Petrological and geochemical constraints on fluid types and formation mechanisms of the Ordovician carbonate reservoirs in Tahe Oilfield, Tarim Basin, NW China

As one of the major hydrocarbon source rocks, deeply buried (4500-7000 m) Ordovician marine carbonates are important reservoir rocks in Tahe Oilfield, Tarim Basin, NW China. Most of the observed diagenetic features are genetically related to the meteoric water due to the multistage structural uplifts and subsequent paleo-exposure, weathering and erosion experienced by the carbonate strata. There is very limited study on the petrological and geochemical constraints on fluids types and formation mechanisms of the Ordovician carbonate reservoirs in this region. In order to investigate to the mechanism and characteristics of karstification, this paper integrated petrographic studies, elements and C, O, Sr isotopes geochemistry and fluid inclusions, and revealed the origin and types of diagenetic fluids, with which the dissolution and development of secondary porosity enhances reservoir properties of the Ordovician carbonates. Through this study, vugs and pores generated from epigenetic meteoric dissolution were observed to partially destroyed due to subsequent compaction, filling and cementation. Meteoric water, hydrothermal fluids and formation water are the three main types of diagenetic fluids in the Ordovician carbonate rocks of Tahe Oilfield. This paper investigated the values of key parameters related to perception, and figured out the precipitation types of calcites by methods of comparisons. Comparing to carbonate matrix, some calcite cements exhibit the features of relatively negative values of delta O-18, a relatively wide range of delta C-13, higher ratios of Sr-87/Sr-86, and low to medium homogenization temperatures. These may indicate the precipitation of calcite cement from meteoric water. Respectively high TREE and obvious positive Eu anomaly, lower delta O-18, higher delta C-13 value, higher Sr-87/Sr-86 ratios, combined with abnormal high homogenization temperatures of some calcites could confirm their precipitations from hydrothermal fluids. Respectively higher delta O-18 values and lower Sr-87/Sr-86 ratios of some calcite cements suggest their precipitations from formation water. The dissolution of meteoric water is the most important karstification and controlled by carbonic acid solution recharge conditions. Hydrothermal activity can promote the thermochemical sulfate reduction (TSR) and have certain dissolution ability. The paper established the models of meteoric water and hydrothermal fluids karstification. In practice, the results would work as a useful reference for the exploration of deeply burial carbonate reservoirs and hydrocarbon in carbonate strata of basins around the world.