Diagenetic Characteristics Of Clay Minerals In Volcaniclastic Rock

This paper systematically described the main types of rocks and clay minerals of Xing'anling Group and Tongbomiao formation in Hailar basin as well as the regional geological setting and analyzed the change of clay minerals with depth,the characteristics of clay mineral assemblages in vertical and horizontal and the genesis and diagenetic stage of clay minerals as well as their effect on container rocks. In the research, we observed the slices of the rocks by polarized light microscope and found that the rocks in Xing'anling Group were mainly sand flow-breccia, eruptive breccia, andesitic tuff, dacitic tuff, tuffaceous siltstone, tuffaceous mudstone and mudstone and that the rocks in Tongbomiao formation are mainly sand flow-breccia, andesitic tuff-breccia, andesitic tuff, rhyolitic ignimbrite, dacitic ignimbrite, rhyolite,sedimentary tuff and tuffaceous sandstone. The forms of clay minerals were observed through scanning electronic microscope.Smectite was pasted on the surface of a grain or the breastwork of a hole and associated with chilkinite as honeycomb complex. Under scanning electronic microscope, the chilkinite was hairlike or slice-like complex and was distributed extensively. The chilkinite was often pasted on the surface of a grain or the breastwork of a hole with bridge among grains.The smectite, chilkinite, ball pyrite and calcite were associated with each other. The chlorite was less observed in this area. The volcaniclastics and volcanic materials were preserved in the original shape but chlorited. The kaolinite often filled the holes between grains with a few on the surfaces of grains and their aggregate were in the shapes of book and helminth. Some of the feldspare were alterated into kaolinite and the kaolinite was metasomatized by iron dolomite. There were printed page self-reproducing kaolinite on the surface of armour self-reproducing quartze.And the kaolinite was connected in pieces. The kaolinite, iron dolomite and chilkinite were aggregated together. The characteristics of clay mineral contents were analyzed through X-ray diffracttion analysis. The relative average contents of minerals in sand grade rocks were gotten for the Xing'anling Group. They were as follows: smectite 5.23%(the highest 47%), chilkinite 25.13%(the highest 76%), kaolinite 21.16%(the highest 68%),chlorite 22.94%(the highest 83%), chilkinite and smectite in combination 4.06%(the highest 14%), smectite and chlorite in combination 21.48%(the highest 98%).At 1878m, kaolinite begun to decrease and chilkinite begun to increase obviously but other clay minerals changed in little.The relative average contents of minerals in mud grade rocks were gotten for the Tongbomiao formation. They were as follows: smectite 12.55%(the highest 92%), smectite and chlorite in combination 1.95%(the highest 30%), chlorite 16.23%(the highest 36%), chilkinite and smectite in combination 13.86%(the highest 49%), kaolinite 7.18%(the highest 52%).At about 1890m, smectite and kaolinite begun to disappear one by one but chilkinite and smectite in combination layer begun to increase slowly, which showed that the smectite and kaolinite begun to change into mixed-layer clay at this depth. Chilkinite and chlorite changed a little showing that the smectite and kaolinite did not change to extremity clay minerals at this depth. The relative average contents of minerals in sand grade rocks were gotten for the Tongbomiao formation. They were as follows: smectite 25.86%(the highest 70%), chilkinite 14.93%(the highest 60%), kaolinite 32.5%(the highest 87%), chlorite 17.86%(the highest 75%), chilkinite and smectite in combination 3.14%(the highest 50%), smectite and chlorite in combination 5.71%(the highest 64%). The content of kaolinite did not change obviously with depth but smectite begun to disappear one by one at 1820m. Smectite and chilkinite in combination layer begun to appear at 1600m and begun to increase slowly at 1815m. At the same time, chlorite content begun to increase obviousely at the depth showing that smectite begun to change intosmectite and chilkinite in combination layer with a larger speed.At 1890m, smectite diappeared wholly. The graph of clay content change with deoth in mud grade and sand grade rocks showed that smectite and kaolinite decreased with depth but chlorite and chilkinite increased with deoth. The distribution characteristics and assemblage types of clay minerals are analyzed for the studied area.There were 7 kinds of clay mineral assemblages in the mud grade rocks of the Xing'anling Group. The clay minerals from depression were mainly chilkinite and chlorite but their contents were different at different well area. Chilkinite was highest at Bei19 well reaching over 75%. Chlorite was highest at Bei2 well reaching more than 80%.In horizon, kaolinite begun to increase from southwest to northeast and it reached 40% at Bei18 well. There were 7 kinds of clay mineral assemblages in the sand grade rocks of the Xing'anling Group. In the northwest part of the depression, the clay mineals were mainly chilkinite reaching more than 80%(the highest) at Bei19 well.From northwest to southeast, chilkinite became less and less but chlorite increased one by one at the same time reaching 75%(the highest) at Bei36 well with the emergence of chilkinite and smectite in complex.There were 6 kinds of clay mineral assemblages in the mud grade rocks of the Tongbomiao formation.In the west and middle parts of the depression, the clay minerals were mainlychilkinite reaching 75%(the highest) at Bei34 well.From northwest to southeast, chilkinite became less and less but smectite increased one by one.In the southeast of the depression, the clay minerals were mainly smectite reaching 80% and mixed clay minerals were very little. Kaolinite increased from the southwest to northeast reaching 40% at the end.There were 7 kinds of clay mineral assemblages in the sand grade rocks.In the north part of the depression,arrounding Bei12 well, kaolinite content was higher than that in other areas reaching 75%.In the middle of the depression, kaolinite became less and smectite and chlorite increased quickly reaching 75% at Bei13 well. At the same time, smectite and chlorite occurred together. Chilkinite were distributed in every part of the depression with the highest content of 50%. The clay minerals in mud grade rocks of the Xing'anling Group were mainly chlorite and chilkinite and the mineral assemblage was mainly chlorite and chilkinite complex with little combination occurrence of the both.But The clay minerals in sand grade rocks of the Xing'anling Group were mainly kaolinite, chlorite and chilkinite and the mineral assemblage was mainly kaolinite, chlorite and chilkinite complex with obvious combination occurrence of them.The clay minerals in mud grade rocks of the Tongbomiao formation were mainly chilkinite with a little kaolinite and the mineral assemblagewas mainly chilkinite complex.But the clay minerals in sand grade rocks of the Tongbomiao formation were mainly kaolinite, chlorite and smectite or chlorite with a little chilkinite and the mineral assemblage was mainly kaolinite complex. According to the analysis on the clay mineral assemblages of the Xing'anling Group and the Tongbomiao formation, it was clear that there were more chlorite in the mud grade rocks of the Xing'anling Group but there were more combination occurrence of clay minerals in the Tongbomiao formation. The content of chilkinite was higher and the content of kaolinite was lower.But in the sand grade rocks, there were more smectite, chlorite and the combination occurrence of the both.However, there were more chilkinites in the Xing'anling Group and there were more kaolinites and little chilkinite in the Tongbomiao formation, which showed that the Tongbomiao formation had a better characteristics of oil and gas collecting. Kaolinites and smectite in the Xing'anling Group became less with the increase of depth but chilkinite, chlorite and combination occurrence of the both became more commen. The aim of comparing the clay mineral assemblages with that in Songliao basin chosing depth as a index was to find the differences of clay assemblages between volcaniclastic rocks and non-volcaniclastic rocks.Chosing Qijia-Gulong depression in themiddle depression of the Songliao basin and Beir depression as comparing partners,the typical clay minerals in mudstone and sandstone were compared.The combination occurrence of two kinds clay minerals happened in a larger depth in the Beir depression not only for mud grade rocks but also for sand grade rocks. The smectites in the Beir depression were more than that in Qijia-Gulong depression and when smectites diappeared in the Qijia-Gulong depression at a depth of 2100m but there were some smectites in the Beir depression. As a matter of fact, there were still some smectites at a depth of 3000m in the Beir depression. The origin of this was that there were more materials in volcaniclastic rocks for the formation of smectite in the Beir deprerssion.The smectites were mainly formed through the hydrolysis of volcanic materials during the early stage of deposition and diagenesis.However, there were little materials for the formation of smectitesin non-volcaniclastic rocks. The smectites in the strata were mainly formed through the hydrolysis of volcanic materials during the early stage of deposition and diagenesis and the smectites changed into chilkinite and chlorite whith the increasing of depth.The chlorites and chilkinites were the products of smectites and kaolinite during the diagenesis stage.With the increasing of depth, smectites and kaolinite became less and less but chilkinite-smectite assemblage, chilkinite-chlorite assemblageas well as chilkinite and chlorite became more and more. The kaolinites were formed through the reaction of desaturated acid water with feldspar or other aluminium silicate. Or the kaolinites were formed from the degeneracy of smectites.If the medium in holes in the sedimentary rocks was alkaline, the kaolinites were not stable and might change into chilkinite and chlorite. The clay mineral assemblages in the studied area were mainly chilkinite-smectite complex and smectite-chlorite complex which were the middle products from smectites to chlorites. The smectite-chlorite complex occurred at a depth of 1697m and the chilkinite-smectite complex occurred at a depth of 1876m,which showed that smectites begun to change into chilkinites at 1697m. Using vitrinite reflectance, cryptogam color index, the highest temperature of decomposition by heat and in accordance on analysis on the clay mineral assemblages as well as the content of smectites in chilkinite-smectite assemblage, the diagenesis stages of the Xing'anling Group and Tongbomiao formation were determined and they were in stage A. The results were consistent with the diagenesis stage division in the profession standards of The People's Republic of China(Economy and Trade department of China,2003).However, if taking clay as division standards, there were some diffenceces between the mineral assemblage determined and that guided in the profession...