| As a clean and efficient energy resource,shale gas exploration and development is being accelerated in China.In 2012,Wufeng-Longmaxi shales of well JYHF1 in Jiaoshiba structure obtained high-yield industrial gas flow,with the gas test yield of 20.3Χ104 m3/d,and the Fuling shale gas field has become the first industrial shale gas exploration and development demonstration area in China.The black shales of the upper Ordovician Wufeng formation-lower Silurian Longmaxi formation in Yudong area has experienced a long geological evolution history and has a complex and special shale gas formation and enrichment mechanism.According to the exploration and development data and the existing researches,the pressure coefficient of the high-evolution Wufeng-Longmaxi shale gas layer in Jiaoshiba structure is about 1.0-1.7,and the magnitude of overpressure has a good positive correlation with gas production.The overpressure of shale gas layer is not only a favorable evidence for the long-term effective preservation conditions,but also a key factor for the enrichment of shale gas in Wufeng-Longmaxi shales.The scientific question that the relationship between the preservation and enrichment of shale gas and the overpressure evolution in Wufeng-Longmaxi shales requires to be studied deeply,and one of the fundamental issues is to find the paleotemperature-pressure conditions in Wufeng-Longmaxi shales and the genetic of the related information carrier of fracture cement veins.In this thesis,quartz and calcite veins filled in fractures of Wufeng-Longmaxi shales in Yudong area are the research object.Using comprehensive approaches including mineralogy,element and isotope geochemistry,geochronology and fluid inclusion techniques,the thesis focuses on the paleofluid sources of fracture cement veins,vein formation time and paleotemperature-pressure information trapped by fluid inclusions.Through reconstructing the formation temperature-pressure-composition-time?p-t-x-t?process of fracture cement veins,the genetic mechanism of fracture cement veins,and the source of paleofluid precipitating veins and the activity time of paleofluid or the formation time of fracture cement veins are expounded;the relationship between the trapping temperature-pressure of fluid inclusion in fracture cement veins and the paleotemperature-pressure environments of the shale gas layer at the time of the formation of fracture cement veins are revealed,and the influence of paleo-temperature and pressure of shale gas layer and paleofluid environment on the enrichment and preservation conditions of shale gas are analyzed.These works lead to the following conclusions:1.Macro-facies analysis of the core of the sampled wells together with micro-facies analysis of thin sections allow distinction of different fracture cement veins.The fracture cement veins in the Jiaoshiba area and Penghsui area consist of calcite and/or quartz veins.The fractures hosted in the Wufeng formation and the bottom of the Longmaxi formation were mineralized with calcite as the predominant fracture cement,and quartz cement as a cement phase postdating calcite cement,distributed in the interior of fracture.In contrast,the fractures hosted in the the middle Longmaxi formation were dominantly mineralized by quartz,and pure quartz veins are common.2.The origin and evolution of paleofluid precipitating veins were studied by various geochemical methods.According to the carbon,oxygen and strontium isotopes characteristic,the calcite veins in Jiaoshiba area can be divided into two groups:calcites within the Longmaxi formation and calcites within the Wufeng formation-sub layer 1.Calcites within the Longmaxi formation have relatively lower?13C and?18O values and higher 87Sr/86Sr values than those of calcites within the Wufeng formation-sub layer 1;?REE+Y?NASC pattern of the two-group calcite veins are similar,but some differences are also identified in Eu anomaly,Y anomaly,Y/Ho value,La/Ho value.Calcites within the Longmaxi formation have relatively higher positive Eu anomaly,lower positive Y anomaly,lower Y/Ho values,and smaller range of La/Ho values than those of calcites within the Wufeng formation-sub layer 1.Two groups of quartzs in Jiaoshiba area were recognised based on the characteristic of their distinct?REE+Y?NASC pattern:a slight enrichment in middle REE?MREE?relative to light REE?LREE?and heavy REE?HREE?was observed for the first-group quartzs,with distinct positive Eu and Y-anomalies,relatively higher Y/Ho values,and a small range of La/Ho values;the fractionation of light and heavy rare earth elements in the second-group quartzs is not obvious,?REE+Y?NASC pattern curves are relatively gentle,with no positive Eu and Y-anomalies,relatively lower Y/Ho values,and a large range of La/Ho values.The 87Sr/86Sr values of two-group quartzs within the Longmaxi formation are similar and consistent with that of their proximal host rocks,while the 87Sr/86Sr values of the first-group quartzs within the Wufeng formation are less radiogenic than that of their proximal host rocks at the time of mineralization.3.Measured carbon values for calcites within the Longmaxi formation and calcites within the Wufeng formation-sub layer 1 become depleted?up to 7?compared to those of late Ordovician-early Silurian marine average?13CPDB?0±1to 2±1.5?and the Jiancaogou formation argillaceous limestones we measured.One mechanism to create depleted?13C values is that the organic carbon in the fluid was converted to HCO3-so that the isotopically light carbon can be incorporated into carbonate.The depleted?13C signatures reflect calcites within the Longmaxi formation and calcites within the Wufeng formation-sub layer 1 deposited from a mixed CO2-and CH4-bearing fluid.The?18OSMOW values of paleofluid precipitating calcites within the Longmaxi formation and calcites within the Wufeng formation-sub layer 1 are mainly distributed in the range of7.8to 9.6,indicating that the vein-forming fluids originated from the evolved formation water under strong water-rock reaction conditions.Pronounced positive Eu anomalies seen in shale-normalised REE+Y patterns of calcites and the first-group quartzes within the Wufeng-Longmaxi formation maybe simultaneously dominated by hydrothermal fluids?>200°C?and the re-dissolution of plagioclase in host-rock mineral composition.The 87Sr/86Sr isotopic features suggest that the fluid precipitating fracture cement veins in the Wufeng-Longmaxi shales was derived largely from the surrounding host-rock source,whereas the fluid precipitating fracture cement veins in the Wufeng Formation-sub layer 1 was derived not only from the surrounding host-rock source,but other sources such as the underlying Jiancaogou formation argillaceous limestones which may release a less radiogenic 87Sr calcium-rich fluid.Diffusional transport or advective transport of the calcium-rich fluids from the underlying Jiancaogou formation argillaceous limestones to the Wufeng Formation-sub layer 1 has caused the calcites within the Wufeng formation-sub layer 1 simultaneously possess the relatively less radiogenic 87Sr/86Sr ratios,higher?13C and?18O ratios,higher Y/Ho ratios and lower positive Eu-anomalies.4.The calcite veins in well PY1 of Pengshui area can be divided into three groups:the first-group calcites within the carbonaceous shales of upper Wufeng formation have obvious positive Eu anomaly??Eu=1.945.98?and negative Ce anomaly??Ce=0.610.97?,the average 87Sr/86Sr value of 0.720641,and the range of?13CPDB and?18OSMOWMOW is-0.61.0,17.018.0,respectively;the second-group calcites within the siliceous shales of lower Wufeng formation are characterized by positive Eu anomaly??Eu=1.281.52?,positive Ce anomaly??Ce=0.991.26?,the average 87Sr/86Sr value of 0.720668,and the range of?13CPDB and?18OSMOW is-0.67-0.9,17.617.7,respectively;the third-group calcites within the argillaceous limestones of Jiancaogou formation have obvious negative Eu anomaly??Eu=0.680.82?and positive Ce anomaly??Ce=1.121.24?,the average 87Sr/86Sr value of 0.712791,and the range of?13CPDB and?18OSMOWMOW is 0.10.3,16.417.0,respectively.The 87Sr/86Sr isotopic features and REE+Y patterns of quartzes within the carbonaceous shales are similar with the first-group calcites.It has obvious positive Eu anomaly??Eu=2.483.64?and slight negative Ce anomaly??Ce=0.870.93?,and the initial 87Sr/86Sr values were 0.7206910.721006.5.The 87Sr/86Sr isotopic features of the fractured veins in well PY1 of Pengshui area suggest that the fluid precipitating the third-group calcites was derived largely from the surrounding host-rock source,whereas the fluid precipitating the first-group and second-group calcites and the quartzs in the Wufeng formation was derived not only from the surrounding host-rock source,but other sources such as the overlying Longmaxi formation carbonaceous shales which may release a more radiogenic terrestrial 87Sr fluid.Measured carbon values for calcites and their proximal host rocks are within those of late Ordovician-early Silurian marine average?13C,while the?13C values of the second-group and third-group calcites are slightly negative than those of their proximal host rocks,and the?13C values of shales(?13CPDB=-0.43-0.30)are less than those of limestones(?13CPDB=1.672.04).Combined with Sr isotope characteristics,it can be inferred that diffusional transport or advective transport of the depleted?13C fluids from the Wufeng-Longmaxi shale gas layer to the lower Wufeng formation and the underlying Jiancaogou formation argillaceous limestone layer has caused the second-group and third-group calcites possess slightly negative?13C values.Pronounced positive Eu anomalies seen in the quartzes and first-group calcites are related to the re-dissolution of plagioclase in host-rock mineral composition,independent of fluid temperature.The negative Eu anomaly and positive Ce anomaly of the third-group calcites suggest that the fluid precipitating the third-group calcites are typical marine fluid source.6.Microthermometry of methane-saturated aqueous inclusions in fracture cement veins of Jiaoshiba area provides trend in trapping temperatures that Th values concentrate in the range of 191.8230.6°C,196.2221.7°C for quartz and calcite,respectively.Tm values of quartz and calcite concentrate in the range of-9.8-3.7°C,-5.3-2.9°C,respectively.There are two different low-temperature transformation modes in the microthermometry of methane inclusions:pure methane inclusions?L?L+V?L?and methane inclusions containing a small amount of CO2?L?L+V+S?L+V or L+S?L?.The Th values of pure methane inclusions are in the range of-99.4-87.0°C.There are two kinds of spectra in laser Raman analysis of methane inclusions:?1?most of the methane inclusions contain CH4 as the only detectable phase during Raman analysis,which are pure methane inclusions;the Raman CH4 symmetric stretching?v1?peak positions of pure methane inclusions range from 2910.57 cm-1 to 2911.27 cm-1.?2?in addition to the high intensity methane peak,some methane inclusions contain the weak intensity CO2 peak.Raman spectroscopy to check the component of two-phase aqueous inclusions indicate that the aqueous inclusions are methane saturated and no ethane and/or carbon dioxide was detected in the aqueous inclusions.7.The Raman spectroscopy technique and microthermometry analytical technique were adopted to determine the density of methane inclusions in fracture cement veins of Jiaoshiba area,which concentrates in the range of 0.2450.302 g/cm3.The density of methane inclusions is higher than the critical density of methane,indicating that the methane inclusions in fracture cement veins of Jiaoshiba area belong to supercritical high-density methane inclusions.Pore-fluid pressures of 91.8 to 139.4 MPa for methane inclusions,calculated using the Raman shift of C-H symmetric stretching?v1?band of methane and equations of state for supercritical methane,indicate fluid inclusions trapped at medium-strong overpressure state.Fluid inclusion analysis indicates that there were supercritical high-density methane fluid phase and methane-saturated water phase pore fluids existing in the Wufeng-Longmaxi shales during the formation of fracture cement veins.We infer that methane inclusions were trapped in the separate immiscible supercritical high-density methane fluid phase,and aqueous inclusion trapping under water-saturated reservoir conditions that were in pressure communication with the supercritical methane phase.Pressure communication between these phases would ensure that any fluid inclusions trapped in the water phase would remain methane-saturated.We interpret trapping Th and pressures of fluid inclusions to represent trapping under methane saturation as the P-T conditions within the reservoir systematically change over time at maximum burial depth.8.Microthermometry of aqueous inclusions in calcite veins within Longmaxi shales of well PY1 indicate that Th values concentrate in the range of 128.5156.4°C,and Tm values concentrate in the range of-9.1-5.2?,corresponding to salinities of 8.1012.97 wt%NaCl equivalent.The Th values of aqueous inclusions in calcite veins within the Wufeng shales of well PY1 range from 132.6 to 162.7?.The trapping pressure of methane inclusions in calcite veins within Longmaxi shales ranges from approximately32.5 to 43.0 MPa.Calcite veins in the Wufeng-Longmaxi shales of well PY1 formed in the tectonic uplift process.A comparison of the fluid inclusion homogenization temperatures with burial and thermal maturity models,we infer that the formation time of fractures in the Wufeng-Longmaxi shales of well PY1 was about 65 Ma.At the time,the Wufeng-Longmaxi shales were in normal pressure state.The Wufeng-Longmaxi shales in Pengshui area and Jiaoshiba area have similar reservoir formation basis and hydrocarbon generation process,fluid overpressure should be generated in geological history period,and the Wufeng-Longmaxi shales in Pengshui area should be in abnormal high pressure state before or at the beginning of Yanshanian uplift.However,the nowadays pressure state of these two areas is obviously different,which is mainly affected by the difference of tectonic uplift time and tectonic strength,and the difference of lateral sealing property of overpressured compartment.9.Isotopic geochronology can provide valuable information for the formation time and genetic mechanism of fracture cement veins.Sm-Nd isochron dating of calcite veins within Longmaxi Formation and Wufeng Formation-sub layer 1 define an age of 160±13 Ma and 133±15 Ma,respectively.The Sm-Nd ages of calcite veins correspond to the main episode time of the early Yanshanian tectonic movement in the study area.According to the above-mentioned temporal relationship,it can conclude that the fracture cement veins in Jiaoshiba area were formed in the early Yanshanian multi-episode tectonic compression environment.The Sm-Nd ages of calcite veins are also consistent with the period of the maximum burial depth of the Wufeng-Longmaxi shale gas layer,in combination with the ubiquitous presence of high-density single-phase methane inclusions and with high porefluid pressures calculated from fluid inclusion methane concentrations in fracture cement veins,it can infer that the Wufeng-Longmaxi shale gas layer of Jiaoshiba area were in a medium-strong overpressure state before or at the initial stage of Yanshanian tectonic uplift.10?According to the REE,C-O-Sr isotopic composition,paleotemperature-pressure trapped by fluid inclusion and isotopic chronology information,the fracture cement veins in the Wufeng-Longmaxi shales of Jiaoshiba area were formed in the early Yanshanian lateral compressional tectonic movement stage,and the fluid precipitating veins originated from its own shale system,which was influenced by the mixing of marine fluid from the underlying Jiancaogou formation argillaceous limestones;the homogenization temperature and trapping pressure of fluid inclusions represent the paleotemperature and pressure conditions of the shale gas layer before or at the beginning of Yanshanian-Himalayan tectonic uplift,and the preservation conditions of the Wufeng-Longmaxi shales of Jiaoshiba area were strictly effective during Yanshanian-Himalayan tectonic compressional uplift.However,the fracture cement veins found in the Wufeng-Longmaxi shales of Pengshui syncline area were formed during the process of Yanshanian-Himalayan tectonic compression and uplift,and there were multi-stage vein-forming fluid activity and vein precipitation and reformation;the fluid precipitating veins were mainly derived from the shale layer and the underlying Jiancaogou formation argillaceous limestones,without the effects of external paleofluids or surface water;the homogenization temperature and trapping pressure of fluid inclusions represent the paleotemperature and pressure conditions of shale gas layer during the Yanshanian-Himalayan tectonic uplift stage,and the Wufeng-Longmaxi shales of Pengshui syncline area still had basic preservation conditions during Yanshanian-Himalayan tectonic uplift. |
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