The Diagenesis Of Pyroclastic Rock And The Effect Of The Injection Of CO₂ On Reservoir In Tanan Depression Of Tamuchage Basin

Pyroclastic rock are a kind of transitional rocks between lavas and sedimentary rocks. As more and more unconventional oil and gas fields are being discovered and developed, volcanic rock reservoirs are becoming increasingly important. This paper take Tanan area Tamuchage Basin for example,studying on the diagenesis of Pyroclastic rock,the reservoir characters and their controlling factors and the effect of the injection of CO₂ on reservoir.Routine thin sections identification,observation of casting thin sections, observation of SEM,image analysis and X-ray diffraction were adopted on the study.Through stating skeleton clastic,the rock types of tongbomiao formation are mainly of tuffaceous sandstone and tuffite,secondary of tuff and sandstone. the rock types of nantun formation is mainly of sandstone and tuffaceous sandstone.The diagenesis characteristics of pyroclastic rock from Tongbomiao Formation and Nantun Formation is investigated by ordinary thin-section, casting thin-section identification and SEM analysis,combining with the data collected. The types of diagenesis include compaction , devitrification , alteration , cementation ,clay minerals,erosion and dissolution;According to the symbiosis of authigenic minerals in,determine the diagenetic paragenesis is alteration,devitrification and authigenic quartz microlite,authigenic clay mineral coating- compaction - smectite - illite,chlorite - analcime - dissolution,kaolinite -ferrocalcite-dawsonite-ankerite. The diagenetic facies can be devideed into piezocrystallization , early weak compaction - authigenic clay mineral coating diagenetic facie;strong compaction dissolution-kaolinite,silica cementation,authigenic quartz microlite,quartz secondary overgrowth diagenetic facie;dissolution-albite-dolomite cementation。Acording to vitrinite reflectance, rock maxima pyrolysis temperature and authigenic clay mineral association, the diagenetic phase of sandstones from Tongbomiao Formation and Nantun Formation is mostly in middle diagenetic A phase.Authigenetic mineral sequences are authigenic quartz microlite,albite,ferro-carbonate mineral and dawsonite.The primary type of reservoir space is secondary pore, and the secondary is original pore. There are different kinds of pore based on the different types of rocks. The whole and solution intergranular pore, and solution intragranular pore are well developed in the common sandstone. Besides the assemblage of remaining intergranular pore, filling inner pore, crack pore, solution pore, there is the assemblage of composite pore, original pore and secondary pore which are well developed in the tuffaceous glutenite. In the tuffaceous sandstone and sedimentary tuff, the original pore and secondary pore are well developed. Tuff is mainly to secondary pore.Based on the analysis of the mercury penetration data, the grade of the reservoir pore structure is mainly to fewer poor in Tongbomiao Formation. The other is poor. The grade of the reservoir pore structure is mainly to fewer poor in Nantun Formation, the other is better. The grade of the Nantun formation is better than the Tongbomiao Formation's. The reservoir pore structure is closely related to the type of rock. The Tongbomiao Formation which is poor in the reservoir pore structure is mainly to athrogenic rock. The Nantun Formation which is better in the reservoir pore structure is mainly to terrigenous clastic rocks.According to Sneider Plate, the original porosity of Tongbomiao Formation is 34%, and the Nantun Formation's is 33%. The surface porosity of Tanan depression reservoir is 2.05-31.47%, and the proportion of secondary pore is 2.08-100%. Based on the depth of Nantun Formation to Tongbomiao Formation, 1542-2532m, the diagenesis stage belongs to subage of type A in the middle stage. There are three stages during the decrease of original porosity. From 1500m to 1700m, the remaining intergranular pore is less than 5%, and the compaction plays an important role in the decrease of original porosity. Form 1700m to 1784m, the remaining intergranular pore is about 10%, and the compaction and cementation play the same role in the decrease of original porosity. From 1784m to 2532m, the remaining intergranular pore is less than 10%, and the cementation play an important role in the decrease of original porosity.The porosity and permeability of reservoir indicate that the Tongbomiao Formation is extra middle-low porosity, extra low permeability.With the changes of burial depth, there are two abnormal high pore belts in Tongbomiao Formation to Nantun Formation of Tanan depression, 1850m-2100m and 2400-2650m. The first abnormal high porosity belts can be discovered in Well Ta19-24, Well Ta19-32, Well Ta19-30, Well Ta19-25, and Well Ta19-45. The second abnormal high porosity belt can be discovered in Well Ta19-42, Well Ta19-49.Regression analysis is proceeded on porosity and depth, and we get the equations of regressionφ=-0.008x+28.666. Regression analysis is proceeded on porosity and depth,the content of carbonate, the equation of regressionφ=29.86-0.008*h-0.248*CC(least square method),φis gained. The porosity will decrease 2.48%, when the content of carbonate increases 10%.The chief controlling factors of reservoir porosity and permeability are the component of detrital material, the sedimentary environment and the diagenesis. In the same depth, the porosity and permeability of pyroclastic rock are better than the common sandstone's. The porosity of the fan-delta front, fan-delta plain and shore-shallow lacustrine is better. The compaction plays a key role in the decrease of porosity. The albitization benefits the formation of secondary pore. The coating of grain constrains the development of the overgrowth quartz that conserves the partial original pores. Based on the observation of thin section, the dawsonite often replace the feldspar crystalloclastic in the athrogenic rock, and there are only a little dawsonite which fill pore. So the injecting of CO₂ is not remarkable during the process of reservoir porosity change in athrogenic rock.