Hydrothermal dolomite reservoirs in a fault system and the factors controlling reservoir formation-A case study of Lower Paleozoic carbonate reservoirs in the Gucheng area, Tarim Basin

The widespread hydrothermal activity in the Lower Paleozoic carbonate reservoirs of the Tarim Basin has received much attention. The discovery of hydrothermal dolomite reservoirs indicates that hydrothermal alteration may have a positive effect on reservoir development. However, the factors controlling hydrothermal dolomite reservoirs and their pore development characteristics are difficult to recognize and interpret. Based on the lithology, petrology, pore fabrics and geochemical characteristics of the Lower Paleozoic carbonate reservoirs in the Gucheng area, this research discloses that the origin of the carbonate reservoirs is related to alteration by hydrothermal brine. Further, hydrothermal dolomite reservoirs are strongly heterogeneous in different fault zone structures. This research divides the fault structures into fault cores (type 1) and damage zones (types 2 and 3). Type 1 is characterized by many breccias cemented by saddle dolomite, locally developed vugs or fractures in the cores, low density (1.7–2.8 g/cm3) and resistance (22.4–1075 Ω) in the log response, and shows abundant black-spotted vugs and sinusoidal fractures in formation microscanner images (FMI). Due to poor connectivity and high heterogeneity (permeability: 3.24 to 0.01 mD, average 0.59 mD; porosity: 5.1 to 0.7%, average 1.8%), type 1 hydrothermal dolomite reservoirs show both positive and negative hydrothermal fluid contributions. In the damage zones, especially in the areas near fault cores, type 2 is rich in fracture-pore systems in both the cores and FMI and is characterized by serrated fluctuations in acoustic waves and densities (2.4–2.8 g/cm3). Type2 performs well in terms of physical properties (permeability: 8.06 to 0.011 mD, average 0.837 mD; porosity: 6.6 to 0.65%, average 2.5%), indicating potentially favorable areas for the development of hydrothermal dolomite reservoirs. At the distal end of a damage zone or in poorly developed areas, type 3 features filling by hydrothermal products or higher densification (permeability: 0.77 to 0.0003 mD, average 0.125 mD; porosity: 1.6 to 0.2%, average 0.2%). The well logging response of type3, which has a density of around 2.8 g/cm3 and flat acoustic wave curves, is similar to that of the host rock. The differences in the development of hydrothermal dolomite reservoirs among diverse fault zones suggest that the damage zone (type 2) may be a potential area for hydrothermal dolomite reservoirs in the fault system. This study illustrates a model in which the hydrothermal fluid dissolves host rock in the proximal area and precipitates it at the distal end of the damage zone. The hydrothermal dolomite reservoir characteristics of different fault zones are also further interpreted and described. Therefore, different locations within a fault zone need to be considered and distinguished in studies of the factors controlling hydrothermal dolomite reservoirs in fault systems. This view may provide new ideas regarding whether hydrothermal activity in the Tarim Basin plays a positive role in the formation of reservoirs and offers new suggestions for the exploration of hydrothermal dolomite reservoirs in fault systems.