Study On The Relationship Of Pyrite And Content Of Organic Matter In Marine Black Shale And Its Significance For Shale Gas Exploration

Shale gas exploration in the Southern China has achieved a breakthrough in many areas in recent years,but the exploration and evaluation technologies for shale gas resource are still in the exploratory stage and the main problems facing are the resources evaluation and locating of target reservoirs.The conventional technologies for oil and gas exploration are aimed mainly on imaging of favorable structures,but it is not enough to only know the spatial distribution of shale formation for shale gas exploration,and to achieve the evaluation of shale reservoir by using the total content of organic carbon(TOC)parameter also necessary.Therefore,it is of great value to study effective methods and techniques to predict the TOC content and spatial distribution in shale reservoirs.At present,TOC parameter can only be obtained through laboratory test and analysis of well logging data analysis,there is no usable surface exploration methods to evaluate or predict spatial distribution of favorable shale gas reservoir.Shale with rich organic matter is the main rock type for shale gas,while pyrite is a characteristic mineral rich in organic deposits.The electric polarizability parameter derived from the complex resistivity exploration method is especially sensitive to content of pyrite,it is suggested to obtain the spatial distribution of pyrite content by the ground resistivity exploration,and then to evaluate TOC according to the quantitative relation model and identify favorable target layers by combining with sedimentary environment and sedimentary facies analysis.The purpose of this study is to clarify relation and control factors of pyrite and TOC content variations in the process of formation and evolution of shale layers at different sedimentary environment by analyzing laboratory testing data of shale samples,and to establish the quantitative model of TOC and pyrite content for identifying and evaluating shale reservoirs using the complex resistivity exploration data.In this study,outcrop black shale samples are collected from four locations in West Hubei-East Chongqing area and 35 samples are finishing for laboratory testing measurements.Large number of data sets are obtained including mineral components,principal elements,trace elements,rareearth elements,the iron content of the components,the contents of organic carbon isotopic and pyrite sulfur isotope,and the wide band complex resistivity measurements of partial samples.In addition,more data from publicly available sources are collected and used to extend the scope of the experimental data,to reinforce and validate credibility of the findingsVarious types of statistical analysis are applied to the experimental data set.Three methods were used to analyze the redox environment based on measured particle size of pyrite.Analysis results of different methods show good consistency and illustrate the feasibility of analysis of sedimentary environment by pyrite particle size parameter.The results show that in the Early Silurian period,the size of framboidal pyrite in Fengxiang area is in general larger than other areas,and the sedimentary water is oxygen poor environment with weak oxygen or partial oxygen;the diameter of framboidal pyrite is moderate in the samples from Shizhu area,the sedimentary water is anoxic,and the bottom water is in a long period of anaerobic environment;the size of the framboidal pyrite from Yushan samples is smaller than that of the Shizhu area even they are quite close,while the sedimentary water environment is similar to that of the Shizhu region;the diameter of framboidal pyrite from Wufeng samples is smaller,and the sedimentary water environment is anaerobic anoxic environment in the bottom water.In the Early Cambrian period,the size of framboidal pyrite is smaller,and the sedimentary water belongs to anoxic or weak sulfide environment.Sedimentary environments for four sampling area are tried to restore by analyzing of the geochemical data.The analysis results show that the four sampling areas have high primary productivity in both the Early Cambrian and Early Silurian period,sedimentary water is a mixing of normal seawater and hot water of magmatic components.All four locations have shown the characteristics of hypoxia iron rich in Early Silurian but with slightly differences on the oxidation environment,except for that samples from Fengxiang area in Early Cambrian is in oxidation reduction environment with hypoxia iron rich and short term sulfide environment.Provenance analysis shown that four sampling areas have certain differences in source materials in different periods: sediments in sampling area of Fengxiang,Shizhu andYushan were derived from multiple sources and was single source in Wufeng area in the Early Silurian;sediments in Fengxiang sampling area was single source material in Early Cambrian period.The source of all the sampling area in both the Early Cambrian and Early Silurian sediments were mainly in neutral-felsic or felsic source;sources of all sampling areas are subject to weathering,transporting and sorting except for Wufeng area in Early Silurian.The overall samples collected in this study can be classified as hypoxia and oxygen poor environment through comprehensive analysis of various methods: the Fengxiang area was in oxygen poor environment in Lower Silurian and possibly in anoxic to weakly sulfurized environment in Lower Cambrian period;and the Shizhu and Wufeng area were in anoxic environment and the Yushan region was anoxic to extremely oxygen poor environment in the Lower Silurian period.The correlative behavior of TOC-primary productivity-sedimentary environment,TOC-carbon and sulfur isotope-sedimentary environment-pyrite content,TOC-pyrite content-electric polarizability were recognized based on testing data analysis.It is confirmed that the relation model between TOC and pyrite content of black shales from different sedimentary environments have the same mathematical form,but the parameters of the model are significantly different;and model parameters for samples from different areas of a broad range but same sedimentary environment show only little difference.This indicates that it is important to correctly distinguish sedimentary environment for quantitative prediction of TOC by means of TOC-pyrite content relation model.The electric polarizability,a parameter of complex resistivity which is considered to be tightly related with the pyrite content of shale samples was obtained by inversion of measured complex resistivity data using multicomponent model of rock complex resistivity responses.It is confirmed that the TOC-pyrite content,TOC-electrical polarizability,pyrite content-electrical polarizability have nonlinear(exponential)positive correlation with high relevancy.The quantitative relation models for pyrite – TOC content,pyrite content-electric polarizability,TOC –polarizability were initially established and prediction coefficients of each model were derived for different sedimentary environments.Therelationship model and parameters was successfully applied to predict TOC value of well core samples by using the measured polarizability parameter,the predict results were compared with TOC value of laboratory measurement and calculating results from well logging data.All three results have shown satisfactory consistency and this gives confidence of application of relation model of the TOC – electric polarizability of shale samples for evaluation of shale reservoir.