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Control of Stratigraphic Sequence on Karst Reservoirs: A Case Study on the Ordovician Carbonate Reservoirs in the North Tarim Basin, China

Abstract

Paleokarstification is one of the key elements in the formation of the Ordovician carbonate reservoirs in the north uplift of Tarim Basin. Based on the regional stratigraphic correlation and comprehensive studies on cores, thin sections, FMI and testing, the significant control of stratigraphic sequence on paleokarst has been uncovered and demonstrated as follows: (1) The primary deposition controlled by stratigraphic sequence laid the material foundation of the karst reservoirs. Organic reefs and biodetritus beaches constituted the primary deposits under the background of a low sea level and high-energy environment, in which case the primary pores provided a relatively large porosity and a good lateral connectivity, contributing to the shaping of the pore-cavity reservoirs and fracture-cavity reservoirs in the karst stages. (2) The sequence evolution enhanced the development of faults and fracture density which played an essential role in the fluids flow and the dissolution of the rocks during the late diagenesis stages. The conversion from extension environment to compression environment in the Middle Ordovician led to the difference in the rocks that the Lower-Middle Ordovician carbonate rocks were relatively pure, whereas the Upper Ordovician carbonate rocks were high in mud. These two types of strata were quite different in lithology and mechanical properties and the Lower-Middle Ordovician was more inclined to form large-scale faults and fractures under the double effects of tectonic uplifts and strike-slip motion. These development areas of fractures and faults provided good conditions for the formation of large-scale fracture-cavity aggregations. (3) The sequence boundaries formed directional zones for fluids movement, which was favorable for the fluids migration and the formation of large karst caves. The fluids movement followed a certain regularity and can be summarized as 2 types: the descending type and the ascending type. For the descending type, the atmospheric water either transformed the exposed surface and the near-surface strata of the carbonate paleo-uplift into karst landform or continued to migrate along the beds laterally or along the faults downward vertically. For the ascending type, the buried fluids that included organic acid, acidic gas or volcanic hydrothermal solution, ascended from the deep through the faults, unconformity surfaces or the passages in beds, reconstructing the pores and caverns that had already existed before.