Comparative study about the characteristics of inclusions in the paleokarst fillings in the northern outcrop and underground areas of Tarim Basin

The Ordovician oil/gas reservoir in Tarim Basin had experienced multiple stages of water-rock interactions, so the distribution and development of the karstic reservoir are very complex. However, the characteristics of inclusions in the paleo-karst fillings can effectively indicate the changes in environments and periods of water-rock actions of the paleokarst. Based on the test of the inclusions in paleokarst fillings sampled from both outcrop area and the underground, the physical and chemical characteristics of the inclusions are summarized. From a new perspective, this paper will carry out a comparative study of the karst development over the outcrop area and the underground, repectively.. The results show that in terms of type of the karstic filling inclusions, the study area is predominated by single liquid phase and gas-liquid phase aqueous inclusions, with only a small quantity of hydrocarbon inclusions occur at the underground in Tahe Oilfield. The physical characteristics of the inclusions reflect the particular environments of the buried paleo-karst. The inclusions can be divided into five combined types according to the interaction between salinity and uniform temperature. Based on the ancient geothermal gradient, the inclusions formed in three kinds of environment (i.e., shallowly buried, medium buried, deep buried). The Ordovician carbonate rock has experienced four periods of karstification, including weathering crust karst, shallowly buried, moderately buried (the early and late) and deeply buried karsts; and the filling of karst reservoir was mainly developed in the medium burial period.