later on when underground rivers developed in the downstream areas,the flood washed the original sand-mudstone into the underground rivers from sinkholes,thus forming sandstone filling in karst caves of underground rivers, resulting in the high-quality oil and gas reservoirs, while almost no sandstone reservoir is present in surface rivers of Ordovician karst in the Tahe oilfield.