Table of Content

28 June 2021, Volume 42 Issue 3

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Petroleum Geology

Theoretical and technological progress and research direction of deep and ultra-deep carbonate reservoirs

Zhiliang He, Yongsheng Ma, Dongya Zhu, Taizhong Duan, Jianhua Geng, Juntao Zhang, Qian Ding, Yixiong Qian, Yujin Wo, Zhiqian Gao

2021, 42(3):  533-546.  doi:10.11743/ogg20210301

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The journey marching into deep and ultra-deep carbonates in China has been rewarded with more and more oil and gas discoveries, thus making the country the most active area for ultra-deep oil and gas exploration and development across the world.As a result, progresses have been made in the study of formation mechanisms, geological models, geophysical prediction and fine geo-modeling of these carbonate reservoirs.Some preliminary consensuses on high-quality deep carbonate reservoir genesis were reached: original high-energy facies and early dolomitization are the basis for the development of high-quality reservoirs; tectonic uplift activities lead to meteoric water karstification associated with unconformity, resulting in karst fractured-vuggy reservoirs; and a combination of early-stage material basis with later burial environment is key to the formation and preservation of high-quality reservoirs.Significant progress identification and tracking of diagenetic fluids and timing of diagenesis shed light on the diagenetic evolution analysis and geo-modeling with high precision and resolution.Substantial advancement has also been achieved in seismic technology for the elastic property variation trend prediction of rock frame under high temperature and high pressure, the construction of rock physics models, the elastic property variation trend prediction of multi-phase pore fluids mixtures and the high-resolution seismic inversion.As for the fine geo-modeling of the reservoirs, the multi-point statistics technology combined with sedimentary process simulation, porous carbonate sedimentary inversion with intelligent optimization, petrophysical facies modeling of multi-scale data fusion, and cellular automata fault-controlled paleokarst process numerical simulation were developed.Despite the progress, the study of deep and ultra-deep carbonate reservoirs is still faced with a series of significant theoretical and technical challenges: disputes on genesis, technical bottlenecks constraining seismic prediction, and a lack of effective and fine characterization and model for reservoirs, to name just a few.It is suggested that the future study be focused on the diagenetic fluid identification based on available petrological, mineralogical and geochemical data, as well as revealing the formation and maintenance mechanisms of reservoir space under complex fluids through high temperature and high pressure dissolution experiments and numerical modeling; on the development of a system for elastic property measurement of carbonates under higher temperature and pressure settings to analyze the scaling effects of seismic wave propragation due to the strong reservoir heterogeneities and establish the reservoir characterization workflow that can be real-timely optimized according to the geological target as well; on forward and inversion numerical simulation of sedimentary reservoirs and diagenesis under multi-field coupling; and on intelligent geo-modeling for multi-scale data fusion and multi-method collaboration with a view to improving the geo-modeling accuracy.

Characterization and evaluation of chert reservoirs in ultra-deep carbonate rock formations-A case study on Well TS 6 in the Tarim Basin

Yongli Liu, Donghua You, Haiying Li, Lijun Gao, Hong Jiang, Weifeng Zhang, Fang Bao

2021, 42(3):  547-556.  doi:10.11743/ogg20210302

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Thick chert reservoirs were revealed by Well Tashen 6 penetrating the Ordovician Penglaiba Formation in the Tarim Basin and have been the focus of research for reservoir development characteristics and storage capacity. Based on lithology and electrical response characteristics, the reservoirs were studied with detailed core observation and description, X-ray diffraction, microscopic petrology analysis, reservoir performance and other reservoir characterization and evaluation methods. The results show that the reservoirs have higher porosity than dolostone reservoirs of the same sequence, with an average porosity as high as 13.5% at a burial depth of more than 7 400 m. The reservoirs are mainly composed of microcrystalline quartz, with a content as high as 83.0%-93.7%, and a small amount of clay, calcite, dolomite and other minerals. The reservoir space is relatively uniformly distributed intercrystalline micropores in quartz crystals with a pore diameter mainly varying from 1 to 10 μm, followed by 0.1-1 μm, and main throat diameter ranging from 10 to 100 μm. The off-line nuclear magnetic resonance of pressure saturated crude samples shows that pores of 0.1-10 μm in diameter serve as the main storage space. And the greater the total porosity, the greater the total amount of crude oil that can be accommodated. The interbedded siliceous-dolomite, the micro-bedding structure, the high initial porosity and the symbiotic siliceous nodules, all indicate that the reservoirs are the co-product of sedimentation and diagenesis in relatively deep water area on the platform. Furthermore, the seismic reflection characteristics reveal that the reservoir body is about 300 m long from north to south and 200 m wide from east to west. The discovery of siliceous rock reservoirs is of great significance to understanding the internal structural differences of the Early Ordovician carbonate platform in the Tarim Basin.

Sedimentary configuration and reservoir distribution in the Cambrian mound-shoal complexes at platform margins of Gucheng area, Tarim Basin

Junlong Zhang, Mingyi Hu, Aiyun Wang, Bin Zhang, Bo Yan, Xiangiang He

2021, 42(3):  557-569.  doi:10.11743/ogg20210303

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The four stages of massive Cambrian platform-margin mound-shoal complexes in Gucheng area are the new exploration targets in the Tarim Basin with some promising reservoirs encountered during wildcatting.However, a lack of understanding on the complexity in sedimentary models and reservoir distribution as well as other reservoir forming factors has limited further exploration.Based on core observation, thin section identification, petrographical and geochemical characterization of reservoirs, seismic facies description, and etc., this paper expounds on the identification of sedimentary configuration patterns of the complexes and major factors controlling the reservoirs, and the establishment of a reservoir distribution model under constructive diagenesis constrained by sedimentary configuration.The results show that there are four microfacies (i.e.mound base, mound core, mound flank and mound flat) developed inside the complexes with the sedimentary energy gradually increasing upward, and two types of sedimentary configurations (i.e."accretion type" and "progradation type").The facies belts formed during stages 1 and 2 are "accreted" mound-shoal complexes of vertical superimposition, and those during stages 3 and 4 are "progradational" mound-shoal complexes of lateral stacking.High-quality reservoirs with dissolved pores and cavities are well developed in multiple cyclic distribution with high-energy facies belts as the basis and meteoric water dissolution as an indispensable factor.Generally, the top of the mound-shoal complexes are better in physical properties compared with the bottom, core and flank.The frequently exposed mound-shoal complexes of accretion type contain large-scale reservoirs in a vertically "sandwich-shaped" superimposition distribution; while the long-term exposed progradation type has reservoirs of better physical properties in a laterally "inverted toothbrush-shaped" staggered distribution.The results may be of geological value to the prediction of potential hydrocarbon play fairways in exploration deployment.

Characteristics, enviornment and geological dating of primary dolomite in the Neoproterozoic Pingwagou Formation at Hongliugou, Ruoqiang County, Xinjiang

Yixiong Qian, Chenglin Chu, Yuejun Li, Qingzhen Zhang, Wangpeng Li, Xin Yang

2021, 42(3):  570-586.  doi:10.11743/ogg20210304

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The Neoproterozoic dolomite, one of the important carriers of ancient global climatic, biological, ecological and environmental information, is of great theoretical significance and application value.This study highlights the progress in characterization, environmental reconstruction and geological dating of a typical primary dolomite: the bitumen-bearing microbial Neoproterozoic dolomite from the Pingwagou Formation (previously believed to belong to the Nierkuli Group of Qingbaikou System) at Hongliugou section Ⅰ in Ruoqiang County.Petrographic and geochemical analyses as well as detrital zircon geochronology show that the Pingwagou Formation is mainly a set of intertidal to subtidal deposition of shallow-water primary dolomite mainly composed of algal oolite-granular dolomite, spongiostromate, laminated fine- and coarse-grained agglutinated stromatolites in wavy, conical, domal and columnar patterns, and microbolites in cemented spherulites intercalated with some tempestites.Two shallowing-up and deepening-down sub-cycles of bioherm and beach facies are developed in inner and mid ramps of the formation. Marine cements in grape-, fiber-, fibrous column-shaped or radial-fascicled patterns are broadly distributed with high ratios of Mg/Ca and a "cap-pattern" rare-earth distribution, indicating typical aragonite sea sediments and primary dolomites.Based on an overall consideration of stromatolite patterns, pronounced positive δ¹³C excursions (ave.7.80‰), δ¹⁸O (ave.-1.82‰) and elevated ⁸⁷Sr/⁸⁶Sr values (ave.0.708 07) as well as zircon U-Pb dating of underlying clastics, we conclude that the formation is a set of primary dolomite deposition by Mg- and Si-rich seawater as well as seafloor hydrothermal inputs against a warm climate in the Neoproterozoic Cryogenian after the Sturtian Ice Age (685-635 Ma).Despite a long geological evolution, the dolomite in the formation is of certain reservoir properties and has formed an effective source-reservoir combination with underlying argillaceous source rocks, posing an interesting yet tough challenge to oil and gas explorers.

Paleozoic horst-twist superimposed fault-fracture body model in Gucheng area of Tarim Basin

Guangyu He, Zicheng Cao, Zewei Yao, Tianqi Liao, Bo Lin

2021, 42(3):  587-594.  doi:10.11743/ogg20210305

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The massive carbonate rocks in the deep Paleozoic of Tazhong area are currently a hot spot for oil and gas exploration.Despite several major breakthroughs in oil and gas exploration made along the NW-, NE-, and NEE-trending fault-fracture zones on the northern slope of Tazhong in recent years, the research on fault-fracture bodies in neighboring Gucheng area is yet to be deepened, which has severely restricted the oil and gas exploration in the area.Based on high-resolution 3D seismic sections, this study proposed a Paleozoic superimposed horst-twist fault-fracture body model in Gucheng area, the features of which are summarized as follows.(1) The fault-fracture bodies are not inherited vertically.From the bottom up, i.e.from the Cambrian, Middle-to-Lower Ordovician, Upper Ordovician to Carboniferous, four zones, including the Early Caledonian extensional fault-fracture zone, the Mid-Caledonian transpressional fault-fracture zone, the Late Caledonian transtensional fault-fracture zone and the Early Hercynian transtensional fault-fracture zone are successively developed and superimposed.(2) Horizontally, the strike of the fault-fracture bodies from bottom to top shows obvious clockwise and counterclockwise rotation with those in the deep Cambrian, Middle-to-Lower Ordovician, Upper Ordovician, and top Carboniferous striking NEE, NNE, NE-NEE and NE, respectively and compositely.(3) The fault-fracture bodies feature a superposition that can be roughly described as "an inverted cone in the upper and a horst in the lower", and "being tensile in upper and lower, but compressional in the middle" from a sectional view.(4) The fault-fracture bodies are also distributed in a "broom-shaped" and en echelon arrangement from a map view.

Discovery and significance of the Ordovician paleo-karst caves in shallow drilling in the southern Ordos Basin

Xiaohui Jin, Juntao Zhang, Dongsheng Sun, Qiang Ding, Jiaqi Yang

2021, 42(3):  595-603.  doi:10.11743/ogg20210306

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The Ordovician sedimentary and karst environment as well as karst reservoir characteristics in the southern margin are completely different from those in the central-east of Ordos Basin.A shallow well drilled into the southern margin revealed the existence of karst caves filled with clastic rocks.Two sets of strata were recognized as the Benxi and Pingliang Formations, with the former dominated by bauxite mudstone and the latter by grainstone and algal limestone interbedded with multiple sets of mudstone layers.Mineral composition, major element, trace element and rare earth element analyses were carried out to compare the source, weathering alteration degree, redox environment, climate and paleo-tectonic environment of the two sets of mudstones.The results show that the Benxi Formation mudstones are of higher-degree weathering and the Pingliang Formation mudstones were deposited in a more reductive setting.The sedimentary environment of both sets represents a transition from sea to land with frequent climate changes against tectonic backgrounds of active continental edge and continental island-arc in the Late Paleozoic.The Pingliang Formation mudstones of terrigenous clast origin that represents cavities and underground river deposits in the pre-Carboniferous karst period, deposited during wet karst development stage when the southern Ordos Basin were uplifting.Considering the widespread fractures at the upper and lower parts of the formations, we suggest that a karst fracture-cavity system once existed in the southern Ordos Basin.Although the caves revealed by the well are filled with clasts, it can be inferred that there may be residual fractures and cavities at certain paleomorphological locations in the southern Ordos Basin, which are of great value to future oil and gas exploration.

Characteristics of deep Cambrian carbonate reservoirs in the Ordos Basin and main control factors

Chunlin Zhang, Jingli Yao, Chengshan Li, Fengcun Xing, Liyong Fan, Wuling Mo, Qiuying Zhu, Xu Zeng

2021, 42(3):  604-614.  doi:10.11743/ogg20210307

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The Cambrian carbonates are important oil and gas exploration targets in the Ordos Basin.An integration of the latest drilling and seismic data as well as outcrop and thin section observation shows that the Cambrian carbonates contain two major categories of reservoirs, i.e.facies-controlled and intensively structure-modified reservoirs.They can be further subdivided into eight types, of which the epigenetic karst and high-energy dolomitized oolitic shoal are the main reservoirs.The Cambrian carbonates are characterized by well-developed pores, cavities and fractures of different types and high diversity.The study proposes that the Cambrian reservoir development is mainly controlled by the dolomite of high-energy facies, epigenetic karstification of the paleo-uplift zone and faulting activity.And a model for the Cambrian reservoir development is also established.In addition, a discussion based on the study was carried out for the exploration direction of the Cambrian carbonates in the Ordos Basin and it is concluded that the Cambrian fractured-vuggy karst reservoirs around the Qingyang paleo-land at the southwestern margin of the basin shall be the next focus for exploration.

Distribution of paleo-karst reservoirs in Ma 4 Member, Hangjinqi area, northern Ordos Basin

Wei Zhang, Xiaoqi Ding, Chuntang Li, Zhuangzhuang Qi

2021, 42(3):  615-626.  doi:10.11743/ogg20210308

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Karst limestone reservoir, a newly recognized reservoir type with hydrocarbon potential in recent years, is also present in the northern Ordos Basin.Its forming mechanism and distribution pattern are of great significance for future gas exploration in the area.Studies were carried out on samples from the area on the relationship between cavern facies and the reservoir through cavern anatomy, on the paleotopography of weathering crust and vertical pattern of the multiple cave layers through cavity logging and levelling interpretation to explain the relationship between multi-layers of caverns and structural uplift, and on the distribution pattern of caverns based on seismic data.The results show that vugs occurring below the unconformity are mostly filled with clay and fine-grained debris, thus incapable of storing.However, those far away from the unconformity are dominant reservoirs.Cave roof and unfilled cavities are also dominant reservoir space.Three layers of caverns, a result of periodic uplifting in northern part of the study area during the Caledonian, are developed in Ma 4 Member in a steady distribution and good well-to-well correlation.The cavities are more developed in the west of study area compared with the east part.The prediction of cavity distribution by well-seismic correlation indicates that sink holes mostly occur in the north, and underground channels mostly occur in the south.Cavities show a network-shaped distribution in plane view, implying a minor controlling effect by faults.Paleo-water table serves to control the stratification of cavities.The discovery of three layers of cavities in Ma 4 Member broadens the horizon of gas exploration in the northern Ordos Basin.

Characteristics and geological significance of mixed sediments in the Lower Cambrian Longwangmiao Formation, central Sichuan Basin

Jinmin Song, Yaxiong Zhang, Kewei Yin, Shugen Liu, Zongyu Chen, Zhiwu Li, Yanhong Tian, Wei Sun, Ying Liu, Xin Jin, Lingli Zhao

2021, 42(3):  627-638, 701.  doi:10.11743/ogg20210309

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The mixed siliciclastic and carbonate sediments that prevail the Lower Cambrian Longwangmiao Formation in the central Sichuan Basin are poorly understood.Their characteristics and geological significances are therefore focused in this study through core description and petrologic analysis.The results show that mixed carbonate-siliciclastic deposition dominates the formation as carbonate-mixed carbonate-siliciclastic sequences and mixed carbonate-siliciclasitic sequences in gradually mixing or type Ⅱ composite mixing styles.Vertically, the sediments are clearly storm-related and controlled by sea level variations in transgression and early regression stages of the third-order eustatic cycle with the mixing range much larger in transgression than that in regression stage.Horizontally, the sediments show higher mixing intensity in the Gaoshiti area than in the Moxi area.A characteristic comparison between the sediments of the formation in the central and northern Sichuan Basin reveals that sediments in the northern part of the basin have higher mixing intensity and greater granular size due to its adjacency to the Qinling Palaeocean and Motianling Palaeoland and a possible influx of terrigenous debris brought by storms through the north part to the center.The mixed sediments in the central Sichuan Basin indicate that Gaoshiti area was lying on a palaeo-slope where silty quartz grains were unloaded to form a mixing deposition center.It is also highly possible that quality reservoirs in the formation occur mostly in the weakly to non-mixing sediments of shoal facies, since highly mixed sediments are not suitable for the formation and development of such reservoirs.

Influence of hydrothermal activity on the Maokou Formation dolostone in the central and western Sichuan Basin

Ting Li, Dancheng Zhu, Minglei Yang, Pingping Li, Huayao Zou

2021, 42(3):  639-651.  doi:10.11743/ogg20210310

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The hydrothermal dolostone in the Permain Maokou Formation of Sichuan Basin has been targeted frequently for natural gas during the past few years.Though quite common in the formation as a result of the Emeishan flood basalt eruption, hydrothermal activities and their effect on dolostone reservoirs are not sufficiently understood.Based on observation of outcrops, cores, and thin sections along with analysis of cathodoluminescence, homogenization temperature of fluid inclusions and geochemistry, we reached the following conclusions.Firstly, dolostone in the Maokou Formation can be divided into laminar dolostone and patchy dolostone.The former, the dominant type, can be further subdivided into very fine to fine-crystallized dolostone (D1) and fine to medium-crystallized dolostone (D2).The latter is composed of medium to coarse-crystallized dolomite (D3).Secondly, D2, mainly developed in D1 as patches, shows inequigranular texture and develops hydrothermal-related structures and minerals.Compared with D1, D2 has higher Fe and Mn content, stronger cathodoluminescence, higher homogenization temperature of fluid inclusions, higher ⁸⁷Sr/⁸⁶Sr, and more negative δ¹⁸ O_PDB value, indicating that D2 is generated by recrystallization of D1 under the influence of hydrothermal fluids.Thirdly, hydrothermal process can be both constructive and destructive to the development of Maokou Formation dolostone reservoirs.It is constructive in that hydrothermal activities led to the formation of rupture system and breccia, hydrothermal dissolution to dissolved pores and cavities, and hydrothermal metasomasis to dolomitization of limestone.It is destructive in that the physical properties of dolostone were affected by mineral sedimentation that plugged previously formed pores, ruptures, and cavities and by recrystallization that added more Mg²⁺ and CO₃^2- to the system.Lastly, the main driving force for the formation of reservoir space in the Maokou Formation is an early-stage meteoric freshwater dissolution, and the footprint of hydrothermal fluid can be seen as a clue to potential dolostone reservoirs.Therefore, importance shall be attached to shoal with early-stage dissolution during the exploration of Maokou Formation reservoirs in Sichuan.

Seismic physical model of the Maokou Formation reservoirs of platform-margin shoal in Yuanba area, Sichuan Basin

Zhenchao Qin, Zhiwei Miu, Jianxin Wei, Bangrang Di

2021, 42(3):  652-660, 746.  doi:10.11743/ogg20210311

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Shoal reservoirs were encountered by Well Yuanba-7 and yielded high industrial gas flow from the Maokou Formation at the platform margins of Yuanba area in Sichuan Basin, marking a major breakthrough in oil and gas exploration frontiers.In order to confirm the seismic response characteristics and identification methods of the reservoirs, the study constructed a three-dimensional physical model of platform-margin shoal reservoirs consistent with the actual formation parameters of the studied area based on the comprehensive geological and seismic interpretations.The model were then modified according to acquired seismic data and processing procedures to obtain physical simulation data for analyzing the seismic response characteristics and assessing prediction methods of the reservoirs.The results show that the seismic attribute analysis based on large time windows for platform-margin shoal reservoirs cannot accurately describe the changes of sedimentary microfacies.Instead, the fine interpretation of strata focusing on reservoirs based on geological models does better.With reservoir thickness and property parameters affecting seismic amplitudes, the method works better with amplitudes clearly revealing the physical properties of reef-shoal reservoirs and frequency attributes mapping the reservoir boundaries.

Characteristics and distribution of platform-margin reef-shoal reservoirs in Changxing-Feixianguan Formations to the west of ocean trough, E'xi-Chengkou area

Guoqi Wei, Wei Yang, Mancang Liu, Saijun Wu, Wuling Mo, Hui Jin, Juehong Shen, Cuiguo Hao

2021, 42(3):  661-672, 764.  doi:10.11743/ogg20210312

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An integrated analysis of outcrop, drilling, log and seismic data of the Changxing-Feixianguan Formations in E'xi-Chengkou area has led to new understandings of the characteristics of the platform-margin reef-shoal reservoirs therein.(1) The Changxing-Feixianguan Formations are a rimmed carbonate platform, composed of basin, slope, platform-margin reef-shoal, open platform, restricted platform and evaporated platform facies from east to west.(2) A platform-margin belt was developed during the sedimentation of Changxing-Feixianguan Formations with the Changxing Formation dominated by platform-margin reef facies and Feixianguan Formation by platform-margin oolitic shoal facies.This massive reef-shoal complex expands broadly and continuously in the area.(3) The platform-margin reef-shoal in the Changxing Formation consists of reef base, reef core and reef cap.The reef core is mainly cemented reef and baffle-frame reef, and the reef base and cap are mainly the biodetritus shoal facies distributed in Fengtong Township in Wanyuan City, Panlongdong Village in Xuanhan County, Manyue Township in Kaixian, Well Fengtan 1 and Jiantianba Township with no obvious signs of migration.(4) The platform-margin reef-shoal facies in the Feixianguan Formation is dominated by widespread oolitic dolomite and oolitic limestone and gradually migrating basinward from the second member to the fourth member of Feixianguan Formation.The facies in the second member is mainly distributed in Batai Town in Wanyuan City, Manyue Township in Kaixian, Shuanghekou of Wanzhou District in Chongqing, Zhaigouwan and Shibaozhai areas.The facies in the third member migrated eastward to Ningchang area, and that in the fourth member to Kongjiagou area.The shoal facies of the second member of Feixianguan Formation is highly dolomitized and superimposed onto the reef-shoal facies of Changxing Formation.

Differential evolution and origin of high-quality reservoirs in the Lower Paleozoic carbonate buried hills in Dagang prospecting area, Huanghua Depression

Xin Cheng, Lihong Zhou, Yingchang Cao, Fengming Jin, Lixin Fu, Hongjun Li, Da Lou, Guanghui Yuan

2021, 42(3):  673-689.  doi:10.11743/ogg20210313

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The exploration for carbonate reservoirs in buried hills has been hampered by a poor understanding of their distribution and genesis.With a combination of seismic profile interpretation, core and thin section observation, logging analysis as well as geochemical and burial history analysis, the types of the Lower Paleozoic buried hills, reservoir characteristics and genesis of high-quality reservoirs in Dagang prospecting area were studied from the perspective of evolutional difference.The buried-hills in the area can be classified into five categories: monadnock, intermittently eroded fault-block buriedhill, fault-block buried hill, compressional folded buried hill and gravitational sliding folded buried hill.The monadnock resulted from a late-stage meteoric water leaching is dominated by dissolution pores and cavities, followed by some fissures.The intermittently eroded fault-block, a result of middle-stage meteoric water leaching and deep burial dissolution, is full of dissolution pores and tectonic fissures.The last three buried hills that shunned the middle- or late-stage meteoric water leaching contain reservoirs linked to faulting and deep burial and formed either by fault-related shattering-deep burial dissolution or by fault connection-TSR related dissolution.The former forms mostly composite reservoirs with dissolution pores, cavities and fissures in areas close to basement faults and would gradually turn into tectonic fissure-dominated reservoirs as they migrate away from the basement faults.The latter forms reservoirs commonly seen in deep buried formations with very high temperatures (higher than 140 ℃).These reservoirs are mainly composed of fissures, followed by some dissolution pores and cavities.It is therefore suggested that high-quality reservoirs are most probably developed in areas with the Upper Paleozoic strata heavily eroded or basement faults well developed or with the Upper Paleozoic strata developed against a high temperature setting (higher than 140 ℃).

Methods and Technologies

Prediction of burial dissolved vugs in carbonates based on dissolution simulation: A case study of the Longwangmiao Formation dolostone reservoirs, Sichuan Basin

Anjiang Shen, Zhanfeng Qiao, Min She, Shaoxing Meng, Jie Zhang, Xin Wang

2021, 42(3):  690-701.  doi:10.11743/ogg20210314

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The prediction of burial dissolved vug distribution in carbonate rocks has always been challenging.This study, taking the Longwangmiao Formation dolostone as an example, reveals the origin of burial dissolved vugs and their distribution in carbonates through dissolution modelling under formation conditions and may serve as a theoretical foundation for the prediction of burial porosity in carbonates.The results show that the burial dissolved vugs in carbonates are formed under the control of the physical properties of rocks, organic acid concentration and temperature.Their distribution is laterally conditioned by geological boundaries (namely, exposed interface, sequence boundary, unconformity and fault system) and high-permeability layers, and vertically showing a vug-generating peak season, during which, burial dissolved vugs occurred in great numbers because of suitable temperature and high concentration of acid fluid at some intervals with certain burial depth and then evolved along with burial, temperature-pressure and fluid changes.Based on the understanding of the origin of burial dissolved vugs and their distribution patterns, this study quantitatively predicts the burial dissolved vug distribution in the Longwangmiao Formation dolostone by mapping the pre-burial porosity distribution of the Longwangmiao Formation, the organic acid and hydrocarbon generation intensity of the Qiongzhusi Formation source rocks at the temperature window of 70-100 ℃, the distribution of paleo-uplifts and faults as well as the burial duration of the Longwangmiao Formation within the window (70-100 ℃, corresponding to a depth of 1 740-2 850 m).This technique and process may be a useful tool in the prediction and assessment of burial dissolved vugs in other carbonate reservoirs.

Pore evolution of the Permian Qixia-Maokou Formations dolomite in Sichuan Basin based on in-situ dissolution simulation experiment

Shiqi Liu, Senran Chen, Bo Liu, Kaibo Shi, Yuyang Liu, Haofu Zheng, Qingqing Luo

2021, 42(3):  702-716.  doi:10.11743/ogg20210315

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A hydrothermal diamond anvil cell (HDAC) equipped with a laser raman spectroscopy (LRS) and an optical microscope was used in an in-situ simulation experiment of dissolution-precipitation process between carbonate minerals (rocks) and pore fluid under continuous closed-open-closed cell (system) conditions that are typical of a deep burial process.In line with the actual geological conditions, the experiment was carried out on the Qixia-Maokou Formations dolomite samples from the Sichuan Basin and successfully generated acetic acid in closed cell through hydrolysis of acetic anhydride.The results show that dissolution occurs at the edge and cleavage of carbonate rock samples in a closed but periodically open system, and that the dissolution range is related to acid fluid flux.After entering the closed system, the relative ion concentration of solution was measured with the (semi-quantitative) Raman spectroscopy and indicates a precipitation tendency.The experimental results and the actual geological conditions of the Permian in the Sichuan Basin jointly indicate that the evolution of intergranular pores and intergranular dissolution pores of coarse-grained dolomites in the Qixia-Maokou Formations in southwestern Sichuan Basin conforms to the experimental simulation result and that transportation of ions in acidic fluids through tectonic fractures facilitates the redistribution of pores and materials through dissolution-cementation in the system.

Key seismic techniques for predicting deep marine carbonate reservoirs and the effect analysis: A case study on the Sinian-Cambrian reservoirs in the Sichuan Basin and the Ordovician reservoirs in the Tarim Basin

Yu Lin, Xiangwen Li, Kang Chen, Yintao Zhang, Dianguang Zang, Zhi Yu

2021, 42(3):  717-727.  doi:10.11743/ogg20210316

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The seismic characterization of carbonate reservoirs with the quantitative prediction at its core is key to reserve growth and production enhancement.Petroleum exploration and development in China have formerly been focused on middle-to-shallow terrestrial clastic rocks.In the past two decades, deep marine carbonate rocks, however, start to shine with the discovery of Tahe, Puguang, Anyue and other large-scale gas fields in the west of the country.In comparison to other countries, the deep marine carbonate reservoirs in China are generally characterized by more complex surface conditions, longer history, and higher heterogeneity, all of which have posed significant challenges to quantitative reservoir prediction.Based on the practices in the Ordovician fractured-vuggy reservoirs in the Tarim Basin and Sinian-Cambrian reef-shoal reservoirs in the Sichuan Basin, we systematically expound on seismic prediction approaches and key supporting technologies targeting at dealing with the difficulties specific to the two types of deep carbonate reservoirs.With continuous progress in technologies such as seismic acquisition with "wide azimuth and broadband as well as high density" (2W1H), high fidelity and broadband data processing for well control, pre-stack impedance inversion constrained by sedimentary facies and other key technologies, the reservoir prediction of deep marine carbonate rocks in China has changed from qualitative description into semi-quantitative prediction, and from simple description into fine depiction of reservoir flow units.The study is also of guiding value to the fine development of carbonate reservoirs in both China and other countries.

Internal architecture characterization of fractured-vuggy carbonate reservoirs: A case study on the Ordovician reservoirs, Tahe Unit S67, Tarim Basin

Xinrui Lyu, Jianfang Sun, Xingwei Wu, Hehua Wei, Fengying Xiao, Cuiyu Ma, Chuanzhen Song

2021, 42(3):  728-737.  doi:10.11743/ogg20210317

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For the purposes of precisely delineating the characteristics of fractured-vuggy carbonate reservoirs marked by diverse genetic types and complicated internal structure, the paleo-karst is subdivided into three types in terms of their major control factors and origins, namely the epi-karst, fault-controlled karst and underground river system.Formed by weathering and leaching, the epi-karst has a wide range of distribution with a reservoir architecture composed of small seam-cavern bodies, predominantly small karst caves as well as dissolved pores and fractures.Fault-controlled karst dominated by complex faults and fractures, large fault controlled karst caves, dissolved pores and dissolved fractures, are formed by dissolution enlargement mainly along the fault zone.The large pipe system of the underground paleo-rivers mainly dominated by the fluctuation of water table, came into being by long-term current corrosion, and its reservoir architecture mainly makes up of karst pipes, chambers, and dissolved fractures and vugs along the pipe.In the light of distinct characteristics of karst in origin, amplitude spectrum gradient seismic attributes combined with sequential indication simulation were applied to characterize the epi-karst distribution.Features of fault-controlled karst were depicted based on impedance inversion and gradient structure tensor attributes.The model of ancient underground river was constructed by means of frequency-division attributes and object-based simulation method.Consequently, logging, geologic and dynamic data were integrated to delineate the connectivity, filling and physical properties of the paleo-karst; in addition, a 3D model was constructed by a fusion of the above-mentioned karst types.The results show that this method can effectively characterize the complex internal structure and strong heterogeneity of such reservoirs.Unit S67 comes in quite complicated reservoir architectures and various connectivity styles.The epi-karst reservoir bodies are quite small in scale and locally connected almost in laminar distribution, with a thickness ranging from 0 to 50 m, better in physical property and connectivity.The fault-controlled karst reservoirs are large in scale in various distribution patterns along fault zones, with great discrepancy in physical property and good connectivity following the fault direction.The pipe system mainly developed in paleo-subterranean river, is marked by two layers of river course, with heavy filling, poor physical property and connectivity.Based on the fusion model, the unit reserve constitution was refined and ascertained.The model was applied to reservoir numerical simulation and development plan adjustment with remarkable effect.

Application of clumped isotopes to restoration of dolomitizing fluids and its limitations

Pingping Li, Chun Wang, Huayao Zou, Xinya Yu

2021, 42(3):  738-746.  doi:10.11743/ogg20210318

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Clumped isotopes (Δ₄₇) can be used to determine formation temperature of carbonate minerals.If combined with the conventional oxygen thermometer and δ¹⁸O of the minerals, the temperature can be used further to calculate the δ¹⁸O of fluids (water), in which the minerals are developed.This provides a potential new method for determining the δ¹⁸O and sources of dolomitizing fluids.However, affected by recrystallization and solid-state reordering after dolomitization, the clumped isotope temperature (T_Δ47) usually reads higher than the actual dolomitization temperature, thus limiting the application of T_Δ47 to the restoration of fluid δ¹⁸O.To determine the sources of the dolomitizing fluids from the Changxing (P₂c) and Feixianguan Formations (T₁f) in northeastern Sichuan Basin, clumped isotopes of dolostone samples from the Puguang and Yuanba gas fields were collected and measured.The T_Δ47 of P₂c and T₁f dolostone samples from the two gas fields ranges between 70 ℃ and 130 ℃, which, however, should be seen as apparent temperature of the samples considering the effect of recrystallization and solid-state reordering.The thermal history and solid-state reordering model results show that the apparent temperature of the samples would increase by about 30-35 ℃ under high-temperature solid-state reordering and the δ¹⁸O of the dolomitizing fluids is 0-4‰ (SMOW) after restoring solid-state reordering, indicating a high-salinity evaporative sea water source.

Seismic prediction of tight sandstone reservoir fractures in XC area, western Sichuan Basin

Junzhou Liu, Lei Han, Lei Shi, Shuangquan Chen, Wenya Lyu

2021, 42(3):  747-754.  doi:10.11743/ogg20210319

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A seismic fracture prediction method is proposed based on the fusion of multiple prestack and poststack seisimc attributes for predicting the development of tight sandstone reservoir fractures in XC area in the Sichuan Basin.The fracutres in the area include both large-scale fracutres that can be identified by seismic responses and small-scale fracutres that can not be readily identified by seismic responses.It is the latter that requires special processing.The geometric seismic attributes related to large-scale fractures are firstly extracted by using the poststack seismic data of high signal-to-noise ratio and then combined with the distribution pattern of the fractures in the target layer characterized with multi-attribute fusion to predict the distribution of small fractures through further combination with frequency-dependent anisotropic parameter inversion.At last, the prediction results are summarized to analyze the fracture distribution pattern of the target layer, with a view to accurately depicting potential targets with well-developed fractures.In all, the actual data obtained are consistent with predicted results, showing that fractured reservoirs can be accurately described by using this integrated multi-scale fracture prediction technology.

Non-deterministic method for tight oil reserves upgrade potential assessment and its application

Jun Li

2021, 42(3):  755-764.  doi:10.11743/ogg20210320

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Reserves upgrade potential assessment of continental tight oil reservoirs in China encounters a series of problems such as high reservoir heterogeneity, great uncertainties caused by limited geological understanding of certain stages, and marked differences between static and dynamic evaluation results.All these are difficult to be dealt with conventional deterministic methods.Therefore, a systematic non-determinstic reserves upgrade potential assessment method was proposed.The method is an improved version in terms of the following three aspects.Firstly, with uncertainties considered, the reserves with upgrading potential are quantitatively determined under diverse reliabilities.Secondly, a mutual correction and comprehensive evaluation of the static and dynamic methods is made possible by applying the posteriori probability theory.Thirdly, reserves upgrade probabilities and scales are quantitatively evaluated based on probability method by introducing the concept of upgrade probability.The reserves upgrade probability and scale under different oil prices and engineering technology conditions were clearly seen when applying the method to the possibility assessment of upgrading 'controlled reserves' to a higher reserves category for tight oil reservoirs in the Chang 8 Member of Well Block A, Ordos Basin.This shows that the method could be of effective technical support to tight oil reserves upgrade evaluation and exploration target optimization in China.