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JOPC 2023 Vol.25 Number 4
2023, Vol.25 Num.4
Online: 2023-08-01
SPECIAL ISSUE ABOUT "RESEARCH ON FINE SEDIMENTS"
LITHOFACIES PALAEOGEOGRAPHY AND SEDIMENTOLOGY
PALA EOGEOGRAPHY AND MINERAL RESOURCES
 
SPECIAL ISSUE ABOUT "RESEARCH ON FINE SEDIMENTS"
729 CAO Yingchang, LIANG Chao, HAN Yu, XI Kelai, WANG Junran, JI Shichao, MEI Junfang
Discussions on classification scheme for fine-grained sedimentary rocks based on sediments sources and genesis
The classification of fine-grained sedimentary rocks is crucial for the deep and systematical study of the fine-grained sedimentary rocks. The classification scheme not only has to reflect the sources,sedimentation types of rock components,it also should be widely applicable. There are various sources of fine-grained sedimentary rocks, including terrigenous clastic sources, endogenic chemical precipitation sources, biological sources and volcanic clastic sources. Among different basins,there are significant differences in the sediment source of fine-grained sedimentary rocks;furthermore,in the same basin,the same mineral can also has multiple sources. Currently,there is no systematic classification scheme for fine-grained sedimentary rocks,a special type of rocks. Based on the components, sediment sources and sedimentation types,we propose a comprehensive four-component three-terminal classification scheme for fine-grained sedimentary rocks. Firstly,the sediment source is considered as the first principle,and three end-members, i.e.,terrestrial clastic substances,endogenic chemical precipitants,and volcanic clastic substances,are used to classify them into four types(terrigenous clastic fine-grained sedimentary rocks,chemical fine-grained sedimentary rocks,pyroclastic fine-grained sedimentary rocks,and mixed fine-grained sedimentary rocks), and according to the content of organic matter, the fine-grained sedimentary rocks can be further divided into three categories: high organic matter, medium organic matter and low organic matter. Secondly, fine-grained sedimentary rocks are divided into 11 subtypes following the three-level naming principle, combined naming principle and compound naming principle. Thus, fine-grained sedimentary rocks could be named according to the relative content of the components.
2023 Vol. 25 (4): 729-741 [Abstract] ( 823 ) [HTML 1KB] [PDF 17766KB] ( 221 )
742 SHAO Longyi, ZHANG Tianchang
Discussion on definition and classification of mudrock
Mudrock is a major category of fine-grained sedimentary rocks,and there have been different understandings of its definition and classification in China,resulting in much confusion for academic research. This paper reviews the evolution of mudrock-related concepts and definitions in the development of sedimentology at home and abroad and attempts to provide a practical classification system for mudrock. Scholars in European and American tend to use the term “mudrock”, which refers to fine-grained terrestrial clastic rocks with a grain size less than 0.063 mm,including siltstone(two-thirds or more silt content),mudstone(neither silt nor clay content greater than two-thirds)and claystone(more than two-thirds clay content),in which the laminated rock is called shale. However, scholars in the former Soviet Union tend to use the term “глинистые породы” (argillaceous rock) to refer to sedimentary rocks containing more than 50% of clay minerals,in which the fissile rock is called shale. There are essentially two classification schemes for mudrock and other fine-grained sedimentary rocks,classified by texture(grain size),composition and sedimentary structure. One is a multi-element comprehensive rock classification,and the other is an end-members diagrammatic classification. We recommend the term “mudrock”as a general name for the fine-grained terrestrial sedimentary rocks consisting mainly of silt-sized(0.004-0.063 mm)and clay-sized(less than 0.004 mm)clasts. A triangular prism diagram classification method is adopted to classify fine-grained sedimentary rocks into fine-grained terrestrial sedimentary rock(i.e. mudrock)and fine-grained endogenous sedimentary rock,based on three end-elements of detrital minerals(mainly felsic detrital minerals),clay minerals and other endogenous minerals. Mudrock is further classified into three subgroups of siltstone,mudstone and claystone,or “-shale” if laminations or fissility are developed. Taking the TOC contents of 0.5% and 2% as the dividing points,fine-grained sedimentary rocks are classified into organic-poor,organic-bearing,and organic-rich ones. It is expected that this classification scheme will contribut to the mudrock classification.
2023 Vol. 25 (4): 742-751 [Abstract] ( 341 ) [HTML 1KB] [PDF 835KB] ( 186 )
752 LIU Huimin, WANG Yong, LI Junliang, ZHANG Pengfei, ZHANG Shun, WEI Xiaoliang, QIN Feng
Main controlling factors and distribution characteristics of shale lithofacies in the Eocene of Jiyang Depression
In recent years,over 10 100 meters of cores have been collected to the study of the shale oil and gas system in the Jiyang Depression. These cores provide excellent materials for investigating the main control factors and distribution of shale lithofacies in continental rifted basins. In this study,diverse shale lithofacies in the upper Member 4(Es4S)and lower Member 3(Es3X)of Eocene Shahejie Formation in the Jiyang Depression are identified by utilizing thin section,XRD mineral composition and major element data. The experiment results show the primary development of four types of shale lithofacies in the Jiyang Depression,namely carbonate shale facies,mixed shale facies,clay shale facies,and felsic shale facies. Moreover,these lithofacies are further subdivided into nine subtypes: organic-rich laminated carbonate shale facies,organic-rich layered carbonate shale facies,organic-rich laminated carbonate mixed shale facies,organic-rich laminated clay mixed shale facies,organic-rich layered felsic mixed shale facies,organic-rich laminated clay shale facies,organic-rich layered clay shale facies,organic-rich laminated felsic shale facies and organic-bearing layered felsic shale facies. The Jiyang Depression,located in a northern latitude Asian monsoon climate zone,experienced intense rifting and was underfilled by sediments during the Paleogene. The lacustrine palaeowater medium(palaeo-water depth,palaeo-salinity,and palaeo oxidation-reduction)and palaeo-provenance variations have jointly controlled the types and distribution characteristics of lithofacies. Among these factors,palaeoclimate and basin evolution controlled the large-scale development of organic-rich carbonate laminated shale lithofacies,while the types of shale lithofacies were controlled by the interplay between palaeo-water body and palaeo-provenance. Additionally,the distribution of shale lithofacies was determined by the interaction of palaeogeomorphology and palaeo-provenance. This study has established a belt distribution model of shale in the Es4S and Es3X of the Jiyang depression,which can provide valuable insights for shale oil and gas exploration in continental rifted basins and contributes to the understanding of shale genesis research under similar geological conditions.
2023 Vol. 25 (4): 752-767 [Abstract] ( 328 ) [HTML 1KB] [PDF 15180KB] ( 106 )
768 XIE Haoran, LIANG Chao, WU Jing, JI Shichao
Impacts of volcanic activity on sedimentary palaeo-environment and organic matter enrichment
Volcanic activity greatly affected the palaeoecological environment and biological evolution process. This study focuses on examining the volcanic activity records during the Ordovician/Silurian period in the Yangtze area. We incorporates the volcanic activity records from the Junggar Basin,Ordos Basin,and Santanghu Basin,as well as modern volcanic activities worldwide to elucidate the influence mechanism of volcanic activity on sedimentary palaeo-environment and organic matter enrichment. Tuff,sedimentary tuff,bentonite and tuffaceous sedimentary rocks formed in geological history are good proxy of volcanic activities. Through the analysis of the petrology,mineralogy and geochemical characteristics from macro-to micro-scale,this study established a comprehensive judgment method of multi-angle assessment of volcanic activity in the section core-microstructure-whole rock minerals-geochemistry. The nutrient elements released by volcanic activities lead to the proliferation of marine organisms and the increase of primary productivity,which is conducive to the accumulation of organic matter,while toxic substances impede the survival of organisms and reduce primary productivity. The relative amounts of nutrients and toxins released by volcanic activity may affect the primary productivity of the ocean. Low intensity volcanic activity can promote biological reproduction,while intense volcanic activity constantly increases the pressure of ecological environment restoration,ultimately resulting in irreversible damage. Volcanic activity intensifies water body hypoxia and enhances organic matter preservation efficiency. Sulphuric acid aerosols derived from volcanic gases increase planetary albedo and lower surface temperatures.
2023 Vol. 25 (4): 768-787 [Abstract] ( 325 ) [HTML 1KB] [PDF 3670KB] ( 209 )
788 SHI Zhensheng, WANG Hongyan, ZHAO Shengxian, ZHOU Tianqi, ZHAO Qun, QI Ling
Rapid transgressive shale characteristics and organic matter distribution of the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations in southern Sichuan Basin,China
The characteristics and formation of the MF(maximum flooding)black shale are one of the focuses of fine-grained reservoir geology research. Seismic interpretation,well correlation,X-ray diffraction whole rock analysis,total organic carbon(TOC)test and field emission scanning electron microscope analysis show that the MF black shale of the Longmaxi Formation in the southern Sichuan Basin is in the basal part of the Longmaxi Formation,corresponding to the graptolite belt LM1. The shale has an average content of 49.3% quartz(85% clay-sized),10.5% calcite,8.4% dolomite and 23.4% clay minerals. The quartz content increases while the clay mineral content decreases basinward. The MF black shale formed during the stage of rapid relative sea level rise,with a thickness of 0.5-2.8 m and gradually thickening basinward. The average TOC content is 5.4%,which exhibits a gradual decrease towards the basin and forms four distinct stacking patterns in the vertical TOC content profile. The mineral composition and thickness distribution of the shale are closely related to the rapid transgression,biology and volcanism during the sedimentary period. The rapid transgression has led to a decrease in terrestrial sediment input,resulting in a reduction in shale thickness. Additionally,biological activity and volcanic influences have resulted in a prevalence of microcrystalline quartz and an increase in shale thickness towards the basin. The high TOC content of this shale is related to anoxic water,low sedimentation rate,and high primary productivity. Anoxic water body enhances preservation of organic matter. Low sedimentation rates can weaken the dilution of organic matter,while high primary productivity can increase the supply of organic matter. The planar variation and stacking style of TOC content in this set of shale are related to water depth. With increasing water depth,there is an amplified degradation and recycling of organic matter during sedimentation,leading to a decline in TOC content. Simultaneously,as the water depth rises,the sediment accommodation space also increases,resulting in distinct stacking patterns of TOC content.
2023 Vol. 25 (4): 788-805 [Abstract] ( 289 ) [HTML 1KB] [PDF 4733KB] ( 83 )
806 ZHANG Meizhou, ZHU Xiaomin, JIANG Zhenxue, ZHU Deyu, YE Lei, CHEN Zhiyuan
Main controlling factors of organic matter enrichment in continental freshwater lacustrine shale: a case study of the Jurassic Ziliujing Formation in northeastern Sichuan Basin
It is of great significance to study the enrichment mechanism of organic matters and the associate main controlling factors of shale strata in continental freshwater lacustrine,which can guide the exploration and development of continental shale oil and gas. Lacustrine shales are developed in Dongyuemiao member and Da'anzhai member of Jurassic Ziliujing Formation in Sichuan Basin,which has great potential for oil and gas exploration. Its organic matter enrichment mechanism needs to be explored. Taking those shales in Ziliujing Formation as the research object,core description,thin section observation,electron microscopy scanning,measuring organic carbon content,kerogen microscopic analysis,and main and trace elements test were carried out to restore the sedimentary palaeoenvironment and then examine the main controlling factors of organic matter enrichment. The results show that the organic carbon content of Ziliujing Formation shale ranges from 0.29% to 3.43%,with an average value of 1.32%. Kerogen type index was mainly distributed in the range of 24-52,with an average of 37,indicating that organic matter was type Ⅱ1 and type Ⅱ2. Organic matter mainly came from the mixture of aquatic organisms,and terrestrial higher plants. The values of C,Sr/Cu,CIA,and Sr/Ba,and palaeo-salinity,proxies of palaeoclimates,indicate that the Sichuan Basin was a freshwater lacustrine in humid climate during the deposition of Ziliujing Formation. The medium productivity of lacustrine water provides sufficient material basis for the enrichment of organic matter. The oxygen-rich bottom water leads to the oxidation of organic matter,which is not favorable for the enrichment of organic matters. The analysis of cross plot of TOC and several palaeoenvironmental proxies show that organic matter enrichment in Ziliujing shales is mainly controlled by the palaeo-productivity,palaeo-climate,and deposition rate. Organic matter can be enriched only under the background of relatively humid climatic conditions,relatively high productivity,and appropriate deposition rate. Redox conditions and salinity are not the main controlling factors of organic matter enrichment in freshwater lacustrine shales,which is quite different from marine shales and saline lacustrine shales.
2023 Vol. 25 (4): 806-822 [Abstract] ( 313 ) [HTML 1KB] [PDF 9569KB] ( 112 )
823 LU Qiqi, XIN Honggang, WANG Lin, LUO Shunshe, DAN Weidong, FENG Shengbin
Sedimentary types,characteristics and model of lacustrine fine-grained gravity flow in the Member 7 of Trassic Yanchang Formation in Ningxian area,Ordos Basin
The lacustrine fine-grained gravity flow sand body is a favorable target area for exploration in deep-water basins. Based on core,drilling,well logging,mud logging and thin section identification,the sedimentary types,characteristics,sedimentary evolution process and sedimentary model of fine-grained gravity flow are studied in the Member 7 of Yanchang Formation in the Ningxian area,Ordos Basin. The findings results show the presence of ten lithofacies,six lithofacies associations,and five types of deep-water sediments,including slide-slump deposits,sandy debris flow deposits,hybrid event bed,turbidity current deposits and deep lacustrine muddy deposits. According to the lithofacies and their combination characteristics,the semi-deep to deep lake subfacies can be further divided into six sedimentary microfacies: confined channel,unconfined channel,levee,lobe,inter-channel and semi-deep to deep lake mud. The delta front sediments destabilized under the action of external forces,and experienced five evolution stages of sliding,slumping,sandy debris flow,hybrid event bed,and turbidity currents. With the“fluid transformation and separation”and“hydroplaning”,the gravity flow deposits gradually evolved from confined channel-levees,unconfined channel-levees,to lobes toward the center of basin. This study will help to deepen the understanding of lacustrine fine-grained gravity flow deposits and provides guidance for deep-water oil and gas exploration and development.
2023 Vol. 25 (4): 823-840 [Abstract] ( 285 ) [HTML 1KB] [PDF 23872KB] ( 97 )
 
LITHOFACIES PALAEOGEOGRAPHY AND SEDIMENTOLOGY
841 XU Kang, GONG Hanning
Lithofacies palaeogeographic characteristics and favorable reservoir facies of the Leikoupo Formation in Sichuan Basin
In recent years,important exploration findings have been made in the Middle Triassic Leikoupo Formation in Sichuan Basin. Based on the database consisting of outcrops,logging data,and seismic data and taking the third-order sequences stratigraphic framework as unit,the lithofacies palaeogeographic map of Leikoupo Formation is compiled and the characteristics of favorable reservoir facies are analyzed. The results suggest that the Leikoupo Formation in Sichuan Basin can be divided into two third-order sequences and that the main sedimentary environments are restricted platform and evaporate platform. In the restricted platform,the upper intertidal zone,the lower intertidal zone,the limited lagoon and the beach within the platform are developed. The supratidal zone and evaporate lagoon are developed in the evaporate platform. The palaeogeographic pattern changed from “high in the west and low in the east”during the SQ1 period through “high in the east and low in the middle”during the SQ2-TST period to “low in the west and high in the east”during the SQ2-HST period. In the early and later periods during the deposition of Leikoupo Formation,the lithofacies are dominated by dolomite and gypsum,however,in the middle period they are dominated by Limestone and gypsum. The north part of Guanghan slope in western Sichuan is a favorable area for the development of algal dolomite flat traps of the third period of the Member 4 of Leikoupo Formation. The Member 3 of Leikoupo Formation in the northwest of Zitong sag in western Sichuan and the Member 1 of Leikoupo Formation around Luzhou ancient uplift are favorable areas for the development of beach facies dolomite reservoir. Under the top unconformity of the Leikoupo Formation in northeastern Sichuan,the karst reservoirs in the structural superimposed unconformity are favorable to be formed.
2023 Vol. 25 (4): 841-855 [Abstract] ( 400 ) [HTML 1KB] [PDF 10304KB] ( 102 )
856 QIAO Dan, SHI Zhiqiang, ZHANG Xiang, LI Changhao
Characteristics of contourites in the Lower Silurian in southeastern Sichuan Basin,China
Modern marine geological surveys show that there are contourites along the slopes of carbonate platforms. However,it is difficult to identify contourite drifts preserved in the strata. In this study,we document contourites from the Lower Silurian Xiaoheba and Shiniulan Formations in the southeastern Sichuan Basin based on the field investigation and microscopic observation. Symmetrical graded beddings and banded structures with periodic variations are found from the Xiaoheba Formation at such sections as Sanquan in Nanchuan County,Jinzhu in Shizhu County,Huangcaochang in Wulong County. Nodular limestones and argillaceous strip-banded limestones are developed in the lower Shiniulan Formation,while thin-bedded sandstones are found in the upper Shiniulan Formation at Baiyangshu and Podu sections in Tongzi County. Compared with modern carbonate platforms at Bahama and Maldivian,and with consider of the typical deep-water isobathic sedimentary features,it is suggested that contourites predominate the sandstones of Xiaoheba Formation. Likewise,the thin-bedded sandstones yielding from the upper Shiniulan Formation might be the contourites controlled by periodic changes of sea level. Meanwhile,the striped limestones in the lower Shiniulan Formation are recognized as the deposits controlled by tractive current(i.e.,contour current). This study provides an actualistic example for ancient contourites which might enhance our understanding of the Lower Silurian palaeogeographic evolution and of oil/gas reservoirs in the southeastern Sichuan Basin.
2023 Vol. 25 (4): 856-871 [Abstract] ( 266 ) [HTML 1KB] [PDF 11742KB] ( 91 )
872 LIU Shengqian, HE Youbin, JIANG Zaixing, LUO Jinxiong, ZHU Xueqing, ZHU Qing, TANG Wei
Sedimentary characteristics, dominant factors and genetic model of lacustrine carbonate reef-shoal depositional system: a case study from the upper 4th Member of Shahejie Formation in western Dongying sag

Lacustrine carbonates are widely distributed in continental lacustrine basins. The development model of a large reef-shoal depositional system is of great significance for understanding the genetic mechanism of lacustrine carbonates. Based on drilling cores,thin sections,well-log,seismic data and geochemistry measurements,the sedimentary characteristics of lacustrine carbonates in the upper Member 4 of Shahejie Formation(Ess4)of the Dongying sag were analyzed in detail. The results show that seven rock types and four lithologic assemblages were developed in the study area,and four types of sedimentary microfacies, including algal reef,shallow shoal,mudflat and storm deposits can be identified. The algal reef and shallow shoal constitute the reef-shoal complex depositional system,which was mainly distributed in the NE-SW direction of the top of the carbonate platform. The mudflat was mainly developed in the platform and near the denudation area,while the storm deposits were distributed in the upper part of the southeast slope of the platform. From the early to late stage of the highstand period,the reef-shoal complex system gradually migrated laterally from the former vertical accretion,and the plane distribution gradually expanded to the upper part of the slope area. It was speculated that the warm climate,the prevailing southeast wind of the palaeo-East Asian monsoon climate,basin structure and palaeogeomorphology,lake level fluctuation,carbonate weathering and seawater transgression integrated controlled the formation and evolution of the lacustrine reef-shoal system. The “climate-provenance-basin”genetic model for lacustrine carbonates was proposed in this paper,which can enrich the understanding of the formation mechanism of large-scale reef-shoal depositional system in lacustrine basins.
2023 Vol. 25 (4): 872-888 [Abstract] ( 284 ) [HTML 1KB] [PDF 12534KB] ( 120 )
889 WANG Shan, CAO Yinghui, YAN Lei, DU Dedao, MA Debo, ZHANG Xiang, CHEN Zhiyong, ZHOU Hui, YANG Min, BAI Ying
Sedimentary characteristics and development model of the Middle and Lower Cambrian evaporite in Tarim Basin
The Middle and Lower Cambrian is an important exploration field in Tarim Basin. Development of the Middle and Lower Cambrian evaporites is one of the key factors to successful Cambrian subsalt exploration and plays an important role in oil and gas accumulation. Therefore,the distribution characteristics and development model of the Middle and Lower Cambrian evaporite are of great significance for oil and gas exploration. Based on the regional geological background,core slice observation and drilling data analysis,this study identifies rock types of the Middle and Lower Cambrian evaporite in Tarim Basin,depicts its spatial distribution and establishes its formative model. The study shows: (1)The evaporites of the Middle and Lower Cambrian in Tarim Basin are mainly salt and gypsum rocks. The Lower Cambrian evaporite is thin and only distributed in the west of the basin without salt accumulation center. The thickness and distribution range of the Middle Cambrian evaporite both increased. Taking Bachu uplift as the salt accumulation center,it is distributed in a large area in Bachu-Tazhong-Tabei area in thick layers. (2)In the Lower Cambrian Xiaoerbulak Formation,the origin of the evaporite was“Sabkha”and“shallow-water evaporative lagoon”in a carbonate ramp. In the Wusongger Formation,the origin of the evaporite was“Sabkha”in a restricted platform. In the Middle Cambrian,the basin was periodically isolated from an open sea and the depositional environment evolved into a drying-evaporative platform. The evaporite distribution had a sedimentary characteristics of salt-gypsum-dolomite-mudstone from inside to outside. The conclusions could deepen the understanding of the dolostone-evaporite paragenesis system in Tarim Basin,providing new reference information for the prediction of Cambrian high-quality caprock and the distribution of effective reservoir cap assemblages in the Tarim Basin. Meanwhile,this study provides an important theoretical support for the Cambrian subsalt exploration.
2023 Vol. 25 (4): 889-905 [Abstract] ( 260 ) [HTML 1KB] [PDF 6374KB] ( 77 )
 
PALA EOGEOGRAPHY AND MINERAL RESOURCES
906 YANG Shangfeng, BAO Zhidong, SHEN Yanwei, GAO Yinshan, LIN Yanbo, ZHU Lihua, CHEN Xiaohong, DING Sheng, ZHU Bohua, QI Qing, LI Fei
Diagenetic evolution and sources of main cements within palaeochannel sandstone reservoirs of the Jurassic Yan'an Formation in Jiyuan area,Ordos Basin
Diagenesis directly affects the evolution of reservoir physical properties,and further controls reservoir quality. It is of great significance for oil and gas exploration to clarify the relative sequence of diagenesis. The diagenetic evolution of Jurassic Yan'an Formation palaeochannel sandstone in Jiyuan area of Ordos Basin and its main sources of cements were studied based on core observation,common thin sections,cast(stained)thin sections,scanning electron microscopy(SEM),cathodoluminescence(CL),X-ray diffraction,fluid inclusion and carbon & oxygen isotopes. Also the impact of cements and compaction on the loss of intergranular pores was quantitatively analyzed. The results show that paleochannel sandstone reservoir of Yan'an Formation is mainly composed of arkoses and lithic arkoses,with average porosity of 15.1% and average permeability of 19.5×10-3 μm2,which belongs to medium-porosity and low-permeability reservoir. The diagenetic process has experienced compaction,early cementation,medium dissolution,late cementation and late replacement. As far as the cements are concerned,the content of ferrocalcite is the highest,and kaolinite is the most widely distributed. Cements are the main contributor to the reduction of original intergranular pore volume,with an average contribution of 37.5%,and compaction was the minor contributor,with an average contribution of 29.4%. In addition,calcites directly derived from diagenetic fluid related to atmospheric water;dolomite was a joint source of early calcite,coal and mudstone thermal evolution products,but ferrocalcite primarily originated from thermal evolution of coal and mudstone interlayer. This study establishes a diagenetic sequence of Jurassic Yan'an Formation palaeochannel sandstone reservoirs in Jiyuan area,which is able to provide a foundation for reservoir assessment for further exploration and development.
2023 Vol. 25 (4): 906-919 [Abstract] ( 359 ) [HTML 1KB] [PDF 5202KB] ( 97 )
920 CHEN Zengyu, LIU Rui, TAN Xiucheng, JING Cui, NIE Zhou, HENG De, JIANG Dingchuan, WEN Ran, CHEN Lei, TANG Yufeng, TANG Kui
Implications of multi-source quartz on shale-gas enrichment in the Wufeng-Longmaxi shale of Changning area in southern margin of Sichuan Basin, China

Quartz is a common mineral in shale,but it has a complex origin,which is of great significance for shale gas enrichment. It is particularly tricky to identify the quartz origin after the diagenetic process. In order to identify the sources and genesis of quartz minerals in the shale of Wufeng-Longmaxi Formation in Changning area,southern Sichuan Basin,environmental scanning electron microscope(SEM),X-ray diffraction(XRD),and helium porosity and total organic carbon(TOC)were studied. The results show that: (1)There are three types of quartz,including extrabasinal,intrabasinal,and authigenic quartz in the Wufeng-Longmaxi shale of the Changning area,the southern Sichuan Basin. (2)The high flux of extrabasinal(terrigenous)quartz diluted the abundance of organic matter in the 3rd layer of Long 1 member,resulting in a general trend of TOC first increasing and then decreasing with the increase of quartz content. (3)Intrabasinal quartz content correlates positively to TOC content,suggesting that the high abundance of plankton opal skeleton was coupled with the high productivity of organic matter. However,due to the higher preservation efficiency of biological opals compared with organic matter in the weak oxidation or oxidation background of the seabed,the correlation coefficient between clastic quartz and TOC is abnormally high. (4)Intrabasinal quartz is positively correlated with helium porosity because of the generation of secondary organic pores by the thermal cracking of organic matter that was initially produced with intrabasinal quartz. However,authigenic quartz generated by clay mineral transformation is negatively correlated with helium porosity,which is due to the filling of some primary or secondary pores by authigenic quartz.
2023 Vol. 25 (4): 920-930 [Abstract] ( 238 ) [HTML 1KB] [PDF 4142KB] ( 91 )
931 YU Zhou, HU Zijian, WANG Qianping, ZHAO Jing, WU Dongxu, WU Xingning, LI Weiling, LU Huili, ZHU Wenbo
Characteristics and main controlling factors of the Ordovician deep dolomite reservoirs in mid-eastern Ordos Basin
The deep Ordovician dolomite in the mid-eastern Ordos Basin is an important target for natural gas exploration. There are few studies on this set of dolomite reservoirs,and the reservoir characteristics and genesis are unclear,which restricts the further exploration. Based on outcrops,cores,thin sections,loggings,physical property data and CT scanning data,the dolomite reservoir is systematically studied here. The dolomit reservoir of gypsum-dissolved pore type,granular beach type and bio-turbated type developed in deep Ordovician in the mid-eastern Ordos Basin. The gypsum-dissolved pore type reservoir is characterized by anhydrite nodule bearing micro-crystalline dolomite,and the pore is composed of gypsum molds and micro-fractures with an average porosity of 2.57%. The grain-beach dolomite reservoir is composed of calcarenite-rudite dolomite,oolitic dolomite and crystalline dolomite. The pores include inter-granular pores and micro-fractures,with an average porosity of 4.65%. The bioturbated dolomite reservoir is porphyritic micro-crystalline dolomite and porphyritic calcareous micro-crystall dolomite. The pores are inter-crystalline pores and micro-fractures, and the average porosity is 2.54%. Development of gypsum-solution pore-type and grain-beach type dolomite reservoirs was mainly controlled by sedimentary facies,quasi-synchronous dissolution and rock salt filling. The gypsum bearing dolomitic flat and grain-beach facies provided the basis for development of the two types of dolomite reservoirs. The quasi-synchronous dissolution was the key to development of the reservoir,and the rock salt filling was the main factor for densification of the reservoir. Development of bioturbated dolomite reservoir was mainly controlled by bioturbation and dolomitization. Bioturbation createed basic conditions for development of bio-turbated dolomite reservoir. Quasi-contemporaneous dolomitization was conducive to formation and preservation of inter-crystalline pores. The above results are conducive to the exploration of deep dolomite natural gas in the Ordovician in mid-eastern Ordos Basin,and may provide technical support for the further exploration.
2023 Vol. 25 (4): 931-944 [Abstract] ( 247 ) [HTML 1KB] [PDF 5438KB] ( 87 )
945 HAN Guomeng, LIU Yanxin, WU Xuesong, JI Youliang, MOU Liangang, YANG Peng, DUAN Runmei, YUAN Xuehua, GUO Qiuxia
Quantitative study on diagenetic characteristics and pore evolution of middle-deep tight sandstone reservoirs: a case study of the Second Member of Kongdian Formation in Nanpi slope,Cangdong sag
Diagenesis plays a crucial role in shaping the evolution of reservoir physical properties and ultimately governs reservoir quality. Understanding the relative sequence of diagenesis is of utmost importance for oil and gas exploration. The diagenetic evolution of the Jurassic Yan'an Formation palaeochannel sandstone in the Jiyuan area of the Ordos Basin and its main sources of cements were studied based on core observation,common thin sections,cast(stained)thin sections,scanning electron microscopy(SEM),cathodoluminescence(CL),X-ray diffraction,fluid inclusion and carbon & oxygen isotopes. In addition,the impact of cements and compaction on the loss of intergranular pores was quantitatively analyzed. The results show that paleochannel sandstone reservoir of the Yan'an Formation is mainly composed of arkoses and lithic arkoses,with an average porosity of 15.1% and average permeability of 19.5×10-3μm2,classifying it as a medium-porosity and low-permeability reservoir. The diagenetic process has experienced compaction,early cementation,medium dissolution,late cementation and late replacement. Among the cements,ferrocalcite exhibits the highest content,while kaolinite is the most widely distributed cement. Cements are the main contributor to the reduction of original intergranular pore volume,accounting for an average contribution of 37.5%,while compaction was the minor contributor with an average contribution of 29.4%. In addition,calcites directly derived from diagenetic fluid related to atmospheric water,whereas dolomite originated from early calcite,coal and mudstone thermal evolution products. Ferrocalcite,on the other hand,primarily originated from thermal evolution of coal and mudstone interlayers. This study establishes a diagenetic sequence for the Jurassic Yan'an Formation palaeochannel sandstone reservoirs in the Jiyuan area,which serves as a fundamental basis for reservoir assessment and provides valuable insights for future exploration and development.
2023 Vol. 25 (4): 945-958 [Abstract] ( 242 ) [HTML 1KB] [PDF 22417KB] ( 83 )
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