Formation mechanism of deep Cambrian dolomite reservoirs in the Tarim basin, northwestern China

• One of the deepest dolomite reservoirs around the world was reported.
• Contrary to the traditional trend, the porosity increases with increasing depth.
• Hydrothermal alteration during deep burial stage is the key controlling factor.
• The porosity model for the deep dolomite reservoir was proposed.

Well TS1, which is located in the north uplift of the Tarim basin, northwestern China, has a total depth of 8408 m and is the deepest petroleum well in the world. The well penetrates a high-quality Upper Cambrian dolomite reservoir from 6884 m to 8408 m. The porosity of this dolomite increases with increasing depth, reaching 9.1% at a depth of 8407.56 m. A large number of dissolution pores are present in the core retrieved from well TS1. Some of the pores are partially filled with coarse dolomite crystals, which display saddle shapes and undulose extinction. The pore-filling dolomites have high concentrations of Fe and Mn, averaging 3158.1 and 172.5 ppm, respectively. The concentration of Ba is up to 4000.3 ppm. The oxygen isotope composition of the pore-filling dolomites is relatively light, and the δ18O values range from −10.9‰ to −5.1‰. The 87Sr/86Sr values are relatively high, ranging from 0.709361 to 0.709975. The mineralogical and geochemical features demonstrate that the pore-filling dolomites were precipitated from hydrothermal fluids that significantly altered the deep Cambrian dolomite reservoirs during upward migration along faults and fractures in the Permian. According to petrologic, geochemical and seismic features, the dolomite reservoirs were first dissolved by meteoric water during or shortly after deposition and were then significantly altered by hydrothermal fluids during burial. A porosity variation model was proposed for the deep dolomite reservoirs that were influenced by hydrothermal fluids. The model of increasing porosity with increasing depth in well TS1 indicates that there may be high-quality dolomite reservoirs in the deep strata of basins where hydrothermal fluids are active. The small amount of liquid hydrocarbon obtained from cores at a depth of more than 8400 m demonstrates that the deep dolomite reservoir is still suitable for hydrocarbon accumulation due to an unusually low geothermal gradient of approximately 20 °C/km. The deep strata in basins with similar geological parameters may therefore be potential exploration targets elsewhere in the world.