Restoration of reservoir diagenesis and hydrocarbon accumulation process by calcite in-situ U-Pb dating and fluid inclusion analysis: A case study on Cretaceous Qingshuihe Formation in Gaoquan Structure, southern Junggar Basin, NW China

The complexity of diagenesis and hydrocarbon accumulation in the deep reservoirs in southern Junggar Basin restricts hydrocarbon exploration in the lower reservoir assemblage. The lithofacies and diagenesis of reservoirs in the Cretaceous Qingshuihe Formation in the Gaoquan structure of the Sikeshu Sag, southern Junggar Basin were analyzed. On this basis, the thermal history was calibrated using calcite in-situ U-Pb dating and fluid inclusion analysis to depict the hydrocarbon accumulation process in the Gaoquan structure. The results show that the Qingshuihe reservoir experienced two phases of calcite cementation and three phases of hydrocarbon charging. The calcite cements are dated to be (122.1±6.4) Ma, (14.4±1.0) Ma - (14.2±0.3) Ma. The hydrocarbon charging events occurred at around 14.2-30.0 Ma (low-mature oil), 14.2 Ma (mature oil), and 2 Ma (high-mature gas). The latter two phases of hydrocarbon charging contributed dominantly to the formation of reservoir. Due to the S-N compressive thrust activity during the late Himalayan period since 2 Ma, the traps in the Gaoquan structure were reshaped, especially the effective traps which developed in the main reservoir-forming period were decreased significantly in scale, resulting in weak hydrocarbon shows in the middle-lower part of the structure. This indicates that the effective traps in key reservoir-forming period controlled hydrocarbon enrichment and distribution in the lower reservoir assemblage. Calcite U-Pb dating combined with fluid inclusion analysis can help effectively describe the complex diagenesis and hydrocarbon accumulation process in the central-west part of the basin.

Chronology and Genesis of S-type Granites in Hetai District, Guangdong Province: Constraints from LA-ICP-MS Zircon U-Pb Dating and Tourmaline Boron Isotope In-situ Analyses

1 Introduction Hetai district,which is a mountainous area,situated on Guangning and Zhaoqing city,west Guangdong Province.Hetai district is generally located on southwest of South China Caledonian fold belt,east margin of Yunkai post-Caledonian uplift.Multiple type granites are widely distributed in Hetai district,including Caledonian,Indosinian and Yanshanian granites.Based on different

Laser ablation in situ U-Pb dating and its application to diagenesis-porosity evolution of carbonate reservoirs

Through the development and calibration of a reference material which is 209.8 Ma old using a newly-developed Laser Ablation(LA)Multi-Collector Inductively Coupled Plasma Mass Spectrometry(MC-ICP-MS)technique,we successfully overcome the difficulty in sampling and dating ultra-low U-Pb ancient marine carbonates,which was previously untenable by isotope dilution(ID)methods.We developed the LA-MC-ICP-MS in situ U-Pb dating technique for ancient marine carbonates for the study of diagenesis-porosity evolution history in Sinian Dengying Formation,Sichuan Basin.By systematically dating of dolomitic cements from vugs,matrix pores and fractures,we found that the burial and diagenetic process of dolomite reservoirs in Sinian Dengying Formation was characterized by progressive filling-up of primary pores and epigenic dissolution vugs.The filling of vugs happened in three stages,early Caledonian,late Hercynian-Indosinian and Yanshanian-Himalayan,while the filling of matrix pores mainly took place in early Caledonian.The unfilled residual vugs,pores and fractures constitute the main reservoir sapce.Based on the above knowledge,we established the diagenesis-porosity evolution history of the dolomite reservoir in Sinian Dengying Formation,Sichuan Basin.These findings are highly consistent with the tectonic-burial and basin thermal histories of the study area.Our study confirmed the reliability of this in situ U-Pb dating technique,which provides an effective way for the investigation of diagenesis-porosity evolution history and evaluation of porosity in ancient marine carbonate reservoirs before hydrocarbon migration.

The dating and temperature measurement technologies for carbonate minerals and their application in hydrocarbon accumulation research in the paleouplift in central Sichuan Basin, SW China

A new method for reconstructing the geological history of hydrocarbon accumulation is developed, which are constrained by U-Pb isotope age and clumped isotope((35)47) temperature of host minerals of hydrocarbon-bearing inclusions. For constraining the time and depth of hydrocarbon accumulation by the laser in-situ U-Pb isotope age and clumped isotope temperature, there are two key steps:(1) Investigating feature, abundance and distribution patterns of liquid and gaseous hydrocarbon inclusions with optical microscopes.(2) Dating laser in-situ U-Pb isotope age and measuring clumped isotope temperature of the host minerals of hydrocarbon inclusions. These technologies have been applied for studying the stages of hydrocarbon accumulation in the Sinian Dengying gas reservoir in the paleo-uplift of the central Sichuan Basin. By dating the U-Pb isotope age and measuring the temperature of clumped isotope((35)47) of the host minerals of hydrocarbon inclusions in dolomite, three stages of hydrocarbon accumulation were identified:(1) Late Silurian: the first stage of oil accumulation at(416±23) Ma.(2) Late Permian to Early Triassic: the second stage of oil accumulation between(248±27) Ma and(246.3±1.5) Ma.(3) Yanshan to Himalayan period: gas accumulation between(115±69) Ma and(41±10) Ma. The reconstructed hydrocarbon accumulation history of the Dengying gas reservoir in the paleo-uplift of the central Sichuan Basin is highly consistent with the tectonic-burial history, basin thermal history and hydrocarbon generation history, indicating that the new method is a reliable way for reconstructing the hydrocarbon accumulation history.

Combined garnet,scheelite and apatite U-Pb dating of mineralizing events in the Qiaomaishan Cu-W skarn deposit,eastern China

Determining the precise timing of mineralization and mineralizing events is crucial to understanding regional mineralizing and other geological events and processes.However,there are a number of mineralogical and analytical limitations to the approaches developed for the absolute dating of mineralizing systems,such as molybdenite Re-Os and zircon and garnet U-Pb,among others.This means that the precise and accurate dating of mineralizing systems that may not contain minerals suitable for dating using existing approaches requires the development of new(and ideally in situ)approaches to absolute dating.This study outlines a new in situ analytical approach that has the potential to rapidly and accurately evaluate the timing of ore formation.Our study employs a novel application of in situ scheelite U-Pb dating analysis using laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS)and samples from the Qiaomaishan deposit,a representative example of skarn mineralization within the Xuancheng ore district of eastern China.Our approach to scheelite dating of the deposit is verified by cross-comparison to dating of cogenetic garnet and apatite,proving the effectiveness of this approach.Our new approach to dating of scheelite-bearing geological systems is rapid,cheap,requires little sample preparation,and is undertaken in situ,allowing crucial geological and mineralogical context to be retained during analysis.The approaches outlined here not only allow the determination of the absolute timing of formation of the Qiaomaishan deposit through the U-Pb dating of scheelite[138.6±3.2 Ma,N=39,mean square weighted deviation(MSWD)=1.17],garnet(138.4±1.0 Ma,N=40,MSWD=1.3),and apatite(139.6+3.3 Ma,N=35,MSWD=0.72),but also further supports the theoretical genetic links between this mineralization and the emplacement of a proximal porphyritic granodiorite intrusion(zircon U-Pb age:139.5±1.2 Ma,N=23,MSWD=0.3).Moreover,our research indicates that the higher the concentrations of U within scheelite,the more suitable that scheelite is for U-Pb dating,with the main factor controlling the U content of scheelite seemingly being variations in oxygen fugacity conditions.This novel approach provides a potentially powerful tool,not just for the dating of skarn systems but also with potential applications in orogenic and intrusion-related gold,porphyry W-Mo,and greisen mineralizing systems as well as other scheelite-bearing geological bodies or geological systems.

Improvement of in situ LA-ICP-MS U-Pb dating method for carbonate minerals and its application in petroleum geology

In situ carbonate U-Pb dating is gaining popularity,and it has great potential for application in petroleum geology.However,the low U content(<10μg/g)and high common Pb content of carbonate minerals,along with the uneven distribution of U and Pb and the lack of matrix-matched reference material make carbonate U-Pb dating inaccurate and less successful,which limits the widespread application in geosciences.This study evaluated the limitations of in situ carbonate U-Pb dating and proposed a method to rationally determine the laser ablation parameters of samples by improving the experimental approach based on the laser ablation sector field inductively coupled plasma mass spectrometry(LA-SF-ICP-MS).By setting a different spot size and laser frequency for the reference material and unknown samples in the same session based on the U content of the sample,the ablation craters of the reference material and unknown samples were given the same depth/width ratio,avoiding systematic offset caused by differences in down-hole element fractionation and reducing the consume of reference material.Depending on the heterogeneous distribution of U and Pb contents in carbonate minerals,the method of grid screening and setting ablation spots during screening were used to quickly select domains with high U and low common Pb,which improves the efficiency of setting laser spots and the success rate of dating,as well as reduces the experimental time and economic cost.The accuracy and success rate of carbonate U-Pb dating were effectively improved by improving the experimental method,and the technique was applied to two carbonate samples with low U and high common Pb contents that were difficult to date by traditional methods.The two samples are the saddle dolomite in the central Sichuan Basin and the calcite cement in the sandstone reservoir of the Cretaceous Qingshuihe Formation in the South Junggar Basin.The robust ages have been obtained,which constrains the timing of the diagenetic and hydrocarbon accumulation process in the studied area.

Evolution of Ordovician YJ1X ultra-deep oil reservoir in the Yuecan oilfield, Tarim Basin, NW China

Based on a combined in-situ calcite U-Pb dating, molecular geochemical correlations of reservoir oil and extract from reservoir rocks, and fluid inclusion analysis, the charge and evolution history of the YJ1X ultra-deep oil reservoir of the Ordovician Yijianfang Formation in the southwestern part of the Tabei Uplift has been determined systematically.(1) The reservoir oil, free oil and inclusion oil have similar geochemical characteristics and are presumably derived from marine source rock deposited in similar sedimentary environment.(2) The reservoir oil, free oil and inclusion oil have similar maturities with calculated equivalent vitrinite reflectance values in the range of 0.80%-0.96%.(3) Two types(Group Ⅰ and Ⅱ) of oil inclusion assemblages(OIAs) have been identified in the reservoir, of which Group Ⅰ represents the original gas-saturated oil entering the trap during the initial oil charge, whereas Group Ⅱ represents undersaturated residual oil retained in the reservoir after minor leakage of light hydrocarbon.(4) The reservoir experienced oil charge only once during the Early Devonian around 425 Ma and has been well preserved after the minor light hydrocarbon leakage in the Middle Devonian. The study shows that there may be old oil and gas accumulations in ultra-deep strata of petroliferous basins with well-developed caprock and stable tectonic background.

Anticlockwise metamorphic paths at ca.890–790 Ma from the NE Baidrag block,Mongolia,indicate back-arc compression at the Rodinia periphery

The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known.In contrast,accretionary orogenic processes occurring at the supercontinent periphery following Rodinia assembly are poorly understood.To fill this gap,we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt,where numerous data testify for Meso-to Neoproterozoic magmatic reworking.The tectono-metamorphic evolution of the periSiberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic–early Cambrian(Baikalian)cycle.However,we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block,previously considered as a typical example of the Baikalian metamorphic belt.This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist.Grt-Sil-Ky gneiss records initial burial to the sillimanite stability field at 720℃ and 6.0 kbar followed by further burial to the kyanite stability field at 750℃ and 9 kbar and decompression to 650℃ and 8 kbar.The Grt-St schist records initial burial to the staurolite stability field at 620℃ and 6 kbar,followed by further burial to 590℃ and 8.5 kbar.The monazite data yield a continuum of ^(207)Pb-corrected ^(238)U/^(206)Pb dates of ca.926–768 Ma in the Grt-Sil-Ky gneiss,and ca.937–754 Ma in the Grt-St schist.Based on monazite textural positon,internal zoning,and REE patterns,the time of prograde burial to 6.0 kbar under a thermal gradient of 27–32℃/km is estimated at ca.890–853 Ma.It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18–22℃/km dated at ca.835–815 Ma.The late monazite recrystallization at ca.790 Ma is related to decompression.Additionally,monazite with dates of ca.568–515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint.Metamorphic zircon rims with Th/U ratios of 0.01–0.06 in Grt-Sil-Ky gneiss with 877±7 Ma age,together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism.The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice–probably of back-arc or arc type.This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri-Rodinian subduction of the Mirovoi Ocean.The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly,followed by strong mechanical coupling potentially related to onset of Rodinia dispersal.