Natural Gas Reservoiring Phases Divided Through Method of Natural Gas Inclusions: A Case from the Upper Paleozoic Reservoirs in the Southeastern Ordos Basin

Key words: 
• Ordos basin /
• inclusion /
• Upper Paleozoic /
• hydrocarbon accumulation time

Abstract: In the southeast of the Ordos Basin, the Upper Paleozoic gas reservoir maily includes Taiyuan Formation, Shanxi Formation, the lower Shihezi Formation and the upper Shihezi Formation and lithology is mainly quartz sandstone. Based on microscope petrographic study, the main diagenetic types of the reservoir are compaction, pressure dissolution, cementation and metasomatism, fracture and dissolution, and four major diagenetic stages can be identified in study reservoir. The first stage is precocious rock early stage and shallow burial diagenesis, whose performances are that the clastic particles present a point -line contact relation and a large number of siliceous cement forms. The second stage is fracture and dissolution, which is the strongest diagenesis in this area and quartz and feldspar are generally affected then a great many crack forms, meanwhile a lot of pores appear. The third stage is the late deep burial diagenesis, whose performances are that the quartz clastic particle dissolves and early crack forms then the recrystallization happens, making the dissolution pore and fracture heal, and multiple growing increased edges form as well as a lot of siliceous cement. The fourth stage is the formation of late tectonic stress fracture, late carbonate and gypsum mineral salts. Two phases of gas inclusions are identified. The first phase of gas inclusions was formed in the late stage of early diagenesis. This period of inclusion hasa higher gas/liquid, which is generally about 20 percent, and fluoresce inclusions are seen. The formation temperatures of inclusions are between 130℃ and 140℃ and the salinity is between 1.0 wt%and 2.0 wt%. The second phase of the natural gas inclusions forming in late diagenetic later period are distributed in late fractures and the late sparry calcite and quartz cement. The organic inclusions in later sparry calcite and quartz cement are more, and the planar distribution, distributed in calcite or plagioclase cleavage plane, which are maily gaseous hydrocarbon inclusions. This inclusion’s gas/liquid ratio is small, generally less than 10% while most of them are the organic gaseous and brine forming the gas and liquid these two phase inclusion with the parcel body wall is thin and the color is shallow. Inclusion’s formation temperature is between 160℃ and 170℃ and its salinities are between 3.0 wt% and 4.0wt %. Studies suggest that the first stage represents the early gas’s primary migration and reservoir injection events, while the second stage represents the natural gas’s massive secondary migration and gas accumulation event.