Determination Of Thermal Maturity Research Articles

Thermal Maturation Trend and Cluster Validation Using K-Means Clustering: Case Study in Salawati Basin

In the last few years, exploration activities have experienced obstacles and have not attracted much attention from researchers in Indonesia. One of the basins in Indonesia that is still interesting is Salawati Basin in West Papua, Indonesia. Several studies related to petroleum systems have been carried out in this basin, especially those that control the migration of hydrocarbon in Salawati Basin. This research was conducted on three wells located in the western of Salawati Basin to evaluate the characteristics and the maturity of the source rock, especially the Sirga Formation. This formation is penetrated by wells MM-02, MM-03, and MM-04, by using the K-Means Clustering method to validate several parameter variations of the maturity level. K-means cluster performs data from various parameters of maturity level with predefined three numbers of cluster. The thermal evolution of source rocks can alter organic matter physical and chemical properties, converting it into hydrocarbon compounds. The selected maturity parameters have good sensitivity to the determination of thermal maturity including phenanthrene (P), in addition to the MPI (methylphenanthrene index) parameter which was obtained from mass chromatograms m/z 178 and m/z 192 based on the peak area of P and methylphenanthrene (MP). Based on the appearance of the crossplot, both Radke and Kvalheim Formulas show that the maturity in studied area is divided into three main zones: peak maturity zone, early maturity zone, and outliers. This study also found that increasing thermal maturity is correlativewith the increasing depth.

Intelligent geochemical interpretation of mass chromatograms: Based on convolution neural network

Gas chromatography-mass spectrometry (GC-MS) is an extremely important analytical technique that is widely used in organic geochemistry. It is the only approach to capture biomarker features of organic matter and provides the key evidence for oil-source correlation and thermal maturity determination. However, the conventional way of processing and interpreting the mass chromatogram is both time-consuming and labor-intensive, which increases the research cost and restrains extensive applications of this method. To overcome this limitation, a correlation model is developed based on the convolution neural network (CNN) to link the mass chromatogram and biomarker features of samples from the Triassic Yanchang Formation, Ordos Basin, China. In this way, the mass chromatogram can be automatically interpreted. This research first performs dimensionality reduction for 15 biomarker parameters via the factor analysis and then quantifies the biomarker features using two indexes (i.e. MI and PMI) that represent the organic matter thermal maturity and parent material type, respectively. Subsequently, training, interpretation, and validation are performed multiple times using different CNN models to optimize the model structure and hyper-parameter setting, with the mass chromatogram used as the input and the obtained MI and PMI values for supervision (label). The optimized model presents high accuracy in automatically interpreting the mass chromatogram, with R2 values typically above 0.85 and 0.80 for the thermal maturity and parent material interpretation results, respectively. The significance of this research is twofold: (i) developing an efficient technique for geochemical research; (ii) more importantly, demonstrating the potential of artificial intelligence in organic geochemistry and providing vital references for future related studies.

Application of Organic Petrology and Raman Spectroscopy in Thermal Maturity Determination of the Karoo Basin (RSA) Shale Samples

An assessment performed using raman spectroscopy has found space in the black shales of the Cisuralian-age rocks of the Karoo Basin in South Africa, particularly those from the Guadalupian Ripon, Cisuralian Whitehill and Prince Albert Formations. It is used in conjunction with geochemical screening techniques such as organic petrology and programmed pyrolysis. In turn, the combination of these techniques is used for the assessment of the thermal maturity of the sedimentary organic matter from the perspective of hydrocarbon generation, retention, and expulsion. To provide further understanding of the black shales in the Cisuralian-age rocks of the Karoo Basin in South Africa, this study focuses on the characterization of samples from the KWV−01 borehole drilled in the southeastern Karoo Basin. In addition, the USA Devonian/Carboniferous Berea Sandstone project samples were included for comparison, and were used as a quality assurance measure. Organic petrology was utilized to assess the organic quality and thermal maturity of the black shales. The results obtained showed that the Karoo Basin shales are overmature, containing an abundance of solid bitumen, and this often characterizes a shale reservoir with moveable hydrocarbons (shale gas). The programmed pyrolysis analysis conducted on the black shales of the Karoo Basin yielded artifact results, as they were determined from a very low and poorly defined S2 peak. This indicated the shales to be overmature and categorized them to be of poor hydrocarbon generation potential. Raman spectroscopy was used to gain insights about the molecular structure of the black shales and to assess if this technique could be used as a complimentary tool to determine the thermal maturity of the shale samples. Raman parameters such as G–D1 Band separation, G and D1 band full width at half maximum (Gfwhm and D1fwhm) and G band position were successfully correlated with vitrinite reflectance (RoV), demonstrating a good potential for Raman spectroscopy to predict the thermal maturity of the shales. Overall, the study provides valuable information and knowledge concerning black shale sample characterization (particularly the thermal maturity and molecular structural characterization) in the Karoo Basin, South Africa.

Thermal evolution behavior of the organic matter and a ray of light on the origin of vitrinite-like maceral in the Mesoproterozoic and Lower Cambrian black shales: Insights from artificial maturation

The overmature Precambrian to Lower Paleozoic marine shales from China contain relatively simple organic matter (OM) composition, which is dominated by in-source solid bitumen (i.e., solid bitumen in the hydrocarbon source rocks). The thermal evolution behavior of the original OM and the determination of thermal maturity in the Precambrian to Cambrian marine shales have been challenging for decades. The vitrinite-like maceral (VLM) is widely present in these marine shales, and its origin is still unknown. To address these issues, the immature Proterozoic Xiamaling shales from China and the immature Cambrian Alum shales from Sweden, and a Chinese immature Carboniferous coal were heat-treated at temperature range of 300 °C to 550 °C. The carbonized residue of the artificially matured samples was examined for their morphological and reflectance variation, and the results were compared with data on the other overmature natural shales from China and Sweden.OM components are similar in the Xiamaling and Alum immature oil shales, consisting of filamentous algae, matrix bituminite, bituminite, VLM and liptodetrinite, and rare thucholites are present in the Xiamaling shales. The algal-derived OM decomposed gradually due to hydrocarbon generation at 300–350 °C. OM is mainly composed of the in-source solid bitumen in the artificially heated shales after 350 °C, similar to the overmature Precambrian to Cambrian natural shales, and the in-source solid bitumen gradually loses its mass with increasing thermal maturity. The in-source solid bitumen is derived from the thermal cracking of the retained oil or the direct conversion of algal-derived liptinite macerals (e.g., the bituminite) or their mixture. VLM in the Xiamaling oil shales can not be observed after 350 °C, but VLM is still present in the Alum oil shales. It can be inferred that there is a different source of VLM in these shales, and VLM in the Xiamaling oil shales contains more volatile material. The VLM in the Xiamaling shales may be biodegradation products of liptinites under anoxic environments. The origin of VLM in the Cambrian Alum shales requires further study to be verified, although it is certain that graptolites are not its sources. The reflectance of in-source solid bitumen (SBRo) increases with heat-treated temperature in both the Xiamaling shales and the Alum shales, but at different rates, which may be due to the difference of the original kerogen composition in these shales. VLM reflectance (VLMRo) and SBRo in sections perpendicular to bedding can be used to determine the maturation level of the Precambrian-Cambrian sediments. Their relation to equivalent vitrinite reflectance (EqVRo) can be expressed by the following equations: EqVRo = 1.07 × VLMRo – 0.18, EqVRo = 0.87 × SBRo + 0.25 (in the Precambrian sediments) and EqVRo = 1.15 × SBRo + 0.01 (in the Cambrian sediments).

Thermal maturity determination for oil prone organic matter based on the Raman spectra of artificial matured samples

Thermal maturity determination for oil prone organic matter (OM) is a very important and difficult work, especially for the source rock with vitrinite absence. In this study, vitrinite reflectance and Raman spectroscopy of 23 coal samples with maturity ranged from 0.55%Ro to 5.32%Ro were measured. The authors proposed that Raman parameters of band D position (Dp) and the distance between bands D and G (D–G) are excellent maturity indexes. A coal sample and two oil prone kerogens of low maturity were matured artificially by heating in gold tube system. The maturities of pyrolysis residues at different pyrolysis temperature were determined by comparing the Raman maturity parameters between of geological and artificial matured coal samples. The pyrolysis residues of oil prone kerogen share a same maturity as that of coal at corresponding pyrolysis temperature as they were heated in same pyrolysis condition. On the base of these works, two equations to determine the maturity of oil prone OM were set up by using Raman maturity parameters of Dp and D-G.

Compositional variations and carbon isotope reversal in coal and shale gas reservoirs of the Bowen and Beetaloo basins, Australia

AbstractStable isotope composition of gas is widely used in hydrocarbon exploration to determine the composition and thermal maturity of source rocks. Many isotope classification systems used for gas to source rock correlation and thermal maturity determination are primarily based on empirical observations made in conventional reservoirs and the kinetic isotope effects observed during pyrolysis experiments performed on source rocks. However, such relationships may not be readily applicable to onshore unconventional reservoirs due to the strong molecular and isotope fractionation that occur during extensive gas expulsion associated with basin uplift and depressurization.Degassing studies of freshly recovered core samples can provide useful insight into the behaviour of gas molecules in unconventional reservoirs during basin uplift. The analyses of Australian coal and marine shale samples demonstrate that during desorption both molecular and isotopic compositions of gas change at variable rates. Gas initially desorbed from the samples is mostly CH4, whereas later desorbed gas becomes increasingly enriched in C2H6 and higher hydrocarbons. Hydrocarbon molecules also fractionate according to their isotopic composition, where the early released gas is enriched in 12C causing the remaining gas in the reservoir to be enriched in the heavier 13C isotope. During the release of gas from the Bowen Basin coals the C isotope ratio of CH4 (δ13C1) changes by up to 21‰ (VPDB), whereas that for C2H6 (δ13C2) and C3H8 (δ13C3) changes by <6‰. Similar changes in the isotope composition can be seen during the release of gas from marine source rocks of the Beetaloo Sub-basin. In a fully gas-mature middle Velkerri shale sample, δ13C1 changes by up to 28‰ and δ13C2 by up to 3‰ with no appreciable change occurring in δ13C3.The extent of molecular fractionation during gas flow through carbonaceous rocks is primarily related to the adsorption–desorption properties of organic matter and diffusivity through the overall rock matrix. Using the current dataset, the magnitude of the contributions exerted by the desorption and diffusion processes cannot be readily distinguished. However, both Bowen Basin coals and Beetaloo Sub-basin shale show similar fractionation effects during gas flow, where the heavier alkane molecules, including those containing more 13C, desorb and move slowly compared with the lighter components, in particular CH4. Different rates of isotope fractionation between hydrocarbon molecules during gas flow cause the shape of compound-specific-isotope (CSI) curve to change with time. Early released gas is characterized by a normal CSI trend where the short-chain hydrocarbons are isotopically lighter compared with the longer-chain hydrocarbons. Because CH4 and C2H6 molecules enriched in 12C desorb and diffuse more readily than the heavier hydrocarbons (including those enriched 13C), the gas remaining in the coal and shale samples after extensive desorption shows a reversed CSI trend where CH4 and C2H6 are isotopically heavier compared with the longer chain hydrocarbons. Reversed isotope trends may also develop over geological time, particularly where the source rock is fully gas-mature and has expelled hydrocarbons due to prolonged degassing. As seen in the Beetaloo Sub-basin, the CSI trend in the dry-gas-mature Velkerri shale is reversed, possibly due to the loss of a large proportion of originally generated CH4 during post-Cambrian basin uplift.

Adventures in pyrolysis II, modeling pyrolysis peaks of petroleum source rocks

Programmed pyrolysis is an integral part petroleum source rock evaluation. The analysis is used to determine the potential of a source rock to generate further petroleum, and with other geochemical measurements, how much it may have generated in the past. An important part of the overall evaluation is an estimate of the source rock thermal maturity. Unfortunately, the addition of any extraneous, non-native hydrocarbons can negatively impact the yield of the samples, and more important, the thermal maturity determination. These extraneous hydrocarbons can be petroleum previously generated and retained within the source rock, fluids that migrated in the subsurface into the rocks, or organic material added during the drilling process. The objective of this study was to determine if mathematical methods could be used to remove these extraneous contributions from the true source rock yields, and arrive at a better determination of thermal maturity.Eighteen samples were studies using the standard pyrolysis method applied in the geochemical services industry. Two aliquots of each sample were analyzed: one as-received and a second after solvent extraction solvents to remove soluble organic matter but not kerogen. The pyrograms typically associate with the conversion of native organic matter in the rock were considered as the sum of up to three underlying components which could be recovered by non-linear least squares methods. Four distribution functions were used to model the pyrograms: classical Gaussian distribution, the Cauchy function, the Logistic function and the Weibull distribution. In addition to the ability of each model to match the measured data, each was evaluated for ease of use. All models reproduced the total pyrolysis yields of the test samples, and generated Tmax values with agreed within 2 °C.•The Gaussian model was marginally better at fitting the measurements than the Logistic or Weibull models, and distinctly better than the Cauchy model. Based upon experience, the Gaussian model was the most forgiving in the choice of initial parameters.•The Cauchy model performed the poorest at fitting the measurements and had the largest differences between extracted and unextracted Tmax values. The deviations from the measured values were most pronounced at high and low temperatures due the longer tail of the Cauchy function.•The Logistic model has marginally lower ability to fit the measured data than the Gaussian model, and similar to the Weibull model. However, the Logistic model was more sensitive to the choice of initial values than the Gaussian model, but less sensitive than the Weibull model.•The Weibull model was marginally poorer at fitting the measurements than the Gaussian model and similar to the Logistic model. It did less well at fitting the four component model for the more complex samples. It was most sensitive to the accurate choices for the initial conditions to insure convergence. For some samples sequential manual choices for these parameters was needed before the Gauss-Newton method could be employed.

Assessing low-maturity organic matter in shales using Raman spectroscopy: Effects of sample preparation and operating procedure

Laser Raman spectroscopy is used to assess the thermal maturity of organic matter in sedimentary rocks, particularly organic-rich mudstones. However, discrepancies exist between quantified Raman spectral parameters and maturity values obtained by vitrinite reflectance. This has prevented the adoption of a standard protocol for the determination of thermal maturity of organic matter (OM) by Raman spectroscopy. We have examined the factors influencing the Raman spectra obtained from low-maturity OM in potential shale gas reservoir rocks. The inconsistencies in Raman results obtained are due to three main factors that are critically evaluated: (1) different operational procedures, including experiment setup and spectral processing methods; (2) different methods of sample preparation; (3) the analysis of diverse types of OM. These factors are scrutinized to determine the sources of inconsistency and potential bias in Raman results, and guidance is offered on the development of robust and reproducible analytical protocols. We present two new Raman parameters for un-deconvolved spectra named the DA1/GA ratio (area ratio of 1100–1400 cm−1/1550–1650 cm−1) and SSA (scaled spectrum area: sum of total area between 1100 and 1700 cm−1) that offer potential maturity proxies. An automated spreadsheet procedure is presented that processes raw Raman spectra and calculates several of the most commonly used Raman parameters, including the two new variables.

Organic matter type, origin and thermal maturity of Paleozoic, Mesozoic and Cenozoic successions of the New Siberian Islands, eastern Russian Arctic

Geological investigations in the offshore area of eastern Russian Arctic attracted an increased interest in the recent years. Data concerning organic and coal petrology is scarce for the shelf and offshore area of the eastern Russian Arctic. Consequently, thermal maturity and depositional environments of sedimentary facies remain less described and examined. A detailed characterisation of organic matter type and origin as well as determination of thermal maturity have been carried out on siliciclastic and carbonate sedimentary samples from Paleozoic, Mesozoic and Cenozoic outcrops of the New Siberian Islands, eastern Russian Arctic. The acquired organic and coal petrological data contribute to the overall assessment of petroleum potential of the studied offshore area and the neighboring shelf area.The investigated organic matter of Silurian and Middle Devonian mudstones and siltstones contain distinct, multi-staged solid bitumen (“dead oil”) preserved either as fracture-, fissure-, and vein-fillings, indicating activation of migrations pathways for re-occurring crude oil-bearing fluids or in form of “streaks”. The examined dispersed organic matter of Triassic deposits is predominantly composed of terrestrially derived macerals followed by alginite macerals (Tasmanite and Leiosphaeridia prasinophytes). The analysed Cretaceous and Tertiary deposits reveal (i) Aptian/Albian inertinite-rich sub-bituminous coal and lignites formed in fresh water lakes and swamps to brackish near-shore depositional environments with phases of wildfires as well as (ii) Cenomanian/Turonian lignites with pollen grains and carbonised debris (pyrofusinite), indicating stages of forest/moor paleofires. The (iii) Paleocene and Eocene deposits bear mineral-rich and xylite-rich lignites characterised by the lowest coal rank stage, containing different fossil conifer species.The thermal maturity of organic matter in Silurian to Tertiary deposits obtained from random vitrinite and bitumen reflectance measurements is an important parameter in determining the petroleum generation stage at the studied localities. The calculated thermal maturities of Silurian to Middle Devonian deposits acquired from random reflectance measurements of bitumen indicate condensate-wet gas to early dry gas generation window. The thermal maturities of sedimentary organic matter in samples collected from Tertiary to Triassic successions ranges from immature to main phase of oil generation.The composition of the organic matter and thermal maturity of the analysed Triassic siliciclastic deposits suggesting mature stage for oil generation can be used among others to evaluate their hydrocarbon potential. The Cretaceous and Tertiary deposits can be described as thermally immature. In two localities, where the thermal maturity is anomalously high, the examined organic matter has been thermally altered beyond the dry gas preservation limit.

A RaMM study of thermal maturity of dispersed organic matter in marine source rocks

The Raman Maturity Method (RaMM) is a non-graphical technique for thermal maturity determination based on the multi-linear regression analysis of parameters derived from carbon Raman spectra of humic macerals. It is calibrated against vitrinite reflectance with a suite of Australian mostly Permian coals. In a previous communication an equation to cover the range 0.4–1.2% equivalent vitrinite reflectance was derived. In this report we extend the range with a second equation for the range 1.0–2.5%, and validate its use by testing a series of Australian Permian and German Carboniferous coals. The special features of the technique are that it is equally applicable to vitrinite and inertinite, it corrects internally for the suppression of vitrinite reflectance effect, grains as small as 5μm are suitable, and it does not require well depth information to be available.The technique has been applied to two wells, Cape Range-2 and Dampier-1, from the rift margin of the North West Shelf of Australia. Both wells intersect large thicknesses of fine-grained marine Jurassic and Cretaceous sediments including the major source rock formation of the Carnarvon Basin. There are no terrestrial sediments for control. There is good agreement between RaMM and VR results for VR>1.2% for Cape Range-2. However within the oil window in both wells RaMM results indicate the vitrinites in many of the samples have reflectance suppression to such a degree that the VR results would be misleading or useless for maturity modeling. It is likely that this problem, which is well documented for marine influenced coals, to some degree affects organic matter in marine source rocks worldwide.

Applicability of Calculated Vitrinite Reflectance for Assessment of Source Rock’s Organic Matter Maturity in Hyperthermal Basins (Banat Depression, Serbia)

Middle and Upper Miocene petroleum source rocks from the Banat Depression, SE Pannonian Basin (Serbia) were investigated. These rocks contain organic matter (OM) in a relatively wide range of maturity (from early to late stage of liquid hydrocarbons generation). OM consists predominantly of Type II kerogen and was deposited under variable redox conditions. This prolific oil and gas production area, as a part of the hyperthermal Pannonain Basin, is characterized by high geothermal gradients (4.0–7.5°C/100 m) and high heating rates (9–22°C/Ma) of the Miocene sediments. Considering complexity of the vitrinite reflection measurements in the source rocks containing Type I and/or Type II kerogen, in this study the values of the measured vitrinite reflectance were compared to calculated vitrinite reflectance, based on maximal palaeotemperature and numerous geochemical maturity parameters. The objective of the study was to assess the applicability of calculated vitrinite reflectance for the Type II kerogen source rocks from a hyperthermal basin. The correlation analysis was performed according to Spearman and Pearson tests. The obtained results showed that calculation of vitrinite reflectance and determination of thermal maturity of OM of the source rocks in hyperthermal basins with high heating rates is best done when based on geochemical parameters calculated from compositions of steranes and methyldibenzothiophenes, and as expected-on maximal palaeotemperature.

OILS IN THE NW NIGER DELTA: AROMATIC HYDROCARBONS CONTENT AND INFRARED SPECTROSCOPIC CHARACTERISATION

The occurrence and distribution of aromatic hydrocarbons in oil samples from an oilfield in the NW Niger Delta was investigated, focussing on naphthalenes, phenanthrenes and mono‐ and triaromatic steroids. Tetramethylnaphthalene is the most abundant of the naphthalene homologues, while dimethylphenanthrene is the most abundant of the phenanthrene homologues. The relative concentration of the sum total of the phenanthrenes and their isomers was greater than that of the naphthalenes.The distribution of naphthalenes and phenanthrenes was strongly controlled by thermal maturation and biodegradation of the oils. Various aromatic hydrocarbon‐ dependent geochemical parameters indicate that the oils are mature and were generated at similar levels of thermal maturation. Plots of these parameters showed that the distribution of homologues of both naphthalene and phenanthrene is sensitive to the biodegradation of the oils.Infrared spectroscopy (IR) was used for geochemical characterisation of the NW Niger Delta oil samples in terms of both source type and thermal maturity. The characterisation was based on the peak intensities of the aliphatic and the carbonyl groups relative to the aromatics. The study showed that IR may be useful for the determination of thermal maturity, but is not a good tool for source characterisation.

Determination of thermal maturity and organic matter type by principal components analysis of the distributions of polycyclic aromatic compounds

The thermal maturity and organofacies sensitivity of polycyclic aromatic compound (PAC) distributions was explored by examination of the aromatic fractions of solvent extracts from a diverse set of 53 shales, coals and kerogen macerals which have undergone either natural or artificial maturation and which represent all three principal sedimentary organic matter (OM) types. Systematic changes with maturation were observed in the following groups of isomers: tri- and tetramethylnaphthalenes, methyl- and dimethylphenanthrenes, methyl- and dimethyldibenzothiophenes, methylpyrenes, and methylchrysenes. The maturity differences were quantified by mathematical ratios of the relative concentrations of the more thermally stable isomers to the less stable, on the basis of theoretical considerations and empirical observations. The PAC maturity parameters, unlike those derived from saturated biomarker stereoisomers, are typically effective across the entire oil generation window. To compensate for the effects of OM type on the maturity parameters, they were combined using principal components analysis. The resulting first principal component was in good agreement with independent indicators of maturity. The relative distributions of C 0–C 3 alkylphenanthrenes, dibenzothiophene, methyldibenzothiophenes and methyldibenzofurans were evaluated by a separate principal components analysis. The results permitted an independent grouping of the samples by OM type and suggested additional, simple molecular ratios that allow graphical recognition of OM type, including the ratio of dibenzothiophenes to dibenzofurans and a ratio using C 2-alkylphenanthrene isomers.

THE CURRENT STATUS OF THE FAMM THERMAL MATURITY TECHNIQUE FOR PETROLEUM EXPLORATION IN AUSTRALIA

The fluorescence alteration of multiple macerals (FAMM™) technique was developed in 1989-91 to overcome problems encountered by existing techniques applied to the thermal maturity determination of North West Shelf organic matter. Subsequently it has been widely used in Australia, SE Asia and elsewhere. This paper reviews the present status of the FAMM technique.Since its inception, the methodology of the FAMM technique has undergone some changes. The most important is a closer integration with vitrinite reflectance (VR) resulting in expansion of the capacity of the technique to solve complex maturity problems. The accumulated data indicate that shales and mudstones are most suitable for FAMM analysis for the reason that more porous lithologies are potentially more susceptible to organic matter oxidation. There is extensive evidence that organic matter in cores and well-protected cuttings samples of shales remain unaffected by oxidation during decades of storage because clay is extremely effective in limiting access of oxygen. Adhering to sampling guidelines can greatly reduce the risk of errors from this source. Of the two recently described thermal maturity techniques based on combining VR with fluorescence intensity measurements (Quick, 1994; Newman, 1997), FAMM is more closely related to Quick's method. As the fluorescence alteration ratio which acts as the thermal maturity indicator in the FAMM method is independent of VR, it is possible to cross check results for consistency using suppression iso-correction curves-a possiblity which does not exist with the other thermal maturity techniques based on fluorescence. A working set of suppression iso-correction curves has been determined specifically for Australian Jurassic vitrinite. Whether from coal or dispersed organic matter (DOM), Australian Jurassic vitrinites are commonly perhydrous and this should be borne in mind when modelling thermal histories of North West Shelf sequences.

Thermal maturity evaluation by sonic log and seismic velocity analysis in parts of Upper Assam Basin, India

Vitrinite reflectance is an important index used in assessment of thermal maturity of petroleum source rocks. Vitrinite reflectance typically increases with depth of burial as a function of time and temperature and is accompanied by a decrease in log-derived interval transit time. The relationship between these parameters has been investigated in the Upper Assam (Tertiary) Basin, India. An equation that relates sonic log transit time to vitrinite reflectance has been derived. Thus, interpretation of the sonic log enables a quantitative estimation of thermal maturity that is not affected by the problems frequently encountered in measurement and interpretation of vitrinite reflectance data. Further, the approach has been extended to the determination of thermal maturity from seismic velocity analysis. The results suggest that seismic data can be used as an additional maturity evaluation tool which can provide preliminary estimates of maturity in undrilled areas of the basin.

Raman spectroscopy of chitinozoans as a maturation indicator

Micro-Raman spectra were acquired from a series of individual chitinozoans of low ( R ch 0.6%) to high thermal maturity ( R ch 5.1%). This provides information on their molecular and structural characteristics. All spectra show two broad bands on a fluorescent background at ∼ 1600 and 1355 cm −1. With increasing rank there is a significant decrease in the line-width of the 1600 cm −1 band, assigned to the out of phase coupling of an in-plane C-C stretching vibration. The rate of change for the reduction in the line-width is most pronounced within the lower rank material as the sample matures through the “oil window”. These data suggest that Raman spectroscopy of chitinozoan maceral, in particular monitoring of the half-width of the 1600 cm −1 band, shows potential as a maturation indicator in sedimentary rocks. Significantly, it offers a link between optical methods of thermal maturity determination and the underlying changes in the physical and chemical structure of the organic material.

Rock-Eval Tmax Suppression: Documentation and Amelioration

The Rock-Eval thermal maturation parameter (Tmax) may occasionally be suppressed relative to surrounding rock units. This is commonly assumed to result from either a bitumen contribution to the S2 peak or the presence of sulfur-rich (and hence thermally labile) kerogen. The phenomenon is frequently coincident with a high natural gamma-ray signature and elevated Rock-Eval hydrogen index (HI) and total organic carbon (TOC) contents. Determination of thermal maturity of such units relies either on the interpolation of Tmax from surrounding units or the direct estimation of the extent of suppression on a given sample. The latter may be required if the available samples are restricted to conventional or sidewall-core material representative of imited stratigraphic intervals. The Second White Speckled Shale (part of the Cretaceous Colorado Group) in the Alberta basin displays Tmax suppression over part of its thickness. A consistent relationship emerges from calculating a linear regression of Tmax as a function of either TOC or HI for several wells at different levels of thermal maturity. Below values of about 453°C (late oil window), Tmax is suppressed by about 1°C for every increase in HI of 50 mg hydrocarbon/g TOC. Thus Tmax suppression in the Second White Speckled Shale can be corrected by extrapolating all Tmax values to a constant HI of 100 to 150 mg hydrocarbon/g TOC, values typical of type III kerogen.

Should fluorescence alteration replace vitrinite reflectance as a major tool for thermal maturity determination in oil exploration?

Should fluorescence alteration replace vitrinite reflectance as a major tool for thermal maturity determination in oil exploration?

Should fluorescence alteration replace vitrinite reflectance as a major tool for thermal maturity determination in oil exploration?

The fluorescence alteration multiple macerals (FAMM) technique is closely related to vitrinite reflectance. Until recently, fluorescence alteration study of dispersed organic matter (DOM) in sedimentary rocks has been regarded as technically difficult. With the introduction of optical microprobes equipped with small continuous wave gas lasers having a very stable output of blue radiation readily focused to 1–2 μm on the sample, this work has now become routine. FAMM has important advantages over vitrinite reflectance stemming from the possibility of resolving the data into two parameters, one related to the level of maturation, and the other related to the hydrogen-richness of a maceral. This enables the resolution of what are often difficult problems for the single parameter vitrinite reflectance technique, namely the identification of indigenous vitrinite in DOM, the recognition of caved and reworked organic matter, and the suppression of vitrinite reflectance. For these reasons, FAMM is a powerful tool for investigating vitrinite reflectance anomalies, especially those associated with unconformities. It is also well suited to investigate the tacit assumption in the vitrinite reflectance technique that all telocollinite matures along a single compositional pathway. The FAMM technique is not without problems. The range is restricted to equivalent mean random vitrinite reflectance Rmo = 0.4–1.2, the results are more sensitive to ambient temperature oxidation than vitrinite reflectance, and the technique demands a high level of technical expertise. The FAMM technique is calibrated on coals and it is therefore more likely to give high quality results on Types II and III kerogens rather than Type I kerogens which have not yet been intensively investigated. Although FAMM is unlikely to replace vitrinite reflectance, considerable confidence may be had in a maturation assessment when consistent results are obtained from these two closely related but independent techniques.

Should fluorescence alteration replace vitrinite reflectance as a major tool for thermal maturity determination in oil exploration?

The fluorescence alteration multiple macerals (FAMM) technique is closely related to vitrinite reflectance. Until recently, fluorescence alteration study of dispersed organic matter (DOM) in sedimentary rocks has been regarded as technically difficult. With the introduction of optical microprobes equipped with small continuous wave gas lasers having a very stable output of blue radiation readily focused to 1–2 μm on the sample, this work has now become routine. FAMM has important advantages over vitrinite reflectance stemming from the possibility of resolving the data into two parameters, one related to the level of maturation, and the other related to the hydrogen-richness of a maceral. This enables the resolution of what are often difficult problems for the single parameter vitrinite reflectance technique, namely the identification of indigenous vitrinite in DOM, the recognition of caved and reworked organic matter, and the suppression of vitrinite reflectance. For these reasons, FAMM is a powerful tool for investigating vitrinite reflectance anomalies, especially those associated with unconformities. It is also well suited to investigate the tacit assumption in the vitrinite reflectance technique that all telocollinite matures along a single compositional pathway. The FAMM technique is not without problems. The range is restricted to equivalent mean random vitrinite reflectance Rm o = 0.4–1.2, the results are more sensitive to ambient temperature oxidation than vitrinite reflectance, and the technique demands a high level of technical expertise. The FAMM technique is calibrated on coals and it is therefore more likely to give high quality results on Types II and III kerogens rather than Type I kerogens which have not yet been intensively investigated. Although FAMM is unlikely to replace vitrinite reflectance, considerable confidence may be had in a maturation assessment when consistent results are obtained from these two closely related but independent techniques.