| Hailaer-Tamtsag Basin is the biggest petroliferous basin in the peripheryof Songliao basin, and also a key area in the exploration and development ofDaqing peripheral oilfield. Wuerxun and Beier depressions are two with abigger exploration area and a higher exploration degree in Hailaer basin;Tanan depression is the biggest in petroleum generation in Tamtsag Basin,with prodigious oil-gas exploration and development potential.Hailaer-Tamtsag Basin has the characteristics in multiple and nearby provenance, providing sediments to the basin from volcanism and normalweathering. In different areas within the basin, the proportion or advantage ofsediments from volcanism and normal weathering are different, thus differentlithological associations and textures had formed. Such difference becamemore diversified and complicated due to diagenesis, resulting in lithologicalcomplexity. Moreover, such complexity has seriously restricted theunderstanding to the four characteristics of the reservoir, which can be mainlyconcluded as “three difficult issues”, i.e. lithology identification difficulty,reservoir parameter obtaining difficulty and fluid property identificationdifficulty. Thus the accuracy of the aspects including lithologicalinterpretation, reservoir type classification and oil-bearing ability predictionwould be difficult to achieve effective improvement. On the other way round,logging has provided us the logging geophysical information related topetrographic composition and texture, while such information may be explored in targeted study. The exploration of such information can help us tosolve the inadaptability between rock and logging response; meanwhile, toprovide effective support for us to deeply understand the petrologymechanism of volcaniclastic rock lithology and its variation.The rock type was identified as per core description and thin sectionanalysis. Volcanic sedimentation analysis was carried out by rock types andgrain size analysis, to explore the accumulation of volcaniclastic sediments.The study on sedimentary microfacies is a basic work for the single wellfacies analysis to obtain the reliable results of core sedimentary facies. Theprovenance-sedimentary system and lithology division was identifiedcombining with heavy minerals, sand factor, paleocurrent analysis andlithology division. Moreover, the diagenesis type, characteristics andsequence of volcaniclastic rock were identified by thin section analysis,scanning electron microscope (SEM) and X diffraction analysis, and the stage classification standard on diagenesis of study area was then further identifiedby combining with the maturity of organic matter, the highest pyrolysistemperature and paleogeotemperature. Under the basic framework ofdiagenesis stage, the factors which had influence to volcanic glassdevitrification and its behavior during the diagenesis stage were deeplydiscussed via electron probe, general thin section analysis and SEM analysis.The inadaptable rock types were identified by logging response analysis onlithology, and the factors influencing rock logging response wascomprehensive considered; meanwhile, under the background of equivalentmixed-layer minerals of illite and smectite and the similar tuffaceous contentbelow the silt particle grade, the study method of the cause of bigger CECdifference was explored, and the influence of tuffaceous composition to rockelectronic conductivity was identified. On such basis, the rock samples withclose electronic resistivity and bigger CEC difference were sought to study the cause of the closed electronic resistivity of rock under bigger CECdifference, and the influence of the occurrence state of tuffaceous below thesilt particle grade to rock electronic resistivity was identified. Basing on thediagenesis change of tuffaceous and combining with the electronic conductivemechanism of tuffaceous, the logging response mode of volcaniclastic rockwas identified. Rock classification of volcaniclastic rock from the view oflogging response still could not reach the satisfactory results, thus thepartition and layered logging response mode analysis has become inevitable,and the study suggests that such analysis mode can further distinguish somelithologies from others. On the basis of pore type analysis, the partition andlayered pressured-mercury testing analysis was carried out; and combiningwith productivity results, reservoir types were classified, logging reservoirclassification plate is established. Targeting at the corresponding relationbetween reservoir and lithology, the difference among the some reservoirs with the same lithology was analyzed, and the cause was also explored, so asto lay a lithologic and sedimentary foundation for the identification of highquality reservoirs.I. Volcanic-sedimentation (accumulation)The lithology of Tongbomiao and Nantun formations is a set of rocksbetween normal terrigenous clastic sedimentary rocks and volcanic lava fromcore observation and thin section analysis. The rock types are volcanic lava,pyroclastic lava, welded pyroclastic rock (ignimbrite), pyroclastic rock, sedvolcanic clastic rock, volcanic sedimentary rock, normal sedimentary rock.The lithofacies zonation involved with volcanism was influenced bypaleo geographic partition. There were obvious volcanic fingerprints insedimentary type, reflecting special transportation-sedimentary medium,which presented as the mixed accumulation of volcanic clasts and normalterrigenous clasts in volcanic sedimentary area. Moreover, in the sedimentary belts (including volcanic belt) from alluvial fan to lakes, specialaccumulations with volcanic characteristics formed. The special deposition ofvolcanic clasts mainly displays in: volcanic clast channel, pyroclastic (fan)delta and deepwater volcanic ash sedimentation, etc. The pyroclasticsedimentation in alluvial plain mainly include three categories, respectively (1)the sheet flow caused by hot base surge in alluvial fan;(2) beach sand in theabandoned channel of braided river caused by hot base surge; and (3)marginal bank in meandering stream caused by hot base surge. Thepyroclastic sedimentation in delta plain mainly represent in theinterdistributary channel and natural levee caused by hot base surge; and thedelta front mainly include underwater channel caused by hot base surge andhot pyroclastic flow, the mouth bar caused by hot base surge and hotpyroclastic flow, and the distal bar caused by hot base surge. The pyroclasticsedimentation of deepwater environment mainly includes mudstone of still water caused by airfall, turbidity and underwater fan sediment caused by hotbase surge.The provenance-sedimentary system can be identified effectively bysedimentary facies, heavy minerals, sand factor, paleocurrent analysis andlithology divisions, moreover, the distribution of the characteristic lithologyand the structure units’ position can be presented by lithology divisions.Wuerxun depression can be divided into three single lithologic areas and amixed area. Three single lithologic areas are sedimentary rock area withmetamorphic rock clastics, pyroclastic rock area and pyroclastic sedimentaryrock area. The mixed area can be divided into two sub mixed areas from thecontents and components of clastics in rocks: the area dominated with normalsedimentary clastic and the area dominated with pyroclastic. Each singlelithologic area corresponds with each structural unit. The sedimentary rocksarea with metamorphic rocks clastics corresponds to Wuxi fault terrace belt, the pyroclastic rocks area correspond to Bayantala structural belt, and thepyroclastic sedimentary rocks area is a part of Wudong arc structural belt.Beier depression can be divided into two single lithologic areas and a mixedarea. Two single lithologic areas are normal sedimentary rocks area andpyroclastic rocks area. The normal sedimentary rocks area corresponds toSunainuoer structural belt, the pyroclastic rocks area corresponds to Sudeerstructural belt. Tanan depression can be divided into three single lithologicareas and a mixed areas. Three single lithologic areas are pyroclastic rocksarea, pyroclastic sedimentary rocks area and normal sedimentary rocks area.The pyroclastic rocks area corresponds to East fault terrace belt, thepyroclastic sedimentary rocks area corresponds to the north of West buriedhill fault belt, and the normal sedimentary rocks area corresponds to the southof West buried hill fault belt. II. DiagenesisThin section analysis and SEM analysis show that the diagenesis types ofthis area include welding, mechanical filtration, compaction-pressure solution,devitrification, recrystalization, cementation, authigenic mineraltransformation, corrosion and dissolution. Of which, welding, devitrification,and the corrosion and dissolution of tuffaceous are the typical diagenesistypes of pyroclastic rocks.As per diagenetic symbiotic relationship embodied in diagenesis, thecombination of diagenetic symbiosis of this area mainly include fourcategories, respectively:(1) authigenic white mica and chlorite;(2)dissolution of quartz and siliceous cementation;(3) smectite, illite andchlorite; and (4) zeolite, authigenic quartz and authigenic feldspar.Based on the occurrence sequence of various diagenesis, diageneticsequence can be classified as welding stage, mechanical filtration stage, devitrification stage, tuffaceous corrosion and dissolution stage, clay minerallayer-mixing stage, authigenic white mica stage, zeolite cementation stage,particle strong cementation stage and ankerite development stage. On suchbasis and combining with vitrinite reflectance (Ro), maturity of organic matter,the highest pyrolysis temperature of organic matter and parageneticassociation of minerals, It is believed that the lower Cretaceous of the studyarea is from eogenetic B phase to telogenetic B phase, mainly in telogenetic Aphase in this paper.There are many factors influencing the devitrification of volcanic glass,including temperature, pressure, Ph value, Eh value, diagenetic fluid andauthigenic constituent structure, etc. The extensively developed acid volcanicglass of the rocks in study area was taken as the key studying object, wecarried out in-depth analysis on the devitrification of acid volcanic glass byvirtue of SEM and electronic probe. The result shows that the sequence of crystallization of mineral in the process of devitrification of volcanic glass is,in fact, from the high to low of the oxygen depolarization, and itsdevitrification behavior can be classified into5stages, respectively:1)hydration phase;2) desilication phase;3) dealumination phase;4)Na-rich/Ka-rich phase;5) final crystallization phase. Such five phases are notthe ones that must be undergone by vitroclastic in the process of alteration.Subjected to the temperature, pressure, Ph, Eh and fluid nature, volcanic glassmay experience several phases, for example, the chemical corrosion anddissolution of tuffaceous may result in the direct dissolution of volcanic glass.III. Logging responseResistivity is the important parameter for well logging interpretation, theconductivity condition of tuffaceous of the study area can not obtain theuniform cognition in well logging interpretation, i.e. not only high resistance tuffaceous exist, but also low resistance tuffaceous exist. For identifying theconductivity ability of tuffaceous, in this study we focus on the rocks withhigh, medium and low tuffaceous content to do electrochemical experiments,including the CEC (cation exchange capacity) and resistivity of rock. Thisstudy suggests that the mechanical and electrical properties of the volcanicglass with Na (or Ka) on the surfaces would have obvious difference from theinside. Since the potassium-sodium ions are apt to move, the hydrogen ions inwater on the surface of vitroclastic shall have ion exchange with sodium(potassium) ion of glass, to generate sodium (potassium) hydroxide or sodium(potassium) carbonate solution, which shall adsorb to the surface ofvitroclastic to form solution film. The sodium ion (potassium ion) in solutionfilm has high transfer ability, and such positive ions can have ion exchangewith external aqueous solution, thus they have high CEC.By systematical microscopic sections analysis and relevant experiments, the petrologic mechanism of special lithology of a series of logging responsewas defined, which including high natural gamma sandstone, high resistancemudstone, low resistance tuff and sandstones that have different relationshipbetween CNL and DEN, and so on. Take high natural gamma sandstone andlow resistance tuff for instance, the factors affecting sandstones with highnatural gamma are tuffaceous matter, clay, feldspar, calcium and oil. Hightuffaceous matter led to the enrichment of thorium, high feldspar and clay tothe enrichment of uranium, thorium and potassium, oil-bearing sandstone touranium. Calcareous by influencing the migration and precipitation ofuranium to change the gamma value of sandstone, high calcareous to thedecrease of uranium. Low resistance tuff has relationship to the volcanic glassgenerated by explosive volcanic eruption, anagenetic volcanic glass have highpotassium ion and sodium ion, and have high CEC, meanwhile, they presentlow resistance. Whatever, the additive electro conductibility generated by clay mineralization of volcanic glass can also make the rock resistivity reduce. Thestudy on petrologic mechanism of these special lithology has enhanced theaccuracy of understanding to the four characteristics of part reservoir inpractical production.Well logging lithology identification research shows that when utilizingdeep lateral logging resistivity value and density to have crossplot withneutron cross value XD-N, rocks can be classified into five types: andesite,volcanic breccia, tuffaceous high resistance conglomerate, mudstone, andother lithology. And then when making the crossplot with gamma and soundwave, other lithology can be classified into (tuffaceous) conglomerate,(tuffaceous) sandstone and tuff class. The layered and partition lithology-welllogging response mode can effectively distinguish tuff (tuff and sedimentarytuff), tuffaceous conglomerate, tuffaceous sandstone, common conglomerate,common sandstone. And the classification system of pyroclastic rocks was established, which corresponds to petrology and the standard of well logginggeophysical.IV. Reservoir ClassificationThe observation on cast thin section shows that the porosity type in thestudied area can be divided into three types: primary porosity, secondaryporosity and mix porosity. Secondary porosity is dominated in the three types.Secondary pores are mainly dissolution intergranular porosity, dissolutionintragranular porosity, and dissolution inter-fillings porosity. Diagenesis hadinfluence to porosity. Compaction and cementation would obviously reduceporosity, while corrosion and dissolution would increase porosity. Thereservoir can be classified into four categories by the pressured-mercury data,for class I reservoir, the drainage pressure is below0.063MPa, radius meanvalue above3μm, porosity at16-21%and permeability above60μm2,belonging to the best reservoir; for class IIA reservoir, the drainage pressure is at0.063-0.485MPa, radius mean value at0.50-3μm, porosity at10-19%andpermeability at0.6-80μm2, belonging to the general reservoir; for class IIBreservoir, the drainage pressure is at0.2-2MPa, radius mean value at0.1-0.5μm, porosity at6-14%and permeability at0.06-0.8μm2, belonging tobad reservoir; for class III reservoir, the drainage pressure is above2MPa,radius mean value below0.1μm, porosity at3-14%and permeability below0.5μm2, belonging to very bad reservoir. It can be found from the oil testingconclusion of Daqing oil field that class I reservoir has natural high yield;class IIA has high yield after fracturing; class IIB has low yield afterfracturing; and class III has no yield after fracturing.The study on reservoir porosity characteristics shows that the porosity ofclass I reservoir are mainly dissolution intergranular porosity, the pore-throatconnectivity is good, clay minerals are mainly kaolinite and illite; kaolinite ismainly scattered in the pores; illite mainly filled in lining. The porosity of class IIA reservoir are mainly intergranular porosity and dissolutionintragranular porosity, pore-throat connectivity is relatively good, clayminerals mainly is illite; illites are filled in pores in crosslink form oradsorbed to skeleton particle surface in lining form. The intergranular porosityin class IIB reservoir is less, the quantity of intragranular pores and interstitialmaterial pores have increased with poor connectivity; clay minerals mainly isillite and filled in pores in crosslink form; secondary quartz has extensivelydeveloped in granules. For class III reservoir, effective pores almost can notbe found in cast thin sections.After study, it is found that acoustic wave, density and neutron loggingcurve can be used as the main measures for classifying the categories ofreservoir. Due to compaction influence, acoustic wave curve is not suitable todirectly reflect reservoir physical property characteristics, density curve canreflect reservoir porosity characteristics, and XD-Ncan indirectly reflect shale content. Therefore, density curve and XD-Ncan be based on to indirectlyassess the fair or foul of physical property connectivity of reservoir and carryout reservoir category judgment. For class I reservoir, DEN value isequivalent to or below2.43g/cm~3, CNL value at12-27%, and XD-Nvalue at-7-10. For class II reservoir, DEN value is at2.43-2.54g/cm~3, CNL value at6-24%, and XD-Nvalue at-10-6.5. For class III reservoir, DEN value is above2.54g/cm~3, CNL value at7-23%, and XD-Nvalue at-15-0.The statistics on the lithology of four reservoirs show that variouslithologies exist in such four reservoirs, coarse fraction has better physicalproperty as compared to fine fraction. The same lithology may occur indifferent reservoir category, of which, conglomerate is most typical, whichexists from class I to class III reservoir with content no less than9%. It isrelated to the formation environment of conglomerate, and the conglomerateformed under different environments has great difference in composition, granularity and separation. |
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