Rock-Eval Pyrolysis Parameters Research Articles

Mechanism of water-induced alterations to nanoscale pores: Implications from anhydrous versus hydrous pyrolysis

In this study, shale samples from the Chang7 Member of Triassic Yanchang Formation in Ordos Basin, China were used to conduct artificially induced thermal maturity experiments from 250 °C to 450 °C for 72 h. The role of water in organic matter (OM) conversion and nanoscale porosity alterations were observed and discussed comprehensively by Rock-Eval pyrolysis, low pressure N2/CO2 adsorption, Fourier transform infrared spectroscopy (FTIR) and water-soluble organic acids (WSOAs) experiments. Results show that Rock-Eval pyrolysis parameters (Tmax, S1, and S2), TOC content, WSOAs and pore volume (PV) show slight changes below 350 °C while these parameters were considerably altered above 400 °C under two different pyrolysis conditions (anhydrous and hydrous), indicating that after 400 °C, the OM conversion and bitumen cracking will accelerate. In comparison to anhydrous pyrolysis, hydrous pyrolysis always has smaller Tmax, implying that water could inhibit OM thermal progression. Moreover, S1 of hydrous pyrolysis is approximately two times higher than that of anhydrous pyrolysis, which originates from water providing an exogenous source of hydrogen to generate excessive volumes of hydrocarbons, enhancing the conversion rate of OM. The WSOAs are 2 to 4 times greater under hydrous pyrolysis than anhydrous pyrolysis, denoting that the presence of water could improve the yield of organic acids via inhibiting the polycondensation of oxygen-containing functional groups and oxidizing organic components by hydroxyl radicals. Moreover, below 350 °C, pyrolysis shale residues exhibit a slightly smaller PV in hydrous pyrolysis, mainly due to the infilling effect of bitumen. However, exceeding 400 °C, PV that was obtained from low pressure N2 adsorption under hydrous pyrolysis conditions is found to be two times as large as the value under anhydrous pyrolysis conditions, which is attributed to the net-PV increase reactions such as thermal cracking, and the dissolution of organic acids. Furthermore, in anhydrous pyrolysis, the polycondensation of oxygen-containing functional groups and the carbon-carbon cross linking could be considered as the two main reaction pathways to release the sources of hydrogen. Collectively, the presence of water is significant for hydrocarbon generation and porosity evolution during thermal progression of OM at various stages.

Molecular and isotopic signatures of indigenous and non-indigenous hydrocarbons in Campanian sediments from the Tano Basin, Ghana

The Campanian source strata of the Tano Basin, Ghana, have garnered exploration interest based on the recent discoveries of hydrocarbon shows in some offshore wells. This study reports on the bulk geochemical composition (Total organic carbon content and Rock-Eval pyrolysis parameters), molecular markers, and stable carbon isotope (δ13C) composition of eleven Campanian sediments from TP–1 and Ankobra–1 well, a shallow and deep-water well in the Tano Basin respectively. Based on the analysis of total organic carbon content and Rock-Eval pyrolysis parameters (S1, S2, and Tmax), it was found that the TP-1 Campanian sediments contain indigenous hydrocarbons. Contrariwise, the Campanian samples from the Ankobra-1 well are mainly composed of non-indigenous hydrocarbons, possibly originating from offshore drilling fluid contaminants. Distribution of saturated and aromatic hydrocarbons, including normal alkanes, acyclic isoprenoids, steranes, naphthalenes, phenanthrenes, dibenzothiophene, dibenzofuran, fluorenes, and their alkyl derivatives, as well as stable carbon isotope (δ13C) values, were analyzed to gain insight into the biological source input, depositional environment, and thermal maturity of the organic matter (OM) in the sediments. The molecular and isotope results indicated that the OM in the indigenous hydrocarbons is from mixed organic sources, with a predominance of terrestrial plants, whereas lower marine organisms dominate the non-indigenous hydrocarbons. Moreover, the molecular ratios obtained from our analysis indicated that the OM present in the indigenous hydrocarbons was deposited under suboxic conditions in a marginal marine environment. Contrarily, the non-indigenous hydrocarbons exhibit a strong association with marine carbonate facies. Additionally, the maturity assessment based on biomarker ratios, indicated that the OM within the Campanian sediments from TP-1 well is in the early phase of oil generation whereas the OM in the non-indigenous hydrocarbons is in the peak-late mature phase. In conclusion, our geochemical findings have revealed the hydrocarbon potential and heterogeneity of hydrocarbons within the Tano Basin's Campanian source strata induced by non-indigenous hydrocarbons. The geochemical data obtained from our study provide evidence of the potential influence of non-indigenous hydrocarbons on the misrepresentation of source input, depositional environment, and thermal maturity of organic matter (OM) in source rock analysis.

Geochemistry of Permian carbonaceous shales from Raniganj sub-basin, Damodar Valley, India: Implications for provenance, weathering, tectonics and source of organic matter

Raniganj sub-basin, the easternmost depository of Damodar Valley encompasses the entire succession of Gondwana Supergroup, the lower and upper Gondwana sequences. The present investigation of Permian carbonaceous shales belonging to Barakar, Barren Measure and Raniganj formations of lower Gondwana sequence from this basin is an integrated study of inorganic and organic geochemical characteristics viz. major/trace elements, stable isotopic composition and kerogen type to ascertain source rock attributes such as provenance, tectonic setting, prevailing weathering/climatic conditions as well as source and type of organic matters. Trace element indices, such as La/Sc, Th/Co, Th/Sc, (Gd/Yb)CN, Eu/Eu*, Cr/V, Y/Ni, reveal that the shales are originated from felsic/granitic source rock. Prominent Eu-anomaly, enrichment of chondrite normalised light rare earth element (LREE) values and a flat heavy rare earth elements (HREE) pattern reflect post-Archean felsic source for the shales. The major element composition, especially relative enrichment of Al over readily soluble elements, Ca, Na and K yielding high CIA (74.9–84.6%) and CIW (87.1–97.5%) as well as low ICV (0.64–1.32) values, establishes significantly weathered and matured nature of the shales. The distribution of redox-sensitive elements (U/Th, V/Cr, Ni/Co, Ce/Ce*) as well as low ICV, depleted K2O/Al2O3 ratios and high CIA values point towards an oxic environment and warm-humid climatic condition during deposition of these shales. The conventional method for determining tectonic setting using major and trace element composition fails to clearly delineate the tectonic setting for the current shales. The samples show wide scattering across active and passive continental margin in the K2O/Na2O–SiO2 diagram, whereas the samples are concentrated near the active in the continental margin in the Th–Sc–Zr/10 diagram contradicting the geological framework of Raniganj sub-basin. Discriminant functions-based approach which categorises the tectonic setting into continental/oceanic arc, continental rift and continental collision settings has been proved to be suitable in the present case and reveals continental-rift setting for the shales from Raniganj sub-basin. The carbon isotopic signature (δ13Corg) of the shales reveals a range (−24.5 − −22.9‰) intermediate between that of marine and terrestrial (C3 plants) organic matters indicating possible mixed type of organic matters. The fluvio-marginal marine depositional condition obtained from K2O/Al2O3 and MgO/Al2O3 ratios further implies the possibility of mixed nature of organic matter. The geochemical parameters of the organic matter (Rock-Eval pyrolysis parameters S1, S2, TOC, HI, OI etc.) indicate that the studied shales contain thermally matured mixed Type III/II gas-prone kerogen and are derived predominantly from terrestrial sources with significant marine input.

Geochemical Features of Lacustrine Shales in the Upper Cretaceous Qingshankou Formation of Changling Sag, Songliao Basin, Northeast China

The organic-rich shale of the Upper Cretaceous Qingshankou Formation is an important hydrocarbon source rock in Northeast China. In this study, taking the lacustrine shale in the Qingshankou Formation as an example, geochemical analysis, including total organic carbon (TOC), Rock-Eval pyrolysis, maceral compositions, X-ray diffraction (XRD), and biomarker analyses, were carried out on twenty-four shale samples to evaluate the geochemistry and environmental features of this shales. The Qingshankou lacustrine shales contains mainly Type I/II1 organic matter and is oil prone, with a good to excellent source rock. Vitrinite reflectance and Rock-Eval pyrolysis parameters show that the Qingshankou lacustrine shales is mainly in the mature stage and within the oil window. Biomarker composition of the shales provide evidence that the Qingshankou lacustrine shales was formed in a reductive sedimentary environment with relatively high salinity water. The organic matter came from a mixture of plankton, bacterial and land plants.

Geochemical evaluation of hydrocarbon generating potential and thermal stress experienced by Patala Formation, Salt Range, Upper Indus Basin, Pakistan

This study focused on the investigation of hydrocarbon potential, thermal maturity and ranking of the coal from the Salt Range in Pakistan. Four different mines located in different areas of the Salt Range were selected for sample acquisition. The samples were further processed and analyzed in laboratory using proximate analysis, ultimate analysis and Rock-Eval pyrolysis. Proximate analysis illustrate that coal of the Salt Range is of low rank because of low moisture content and high volatile material. The ultimate analysis indicates that the coal is immature with lower carbon contents. The coals have total organic carbon contents in the range of 36.45 to 48.68 wt.% and hydrogen index values between 277 and 445 mg HC/TOC indicate that the organic matter type is dominated by type II. Moreover, Rock-Eval pyrolysis parameters further support it as an excellent source rock having type II kerogen, oil-prone nature with some gas-prone character. These data suggest that the coal samples are in low conversion state and thermally immature.

Comparisons of pyrolysis parameters between source rocks and their clay-sized fractions: Implication for source material of hydrocarbon generation

In natural environments, organic-clay interactions are strong and cause organo-clay composites (a combination between organic matter [OM] and clay minerals) to be one of the predominant forms for OM occurrence, and their interactions greatly influence the hydrocarbon (HC) generation of OM within source rocks. However, despite occurring in nature, dominating the OM occurrence, and having unique HC generation ways, organo-clay composites have rarely been investigated as stand-alone petroleum precursors. To improve this understanding, we have compared the Rock-Eval pyrolysis parameters derived from more than 100 source rocks and their corresponding <2 μm clay-sized fractions (representing organo-clay composites). The results show that all of the Rock-Eval pyrolysis parameters in bulk rocks are closely positively correlated with those in their clay-sized fractions, but in clay-sized fractions the quality of OM for HC generation is poorer, in that the pyrolysable organic carbon levels and hydrogen index values are lower, whereas the residual organic carbon levels are higher than those in bulk rocks. Being integrated with the effects of organic-clay interactions on OM occurrence and HC generation, our results suggest that organo-clay composites are stand-alone petroleum precursors for HC generation occurring in source rocks, even if the source rocks exist in great varieties in their attributes. Our source material for HC generation comprehensively integrates the original OM occurrence and HC generation behavior in natural environments, which differs from kerogen and is much closer to the actual source material of HC generation in source rocks, and it calls for further focus on organic-mineral interactions in studies of petroleum systems.

Hydrocarbon source potential and paleodepositional environment of the (Devonian) Barreirinha formation on the south edge of the Amazonas basin border, Brazil

The present work aims to study the depositional paleoenvironment and the hydrocarbon source potential of the (Devonian) Barreirinha Formation, in the southern portion of the Amazonas basin. To accomplish this task, the geochemical parameters (total organic carbon, Rock-Eval pyrolysis, total sulfur and total nitrogen, whole oil hydrocarbons, and biomarkers) and palynofacies of 22 samples distributed in three outcrops are studied. Total organic carbon (TOC) values in the collected samples vary between 1.42% and 5.56%, indicating great variation in the production and preservation of organic matter. The sulfur content, ranging from 0.05 to 2.90%, indicates deposition in a marine environment. Rock-Eval pyrolysis parameters also reflect variations in the depositional conditions of this formation. The potential for hydrocarbon generation varies from low to excellent, noting that the samples that have the highest values of generation potential (S2) have higher TOC levels. This characteristic is justified by the good quality of the organic matter, since such samples have high hydrogen index (HI) values and low oxygen index (OI) values, indicating type I/II kerogen. The low free hydrocarbon (S1) values present in the samples (<0.5 mg HC/g rock) indicate immaturity, as shown by the Tmax (<440 °C) and by the production index (PI < 0.1). For total oil hydrocarbon chromatographic analysis, the largest contribution of algal organic matter is explicit for terrestrial-aquatic reasons (TAR <1). Furthermore, the pristane and phytane isoprenoid ratios (Pr/Phy) indicate differences between anoxic and oxic environments. Saturated biomarkers allow us to state that the organic matter present in the samples has a marine origin with terrestrial contributions and low thermal maturation. Palynofacies analysis shows that there is a predominance of amorphous organic matter (AOM), with a moderate amount of palynomorphs and subordinate amounts of phytoclasts. In the ternary diagram, these percentages allow us to infer that the depositional paleoenvironment was a distal platform with oxic-anoxic variation.

Geochemical characterization and origin of kerogens from source-rock of Devonian in the Amazonas Basin, Brazil

Thirty-seven devonian shale samples from the Barreirinha Formation were collected from the northern and southern borders of the Amazonas Basin and were geochemically analyzed to evaluate their hydrocarbon source potentials and depositional paleoenvironments and to estimate the thermal effects of igneous intrusions on kerogens. The results showed that samples from the northern border exhibited total organic carbon (TOC) and Rock Eval pyrolysis parameters that indicated excellent hydrocarbon source potential and type III kerogen. These TOC values were residual due to the intense hydrocarbon generation from heat from intrusive rocks present in that area. Samples from the southern border had poor to excellent hydrocarbon source potential and type II and III kerogens. Chromatograms of organic extracts and saturated biomarkers suggested marine anoxic to oxic and suboxic to oxic depositional paleoenvironments in the northern and southern borders, respectively. The saturated biomarkers and stable carbon isotope ratios (δ¹³C) suggested that the original organic matter was marine with some continental contributions. The maximum pyrolysis temperature (Tmax), Spore Color Index (SCI), kerogen color, Pr/n-C17 and Ph/n-C18 ratios, and biomarker data indicated that the northern border shales are at the end of the oil generation window, while those from the southern border are at the beginning of the oil generation window.

NMR relaxometry interpretation of source rock liquid saturation — A holistic approach

Fluid types and saturation are essential factors in assessing oil recovery resources and potential in organic-rich mudstones. One of the methods for analyzing tight rocks' saturation is low-field nuclear magnetic relaxometry (NMR). Interpretation of NMR results, particularly the data of T1-T2 maps, delivers different fluid saturation and estimates hydrogen populations. Although previous research proposed a qualitative interpretation of T1-T2 distributions, there is still no obvious scheme that can be directly applied in routine petrophysical studies. However, current NMR-based workflows and manual interpretation routines are subjective and laborious. We attempt to fill this gap by identifying the best interpretation scheme for three distinct types of organic-rich source rock of the Bazhenov formation and modifying a well-known T1-T2 map interpretation scheme. Experimental results include NMR 2-MHz T1-T2 datasets (original and differential maps for samples in as-received, dried, and saturated states) with the corresponding Rock-Eval pyrolysis parameters and mineral composition from X-ray diffraction. The analytical part compares the existing schemes and modifies the interpretation scheme based on the rock petrophysical background. The paper reviews the features of the manual interpretation and previously established schemes and proposes a modified scheme best fitted to the petrophysical and geochemical data. In addition, we study the impact of mineral composition and geochemical properties on T1-T2 signal interpretation and analyze cross-correlations. The study confirms the presence of a significant amount of clay-bound water from a detailed analysis of the corresponding regions in T1-T2 regular and differential maps. In the paradigm, we propose a semi-automated processing of NMR T1-T2 data using a custom interpretation scheme composed of regions from multiple interpretation schemes and consider their modification for high-quality results.

Depositional paleoenvironment and source rock characterization across the Toarcian Oceanic Anoxic Event from the eastern Tethys, Tibet, SW China

An integrated approach based on organic petrographic and geochemical proxies along with palynological and palynofacies analysis were employed using 24 rock samples representing the upper part of the Quse and Sewa formations in the southern Qiangtang Basin, Tibetan Plateau, SW China. This interval was deposited during the Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE) and is investigated to assess the biostratigraphic age control based on palynomorphs composition. The current approach allows for the interpretation of the local paleoenvironmental response to T-OAE climate variability that still unraveled based on palynofacies analysis of the Bilong co succession. Visual palynofacies analysis, vitrinite reflectance (VRo), and total organic carbon (TOC)/Rock-Eval pyrolysis parameters enables a detailed assessment of probable source rock characterization.The palynomorph content comprises mainly terrestrially-derived land-plants, mostly sphaeromorphs pollen grains, with minor content of marine dinoflagellate cysts (dinocysts). Marker dinocysts and pollen taxa allow the subdivision of the studied succession into two age-distinctive units of early Toarcian and late Bajocian-Bathonian. Additionally, carbon isotope profile of the studied succession observed a large negative carbon isotope excursion (CIE) within the lower Toarcian oil shale interval, which is compatible with regional δ13Corg records from adjacent basins at this time. The regional TOC contents of lower Toarcian reached up to 21 wt% in areas of the eastern Tethys compared to lower values in the western Tethys and Boreal (ca. 3 wt%). Two palynofacies assemblages revealed deposition of these deposits in distal inner neritic to fluvio-deltaic environment and to some extent in brackish marginal marine setting. Geochemical screening indicated that most samples were dominated by very good to excellent organic matter richness of kerogen Type II and mixed Type II/III with very good to excellent hydrocarbon generation. The calculated VRo-eq from Rock-Eval Tmax and measured VRo values are in good agreement and showed that the organic matter of the studied interval is in the early to middle stages of the oil window.

Examining the relationship between Tmax and vitrinite reflectance: An empirical comparison between thermal maturity indicators

For over 40 years Rock-eval pyrolysis data have been used to assess thermal maturity and source rock potential. There have been many published equations representing the transformation of Tmax to vitrinite reflectance (Ro) equivalence, but there has not been an attempt to investigate the empirical relationship using a large dataset. In this study, 33,732 approximately coincident measurements from 903 publications were utilized to test the applicability and practicality of Tmax, hydrogen index (HI), and production index (PI) as thermal maturity indicators. Additional sample metadata (sample type, lithology, organofacies, sample age, and other Rock-eval pyrolysis parameters) were also evaluated to ascertain if there were parameters that more frequently led to higher confidence measurements.There was a positive correlation between Tmax and Ro, but there is considerable scatter in the data. A stronger trend was revealed in the mean, median, and mode values for each Tmax versus Ro pair that can be represented by the equation Ro = (0.013 x Tmax) - 5.0 with a R2 value of 0.22, which may be an improvement upon previously published equations. There was a breakdown in the correlation between Tmax and Ro at the beginning of the dry gas window (Ro values ~1.5; Tmax ~500 °C), which was not unexpected since the S2 peak significantly decreases through the wet gas window. Tmax-derived thermal maturity estimates are therefore most reliable between the values of ~430 °C and ~500 °C. It was also observed that coals with high total organic content (TOC) and S2 values had the least amount of data scatter among the different source rock organofacies. This may be due to the higher initial S2 values and these samples having a more prominent S2 peak allowing them to yield more reliable Tmax determinations.

Evaluation of the hydrocarbon generation potential of the Pimenteiras Formation, Parnaiba Basin (Brazil) based on total organic carbon content and Rock-Eval pyrolysis data

The Parnaiba Basin is a Paleozoic to Mesozoic intraplate volcano-sedimentary basin where the Pimenteiras Formation (Devonian) is the main sequence with potential of hydrocarbon generation, mostly natural gas. The present paper evaluates the potential of hydrocarbon generation of Pimenteiras Formation based on Total Organic Carbon (TOC) and Rock-Eval pyrolysis parameters. In this work, 1077 shale samples of the Pimenteiras Formation distributed in 32 wells were evaluated. The TOC content varies between 0.1 to 4.7 wt.%, partially reflecting the accumulation and preservation rates of the organic matter in marine and coastal depositional environments controlled by regressive-transgressive cycles. The oxic and anoxic conditions vary significantly with deposition in this situation, which were evidenced by HI and OI variations through sample profiles. In the north and center of the basin, the Pimenteiras Formation has a higher potential for hydrocarbon generation relative to the south, probably due to higher anoxic conditions during deposition. The Hydrogen Index indicates the predominance of kerogen types II and III with minor occurrences of types I and IV. The Tmax values indicate general immature conditions and locally postmature, where the lowest temperatures represent the basin´s burial history, whereas the higher ones were influenced by igneous intrusions and thermogenic anomalies related to the Transbrasiliano Lineament. In addition, the excessive heat around the intrusions altered the Rock-Eval pyrolysis parameters as well as the type of organic matter, resulting in a relative increase of the kerogen types III and IV, which explains the great potential for gas generation in this basin.

Prediction of vitrinite reflectance values using machine learning techniques: a new approach

Vitrinite reflectance (VR) is considered the most used maturity indicator of source rocks. Although vitrinite reflectance is an acceptable parameter for maturity and is widely used, it is sometimes difficult to measure. Furthermore, Rock-Eval pyrolysis is a current technique for geochemical investigations and evaluating source rock by their quality and quantity of organic matter, which provide low cost, quick, and valid information. Predicting vitrinite reflectance by using a quick and straightforward method like Rock-Eval pyrolysis results in determining accurate and reliable values of VR with consuming low cost and time. Previous studies used empirical equations for vitrinite reflectance prediction by the Tmax data, which was accompanied by poor results. Therefore, finding a way for precise vitrinite reflectance prediction by Rock-Eval data seems useful. For this aim, vitrinite reflectance values are predicted by 15 distinct machine learning models of the decision tree, random forest, support vector machine, group method of data handling, radial basis function, multilayer perceptron, adaptive neuro-fuzzy inference system, and multilayer perceptron and adaptive neuro-fuzzy inference system, which are coupled with evolutionary optimization methods such as grasshopper optimization algorithm, bat algorithm, particle swarm optimization, and genetic algorithm, with four inputs of Rock-Eval pyrolysis parameters of Tmax, S1/TOC, HI, and depth for the first time. Statistical evaluations indicate that the decision tree is the most precise model for VR prediction, which can estimate vitrinite reflectance precisely. The comparison between the decision tree and previous proposed empirical equations indicates that the machine learning method performs much more accurately.

Transformation of Terrestrial Organic Matter during Mesocatagenesis and Apocatagenesis

Abstract —A combination of modern analytical methods was used to study terrestrial organic matter from upper Paleozoic rocks in the central part of the Vulyui hemisyneclise (East Siberia) sampled from the depth interval 3370–6458 m penetrated in the Srednevilyuiskaya-27 superdeep well. In this study, we used 71 core samples to trace depth variations in the geochemical characteristics of organicmatter during middle–late mesocatagenesis and apocatagenesis: organic carbon content (Corg), Rock-Eval pyrolysis parameters (HI, Тmax), carbon isotope composition, thermal maturity (RVto, %), hydrocarbon type composition (hydrocarbons, resins, and asphaltenes), distributions of n-alkanes (TIC), steranes (m/z 217), and terpanes (m/z 191), and standard biomarker indices with increasing degree of thermal maturity. It is shown that during late mesocatagenesis and apocatagenesis, the pyrolysis parameters and biomarker indicators are no longer effective as indicators of organic-matter maturity and genesis.

A study on fractal characteristics of lacustrine shales of Qingshankou Formation in the Songliao Basin, northeast China using nitrogen adsorption and mercury injection methods

In order to research the fractal characteristics of lacustrine shale, we conducted low-pressure N2 adsorption and mercury injection measurements on 35 shale samples of Cretaceous Qingshankou Formation in the Songliao Basin, northeast China. The fractal dimensions (D1, D2, and D3) were obtained from N2 adsorption (P/Po < 0.5 and P/Po > 0.5) and mercury injection data, respectively. Accordingly, we analyzed the relationships between fractal dimensions and petrophysical and geochemical parameters. The results show that the fractal dimensions (D1, D2, and D3) are in the ranges of 2.0174–2.5705, 2.5589–2.8425, and 2.1461–2.9728, respectively. There exists a positive relationship between D1 and D2, suggesting that micropores and mesopores have similar surface characteristics. The poor correlations between D3 and D1 or D2 may be related to the different principles of low-pressure N2 adsorption and mercury injection. The fractal dimensions (D1 and D2) correlate positively with pore volumes and surface areas, which indicates that the fractal dimensions can well reflect the pore structure and surface characteristics. The fractal dimensions (D1 and D2) decreased with the increasing TOC content, which may be aroused by the low abundance of pores in organic matters. The fractal dimensions (D1 and D2) decreased firstly and then increased with the increasing maturity, which is aroused by liquid hydrocarbons generation and cracking. And the fractal dimension D3 decreased during maturation, which may be induced by compaction. The fractal dimensions (D1 and D2) decreased with the increasing Rock-Eval pyrolysis parameters S1 and S1/TOC, which suggests that the fractal dimensions can be indicators of oil content of shales.

Methane adsorption characteristics of shale from different basins and its geological significance

The CH4 adsorption characteristics of shale from different basins was studied based on determination of mineral compositions, Rock-eval pyrolysis parameters and CH4 gas adsorption isotherms. The results show that TOC contents, contents of quartz and maturity stage of shale can influence the CH4 adsorption capacity. Whereas, the results show that there exists unobvious relationships between CH4 adsorption capacity and the contents of clay and carbonate materials can be obtained in this study.

Origin and genetic family of Huhehu oil in the Hailar Basin, northeast China

The Huhehu Sag is one of the most important oil and gas depressions in the Hailar Basin. However, the origin of Huhehu oil is still controversial. Previous studies on source rocks have mainly focused on the Nantun Formation (K1n); a few studies have investigated the Damoguaihe Formation (K1d). Based on the Rock–Eval pyrolysis parameters, 172 drill cutting samples from the Huhehu Sag were analyzed to evaluate their geochemical characteristics. Based on the Rock–Eval data, the mudstones from the first member of the Damoguaihe Formation (K1d1) and the second member of the Nantun Formation (K1n2) have moderate to high hydrocarbon generation potential, while mudstones from the first member of the Nantun Formation (K1n1) have poor to good hydrocarbon generation potential. Additionally, both the K1n1 and K1n2 coal members have poor to fair hydrocarbon generation potential, but the K1n2 coal member has a better generative potential. Fifteen Huhehu oils were collected for molecular geochemical analyses to classify the oils into genetic families and to identify the source rock for each oil using chemometric methods. The Huhehu oils were classified into three groups with different maturity levels using hierarchical cluster analysis and principal component analysis. Group A oils (high maturity) are characterized by relatively moderate ratios of Pr/Ph, Pr/n-C17, and Ph/n-C18, as well as an abundance of C29 steranes, mainly derived from the K1n2 and K1n1 mudstone members. In comparison, group B oils (moderate maturity) have relatively low Pr/Ph ratios, moderate Pr/n-C17 and Ph/n-C18 ratios, and low concentrations of C29 steranes. Group C oils (low maturity) show relatively high ratios of Pr/Ph, Pr/n-C17, and Ph/n-C18, as well as high concentrations of C29 steranes. Furthermore, group B oils derived from the K1d1 mudstone member and group C oils derived from the K1n2 coal member were also identified by principal component analysis score plots. Correlation studies suggest a major contribution from the K1n mudstone Formation and the K1d1 mudstone member to the oils of the Huhehu Sag. So, the Nantun Formation and relatively shallow strata of the Damoguaihe Formation (e.g., the K1d1 member) represent important targets for future oil-reservoir exploration in the Huhehu Sag.

Organic geochemistry of the Eagle Ford Group in Texas

A comprehensive regional organic geochemical study was performed on outcrop and core samples from the Eagle Ford Group with the aim of determining variations in organic matter source, thermal maturity, and depositional environments. Total organic carbon (TOC) and Rock-Eval pyrolysis parameters show that the Eagle Ford Group has excellent hydrocarbon source rock potential and is dominated by type II kerogen. Distributions of regular steranes, hopanes, and monoaromatic steroids confirm a marine carbonate depositional environment. Aryl isoprenoid ratios suggest the occurrence of intermittent photic zone anoxia. The n-alkane and sterane distributions and the presence of pregnanes, homopregnanes, and gammacerane suggest deposition under hypersaline conditions particularly in west and east Texas. In east Texas, biomarkers indicate that the Eagle Ford Group was partially sourced from terrigenous organic matter and deposited under oxic–suboxic conditions. Thermal maturity parameters including the temperature at which the peak corresponding to the compounds produced from the thermal degradation of kerogen in the Rock-Eval chromatogram maximizes (Tmax), vitrinite reflectance, and biomarker ratios indicate that in west and central Texas, the Eagle Ford Group is immature to marginally mature; in southwest Texas it is in the oil to dry gas window; and in east Texas, the Eagle Ford is in the main stage of oil generation. Geochemical logs of biomarker ratios show that the Lower Eagle Ford Formation has the highest TOC and hydrogen index, particularly in the Lozier Canyon Member; experienced stronger anoxic conditions during deposition; and has lower clay content compared with the Upper Eagle Ford Formation. Geochemical parameters show that the latter may have received an additional siliciclastic and terrigenous organic matter input under oxic–dysoxic conditions. Isotope data indicate a marine organic matter source for the Eagle Ford Group, but surprisingly, δ13C values do not show significant organic facies, depositional environment, or thermal maturity variations.

Clay mineral diagenesis and source rock assessment in the Bornu Basin, Nigeria: Implications for thermal maturity and source rock potential

An understanding of the mineral and organic matter modifications during burial are critical in assessments of potential source rocks in petroleum exploration studies. Here, we examined the relationships between clay mineral diagenesis and organic thermal indicators (vitrinite reflectance and Rock-Eval Tmax) to constrain the thermal maturity and hydrocarbon potential of source rocks in the Mbeji-1 well in the northeastern part of the Cretaceous/Tertiary Bornu Basin. X-ray diffraction mineralogical data show that the <2 μm clay-sized fraction consist of mixed layer illite/smectite (I/S), kaolinite, illite, and Fe-chlorite clay mineral assemblages. The major diagenetic changes in the clay mineral evolution are transformation of random (R0) to ordered (R1) illite/smectite (I/S) mixed layer between ∼2500 m and ∼3640 m depths, decrease in smectite in I/S with burial, and appearance of chlorite at ∼2500 m. Total organic carbon and Rock-Eval pyrolysis parameters (HI, S2, Tmax) indicate presence of lean shales consisting of gas prone to inert terrestrial organic matters. Source rocks at shallower depths below 2500 m are immature while those below this level within lower Fika Formation to upper Gongila formations are marginally matured with respect to hydrocarbon generation. The variation between I/S transformation and VR for the Mbeji-1 well shows a delay in illitization process compare to organic matter maturity. We considered that other than temperature, insufficient and variable potassium availability, and short-heating time resulted in retarding the smectite illitization reaction relative to the organic matter parameters (vitrinite reflectance and Tmax). These differences further demonstrates that organic matter responds quickly to increasing temperature than mineral reactions at short heating time in young sedimentary basins.

Determination of organic degradation rates in 100 My old sediments: Application to Cretaceous black shale intervals from Demerara Rise, ODP Leg 207

Thick organic-rich, Albian to Santonian, black shale sequences were recovered from Ocean Drilling Program Sites 1261 and 1257 on the Demerara Rise, offshore Surinam, at depths of 564 and 174m below seafloor (bsf) respectively. Total organic carbon (TOC) concentration varies between 3.9 and 17.3wt%. Rock-Eval pyrolysis parameters and elemental analysis indicate that the kerogen samples are of Type II, thermally immature and with well preserved organic matter. The hydrogen index (HI) values fluctuate between 608 and 765mghydrocarbon(HC)/gTOC and correlate well with C/N and the Sorg/C atomic ratios, indicating varying early diagenesis conditions between samples. Higher C/N and Sorg/C values for kerogen from Site 1261 suggest more reducing depositional conditions than at Site 1257. Long term organic diagenesis is marked by a linear decrease in the oxygen index (OI) at Site 1261, from 34 to 22mgCO2/gTOC. Ester and carboxyl functional groups are the main target of the degradation as inferred from the decrease in CO and OCC Fourier transform infrared spectroscopy (FTIR) bands as well as in the CHOH and the OCH3/CHxN bands in 13C nuclear magnetic resonance (NMR) spectra. Degradation rate at Site 1261, determined combining quantitative NMR spectra and bulk elemental characterization, is between 1.7 and 12.1µmol/m3/yr per 1% TOC, at the lower end of reported rates in a similar context. Kerogen samples from Site 1257 do not show any decrease in oxygen content with depth, indicating significantly lower degradation rate. We propose that the long term exposure to moderate temperature (30°C) at Site 1261 activated the organic matter, thereby sustaining microbial diagenesis.