Source Rock Evaluation Research Articles

Source Rock Evaluation of Some Mesozoic and Paleozoic Rocks, Faghur Basin, Neith Field, North Western Desert, Egypt

Faghur Basin is one of the most important hydrocarbon provinces inthe northern Western Desert of Egypt. Some of the Mesozoic (Alam ElBueib and Khatatba formations) and Paleozoic rocks at this basin wereevaluated as source rocks based on certain parameters including: thequantity, quality and maturation of the contained organic matter.PetroMode 1D software was used for the construction and prediction ofgeological and geochemical models. Results from organic geochemicalanalysis have clarified that the TOC, HI, S1, S2, %Ro and Tmax values ofAlam El Bueib source rock units range from 0.27 to 6.39 wt.%, 34 to114 mg/g, 0.12 to 0.38 mg/g, 0.23 to 4.3 mg/g, 0.58 to 1.07 % and 409to 450 oC, respectively. While for the Khatatba source rock these valuesrange from 0.41 to 30.09 wt.%, 27 to 155 mg/g, 0.1 to 0.53 mg/g, 0.15 to26.41 mg/g, 1.02 to 1.09 %, 363 to 454 oC, respectively. As for thePaleozoic source rock they are ranging from 0.53 to 4.83 wt.%, 52 to136 mg/g, 0.06 to 0.4 mg/g, 0.29 to 6.57 mg/g, 0.97 to 1.1 % and 406 to467 oC, respectively. Such results indicate that the Alam El Bueib sourcerocks are characterized by type (III) kerogen while Khatatba andPaleozoic source rocks are of type (III) kerogen with inputs of kerogentype (II). Burial history analysis indicates that most of these source rocksare mature and reached their maximum temperature at Neogene time.

Effects of lacustrine depositional sequences on organic matter enrichment in the Chang 7 Shale, Ordos Basin, China

Chang 7 Shale, characterized by lacustrine black organic-rich shales and interbedded siltstones, has become a target for shale oil exploration and development in the Upper Triassic Ordos Basin, China. However, the heterogeneous enrichment of organic matter (OM) has led to controversy regarding source rock evaluation and difficulty in accurate prediction of favorable zones. Sequence stratigraphy, as a tool for prediction of sedimentary deposits has encountered challenges in lacustrine deep-water environments because of rapid facies changes and limited lateral continuity of key beds. In this study, a novel method is proposed to predict OM enrichment based on parasequences division and calibration of well logging and core lithology through analyses of thin sections and geochemical data from 238 core samples, as well as available well-logging data from a selected well. Chang 7 shales were simplified into three facies in terms of sedimentary structure and mineral composition, combining geological genesis with lateral correlation. Argillaceous laminated shale (Facies 2), with the highest average total organic carbon and S1 + S2 values, is the most favorable facies for shale oil exploration. The Chang 7 Member was divided into 15 parasequences and six parasequence sets, within one transgressive-regressive sequence. Depositional sequences have a significant effect on the enrichment of OM, given 1) the influence of lake-level fluctuations on OM accumulation, 2) the OM enrichment determined by shale facies, and 3) the distribution of shale facies controlled by parasequences. In particular, the most favorable locations are the bases of parasequences (generally the lower third to half from the bottom) in the TST and early RST. The application of this method to a test well showed excellent performance based on verification from the organic geochemical data of 86 samples. Five favorable targets of Facies 2 were marked within fifteen favorable locations. By linking meter-level depositional sequences with centimeter-level OM enrichment, this study provides an alternative approach to determine the target area in a single well profile and realize isochronous inter-well prediction for deep-water shales.

Geochemical studies on high-efficiency hydrocarbon expulsion of Triassic lacustrine shale in the Ordos Basin, China: implications for tight and shale oil accumulation

The organic-rich sediments developed at the bottom of Chang7 Member of the Upper Triassic Yanchang formation, which can be divided into oil shale and black mudstones, are dominate source rocks for low permeability and tight oil reservoirs in Mesozoic petroleum system of Ordos Basin. The hydrocarbon expulsion characteristics of the organic-rich sediments have been investigated by geology and geochemistry data. By comparison with the geochemical parameters of crude oil samples and mudstone extracts, Chang7 shale has many geochemical evidences on high rates of hydrocarbon expulsion. Lower conversion rate of chloroform extracted bitumen “A”, especially for the samples with TOC higher than 10%; lower productivity indexes and lower hydrogen indexes reducing with TOC increasing; lower ratios of saturates/aromatics but higher contents of polar components for residual bitumen “A”; heavier stable carbon isotopes of residual hydrocarbons. It should be noted that the source rock evaluation and oil-source correlation would be misunderstood based on these abnormal geochemical data caused by high-efficiency hydrocarbon expulsion. Calculation for rock samples have shown that the expelled rates of the shale samples averaging 70% are significantly higher than those of the mudstone samples. Chang 7 shale with large amounts of hydrocarbons generated and expelled is critical for hydrocarbon accumulation in low-permeability and tight oil reservoirs. Also, Chang7 organic-rich rocks is an important exploration field of shale oil in the future.

Evaluation and formation model of Cambrian high-quality source rocks in the southeast margin of North China Plate

In response to the question of whether effective Cambrian source rocks are developed in the North China Plate, field investigations on the southern margin of North China have been carried out. High-quality marine argillaceous source rocks of continuous thickness of 35 m in the Lower Cambrian were discovered in the south-eastern margin of North China Plate. Phosphorus nodules are common in the source rocks. The TOC is 1.19%∼29.70%, with the average of 7.31%. The organic matter comes from low-level biological algae. Because the carbon isotope of kerogen is relatively more negative, it is judged to be mainly benthic algae. The converted vitrinite reflectance (Ro) from measured bitumen reflectance is 2.24%∼3.45%, which is an over-mature source rock. The Lower Cambrian source rocks in the southern margin of North China occurred under the background of rising sea levels. After the glaciers on the southern margin of the North China Plate melted, the Qinling rift trough developed and the stratum subsidence caused rapid transgression of seawater from southeast to northwest. At this time, algae multiply in large numbers, and after death they accumulate with phosphorus-containing materials to form phosphorus-containing organic matter. Benthic algae mainly live in deep-water shelf not affected by waves and tides. Therefore, the development of source rocks is mainly in the deep water shelf.

Palynofacies analysis and source rock evaluation of the Upper Cretaceous-Oligocene succession in the Drazia-1 well, Alamein Basin, Egypt

Visual palynofacies analysis and petroleum source rock assessment are reliable and widely used applications for hydrocarbon potential. The Alamein Basin located in the north Western Desert is one of the promising hydrocarbon provinces all over Egypt. In this context, a total of 59 cutting samples representing the Upper Cretaceous Khoman formation and the Paleocene-Oligocene Apollonia and Dabaa rock units from the Drazia-1 well drilled in the East Yidma Oil Field, Alamein Basin, were analyzed. These samples were investigated for their palynological and palynofacies microscopy, including 39 samples for geochemical screening of their organic carbon content and 10 samples for Rock-Eval® 6 Turbo. Palynofacies analysis defined two palynofacies types of organic matter (OM). The first was characterized by phytoclasts-AOM composition of kerogen type III (gas-prone) and represents the Dabaa and Apollonia formations, and was deposited in a highly proximal shelf setting in marginal oxic-dysoxic conditions. The second was dominated by amorphous organic matter (AOM) and recognized in the Khoman formation, demonstrating a distal suboxic-anoxic basin with oil-prone kerogen type II. Moreover, geochemical analyses of the total organic carbon (TOC) content and the Rock-Eval pyrolysis determine the characteristics of possible source rock intervals and defined their hydrocarbon potentials. The investigated interval (4200–5150 ft) is suggested to be a poor source rock due to poor hydrocarbon potential of S2 values. The organic matter was thermally immature of kerogen type III (woody material typically generates gas). The palynofacies distributions and thermal hydrocarbon maturation in the Alamien Basin were affected by the tectonic movements that happened in the northern Egypt, mainly the Syrian Arc System.

PETROLEUM POTENTIALITY AND PETROPHYSICAL EVALUATION OF THE MIDDLE-JURASSIC SARGELU FORMATION, NORTHERN IRAQ

This study indicates the source rock evaluation and reservoir properties of the Middle-Jurassic Sargelu Formation in two selected sections (well Atrush-3 and Warte outcrop) northern Iraq. The well log data display that the shale includes 30% of the upper part of the Sargelu Formation. This extraordinary ratio of shale content has negative impacts on the reservoir properties in this part of the formation. The lower part of the formation has a good porosity ratio that reaches 15% as calculated from the log data. The porosity is obtained from analyzed core samples support the same conclusion that obtained in the log data analysis and it is measured at 9-17.5% for interval 1282-1301 m in Atrush-3 well and 5-7.9% in the interval 54-75 m Warte section. The formation has very good organic matter quality for black shales as the average total organic carbon (TOC) measured from the pyrolysis assessments is about 2.61 wt% in both the well and the section. The low oxygen index (average 26 mg CO2/g TOC) and high hydrogen index (average 423 mg HC/g TOC) indicate the variation of the kerogen class from type II in the Atrush-3 to type II-III in the Warte section. These two forms of kerogen can produce oil and condensate gas in a suitable state. Pristine/phytane (Pr/Ph) ratios are less than one (0.67-0.83) and this reveals the anoxic condition for the formation. The colors of the palynomorphs in the examined samples under the microscope changes from yellowish brown to brown (TAI=+2, -3), and the average Tmax and vitrinite reflectance are 444 oC and 0.80, respectively. Sargelu Formation is located within the mature and oil window. This is also supported by the carbon preference index which is around one (average 0.98).

Total organic carbon (TOC) enrichment and source rock evaluation of the Upper Jurassic-Lower Cretaceous rocks (Barents Sea) by means of geochemical and log data

The Barents Sea is a typical example of a multiple sourced hydrocarbon system, with several gas and oil fields. Previous studies have suggested a gas and oil-prone character of the source rocks, but a lack of success in oil discoveries characterizes this area. In this work, the geochemical characterization of source rocks using well data from six exploration wells in the south-western part of the Barents Sea is presented. We studied the Upper Jurassic-Lower Cretaceous Knurr, Hekkingen and Fuglen Fms. mainly formed by shales and mudstones. Here, the Passey’s method is applied to characterize the geochemical properties and organic matter of these formations by estimating total organic carbon content (TOC), the quantity of thermally generated hydrocarbons (S2), and hydrogen index (HI). Shale volume was also estimated to check the reliability of the calculated parameters. High shale volume values were found in all the formations, which are generally around 0.8 (volume fraction), indicating a predominance of shaly/muddy lithologies. However, some interbedded sandstones and siltstones are present, together with thinner dolomite and limestone layers. Our results show that Upper Jurassic source rocks such as Knurr Fm., which are often not object of geochemical studies, are mature and characterized by good organic carbon content. Good and very good organic carbon content are measured in the Fuglen and Hekkingen Fms., respectively. The hydrogen index suggests that all the source rocks are mixed gas-oil prone, with the Fuglen Fm. standing out as the most gas-prone between the studied formations. This study demonstrates that the Passey’s method can be an alternative to infer geological information when geochemical data are not available.

Three‐Dimensional Structural Modelling and Source Rock Evaluation of the Quseir Formation in the Komombo Basin, Egypt

AbstractThe Quseir Formation consists mainly of dark gray mudstones with a high organic matter content and excellent hydrocarbon‐generating potential. The main objectives of this study are to highlight the dominant structural elements in the Komombo Basin, Egypt, and evaluate the geochemical characteristics of the Quseir Formation. Depth maps and a 3D structural model indicate two normal fault trends NW–SE and ENE–WSW. The NW–SE trend is the dominant one that created the primary half‐graben system. The depth to the top of the Quseir Formation gradually decreases from the eastern and central parts towards the corners of the basin. The thickness of the Quseir Formation ranges from about 300 to 1000 ft. The 3D facies model shows that the shale has a large probability distribution in the study area, compared with the sandstone and siltstone. The source rock potential varies between good in the western part to very good in the eastern part of the basin. The organic‐rich interval is dominated by gas‐prone kerogen type III based on TOC and Rock‐Eval. The pyrolysis data vitrinite reflectance (%Ro) (0.5–0.74%) and Tmax values (406–454C°) suggest a maturity level that ranges from immature to early maturity stage for hydrocarbon generation.

Sedimentary environments and mechanism of organic matter enrichment of dark shales with low TOC in the Mesoproterozoic Cuizhuang Formation of the Ordos Basin: Evidence from petrology, organic geochemistry, and major and trace elements

Proterozoic strata have become a new target for hydrocarbon exploration worldwide. They exhibit the potential to become a resource replacement target. To provide a reference for hydrocarbon source rock evaluation, the sedimentary environment and mechanism of organic matter enrichment of dark shales in the Mesoproterozoic Cuizhuang Fm. were investigated for the first time by combining organic and inorganic research concepts. Although the total organic carbon (TOC) content in the Cuizhuang Fm. is relatively low (mean value of 0.59%), it still can be considered as a potential source rock. Thin-section observations, main and trace element contents, and biomarker compounds revealed the following: (1) the Cuizhuang Fm. was deposited in a neritic shelf sedimentary environment at a passive continental margin, and the climate was warm and humid. The hydrocarbon-generating organisms mainly comprised prokaryotes such as cyanobacteria and other bacteria. All these characteristics meet the conditions of source rock formation. (2) The low oxygen level, felsic source provenance, and absence of submarine hydrothermal activity led to low productivity, and the low productivity and low-grade hydrocarbon-generating organisms led to low TOC content in the Cuizhuang Fm. (3) The enrichment of V in the Cuizhuang Fm. is apparent, but does not reach the hyper-enrichment threshold of 500 ppm associated with euxinic conditions, revealing anoxic ferruginous conditions. Moreover, the V content is positively correlated with TOC content, indicating that organic matter enrichment was mainly controlled by the redox conditions of the bottom water. Therefore, the sedimentary areas with high productivity were related to basic source provenance and submarine hydrothermal activity, and anoxic bottom water may have promoted the development of better source rocks. This findings may represent the main direction for identifying effective source rocks in subsequent work.

Hydrocarbon source-rock potential of the Jurassic succession in eastern part of the North Western Desert, Egypt

The hydrocarbon potential is determined by the quantity and quality of organic matter encountered in the Jurassic sediments in two wells at the Northern Western Desert. It utilizes to define the zones of oil and gas using the well logging data for calculates the total organic carbon (TOC). The evaluation of source rock has been based on two steps; the first one depended on the geochemical parameters including TOC, S1, S2, Tmax, and Vitrinite reflectance (Ro %) of two wells JG-1 and JD-4. The second step was to calculate (TOC) from wireline logs. The well log types utilized in such kind of analysis are the density log, sonic log, resistivity log and gamma-ray log. The stratigraphic sequence, in the studied wells ranges in age from Paleozoic to Recent. The present work focuses on the Jurassic rocks represented by Khatatba Formation as they include the main source horizon. Based on the obtained results, Jurassic sediments called as fair to excellent source rock potential. The genetic type of organic matter can be identified through the study of pyrolysis data, which indicate that is rich in mixed oil and gas-prone kerogen except few samples reflect type I organic facies.

Source rock evaluation of the Cenomanian Raha Formation, Bakr oil field, Gulf of Suez, Egypt: Observations from palynofacies, RGB-based sporomorph microscopy, and organic geochemistry

The Gulf of Suez area in eastern Egypt is one of the most prospective and promising hydrocarbon provinces all over the world. Sixty drill cuttings samples of the Upper Cretaceous Raha Formation (70–120 m) have earlier been processed for their palynomorph and palynofacies contents, including 36 geochemically screened samples for their source-rock potential using TOC, Rock-Eval®. 6 Turbo, the new Rock-Eval® 7S instrument along with one of its latest modules, the CleanSim method. The combination between visual palynofacies and RGB-based sporomorph color image analysis along with the geochemical results are presented as part of a detailed assessment of the organic matter (OM) types, OM-richness, and thermal maturity levels.Palynofacies analysis defined three associations of OM types. The first and third associations consist of kerogen Type II; the former is dominated by a moderate abundance of amorphous organic matter (AOM) and phytoclasts, whilst the latter is dominated by high AOM content. Conversely, the second association is dominated by terrigenous fragments of gas-prone Type-III kerogen. The application of RGB analysis indicated a thermally immature to mature stage for some intervals in one of the three studied wells, the B-109.Geochemical screening results led to divide the Raha Formation into three distinctive groups, which are characterized by varied OM qualities and quantities, hydrocarbon potential, and thermal maturity levels.Varied thermal maturity levels was related directly to the Late Cretaceous tectonics like the effect of Syrian Arc System and the Oligocene-Miocene syn-rift activity that led to the opening of the Gulf of Suez. Differences in kerogen types were observed between geochemical and palynofacies analyses. Microscopic investigations of palynofacies-dominated AOM suggested that most AOM particles were of terrigenous origin due to the effect of biodegradation and gradual transformation of phytoclasts into AOM, reflecting a kerogen Type III instead of kerogen Type II that was indicated by palynofacies analysis ternary plots.

Impact of marine incursions on lacustrine source rocks: organic matter quantity, quality, and kinetics in the Paleocene South Yellow Sea Basin, offshore eastern China

The impact of marine incursions during transgression, on the quantity and quality of organic matter in lacustrine source rocks, is critical to understanding their source rock properties and therefore on their hydrocarbon generation potential, but is also challenging and controversial. Here we examine this issue based on a case study in the South Yellow Sea Basin (SYSB) offshore eastern China, addressing organic matter (OM) quantity, quality and petroleum generation kinetics. A total of 139 lacustrine source rock samples was collected from the fourth member of the Paleocene Funing Formation (E1f4) in the SYSB, and investigated for organic petrographic and geochemical characteristics. The samples are immature to early oil window mature and the OM in all samples is dominated by terrigenous material, as shown by the organic petrography, relatively low hydrogen indices and biomarker distributions. The sediments are classified as inert to gas-prone. However, the presence of marine fossils (red and brown algae, coccolithophores and probably Neomonoceratinabullata) and the biomarker results, suggest that marine incursions occurred during the deposition of the lower unit of the E1f4 (E1f4L). The samples from this unit display higher HI values and more depleted δ13Corg ratios, but lower TOC and S1 + S2 contents, than the upper unit of the E1f4 (E1f4U) source rocks without marine influence. This suggests that marine incursions primarily improved the quality, rather than quantity, of organic matter. Bulk kinetics indicated that marine incursions also led to a more homogenous OM and compositional kinetics and phase properties suggest that black oil is generated throughout the process of kerogen conversion. Thus, our data suggest that marine incursions had a positive effect on the OM quality and oil generation potential of these lacustrine source rocks. Therefore, the impact of marine incursions should be fully considered during source rock evaluation of coastal lacustrine basins, especially during high sea-level stages. In addition, the greatest hydrocarbon exploration potential is observed in the South 4th Sag of the Southern Depression and thus this study provides valuable data for shaping regional exploration strategy in the SYSB.

Source rock evaluation of Afowo clay type from the Eastern Dahomey Basin, Nigeria: insights from different measurements

The Cretaceous Afowo Formation in the Eastern Dohamey Basin is characterized by an admixture of lithofacies ranging from sandstones, claystones, shales, clays, sand/shale, and sand/clay intercalations. The sandy facies, a mix of sandstone, clay, shale, and intercalations, contain biodegraded hydrocarbons while the shales and claystones that underlie it are rich in organic matter. The hydrocarbon-bearing interval is commonly referred to as the oil sand or tar sand. In this study, Afowo clay type underlying an outcrop of the oil sand was appraised for its hydrocarbon potential with loss on ignition, thermogravimetry, and rock evaluation pyrolysis. Results obtained from loss on ignition showed that total organic matter content, a proxy to total organic carbon, for the Afowo clay type ranged from 9.410 to 38.750 wt%. The organic maturation temperature (Tmax) was determined using both thermogravimetry and rock evaluation pyrolysis (Rock–Eval). Thermogravimetric analysis produced reliable Tmax within the range of 417–424 °C for all the samples. The results from rock evaluation pyrolysis on the same samples showed that total organic carbon ranged from 0.81 to 18.46 wt% with Tmax ranging from 417 to 424 °C. It was not possible to determine Tmax for one of the samples with Rock–Eval due to a small S2 value (0.22 mg Hc/g). The variations in organic matter contents from loss on ignition agree with total organic carbon computed from rock evaluation pyrolysis; samples with high organic matter contents have corresponding high TOC values. This study demonstrates that loss on ignition and thermogravimetry could complement and augment rock evaluation pyrolysis data for petroleum source rock characterization.

Hydrocarbon source rock evaluation and quantification of organic richness from correlation of well logs and geochemical data: A case study from the sembar formation, Southern Indus Basin, Pakistan

Sembar Formation of Cretaceous encountered in Khajari-01 and Miran-01 wells, which are located in the Southern Indus Basin (SIB), Pakistan. Total organic carbon (TOC) content is generally measured by traditional methods such as the evaluation of side wall cores and formation cuttings in a geochemical lab. Data obtained from these experimental techniques are not continuous and also a time-consuming process. A solution to this problem is high resolution and continuous information obtained from well logs. Different methods were used to estimate TOC through well logs. Among those methods, four methods have been chosen to estimate TOC in the current research, i.e., Density Log, Spectral Gamma-Ray Log, ΔlogR, and Multivariate Fitting methods. The TOC estimated values have been correlated with well cuttings TOC values to optimize a method for TOC estimation through well logs. Fourteen well cutting samples of Sembar Formation from Khajari-01 well and seven samples from Miran-01 well have been selected for measurements of geochemical parameters. These parameters were used for source rock evaluation of the formation to determine the quality, quantity, and maturity of the Sembar Formation. Modified van-Krevlen diagram of Hydrogen Index (HI) versus Oxygen Index (OI) and Langford diagram of S2 versus TOC indicates that Sembar Formation in Khajari-01 well contain kerogen type-III (gas prone) compare to Miran-01 well that contains kerogen type-IV which makes it non-productive. The organic richness results indicate that TOC in Khajari-01 well is poor to very good while in Miran-01 a fair to good generation potential exist. Maturity indicators i.e., Pyrolysis Temperature (Tmax) and Production Index (PI) values show an immature to early mature zone for Khajari-01 well and over-mature zone in Miran-01 within the Sembar Formation. Density Log, Spectral Gamma-Ray Log, and Multivariate Fitting methods have poor coefficient of determination (R2) values between the estimated TOC (from well logs) with well cuttings tested samples TOC values. It has been inferred from data obtained using ΔlogR method that it has good R2 value between well logs estimated TOC and actual laboratory measured TOC in both wells which is optimized for future study in Southern Indus Basin, Pakistan.

The time-temperature-maturity relationship: A chemical kinetic model of kerogen evolution based on a developed molecule-maturity index

Kerogen maturity is an important indicator for evaluating source rocks. We propose a kerogen maturity index (molecule-maturity index, MMI) based on the changes in the molecular structure of organic matter during thermal evolution. The MMI has a positive correlation with vitrinite reflectance. Molecular dynamics simulations indicate that the weight loss is with a positive linear correlation with the MMI, demonstrating the ability of the index to reflect the hydrocarbon generation of organic matter. In addition, a new conversion is introduced by the MMI to analyze the kinetics of kerogen thermal evolution. The activation energy is expressed as a continuous function of the MMI linking by the new conversion. The activation energy was calculated at the isoconversional points of previous conversion, which was not defined as a continuous function before. A linear relationship between the activation energy of kerogen is observed, and a new chemical kinetic model of kerogen thermal maturity is established based on the MMI (kinetic model of the maturity evolution, MEKM). The MEKM equation has a simple form and is convenient for engineering applications. Our findings provide insights into the kinetics of kerogen thermal maturation and guide the choice of temperature and time for artificial maturity.

Palynofacies and Source Rocks Evaluation for Selected Samples of Subba Oil Field, Southern Iraq

This study includes a palynological and organic geochemical evaluation of Nahr Umr, Zubair, and Yamama Formations in the Subba field, Southern Iraq, represented by three wells (Su-14, Su-9 and Su-8). The determination of quantity organic matter showed that the rocks of Nahr Umr Formation had a total organic carbon of 4.76%, indicating very good production of hydrocarbons where type (II) kerogen was dominant, but the rocks were thermally immature. Zubair Formation had a total organic carbon of 1.91% to 2.26%, indicating good to very good production of hydrocarbons where kerogen of types (I) and (II/III) were dominant, with low thermal maturity. Yamama Formation showed a total organic carbon of 1.68%, revealing good production of hydrocarbons where type (I) kerogen was dominant, with low thermal maturity. Facies analysis of Nahr Umr, Zubair and Yamama Formations demonstrated the presence of two facies, namely the distal dysoxic-Oxic shelf and the distal suboxic-Anoxic basin. It is clear that the environment of rock deposition is a marine environment, which is far from the coast.

ОРГАНИЧЕСКАЯ ГЕОХИМИЯ КАЙНОЗОЙСКИХ ПОРОД УШУМУНСКОГО БУРОУГОЛЬНОГО МЕСТОРОЖДЕНИЯ СРЕДНЕАМУРСКОГО ОСАДОЧНОГО БАССЕЙНА

Актуальность исследования обусловлена необходимостью расширения российской ресурсной базы Дальнего Востока. Изучение осадочных бассейнов и их углеводородного потенциала, разработка новых технологий прогноза традиционных и нетрадиционных месторождений углеводородного сырья остается приоритетным направлением Российской АН на многие годы. Оценка обогащенности пород органическим веществом и его генерационного потенциала поможет в работах по бассейновому моделированию. Настоящие исследования нужны также для: уточнения обстановок накопления органического вещества; реконструкции ландшафтов и биологического мира в кайнозое. Цель: охарактеризовать биологический источник органического вещества, условия его накопления, обстановки диагенеза и уровень катагенетической зрелости в породах Ушумунского буроугольного месторождения. Объекты: угли, глины и алевриты Ушумунского буроугольного месторождения Среднеамурского осадочного бассейна. Методы: сопоставление данных пиролиза и изотопного анализа органического вещества и характеристик углеводородов-биомаркеров, полученных с помощью хромато-масс-спектрометрии в насыщенной и ароматической фракциях битумоидов, оценка источников, диагенетических обстановок и катагенетической преобразованности органического вещества. Результаты. Породы палеогена и неогена Ушумунского буроугольного месторождения Среднеамурского осадочного бассейна впервые изучены методами органической геохимии. Органическое вещество исследованных образцов имеет признаки как террагенного (гумусового), так и аквагенного (сапропелевого) типов. Изотопный состав углерода органического вещества, распределение н-алканов, наличие каурана, биерана, филлокладанов (биомаркеров высших растений Podocarpaceae и Araucareaceae) в насыщенной фракции битумоидов, ретена (биомаркера хвойных растений) в ароматической фракции свидетельствуют о террагенной природе органического вещества. Вместе с тем в большинстве образцов характеристики ароматической фракции соответствуют аквагенному органическому веществу: низкие концентрации фенантренов, высокие – моно- и триароматических стероидов, распределение метилдибензотиофенов, вероятное присутствие длинноцепочечных алкилнафталинов – биомаркеров озерных водорослей Gloeocapsomorpha prisca и Botryococcus braunii. Также в большинстве образцов концентрации фитана значительно выше, чем пристана, и половина образцов согласно водородному индексу HI относится к типу II (аквагенный морской). Морских отложений в районе исследований нет, по-видимому, тип II является смесью типов I (аквагенный озерный) III (континетальный). Причем, судя по изменениям HI по разрезу, режим осадконакопления менялся неоднократно с озерного до торфяно-болотного и обратно. В ряде образцов максимумы в составе ациклических изопреноидов приходятся на i-С21 и i-С22, возможно, из-за значительного вклада организмов домена археа в исходное живое вещество. Согласно высоким концентрациям дибензотиофенов, органическое вещество пережило диагенез с избытком серы в осадке. Вероятно, двойственность признаков (террагенные и аквагенные) связана со специфичностью исходной биоты этого бассейна и является характерной для ее возрастной и/или географической локализации. Низкие значения показателей отражательной способности витринита, пиролитические характеристики, резкое преобладание соединений с нечетным числом атомов в молекуле в н-алканах, наличие биогопанов и гопенов при почти полном отсутствии типичных стеранов и терпанов, распределение триароматических стероидов соответствуют катагенетически незрелому органическому веществу, и, возможно, недостаточная зрелость усиливает эту двойственность характеристик.

Geochemical Evaluation and Pore Type Characterization of Carbonaceous Rich Facies in Brown Shale Formation, Central Sumatra Basin

DOI:10.17014/ijog.7.2.121-133Brown Shale Formation of Pematang Group is a key source rock in the Central Sumatra Basin. The formation consists of three lithofacies: algal rich facies, mixed algal-carbonaceous facies, and carbonaceous rich facies. This paper focuses on evaluating the geochemistry and on characterizing the porosity of the carbonaceous rich facies. Geochemical evaluation of the carbonaceous source rocks was conducted using Rock-Eval pyrolysis, while mineralogy and microtextural characterization were assessed by incident light microscopy and scanning electron microscopy (SEM). The results show that the organic components of the carbonaceous source rocks are predominantly composed of vitrinite with minor inertinite and liptinite. The geochemical characteristics indicate that the carbonaceous rich facies could be ranked as a good to excellent level in terms of source rock potential with high TOC content, mainly containing kerogen type III as a gas prone potential. SEM results show that the pore types in the carbonaceous source rocks can be classified into three main types: porous floccules, organic-porosity, and intraparticle pores. Thus, the carbonaceous rich facies of the Brown Shale Formation contains gas prone source rocks with good generation potential, as well as organic rich shale unconventional reservoirs.

Modeling hydrocarbon accumulation based on gas origin and source rock distribution in Paleozoic strata of the Ordos Basin, China

Paleozoic gases in the Ordos Basin, China, are produced from upper Paleozoic tight sandstones, Ordovician weathered-crust, and ‘pre-salt’ (beneath gypsum deposits) dolomite reservoirs. Their origins have been debated, particularly as to whether Carboniferous–Permian (CP) or Ordovician source rocks contributed more gas to the lower Paleozoic gas reservoirs. The expanded exploration of lower Paleozoic gas to the eastern, western, northern, and southern regions of the basin has re-initiated the debates. In this study, geochemical characteristics of gas samples from newly drilled wells in different regions of Ordos Basin in combination with the source rock evaluation were used to determine the gas origin. The carbon and hydrogen isotope of alkane indicate that gas mixing occurred in both upper Paleozoic and Ordovician weathered-crust gases, and that TSR occurred in pre-salt gases. A modified δ13C–1/Cn plot was suggested to be useful to determine the carbon isotopic patterns of gases generated from the different source rocks. The light hydrocarbon characteristic shows that oil-type gas mainly distributes in the eastern basin. A study of source-rock distributions also indicates that the most favorable Ordovician source rock is located in the eastern basin. Based on the distribution of CP coal measures, limestones and lower Paleozoic carbonate source rock, a gas accumulation history was established. The results of this study provide applicable method for identifying gas origin in Ordos Basin. Moreover, the accumulation history proposed in this study is useful for further exploration of the lower Paleozoic gas in Ordos Basin.

Molecular Organic Geochemical Characteristics and Coal Gas Potential Evaluation of Mesozoic Coal Seams in the Western Great Khingan Mountains

AbstractCoal‐bearing strata are widespread in the western Great Khingan Mountains. Abundant coal resources have been found in the Jurassic Alatanheli Groups, the Cretaceous Bayanhua Groups, the Damoguaihe Formation and the Yimin Formation. The organic geochemical characteristics were analyzed in combination with hydrocarbon source rock evaluation and molecular organic geochemistry experiments, and the coal gas potential of coal seams was evaluated. The source rock evaluation results indicated that the Mesozoic coal samples have the characteristics of high organic matter abundance (TOC>30%), low maturity (Ro values of approximately 0.6%), and type III composition. The hydrocarbon generation potentials of the Alatanheli Groups and Bayanhua Groups are high, while the generation potentials of the Damoguaihe Formation and the Yimin Formation are low. The results of geochemistry show that the depositional environment of the coal seam was a lacustrine, oxidizing environment with a low salinity, and the source of the organic matter was mainly higher plants. Affected by weak degradation, the coal seams mainly formed low‐maturity gas of thermal catalytic origin. The Cretaceous coal seams contain a large amount of phytoplankton groups deposited in a low‐stability environment affected by a transgression event, and the potential range varied widely. For the Jurassic coal seams, the depositional environment was more stable, and the coal seams feature a higher coal‐forming gas potential.