Table of Content

28 August 2019, Volume 40 Issue 4

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Petroleum Geology

Sedimentary dynamics of basin: Status and trend

Lin Changsong

2019, 40(4):  685-700.  doi:10.11743/ogg20190401

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Sedimentary dynamics of basin or sediment filling dynamics of basin, a frontier in sedimentary basin analysis and petroleum geologic research, highlights the response relationship between basin filling characteristics and formation processes, as well as its controlling mechanisms.The status and trend of the study were documented considering the geographical features and research practices of sedimentary basins in China.The main themes include:① sequence architecture of sedimentary basins, as well as structure, evolution and process responses of depositional system tracts;② tectono-sedimentology or tectono-stratigraphy;③ "Source-to-Sink" system and sediment dispersal systems in basins;④ simulation of basin filling processes and so on.Analysis of sedimentary basin filling dynamics will continue to provide high-resolution sedimentary records for geodynamics, and theoretical support for the exploration and development of depositional geological resources, which have become increasingly difficult.The study betokens profound and new prospects.

Sedimentary environment of the Lower Cambrian Qiongzhusi Formation shale in the Upper Yangtze region

Zhao Jianhua, Jin Zhijun, Lin Changsong, Liu Guangxiang, Liu Keyu, Liu Zhongbao, Zhang Yuying

2019, 40(4):  701-715.  doi:10.11743/ogg20190402

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The lower Cambrian shale shows a bright potential for shale gas exploration and is an important strategic successor play for oil and gas exploration in the Upper Yangtze region.The sedimentary environment of the Qiongzhusi Formation shale was comprehensively studied through outcrop observation, core description, thin section observation, SEM analysis, and geochemical analysis of major and trace elements.According to mineralogy, rock fabrics and sedimentary structures, five major- and 12 sub-lithofacies types were identified in the Qiongzhusi Formation shale, six typical sedimentary sequences developed in shallow-water shelf, deep-water shelf, and slope-basin sedimentary systems were summarized, and the corresponding sedimentary models were established.Under the paleoceanographic conditions of oxidation and relatively low productivity, phosphorus-dolomitic-argillaceous sedimentary sequences and sandy-argillaceous-calcareous sedimentary sequences were developed in shallow shelf facies with low TOC.In contrast, organic-rich argillaceous-phosphorus-dolomitic, siliceous-argillaceous, and argillaceous sedimentary sequences were developed in deep shelf and slope-basin facies with sediments deposited in oxygen-poor, anoxic oceans with relatively high productivity.The majority of sedimentary zones in deep shelf facies is favorable for the development of organic-rich shales in the interior of intracratonic rift along Ziyang-Changning trend within Sichuan Basin, central and northern Guizhou and western Hubei-Eastern Chongqing areas in southeastern Sichuan Basin.In addition, hydrothermal activities provide necessary materials and preservation conditions for the black shales of the Lower Cambrian Niutitang Formation in northwestern Guizhou, which in turn may generate organic-rich shales.

The rock physics modeling experiment under overpressure and theoretical model for overpressure prediction in carbonate rocks

Liu Yukun, He Sheng, He Zhiliang, Zhang Dianwei, Li Tianyi, Wang Xiaolong, Guo Xiaowen

2019, 40(4):  716-724.  doi:10.11743/ogg20190403

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Prediction of overpressure in carbonate formations is still a difficult problem in overpressure researches.The most common methods on overpressure prediction in clastic formations are empirical based on Terzaghi effective stress theory as well as logging and seismic parameters(mostly P-wave velocity) which have a clear response to overpressure.These empirical methods are not applicable to predict overpressure in carbonate formations with dense matrix and extremely heterogeneous physical properties.We selected carbonate rock samples to carry out the rock physics modeling experiment with overpressure, and analyzed the relationship of rock elastic property with pore fluid pressure and effective stress.Based on the poroelasticity theory and generalized Hooke's law in fluid-bearing rocks, a theoretical model for overpressure prediction(a quantitative model of overpressure prediction) was established to reflect the quantitative relationship between pore pressure and rock elastic parameters by analyzing the constitutive relation between stress, strain and pore pressure in carbonate formations.The elastic modulus of rock matrix was calculated by Voigt-Reuss-Hill model and mineral composition content in carbonate rocks is obtained through lab measurements.The Wood and Patchy models were used to calculate the elastic modulus of pore fluids, and then the equivalent elastic modulus of rock framework is calcula-ted indirectly by the Biot effective stress theory and the correlation between effective stress and elastic modulus of rock framework was acquired through the rock physics modeling experiment.The above obtained elastic parameters were in turn used to calculate overpressure in carbonate rocks by the quantitative overpressure prediction model.In comparison with the pore fluid pressure artificially loaded in the carbonate rock physics modeling experiment, we verified the applicability of the quantitative model of overpressure prediction in carbonate rocks, and proposed the idea for correction of the theoretical model of overpressure prediction.The rock elastic parameters required for the theoretical model of overpressure prediction can also be calculated by studying logging and seismic data, and the pre-drilling overpressure prediction with seismic data could also be achieved in carbonate formations.

The “2W1H” seismic exploration technique and its application in Pubei area, Turpan-Hami Basin

Ran Jianbin, Zhang Yishan, Li Haiyin, Chen Jia, Li Wenge, Xing Hongge, Dai Yu

2019, 40(4):  725-737.  doi:10.11743/ogg20190404

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Precise recognition of trap and accurate prediction of reservoir distribution have always been the key factors res-tricting oil and gas discovery in Taibei sag of Turpan-Hami Basin.The signal-to-noise ratio, frequency bandwidth and resolution of seismic data have been significantly improved through seismic data acquisition of vibroseis with the "wide-band, wide-azimuth and high-density" technique, OVT domain time migration, establishment of initial velocity models for geosteering depth migration, and iterative processing of anisotropic depth migration.A good data base has been provided for the prediction of fine characterization of fracture system and sand reservoir in this area with six third-order sequences identified in the Jurassic, within which sand bodies at the bottom of Qiketai Formation and on top of Sanjianfang Formation are identified as two fourth-order sequences characterized by steadily rising semi-cycles under the guidance of high resolution sequence stratigraphy, and the comprehensive analysis of high-resolution seismic and well log data.The structure of "four belts and two depressions", together with its formation mechanism in the study area, has been revealed through analyses of structural geometric attributes and fine structure interpretation.The distribution of sand bodies in the petroliferous zones of Qiketai and Sanjianfang Formations is predicted by multiple broadband seismic inversion techniques.Based on the comprehensive analyses of structure, reservoir and hydrocarbon accumulation pattern, it's pointed out that the hydrocarbon accumulates along the ridge lines of the structural belts in the study area, and the fault system is the dominant route of hydrocarbon migration.The Qiketai and Sanjianfang reservoirs are developed in Pubei tectonic belt, thinning out to Yuguo and Shengbei sub-sags.The lithologic traps with nose-shaped structures, located at the joint of the Pubei and Yuguo tectonic belts, are favorable for making breakthroughs in oil and gas exploration.

Formation of fault bend fold-controlled meteoric karst reservoir in the middle and late Himalayan movement: A case study of the Dawangou Formation carbonate rocks at Yangjikan section, Keping, northwestern Tarim Basin

Qian Yixiong, Tian Mi, Li Huili, Chen Yue, Sha Xuguang, Li Hongquan

2019, 40(4):  738-751.  doi:10.11743/ogg20190405

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The karst profile at Yangjikan section, Keping, northwestern Tarim Basin, provides an excellent opportunity to evaluate the relative importance of reaction between fault bend fold-controlled uplift and meteoric water dissolution-controlled karstification during the middle and late Himalayan movement; and then the porosity caused by interlayer decollement under structural compression and dissolution was estimated, in line with the comprehensive study of petrographic, geochemical and fold disengaging analyses.The fold disengaging(simulated with VSD)facilitated the result of 0.2%-0.5% porous zones, accounting for 1/50-1/100 of the effective and actual empty-cavity rate.However, the porous zones have played a significant role in Darcy flow channel formation and karstification.The breccias related to mechanical fracture filling, calcareous tufas and megacrystalline calcites are well developed in the Yangjikan section karst reservoir.Compared to the surrounding rocks of the Dawangou Formation, the negative and linear correlation of δ¹⁸O(PDB)and δ¹³C(PDB)in the formation is distinct, and single-phase aqueous inclusions, typical products of meteoric water karstification, grew in the megacryst calcites within fractures and cavities.A successive tufa depositions from the lower to the upper section of karst profile, are characteristic of weakening negative depleted δ¹³O(PDB), δ¹⁸O(PDB) and δ³⁴S(V-CDT).The content of Al₂O₃, Fe₂O₃, MgO, K₂O, and Na₂O decreases from tufas to megacrystalline calcites to(long)cylindrical scalenohedron calcites, indicating a transition in climate conditions from semi-wet to humid, to semi-humid and then to drought, as well as changes of hydrodynamic conditions.Moreover, the karst cavern of 1.5m-2.0m high can be formed within less than 5ka under a hot humid and strong hydrodynamic condition, while it may take a longer time in semi-humid and drought climate.

Controls of facies-potential coupling on oil accumulation in the Mesozoic Ordos Basin

Qu Hongjun, Pu Renhai, Chen Shuo, Gao Shengli, Zheng Yanrong

2019, 40(4):  752-762,874.  doi:10.11743/ogg20190406

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Oil in Ordos Basin mainly accumulates in the Mesozoic, so it is of great significance to study the oil accumulation patterns of the Mesozoic basin.This study provided an overview of the history of hydrocarbon exploration theories and discussed the oil accumulation patterns in the Mesozoic Ordos Basin based on the systematic analysis of the relationships between the Mesozoic source rocks, sedimentary facies belts, hydrocarbon migration and accumulation, and the oil reservoirs.The results show that the sedimentary facies control the distribution and development characteristics of hydrocarbon source rocks and reservoirs, whereas the diagenetic facies control the distribution and reservoir space of quality reservoirs.Pressure or fluid potentials determine the migration directions and pathways of oil and gas, thereby the coupling of facies and potentials controls the locations of hydrocarbon accumulations and traps.The Ordos Basin was a large inland down-warped basin during the deposition of the Yanchang and Yan'an Formations as the main oil-bearing layer series, featuring relatively simple structures, relatively flat-lying strata, and poorly developed fractures.Therefore, among the six major requirements for hydrocarbon accumulation, risks associated with cap rocks, traps and preservation are almost negligible.When the source rocks are ascertained in the Mesozoic Ordos Basin, the main risks are related with reservoirs, as well as hydrocarbon migration and accumulation.Thus, facies-potential coupling controls oil accumulation in the Mesozoic Ordos Basin, revealing the accumulation pattern as follows:"source rocks control the distribution of play, sedimentary facies control the occurrence of play fairways, and potentials control the location of oil reservoirs".To be specific, the source rocks in Chang 7 and Chang 9 in the Mesozoic basin control the distribution of play, and the 11 fluvial-delta systems around the basin control the distribution of play fairways.For shallow reservoirs controlled by buoyancy, the nose structural uplift plays a significant role in controlling the formation and distribution of oil reservoirs, while for the lower assemblage reservoirs controlled by abnormal pressures, the migration directions and accumulation locations are determined by the diffe-rential pressures or fluid potentials between source rock and reservoir.In addition, facies-potential coupling controls the diversity of hydrocarbon accumulation in the Mesozoic Ordos Basin seen from both planar and vertical directions, which is embodied by the "compensatory accumulation" in the shallow layers of the eastern Ordos Basin, "vertical bead-like accumulation" in the western Ordos Basin, and "abnormal pressure-controlled accumulation" in the deep Yanchang Formation.This understanding is also applicable to the exploration targets in other large down-warped petroliferous basins with higher exploration maturity.

Characteristics of major unconformities and paleo-geomorphology during the Mesozoic key transformation stages in Kuqa-Tabei area

Wu Gaokui, Lin Changsong, Liu Yongfu, Liu Jingyan, Yang Xianzhang, Li Hao

2019, 40(4):  763-777.  doi:10.11743/ogg20190407

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Research on major unconformities and large-scale palaeo-uplifts developed during the key transformation stages in sedimentary basins is key to revealing the basin dynamics evolution and hydrocarbon accumulation.Based on the integrated analysis of seismic, logging and outcrop data, we demonstrated the characteristics of the Mesozoic major unconformities and palaeo-geomorphology, as well as how they are influenced by the dynamics evolution of Kuqa-Tabei basins through tracking unconformity truncation and onlap points, etc.Results show that the study area in the Mesozoic undergone four key transformation stages, namely the Late Permian, Late Triassic, Late Jurassic, Late Cretaceous stages; in turn four regional angular unconformities, namely TT, TJ, TK and TE, which have brought significant impact on the basinal tectonic palaeo-geomorphology.The transformation occurring at the Late Permian was mainly characterized by the development of large-scale palaeo-uplift belts in the central and western parts of the study area, and the unconformity TT was mainly distributed around the palaeo-uplifts with the maximum denudation thickness of 1 200 m.The tectonic movement of the Late Triassic resulted in a significant reactivation of the palaeo-uplift belts in the study area, and the unconformity TJ produced is of wide range with the maximum denudation amounting to 400 m.The tectonic movement taking place in the Late Jurassic resulted in another stage of uplifting and denudation in the central and western of the study area, and consequently the unconformity TK was developed with the maximum denudation of 500 m.The transformation of the Late Cretaceous is mainly characterized by the development of the Wensu palaeo-uplift at the western margin of the study area, and its associated unconformity TE is of limited distribution with the maximum denudation amounting to 300 m.Three geomorphologic units are recognized in the Mesozoic palaeo-highs, that is the uplift high, slope and depression areas.Around the slope area formed unconformity triangle zones, which are favorable for the development of litho-stratigraphic traps.In addition, it's believed that the four collisional orogeneses occurring around the basins had driven the formation of four transformation stages in the Mesozoic.

Characteristics of the Neogene strike-slip faults and their controls on hydrocarbon accumulation in Shaleitian uplift, Bohai Bay Basin

Zhang Zhengtao, Lin Changsong, Li Huiyong, Huang Zhi, Zhang Guokun

2019, 40(4):  778-788.  doi:10.11743/ogg20190408

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Shaleitian uplift is the most abundant area for Neogene hydrocarbon accumulation in the western Bohai Sea.In order to clarify the characteristics of strike-slip fault growth and their controls on hydrocarbon accumulation, the fault types and development characteristics were described in detail using high resolution 3-D seismic and core data, and the relationship between strike-slip faults and hydrocarbon play elements was also discussed.The results show that there are three types of strike-slip faults developed in the study area, namely the basement involved strike-slip faults, caprock strike-slip faults and extensional strike-slip faults.Various types of traps are formed within or between fault zones under the influence of different types of strike-slip faults.Among others, the large-scale drape anticline, fault-nose and faulted anticline traps are favorable traps, but the small-scale fault-block traps are adverse ones.Strike-slip faults control the formation and development of skeletal sand body and fault conduit systems, which are pathways for hydrocarbon expulsion and migration from mature source rocks from adjacent sags.High-angle fractures caused by strike-slip improve reservoir qualities such as permeability.Strike-slip faults widespread in the Neogene formations are the main factors controlling hydrocarbon accumulation in the study area.

Features and sedimentary evolution of high-frequency sequence in continental lacustrine rift basin: Example of the lower Shahejie member 3 in Jiyang Depression，Bohai Bay Basin

Peng Li, Wu Yiming, Lian Zhanggui, Peng Peng, Wang Jian, Su Zhou, Yi Zhenli

2019, 40(4):  789-798.  doi:10.11743/ogg20190409

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Lithofacies along with its assemblages is distributed regularly within the sequence stratigraphic framework.Based on T(transgression)-R(regression) sequence division features, variations, architectures and distribution patterns of the key sequential interfaces have been analyzed systematically in fault continental lacustrine basin in the lower part of the third member of the Paleogene Shahejie Formation(Es^3L) in Jiyang Depression, Bohai Bay Basin, by core observation and sample test data.And the evolution pattern of sedimentary sequences in Es^3L has been summarized furtherly.The results show that ① the major types of key sequential interfaces in Es^3L are the regional maximum flooding surface(MFS) and sequence interface.② Three variations of sequences are identified, namely the shallowing-upward R-shaped semi-cycle, deepening-upward T-shaped semi-cycle and stable cycle.While the sequence units can be reduced to three types, namely the type characterized by fast rise but slow drop, the type characterized by slow rise but fast drop, and the type characterized by constant variation.③ As an integrated third-order sequence, the Es^3L can be sub-divided into four forth-order sequences, that is, the early lacustrine transgressive system tract(early LTST), late lacustrine transgressive system tract(late LTST), early lacustrine regressive system tract(early LRST) and late lacustrine regressive system tract(late LRST).These forth-order sequences together with the sub-divided fifth-order sequence units feature comparability and integrity. ④ The lake level of the Es^3L in Jiyang Depression generally fluctuates in a fast-rise-but-slow-fall pattern, and the fluctuation can be classified into four stages:the fast-rise stage during the early LTST, the slow-rise stage during the late LTST, the slow-fall stage during the early LRST, and the fall stage during the late LRST.In the process of sequence evolution, factors, such as tectonic subsidence, sediment supply and climate conditions, play specific roles in different stages, resulting in differential distribution of lithofacies and their assemblages within the sequence stratigraphic framework.

Lithofacies characteristics and sedimentary environment of Es^4U and Es^3L: A case study of Well FY1 in Dongying sag, Bohai Bay Basin

Pang Shuyi, Cao Yingchang, Liang Chao

2019, 40(4):  799-809.  doi:10.11743/ogg20190410

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Characteristics summarized through core and microscopic thin section observations, including the organic matter content, inorganic mineral composition and sedimentary structure, were used to categorize the lithofacies types of the fine-grained sedimentary rocks in the upper section of the fourth member of Shahejie Formation(Es^4U) and the lower section of the third member of Shahejie Formation(Es^3L) in Dongying sag, Bohai Bay Basin.As a result, 11 lithofacies types were identified.The vertical variation of the sedimentary environment was analyzed in terms of paleoclimate, palaeosource, paleo-water-depth, paleosalinity and paleo-oxidation-reduction, based on the content of major and trace elements as well as vertical variation of specific element ratios.The fine-grained sedimentary rocks formed in semi-deep to deep brackish-to-saline lacustrine environments with relatively humid climate and reduction-to-strong reduction.In addition, the evolution of sedimentary environment is obviously periodic, featuring increase in water depth, decrease in salinity, climate change from arid to humid, change from strong reduction to weak reduction then to reduction, and terrigenous sediment supply change from large to small then to large quantities.All the five elements of sedimentary environment collectively influence the vertical distribution and combination of lithofacies types.With the water depth increasing, the proportion of carbonates to lithofacies assemblages will decrease.Without considering the gravity flows, a combination of water depth, offshore distance and terrigenous debris supply will affect the contents of quartz and feldspar, which will in turn influence the development of siltstone and silty fine-grained sedimentary rocks.The setting featuring high salinity and strong reduction facilitates the preservation of organic matters, prone to produce organic-rich lithofacies types.Variation of paleoclimate makes other paleoenvironmental elements change correspondingly, then affecting the distribution and combination of lithofacies types.

The hydrocarbon expulsion efficiency of medium-low mature source rocks and tight-shale oil potential in the Es⁴, Leijia area in West Depression, Liaohe Basin

Wang Yuan, Wang Shaoyong, Li Jianzhong, Zhang Yijie

2019, 40(4):  810-821.  doi:10.11743/ogg20190411

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Thick dolomitic/analcite mudstones and shales at the stage of lower maturity(R_o:0.3% -0.8%) are deve-loped in saline and restricted bay environment of the 4^th member of Shahejie Formation(Es⁴), Leijia area in West Depression, Liaohe Basin.Based on the analysis of casting thin sections and SEM images, it is found that structural fractures and constricted fissures are highly developed in shale reservoirs, intergranular pores in clastic minerals are also developed, and dissolved nanopores in dolomites and analcites are present without organic nanopores.What's more, the distribution of hydrocarbon source rocks with high abundance of organic matters(TOC>4%) is determined with the quantitative evaluation of well logging using TOC data(ΔlgR method).It reveals that quality hydrocarbon source rocks in Dujiatai and Gaosheng reservoirs are distributed mainly in Leijia and Chenjia sags.In addition, the experimental results of hydrocarbon expulsion efficiency show that at the current maturity stage, the expulsion efficiency of hydrocarbon source rocks ranges from 12% and 17%, with the hydrocarbon production totaling 10 to 35 mg/g.As for the high abundance and large cumulative thickness of hydrocarbon source rocks, it is estimated that the tight-shale oil reserves can reach 1.37×10⁸ t in Es⁴, Leijia area.According to the integrated analysis of source-reservoir assemblages, the Dujiatai reservoir in Es⁴, Leijia area is favorable for tight-shale oil exploration, while the Gaosheng reservoir is favorable for shale oil exploration.In a word, the studied area is favorable for integrated exploration of tight-shale oil since different types of hydrocarbon source rocks have certain hydrocarbon-generating and reserving potentials.

Triassic sedimentary model in Block T of Tahe oilfield, Tarim Basin

He Tingting, Duan Taizhong, Zhao Lei, Liu Yanfeng

2019, 40(4):  822-834.  doi:10.11743/ogg20190412

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The Triassic sedimentary system in Block T of Tahe oilfield is rarely documented.Based on the sequence stratigraphic division, the core, logging and seismic facies, as well as normalized seismic waveform attributes were integrated to analyze the relationship between sedimentary systems and tectonic movements of the foreland basin, and to establish the Triassic sedimentary model of Block T in Tahe oilfield, Tarim Basin.The results show that the Triassic in Block T of Tahe oilfield can be divided into six long-term base-level cycles, inside which the braided river delta, fluvial and lacustrine se-dimentary facies are identified;the normalized seismic waveform attribute analysis shows that the braided river delta-lacustrine sedimentary system presents a migration from the near north-east to the north, while the fluvial sediments were deposited in a nearly east-west direction.During the active period of thrusting, the tectonic subsidence in the proximal realm of the basin resulted in the rapid proximal accumulation of a large amount of coarse-grained sediments, forming a braided river delta-lacustrine sedimentary system with a transportation direction approximately perpendicular to the orogenic belt;while during the quiescent period of thrusting, tectonic uplifting occurred in the proximal realm of the basin, the terrain grew gentle and the fluvial transportation got weak.As a result, the fine-grained sediments were deposited in the nearly EW-trending fault zones formed during the thrusting period, forming the fluvial sedimentary system with a sediment transportation direction approximately parallel to the orogenic belt.Therefore, the thrusting in the foreland basin ge-nerated two sedimentary models for the Triassic sedimentary system in Block T of Tahe oilfield, with transportation directions being approximately perpendicular to and parallel with the orogenic belt, respectively.The sedimentary models are of guidance value for the exploration and development of the Triassic clastic reservoirs in the Tarim Basin.

Carbonate sequence architecture, sedimentary evolution and sea level fluctuation of the Middle and Lower Ordovician on outcrops at the northwestern margin of Tarim Basin

Wang Qinglong, Han Jianfa, Li Hao, Sun Yanda, He Haiquan, Ren Shijun

2019, 40(4):  835-850,916.  doi:10.11743/ogg20190413

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Carbonate deposits of the Middle and Lower Ordovician develops well on Kalpin and Bachu outcrops, north-western margin of Tarim Basin.Conodont biostratigraphy shows that the unconformable contact exists between the Ying-shan Formation and its underlying Penglaiba Formation with a 5.9 Ma hiatus, while the Yingshan Formation and its overlying Yijianfang Formation are conformable with a total of 14.4Ma deposition.Comprehensive analysis of outcrops, existing geochemical indicators, and 3-D seismic and well logging data in Tazhong uplift, indicates that the carbonate rocks of the Yingshan and Yijianfang Formations can be divided into two composite (second-order) sequences (CS1 and CS2) along the major unconformity.From within, five third-order sequences (Sq1-Sq5) are identified along sub-level exposed unconformities or lithofacies mutation boundaries, which may be used for regional correlation.Eleven and six microfacies types are recognized for limestones and dolomites respectively through observation of hand specimens and thin section, which have in turn combined into seven micro-facies associations according to specific facies sequence, namely the reef-shoal complexes at platform margin, clastic shoals within tidal flat, intra-platform shallows and back shoals, bioclast shoals, inter-shoal sea and dolomite flats, indicating a transition from the arid-to-semi-arid restricted platform lagoon and dolomite flat sedimentation of the CS1 to the open intra-platform clastic shoal, bioclast shoal and reef-shoal complex at platform margin of the CS2, as well as the platform drowning in the late of Middle Ordovician.In addition, the fluctuation of relative sea level was reconstructed for the outcrop areas by means of Fischer plot.It's highly consistent with the paleo-water depth evolution demonstrated by facies cycle and geochemical indicators, and can also be correlated with Haq eustatic curve.The eustatic variation dominates the evolution of sequence architectures and deposits, despite that local architectures and deposits are restricted by both eustatic variation and tectonic movement.And the fourth-order eustatic variation starkly controls the facies sequence association.

Characteristics and geological significance of unconformities at the late Early Oligocene in the Pearl River Mouth Basin, northern South China Sea

Gong Yue, Lin Changsong, He Min, Zhang Zhongtao, Zhang Bo, Shu Liangfeng, Feng Xuan, Hong Fanghao

2019, 40(4):  851-863.  doi:10.11743/ogg20190414

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The widespread unconformity(T₇) in the Pearl River Mouth Basin(PRMB), the origin of which is closely related to dynamic basin evolution, eustatic variation and hydrocarbon accumulation, is one of the most important unconformities of the basins located at the northern margin of South China Sea.The provenances and sedimentary environments above and below the interface show significant differences.Based on seismic and logging data, we documented the distribution and structural features of the unconformity in the PRMB during the late Early Oligocene T₇(32~30 Ma), and discussed the mechanism of the unconformity and the basinal evolution features indicated herein.Results show that the T₇ boundary is a stage boundary unconformity with most faulting terminating at T₇ or the fault displacement diminishing markedly across the unconformity, while the underlying Enping Formation formed in the shape of a wedge with its stratum thickness obviously controlled by faulting.The T₇ unconformity can be divided into three zones:the angular unconformity zone, local unconformity and onlap zone, and conformity zone in the north, corresponding to the strong erosion zone, weak erosion zone, and non-erosion zone respectively.The angular unconformity zone is characterized by truncation at the shoulder part due to fault block rotation; the local unconformity and onlap zone shows onlap features and local denudation that is significantly gentle compared with angular unconformity zone; the conformity zone is located on the south of the southern uplift in PRMB and characterized by onlapping at the depression margin and conformity contact in the depression center.The distribution pattern of the stage boundary unconformity in this study area is controlled by tectonism and palaeogeomorphology in different zones of the basin.The angular unconformity zone in the north is mainly controlled by fault rotation and uplifting, but in local unconformity and onlap zone, the erosion is mainly caused by diapir or local uplifts.The conformity zone was located under paleowater level and at the depression center.The pattern indicates that the formation mechanism of the stage boundary unconformity is related to the erosion on shoulder uplift structure at the basin margin at the rifting stage.

Late Miocene to Quaternary sequence architecture and unidirectionally-migrating channels in the northeastern continental slope, Pearl River Mouth Basin

Jiang Jing, Zhang Zhongtao, Li Hao, Luo Yi, Tian Hongxun, Liu Hanyao

2019, 40(4):  864-874.  doi:10.11743/ogg20190415

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Based on the integrated analysis of 3D and 2D seismic, well-logging and paleontological data, two second-order sequences(CS1 and CS2), eight third-order sequences(S1-S8), as well as three main depositional systems(unidirectio-nally-migrating channels in continental slope, shelf margin clinoforms, and shelf-edge deltas) have been recognized in the northeastern shelf edge of the Pearl River Mouth Basin.From outer shelf to lower slope, four types of continental slope channels can be identified:(1) the partly confined outer-shelf unidirectionally migrating composite channels;(2) the broad and shallow outer-shelf to shelf-break erosional composite channels that are unconfined or partly confined;(3) the confined and deeply incised unidirectionally migrating slope channels with vertical and lateral accretion;(4) the U-shaped and unconfined or partly confined vertical accretional composite channels in lower slope.In addition, in view of characteristic variations of sequence boundary, sequence architecture, and depositional system, two types of compositional models of the system tracts have been concluded:(1) the one mainly composed of widespread shelf-margin unidirectio-nally-migrating channels in the Yuehai and Wanshan Formations, which are quite different from the EXXON sequence model;(2) the one that consists dominantly of the Quaternary high-angle shelf margin clinoforms and slope channels.The sequence architecture and depositional evolution in the continental slope in the study area are controlled by a combination of different factors, such as sea level change, tectonism, subsidence rate, and sediment supply.

Sequence architecture and evolution of shelf-margin deltaic systems of the Late Oligocene in Pearl River Mouth Basin

Zhang Manli, Lin Changsong, He Min, Zhang Zhongtao, Li Hao

2019, 40(4):  875-885.  doi:10.11743/ogg20190416

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Outer-shelf to shelf-margin delta and deep-water slope fan systems, the most important petroleum reservoirs in the Pearl River Mouth Basin(PRMB), were widely developed in the Upper Oligocene of the northern South China Sea.Sequence architecture, sedimentary genetic types, as well as sedimentary evolution and its controlling factors of the shelf-margin deltas and associated slope fans in the Upper Oligocene(Zhuhai Formation) in the southern PRMB were studied based on the integrated analysis of seismic, core and well logging data.The Zhuhai Formation is a composite sequence which can be further divided into four sequences.The shoreline trajectories were mapped by trac-king the changes of shoreline breaks, and the depositional genetic units(transgression, slightly-ascending normal regression, apparently-ascending normal regression and locally-descending forced regression, etc.) were identified.The outer-shelf deltas are characterized by tangential reflections on seismic profiles with the thickness of foresets ranging from100 m to 150 m.In contrast, the shelf-margin deltaic clinoforms, with a thickness of 200-1 000 m, display as S-shaped or sigmoid-tangential seismic reflections, and generally consist of thick beds of delta front bar deposits and large-scale slump and deformed sediments, which are associated with prodelta-slope fan systems.In addition, six deltaic clinoforms were identified in the study area, and their areas tend to extend along their strikes.The composite sequence(CS3) is controlled by both relative sea level changes and tectonic subsidence.The massive sediment supply and regional sea level fall are dominant factors controlling the wide-spread distribution of the shelf-margin delta and prodelta slope fan systems.

Diagenetic evolution characteristics constrained by lithofacies in deep tight sand gas reservoir

Lin Jianli, Zhang Xianguo, Lin Chengyan, Duan Dongping, Huang Xin, Sun Xiaolong, Dong Chunmei

2019, 40(4):  886-899.  doi:10.11743/ogg20190417

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The sedimentary and diagenetic evolution of the deep tight gas reservoir in the East China Sea Basin is complex.The diagenetic evolution of different lithofacies reservoirs is key to revealing the sedimentary-diagenetic coupling, to understand the genetic mechanism of "sweet spot" in tight reservoirs, and also to lay a sound foundation for "sweet spot" prediction.A case study of the typical tight gas reservoir in the Huagang Formation located in the deep central structural inversion belt of the Xihu Depression, East China Sea Basin, reveals the diagenetic evolution of various lithofacies reservoirs through core observation, SEM, casting thin section, granularity and X-ray diffraction analyses.The control of deposition on diagenesis is revealed by our diagenetic evolution models for different lithofacies reservoirs, which are based on petrological characterization, classification of main lithofacies types, and analysis of diagenetic evolution sequence and burial-thermal evolution history.The results show that the sediments in underwater distributary channels constituting the main body of the Huagang Formation reservoirs in the braided-river delta front, have mainly developed four lithofacies types, namely massive medium-to-coarse grained sandstones, gravel sandstones, medium-to-fine grained sandstones with parallel bedding and muddy gravel sandstones.The massive medium-to-fine grained and gravel sandstones is high in rigid quartz content, but low in clay content, with dominant thin chlorite and siliceous cements.However, the best reservoir quality is observed in massive medium-to-coarse grained sandstones due to their relatively complete diagenetic process and growth of primary and secondary solution pores at the later buried stage.In contrast, the gravel sandstones are prone to generate tight reservoirs characterized by greater compaction and intense siliceous cementation due to their poor sorting.In addition, the medium-to-fine grained sandstones with parallel beddings, mainly associated with thick chlorite, I/S mixed layer and illite, feature low content of rigid quartz but high clay content.Clastic particles tend to arrange directio-nally in lamina due to sedimentary differentiation:the fine-grained laminae favor stronger compaction at later stage of diagenesis that helps the preservation of local primary porosity in the coarse-grained part, generating reservoir pockets.Muddy gravel sandstones contain lots of pelinite clasts as well as argillaceous matrix, and they have undergone stronger compaction at the eogenetic stage, resulting in destruction of porosity and reservoir quality.

Oil-sources rock correlation of Bayingebi Formation in Hari sag, Yingen-Ejinaqi Basin

Chen Zhijun, Ma Fangxia, Xiao Gang, Zhang Yong, Gao Yiwen, Wang Xiaoduo, Han Changchun

2019, 40(4):  900-916.  doi:10.11743/ogg20190418

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In recent years, some achievements have been made in oil & gas exploration in Hari sag, Yingen-Ejinaqi basin, but the exploration here is very difficult due to its complex geological conditions.Especially, with the deepening of exploration, previous research results of oil & gas sources can not meet the needs of fine exploration research.Based on the test and analysis data of crude oil, oil sand and source rock samples, Bayingebi Formation is subdivided into three segments(K₁b¹, K₁b² and K₁b³ from bottom to top), then the fine oil source analysis is carried out, and the distribution prediction and characteristics evaluation of the main source rocks are carried out in order to clarify the source of oil and the characteristics of the main source rocks.The results show that carbon isotopes have a good indication for oil sources in the study area, the crude oil in main oil-producing segments of K₁b¹ and K₁b² mainly comes from K₁b¹ and K₁b² source rocks, respectively.Biomarkers such as acyclic alkanes, steroids, terpenes and aromatics also indicate the same conclusion.In particular, aromatic compounds have achieved good application results in this study.The maximum thickness of effective source rocks in K₁b¹ is 151 m, and K₁b² is 222 m, the two sets of main source rocks both with a large thickness in the study area.As for geochemistry characteristic of source rock in K₁b¹, TOC averages at 0.89%, δ¹³C of kerogen ranges from -29.1‰ to -24.5‰, R_o from 0.60% to 2.01%, the source rocks are characterized by medium-good organic mater abundance, Type Ⅱ₁-Ⅱ₂ and maturity-high maturity.For source rocks in K₁b², TOC averages at 1.18%, δ¹³C of kerogen ranges from -30.3‰ to -23.8‰, R_o from 0.60% to 2.17%, the source rocks are good-excellent organic mater abundance, Type Ⅱ₁-Ⅱ₂ and maturity-high maturity source rock.With high abundance of organic matter, and in the thermal evolution stage of the peak period of hydrocarbon generation, the two sets of main source rock both have a good hydrocarbon generation potential, and can provide a good hydrocarbon basis for oil & gas accumulation.

Characteristics and controlling factors of deep-water channel sedimentation in Lower Congo Basin, West Africa

Li Quan, Wu Wei, Kang Hongquan, Ren Shijun, Pang Lin'an, Yang Ting, Cai Lulu, Liu Xiaolong

2019, 40(4):  917-929.  doi:10.11743/ogg20190419

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Deep-water channel system is an important part of the "Source-to-Sink" system from continental slope to deep-sea basin, and also a critical reservoir type in international deep-water petroleum exploration.The study of sedimentary characteristics, filling processes and controlling factors of the Miocene deep-water channels in Lower Congo Basin is of great help to deepen the understanding of basin dynamics and improve the prediction accuracy of deep-water reservoirs.Based on the integrated analysis of 3D seismic and logging data, we carried out in-depth study on the evolution of the Miocene high-resolution sequence stratigraphic framework, deep-water sedimentary units, types of deep-water channels, and the initiation, development, decline and extinction of channel complex systems, as well as the controlling factors in Lower Congo Basin.The technical means in terms of sequence stratigraphy, seismic stratigraphy and seismic geomorphology were resorted to in this study.The Miocene strata in the Lower Congo Basin can be divided into 14 fourth-order sequences with the maximum regression surface and the initial transgression surface serving as the sequence boundary.Among others, the fourth-order sequences SQ1L, SQ2L, SQ3L, SQ4L, SQ5L, SQ6L and SQ7L embody the major stages of deep-water channel development.Seven types of deep-water sedimentary units were identified, namely the erosional surface, channel filling, inner levee, outer levee, terminal lobe, pelagic deposit and slump block.And the channel filling and terminal lobe are arenaceous sedimentary units.The deep-water channels in the study area can be divided into four basic types, namely the single incised-channel, single aggradation channel, vertical aggradation channel complex and lateral migration channel complex.And the vertical aggradation and lateral migration channel complexes occur simultaneously in the fourth-order sequence SQ7L, indicating the controls of "autocyclicity" on channel filling processes.In conclusion, tectonic uplifting is a driving force for the formation of deep-water sand-rich channel systems, and the eustatic fluctuation in West Africa controls the development of third-order sequences and the scale of deep-water channel complex.The alternating climate change between greenhouse and icehouse controls the occurrence of deep-water arenaceous gravity flow deposits and deep-water argillaceous deposits in the fourth-order sequences.

Prediction of preservation conditions for Nanchuan shale gas reservoirs based on distribution of seismic quality factor Q

Tang Jianxin

2019, 40(4):  930-937.  doi:10.11743/ogg20190420

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The distribution of seismic quality factor Q and conditions for shale gas preservation of the lower shale interval of Wufeng-Longmaxi Formations were studied in Nanchuan area based on seismic and geologic data as well as seismic attributes.In regard to the faulted fracture zones in the Nanchuan shale gas block, the workflow and its iterative algorithm were proposed to work out the stable seismic quality factor Q and its limits.Results show that the internal features of the faulted fracture zones and distribution of different types of shales can be recognized rapidly and directly from the distribution of seismic quality factor Q.Thereinto, the seismic quality factor Q of the faulted fracture shale zones is generally low, with its value mostly below 0.2 while locally over 0.5, and shows chaotic or porphyritic distribution; the shales are distributed in the shape of stripes, trending north-east, but ripped and crushed by the fault zones, resulting in poor preservation conditions.The seismic quality factor Q of the stably-distributed shale zones is higher, with its value being more than 0.7, and shows stable distribution in plane without any abrupt changes, leading to good preservation conditions.The seismic quality factor Q of the shale zones with well-developed micro-fractures tends to change gradually, with its value diminishing regularly from 0.7 at the margin to 0.1 at the centre of the shale zones, implying the best preservation conditions.Practices have shown that most high-yield shale gas wells are located within the stably distributed shale zones or stable shale zones with well-developed micro-fractures, and the method of evaluating shale with seismic quality factor Q is a practical tool for assessing preservation conditions for shale gas and optimizing the deployment of development wells.

The impact of organic matter content on overpressure analysis and its correction method in the first member of Cretaceous Qingshankou Formation, Placanticline area, Songliao Basin

Xu Zeyang, Zhao Jingzhou, Li Jun

2019, 40(4):  938-946.  doi:10.11743/ogg20190421

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The organic-rich shale is composed of three parts:non-organic mudstone system, solid organic matter and pore fluid.Given that the empirical analysis of source rock overpressure by logging parameters tends to be influenced by orga-nic matter, we proposed a set of logging parameter correction methods based on the content of organic matter for the identification of source rock overpressure origin.A case study of the shale in the first member of Qingshankou Formation (K₁qn₁) in Well Pu532 in Placanticline area, Songliao Basin shows that the knowledge of overpressure caused by disequilibrium compaction has been proven wrong by both porosity comparison and velocity-density cross plot under the effect of organic-matter correction.Furthermore, when measuring the pressure of a research target with its predicted buried depth less than the largest buried depth in history, it is necessary to correct the normal compaction trend by the difference between the maximum palaeoburied depth and the current one.Subsequently the prediction precision of typical wells' pressure will be better than 95% in line with corrected logging parameters.