| As fundamental unit of the Earth materials,Rock and mineral is products of geological processes,and records of the processes or plays as well.Recent studies on petrology are mainly focused on petrochemistry.However,petrophysics has always been an important subject due to its significance.Acting as a branch of petrophysics,petro-electricity focuses on the electrical and magnetic properties of the rock.It is one of the very important methods to the study of the structure and physical properties of the interiors as well as the near surface of the Earth.As a result,petro-electricity highlights the research hot-spot both in petrology and geophysics,and it has been widely used in geology exploration,economic geology research,resource development and environmental engineering.However,given the development history and hard works of nearly 200 years,too many fundamental and technical problems,such as measurement technique and precision,understanding the properties and its origin,modeling and inversion and so forth,still remain as open issues and require further study.In this dissertation,we focus on two fundamental problems:one is the petro-electricity properties on room condition and its origin,the other is the corelation between the petrochemistry and the petro-electricity properties.Luobusha ultramafic rock and the Upper Yangtze black shale are taken as research examples,and the purposes of the research are finding a new way for the exploration of Luobusha chromium and monitoring the hydrofracture as well as helping the delimiting of the favorite shale gas reservior using petro-electricity.Main research contents and results are listed as follows.1.Features of the complex impedance of rock in broad frequencies band and room condition.Features of the complex impedance of rock varies with frequency is one of the basic physical properties of rock as Earth or the other planet materials,and it is also one the foundations of geo-electrical study or exploration.We measured the complex impedance of rocks without soaking,and consider they reflect the rocks in condition of near surface better than those after soaking process.A new measurement method,Impedance Matching mothed,was invented for high impedance rock samples measurement.Cole-Cole model was widely used to describe the features of the complex impedance of rock.According to this model,modulus of the complex impedance or the resistivity of rock should increase with the descending of the frequency in lower frequencies band.However,the measured results of some samples do not absolutely agree with the Cole-Cole model.We observed the obviously descend of the resistivity with frequency descending in the band of lower than 0.01 Hz in some samples.This phenomenon has been measured in several samples of different types such as shale,ore,peridotite or sandstone.We have done repeated measurements with different equipment,conducting media,electrodes and acquired almost the same result.After carefully studied the polarization effect between the samples and the measurement electrode system,we conclude that some rocks have the complex impedance features as descending with the descending of the frequency in lower frequencies band is result from the polarization between the samples and the measurement electrode system,and it could be reduced by prolonging the conduct time between samples and the measurement electrode system before the impedance measurement.2.Features and origin of petro-electricity anomalies at Luobusa Ophiolite in Southern Tibet and its geological implication and application.The Luobusa Ophiolite in Southern Tibet,lies in the eastern portion of Indus-Yarlung Zangbo suture zone which separates Eurasia from the Indian continent.Its distribution and origin of the petro-electricity anomalies,including high magnetic anomaly and electrical anomalies,is of important geology implications on the interior structure and tectonic evolution of the Luobusha mantle peridotites,and even on the tectonics between Eurasia and Indian continent.On the high magnetic anomaly,the ground magnetic survey on Luobusa Ophiolite and the 3D susceptibility inversion result of the observed data indicates that a major portion of the rock mass features normal magnetic anomalies,and that high magnetic anomalies mainly distribute in the south of study area with a width of 100 to 400 m.We also carried out the micro-structure,and mineral composition analysis of rock samples.Based on the analyses of the magnetic properties and the analysis result of rock samples,we conclude that the precipitation of magnetite in the process of serpentinization of the olivine caused by the geological process in the Luobusa Ophiolite is the origin of the Luobusa High Magnetic Anomaly Complex.On the electrical anomalies,we measured the complex impedance and micro-structure,and mineral composition analysis of the rocks samples,processed the AMT survey data on the Luobusha Ophiolite.Based on the analysis result of petrochemistry and rock’s impedance measurement,relationships between the petrochemistry and petro-electricity have been found.For example,the content of calcium or silicone,the content of pyroxene and olivine,in rocks from Luobusha Ophiolite is negatively correlated with the rocks’ resisistivity,which is,however,positively correlated with the content of serpentine and LOI.We consider the metamorphism,deformation and serpentinization is the origin of the decreasing of the rock’s resisistivity.Based on these general observations,we applied petro-electricity to discover the Luobusha Ophiolite and find the largest hidden chromite deposit since ever the past three decades in China.Meanwhile,we answered three vital questions on the economic geology of the Luobusha Ophiolite whereby our petro-electricity research.The first one is that the large-scale hidden chromite deposit might locate in the Transition Zone from large-scale fresh harzburgite to the metaperidotite.The second is there underlies economic chromite deposit under depth of 400 meter.The third is the Dapingtai,an area cover with Glacial Deposits in Luobusha Ophiolite,is a potential favorite area of economic chromite deposit,because the petro-electricity research result indicates that Dapingtai underlies the "Central Chromite-Bearing Zone".3.Petro-electricity features and origin of the Upper Yangtze organic shale.Research on shale gas sparked a revolution all through the world,and the organic shale from Upper Yangtze is the focus of the shale gas research in China.We measured the petrochemical and impedance data of the organic shale from Silurian Longmaxi group(including Ordovician Wufeng group)and Cambrian Niutitang group,and the impedance data indicate that the rocks’ resistivity of the organic shale decreases with the increasing of the TOC and brittle mineral content as well as pyrite content of the same shale.The relatively low resistivity from the result by petro-electricity of organic shale could be used to help to delimit the "sweet spots" in shale gas plays,because the gas-bearing shale features higher TOC,brittle mineral and pyrite content.4.Time-lapse petro-electricity feature of rock and its application in monitoring hydraulic fracturing.Hydraulic fracturing is widely used for initiating and subsequently propagating fractures in reservoir strata by means of a pressurized fluid to release oil and gas or to store industry waste.Downhole or surface microseismic monitoring is commonly used to characterize the hydraulically induced fractures.However,in some cases,down hole microseismic monitoring can be difficult due to the limitation imposed by boreholes.Surface microseismic monitoring often faces difficulties acquiring high S/N ratio data because of the onsite noise from hydraulic fracturing process.Research and field observations indicate that injecting conductive slurry or water into strata may generate distinct time-lapse electromagnetic anomalies between pre-and post-hydraulic fracturing.These anomalies provide a means for characterizing the hydraulic fracturing using time-lapse electromagnetic methods.The time-lapse variation over an hour,one day,one month,and two years of observed audio-magnetotellurics(AMT)resistivity,the 1D and 3D AMT modeling results of the variation pre-and post-hydraulic fracturing are presented in this dissertation.A successful case of applying the time-lapse AMT to map hydraulic fractures is also presented.Observed data indicate that the variation of AMT resistivity is normally less than 6%apart from the data of the dead band and some noisy data.Modeling results show the variation pre and post hydraulic fracturing is larger than 30%at the frequency point lower than 100 Hz,and field observation indicates those are larger than 100%and top to 450%.The case indicates that time-lapse magnetotelluric monitoring may form a new way to characterize the hydraulic fracture. |
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