Hydrothermal dissolution of deep-buried carbonate rocks and its significance for hydrocarbon exploration in Shunnan area, the Tarim Basin: taking well Peng-1 in Shunnan area as a case

Abstract

In order to reveal the genetic mechanism of deep-buried carbonate reservoirs, a study was made based on well Peng-1 in Shunnan area of the Tarim Basin. The core section of well Peng-1 revealed that a large number of fractures and dissolution pores were developed in the carbonate rocks with buried depth of over 7 500 m. Based on detailed core observation and description, it was found that the dissolution pores were mainly distributed in the Upper Cambrian dolomites in the lower part of the core section and few in the Lower Ordovician carbonates in the upper part of the core section. The diameter of pores increases gradually with the increase of burial depth, which indicated that the pores were formed by infiltration of deep-seated hydrothermal fluids rather than meteoric water. The similar δ13C values and 87Sr/86Sr ratios of different types of minerals (including dolomite and calcite) with host rocks indicated that the diagenetic fluids inherited the geochemical characteristics of original sequestered pore water through intense water-rock action with the original carbonate rocks. However, the significantly negative δ18O values (with an average of -13.26 ‰) suggested that they were precipitated from fluids with high temperatures. This inference was verified by the results of fluid inclusion microthermometry, which confirmed that dolomite recrystallization and subsequent calcite precipitation were closely associated with fault-related deep-seated hydrothermal fluids. In addition, whole diameter CT scanning revealed that fractures can significantly improve the reservoir property and permeability of the dolomite formed by hydrothermal dolomitization. The permeability can be improved by at least one order of magnitude, i.e. from 0.02×10-3 μm2 to 0.39×10-3 μm2. Hydrothermal-related dolomite reservoirs may be extensively developed in the deep/ultra-deep buried carbonates in the Tarim Basin in view of the occurrence of such phenomenon in the well TS1 and Gucheng area. Thus, in the future hydrocarbon exploration in the Tarim Basin and elsewhere, hydrothermal-altered dolomite reservoirs deserve more attention.

Keywords

• hydrothermal dissolution
• deep-buried dolomite reservoir
• hydrocarbon exploration
• shunnan area
• tarim basin