Table of Content

25 April 2006, Volume 27 Issue 2

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Occurrences of multiple energy mineral deposits and mineralization/reservoiring system in the basin

Liu Chiyang, Zhao Hongge, Tan Chengqian, Wang Jianqiang

2006, 27(2):  131-142.  doi:10.11743/ogg20060201

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Sedimentary basin is the basic unit of the generation and occurrence of various sedimentary mineral deposits,and is also a large reservoiring/mineralization system.It has both relatively independent and unified mineralization environments and dynamic settings,and has its own mineralization characteristics and patterns.It can be named as basin reservoioring/mineralization system/unit,for differentiating from the other mineralization systems,such as orogen mineralization system.Recent mineral exploration,exploitation and research both at home and abroad show that oil,gas,coal and uranium deposits mainly occur in sedimentary basins.These four major mineral deposits commonly coexist and get enriched in the same basin.Their producing formations are closely related and their space distributions are complicated but ordered.The environments of their occurrences are intrinsically related,with reservoiring/mineralization processes and the time of reservoiring/mineralization and finalization to be the same or similar.All these features indicate that they have close intrinsic relationships and a unified geodynamic setting.The sedimentary minerals,such as oil,gas,coal and uranium,are generated and coexist in the same basin,interact with each other and separately accumulate and are finally allocated through reformation in the process of basin evolution.The main times and stages of generation-reservoiring/mineralization have evident responsive relations and close coupling relations with the main geologic events and basin evolution and reformation stages.This paper discusses some relevant frontier scientific issues and preliminarily establishes the chronogenesis and corresponding relationships between basin's evolution-reformation stages and major geologic events and the reservoiring/mineralization of oil,gas,coal and uranium deposits.

Depositional sequence and filling response characteristics of Triassic Yanchang Formation In Longdong area

Chen Hongde, Ni Xinfeng

2006, 27(2):  143-151.  doi:10.11743/ogg20060202

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Triassic Yanchang Formation in Longdong area is a super-secular cycle bounded on top and base by regional unconformities/disconformities.It can be divided into four secular cycles according to subsidiary structure,climate,provenance condition and fluctuation of lake level during the sedimentary evolutionary cycle of the sequence.Sequence filling dynamics of Yanchang Formation is studied through dynamic genesis analysis of the sequence-basin system.It is believed that the development and evolution of the 4 secular cycles would have been controlled by pulsativity of tectonic subsidence,cyclicity of climate and the resulting differences in sediment yield.The responses of sequence filling dynamics occurring during evolution of the basin have further been discussed in this paper.It is pointed out that there are different combinations of sedimentary subfacies,different types of sediments and different superimposition patterns of sequences in different evolutionary periods of the lake basin.

Evolution of Gan-Shaan paleochannel during Early and Middle Jurassic in Ordos basin

Zhao Junxing, Chen Hongde

2006, 27(2):  152-158.  doi:10.11743/ogg20060203

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The Upper Triassic Yanchang Fm underwent denudation and incision of river valley to different degrees before the deposition of Lower and Middle Jurassic in Ordos basin due to the strong tectonic uplifting du-(ring) Indosinian movement,resulting in rolling paleotopography.Five major paleo-streams,including Gan-Shaan and Qingxi,were developed in early Jurassic'of which Gan-Shaan paeo-stream was the main stream channel.In the Gan-Shaan paleo-stream drainage area, the strata from Fuxian Fm to Yan'an Fm have different sedimentary and palaeogeographic characteristics,and they altogether reflect the structural and sedimentary evolution history of the basin.

Sedimentary tectonic features of Ordos basin in Middle Jurassic Zhiluo-Anding stages

Zhao Junfeng, Liu Chiyang, Yu Lin, Liang Jiwei, Huang Lei

2006, 27(2):  159-166.  doi:10.11743/ogg20060204

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Along with the breakup of Grand North China basin in Middle Jurassic Zhiluo-Anding ages,the Zhiluo-Anding Fms were relatively completely preserved in Ordos basin,with many scattered and contemporaneously correlated strata occurring in adjacent areas.In present Ordos basin,the Zhiluo Formation is dominated by fluvial deposits,while the Anding Formation is dominated by fluvial and lacustrine sediments deposited in arid climate.The lacustrine sediments in Zhiluo-Anding Fms were mainly deposited in the palaeo-depocenter in the southeastern part of the basin,while its accumulation center was located along Wuhai-Otog QiangqiPingliang in the western part of the basin,deviating from the depocenter.The palaeotectonic framework prior to deposition of Zhiluo Fm was characterized by being high in the south and low in the north,as well as high in the east and low in the west.The palaeogeologic features in Zhiluo-Anding ages were characterized by being high in the northwest and low in the southeast,with very slight south-north differentiation.Based on comprehensive analyses of the lithology and lithofacies of equivalent layers and the timing of present tectonic units in the adjacent areas,it is believed that the primary basin in Zhiluo-Anding ages extended to the west of Helan mountain in the west,reached somewhere between east of Luliang mountain and west of Taihang mountain in the east,and was bounded by Daqing mountain in the north and Qinling mountain in the south.

Cretaceous sedimentary formation in Ordos basin

Yang Youyun

2006, 27(2):  167-172.  doi:10.11743/ogg20060205

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Ordos basin was separated from the adjacent Liupanshan and Hetao basins in Cretaceous,thus it was a separated non-equilibrium downwarped basin.The basin has a north-south axis with west-dipping axial plane and the basin-margin slope being gentle in the east and steep in the west.The center of subsidence is located in the western side of the basin,where the sedimentary thickness is obviously larger than the surrounding uplifted zone.The asymmetrical structure and evolution of sedimentary environment have not only controlled the lithology,sedimentary facies and distribution of strata in the basin,but also resulted in the basin margin to be dominated by glutinites and sandstones of alluvial fan,fluvial,delta plain and distributary channel facies,while the inner part of the basin to be dominated by sandstones and mudstones of eolian desert,delta front and lake facies.Elevation and subsidence,denudation and relief would have influenced the plane distribution of Lower Cretaceous.The thickness of residual sediment is thin in the eastern part and margin of the basin,while it is thick and completely preserved in the downwarping regions in central and western parts of the basin.The Luohe Formation is the most widely distributed,followed by Huanhehuachi Formation,while Luohandong and Jingchuan Formations have only been partially preserved in the downwarping regions in the west and in the north.

Structural division and characteristics in western edge of Ordos basin

Zhao Hongge, Liu Chiyang, Wang Feng, Wang Jianqiang

2006, 27(2):  173-179.  doi:10.11743/ogg20060206

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The geologic structures in the western edge of Ordos basin are complicated due to the long active movements of Qinling-Qilian tectonic belt and Alashan massif. Based on comprehensive analysis of gravity,aeromagnetic,remote sensing,seismic and other data,the middle sector of the western edge of Ordos basin can be divided into the northern,middle and southern zones,according to the distinct characteristics of latitudinal structural zones.The northern structural zone can be further divided into Zhuozishan and Hengshanpu segments,the middle zone can be further divided into transform zone and Majiatan segment,while the southern zone can be further divided into Shazijing north,Guyuan and Huating south segments,with each segment having remarkably different structural features.Most of the gas reserves have been discovered in the thrust blocks and anticlines in the Hengshanpu segment,while most oil are commonly distributed in Majiatan segment and in the adjacent thrust nappe structures.

Restoration of eroded thickness and its influence on thermal evolution of Upper Paleozoic source rocks in Ordos basin

Gao Shengli, Ren Zhanli

2006, 27(2):  180-186.  doi:10.11743/ogg20060207

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The eroded thickness of Mesozoic in Ordos basin is restored by using vitrinite reflectance-depth profile(R_o-H),and the thermal evolution of the Upper Paleozoic is comprehensively studied based on thermal simulation experiment of source rocks.The R_o-H profile models can be divided into 3 types:single segment,double segments and three segments.The eroded thickness in the eastern part of the basin is large,in the range from(1 400m) to 2 200m;while that in the western part is relatively small,in the range from 400m to 1 000m.Isothermal-complementary thermal evolution simulation experiments of source rocks show that the R_o increases with time,but the incremental rate decreases when heating the rock samples to a certain temperature and then cooling them;in the same time interval,the larger the temperature drop is,the lower the hydrocarbon-generating capacity would be and the hydrocarbon generating capacity would be getting very low with time when the temperature lowers to a certain value.These results show that temperature plays a key role between the time scale of heating and thermal effects of source rocks.In the eastern and middle parts of the basin,the erosion thickness is much larger than the minimum erosion thickness causing the stagnation of thermal evolution of source rocks, which indicates that the thermal evolution of source rocks in these areas would have been in stagnation.While in the western part,the thermal evolution of source rocks might have lasted slowly till the Cenozoic in the area from Bu1 to Tian1 wells where the erosion thickness is relatively thinner.

Relationship between thermal history and various energy mineral deposits in Dongsheng area,Yimeng uplift

Ren Zhanli, Zhang Sheng, Gao Shengli, Cui Junping, Liu Xinshe

2006, 27(2):  187-193.  doi:10.11743/ogg20060208

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Geothermal gradient was low in Paleozoic and Early Mesozoic in Dongsheng area,Yimeng uplift,resulting in a low level of thermal evolution of source rocks,and it increased up to 3.3℃/100m in Late Mesozoic.The Late Mesozoic tectonic thermal events and the later continuous uplifting in Dongsheng area would have resulted in the active movement of thermal fluid.As a result,the coalification of Permo-Carboniferous,Triassic and Jurassic in the study area mainly occurred during Middle Jurassic to Early Cretaceous.Gas was mainly ge-(nerated) from the Paleozoic coal measure strata in Early Cretaceous,and the humic gas have mainly migrated and dissipated since Early Cretaceous,which is also the main mineralization period of the sandstone uranium impregnations in the area.Based on analyses of tectonic evolution and hydrocarbon generation history in the study area by using vitrinite reflectance and apatite fission track data,the hydrocarbons acting as the reducing agents for enrichment of uranium can only be sourced from the Permo-Carboniferous coal measure strata in the area with relatively high maturity to the south of Yimeng uplift,because the Permo-Carboniferous and Jurassic coal measure strata in the study area are still in immature to low mature evolutionary stages.

Oil/gas reservoiring phases determined through petrographic analysis of hydrocarbon inclusions in reservoirs taking Longdong oilfield,Ordos basin,as an example

Li Rongxi, Xi Shengli, Di Lingjun

2006, 27(2):  194-199,217.  doi:10.11743/ogg20060209

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The main diagenetic types of the sandstone reservoirs in Chang-3 reservoir in Longdong oilfield,Ordos basin,include compaction,cementation,corrosion and fracturing.The diagenetic authigenic minerals are dominated by chlorite,authigenetic quartz,calcite and gypsum.Two different phases of hydrocarbon inclusions have been identified according to the formation time series of the host diagenetic minerals.The hydrocarbon inclusions formed in the first phase would have been formed during the burial diagenesis in late early diagenetic stage,and are distributed in the dissolution pores and bottom of overgrowth edge of minerals,such as quartz and feldspar,as well as in earlier fractures.They are characterized by irregularity in shape and small in size,gene-(rally) in the range of 3-8 μm,and are dominated by multiple phase hydrocarbon inclusions.The hydrocarbon inclusions formed in the second phase would have been formed during basin uplifting stage in late latediagenetic stage,and are distributed in the late-formed fractures,siliceous cement and sparry calcite cement.They gene-(rally) contain saline water,and the boundaries of oil,gas and water are clear.It is believed that the former represents the process of hydrocarbon migration and charging,while the latter represents the process of large scale oil/gas accumulation and reservoiring.Based on the temperatures of hydrocarbon inclusion formed in various phases and analyses of the sedimentary and thermal evolution history of reservoirs,the time of hydrocarbon migration and charging is determined to be about 122 Ma(Early Cretaceous),while that of large-scale accumulation and reservoiring is about 80 Ma(late Late Cretaceous).

Diagenesis and diagenetic facies of gas reservoirs in the Upper Paleozoic 8th member of Shihezi Fm and 2nd member of Shanxi Fm in Yulin-Shenmu area,Ordos basin

Liu Xiaohong, Luo Jinglan, Zhang San, Lin Tong

2006, 27(2):  200-208.  doi:10.11743/ogg20060210

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Sandstone reservoirs in the Upper Paleozoic 8th member of Shihezi Fm and 2nd member of Shanxi Fm in Yulin-Shenmu area are dominated by quartz sandstone,lithic sandstone and lithoclastic quartz sandstone,and the diagenetic types includes compaction,cementation,metasomatic alteration,recrystallization and corrosion.The sandstones have experienced diagenetic evolution processes of A and B phases during early diagenesis and A₁,A₂ and B phases during late diagenesis.Decrease of sandstone porosity is mainly caused by compaction and cementation,which contribute 22% and 15% to the loss of primary pores,respectively.Intercrystalline micropores of clay minerals in sandstone are the main reservoir space and account for more than 60% of the total porosity.Secondary pores resulted from dissolution improved the poroperm of sandstone to some extent.While diaclases contribute less in improving the poroperm of sandstone.There are 6 diagenetic facies zones with various genetic types in the study area,of which combined quartz overgrowth-cementation pore facies and authigenic clay cement dissolution pore facies are the most favorable for gas accumulation and the superimposition and reformation of the two would be the most favorable for gas reservoir development.The micropore facies filled with clay matrix and intercrystalline pore facies cemented by authigenic clay are relatively favorable diagenetic reservoir facies zones.The compacted tight facies filled with matrix-altered hydromica and the calcarinate and metasomatic tight facies are unfavorable for development of pores and gas accumulation.

Diagenesis of Chang 8₁ reservoir with low permeability in Xifeng oilfield,Ordos basin

Li Hong, Liu Yiqun, Liu Linyu

2006, 27(2):  209-217.  doi:10.11743/ogg20060211

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The Chang 8₁ sandstone reservoir in Xifeng oilfield,Ordos basin,is mainly composed of lithic arkose and feldspathic lithic sandstone,and is of typical low porosity and low permeability reservoir with an average porosity of 8.32% and average permeability of 1.33×10^-3 μm².The reservoir has experienced diagenesis,metasomasis,dissolution and cataclasis,and is now in the late diagenetic phase A to B.The mechanical compaction in the early diagenetic phase and the ferruginous carbonate cementation in the late diagenetic phase are the main contributors to the poor poroperm characteristics.According to the characteristics of diagenesis,the braided ri-(ver) delta front sandbody in the study area can be divided into 4 diagenetic facies belts,including compaction and pressolution,weak compaction-carbonate cementation,weak compaction-chlorite-film cementation,and chlorite-film cementation-feldspar corrosion.Sandstones in the latter two facies belts in the core of the main sandbody have good poroperm characteristics and oil-bearing properties,thus they are favorable for hydrocarbon accumulation.

Geochemical behaviors of oil and oil-source correlation in Yanchang Yan'an Formations in Ansai-Fuxian area,Ordos basin

Guo Yanqin, Li Wenhou, Chen Quanhong, Cao Hongxia, Zhang Daofeng

2006, 27(2):  218-224.  doi:10.11743/ogg20060212

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Crude oil analyses,such as group compositions,carbon isotopes and gas chromatograms of saturated hydrocarbon,as well as characteristics of terpane,sterane and other biomarkers,indicate that crude oil in Yanchang-Yan'an Formations in Ansai-Fuxian area have similar geochemical behaviors and are of low-mature-mature oil.The content of saturated hydrocarbon is over 70%,and the carbon isotope value is relatively light with δ¹³C commonly lower than-31‰.The distribution of normal alkane is complete and appears as single-peak type,with the main peak carbons to be C₁₇-C₂₃.The biomarkers of the oil are characterized by the followings:the content of pentacyclic terpane is larger than that of tricyclic terpane,with C₃₀ hopane and C₂₉ norhopane as the dominant components,and gammacerane as the minor component;sterane is dominated by C₂₇ cholestane and C₂₈ ergostane and C₂₉ sitostane;the homosteranes appear to be in asymmetrical"V"distribution,with relatively low C₂₈ sterane content and about the same C₂₇ and C₂₉ sterane contents.Analyses of geochemical beha-(viors) and correlation of oil and source rocks show that the semideep-deep lake facies sapropelic-mixed type mudstones that occur between Chang 8 and Chang 4+5 reservoirs are the main source rocks.The characteristics of biomarkers reveal that the oil has a certain migration effects.

Geochemical behavior of geologic process of natural gas during mineralization of Dongsheng sandstone-type uranium deposit

Wu Bolin, Wang Jianqiang, Liu Chiyang, Wang Feiyu

2006, 27(2):  225-232.  doi:10.11743/ogg20060213

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Observation and testing of gas inclusion,fluid temperature,mineralogy and geochemical behaviors of minerals from epigenetic alteration,tracing of inclusion's hydrogen and oxygen stable isotopes,and correlation of oil/gas charging and uranium mineralization phases indicate that the bleaching phenomena and green epigenetic alteration,as well as uranium mineralization in Dongsheng deposit are mainly related with geologic process of natural gas.The natural gas is mainly of humic gas from the Upper Paleozoic.The main reducing agents for uranium mineralization are organic hydrocarbon gas,such as CH₄,and some CO,H₂S and H₂.Therefore,the genesis of Dongsheng Uranium deposit is mainly related with geologic process of natural gas.

Effects of hydrocarbon migration and dissipation in later reformation of a basin:formation of Mesozoic bleached sandstone in northeastern Ordos basin

Ma Yanping, Liu Chiyang, Wang Jianqiang, Zhao Junfeng, Fang Jianjun, Gui Xiaojun, Yu Lin

2006, 27(2):  233-238,243.  doi:10.11743/ogg20060214

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Study on bleaching of sandstone of different ages in the world indicates that the formation of bleached sandstone is relevant to organic fluid flows.It can provide some clues to hydrocarbon migration,accumulation and dissipation,thus can be used as an evidence for predicting the existence or destruction of oil/gas reservoirs(or fossil oil/gas reservoirs) in nearby areas.The occurrence of oil and gas in Ordos basin would have been altered during the later reconstruction of the basin.Based on analysis of the major elements,rare-earth elements and petrologic characteristics of Mesozoic bleached sandstone in the northeastern Ordos basin,it is preliminarily believed that the bleached sandstone in the study area would have been formed in an acidic and reducing environment.The bleached sandstone is characterized by its relatively high content of kaolinite and pyrite,and the color of ferric oxide minerals on the edge is lighter than that in the center.The major element analysis of sandstone samples shows that they have relatively high Al₂O₃ content and low Fe³⁺/Fe²⁺ ratio;the REE analysis shows that the Eu contents in both core and outcrop samples are negative;all these indicate a reducing environment.At the same time,the geochemical characteristics of oil sand in the northern part of the basin indicate that the oil is exactly condensate as a result of the migration and dissipation of Permo-Carboniferous humic gas in the south to the surface.In a word,the comprehensive study suggests that the bleaching of Mesozoic sandstone in the study area indicates dissipation of natural gas.

Quantitative division of flow units in Panguliang Chang 6 reservoir,Jing'an oilfield

Shi Chengen, Xie Wei, Sun Wei, Liang Xiaowei, Yan Lin

2006, 27(2):  239-243.  doi:10.11743/ogg20060215

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Taking Panguliang Chang 6 reservoir in Jing'an oilfield as an example,six parameters,including reservoir thickness,porosity,permeability,shale content,flow-zone index,and the product of reservoir thickness and porosity,are selected for quantitative division of flow units,based on comprehensive quantitative analysis and evaluation of sedimentary microfacies and reservoir.Panguliang Chang 6 reservoir is finely divided into 15 sublayers,and all the sandbodies can be grouped into 4 types of flow units,including E,G, M and P,by using high resolution sequence stratigraphy.The results show that the bottom of Panguliang Chang 6₁ and the top of Panguliang Chang 6₂ are the major pay zones of Panguliang Chang 6 reservoir in the study area,and all the flow-unit types are well correlated with lithologies,physical properties,sedimentary microfacies and productivity of the reservoir.The selected values of parameters,comprehensive evaluation functions and indexes for the flow unit division can truly and objectively reflect the geologic features of low permeability and low porosity reservoir,such as poor poroperm characteristics and strong heterogeneity,and can meet the accuracy requirements of flow unit evaluation and division for this kind of reservoir.

Types of original sedimentary basins and tectonic evolution on southeastern margin of North China plate

Yu Hezhong, Han Shouhua, Xie Jinlong, Guo Qingxin, Wu Jinyun

2006, 27(2):  244-252.  doi:10.11743/ogg20060216

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The types of original sedimentary basins on southeastern margin of North China plate were controlled by tectonic evolution. North China and Yangtze plates experienced the changes from"divergence"to"convergence"in Paleozoic-Middle Triassic. The southeastern margin of North China plate correspondingly experienced the changes from passive continental margin in Sinian-Middle Ordovician(Z-O₂)to uplifting and denudation in Late Ordovician to Early Carboniferous (O₃-C₁). During late Early Carboniferous-Middle Triassic (C₂-P₂), foreland flysch depositional basins were formed on the continental margin due to convergence of plates. During Late Triassic ?Early Jurassic, Qinling-Dabie orogenic zone was formed through the collision of the two plates, coupled with the formation of molasses sedimentary basins. During Middle Jurassic-Early Cretaceous, the margin of the orogenic belt detached while the epiorogenic subsidence occurred within the orogenic belt, resulting in a complicated structural pattern with outstretching on the trailing edge and thrusting on the leading dege. Outstretched fault-depression and down-warped basins were formed later under the control of the activities of Palaeo-Pacific plate and Pacific plate.

Characteristics of maturity and gas generating yield of source rocks in thermal simulation experiment with lowering temperature

Hu Guoyi, Li Zhenduo, Li Zhisheng, Li Jian, Luo Xia, Wang Chunyi

2006, 27(2):  253-257.  doi:10.11743/ogg20060217

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Thermal simulation experiments with lowering temperature and extending heating time have been performed on 3 coal samples with low maturity (R₀=0.29%), normal maturity (R₀=0.66%) and high maturity (R₀=1.71%), respectively, to study the change of maturity and gas generating yield of source rocks led by extending heating time while lowering the temperature. The vitrinite reflectance (R₀) of source rocks still can increase if the heating time is extended with lowering temperature to a certain point after heating. The increment is in the range of 0.15%-0.40%. The rates of increase of R₀ value vary at different time interval at the same temperature. It is relatively large at the initial stage, but then gradually lowers with extending heating time. Gas generating yield of source rocks measured through extending heating time with lowering temperature is relatively low at the initial stage, but the cumulative gas generating yield increases gradually along with the extension of heating time. Just like R₀value, the rate of gas generation is larger at the initial stage but then lowers with extending heating time. The experiment results show that hydrocarbon generation can go on after the temperature has been lowered to a certain point after heating if the heating time is extended, but the rate of gas generation would decrease gradually.

Neogene hydrocarbon reservoiring in slope zone of high sah'ent in Jiyang depression and exploration practice

Lu Guoming, Liu Kuiyuan

2006, 27(2):  258-262.  doi:10.11743/ogg20060218

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Slope zone of the high salient in Jiyang depression occurs structurally as a monocline uplifting toward the high salient, and is often neglected because it is usually treated as a pathway for hydrocarbon migration toward the main body of the high salient. Being influenced by the basement palaeotopography with alternation of gullies and ridges and changes of Neogene sedimentary facies, five types of small but prolific secondary reservoirs tend to occur in the slope zone of the high salient, i. e. stratigraphic overlapped oil reservoir, reversed roof fault-block oil reservoir, small and low-amplitude secondary oil reservoir, small and low-amplitude buried hill drape oil reservoir and lithologic oil reservoir. Hydrocarbon migration pathways have relatively large influences on these reservoirs. Most of the reservoirs with unconformities as migration pathways are of relatively large, layered, stratigraphic overlapped oil reservoirs, characterized by zonal distribution with narrow oil-bearing zone. While reservoirs with inherited active boundary faults as migration pathways are characterized by long oil-bearing intervals, multiple oil-bearing series and complicated oil-water relationship. Suitable exploration methods and strategies should be taken for different types of reservoirs in the slope zone.

A study on reservoir damage during waterflooding in Yan10 Reservoir of North-Ⅲ block, Maling oilfield

Zhu Yushuang, Li Qingyin, Wang Xiaomeng

2006, 27(2):  263-268.  doi:10.11743/ogg20060219

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In YanlO reservoir of North-Ⅲ block, Maling oilfield, the clay minerals are dominated by illite, mixed illite/smectite, kaolinite and chlorite. The potential damages to the reservoir are water and velocity sensitivities caused by illite, mixed illite/smectite and kaolinite. Based on reservoir damage study by using real sandstone model in the study area, the following understandings are achieved. (1) The velocity sensitivity of reservoir is weak in the study area, and the critical velocity is in positive correlation to permeability. The reservoir damage caused by velocity sensitivity is actually stronger than the experimental results, as the salinity of injection water is lower than that of the formation water used in the experiment. (2) The reservoir in the study area is characterized by weak-to-moderate water sensitivity, and the existence of K⁺ and the increase of injection water salinity may alleviate reservoir damage from water sensitivity. The injection water salinity would be appropriate in the range from 5 to 10 g/L. (3) The oil displacement efficiency with water sensitivity damage is 11.35% lower than that without water sensitivity damage, and the overall displacement efficiency also remarkably lowers, resulting in a lower recovery ratio.

Identification and description of complicated small fault-block traps in Subei basin

Mu Rong

2006, 27(2):  269-274.  doi:10.11743/ogg20060220

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Exploration of complicated small fault blocks has always been a hard nut to crack in the mature oil regions in eastern China. In view of the complicated surface and subsurface characteristics of the small fault blocks in Subei basin, the idea of integrated seismic processing and interpretation is adopted and full 3D data processing method is applied to improve the imaging quality of complicated fault blocks. Taking Gaoyou sag in Subei basin as an example, detailed interpretation of faults, study of structural patterns and mode-guided analysis of complex structures are performed in structural interpretation. Full 3D data interpretation methods, such as 3D spatial horizon calibration, coherence volume, 3D visualization and variable velocity mapping, are used in the identification and description of the complicated small fault-block traps to improve precision. The desired results have been achieved in practical application.

A further discussion on concept and method of making oil/gas trap exploration decision

Wang Chuan, Peng Suping

2006, 27(2):  275-280.  doi:10.11743/ogg20060221

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In view of petroleum exploration would usually be made by stages, petroleum exploration stage can be divided into basin selection, play selection and trap or prospect selection three decision phases, each with different characters, goals, principles and methods. Prospect exploration decision is to make a short-term and phasic tactical decision, without considering earlier exploration expenditures and later development expenditures, so as to avoid falling into previous redundant circumstances without operability. Prospect exploration decision-making takes selection of the best traps or prospects as the goal, and takes obtaining maximum reserves with minimum risk and cost as the principle. If exploration investment is given, a batch of prospects with the lowest exploration risk and largest reserves will be screened out from the selected prospects through determination of the exploration investment limits, comprehensive ranking of the prospects and analysis of exploratory drilling risk of the prospects. Alternatively, if a reserves target is given, prospects with the lowest exploration risk and exploration cost will be selected as the next drilling targets. Ten prospects that are located in Mianzhu-Yanting central uplift zone, Xiongpo fault-folded belt in Longquanshan area, Yazihe-Pingluoba buried fault-folded belt and Shi-fang-Qionglai central synclinal zone are selected as the drilling targets in current exploration phase, by using the concept and method of prospect exploration decision-making and in combination with the actual data in the western depression of Sichuan basin.