Characteristics and Mechanism of Permian Shanxi Tight Reservoir of Changbei Gas Field，Ordos Basin

Key words: 
• tight reservoir /
• mechanism /
• diagenisis /
• Ordos Basin /
• Shanxi Formation

Abstract: Changbei gas field is located in Yishan Slope of the northern center of the Ordos Basin, China. The main pay zone reservoir is Upper Paleozoic Lower Permian Shanxi P1S2 quartz arenite, which deposited in distributary channel of delta plain. The reservoir is low porosity low permeability tight reservoir with average porosity of 5.2% and average permeability of 0.7×103 μm2in a current depth of 2 700~2 950 m. Integrated study on depositional environment, lithology, diagenesis and tectonic background has been conducted. The reasons cause this low porosity low permeability tight reservoir features are considered as the following two factors. (1) On one hand, the reservoir was deposited in a coaly bearing acid environment of deltaic distributary channel, the original pore water is acid water with non saturated calcium carbonate, which can not form earlier cementation within the sandstone reservoir. However, compaction has significantly reduce the original pore and pore volume. On the other hand, this acid diagenesis environment cause large amounts of secondary quartz cementation fill the pores left out during compaction of sandstone reservoir. This is the main reasons cause the low porosity low permeability tight reservoir features in the region. Observation from thin section indicate that calcium carbonate cementation is not developed in the pore of sandstone reservoir especially in the earlier stage, the average component of calcium carbonate cementation is 1%~3%， authigenic kaolinite cementation is well developed with an average component of 7%~10%，illite segmentation is also less developed <3%，secondary quartz cementation well develop, mostly reach Ⅱ—Ⅲ level with an average component of 5%~10%，some layer can reach 10%~18% and maximum is 23%，this phenomenon indicated the acid diagenesis environment of P1S2 quartz arenite reservoir in Changbei gas field. (2) Tectonic study indicated that Changbei gas field is located in Ordos Basin tectonically stable at the time when Lower Permian Shanxi Formation was deposited. No tectonic movement and significant structure activities developed so that the formation layers are flat and rare faults and folds seen in the Changbei field. Buried history analysis also indicated that there is no apparently uplift and subsidence after P1S2 Shanxi Formation deposited. Underground water especially organic acid and CO2 acid water is not active, and pore is reduced by compaction and silicon dioxide cementation, acid water can't effectively dissolve the reservoir and displacement of materials in the pore. On the other hand, there are few dissolvable components e.g. feldspar (<3%), lithic and carbonate cementation in the rock, hence limited amount of secondary pores generated, this is the second reason that cause poor quality reservoir in Changbei gas field.