Diagenesis And It's Comparative Study Of Pyroclastic Rock, Dowsonite-Bearing Sandstone And Common Sandstone In Hailaer Basin

Hailaer Basin is located in the south-west of Hulunbeier Prefecture, Inner Mongolia Autonomous Region. Beier depression and Wuerxun depression are the biggest areas that have been explored and have the most potential for further exploration. The main reservoir of Beier depression consists of pyroclastic rock of Budate Group, Xinganling Group and Tongbomiao Formation and common sandstone in Nantun Formation and Damoguai Formation. The main reservoir in Nantun Formation and Damoguai Formation of Wuerxun depression is sandstone among which the dawsonite-bearing sandstone is an important oil-gas reservoir and inorganic-origin CO₂ reservoir in Wuerxun depression. In this article I carried on the detailed description and the comparative studies of diagenetic of pyroclastic rock, common sandstone in Beier depression and dawsonite-bearing sandstone in Wuerxun depression of Hailaer Basin. Therefore not only widened our research scope in diagenesis study theoretically, moreover provided theory for actual exploration accordingly. Lithological characteristic is the basic of diagenetic study. By appraisive experiment and description of well core and thin sections from Beier depression, it is found that the main rock type of Budate Group are andesite tuff, dacite tuff, liparite-tuff, andesite and small quantity of sedimentary tuff, etc, . The main rock type in Xinganling Group are: dacite tuff-lava breccia, andisite tuff and dacite-tuff, etc, . The main rock types in Tongbomiao Formationare rhyolite tuff-breccia, dacite tuff-breccia, andesite tuff, rhyolite flood tuff, dacite flood tuff, sedimentary tufaceous and tufaceous sandstone, etc, . The 5 porosity types of pyroclastie rock are: Intergranular porosity, Intragranular porosity, filling porosity, fracture porosity and dissolution porosity. Among them dissolution porosity includes intergranular dissolution porosity, intragranular dissolution porosity( feidspar dissolution porosity and fragment dissolution porosity), filling dissolution porosity and fracture dissolution porosity, etc, The types of porosity of pyroclastic rock are generally centralized in low (IIIA) class and medium (IIB) class. Through out the study of 40 exploratory wells and 8 development wells, altogether 2450 porosity statistics in Beier depression, it is concluded that the peak value of porosity is centralized between 8% to 16% (which occupies 50% of the total). The peak value of permeability is generally centralized less than 1×10-3μm2 (which occupied 70% of the total), with the porosity property as medium-size porosity and low permeability. According to the statistical system by Gazzi-Dickinson(1979), the dawsonite-bearing sandstones in Wuerxundepression are arkose sandstone and lithic feldspathic sandstone.Lying on the basic of the statistical study of framewor composition, and applying W.R. Dickinson's triangle discriminant pattern chart which uses the composition of framewor detrital composition as the termination point to determine the provenance tectonic setting, which is, quartz extinction mode-provenance analyzing diagram QFL of QmFLt, QPLVLS, and Basu (1975) and cathode luminescence characteristic of heavy mineral and quartz extinction, I naked the following conclusion the source rock of dawsonite-bearing sandstone in Wuerxun depression was generated from the granite of Proterozoic Era of Eerguna landmass, 340 ～312 Ma subduction-collision type granite in early-medium Hercynian period or 310～250 Ma subduction type granite body. Based on the method of reservoir layer and porosity classification, and by the comprehensive study of observation of normal thin section, casting section, scanning electronic microscope and photographic analysis, we inducted the porosity of the dawsonite-bearing sandstones in Wuerxun depression into the following four types: intergranular porosity, intragranular porosity, filling porosity and dissolution porosity. I also applied photographic analyzing technique by using Laser Scanning Confocal Microscope (LSCM) for 8 samples from 2 wells Tong-6 and Su-16 in Wuerxun depression to study their porosity texture, we naked the conclusion that they belonged to medium-tiny size porosity and thin pore throat sandstone. The physical feature of reservoir layer shows as medium-low porosity with low permeability.By observation through polarizering microscope and scanning electronic microscope, we discovered that the types of diagenetic of pyroclastic rock in Beier Depression consist of diagenetic phase of compaction-recrystallization, clinkering-welding, agglutination and authigenesis (including specific jacketing clay mineral and corslet-shape authigenic quartz), devitrification, metasomatic alteration and erosion-dissolution. Each different type of diagenetic shows different diagenetic phenomenon, and therefore defines different diagenetic paragenetic sequence. In Xinganling Group and Tongbomiao Formation it can be partitioned into 4 diagenetic phases: Patch shape ankerite diagenetic phase, erosion-dissolution kaolinite diagenetic phase, clinkering-welding corslet-shape anthigenic quartz microlite diagenetic phase and adhesion-alteration ankerite diagenetic phase. The diagenetic stage in Xinganling Group is mainly Phase A of middle diagenetic stage, but the possibility of the existence of Phase B and Phase A of early diagenetic stage could not be excluded. The diagenetic stage of Tongbomiao Formation belongs to Phase A of middle diagenetic stage. The diagenetic types of dawsonite-bearing sandstone can be classified to mechanical compaction, agglutination and authigenesis, Group of anthigenic clay mineral, alteration and dissolution. The diagenetic sequence is as follows: quartz overgrowth, authigenic quartz, authigenic kaolinite, anthigenic ledikite, calcite, spathic iron, sulfite ore, sodium feldspar, potash feldspar and dawsonite. This can be clasified into 3 diagenetic combination and 4 diagenetic phase, which are: Feebleness mechanical compaction-kaolinite fill up diagenetic phase, relativelyviolence compaction-unstable component and dissolution diagenetic phase, quartz overgrowth agglutination diagenetic phase and dawsonite agglutination or alternation diagenetic phase. The diagenetic stage contains Phase A and Phase B of early diagenetic stage and Phase A in middle diagenetic stage, among which Phase B of early diagenetic stage and Phase A in middle diagenetic stage are the main diagenetic stages. Comparing pyroclastic rock with feldspathic sandstone, the compaction of pyroclastic rock not only provides similar compaction of terrigenous clastic rock, but also shows the feature of compaction and clinkering-welding. Devitrification is a special type of diagenesis of pyroclastic rock and mainly developed in shard tuff.Alteration and replacement is another special type of diagenesis of pyroclastic rock and mainly developed in tuff and tuff lava. The specific agglutination and authigenic mineral of pyroclastic rock is jacketing clay mineral (rhagas into corslet-shape authigenic quartz and illite). In erosion and dissolution, besides the dissolution of unstable components like arkosic and rock fragments, it is common to find the phenomenon of the dissolution of shard and crystallinoclastic. Considering of diagenesis paragenetic sequence, the diagenesis paragenetic sequence of pyroclastic rock includes not only agglutinate and authigenic mineral (such as patch shape calcite, adhesion-alteration calcite and dolomite etc, ), but also jacketing clay mineral, clinkering-welding, compaction and piezocrystallization, alteration and erosion and other specific diagenetic features. By the study of diagenetic phase feature, we nakedthe conclusion that the diagenetic phase of pyroclastic rock can be mainly clasified into the follow 4 postures: Patch shape ankerite diagenetic phase, erosion-dissolution kaolinite diagenetic phase, clinkering-welding corslet-shape anthigenic quartz microlite diagenetic phase and adhesion-alteration ankerite diagenetic phase. By the study of diagenesis stage we found that the variation in longitudinal direction of clay mineral in pyroclastic rock did not have a certain regularity that vary with the increasing of the depth like the clay mineral in arkosic sandstone. Besides the combination of agglutinate and authigenic mineral as normal sandstone has, the reservoir of dawsonite-bearing sandstone also has the specific combination of agglutinate and authigenic mineral: authigenic kaolinite, quartz secondary enlargement, dawsonite and ankerite. Although the high agglutination level of dawsonite-bearing sandstone greatly effected the development of reservoir porosity, the dawsonite-bearing sandstone arkosic generated violence solution and the solution of authigenic kaolinite is a common phenomenon. According to the feature of diagenetic paragenetic sequence, fluid inclusion and present group water, I preceded analysis on the evolution of diagenetic fluid of pyroclastic rock and dowsonite-bearing sandstone. During the process of diagenesis, rocks in Budate Group generally superimposed low temperature hydrotherm alteration. Diagenetic fluid can be partitioned into early acidic fluid stage and advanced alkali fluid stage in this period of the process. The diagenesis of pyroclastic rock in Tongbomiao Formation shows mechanicalimpaction at the early stage, accompanied with sedimentation of authigenic kaolinite, and then it shows filling up of ankerite spare and agglutination of patch shape ankerite. In this stage the fluid of diagenetic fluid shows alkali feature. With the deepening of diagenetic level, the feature of porosity fluid converted from alkality to acidity. Feldspar and volcanic detritus occured alteration and dissolution. Authigenic jacketing clay mineral and corslet-shape authigenic quartz microcrystallite had been formed. At the final stage the fluid converted again from acidity to alkality and adhesion ankerite developmented. The fluid in dawsonite-bearing sandstone experienced the following 4 stages sequentially, acidic fluid at the early stage, alkali fluid at the early stage, acidic fluid at the final stage and alkali fluid at the present stage.