Oil & Gas Geology-2018, Volume 39 Issue 2

Significance and prospect of ultra-deep carbonate fault-karst reservoirs in Shunbei area, Tarim Basin

Jiao Fangzheng

2018, 39(2): 207-216. doi:10.11743/ogg20180201

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Oil and gas reservoirs discovered in the ultra-deep carbonates of a burial depth larger than 7 000 m in Shunbei area of Tarim Basin have gained attention from the industry.A comprehensive study was then carried out on the tectonic evolution,reservoir characteristics and oil and gas accumulation patterns in the area.The results show that the reservoirs are of fault-karst type,featuring pervasive distribution but uneven enrichment of oil in the whole fault zone.With caverns,fractures and along-fracture dissolved pores and vugs formed by multi-stage strike-slip faulting as well as dissolution and alteration,the Middle-Lower Ordovician carbonates were wonderful reservoirs with favorable properties.Tight carbonates at the periphery of the faulting zone served well as lateral barriers for the formation of lithologic traps.The overlaying massive mudstone in Queerqueke Formation was the regional seal,which caped the hydrocarbons being sourced from the Lower Cambrian and migrating up along the strike-slip faults. The accumulations were mostly formed at late stage.The faults acted both as hydrocarbon migration pathways and accumulation spots.The discovery of the reservoirs demonstrated a new hydrocarbon reservoiring pattern in the ultra-deep sequences.About 18 strike-slip faulting zones were identified in the area with a resource volume as high as 17×10⁸ t oil equivalent,indicating a bright exploration and development future in the area.

Characteristics of saturated hydrocarbons from Lower Paleozoic marine shales in western Hubei-eastern Chongqing area and their indications

Xu Shuhui, He Sheng, Zhu Gangtian, Liu Ruobing, Lu Yaqiu, Wei Sile

2018, 39(2): 217-228. doi:10.11743/ogg20180202

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Organic-rich marine shales in the Lower Cambrian Shuijingtuo (Niutitang) Formation,the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation of western Hubei-eastern Chongqing area are very attractive targets for shale gas exploration in southern China.This research shows that the maximum paleogeotemperature experienced by the Shuijingtuo Formation ranges from 280℃ to 300℃,while the maximum paleogeotemperature of the Wufeng Formation and the Longmaxi Formation ranges from 200℃ to 220℃.The organic matter of the three formations is over-mature,with the vitrinite reflectance (R_o) for the Shuijingtuo Formation varying from 3.90% to 4.50%,while that for the Wufeng Formation and the Longmaxi Formation from 2.45% to 3.00%.However,nearly a complete series of n-alkanes mostly with bimodal distributions,isoprenoid alkenes,steranes,and terpanes have been observed in saturated hydrocarbons from the sampled shales' chloroform extract.This demonstrates that the conversion from liquid hydrocarbons to gaseous hydrocarbons (i.e. cyclo-polycondensation and hydrocracking) has been restricted within the shale strata,and thus the carbon skeleton of typical saturated hydrocarbon biomarkers could preserve at 200℃ to 300℃ for 50 to 180 Ma.What is also shown by the research is that some saturated hydrocarbon biomarkers in over-mature shales have not lost their ability to indicate maturity and biogenesis completely.Judging from the carbon number range and bimodal distributions of n-alkanes,and C₂₃ tricyclic terpane/C₃₀ hopance ratios in the sampled shales,the shales in the Shuijingtuo Formation have a higher level of organic maturity and the prokaryote is a major source of their organic matter,while the organic parent substance for Wufeng shales and Longmaxi shales are mainly derived from bacteria and alga.

Bedding decollement deformation domain in the Lower Paleozoic shales in Chongqing:Formation and geological significance

Yue Feng, Li Yongchen, Zhao Baoshan, Zeng Xiangliang

2018, 39(2): 229-238. doi:10.11743/ogg20180203

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The study presents the types,characteristics and the forming process of the bedding decollement deformation domain in the Lower Paleozoic shales in southeast Chongqing via mechanical analysis based on the observation of shale outcrops and cores,and discusses its geological significances in combination with shale gas exploration practices.Two types of bedding decollement deformation domains,namely compression-type and gravity-type,are recognized.The compression-type is formed when horizontal compressive tectonic stress dominates and shale layers are under the action of reverse fault;on the contrary,the gravity-type is formed when the overburden pressure dominates and the shale layers are under the action of normal fault.Compared with the gravity-type,the compression-type is more intense in stratum deformation with larger scale of influence on shales.Burial depth and dip angles are the key geological factors that control the formation of bedding decollement deformation domain in the same shale layer in a certain area.The critical dip angles for the compression-type expand gradually with the shale burial depth increasing,whereas that for the gravity-type decreases gradually with the increasing burial depth.The gravity-type is destructive to the preservation of shale gas.The bedding crushed zones and net fracture systems resulted by compression-type in an anticline are interpreted as the main migration pathways and reservoir space for shale gas,while those in an syncline are the main pathways for shale gas migrating to adjacent anticlines or releasing in the atmosphere.

Detrital zircon genesis and provenance tracing for reservoirs in the Lower Zhujiang Formation in Hanjiang-Lufeng Sag,Pearl River Mouth Basin

Jiao Peng, Guo Jianhua, Wang Xikai, Liu Chensheng, Guo Xiangwei

2018, 39(2): 239-253. doi:10.11743/ogg20180204

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High-quality reservoirs in the Lower Zhujia Formation in eastern Pearl River Mouth Basin have trigged many disputes over their provenance distribution patterns and lithologies.As an attempt to dispel the doubts over the issue,we performed U-Pb dating on some detrital zircons in sandstone samples from the Lower Zhujiang Formation in the Lower Miocene of 7 different structural zones in eastern Pearl River Mouth Basin with LA-ICP-MS dating technology.The results show that the zircons were mostly magmatic origin,followed by metamorphic origin.The zircons of magmatic origin showed up during four age intervals:98-114,137-163,211-247 and 387-428 Ma.By comparing the ages of the zircons with those of potential provenance rocks and combining the age spectrum,we suggest that there were at least 4 water systems serving as detrital material providers during the early Zhujiang time of the Early Miocene.Hanjiang provenance was the source of sediments in Haifeng 28 and Lufeng 2 structural zones,Dongsha uplift provenance provided sediments for Lufeng 22 structural zone,Shanwei provenance generated materials for Huizhou 10 structural zone and joined Zhujiang provenance at Huizhou 08 structural zone,and Dongsha uplift provenance,possibly together with Hanjiang provenance,provided materials for Lufeng 13 structural zone.Based on the above analyses,we propose that Hanjiang-Lufeng area,with favorable geological conditions for the formation of shallow and medium to deep reservoirs,be the key future exploration target in Pearl River Mouth Basin.

The identification and characteristics of migration system induced by high pressure, and its hydrocarbon accumulation process in the Yingqiong Basin

Fan Caiwei

2018, 39(2): 254-267. doi:10.11743/ogg20180205

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The Yingqiong Basin is a Cenozoic high-temperature and high-pressure basin with rapid subsidence,usually accompanied by the development of thermal fluid diapirs.This formed a unique hydrocarbon migration system,with the diapirs and fractures caused by the pressure release of deep strata functioning as the major transporting pathways of natural gas in the basin.The research distinguished 2 types of migration systems in the Yingqiong Basin:the high-pressure-diapir system and the fracture system,based on 3D seismic coherence analysis and curvature analysis along the formation.The spatial distribution characteristics of these two systems and their evolution processes have been investigated,and the relationship between the overpressured migration systems and the gas charging,dissipating and accumulating in the study area is analyzed by geochemical means such as natural gas isotopes and inclusions.The results indicate that,in spatial distribution,the two types of migration systems are characterized by stepwise development from deep to shallow:the former usually develops in the central depression zone of the Yinggehai Basin and the latter mainly distributes in the margin of thermal fluid diapirs and the high-pressure strata in the subsidence center proximity part of the steep slopes.According to the evolution history,the diaper migration system consists of five stages (types):pressurization,puncture,equilibrium,release and collapse,and at each stage the gas accumulation style features deep gas charging,shallow gas accumulating,simultaneous deep and shallow accumulating,flank accumulating,and accumulation destruction respectively.The fracture migration system originates from either high pressure or certain deformation processes,characterized with early gas accumulation in shallow strata and later gas accumulation in deep ones.

Hydrocarbon accumulation analysis of tight sandstone based on reservoir architecture theory-A case from Putaohua reservoir in the Gulong syncline,Songliao Basin

Liu Fang, Liu Xingquan, Liu Zongbao, Guan Xianli, Zhang Yunfeng, Liu Yunyan

2018, 39(2): 268-278. doi:10.11743/ogg20180206

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Tight sandstone reservoir is highly heterogeneous and its hydrocarbon accumulation process is complicated.The petrologic features,physical properties,pore throat structures and diagenesis of the Putaohua oil reservoir in Gulong syncline,Songliao Basin,were studied via a combination of core,thin section,SEM and experimental data.Then the spatio-temporal coupling relationship between the tightening history and hydrocarbon accumulation history of tight sandstone reservoir was analyzed based on the hierarchical system of architecture theory and the identification of genetic units.Results indicate that (1) hierarchically the Putaohua oil reservoir is composed of 8 lithofacies units,1 subfacies unit,and 4 microfacies units in terms of genesis;(2) the reservoir sandstone is mainly feldspar lithic sandstone with fine-grain size,medium sorting and rounding,mostly calcite cement filling,an average porosity of 7.63% and an average permeability 0.42×10^-3 μm².The majority of the pores are intergranular dissolution pores,developing shrinkage,fracture,and curved-plate throats;(3) the Putaohua oil reservoir is currently in Period A of the anadiagenetic stage after two periods of cementation and one period of dissolution during the diagenetic evolutionary history;(4) the oil enrichment of the Putaohua reservoir is attributed to the dynamic evolutionary process of multiphase accumulation and selective charging.And we come to the conclusion that microfacies of the underwater channel,thick sheet sandstone,and the interconnected thin sheet sandstone close to the above two areas are high-quality hydrocarbon-enriched reservoirs in the syncline.

Impact of sedimentation and diagenesis upon reservoir quality under constraint of tectonic activities and sequence stratigraphic framework-A case study of Putaohua oil layer in Daqingzijing area of southern Songliao Basin

Cao Zheng, Lin Chengyan, Dong Chunmei, Ren Lihua, Song Lei, Xing Xinya, Ma Xiaolan

2018, 39(2): 279-290. doi:10.11743/ogg20180207

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The impact of sedimentation,diagenesis and tectonic activities upon reservoir quality in Putaohua oil layer of Daqingzijing area,southern Songliao Basin,was studied based on sequence stratigraphic theories through analyses of drilling and logging data,core and thin section observation,SEM images as well as lab testing results.The result shows that the sequence stratigraphic framework of Putaohua oil layer is mainly composed of a low stand system tract and a lake transgression tract,with the former containing some high-quality sandstone reservoirs in distributary channels and forks of the main stream and the latter with quality sandstone reservoirs in underwater distributary channels.Sedimentation constrained by the framework was found to control the vertical and horizontal distribution of sedimentary microfacies and the scales of sandbodies in these system tracts.Vertically,sandstone layers at the upper part of the low stand system tract are large in scale and good in connectivity,thus contain relatively high-quality reservoirs; and horizontally,reservoir properties of the main distributary channel sandstone and fork sandstone are better for oil and gas accumulation.Diagenesis and compaction were suggested to be the direct reason behind the low-permeability of Putaohua oil layer,while the sequence stratigraphic framework,which controlled an early diagenesis environment in both system tracts,was regarded to be a factor that strengthened to some extent the carbonate cementation of sandstone layers in the lake transgression system tract.The range and extent of dissolution there were thought to be determined by the properties of reservoirs,distribution of source rock-rooted faults and distance to these faults.Relatively high-quality reservoirs in the oil layer were controlled jointly by four factors,namely the sequence stratigraphic framework,sedimentation,diagenesis and tectonic activity,i.e. the main distributary channels at the central and upper part of the low stand system tract that were near to the source rock-rooted faults or bounded by the faults served as the major spots for high-quality reservoirs to take shape.

A study on volcanic eruption edifices and stages and their control over hydrocarbon accumulation:A case study of the first member of the Yingcheng Formation in Yingshan Sag,northern Songliao Basin

Zhang Jiao, Xin Chaokun, Zhang Junyong

2018, 39(2): 291-299. doi:10.11743/ogg20180208

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As one of the important controlling factors over the distribution of high-quality reservoirs,volcanic edifices have been the major study objects of explorers for gas reservoirs.The first member of the Yingcheng Formation in Yingshan Sag of northern Songliao Basin,being dominated by acidic volcanic rocks and having a porosity less than 5% with intensive later alteration and less explored with drilling,was studied based on seismic data (the continuity of seismic events,amplitude intensity),layered properties,lithology and logging data,to establish boundary marks of volcanic eruption cycles and stages.Measures such as coherence volume interpretation,trend-surface analyses and manual identification,were used to discern volcanic edifices of different eruption stages and to determine their significances of controlling the quality of reservoirs based on their layout relations with hydrocarbon distribution.The result shows that six stages can be recognized in the volcanic rocks of the member from bottom up.Three massive edifices (the east,central and west edifices) were formed as the volcanic rocks erupted while migrating from east to west along faults in the area.The quality of the reservoirs there was conditioned by the preservation state of the edifices and the internal lithologic distribution.The middle parts of the central and west edifices were found to be well preserved with favorable porosity and permeability (due to the highly developed fractures and dissolved pores) for hydrocarbon to accumulate.But the eastern part of the east edifice was less preserved after a later uplifting and erosion as well as weathering and leaching,and its reservoir properties were poor due to the filling-up of dissolved pores and fractures by the overlaying sedimentary tuff and secondary minerals,thus being considered a less desired exploration target for gas.

Lithologic gas reservoir formation and exploration potential of the Xujiahe Formation in gently structured zones,southern Sichuan Basin

Jin Hui, Yang Wei, Xia Jiwen, Ma Yanliang, Shi Zhensheng, Shen Juehong

2018, 39(2): 300-308. doi:10.11743/ogg20180209

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There is no significant breakthrough in the exploration and development of the Xujiahe Formation in Shunan area in recent years compared with that of lithologic gas reservoir of the Xujiahe Formation in the central Sichuan Basin.Thus the lithologic gas reservoir forming conditions and exploration potential of the Xujiahe Formation in gently structured zones of Shunan area have always been the focus of attention.With the integrated analysis of outcrop,core and logging data,and the investigation of various accumulating conditions and major controlling factors,such as tectonic setting,sedimentary sandbody development,source rock presence,reservoir characteristics and etc.of the Xujiiahe Formation in this area,we believe that:(1) a large area of flat laying strata in the tectonically quiescent area is favorable for forming lithologic gas reservoirs;(2) the source rocks capable of large area hydrocarbon generation are distributed widely in the 1^st,3^rd,5^th member and the 2^nd submember of the 6^th member of the Xujiahe Formation,providing the gas source for lithologic accumulation;(3) the 2^nd,4^th member and the 1^st,6^th submember of the 6^th member of the Xujiahe Formation in the large-scale open flow lacustrine basin mainly are sandstones that are widespread and present in multiple zones,favorable for reservoir formation;(4) source beds and reservoirs interbeddedly deposit,resulting in similar accumulating features to the Xujiahe Formations in the central Sichuan Basin,i.e.large area continuous source kitchen hydrocarbon supply and efficient near-source charge.We believe that the Xujiahe Formation in the Shunan area has the conditions favorable for lithologic gas accumulating and may have formed a large-scale gas accumulation area.The understanding provides the geological basis for the exploration deployment of the Xujiahe Formation in tectonically quiescent zones,and indicates that the gently structured area between the low structures will be a focus for further exploration.

Origin,migration,and accumulation evolution of reservoir water in the gas field with Xu 2 gas reservoir,western Sichuan Depression,Sichuan Basin

Kang Baoping, Huang Xiaoyan, Guo Shuping, Li Zhen

2018, 39(2): 309-317. doi:10.11743/ogg20180210

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The gas reservoirs of Xu 2 developed by SINOPEC Southwest Company in Xinchang and Dayi were put into production,with the quite complicated distribution of gas/water.Therefore,adopting American hydrogeologists Novk's Phase diagram analysis and characteristics of deuterium and oxygen isotopes in gas field water,we identified the production formation of gas field water and its origin,analyzed the dynamic migration of gas field water,and investigated the evolution of gas accumulation in the western Sichuan Depression.The results show that (1) the gas field water of Xu 2 gas reservoir in Xinchang is originally marine formation water in the lower strata of the Xuxiahe Formation and Leikoupo Formation,which was in closed conditions initially but migrated into Xu 2 gas reservoir due to the change of preservation conditions caused by structural movement of later period,resulting in the present gas field water;(2) the gas field water of Xu 2 gas reservoir in Dayi changes gradually from the original deposition water in Xujiahe Formation to the marine formation water in the strata lower of the Xujiahe Formation and Leikoupo Formation.The water was well sealed and under thorough reduction conditions,which is typical of paleo-sedimentary water.But due to the increasing production of Xu 2 gas reservoir there,the marine formation water of Leikoupo Fomation in the lower strata has migrated into the 2^nd member of the Xujiahe Formation along faults controlling Dayi structure,replacing the paleo-sedimentary water of Xujiahe Formation.This scenario explains the present dynamic characteristics of gas field water in Xu 2 gas reservoir of Dayi.The accumulating evolution shows that Xu 2 gas accumulating process and the influence from fractures in Xinchang are different from those in Dayi,thereby generating different types of gas accumulations.Accordingly,the types and dynamic characteristics of gas field water in Xu 2 gas reservoir in Xinchang are different from those in Dayi.

Typological analysis and genetic mechanism of dolomite in the lower Cambrian Longwangmiao Formation,eastern Sichuan Basin

Sun Haitao, Zhang Yuyin, Liu Huilin, Xie Rui, Yang Xueqi, Ren Ying

2018, 39(2): 318-329. doi:10.11743/ogg20180211

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The dense sampling of the dolomite from the Bandenggou outcrop of Longwangmiao Formation in the eastern Sichuan Basin was carried out with the samples selected through X-ray diffraction.The classification and genesis of the dolomite were demonstrated based on test results such as major and trace elements,rare earth elements (REEs) and carbon and oxygen isotopes.The results show that the dolomitization of the study area could be divided into two types in terms of origins.One is the micritic dolomite with low order degree,which developed in evaporating environment and was influenced by para-syngenetic Sabkha dolomitization,seepage-reflux dolomitization,and terrigenous clastic minerals.For this kind of dolomite,MgO and CaO contents are positively correlated,and Mg/Ca ratio is high in general,with low content of Sr,high content of Fe,Mn and REE+Y,and significant δEu & δCe negative anomalies.Additionally,the δ¹³C & δ¹⁸O values are consistent with those of the limestone deposited in coeval sea water.The other is the powder fine crystal dolomite,formed by shallow burial dolomitization,with negatively correlated MgO and CaO content,low Mg/Ca value,high content of Sr,Fe and Mn,low content of REE+Y,δEu & δCe negative anomalies,and the same δ¹³C value of the limestone deposited in coeval sea water.However,the value of δ¹⁸O is a little bit lower than that of the limestone in coeval sea water.According to the petrographic characteristics of the dolomite,we believe that the rock has undergone the syndepositional dolomitization (including Sabkha effect and reflux infiltration)superimposed by shallow burial dolomitization at later time.As a result,the values of carbon and oxygen isotopes became different through the processes.

Factors controlling medium-to-deep coarse siliciclastic reservoirs of high quality at the southeastern margin of Qinnan Sag, Bohai Bay Basin,China

Wang Guanmin, Zhang Jie, Wang Qingbin, Li Jiawei

2018, 39(2): 330-339. doi:10.11743/ogg20180212

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Coarse siliciclastic reservoirs with low porosity and low permeability of fan-delta facies are common in the medium-to-deep first and second members of the Eocene Shahejie Formation (Es_1-2) at the southeastern margin of Qinnan Sag in the Bohai Bay Basin,but there are also some zones with high porosity and high permeability in some intervals.The reservoir spaces are largely dissolution pores and biological coelomopores,and the mechanisms of favorable reservoir development are different from that of the common tight coarse siliciclastic reservoirs.So we integrated data from core samples,casting thin sections,burial history,and fluid inclusion to investigate the characteristics and controlling factors of the high-quality coarse siliciclastic reservoirs in the Es_1-2.The results indicate that spiral-shell-bearing coarse siliciclastic interbeds are common in the Es_1-2 coarse siliciclastic reservoirs of fan-delta facies in the study area and are interpreted as wave-modified,intermittently abandoned underwater distributary channel deposits.Intensive calcite cementation plays a major role in the formation of tight reservoirs.The spiral-shell-bearing coarse siliciclastic rock is the major rock type for the development of high-quality reservoirs.Both lithological factors and diagenetic factors contributed to the development of high-quality reservoirs.The former include particle supporting,good sorting,coelomopores being rich in spiral shell fossils and early cryptocrystalline dolomite cementation,while the latter are mainly poor compaction and infiltration of diagenetic fluids due to particle supporting,strong dissolution of intermediate-acidic volcanic detritus,dolomite cements and feldspars by acidic pore-space waters as well as protection of pores by early charged hydrocarbons.This mechanism of favorable reservoir formation is of great referential significance for petroleum exploration in tight coarse siliciclastic reservoirs formed in brackish settings on steep-slope zones.

Mixed sedimentation in saline lacustrine basins during initial Eocene thermal maximum period:A case study on Kongdian Formation in Dongying Sag,Bohai Bay Basin

Tan Xianfeng, Wang Ping, Wang Jia, Luo Long, Liang Mai, Tan Donping, Kuang Hao

2018, 39(2): 340-354. doi:10.11743/ogg20180213

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Mixed sedimentation,a special sedimentary phenomenon,has always been a hot topic in sedimentology domain.This article documents a systematic study on the mixed sedimentation in saline lacustrine basins during the initial Eocene thermal maximum (IETM) period and the formation of Dongying Sag,Bohai Bay Basin based on analyses of core,thin section,isotopic and element data.The results reveal that there are two kinds of mixed sedimentation in the Kongdian Formation of Dongying Sag:mixed sedimentary sequences and structure-mixed deposits.The former mainly includes deposits of continental siliciclatic rocks interbedded with carbonates and that interbedded with sulfates.The latter mainly include deposits of structurally mixed continental siliciclastic with carbonates and that with sulfates.Various mixed siliciclastic-carbonate facies zones are identified in different stages of the deposition of the Kongdianzu Formation.Mixed sedimentation in the formation occurred in shore-shallow lake mud flats and sand bars during the early stage,and then in mud flats,sand bars and high-salinity deep water settings during the interim stage due to the thermal maximum.Later on,with a continuous salinization of the lake basin,sedimentation in the place went on in such settings as mud flat,sand bar,high-salinity deep water as well as bioclastic shoal and etc.It was dominated by facies-margin mixing,with accidentally discrete or autochthonous mixing.Controlling factors include sediment input,water hydrochemistry,hydrodynamic force,and fluctuation of water-level.The study confirms that,during the initial Eocene thermal maximum period,the lake basin in the area had gone through an intense salinization process,which occurred simultaneously with clastic input into the basin and formed typical mixed siliciclastic-carbonate deposits in the saline lacustrine basin.Dissolution of carbonate and sulfate during burial process of the mixed sediments and fracturing contributed to the formation of hydrocarbon-bearing reservoirs.The original chemical constituents in the mixed deposits could be used to restore the paleo-environment of the lake.

Characteristics and development patterns of reservoir space of the Lower Paleozoic buried hills in Dongying Sag,Bohai Bay Basin

Zan Nianmin, Wang Yanzhong, Cao Yingchang, Yuan Guanghui, Cheng Xin, Jiang Wei, Jia Guanghua, Song Mingshui

2018, 39(2): 355-365. doi:10.11743/ogg20180214

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The Cambrian and Ordovician carbonate reservoirs in Dongying Sag of the Bohai Bay Basin are highly heterogeneous due to multiple stages of tectonic activities and diagenesis they have experienced.Pore space and its development patterns in the Lower Paleozoic buried hills in Dongying Sag were systematically studies through an integration of logging,core,thin section,carbon and oxygen isotope,fluid inclusion as well as seismic data.The pore space is dominated by fracture,solution cavities and dissolved pore.The solution cavities are mostly of lime mud type and inter-breccia type.The fractures are mainly tectoclases and solution fractures.And the pores include inter-breccia dissolution pores,intra-vein dissolution pores and intercrystalline dissolution pores.The Lower Paleozoic buried hills in Dongying Sag are largely of residual mound-type and fault-block type.The former is distributed in Caoqiao and Zhengjia-Wangzhuang areas and nurtured weathering crust reservoirs with its vertical vadose zone and horizontal phreatic zone caused by unconformities.Pore space there is mostly high-angle fracture,inter-breccia solution cavity and horizontal layered solution cavity.They usually have massive epigenetic karst belt and are high-quality reservoirs for hydrocarbon with well-connected large fractures and pores.The hills dominated by limestone have epigenetic karst belts as thick as 170 m,while those controlled by dolomites have thinner epigenetic karst belts of only 70 m in thickness.The latter is distributed in Gaoqing-Pingnan and Wangjiagang areas.Influenced by multi-stage faulting and deep hydrothermal activities,this type of hills contains mostly reservoirs with pore space dominated by fractures near faults and on crest of folds,as well as dissolution pores and vugs developed along the fractures.

Favorable and major geological controlling factors for coalbed methane accumulation and high production in the Chengzhuang Block,Qinshui Baisn

Wang Bo, Yao Hongxing, Wang Hongna, Zhao Yang, Li Mengxi, Hu Qiujia, Fan Meirong, Yang Chunli

2018, 39(2): 366-372. doi:10.11743/ogg20180215

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As one of the most successful blocks of coalbed methane (CBM) development in Qinshui Basin,the exploration and development in Chengzhuang coal block can provide useful guidance for the development of other CBM blocks with geologically similar backgrounds.In combination of plenty of practical data on exploration and development and methods such as mathematical statistics,laboratory experiment,wire line logging interpretation,structure analysis,numerical modeling,decline analysis,we analyzed the genetic mechanism,accumulation conditions of CBM,and patterns of gas production,in the Chengzhuang Block.The study indicates that:(1) the Chengzhuang Block is characterized by favorable accumulation conditions with its simple structure,extensive reservoir distribution,shallow burial depth of coal reservoir,organic thermogenic gas,high gas content and high permeability;(2) the high productivity wells account for 47% of the total wells of the block,with high average production of 4 000~7 000 m³/d per well and high expected recovery of 55.1%.Furthermore,we analyzed the controlling mechanisms for CBM gas accumulation and high production,and concluded that (1) the tectonic thermal events would boost gas generation,reservoir quality,seal-capping capacity and optimal distribution of hydrodynamic force,which would be beneficial for CBM accumulation;(2) the coal reservoirs with horizontal extensional stress and in the coal mine pressure relief zone are of high permeability,favorable for CBM production.

Characterization and fractal nature of adsorption pores in low rank coal

Zhou Sandong, Liu Dameng, Cai Yidong, Yao Yanbin, Jiao Yongyan, Ren Shijun

2018, 39(2): 373-383. doi:10.11743/ogg20180216

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We collected 8 kinds of low rank coal (LRC) with distinguished macrolithotypes,carried out some experiments like the low-temperature liquid nitrogen adsorption and isothermal methane adsorption,discussed the LRC adsorption pore characteristics,and established the relationship between fractal model suitable for LRC adsorption pores and fractal dimension with pore structures and adsorption capacities.From this,the LRC absorption/desorption curves are divided into four categories with 4 corresponding pore types:taper-shaped pores with one end closed being dominant in bright coal,ink-bottle-shaped pores dominating semi-bright coal,open-cylindric pores and wedge-shaped ones occurring in semidull coal,and open-cylindric pores being predominant in dull coal.Results show that (1) compared with the BET model,the FHH model is more appropriate for the characterization of LRC pores in respect of fractal nature;(2) the various macrolithotypes in FHH model show different adsorption characteristics at relative pressure 0~0.5 and 0.5~0.5 and have fractal dimensions D₂₁ and D₂₂ respectively,with D₂₁ representing the surface roughness of adsorption pores and D₂₂ the irregularity of the pore structures.As D₂₁ gets bigger,the pore specific surface area increases,whereas the bigger D₂₂ gets,the smaller the average pore size becomes,the higher the micropore content is,and the larger the pore heterogeneity is;(3) D₂₁ is more effective than D₂₂ in controlling the adsorption capacity of LRC.When the coal is characterized by high D₂₁ and low D₂₂,the adsorption capacity is the strongest,and the adsorption capacity enlarges as the fractal dimension of cumulative specific surface area increases.The LRC adsorption process subsequently shows transition from predominance of monomolecular layer adsorption to predominance of polymolecular layer adsorption with increasing pressure.

Sedimentary characteristics and significance of reworked sands in the Ordovician Pingliang Formation,southwestern margin of Ordos Basin

Li Hua, He Youbin

2018, 39(2): 384-397. doi:10.11743/ogg20180217

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The principles of sedimentology were used to study the sedimentary characteristics,forming process,dictating factors and petroleum exploration significances of reworked sands in the Ordovician Pingliang Formation at southwestern margin of Ordos Basin based on outcrops,restoration of paleocurrent and geochemistry data.The result shows that reworked sands are relatively well developed in the study area and they are mainly affected by factors including provenance,relative sea level,topography and tectonic activities.The Pingliang Formation in the study area is unitary in lithology and various in sedimentary structures and can be divided into three typical facies.Facies 1 of mudstone with horizontal bedding represents deep water autochthonous sediments.Facies 2 of sandstone with cross bedding indicates turbidity sediments.Facies 3 of sandstone with bimodal cross bedding points to reworked sands.The typical characteristics of reworked sands are as follows:1) relatively well-sorted and -rounded grains of bimodal grain-size distribution;2) two predominant directions of paleocurrent,with one going down along slopes,indicating the direction of turbidity current movement,and the other running parallel to slopes,showing the direction of contour current flowing;3) obvious bioturbation;4) one to tripartite divisions of cumulative frequency curves,showing both gravity and tractive current features;5) upward-fining normal grading with erosive top;and 6) relatively abundant in trace elements (B,Cr,V,and etc).While flowing westward roughly parallel with the slope,the contour current in the study area could have helped to improve reservoir properties of the reworked sands through winnowing,transporting,reworking and re-deposition of the southeastward running turbidity sediments down the slope.Both turbidity sediments and coarse-grained contour current sediments could serve as good oil and gas reservoirs.At the same time,fine-grained deep water autochthonous sediments and contour current sediments could be quality source rocks.It is very likely for the reworked sands,contour current sediments and autochthonous sediments to combine with one another to form ideal hydrocarbon generation-accumulation-sealing assemblages in the study area.

Characteristics and sensitivity of the Middle Jurassic Toutunhe Formation in Fudong slope,Junggar Basin

Cui Cui, Zheng Rongcai, Jiang Yiqin, Jin Jun, Wen Huaguo

2018, 39(2): 398-408. doi:10.11743/ogg20180218

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The reservoir characteristics,controlling factors and sensitivities of the Middle Jurassic Toutunhe Formation in Fudong slope,Junggar Basin,were studied based on core observation,casting thin section identification,assisted with some analytical and experimental data on cores,such as SEM data,physical property depiction,x-ray diffraction results,pore structure and sensitivity test.Results show that:(1) the most favorable micro-facies for reservoir development is underwater distributary channel at delta front and its lithology is mainly composed of feldspathic litharenite that is low in compositional maturity and medium high in textural maturity.The reservoir space is mainly primary residual intergranular pores,partly intergranular and intragranular dissolved pores;(2) displacement pressure and mean saturation pressure are relatively low,but the pore throats are well-sorted,narrow as a whole,and are dominated by capillary throats;(3) reservoir development is controlled by both sedimentary microfacies and diagenesis;the early diagenetic chlorite rim cementation is beneficial to the preservation of some primary pores and throats,but compaction and pressure-solution causes the shrinkage of the primary pore space,the lowering of reservoir permeability,and the tightening of reservoir rocks.In addition,the sandstone reservoirs of the Toutunhe Formation generally have various sensitivities,which is one of the main causes of reservoir damage.Among them,the medium strong water sensitivity is the strongest in reservoir damage,followed by the medium weak acid sensitivity and salt sensitivity,while weak velocity sensitivity is hardly harmful.

Quantification of braided-river lithofacies units and sandbody based on horizontal well data:A case of MPE3 block in Orinoco heavy-oil zone,Venezuela

Huang Wensong, Chen Heping, Li Shengli, Wan Guangfeng, Xu Fang, Meng Zheng, Liu Chang

2018, 39(2): 409-418. doi:10.11743/ogg20180219

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When quantifying sandbody with vertical well data,the volume of sandbody often cannot be defined accurately due to the uncertainty of the interwell correlation.Horizontal well data are of significant referential value for lateral prediction of reservoir sandbody,which is difficult to be achieved by conventional vertical well data.Horizontal wells can reveal lateral variation of sandbodies,so as to assist us in quantitative characterization of sandbodies and quantitative classification of lithofacies units.This study is composed of 4 steps.Firstly,it indentified 4 types of lithofacies units according to the braided-river characteristics in the study area,coring data in vertical wells and well logging data in vertical and horizontal wells,with a view to classify lithofacies units in vertical and horizontal wells.Secondly,a quantitative research on the sandbodies was carried out with horizontal and vertical well data,and thus confirmed the configuration and scale of channel bars and braided channel sandbodies.Besides,the length-width ratio of channel bars,the width ratio of channel bar to channel and that of interbed to channel bar were well established.Subsequently,the seismic wave impedance inversion on braided-river reservoirs was carried out under the constraint of information from sandbodies encountered by horizontal well.Finally,prediction of lithofacies units was done based on the quantitative characterization of sandbodies and the results of wave impedance inversion.Results show that:(1) the horizontal well data can be used to finely characterize lithofacies distribution within reservoirs and improve the accuracy of sandbody quantification and description of internal reservoir heterogeneity;(2) the information from the laterals can increase the number of sample points in seismic inversion,thereby lower the uncertainty of interwell reservoir prediction,greatly improve the accuracy and reliability of internal reservoir seismic inversion,and increase the reliability of the reservoir lithofacies model.

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