Abstract:

The Ordovician carbonate rocks in Tarim basin, which located in the northwestern China, are considered as important reservoirs. Observation of cores and slices indicates that the buried fluid plays an important role in controlling the reservoir. The dissolved pores and secondary minerals during the burial period (calcite, pyrite, etc.) are all related to the activity of the buried fluid. Taking the Ordovician carbonate rock fillings and surrounding rocks in the Tazhong area as the research objects, the research on the properties of buried fluids in the study area was carried out using the methods of carbon, oxygen, strontium isotope, rare earth elements, trace elements and inclusions temperature. It was found that the main fluid types in the study area were normal formation waters, and there were hydrothermal fluid activities in some areas. Due to the slightly different geochemical characteristics, hydrothermal fluids were divided into two types: magmatic hydrothermal fluids and deep circulating hot brines. The geochemical characteristics of normal formation water fluids are that the homogenization temperature of inclusions is lower than the highest burial temperature (120 ℃) in this area, the oxygen isotope composition is from -6.64‰ to -4.23‰, and the strontium isotope value is ⁸⁷Sr/⁸⁶Sr<0.709. The distribution pattern of rare earth elements is consistent with the surrounding rock, and the variation range of δEu is from 0.627 to 0.921. The geochemical characteristics of hydrothermal fluids are the homogenization temperature of the inclusions is higher than the maximum burial temperature of surrounding rocks by more than 5 ℃(125 ℃) and it is of the lighter oxygen isotope composition (from -15.18‰ to 7.09‰). The distribution pattern of rare earth elements is different from the surrounding rock limestone. Some of the hydrothermal fluid samples have ⁸⁷Sr/⁸⁶Sr values (from 0.707,82 to 0.709,03) consistent with the international Ordovician sedimentary carbonate rock standard, with obvious Eu positive anomaly (δEu varies from 1.24 to 20.906), and has a low Th/U value (<0.45). This part of the hydrothermal fluid is magmatic hydrothermal fluid. Others hydrothermal fluid samples have a high ⁸⁷Sr/⁸⁶Sr value (>0.709), and inclusions have high salinity (≥15%NaCl). The main intrusive rocks of the Ordovician in the Tazhong area are diabase, and the diabase should have a lower ⁸⁷Sr/⁸⁶Sr value, which conflicts with the higher ⁸⁷Sr/⁸⁶Sr value in the sample. It is considered that the fluid with the high strontium isotope value in the clastic rock formation is circulated to the carbonate rock formation under the influence of magmatic heat and geothermal gradient, causing some samples to have higher ⁸⁷Sr/⁸⁶Sr values (from 0.709,185 to 0.709,853). This part of the hydrothermal fluid is hot brine circulating in deep. The movement of hydrothermal fluids is mainly controlled by deep faults. Magmatic activities lead to the formation of hydrothermal fluids directly or indirectly.

Key words: carbonate reservoir, hydrothermal fluid, geochemistry, Ordovician, Tazhong region, Tarim Basin