Surface organic geochemical prospecting for hydrocarbons: multivariate analysis

A database of analyses of C₁ and C₇ hydrocarbons from an oil and gas producing region in Mexico has been assembled from gas samples collected at depths of 3, 15 and 30 meters from surficial holes drilled in traverses over producing and barren structures. The surface consisted of subtropical swamps; depth to structure was 3500 to 5800 meters.Hydrocarbon analysis from six structures (three producing and three barren) selected from the database were subjected to multiple discriminant function analysis to produce a retrospective statistical test of the ability of geochemical prospecting to distinguish producing from non-producing structures. The hydrocarbon spectra from 3 meters depth yielded ambiguous results; those from 30 meters produced clear distinction between barren and producing structures. Further, the discriminant functions established a base of geochemical characteristics, founded on known areas (retrospective), to which additional unknown areas (prospective) may be compared for classification. This suggests a bootstrapping approach to exploration in which an increasing number of “known” results can be used to continually update and refine the predictive power of the discriminant function.This indicates the practical ability of a combined geochemical-multivariate statistical prospecting approach as an exploration tool, particularly within a single geochemical/geological province. Geochemical prospecting, perhaps with relatively deep (30 m) penetration, combined with multivariate data analysis is a rapid, potent and relatively inexpensive additional tool for petroleum exploration.     

Geochemical background in soils: a linear process domain? An example from Istria (Croatia)

Definition of geochemical background in exploration and environmental geochemistry has always been regarded as contingent upon scale and investigated locality but mostly under assumption that hosts of processes that produce the data more or less conform individually to Gaussian law of “central tendencies”. Recently, understanding of pedogenesis as synergetic process being characterized by non-linear dynamics renders thermodynamic approach directly applicable in assessment of geochemical thresholds, with concepts of linearity and normality set alongside in solving the problems of soil geochemistry. Seen from this perspective the work is an attempt to relate conceptual fundamentals of non-linear dynamical theory to basic statistical methods in order to elucidate the nature and origins of element subpopulations hidden in the original geochemical data from the soils of Istrian Peninsula (western Croatia). To this purpose the two major groups of soils were selected for analysis depending on the type of bedrock as one of the main soil-forming factors. Geochemical data were subjected to the trimming procedure by which the outliers were removed from the total data collective and attributed to non-linear causes precluding simple cause-and-effect relationships as the sine qua non of Gaussian distribution. Geochemical background is then defined as the normal range of data of the remaining (trimmed) dataset indicating the “thermodynamic branch” of the specific soil processes as opposed to outliers being described as dissipative structures.     

Adjustment of geochemical background by robust multivariate statistics

Conventional analyses of exploration geochemical data assume that the background is a constant or slowly changing value, equivalent to a plane or a smoothly curved surface. However, it is better to regard the geochemical background as a rugged surface, varying with changes in geology and environment. This rugged surface can be estimated from observed geological, geochemical and environmental properties by using multivariate statistics.A method of background adjustment was developed and applied to groundwater and stream sediment reconnaissance data collected from the Hot Springs Quadrangle, South Dakota, as part of the National Uranium Resource Evaluation (NURE) program. Source-rock lithology appears to be a dominant factor controlling the chemical composition of groundwater or stream sediments. The most efficacious adjustment procedure is to regress uranium concentration on selected geochemical and environmental variables for each lithologic unit, and then to delineate anomalies by a common threshold set as a multiple of the standard deviation of the combined residuals. Robust versions of regression and RQ-mode principal components analysis techniques were used rather than ordinary techniques to guard against distortion caused by outliers Anomalies delineated by this background adjustment procedure correspond with uranium prospects much better than do anomalies delineated by conventional procedures. The procedure should be applicable to geochemical exploration at different scales for other metals.     

Semi-hierarchical correspondence cluster analysis and regional geochemical pattern recognition

Semi-hierarchical correspondence cluster analysis (SHCCA), firstly developed in this paper, extracts the main advantages of correspondence analysis, hierarchical and non-hierarchical cluster analysis, and unifies the R- and Q-mode cluster analysis of large data set. A systemic program to recognize the regional geochemical patterns is built up based on this method. With this program, the complex tasks for data interpretation can be achieved by simple processes, and important geochemical information can be displayed by a single diagram, i.e. the multivariate regional geochemical image. As one of the applied examples of this program, the regional geochemical pattern recognition for a shallow covered area around Tahe in Heilongjiang Province is introduced. The results show that many hidden geochemical patterns related to the lithologies, structures, ore-forming conditions and prospecting targets etc are revealed by the geochemical image, and that the main geochemical patterns are related with certain geological and gravitational patterns. By finding contrasts between geochemical patterns and geological or gravitational patterns, the SHCCA results assist the geological mapping in this area. Geochemical data obtained in Chinese regional geochemical exploration provides useful information regarding geology and minerals, and the method described in this paper provides a new way to examine this type of resource.     

Separating anthropogenic from natural anomalies in environmental geochemistry

Environmental geochemistry has attracted increasing interest during the last decade. In Sweden, geochemical mapping is carried out with methods that allow the data to be used in environmental research, including sampling plant roots and mosses from streams, soils and bedrock. These three sample types form an integrated strategy in environmental research, as well as in geochemical exploration. However, one problem that becomes prominent in geochemical mapping is to distinguish the signals derived from natural sources from those derived from anthropogenic sources. So far, this has mostly been done by using different types of samples, for example, different soil horizons. This is both expensive and time-consuming.We are currently developing alternative statistical solutions to this problem. The method used here is PLSR (partial least squares regression analysis). In this paper, we present an initial discussion on the applicability of PLSR in differentiating anthropogenic anomalies from natural contents.PLSR performs a simultaneous, interdependent principal component analysis decomposition in both X- and Y-matrices, in such a way that the information in the Y-matrix is used directly as a guide for optimal decomposition of the X-matrix. PLSR thus performs a generalized multivariate regression of Y on X overcoming the multicollinearity problem of correlated X-variables. The advantage of PLSR is that it gives optimal prediction ability in a strict statistical sense.Bedrock geochemistry from different lithologies in the mapping area in southern Sweden (Y-matrix) is analyzed together with stream or soil data (X-matrix). By modelling the PLS-regression between these two data sets, separate multivariate geochemical models based on the different bedrock types were developed. This step is called the training or modelling stage of the multivariate calibration. These calibrated models are subsequently used for predicting new (X) geochemical samples and estimating the corresponding Y-variable values. Information is obtained on how much of the metal contents in each new geochemical sample correlate with the different modelled bedrock types.By computing the appropriate X-residuals, we obtain information on the anthropogenic impact that is also carried by these new samples. In this way, it is possible from one single geochemical survey to derive both conventional geochemical background data and anthropogenic data, both of which can be readily displayed as maps.The present study concerns development of data analysis methods. Examples of the applications of the methodology are presented using Pb and U. The results show the share of these contents in different sampling media that is derived from bedrock on the one hand, and from anthropogenic sources, on the other.     

Surface geochemical methods used for oil and gas prospecting — a review

The majority of the world's oil and gas deposits have been discovered by drilling in the vicinity of natural petroleum seeps, and to date the most successful geochemical prospecting methods still rely upon the surface detection of hydrocarbons. Gas chromatographic techniques are now commonly used in the analysis of hydrocarbon gases for prospecting both onshore (analysis of soils and rocks) and offshore (analysis of near-bottom waters and sediments). Detection of helium fluxes has been partially successful as a geochemical prospecting technique. Many indirect techniques such as the determination of isotope and metal-leaching anomalies in surface rocks and the measurement of radon fluxes have not been widely used.Onshore geochemical prospecting appears to have more problems associated with it than offshore prospecting due to the more complex migration mechanism of near-surface waters containing dissolved gases. No onshore prospecting studies have been published which thoroughly consider this factor and the success of onshore prospecting remains equivocal. In offshore prospecting “sniffers” have been used to detect hydrocarbon anomalies in near-bottom waters, and coring equipment has been used for the detection of hydrocarbons in near-surface sediments. Success is claimed using these techniques.Geochemical prospecting methods are complementary to the widely used geophysical methods. Geochemical methods can provide direct evidence for the presence of petroleum accumulations and are relatively cheap and rapid. Failures in prospecting to date are attributable to the simplistic manner in which data have been interpreted; insufficient attention has been paid to the hydrological and geological factors which modify the upward migration of indicator species to the surface. As oil and gas deposits become more difficult to locate, greater attention should be paid to geochemical prospecting techniques, especially as a regional exploration tool.     

Borborema Pegmatitic Province: geological and geochemical characteristics

The geological features of the pegmatites from the Borborerna Pegmatitic Province (BPP) are described, combining data from the literature with new field and laboratory observation. A geochemical study was performed against this geological background to test the crystallization model of mineralized pegmatites against barren pegmatites and to compare the BPP with other provinces, fields or individual pegmatites throughout the world. The field evidence (mode of emplacement, textural relationships, zonation and mineralogy) as well as the geochemical characteristics support the pegmatite crystallization model proposed by London (1990). The geochemical and mineralogical evidence places these pegmatites as “medium Ta-mineralized” compared to other pegmatite provinces.     

新疆哈拉奇地区地球化学分区特征及地质意义

水系沉积物测量是勘查地球化学一种重要方法。随着勘查地球化学的深入发展,有关地球化学异常圈定的方法已成为人们关注的焦点。采用R型因子分析方法,对哈拉奇地区水系沉积物测量所取得的数据进行统计处理,提取了具有代表性的5种因子组合类型,同时以因子得分为综合指标,绘制了因子得分等值线图并制作了元素组合分区图。通过综合分析,地球化学分区图反映了不同地段中以相应的元素组合类型为主的地球化学异常及地质成因信息,同时与主要控矿因素的空间分布比较一致,为研究区地质找矿提供新方向。     

Aspects of tin metallogenesis in the Tasman Geosyncline, eastern Australia, as reflected by cluster and factor analyses

Geochemical classification of 284 granitic rocks from the Tasman Geosyncline, eastern Australia, demonstrates the association of tin deposits with leucocratic granites of high tin content. The mean Sn content (26 ppm) in such rocks is about seven times higher than in “barren” granites.Factor analysis of geochemical data indicates differences in the geochemical evolution of various rock types and the relative importance of magmatic differentiation and postmagmatic alteration in determining the composition of rocks in various areas of the geosyncline.     

一种适用于多目标地球化学调查的土壤采样记录卡

土壤地球化学测量是勘查地球化学一种重要的方法手段。传统的土壤地球化学测量主要服务于地质找矿和从事基础地质研究,因此,其野外调查收集的观察数据主要是与找矿有密切关系的相关信息,其信息量十分有限,满足不了多目标地球化学调查的需要,为此,作者在本文中通过野外调查实践,设计出了一种新的土壤测量记录卡,同时,也列述了野外观测数据的采集方法与目的。     

如何利用Excel处理化探数据

利用Excel可以实现对化探数据多种项目的处理,如正态性检验;背景值及异常下限的确定;一元及多元回归分析;相关系数(R型聚类分析)以及一次趋势分析等,操作直观,目前在没有流行地质行业软件的情况下,不失为一种较为可取的选择。通过这种方法,可以较为简洁地实现对一定数量化探数据的处理,为地质找矿工作提供了化探方面的可靠依据。     

数学方法在矿产地质工作中的应用

随着数学地质的发展,数学方法和电子计算机在矿产地质工作中的应用日益广泛和深入,不断解决愈来愈多的矿产地质实践问题和理论问题。这种应用大体上可归纳为以下三个方面(1)数学方法和电子计算机应用于矿产地质工作的每一个阶段。由矿产的普查勘探、详查评价、储量计算直至矿产开发,每一个阶段的矿产地质问题都能用数学方法和电子计算机帮助解决,以提高每一个阶段中矿产地质工作的速度和质量。     

Tectonic terranes, metallogeny and regional geochemical surveys: an example from northern British Columbia

This study utilizes three major data sources: distribution of geological units; density, type, age and distribution of mineral deposits; and elemental analyses from regional geochemical stream sediment surveys to define parameters that ‘characterize’ tectonic terranes in northern British Columbia. A similar approach could be applied anywhere in the Canadian Cordillera.This area, NTS map sheets 104N, 104O and 104P along the British Columbia-Yukon border, forms a transect through allochthonous terranes into North American rocks. These are: the allochthonous island-arc Stikine, oceanic Cache Creek, cataclastic Yukon/Tanana, and island-arc Quensel terranes, the pericratonic Dorsey terrane; the parautochthonous oceanic Sylvester allochthon; and the autochthonous miogeoclinal North American Cassiar terrane. Plutonic rocks of Jurassic-Cretaceous to Tertiary age intrude all terranes.Data sources used in the study are geological base maps and reports, the Ministry of Energy Mines and Petroleum Resources' mineral deposit database (MINFILE) and analytical data from the National Regional Geochemical Survey stream sediment and water sampling program.Geological maps were compiled from various sources and plotted to act as bases for geochemical and mineral deposit overlays for analysis and interpretation.Geochemical samples were separated into background and anomalous populations and compared according to their source terranes. We found that mean concentrations from background sample populations for some elements are statistically distinctive for different terranes. Unfortunately, elemental correlation coefficients for the terranes are similar so cannot be used to characterize each terrane.Data on mineral deposits and occurrences were compiled from minfile and other sources. Particular attention was paid to deposits with histories of production or significant reported reserves. Deposits were sorted by type and commodity to produce synoptic metallogenic maps.The combined data from geological, geochemical and mineral deposit databases form a strong tool for interpreting and predicting patterns of mineralization.     

Use of partial dissolution techniques in geochemical exploration

Application of partial dissolution techniques to geochemical exploration has advanced from an early empirical approach to an approach based on sound geochemical principles. This advance assures a prominent future position for the use of these techniques in geochemical exploration for concealed mineral deposits. Partial dissolution techniques are classified as single dissolution or sequential multiple dissolution depending on the number of steps taken in the procedure, or as “nonselective” extraction and as “selective” extraction in terms of the relative specificity of the extraction. The choice of dissolution techniques for use in geochemical exploration is dictated by the geology of the area, the type and degree of weathering, and the expected chemical forms of the ore and of the pathfinding elements. Case histories have illustrated many instances where partial dissolution techniques exhibit advantages over conventional methods of chemical analysis used in geochemical exploration.     

Regional mineral resources assessment based on rasterized geochemical data: A case study of porphyry copper deposits in Manzhouli,China

This study used regional geochemical survey data (1:200,000 scale) from the Manzouli area of China to assess mineral resources. Geochemical survey data was rasterized and a geochemical atlas was generated, with the image pixel size determined according to geochemical exploration sample point spacing. The Wunugetushan, Babayi, and Badaguan porphyry copper deposits were selected as model areas for the assessment of copper mineral resources. Three parameters were considered for the calculation of the mineral resources. An ore-bearing hydrothermal alteration coefficient was determined based on geological characteristics and geochemical characteristics of the model area, in order to determine alteration intensity; a denudation coefficient was calculated to determine denudation extent; and a mineralization intensity coefficient was calculated to determine the intensity of mineralization within each pixel. Resource estimation was conducted through regression analysis of model deposit resources and coefficients. The results can be used to determine prospecting target areas based on frequency classification and can be used to estimate the number of ore deposits. Results show that resource estimation using rasterized geochemical data provides high prediction precision and accurate positioning.     

Using surface regolith geochemistry to map the major crustal blocks of the Australian continent

Multi-element near-surface geochemistry from the National Geochemical Survey of Australia has been evaluated in the context of mapping the exposed to deeply buried major crustal blocks of the Australian continent. The major crustal blocks, interpreted from geophysical and geological data, reflect distinct tectonic domains comprised of early Archean to recent Cenozoic igneous, metamorphic and sedimentary rock assemblages. The geochemical data have been treated as compositional data to uniquely describe and characterize the geochemistry of the regolith overlying the major crustal blocks across Australia according to the following workflow: imputation of missing/censored data, log-ratio transformation, multivariate statistical analysis, multivariate geospatial (minimum/maximum autocorrelation factor) analysis, and classification. Using cross validation techniques, the uniqueness of each major crustal block has been quantified. The ability to predict the membership of a surface regolith sample to one or more of the major crustal blocks is demonstrated. The predicted crustal block assignments define spatially coherent regions that coincide with the known crustal blocks. In some areas, inaccurate predictions are due to uncertainty in the initial crustal boundary definition or from surficial processes that mask the crustal block geochemical signature. In conclusion, the geochemical composition of the Australian surface regolith generally can be used to map the underlying crustal architecture, despite secondary modifications due to physical transport and chemical weathering effects. This methodology is however less effective where extensive and thick sedimentary basins such as the Eromanga and Eucla basins overlie crustal blocks.     

老挝表层沉积物69种元素地球化学背景值

老挝地质条件优越,资源潜力巨大.为响应国家“一带一路”倡议,在老挝全境开展1∶100万国家尺度地球化学填图工作.采集了大量地球化学数据,为研究区元素分散富集、矿产开发、环境保护、农业生产提供了高质量的基础地球化学数据.共采集地球化学样品2 079件,采用高精度分析技术及严格的质量控制,分析了69种元素.采用X±3S一次性剔除异点后数据集的中位值作为估值,首次给出了老挝全国69种元素地球化学背景值,填补了老挝国家尺度地球化学填图工作的空白.初步讨论了老挝全境、7个三级大地构造单元,6个三级成矿带69种元素的背景值特征.研究表明不同构造单元中受地质背景、构造及岩浆活动的影响,元素分布具有各自特征,同时不同成矿带受成矿作用及构造岩浆活动的影响,元素在各个成矿带分布特征不同,元素的富集对矿床有很好的指示意义.这些背景值的获得为下一步深入研究老挝地球化学填图数据提供了基础对比数据.     

Robust Feature Extraction for Geochemical Anomaly Recognition Using a Stacked Convolutional Denoising Autoencoder

Mathematical Geosciences - Deep neural networks perform very well in learning high-level representations in support of multivariate geochemical anomaly recognition. Geochemical exploration data...     

巨型矿床勘查新战略一一信息找矿

直接信息是最可靠的找矿信息,在矿产勘查中必须起先导的作用。直接信息与间接信息在一定条件和环境下可以互相转化的,只有在信息具有直接指示矿床存在和分布的特性时,它才会发挥实际的找矿效能。因此,多学科信息的收集和综合分析,是信息找矿战略实施的核心。信息找矿战略可以表述为“针对巨型矿床勘查,我们应当瞄准稳陷伏矿和难识别矿,以直接找矿信息（化探资料）为先导,综合地质和地球物理信息,迅速掌握全局,逐步缩小靶区     

Evaluation of geochemical data acquired from regular grids

Geochemical data obtained during mineral exploration often are biased by systematic as well as random errors; these may result in failures when usual methods of evaluation are used. This is true particularly in soil surveys carried out in regions where a long history of prospecting and mining activity has occurred and/or where aerial chemical pollution is likely to have occurred.A satisfactory evaluation of geochemical data even in such an unfavorable case requires sampling on a relatively dense grid and utilization of all available knowledge of types of mineralization. The evaluation procedure proposed consists of five consecutive phases: (1). Dividing the area of interest into subareas of a relatively homogeneous geological nature. (2). Processing by multivariate methods (factor analysis, in particular) without consideration of geographic relations. (3). A preliminary interpretation and search for a geochemical explanation of factors. (4). Processing of individual factors in two-dimensional geographic space by directional and frequency linear filtering methods. (5). Final interpretation and construction of a geochemical model. The procedure is illustrated by an example from a geochemical exploration survey in the vicinity of Píbram (Middle Bohemia).