Table of Content

28 June 2011, Volume 32 Issue 3

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Differences in reservoir forming between porous and fractured gas pools in carbonates,the northeastern Sichuan Basin

Guo Tonglou

2011, 32(3):  311-317，326.  doi:10.11743/ogg20110301

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Abstract: There are two types of gas pools in the Upper PermianLower Triassic in northwestern Sichuan Basin,namely porous and fractured gas pools.They show obvious differences in bitumen content and chemical compositions of gas.Geological,geochemical and basin modeling methods are used to analyze their gas sources,carrier systems and hydrocarbon charging history as well as their influences on hydrocarbon accumulation.The carrier system of poroustype gas pools is composed of faults and porous reservoirs and its formation is in good timing with peak hydrocarbon generation of source rocks,leading to the formation of paleooil/gas pools.During subsequent deep burial,the paleooil pools became gas pools due to pyrolysis.In the same time,H₂S and CO₂ were generated because of thermochemical sulfate reduction between hydrocarbon gases and sulfates in pore fluids.In contrast,the carrier system of fracturedtype gas pools consists mainly of faults and fractures and their formation is later than the peak hydrocarbon generation but is good timing with the late stage of hydrocarbon generation.Therefore,only pyrolysis gas from kerogen accumulate in the reservoirs.Later deep burial of source rocks and tectonic evolution are primary causes for the differences between these two types of gas pools.

Hydrocarbon accumulation patterns of Moxizhuang oilfield,the Junggar Basin

2011, 32(3):  318-326.  doi:10.11743/ogg20110302

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Abstract: This paper discusses hydrocarbon accumulation patterns of Moxizhuang oilfield of the Junggar Basin based on a comprehensive analysis of oilwater relationship,fault occurrence and hydrocarbon accumulation background.The reservoir of J₁s²⁽²⁾(the lower second member of the Jurassic Sangonghe Formation) in Moxizhuang oilfield has a uniform oilwater system with a normal instead of inverted oilwater relationship.Seismic attributes of intervals with welldeveloped coal beds reveals that the previously interpreted major faults are artifacts resulted from seismic reflection of the coal seams,and no large faults are developed in the Sangonghe Formation,no faultsealing trap conditions exist.Hydrocarbon accumulation conditions and distribution characteristics are different inJ₁s²⁽¹⁾ (the upper second member of the Jurassic Sangonghe Formation) and J₁s²⁽²⁾ reservoirs.The J₁s²⁽¹⁾ reservoirs are developed in underwater distributary channel sandbodies of meandering river deltaic front facies,with the favorable reservoirs laterally being in strip distribution.In contrast,the J₁s²⁽²⁾ reservoirs are developed in distributary channel sandbodies of braided river delta facies,with the favorable reservoirs laterally in continuous distribution.The Moxizhuang oilfield is a structurallithologic reservoir that is controlled by both structure and sedimentary microfacies and is still in the posttilting dynamic adjusting process.There exists a uniform oilwater contact in J₁s²⁽²⁾ with occurrence the same as that of the updipping sandbody.

New understandings of hydrocarbon accumulation in Penglai 193 oilfield,the Bohai waters

2011, 32(3):  327-332.  doi:10.11743/ogg20110303

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  Abstract: As the biggest oilfield discovered offshore China so far，the Penglai 19-3 oilfield has aroused constant debates on oil sources and characteristics of massivescale hydrocarbon accumulation in the late stage.From this study，we have brought up with three new understandings.First，oil in the Penglai 19-3 oilfield mainly originates from the source rocks in the Shahejie Formation and partially from source rocks in the Dongying Formation (in the Bozhong sag).Hydrocarbons charged reservoirs from two apparent directions，i.e.,the NW direction (from the Bozhong sag) and the SE direction (from the Miaoxi sag).The favorable factors for the development of the Penglai 19-3 oilfield are extremely abundant oil source in the Bozhong sag，longexisting lowpotential energy area and the energy release from the TanLu Fault as well as favorable carrier system dominated by unconformities.Third，The significant differences of interwell temperature，change patterns of oil density，viscosity and gases and largescale existence of “gas chimney” rooted in ocean floor reveal that this oilfield is obviously characterized by abundant hydrocarbon sources and late hydrocarbon accumulation.Oil charging is still intensive in the northern part of the field at present.All leads to the conclusion that the Penglai 19- oilfield is a typical massivescale，rapid hydrocarbon accumulation in the late stage in Bohai Sea. Key words： biomarker compounds,TanLu Fault,gas chimney,Neogene oil pool,hydrocarbon accumulation in the late stage,Penglai 19-3 oilfield,Bohai Sea New understandings of hydrocarbon accumulation in Penglai 19-3 oilfield,the Bohai waters Guo Yonghua,Zhou Xinhuai,Ling Yanxi,Li Jianping,Wang Feilong and Wang Jun (Exploration & Production Research Institute,Bohai Oilfield,CNOOC Tianjin Company,Tianjin 300452,China) Abstract: As the biggest oilfield discovered offshore China so far，the Penglai 19-3 oilfield has aroused constant debates on oil sources and characteristics of massivescale hydrocarbon accumulation in the late stage.From this study，we have brought up with three new understandings.First，oil in the Penglai 193 oilfield mainly originates from the source rocks in the Shahejie Formation and partially from source rocks in the Dongying Formation (in the Bozhong sag).Hydrocarbons charged reservoirs from two apparent directions，i.e.,the NW direction (from the Bozhong sag) and the SE direction (from the Miaoxi sag).The favorable factors for the development of the Penglai 19-3 oilfield are extremely abundant oil source in the Bozhong sag，longexisting lowpotential energy area and the energy release from the TanLu Fault as well as favorable carrier system dominated by unconformities.Third，The significant differences of interwell temperature，change patterns of oil density，viscosity and gases and largescale existence of “gas chimney” rooted in ocean floor reveal that this oilfield is obviously characterized by abundant hydrocarbon sources and late hydrocarbon accumulation.Oil charging is still intensive in the northern part of the field at present.All leads to the conclusion that the Penglai 19-3 oilfield is a typical massivescale，rapid hydrocarbon accumulation in the late stage in Bohai Sea.

Hydrocarbon accumulation stages and their timing in multicyclical superimposed basins —a case study of the Upper Paleozoic in the eastern Linqing Depression,the Bohai Bay Basin

2011, 32(3):  333-341.  doi:10.11743/ogg20110304

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Abstract:Complexity of hydrocarbon accumulation process is a major factor influencing petroleum exploration in a multicyclical superimposed basin.Modeling of hydrocarbon generation in the PermoCarboniferoussource rocks in the Linqing Depression,the Bohai Bay Basin reveals that four stages of hydrocarbon generation occurred in the middlelate Triassic,the late Cretaceous,the Paleogene,and the NeogeneQuaternary,respectively.The volume of hydrocarbons generated in the first stage was very small and that generated in the second stage is insignificant too.The peak hydrocarbon generation occurred in the Cenozoic.Systematic analyses of fluid inclusions from 76 samples of six wells show that three stages of hydrocarbon accumulations dominated by gas charging events occurred in the late Jurassicearlier Cretaceous (141.9-95.7Ma),the Paleogene (51.0-33.1Ma),and the NeogeneQuaternary (19.7-0Ma),respectively.All the hydrocarbon charging events initiated during the depression of the superimposed basin and terminated during its uplifting.The hydrocarbons charged in the first stage were almost completely destroyed and turned into bitumen,while the hydrocarbons charged in the second and third stages might accumulate in the Upper Paleozoic stratigraphicstructural traps in the central sags compartmentalized by the NWtrending faults.These traps might be favorable exploration targets.

Hydrocarbon accumulation conditions and distribution patterns in the Liaodong Bay Depression, the Bohai Sea

2011, 32(3):  342-351.  doi:10.11743/ogg20110305

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Abstract:A study on hydrocarbon accumulation conditions in the Liaodong Bay depression is performed based on analysis of source rock，reservoirs，reservoirseal assemblage，traps and carrier systems.The results show that there are three sets of source rocks，including the 3rd and 1st member of the Shahejie Formation and the 3rd member of the Dongying Formation.Among them，the 3rd member of the Shahejie Formation is the major source rock.Both basement and clastic reservoirs exist and are of high quality.The main types of sourcereservoircap rock assemblages include indigenous generation and accumulation,，lowergeneration and upperaccumulation and generationinthenew and accumulationintheold.Faults dominate the carrier system and connect source rocks and unconformities which are separate in time and space，forming a faultunconformity carrier system.Traps were mainly developed during the deposition of the Paleogene Shehejie and Dongying Formations.They are mostly syndepositional structural traps and are rarely pure lithologic traps.Horizontally，oil and gas pools distribute along the TanLu Fault zone and are featured by zonation in NS direction and segmentation in EW direction.Low salient structures are rich in oil and gas.Vertically，oil/gasbearing layers become younger and shallower from north to south.

Characteristics of hydrocarbon accumulation in the Paleogene Kongdian Formation of Bamianhe area,the Jiyang Depression

2011, 32(3):  352-359.  doi:10.11743/ogg20110306

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Abstract:The Kongdian Formation in the Bamianhe area has necessary conditions for oil and gas accumulation,such as reservoirs,traps and sealing conditions,but it lacks hydrocarbon generation conditions.Oil in the Kongdian Formation of the Bamianhe area is characterized by low density,low viscosity,low sulfate content,high wax content and high freezing point and can correlate with that in the Kongdian Formation of the Wangjiagang area in the Dongying depression.According to the contrast of geochemical indicators,such as the nature of crude oil,the constitution of component and the biomarker between the Kongdian Formation and Shahejie Formation of the Bamianhe area,we believe that crude oil sourced from the lower 4^th member of Shahejie Formation ( Es⁴) and the 2^nd member of the Kongdian Formation (Ek²) of the Dongying depression migrated to the Bamianhe area along unconformities and favorable passageways which consisted of the Northwest palaeostructure crest and channel sand body.Therefore,a variety of reservoir types were formed with the majority of overlapping unconformity stratigraphic reservoirs and structural reservoirs.Controlled by structural setting,carrier system,reservoir distribution,trap distribution and fault activities,oil/gas in the Kongdian Formation are mainly distributed in the southwestern part of the nose structure zone horizontally and in the lower part of the Kongdian Formation vertically which are within 80 m of distance from the unconformity.These oil pools are featured by insufficient charging and small scale.

Distribution patterns and controlling factors of volcanic gas pools in the Changling fault depression,the Songliao Basin

2011, 32(3):  360-367.  doi:10.11743/ogg20110307

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Abstract: Based on a comprehensive analysis of gas accumulation conditions and temporalspatial combinations of accumulation elements,in this paper we make a study of enrichment pattern and main controlling factors of volcanic gas reservoirs in the Changling fault depression.The results show that gas pools are mainly distributed in the Yaoyingtai and Da’erhan structures in the low salient of central sag and the Shuanglong structure in the eastern slope belt.Volcanic tuff of explosive phase and rhyolite of overflow phase in the Yingcheng Formation are the most favorable reservoirs.The major controlling factors for gas accumulations are as follows:1)distribution of deep source rocks controls the oil and gas distribution in a macro way;2)volcanic rocks and their favorable reservoir facies control the development and distribution of gas pools;3)discordogenic faults are important conditions for the development of oil/gas pools;4)quality cap rocks and lateral seals determine the vertical distribution of gas;5)the early charged hydrocarbons get mixed with latergenerated CO2 when basement faults have good conductivity,leading to the formation of mixed gas pools with both organic and inorganic origins;6)favorable targets are compound traps formed by largescale inherent palaeohighs(palaeoslope),onlapping pinchouts and volcanic rocks

New understandings of the Upper Paleozoic sedimentary facies and gas accumulation patterns in Daniudi gas field

2011, 32(3):  368-374.  doi:10.11743/ogg20110308

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Abstract:By using sedimentary facies analysis method that unifies macroscopic geologic knowledge and microcopic geologic evidences,we conduct the analysis,interpretation and correlation of logging data and observation of cores more than 400 wells.It is thought that the sedimentary facies of the Paleozoic Taiyuan Formation in the Daniudi gas field is debris coastalcarbonate platform and delta in a regional epicontinental sea setting where delta distributary channel and tidal sand bar are deposited.The sedimentary facies of the 1^st member of the Lower Shihezi Formation is lakebraided river delta in a paleostructuresedimentary background with high relief where distributary channel and interchannel microfacies are developed.These sedimentary facies and sand distribution features determine the worsening of reservoir quality and productivity of sandbodies in the 2^nd member of the Taiyuan Formation from northeast to southwest.The plays with high production potential mainly occur in the northcentral part of NNEtrending sandbody in the study area.The chance is bad for them to occur in the 1^st member of the Shihezi Formation in the study area.Plays with moderate production potential distribute in the northcentral part of the study area.

Heterogeneity characterization of fluvial facies reservoirs through integration of seismic and logging data —an example from Bozhong263 oilfield in the Huanghekou Sag, the Bohai Bay Basin

2011, 32(3):  375-381.  doi:10.11743/ogg20110309

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Abstract: Strong heterogeneity exists in multistage sandstone reservoirs of underwater distributary channel microfacies of shallow water delta in the lower Minghuazhen Formation in the Bozhong263 oilfield,Bohai Bay Basin.In this paper,we quantitatively describe the plane heterogeneity of reservoirs through integration of seismic and logging data.The quantitative description of heterogeneity is then to be taken into consideration in geologic modeling and well emplacement,so as to reduce reserve risk,confirm the resources,optimize the location of development wells,direct the composing of Overall Development Plan and make other decisions.Petrophysical analysis indicates that the pseudoimpedance can differentiate the variety of sandstone and shale perfectly.The amplitude and coherence information extracted from the impedance volume can be used to identify the plane variety of lithofacies and the distribution of abandoned channel,point bar,and small faults.They also can be used to constrain the variety of plane permeability during petrophysical modeling.Some virtual faults can be introduced to reflect the interior disconnectivity of sand body.Besides,by using the spectral decomposition technique deriving from short time window Fourier transformation and the absorbing attenuation phenomenon in differential frequency spectrum,this paper studies the latteral variety of reservoir,quantitatively calculates their thickness.Hydrocarbonbearing sand body is successfully detected in this way.

Control of paleogeomorphology on reservoir distribution in Block 4 of Tahe oilfield Wang Ping，Yuan Xiangchun，Li Jianglong，Hu Xiangyang and Xie Lihui

2011, 32(3):  382-387.  doi:10.11743/ogg20110310

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Abstract: Prediction of carbonate reservoirs is challenging all over the world.The reservoir spaces of the Ordovician carbonates in Block 4 of Tahe oilfield are dominated by dissolved vug and fracture systems developed from karstification and tectonic movement.These reservoirs feature in deep burial and high heterogeneity.Fine structural interpretation with 3D seismic data reveals that hydrocarbon accumulation is controlled by vertical and horizontal fault distribution,and the dissolved vugs are well associated with faults.Restoration of the Early Hercynian karst paleogeomorphology on the top of MiddleLower Ordovician in Tahe oilfield indicates that the distribution of the Ordovician buriedhill reservoirs in Block 4 of Tahe oilfield is mainly controlled by the karst paleogeomorphology.The areas favorable for the development of fracturedvuggy reservoirs are delineated.Comprehensive analysis shows that the lowlying areas of the ancient terrain should be avoided in the latter development plan refinement,and parameters for fine reservoir description should be adjusted according to the palaeogeomorphology characteristics.Paleogeomorphology restoration lays a basis for reservoir plan refinement and effective development.This method is helpfull for the study of other similar blocks and the discovery of highly productive Ordovician reservoirs in Tahe oilfield.

Sedimentary facies of the Shanxi Formation in Daniudi gas field,northeastern Ordos area

2011, 32(3):  388-396.  doi:10.11743/ogg20110311

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Abstract:Based on interpretation and correlation of logging data over 300 wells and in combination with rock assemblage,sedimentary structure,profile structure,electrofacies combinations,and the regional geological settings of the Late Carboniferous〖KG-*4〗-Early Permian carton basin in North China,this paper analyzes the lithologic association and distribution characteristics of the Shanxi Formation by using the sedimentary facies analysis technique that unifies macroscopic and microscopic geological evidence.The first member of Shanxi Formation was deposited in an epicontinental environment as evidenced by marine fossils.The study area experienced marine transgression,leading to the deposition of shore delta systems.The second member of Shanxi Formation was deposited during the transition from epicontinental sea to intracontinental depression,when the subduction of Manchuria and Mongolia oceanic crust to the North China plate intensified and the tilting of the northern part of the plate continued,leading to the almost complete regression of sea water from the study area and the deposition of offshore delta system.The first member of Shanxi Formation is dominated by epicontinental sea delta front deposits and its sedimentary facies mainly include underwater distributary channel,bay between underwater distributary channels and peat swamps.In contrast,the second member is dominated by delta plain deposits and its sedimentary facies mainly include distributary channel and bay between distributary channels.The underwater distributary channel and the lower distributary channel are favorable reservoir facies.

Geologic origin of the internal seismic reflection of marine strata in the Lower Yangtze region

2011, 32(3):  397-403.  doi:10.11743/ogg20110312

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Abstract: Data quality of internal seismic reflection in marine strata varies greatly in different places in the Lower Yangtze area.Based on analysis of drilling data,sedimentation,diagenesis and strata velocity,we think that there is too strong diagenesis in the Lower Paleozoic to form any evident velocity interface and hence there are no obvious seismic reflections.While in the CarboniferousPermian of the Lower Paleozoic,there are several evident velocity interfaces to form intense seismic reflections.Residual condition of the CarboniferousPermian and structural deformation are the main factors to control the internal seismic reflection of the marine strata.There are continuous seismic interfaces due to weak structural deformation in parts of southern Yellow Sea and discontinuous seismic interfaces due to great structural deformation in the land of Jiangsu and Anhui provinces.But there is chaotic or jumble seismic reflection in areas without the CarboniferousPermian strata.The areas with the CarboniferousPermian strata and weaker structural deformation are the best places to do seismic survey.

Seismic lithofacies interpretation of delta deposits in the Linnan subsag

2011, 32(3):  404-410.  doi:10.11743/ogg20110313

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Abstract: Turbidite in the third member of Shahejie Formation in Linnan subsag features in small,size,large number and wide distribution.It is hard to fully predict turbidites,because the sandbodies with ambiguous seismic features could be easily missed in reservoir prediction when taking sand group as unit.Based on the principles of isochronism in sequence and sand group and diachronism in sedimentary isochronfacies,this paper establishes the recognition modes of lithofacies with ternary information including geologic facies,well log facies and seismic facies.By studying the structure,thickness and seismic attribute of the reservoir based on the unit of sequence,lithofacies and sandgroups individually,a threestep method is developed for the prediction of the turbidite reservoir in the unit of lithofacies: defining palaeotopography according to thirdorder sequence,determining plays according to lithofacies,and delineating reservoir distribution according to sand groups.The method has been successfully applied in Lunnan sag: four kinds of contact models between the delta front and prodelta subfacies were established,i.e.distal sandbarmudstone,distal sandbarturbidite,mouth barmudstone,and mouth barturbidite.The shape of multiple prodelta subfacies is defined,and the favorable turbidite reservoirs are predicted,laying a reliable basis for turbidite evaluation and exploration deployment in Linnan subsag.

Controlling factors and distribution patterns of lithologic pools in the fluvial facies of the 3^rd and 4^th members of the Quantou Formation in the Songliao Basin

2011, 32(3):  411-419.  doi:10.11743/ogg20110314

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Abstract: Based on data from drilling,logging,seismic and lab,we make a study of the Fuyang oilbearing interval in Songliao Basin by using theories of continental sequence stratigraphy,sedimentary geology and hydrocarbon accumulation.The Fuyang oilbearing interval in Songliao Basin is a lowporosity and lowpermeability reservoir of fluvial facies.The overlapping mudstone of lacustrine facies in the 1^st member of the Cretaceous Qingshankou Formation is both source rock and regional seal,and therefore hydrocarbons generated in the upper source rocks migrated downward and accumulated in the lower reservoirs.Oilsource correlation shows that average distance of downward vertical migration of hydrocarbons is 300 m with the longest distance of 500 m.At the end of the Mingshui Formation deposition,stress field inversion resulted in various faults cutting through source rocks and the Fuyang oilbearing interval and many fractures inside the source rocks and reservoirs.At the same time,source rocks reached the peak of hydrocarbon generation.Driven by tectonic stress and overpressure due to hydrocarbon generation,the hydrocarbons migrated downward along the faults and fractures and accumulated in the Fuyang oilbearing interval.Hence the major period for hydrocarbon accumulation is the late period of the Mingshui Formation deposition.With favorable reservoir quality,meandering river point bars are major reservoirs for oil and gas.The existence of mudstone seals of shallow lacustrine facies leads to the formation of two sets of oilbearing intervals.Oil pools mainly developed in the Fuyang oilbearing interval which was close to the oil source and have a uniform oilwater contact in the basin.High mature source rocks in the 1^st member of the Qingshankou Formation determine that oil pools are mainly distributed in the central depression zone.Relatively low nettogross ratio leads to the predominance of lithlogic oil pools in the Fuyang oilbearing interval.

Bitumen impregnation patterns of the Lower Ordovician Penglaiba Formation in Well Lunshen-2 in Lungu area,the Tarim Basin

2011, 32(3):  420-429.  doi:10.11743/ogg20110315

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Abstract: Bitumen impregnations is common in cores of the Lower Ordovician Penglaiba Formation of Well Lunshen-2 in Lungu area,the Tarim Basin.Through detailed observation of two cored intervals in the Formation,it is inferred that there exist six types of bitumen impregnation patterns,i.e.lamination impregnation,fissure and lamination composite impregnation,fracture impregnation,vug impregnation,irregular stylolite impregnation,and pullapart space impregnation.Different types of bitumen impregnations have different origins and hydrocarbon accumulation significances.It is indicated that the Penglaiba Formation experienced hydrocarbon charging and the different types of bitumen impregnation were caused by the differences of space types for hydrocarbon enrichment.The recognition of bitumen impregnation patterns and analysis of their relationship with reservoir genesis provide a geological basis for fine characterization of the dolomite reservoirs in the study area.Study shows that the silicate in the Penglaiba Formation is the main factor leading to the increase of drilling time and the previously predicted “catenular” dolomites are not favorable reservoirs.

Restoration of thermal history of the PermoCarboniferous basement in the Songliao Basin

2011, 32(3):  430-439.  doi:10.11743/ogg20110316

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Abstract: Vitrinite reflectance of the PermoCarboniferous in the north of Songliao Basin is generally high,usually more than 30%,and varies greatly in different tectonic units.The curve of vitrinite reflectance(Ro)depth(H)of the PermoCarboniferous basement in Songliao Basin has two types,which means that they have different thermal history.One is a twosection type.Vitrinite reflectance values change abruptly across the unconformity on top of the PermoCarboniferous.The Ro values and paleotemperature of the PermoCarboniferous strata under the unconformity are obviously higher than that of the overlying strata,indicating that the maximum paleotemperature of PermoCarboniferous strata had been reached before the deposition of the Early Cretaceous Denglouku Formation.The sedimentary strata deposited since the Cretaceous have no influences on the improvement of the PermoCarboniferous paleotemperature and thermal maturity.The other is a pseudolinear type.The Ro values and paleotemperature vary successively across the unconformity on top of the PermoCarboniferous strata,which shows that maximum paleotemperature of the PermoCarboniferous strata had been reached betwen the Late Cretaceous and Oligocene.The sedimentary strata since the Cretaceous contribute to the improvement of PermoCarboniferous paleotemperature and thermal maturity.Based on comprehensive analysis on vitrinite reflectance,fluid inclusion,fissiontrack age and so on,paleotemperature and thermal history of Songliao Basin is restored,and the relation between basin thermal history and hydrocarbon generation is analyzed.

Structure features and its evolution of Lunnan low uplift, the Tarim Basin

2011, 32(3):  440-447.  doi:10.11743/ogg20110317

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Abstract:Lunnan low uplift,located in Tabei uplift of the Tarim Basin,is a huge complex play fairway with multiple oilbearing layer series and large hydrocarbon reserves.Lunnan low uplift has undergone multistage complex tectonic movement since its formation.According to the fine interpretation of twodimensional seismic lines and the reflectance signature of the truncation and onlap on seismic profiles,this paper identifies 11 important unconformities and six major faults including Luntai,Lunnan,Sangtamu,etc.These faults were mainly thrusts at early stage,and they kept active in the Late HercynianIndosinian period and their activity peaked during the Permian.This is different from the previous suggestion that the faults were developed in the Late CaledonianEarly Hercynian.Tectonic inversion occurred at the end of Paleogene,and this is consistent with previous studies.Tectonic evolution profiles show that the study area mainly goes through five stages of tectonic evolution.Among these 5 stages,the Late HercynianIndosinian was a period when intense compression,uplifting and erosion occurred and exerted great effect on the features of the study area.While in the Himalayan period,strata of the study area were tilted and changed from previously “low in the south and high in the north” to nowadays “high in the south and low in the north” due to the rapid wedging of Indian Plate into Eurasian Plate,and the features of the study area were finalized.

Source rock characteristics and potential plays of marine sequences in North and Northeast China

2011, 32(3):  448-458.  doi:10.11743/ogg20110318

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Abstract: Based on outcrop observations and core sample analysis,we make a preliminary study of the distribution of quality source rocks in the Middle and Upper Proterozoic in northern Hebei province,the Lower Cambrian in the sorthern margin of North China and the Middle Ordovician in southwestern margin of Ordos Basin. Quality source rocks in the Middle and Upper Proterozoic in northern Hebei province are mainly distributed in the lower part of the Hongsuizhuang Formation in the Mesoproterozoic Jixian System;quality source rocks in the Lower Cambrian in the northern margin of North China are mainly distributed in the base of the Xinji Formation(or the Yuhuatai Formation);quality source rocks in the Middle Ordovician in southwestern margin of Ordos Basin are mainly distributed in the lower part of the Pingliang Formation. They are all dominated by black mudstone/shale. We think that carbonates in the Ordovician Majiagou Formation in intra North China platform is poor source rocks as a whole. The southern part of North China is better for the development of coalrelated source rocks in the PermoCarboniferous than Ordos Basin and Bohai Bay Basin,since the former has the coalbearing Upper and Lower Shihezi formations but the latter ones do not have. Considering of metamorphism,we examine the Upper Paleozoic in Northeast China and conclude that source rocks without metamorphism are mainly distributed in the middle and western parts of Erlian Basin and the southeastern part of the Songliao Basin. In conclusion,we think that 1)southwestern margin of Ordos Basin is favorable to explore conventional oil/gas;2)southern part of North China is favorable to discover coalderived oil/gas in the Upper Paleozoic;3)the Middle and Upper Proterozoic in northern Hebei province and the Upper Paleozoic in Northeast China are frontiers that are worthy of further research and exploration.

Petroleum exploration potential of the Ordovician in the Sichuan Basin

2011, 32(3):  459-471.  doi:10.11743/ogg20110319

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Abstract:Based on a detailed comparative study on reservoir petrology and fluid flow characteristics of waterrock between paleohigh and slope in the Sichuan Basin,the author characterized the features of the Ordovician reservoir and its major controlling factors by using petrographic and geochemical methods,and discussed the exploration potential of the Ordovician in the study area from the perspective of hydrocarbon accumulation.The matrix of the Ordovician reservoir is relatively tight.The reservoir spaces in central Sichuan Basin are dominated by intragranular dissolution pores,intergranular dissolution pores and intercrystalline pores,while that in southwestern Sichuan Basin are dominated by intercrystalline pores and mircofractures.Reservoir development is significantly controlled by the initial sedimentary environment,paleokarstification,oil charging and cracking,and fracturing.The sealing conditions were relatively good during oil and gas generation,leading to the formation of paleooil/gas pools which were completely destructed later due to the uplifting since the YanshanianHimalayan period.Shallow shoal limy or dolomitic grainstone occur in the Honghuayuan and Nanjinguan Formations in southern and eastern Sichuan Basin,and are potential highquality reservoirs.In addition,the gas from cracking of oil in the paleo oil pools in the Sinian and Cambrian is an important gas sources for the gas accumulation in overlying Ordovician.Therefore,the southern and eastern Sichuan Basin may be the most favorable areas for oil and gas exploration in the Ordovician.

A discussion on petroleum resource concepts and assessment approaches

2011, 32(3):  474-480.  doi:10.11743/ogg20110320

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Abstract: By analyzing different understandings of petroleum resources，we discuss the differences in connotations of the various concepts of petroleum resources and put forward two different resource assessment approaches.One is hydrocarbongenerationbased approach which takes hydrocarbon generation units as basic appraisal units.Based on principles of hydrocarbon generation，migration and accumulation，it establishes geological models and mathematical models to calculate hydrocarbon generation amount and eventually total resources.The other is hydrocarbonaccumulationbased approach which takes hydrocarbon accumulation units as basic appraisal units.On the basis of comprehensive analysis of petroleum geology，it figures out the number and size distribution of oil/gas accumulations or cells and then calculates undiscovered resources.We also discuss main factors that influence resource assessment.At last，suggestions are made to improve petroleum resource assessment in China：1）to strengthen research on resource assessment methods and complete petroleum resource assessment system；2）to complete classification criteria and system for oil/gas resource in order to bring them in line with international standards；3）to set up a dynamic resource assessment mechanism and to establish the national team，system and database for petroleum assessment.

SINOPEC’s coalbed methane development potential

2011, 32(3):  481-488.  doi:10.11743/ogg20110321

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Abstract: Large amount of coalbed methane(CBM)is produced in the US,Canada,and Australia.From the development of CBM industry at home and abroad,we can see that 1)the development of CBM industry is a long and rough road;2)favorable policies will help to promote the development of CBM industry;3)rapid development of CBM industry needs the involvement of big oil companies.SINOPEC has more than 8 trillion cubic meters of CBM resources.Analysis of geology and block screening reveal that the Heshun block in the Qinshui basin,the southern margin and the northern part of Ordos Basin and the eastern Yunnanwestern Guizhou region are the key areas for developing CBM in the near term.We suggest Sinopec see CBM as important as conventional oil/gas and adopt positive investment strategies.First,get access to as many as CBM resources both at home and abroad through block leasing,acquisition and cooperation with other companies.Second,treat existing blocks differently.For the Yanchuannan,Heshun and Zhijin blocks which have already found CBM,pilot production and further appraisal should be conducted in order to realize commercial production.For blocks in the northern part and the southern margin of Ordos Basin and in the eastern Yunnanwestern Guizhou region,exploration should be deepened in order to make a breakthrough.For blocks in southern North China Basin and Northwest China,exploratory work should be carried out in order to appraise their CBM potential.Third,intensify investment in R&D and build an integrated team of management,production and scientific research to ensure the fast and healthy development of SINOPEC’s CBM exploration and production.