Vitrinite Reflectance Measurements Research Articles

Organic geochemical characteristics of Jurassic petroleum system in Abadan Plain and north Dezful zones of the Zagros basin, southwest Iran

Jurassic succession has resulted in prolific oil and gas reservoirs in southwest Iran. In order to geochemically investigate the Jurassic petroleum system and assess the source rock of these hydrocarbons, 32 source rock samples (from well cutting and surface section) and 4 condensate samples were analysed using geochemical approaches. Rock-Eval pyrolysis and vitrinite reflectance measurement were done on rock samples. Then, condensate samples and source rock extracts were subjected to gas chromatography, gas chromatography–mass spectrometry and isotopic analyses in order to establish oil–oil and oil–source rock correlation. The hydrocarbon generation prediction was then confirmed by petroleum system modelling. The results indicated that the middle Jurassic Sargelu Formation with average total organic carbon (TOC) and hydrogen index values of 3.8% and 600 mg HC/g TOC, respectively, can be classified as good to excellent source rock with kerogen of type II. The maturity indicators revealed late oil to wet gas generation window for the Jurassic Sargelu Formation. Biomarker parameters showed marine carbonate source rock with advanced level of thermal maturity for the source rock of the condensates. Furthermore, there were good correlations between Jurassic Sargelu Formation and the studied oil and condensate samples which proved the Sargelu Formation as the main source rock of the Jurassic petroleum system in the study area. The basin modelling results also confirmed late oil to wet gas window maturity for the Sargelu Formation enabling it as a source rock for generating gas condensate accumulation in southwest Iran. The hydrocarbon generation from Sargelu Formation began in the late Cretaceous and its peak expulsion occurred in the Miocene time.

Tertiary hydrothermal activity and its effect on reservoir properties in the Xihu Depression, East China Sea

Three large-scale episodes of volcanic activity occurred during the Tertiary in the Xihu Depression, located in the East China Sea. Intermediate-felsic magmas intruded along faults and the associated hydrothermal fluids resulted in the hydrothermal alteration of the clastic country rock. To better describe reservoir characteristics, reservoir samples were subjected to the following investigations: thin section examination, scanning electron microscope-energy dispersive spectrometer analysis (SEM–EDS), fluid inclusion homogenization temperature tests, vitrinite reflectance measurements, and X-ray diffraction. The results of this study provide evidence of the following hydrothermal alteration phenomena: brittle fracturing, clastic particle alteration, precipitation of unique hydrothermal minerals (celestite, zircon, apatite, barite, and cerous phosphate). The presence of abnormally high temperatures is indicated by fluid inclusion analysis, the precipitation of high-temperature authigenic minerals such as quartz, illite alteration, and anomalous vitrinite reflectance. Two aspects related to hydrothermal effects on reservoir properties have been investigated in this study: (1) Deep magmatic hydrothermal fluids carry large amounts of dissolved carbon dioxide and sulfur dioxide gas. These fluids percolate into the country rocks along fault zones, resulting in dissolution within the sandstone reservoirs and the development of significant secondary porosity. (2) Magma intrusions increase the temperature of the surrounding rocks and accelerate the thermal evolution of hydrocarbon source rocks. This results in the release of large amounts of organic acids and carbon dioxide, leading the dissolution of the aluminosilicate minerals and volcanic fragments in the reservoirs, and the generation of significant secondary porosity.

Enhancement of organic matter maturation because of radiogenic heat from uranium: A case study from the Ordos Basin in China

Carbonaceous debris (CD) within uranium-bearing strata has been studied in the Daying uranium deposit of the northern Ordos Basin, northern China. The influence of radiogenic heat from uranium on organic matter maturation was investigated through a series of tests including measurements of vitrinite reflectance (Ro), fission-track (FT) analysis in quartz grains, and the calculation of the radiogenic heat production rate of the samples. The results show that Ro in uranium-bearing strata generally increases as the burial depth increases, indicating that CD experienced normal burial coalification. However, Ro values of the samples rich in uranium are 0.062% Ro higher than those without uranium mineralization. Vitrinite reflectance bears a positive relationship with uranium content, and an inverse relationship with distance to the closest sandstone rich in uranium, indicating that uranium enrichment enhances organic matter maturation. The production of uranium decay makes FT observable in quartz grains, and the intensity of decay increases with proximity to the uranium ore body. The calculated radioactive heat production rate from the uranium ore body is 6.857 × 10−5 W/m3. During the long-term stable decay, as the uranium ore body theoretically results in an abnormal increase in temperature of 52°C without consideration of the loss of heat conduction, heat convection, and thermal radiation, this would yield a theoretical Ro increase of 0.209% Ro, reasonably greater than the observed. Therefore, the long-term stable radiogenic heat produced by uranium ore body can slightly enhance organic matter maturation, which is instructive in uranium prospecting.

Geochemical characteristics of Carboniferous coaly source rocks and natural gases in the Southeastern Junggar Basin, NW China: Implications for new hydrocarbon explorations

The Upper Carboniferous coaly source rocks are verified to be developed in the Southeastern Junggar Basin, which is lithologically composed of carbonaceous mudstones and mudstones. According to the geochemical (total organic carbon, Rock–Eval, and biomarker fingerprints) and organic petrology analyses, the Carboniferous source rocks are primarily gas-prone characterized by kerogen type III, and were deposited in an oxidized–sub-oxidized brackish environment significantly contributed by terrigenous higher plants. Carbonaceous mudstones are generally more oil-prone in comparison with the mudstones. The measured vitrinite reflectance, Tmax, and sterane isomers indicate that the source rocks are at immature–early gas generation stage, and have not entered the “peak gas generation stage”. Natural gases are coal-type gases derived from the Carboniferous source rock. The maturities of them are highly inconsistent with those of source rock samples. Based on 2-D basin modeling, natural gases were generated from the highly mature coaly source rocks in the Fukang Depression and migrated upward to the uplift via the P–C regional inconformity.

New insights on measured and calculated vitrinite reflectance

AbstractMeasurement of dispersed vitrinite reflectance in organic sediments is one of the few regional data sets used for placing bounds on the thermal history of a sedimentary basin. Reflectance data are important when access to complementary information such as high‐quality seismic data is unavailable to place bounds on subsidence history and in locations where uplift is an important part of the basin history. Attributes which make vitrinite reflectance measurements a useful data set are the relative ease of making the measurement, and the availability of archived well cores and cuttings in state, provincial, and federal facilities. In order to fully utilize vitrinite data for estimating the temperature history in a basin, physically based methods are required to calibrate an equivalent reflectance from a modelled temperature history with measured data. The most common method for calculating a numerical vitrinite reflectance from temperature history is the EASY%Ro method which we show systematically underestimates measured data. We present a new calculated reflectance model and an adjustment to EASY%Ro which makes the correlation between measured vitrinite values and calculated vitrinite values a physical relationship and more useful for constraining thermal models. We then show that calibrating the thermal history to vitrinite on a constant age date surface (e.g., top Cretaceous) instead of calibrating the thermal history in depth removes the heating rate component from the reflectance calculation and makes thermal history calibration easier to understand and more directly related to heat flow. Finally, we use bounds on the vitrinite–temperature relationships on a constant age date surface to show that significant uncertainty exists in the vitrinite data reported in most data sets.

Organic geochemistry of the Upper Triassic T3x5 source rocks and the hydrocarbon generation and expulsion characteristics in Sichuan Basin, central China

The Upper Triassic Xujiahe Formation is a key target layer for development of tight sandstone gas reservoirs in the Sichuan Basin. Most of the previous work focused on the main reservoirs such as T3x2 and T3x4 with lack of research attention on T3x5, an equally important source-reservoir unit. Here we examine the organic matter and source rock distribution in the T3x5 using rock pyrolysis, asphalt extraction, and gas chromatography. The thickness of T3x5 source rocks reduces from west and northwest Sichuan Basin to east and southeast Sichuan Basin. The main sedimentary facies are salt lakes and deltas, of which the organic matter originated from aquatic organisms and a small amount of terrestrial matter. The abundance of total organic carbon (TOC) in T3x5 is relatively high, at values of 0.07%–7.2%. According to the vitrinite reflectance (Ro), pyrolysis, and gas chromatography measurement results, the T3x5 source rocks are at the stage of mature to over-mature, and their hydrocarbon expulsion threshold and hydrocarbon expulsion peak correspond to Ro of 1.05% and 1.23%, respectively. Furthermore, a hydrocarbon generation center and a hydrocarbon expulsion center are formed horizontally, with the maximum hydrocarbon generation intensity of 50 × 108 m3/km2 and the maximum hydrocarbon expulsion intensity of 48 × 108 m3/km2. The natural gas resource is 3.19–5.36 × 1012 m3, indicating great exploration potential.

Data for: Organic geochemistry of the Upper Triassic T3x5 source rocks and the hydrocarbon generation and expulsion characteristics in Sichuan Basin, Central China

Abstract The Upper Triassic Xujiahe Formation is a key target layer for development of tight sandstone gas reservoirs in the Sichuan Basin. Most of the previous work focused on the main reservoirs such as T3x2 and T3x4 with lack of research attention on T3x5, an equally important source-reservoir unit. Here we examine the organic matter and source rock distribution in the T3x5 using rock pyrolysis, asphalt extraction, and gas chromatography. The thickness of T3x5 source rocks reduces from west and northwest Sichuan Basin to east and southeast Sichuan Basin. The main sedimentary facies are salt lakes and deltas, of which the organic matter originated from aquatic organisms and a small amount of terrestrial matter. The abundance of total organic carbon (TOC) in T3x5 is relatively high, at values of 0.07%–7.2%. According to the vitrinite reflectance (Ro), pyrolysis, and gas chromatography measurement results, the T3x5 source rocks are at the stage of mature to over-mature, and their hydrocarbon expulsion threshold and hydrocarbon expulsion peak correspond to Ro of 1.05% and 1.23%, respectively. Furthermore, a hydrocarbon generation center and a hydrocarbon expulsion center are formed horizontally, with the maximum hydrocarbon generation intensity of 50 × 108 m3/km2 and the maximum hydrocarbon expulsion intensity of 48 × 108 m3/km2. The natural gas resource is 3.19–5.36 × 1012 m3, indicating great exploration potential.

Unconventional gas resources in theCantabrian Zone(NW Spain): A comprehensive preliminary assessment

With the aim of acquiring a better knowledge of the potential for unconventional gas resources in Spain, 134 rock samples were taken from 12 lithostratigraphic units in the Cantabrian Zone (north‐west Spain) and studied for a 3‐year period. The selected units gather, a priori, the basic conditions to constitute potential source rocks for unconventional hydrocarbons. Petrographic studies and total organic carbon analyses were carried out for all samples, whereas Rock‐Eval pyrolysis and vitrinite reflectance measurements with kerogen visual analysis were performed on selected samples. Lutites, litharenites, limestones, and slates are the predominant petrographic types. In most cases, the mean total organic carbon content is below the 2% limit for good quality potential source rocks. Only the Jurassic Rodiles and Tereñes limestones show total organic carbon values above this threshold. Rock‐Eval data reveal very low potential for hydrocarbon generation in almost all cases, with the exception of the dark mudstones of the Carboniferous Barcaliente Formation and the Rodiles Formation (fair and good potential for hydrocarbon generation, respectively). As regards thermal maturation, only the Ordovician formations are located in the peak of the gas generation interval. The Barcaliente and Rodiles formations are near the frontier oil/wet gas maturation areas, and the rest of the units are immature for gas generation.

Mineralogy and facies variations of Devonian and Carboniferous shales in the Ukrainian Dniepr-Donets Basin

Abstract The mineralogy of Devonian to Carboniferous shales from the Ukrainian Dniepr-Donets Basin (DDB) was investigated during this study. These shales show a high compositional variability in vertical and lateral directions. Furthermore, stratigraphic trends were found to be controlled both by climatic factors as well as by changing detrital input from the hinterland. High kaolinite contents and predominance of kaolinite over illite in the Tournaisian and partly in the lower Visean units are likely a result of intense chemical weathering related to the Hangenberg climatic event at the Devonian/ Tournaisian boundary. In contrast, abnormally high kaolinite contents in upper Visean and Serpukhovian samples at the basin center might be caused by different transport properties of kaolinite and illite, leading to selective concentration of small detrital kaolinite particles, which are often in the sub-micrometer range according to scanning electron microscopy observations. K/Al elemental ratios correlate well with illite/kaolinite ratios for samples in which significant amounts of both clay minerals are present, which enables a pre-evaluation of the relative kaolinite content based on bulk geochemical data. As kaolinite is suggested to decrease the fraccability of shales and to have a great influence on their wetting behaviour, this is useful information for explorational purposes. Higher feldspar contents in Devonian and Tournaisian samples, especially along the NE basin margin and in the shallow NW part of the DDB, are likely related to increased detrital input from magmatic precursors (e.g. in the Voronezh Massif ) during (and shortly after) the active rift stage of the DDB. In general, feldspar contents are higher in proximal positions compared to the basin center, which is likely a result of shorter transport distances of the comparably large feldspar grains. Finally, the presence of expandable clay minerals down to depths of 6 km and the fact that no thermal maturity trend is visible down to these depths, proves, that a low post-depositional heat flow was present in the DDB. This is in good agreement with vitrinite reflectance measurements and thermal modelling results from previous studies, which suggest a low Mesozoic heat flow.

Depositional environment and petroleum potential of Oligocene rocks in the Waschberg Zone (Austria)

Abstract Oligocene successions in the North Alpine Foreland Basin (NAFB) and the Western Carpathians reflect Paratethys-wide paleogeographic changes, which also control their petroleum potential. Whereas these rocks have been studied in detail in both areas, the transition zone is still under-researched. In order to fill this gap, the Oligocene succession in the Waschberg Zone, comprising the Ottenthal Formation (NP21–23) and the overlying Thomasl Formation (NP23–24) has been studied using outcrop (Waldweg section) and borehole samples (Thomasl, Poysdorf) and a multidisciplinary approach. The Ottenthal Formation is subdivided from base to top into marls and shales (Ottenthal Mbr.), diatomaceous shales (Galgenberg Mbr.) and marlstones (Dynow Mbr.). Biogenic silica contents, determined using atomic absorption spectroscopy, reach 30 wt. % in the carbonate-free Galgenberg Member, but also in the Dynow Members, which is characterized by upward decreasing productivity of calcareous nannoplankton. Close lithological relations exist with the Oligocene succession in the NAFB, but diatoms are largely missing in the latter. Organic matter contents are surprisingly low in the Ottenthal and Thomasl formations in the Waldweg section, which therefore are poor hydrocarbon source rocks. In contrast, the Thomasl Formation, encountered in the Thomasl and Poysdorf boreholes, holds a fair to good hydrocarbon potential (~ 2.2–2.5 wt. % TOC; type III and type II kerogen) and may generate 1.0 to 1.6 tons of hydrocarbons/m2. Obviously TOC contents of borehole samples are significantly higher than in outcrop samples. Because of severe indications of weathering (e.g., presence of gypsum and jarosite), a detrimental effect of weathering on the samples from the Waldweg section cannot be excluded. Biomarker data suggest a nearshore depositional environment with changing oxygen-availability and salinity. Vitrinite reflectance measurements show that the investigated sections are thermally immature.

Hydrocarbon potentiality and thermal maturity of the Cretaceous rocks in Al Baraka oil field, KomOmbo basin, south Egypt

This paper aims to evaluate the hydrocarbon potentiality and thermal maturity of the Cretaceous source rocks in Al Baraka oil field in KomOmbo basin, south Egypt. To achieve this aim, geochemical analyses (TOC), Rock eval pyrolysis and vitrinite reflectance measurements (R0) were carried out on the studied rocks. The analytical results of the samples that were collected from five exploratory oil wells revealed that almost Lower Cretaceous formations (Sabaya, Abu Ballas, Six Hills and KomOmbo C, B, A) and Upper Cretaceous formations (Dakhla, Duwi, Quseir, Taref and Maghrabi) are ranged from fair to excellent source rocks for hydrocarbon generation. Oil and gas are mainly the future products of the thermally transformed organic matters within almost samples of the Cretaceous formations, where the Lower and Upper Cretaceous formations contain mixed type II/III and III kerogen besides type II kerogen in KomOmbo (B) and Dakhla formations. The thermal maturity parameters clarified that the Lower Cretaceous formations are belonged to marginally mature (in Sabaya and Abu Ballas formations), whereas the rocks of KomOmbo (B) Formation are mature source rocks and fall in the stage of oil generation and reach to the late stage of oil generation (R0 = 1.25). On the contrary the Upper Cretaceous formations are ranged from immature to marginally mature source rocks and reach only the early stage of oil generation in Maghrabi Formation. This study indicated that there is still a good chance to find oil generated from the Dakhla, Duwi, Maghrabi, Sabaya and Abu Ballas formations if buried in greater depths as well as, KomOmbo B and A intervals which are source rock potentials.

Enrichment of Bi-Be-Mo-Cd-Pb-Nb-Ga, REEs and Y in the Permian coals of the Huainan Coalfield, Anhui, China: Discussion

The paper “Enrichment of Bi-Be-Mo-Cd-Pb-Nb-Ga, REEs and Y in the Permian coals of the Huainan Coalfield, Anhui, China” by Munir et al. (2018) provides new data on the geochemical compositions of the late Paleozoic coals from five different mines in the Huainan Coalfield, China, as well as proximate analyses, vitrinite reflectance measurements, maceral contents, and mineralogical constituents. A critical examination, however, suggests that a large proportion of the reported data are questionable, and also that references cited at a number of points are either improper or incomplete.

2D BASIN MODELLING IN THE EASTERN VARISCAN FOLD BELT (CZECH REPUBLIC): INFLUENCE OF THRUSTING ON PATTERNS OF THERMAL MATURATION

This study evaluates the geothermal history of Palaeozoic sedimentary rocks from the easternmost part of the Variscan external zone in the NE Czech Republic. The objective was to investigate the geothermal history of pre‐Variscan Palaeozoic carbonates in the study area, and to assess its relationship with that of the overlying Variscan flysch. In the study area, the Palaeozoic succession occurs at the surface or is overlain by Miocene sediments of the Carpathian Foredeep. Palaeozoic nappes and the main Variscan overthrust have been documented in the subsurface at the deep Potštát‐1borehole.Vitrinite reflectance measurements on 38 samples from the Potštát‐1 well and 19 samples from nearby surface outcrops and shallow boreholes were available. A 2D thermal model was created using PetroMod and the thermal maturity evolution was modelled by EASY%Ro. The thermal model was constructed based on interpretations of two NW‐SE seismic profiles (lines 5/83 and 5/84) oriented perpendicular to the main Variscan thrusts. The results were calibrated using measured vitrinite reflectance and were adjusted with 1D models from three shallow boreholes. At the Potštát‐1 borehole, modelled maximum palaeo‐temperatures of the Variscan flysch (Moravice Formation) ranged from 310°C at a depth of 7.3 km (the top of the preserved succession) to 395°C at the base of the succession, resulting in thermal maturities of >4%Rr. Peak maturation occurred prior to the end of Variscan thrusting. Modelling suggests that the basal heat flow for these thrust units reached a maximum value of 63 mW/m2 at 325 Ma. In addition, the modelling suggests that the maturity of the Palaeozoic carbonates was controlled by the thickness of the overlying Variscan flysch nappes. Maximum palaeo‐temperatures for the Palaeozoic carbonates ranged from 265°C at the top of the interval (at a depth of 7.1 km) to 290°C at the base, resulting in a maturity of 3.8 to >4%Rr which is within the dry gas window. The study suggests that basal heat flows in the original (pre‐thrust) Early Carboniferous sedimentary basin were slightly higher than those in the post‐thrust location for the Variscan flysch nappes. This should be taken into account when evaluating the petroleum system in the South Moravian oil province (SW Czech Republic) where a complete sedimentary sequence has not been preserved.

Present-day thermal field and Mesozoic-Cenozoic thermal evolution of the Western Bredasdorp Basin (South Africa): An integrated 3D numerical forward modelling approach

Abstract We analyse a crust-scale 3D structural model of the Western Bredasdorp Basin (WBB) located offshore southern South Africa to assess both the spatial and temporal development of geothermal gradients in the Southern African ‘passive-transform’ continental margin. The results of simulations of the present-day conductive thermal field were used to constrain the heat flow evolution during 3D numerical modelling of the paleo-thermal regime. Besides conforming to the regional geotectonic framework, the ensuing models are largely consistent with measured temperatures. The largest control on present-day temperatures is exerted by the basin's present-day geometry and distribution of thermal properties which, in turn, trace back to the earliest tectonic event involving crustal thinning and significant syn-rift sedimentation. Implying from the modelled shallow and deep geotherms, we find that the variations of present-day surface heat fluxes in the WBB are much more dictated by the long-term crustal radiogenic heat production in concert with the interaction of low thermally conductive sediments and high thermally conductive crustal rocks rather than by the present-day mantle heat flow. In addition, model calibration with measured vitrinite reflectance trends suggests that three major phases of thermal disequilibrium resulting in high heat flow during syn-rift and post-rift times characterise the distribution of maximum paleo-temperatures and paleo-geotherms. In terms of subsidence, the syn-rift tectonic event plays the most significant role in basin evolution. The height and width of the syn-rift heat flow anomalies and their lateral variations reflect a direct consequence of instantaneous crustal stretching leading to rapid burial and lateral differences in crustal thicknesses and amounts of radiogenic heat produced. The post-rift heat flow peaks are interpreted to be related to a Late Cretaceous mantle-related hotspot event and a late Miocene episode of margin uplift. As indicated by validated vitrinite trends and the modelled distribution of eroded sediments, the hotspot event represents a more localised thermal event, while the late Cenozoic erosion and inferred uplift depict a regional character. The modelled evolution of deep heat flow further suggests that ancient thermal perturbations have little influence on the present-day thermal field.

Late Paleozoic subsidence and burial history of the Fort Worth basin

ABSTRACT The Fort Worth basin in northcentral Texas is a major shale-gas producer, yet its subsidence history and relationship to the Ouachita fold-thrust belt have not been well understood. We studied the depositional patterns of the basin during the late Paleozoic by correlating well logs and constructing structure and isopach maps. We then modeled the one-dimensional (1-D) and two-dimensional subsidence history of the basin and constrained its relationship to the Ouachita orogen. Because the super-Middle Pennsylvanian strata were largely eroded in the region, adding uncertainty to the subsidence reconstruction, we used PetroMod 1-D to conduct thermal-maturation modeling to constrain the post-Middle Pennsylvanian burial and exhumation history by matching the modeled vitrinite reflectance with measured vitrinite reflectance along five depth profiles. Our results of depositional patterns show that the tectonic uplift of the Muenster uplift to the northeast of the basin influenced subsidence as early as the Middle Mississippian, and the Ouachita orogen became the primary tectonic load by the late Middle Pennsylvanian when the depocenter shifted to the east. Our results show that the basin experienced 3.7–5.2 km (12,100–17,100 ft) of burial during the Pennsylvanian, and the burial depth deepens toward the east. We attributed the causes of deep Pennsylvanian burial and its spatial variation to flexural subsidence that continued into the Late Pennsylvanian in response to the growth of the Ouachita orogen and southeastward suturing of Laurentia and Gondwana. The modeling results also suggest that the Mississippian Barnett Shale reached the gas maturation window during the Middle–Late Pennsylvanian.

Characterization of organic-rich shales for petroleum exploration & exploitation: A review-Part 2: Geochemistry, thermal maturity, isotopes and biomarkers

As shale exploitation is still in its infancy outside North America much research effort is being channelled into various aspects of geochemical characterization of shales to identify the most prospective basins, formations and map their petroleum generation capabilities across local, regional and basin-wide scales. The measurement of total organic carbon, distinguishing and categorizing the kerogen types in terms oil-prone versus gas-prone, and using vitrinite reflectance and Rock-Eval data to estimate thermal maturity are standard practice in the industry and applied to samples from most wellbores drilled. It is the trends of stable isotopes ratios, particularly those of carbon, the wetness ratio (C1/Σ(C2+C3)), and certain chemical biomarkers that have proved to be most informative about the status of shales as a petroleum system. These data make it possible to identify production “sweet-spots”, discriminate oil-, gas-liquid- and gas-prone shales from kerogen compositions and thermal maturities. Rollovers and reversals of ethane and propane carbon isotope ratios are particularly indicative of high thermal maturity exposure of an organic-rich shale. Comparisons of hopane, strerane and terpane biomarkers with vitrinite reflectance (Ro) measurements of thermal maturity highlight discrepancies suggesting that Ro is not always a reliable indicator of thermal maturity. Major and trace element inorganic geochemistry data and ratios provides useful information regarding provenance, paleoenvironments, and stratigraphic-layer discrimination. This review considers the data measurement, analysis and interpretation of techniques associated with kerogen typing, thermal maturity, stable and non-stable isotopic ratios for rocks and gases derived from them, production sweet-spot identification, geochemical biomarkers and inorganic chemical indicators. It also highlights uncertainties and discrepancies observed in their practical application, and the numerous outstanding questions associated with them.

A dry polishing technique for the petrographic examination of mudrocks: Discussion

After the paper “A dry polishing technique for the petrographic examination of mudrocks” by Gorbanenko (2017), it is important to stress some differences between the proposed polishing procedure and the existing standards for sample preparation for microscopic examination of organic matter, which may lead to less bright surfaces than required for measuring vitrinite reflectance by the most widely used standards (7404-5 ISO, 2009 or ASTM D7708, 2014).

Geochemistry, reservoir characterization and hydrocarbon generation potential of lacustrine shales: A case of YQ-1 well in the Yuqia Coalfield, northern Qaidam Basin, NW China

Ever since a breakthrough of marine shales in China, lacustrine shales have been attracting by the policy makers and scientists. Organic-rich shales of the Middle Jurassic strata are widely distributed in the Yuqia Coalfield of northern Qaidam Basin. In this paper, a total of 42 shale samples with a burial depth ranging from 475.5 m to 658.5 m were collected from the Shimengou Formation in the YQ-1 shale gas borehole of the study area, including 16 samples from the Lower Member and 26 samples from the Upper Member. Geochemistry, reservoir characteristics and hydrocarbon generation potential of the lacustrine shales in YQ-1 well were preliminarily investigated using the experiments of vitrinite reflectance measurement, maceral identification, mineralogical composition, carbon stable isotope, low-temperature nitrogen adsorption, methane isothermal adsorption and rock eval pyrolysis. The results show that the Shimengou shales have rich organic carbon (averaged 3.83%), which belong to a low thermal maturity stage with a mean vitrinite reflectance (Ro) of 0.49% and an average pyrolytic temperature of the generated maximum remaining hydrocarbon (Tmax) of 432.8 °C. Relative to marine shales, the lacustrine shales show low brittleness index (averaged 34.9) but high clay contents (averaged 55.1%), high total porosities (averaged 13.71%) and great Langmuir volumes (averaged 4.73 cm−3 g). Unlike the marine and marine-transitional shales, the quartz contents and brittleness index (BI) values of the lacustrine shales first decrease then increase with the rising TOC contents. The kerogens from the Upper Member shales are dominant by the oil-prone types, whereas the kerogens from the Lower Member shales by the gas-prone types. The sedimentary environment of the shales influences the TOC contents, thus has a close connection with the hydrocarbon potential, mineralogical composition, kerogen types and pore structure. Additionally, in terms of the hydrocarbon generation potential, the Upper Member shales are regarded as very good and excellent rocks whereas the Lower Member shales mainly as poor and fair rocks. In overall, the shales in the top of the Upper Member can be explored for shale oil due to the higher free hydrocarbon amount (S1), whereas the shales in the Lower Member and the Upper Member, with the depths greater than 1000 m, can be suggested to explore shale gas.

Geochemical characterization of Neogene sediments from onshore West Baram Delta Province, Sarawak: paleoenvironment, source input and thermal maturity

AbstractThe Neogene strata of the onshore West Baram Province of NW Borneo contain organic rich rock formations particularly within the Sarawak basin. This basin is a proven prolific oil and gas province, thus has been a subject of great interest to characterise the nature of the organic source input and depositional environment conditions as well as thermal maturation. This study is performed on outcrop samples of Lambir, Miri and Tukau formations, which are of stratigraphic equivalence to the petroleum bearing cycles of the offshore West Baram delta province in Sarawak. The investigated mudstone samples are organic rich with a total organic carbon (TOC) content of more than 1.0 wt.%. The integration of elemental and molecular analyses indicates that there is no significant variation in the source input between these formations. The investigated biomarkers parameters achieved from acyclic isoprenoids, terpanes and steranes biomarkers of a saturated hydrocarbon biomarkers revealed that these sediments contain high contribution of land plants with minor marine organic matter input that was deposited and preserved under relatively oxic to suboxic conditions. This is further supported by low total sulphur (TS), high TOC/TN ratios, source and redox sensitive trace elements (V, Ni, Cr, Co and Mo) concentrations and their ratios, which suggest terrigenous source input deposited under oxic to suboxic conditions. Based on the analysed biomarker thermal maturity indicators, it may be deduced that the studied sediments are yet to enter the maturity stage for hydrocarbon generation, which is also supported by measured vitrinite reflectance values of 0.39-0.48% R

Palaeoenvironmental implications of coal formation in Dobrudzha Basin, Bulgaria: Insights from organic petrological and geochemical properties

The Dobrudzha Basin in northeast Bulgaria contains 86 coal seams distributed within three coal-bearing formations of Late Carboniferous age. This study presents the organic petrological and geochemical characteristics of coals from the Makedonka and Krupen Formations, containing some of the most prominent coal seams. These were characterized by means of proximate and Rock-Eval analyses, vitrinite reflectance measurements, organic petrography and GCMS analysis in order to establish variations in vegetation and depositional settings. Based on random vitrinite reflectance (avg. 0.7% Rr) and TOC contents (avg. ~80%, daf) the coals can be classified as low rank bituminous (high volatile B-C). Low Tmax values (avg. 430°C) further indicate a low degree of organic matter maturity. The low to moderately high ash yields, together with the presence of shaly partings, and generally low sulfur contents are consistent with peat formation in a fluviatile sedimentary environment. Petrographic analysis reveals complex maceral composition with abundant liptinite and inertinite. The predominance of detrovitrinite and the almost complete lack of strongly gelified macerals argue for enhanced humification of the plant tissues and peat formation in relatively dry environments (above water table). The distributions of n-alkanes, diterpenoids, and steranes indicate that the organic matter originated from terrestrial vascular plants. The high pristane/phytane ratios point to peat deposition under oxic conditions. Low concentrations of hopanes argue for limited bacterial degradation of the organic matter, possibly due to low pH conditions during peatification. The presence of tetracyclic diterpanes and their aromatic counterparts is attributed to pre-gymnosperm (cordaitean) input. The existence of cordaiteans, although as minor component of the palaeovegetational communities, is further evidenced by the presence of trace amounts of aromatic arborane/fernane derivatives in almost all studied samples. The occurrence of combustion-derived PAHs in all studied samples is consistent with the abundant inertinite and indicates frequent forest fires during peat formation.