The Middle Miocene lobe-shaped and band-shaped submarine fans in the Lingshui Sag,Qiongdongnan Basin:source-to-sink system,genesis and implication

Deepwater oil and gas exploration is the key to sustainable breakthroughs in petroleum exploration worldwide.The Central Canyon gas field has confirmed the Lingshui Sag is a hydrocarbon-generating sag,and the deepwater reservoirs in the Lingshui Sag still have more fabulous oil and gas exploration potential.Based on drilling data and three-dimensional(3D)seismic data,this paper uses seismic facies analysis,seismic attribute analysis,and coherence slice analysis to identify the types of submarine fans(lobe-shaped and band-shaped submarine fans)that developed in the Lingshui Sag during the Middle Miocene,clarify the source-to-sink system of the submarine fans and discuss the genesis mechanism of the submarine fans.The results show that:(1)the deepwater source-to-sink system of the Lingshui Sag in the Middle Miocene mainly consisted of a“delta(sediment supply)-submarine canyon(sediment transport channel)-submarine fan(deepwater sediment sink)”association;(2)the main factor controlling the formation of the submarine fans developed in the Lingshui Sag was on the relative sea level decline;and(3)the bottom current reworked the lobe-shaped submarine fan that developed in the northern Lingshui Sag and formed the band-shaped submarine fan with a greater sand thickness.This paper aims to provide practical geological knowledge for subsequent petroleum exploration and development in the deepwater area of the Qiongdongnan Basin through a detailed analysis of the Middle Miocene submarine fan sedimentary system developed in the Lingshui Sag.

Mechanism of carbonate cementation and its influence on reservoir in Pinghu Formation of Xihu Sag

Carbonate cements are the most abundant authigenic mineral and impact on physical properties greatly in sandstone reservoir.In this paper,Pinghu Formation of Xihu Sag was taken as a target.Characteristics,distribution and formation of carbonate cements were investigated via optical microscopy,cathodoluminescence(CL),electron probe and in-situ carbon-oxygen isotope.The results showed that carbonate cements varied in types and shapes.Calcite/dolomite mainly present as poikilotopic cements,while ferrocalcite/ferrodolomite/ankerite generally present as pore-filling cements.Carbon isotope(δ^(13)C)values of carbonate cements were ranging from–7.77‰to–2.67‰,with an average of–4.52‰,while oxygen isotope(δ^(18)O)values were ranging from–18.94‰to–12.04‰,with an average of–14.86‰.Theδ^(13)C/δ^(18)O indicated that the paleo-fluid of carbonate cement was mainly freshwater.Organic carbon mainly came from organic matter within mature source rocks,and inorganic carbon came from dissolution of carbonate debris and early carbonate cements.Distinctiveδ^(13)C/δ^(18)O values manifest that carbonate cements with different types formed in different periods,which make different contributions to the reservoir properties.Calcite/dolomite formed during eodiagenesis(70–90℃)and early mesodiagenesis stage(90–120℃),and were favorable to reservoir owing to their compacted resistance and selective dissolution.Ferrocalcite/ferrodolomite/ankerite formed during middle-late mesodiagenetic stage(above 120℃),and were unfavorable to reservoir due to cementing the residual intergranular pores.Hence,in order to evaluate the reservoir characteristics,it is of significantly important to distinguish different types of carbonate cements and explore their origins.

Complex genesis of multiperiod fractures in the Middle Triassic Leikoupo Formation in the Pengzhou gas field, western Sichuan Basin, China

The cumulative expression of multistage deformation is complex multiperiod fractures,which are commonly seen in tectonic zones.The Middle Triassic Leikoupo Formation in the western Sichuan Basin Depression,China,is a typical marine carbonate reservoir with natural fractures caused mainly by tectonic movements.According to outcrops,drill cores,image logging,and fluid inclusions,the fracture characteristics,types of natural fractures,and interactions of fractures are determined.In total,419 natural fractures in 493.2 m of cores from 7 wells are investigated,which are mainly shear and tensile fractures with a small number of weathering generated fractures.Meanwhile,the results of the stable isotope analysis of δ13C and δ18O,as well as the flow fluid inclusion data,reveal four tectonic periods of fractures with different occurrences.Based on the history of regional tectonic evolution,indicating one period of weathering fractures ascribable to stratal uplift and three periods of structural fractures related to the sequential tectonic movements of the Longmenshan fault belt.By analyzing the interaction relationships of fractures,three types of fracture interaction relationships are observed:cutting,restraining,and overlapping.The four stages fractures are chronologically assigned to(1)the early Indosinian N-S trending compression,(2)the late Indosinian NW-SE compression,(3)the middle Yanshanian NE-SE compression,and(4)the early Himalayan E-W compression.The influence of natural fractures on gas migration and well production in marine carbonates is discussed,and indicates that tectonic fractures could provide seepage channels for gas migration and accumulation from near or distant hydrocarbon source rocks into the Middle Triassic Leikoupo Formation.This study utilizes a pragmatic approach for understanding the fracture genesis mechanism in oil and gas field with multiperiod fractures.

Two stages power generation test of the hot dry rock exploration and production demonstration project in the Gonghe Basin,northeastern Qinghai-Tibet plateau,China

The Hot Dry Rock(HDR)is considered as a clean and renewable energy,poised to significantly contribute to the global energy decarbonization agenda.Many HDR projects worldwide have accumulated valuable experience in efficient drilling and completion,reservoir construction,and fracture simulation.In 2019,China Geological Survey(CGS)initiated a demonstration project of HDR exploration and production in the Gonghe Basin,aiming to overcome the setbacks faced by HDR projects.Over the ensuing four years,the Gonghe HDR project achieved the first power generation in 2021,followed by the second power generation test in 2022.After establishing the primary well group in the initial phase,two directional wells and one branch well were drilled.Noteworthy progress was made in successfully constructing the targeted reservoir,realizing inter-well connectivity,power generation and grid connection,implementing of the real-time micro-seismic monitoring.A closed-loop technical validation of the HDR exploration and production was completed.However,many technical challenges remain in the process of HDR industrialization,such as reservoir fracture network characterization,efficient drilling and completion,multiple fracturing treatment,continuous injection and production,as well as mitigation of induced seismicity and numerical simulation technology.

Distribution and mechanism of gently dipping fractures subjected to river incision: A case study from Nujiang River, China

Gently dipping fractures subjected to river incision are widely distributed on rock slopes.In this paper,a rock slope on the Nujiang River(China)is investigated to study the role of gently dipping fractures in the rock slopes evolution.Detailed field surveys indicate that gentle fractures are concentrated in four main zones.Moreover,the kinematics of the fracture system suggest that the genesis of these fractures can be synthesized into a progressive evolution model.This model indicates that the joints begin with the formation of an array of en echelon cracks that are subjected to continued crack elongation and shearing before ultimately approaching one another and interacting to form a complex joint system.Geomechanical analysis is performed to reveal the mechanisms of this genesis,and three main fracture patterns are identified based on the slope stress and are classified with respect to the slope evolution.Based on the detail field investigations and the evolutionary history of the river valley,we propose that intermittent incision by the river was the main factor contributing to the concentrated distribution of gently dipping joints.

Mechanical genesis of Henan(China) Yima thrust nappe structure

Considering the serious coal and rock dynamic disasters around the main slip plane called F16 in the coal mining area) of Henan Yima(China) thrust nappe structure,the mechanical genesis of the Yima thrust nappe structure was studied comprehensively using geomechanics,fault mechanics,elastic mechanics,and Coulomb's law of friction.First,using the centrifugal inertia force of Earth's rotation as a source,a mechanical model of N-S compression superimposed with W-E reverse torsion was established to explain the formation of the early Yima coal basin and Jurassic Yima Group coal measures.Second,an equation for the ultimate stress in the forming stage of F16 was derived using the plastic slip-line field theory and the parabolic Mohr failure criterion.Moreover,the distribution of ultimate stress and the geometric characteristics of the fault profile were obtained using the field model parameters.Finally,the stress field of F16 and the mechanical genesis of the large-scale reverse thrust sheet were discussed based on elastic mechanics theory and Coulomb's law of friction.The results show that the tectonic framework of the early Yima coal basin and the formation pattern of Jurassic Yima Group coal measures given by the model are consistent with the in-situ explorations.The geometric characteristics of the fault profile obtained by numerical calculation can better reflect the shape of F16 in its forming stage,and the mechanical genesis of the large-scale reverse thrust sheet also concurred with the field situations.Thus,this work can provide a foundation for further studies on the genesis of the thrust nappe structure,the mechanism of rock bursts induced by F16,and the characteristics of the residual stress field in the Yima mining area.

Hydrochemical insights on the signatures and genesis of water resources in a high-altitude city on the Qinghai-Xizang Plateau, South-west China

Water resources have crucial implications for the steady development of the urban social economy.This study investigated the hydrochemical signatures and genesis of water resources in the urban area of Lhasa City(UALC).To this end,several analyses,such as ion ratio analysis and correlation analysis,were performed by comprehensively applying mathematical statistics and integrated hydrochemical methods.The results show relatively low concentrations of major ions in the groundwater and surface water(GSW)of the UALC.The primary anions and cations are HCO3-and Ca2+,reflecting the HCOg-Ca water type.Nevertheless,groundwater exhibits higher concentrations of key chemical components compared to surface water.GsW are weakly alkaline,with pH values of 7.78 and 7.61,respectively,and they have low salinity with average concentrations of total dissolved solids being 190.74 and 112.17 mg/L,respectively.Anthropogenic inputs have minimal influence on the hydrochemical features of GSw,whereas rock weathering is the dominant controlling factor.Furthermore,cation exchange.is a significant hydrogeochemical.process influencing their hydrochemical features.According to the isotope analysis(H and 180),the primary source of recharge for GSW is atmospheric precipitation,with some input from melted ice and snow.Moreover,GSW samples from the UALC show relatively similar 2H and 18O isotopic compositions,indicating the existence of a discernible hydraulic connection linking the two water sources.The research findings can serve as a valuable scientific reference and foundation for the sustainable development,effective utilization,and proper safeguarding of regional water resources in high-altitude areas.

Characteristics and origin of low-organicmatter carbonate source rocks in the Middle-Upper Ordovician, Tarim Basin, northwestern China

The continually-discovered large volumes of marine hydrocarbons indicate a huge exploration potential within the carbonate rocks of Ordovician in the Tarim Basin,northwestern China.The Middle-Upper Ordovician(O2+3)carbonate source rocks are at a highly mature stage.These rocks presently contain a low amount of total organic carbon(TOC_(pd)≤0.5%)(TOC_(pd) means the present-day TOC).A mass balance approach is used to identify the source rocks that have expelled hydrocarbons.The characteristics and origin of hydrocarbon expulsion from low-TOC_(pd) source rocks are studied,which are significant for oil exploration in the basin.The results showed that a low-TOC_(pd) O2+3 source rock having expelled hydrocarbons was type I and had a very narrow oil window.With a high rock maturity and a high original TOC(TOCo)value exceeding 1%,nearly 80%of the generated hydrocarbons were expelled.The content of gammacerane and C28 steranes in the oil and source rock extracts were relatively lean,with a V-shaped distribution of regular steranes,suggesting a possible genetic relation between the low-TOC_(pd) source rocks and the marine oil in the Tabei area.Hydrogenrich planktonic algae or acritarchs were the main hydrocarbon parent materials,distributed in the subsiding platform-slope facies.Organic matter was preserved under reducing conditions,and source rocks were formed with a favorable kerogen type and a moderate hydrocarbon generation potential.This study illustrates the hydrocarbon expulsion of low TOC_(pd) source rocks in northern Tarim Basin,which is greatly significant for analyzing the genesis of marine crude oil in Tarim Basin,and evaluating the distribution of marine source rocks.This research method is theoretically significant for oil and gas exploration in the same type of highlymature carbonate paleobasins.