Marcos Escobar

Effect of coal weathering on some geochemical parameters

The effect of coal weathering on some geochemical parameters was studied. Seven pairs of fresh and altered samples were selected from different Venezuelan coal deposits. The following analyses were performed: water and ash percent, heating value, bitumen content and composition, infrared spectroscopy, n-alkane distribution and humic substances concentration (HS). As weathering proceeds, there is a corresponding change in HS concentration. Most geochemical parameters show small changes with HS concentrations below 15%. For intense weathering, all parameters measured show important modifications. Such changes are due to the breaking of the coals macrostructure.

Prediction of coal properties by derivative DRIFT spectroscopy

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was evaluated as a method for the determination of volatile matter, fixed carbon, ash, heating value, carbon, hydrogen, nitrogen, total sulfur and maximum vitrinite reflectance. Fifty-five samples (mainly bituminous coals) were analysed by ASTM methods and the results correlated with multiple linear regression (MLR) and principal-components regression (PCR) with the second-derivative DRIFT spectra. Good correlations (R > 0.9) were obtained for volatile matter, fixed carbon, ash, carbon, hydrogen and vitrinite reflectance.

A numerical procedure for curve fitting of noisy infrared spectra

A method for the detection of overfitting of noisy spectra is presented. When fitting data that contains random noise, the autocorrelation function (RL) of residuals at lag 1 (R1) approaches zero and then shows a tendency toward more negative values, while the Wald–Wolfowitz test tends to give more positive values, as the data is overfitted. Models that give residuals with a non-random ordering may be rejected. The use of these functions for the determination of the “best” fitting of several approximations (Savitzky–Golay smoothing, segmented osculating polynomials (SOPA), Fourier series, and Lorentzian bands) to a noisy synthetic spectrum, and the application to infrared spectra of coal, is shown.

The prediction of coal properties using compressed infrared data from osculating polynomials

Abstract Reduction of the representation of infrared spectra from coal samples by osculating polynomials of degree nine is discussed. The reduced representation contains polynomial coefficients of order zero to four. Mathematical models of the original spectra are obtained by linear combination of the coefficients. These compressed models are statistically correlated to coal properties, namely, volatile matter, fixed carbon, ash content, heating value, hydrogen, carbon, sulphur, nitrogen, and maximum vitrinite reflectance, and the results are compared with those previously obtained from second derivatives of the same spectra. The use of compressed data, while giving slightly better correlations for some of the properties, has the advantage of requiring less computational time.

Determination of chemical properties of pyrolysis products from coals by diffuse-reflectance infrared spectroscopy and partial least squares

Abstract Typical coal samples from Venezuela were pyrolysed in nitrogen atmosphere at temperatures between 150 and 900°C. Structural changes were monitored by diffuse-reflectance infrared Fourier transform spectroscopy, i.e. DRIFTS. Segmented osculating polynomial approximation (SOPA) and numerical differentiation by the Savitzky–Golay procedure were used as spectroscopic data pre-treatment. The data was then correlated by partial least squares (PLS) and iterative predictor weighting (IPW) with volatile matter, hydrogen, carbon, nitrogen, and total sulphur contents. The method was found to be useful for the prediction of volatile matter and hydrogen contents, but not to the other physical and chemical properties of the pyrolysed products.

Trace elements of Paleocene Táchira coals, southwestern Venezuela: a geochemical study

Abstract The concentration and distribution of 35 elements in 78 coal samples from the Tachira State coal fields belonging to Los Cuervos Formation (Paleocene) of western Venezuela were obtained using atomic absorption spectrometry (AAS) and inductively coupled plasma atomic emission (ICP-AES). The aim of this work was to determine, through a multivariate statistical approach (factor analysis), if there is a correlation between the trace element content in coal beds and the provenance rocks at the time of peat deposition. Comparison with world averages and geometric means for trace element concentrations in coal-bed samples shows that B, Ba, La, V, Mn, Zn, and Pb are depleted. However, Tachira coal samples show a perceptible enrichment in Bi, Sb, As, Cd, and Mo, and they are highly enriched in Ag and Co. Nicholls plot suggests that only B and Co show a distinctive organic affinity. Enriched elements (Ni, Ag, Cd, Mo, Co) are both statistically and genetically related, and we attribute their origin to a volcaniclastic Jurassic unit. Other statistical factors (Th–V–P and Ca–Mg–Mn) reveal a different provenance, indicating a felsic plutonic source and a sedimentary limestone, respectively. A fourth factor (K–Mo–Th–S) is composed mainly of clay minerals and authigenic sulfides.

Geochemistry of oil seeps and rock samples of the Early Tertiary section from the Northandean Flank of the Venezuelan Andes

Abstract An organic geochemical study of oil seeps and Early to Middle Tertiary source rocks of the Northandean Flank (NAF) of the Venezuelan Andes range was performed to establish the possible contribution of terrestrial source rocks to the oil accumulations in the unexplored southern part of the Maracaibo Basin. Source rocks were identified within the Paleocene Barco and Los Cuervos Formations, and the Late Eocene-Oligocene Carbonera Formation. Biomarker distributions indicate a terrigenous organic source for these units. Geochemical modeling performed by Parnaud et al. (1995), indicated Plio-Pleistocene oil generation for the Orocue Group and Middle Miocene to Plio-Pleistocene oil generation for the Carbonera Formation. Geochemical modeling also indicates that the quantity of crude oil expelled by the Carbonera Formation is four times that expelled by the Orocue Group (Barco and Los Cuervos Formations). A northeast to southwest variation from marine to terrestrial oil seeps was identified based on sterane and terpane distributions. The marine oil seeps appear to be derived from the La Luna source rocks. The characteristics of the mixed oils suggest contributions from different sources (La Luna and a terrestrial source rock) most likely the Orocue Group or Carbonera Formation coals. Source facies similar to those in the Los Cuervos and Carbonera Formations coals are most likely the main source rocks of terrestrial oil seeps in the NAF.

Organic Geochemical Investigation and Coal-bed Methane Characteristics of the Guasare Coals (Paso Diablo Mine, Western Venezuela)

Abstract The aim of this work was to carry out a geochemical study of channel samples collected from six coal beds in the Marcelina Formation (Zulia State, western Venezuela) and to determine experimentally the gas content of the coals from the Paso Diablo mine. Organic geochemical analyses by gas chromatography-mass spectrometry and isotopic analyses on-line in coalbed gas samples were performed. The results suggest that the Guasare coals were deposited in a continental environment under highly dysoxic and low salinity conditions. The non-detection of 18α(H)-oleanane does not preclude that the organic facies that gave rise to the coals were dominated by angiosperms. In addition, the presence of the sesquiterpenoid cadalene may indicate the subordinate contribution of gymnosperms (conifers) in the Paleocene Guasare mire. The average coalbed gas content obtained was 0.6 cm3/g. δ13C and δD values indicate that thermogenic gas is prevalent in the studied coals.

Ti/Zr ratio as a geochemical tool for the correlation of weathered coals from geological formations in two Latin American sedimentary basins

ABSTRACT Major and trace element concentrations in fresh and altered coals in outcrops and coalfields from Táchira and Mérida states (Venezuela), as well as from Napo and Pastaza provinces (Ecuador), have been determined to establish inorganic parameters not dependent of weathering effects. Noteworthy is that the Ti/Zr ratio is approximately constant in each sedimentary basin even for great distances (>100 km). Worldwide data of trace elements analysis in coal samples support the stated Ti/Zr ratio constancy in different basins for fresh and weathered ones. Therefore, Ti/Zr ratio can be considered a rapid tool to easily identify coal seam correlations.

Characterization of Physicochemically Abnormal Coals in the Matarere Formation (Cerro Saroche National Park, Venezuela)

Proximate and ultimate analyses, % Rm , vitrinite anisotropy, Rock-Eval T max, and biomarkers have been studied on several coal samples collected at two outcrops in the Cerro Saroche National Park (Venezuela) in order to determine their geochemical characteristics. The results indicate that these coals were deposited in a transitional paleoenvironment under weakly reducing conditions (Palaeogene Matatere Formation), and were derived from mixed organic matter input. Carbon content and other data (%Rm and anisotropy) suggest high levels of maturity (matagenesis to metamorphism) of these coals. However, this does not correspond to the low-volatile bituminous stage (late catagenesis) according to the ASTM classification by rank. These coals might have reached overmaturity as a result of heating by Neogene igneous events.