Types and quantitative characterization of reservoir spaces of the ultra-deep limestone reservoirs in the Yijianfang Formation during the Middle Ordovician, Shuntuoguole area, Tarim basin

The Shuntuoguole area is located in the Shuntuoguole low uplift of the northern depression of the Tarim basin. The Yijianfang Formation of the Middle Ordovician in this region deposited in the carbonate platform sedimentary environments, with 168 similar to 220m thickness and being buried in the depth of 6300 similar to 7850m and even deeper, the property of ultra-deep reservoirs is the key to the study and hydrocarbon exploration. With systematic observation of cores, casting thin sections and SEM, and micro-nano CT detection and analysis of the cores, we found that the rock types of these reservoirs are dominated by compact limestones, which include calcarenites, clotted limestones, boundstones, algal limestones, and so on. The reservoir spaces include the bioclastic blind holes, mould pores, intragranular pores, intracrystalline pores, microfissures and dissolved pores, in which the intragranular pore is the main type. The 3D quantitative characterizations of reservoirs indicate that there are preferably two types of reservoirs in this area They are the bright crystalline algalclastic calcarenites in the intraplatform beach facies and clotted limestones, with the average of porosities ranging from 2% to 4% , some of them up to 12.14%. The secondary reservoirs are algal boundstones and algal limestones. The micro pores and cavities resulted from the tectonic hydrothermal metasomatic alteration and corrosion effects, and microfissures collectively constructed the reservoir space systems, formed the high quality reservoirs. It is of great value for the natural gas exploration of ultra-deep layer.