Organic Matter In Source Rocks Research Articles

Control of Earth system evolution on the formation and enrichment of marine ultra-deep petroleum in China

Taking the Paleozoic of the Sichuan and Tarim basins in China as example, the controlling effects of the Earth system evolution and multi-spherical interactions on the formation and enrichment of marine ultra-deep petroleum in China have been elaborated. By discussing the development of “source-reservoir-seal” controlled by the breakup and assembly of supercontinents and regional tectonic movements, and the mechanisms of petroleum generation and accumulation controlled by temperature-pressure system and fault conduit system, Both the South China and Tarim blocks passed through the intertropical convergence zone (ITCZ) of the low-latitude Hadley Cell twice during their drifts, and formed hydrocarbon source rocks with high quality. It is proposed that deep tectonic activities and surface climate evolution jointly controlled the types and stratigraphic positions of ultra-deep hydrocarbon source rocks, reservoirs, and seals in the Sichuan and Tarim basins, forming multiple petroleum systems in the Ediacaran–Cambrian, Cambrian–Ordovician, Cambrian–Permian and Permian–Triassic strata. The matching degree of source-reservoir-seal, the type of organic matter in source rocks, the deep thermal regime of basin, and the burial-uplift process across tectonic periods collectively control the entire process from the generation to the accumulation of oil and gas. Three types of oil and gas enrichment models are formed, including near-source accumulation in platform marginal zones, distant-source accumulation in high-energy beaches through faults, and three-dimensional accumulation in strike-slip fault zones, which ultimately result in the multi-layered natural gas enrichment in ultra-deep layers of the Sichuan Basin and co-enrichment of oil and gas in the ultra-deep layers of the Tarim Basin.

Characterization of a structural trap associated with an intrusive complex: the El Trapial oilfield, Neuquén Basin, Argentina

Abstract Volcanic plumbing systems emplaced in sedimentary basins may exert significant mechanical and thermal effects on petroleum systems. The last decade of research has evidenced that igneous intrusions may enhance thermal maturation of organic matter in source rocks and lead to both small- and large-scale structures that can deeply impact fluid migration or trapping. This contribution describes how the emplacement of a whole intrusive complex generated a dome structure of the overburden, which is the main trapping structure of a large producing oilfield. Our case study is the lower Miocene Cerro Bayo de la Sierra Negra (CBSN) intrusive complex, Neuquén Basin, Argentina, associated with the El Trapial oilfield where the main trapping structure is a large domal antiform centred on the CBSN complex. This study integrates the large subsurface dataset produced during the development of the El Trapial oilfield. More than 1200 vertical wells (producers and injectors) have been drilled in the flanks of CBSN complex. In addition, five 3D seismic cubes have been acquired over the years that have been merged and reprocessed into a single volume. Such a dataset allows a detailed characterization of both the structure affecting the Mesozoic strata and the geometry of the intrusive complex. Igneous rocks have been recognized along the entire stratigraphic section. Sill intrusions appear to concentrate in the shale units and the stacking of them has a direct impact on the doming structure generation. Our study allowed us to establish a direct correlation between the distribution of the intrusions and the extent, amplitude and style of doming, showing that the dome structure results from the emplacement of the intrusive complex. We also show that part of the doming is related to intrusions emplaced in the Mesozoic formations of the Neuquén Basin, whereas the other part of the doming is related to deeper structures not imaged on the geophysical data. We estimate that the amplitude of the doming reaches up to c. 500 m. The voluminous subsurface data, combined with exposed outcrops, makes the CBSN complex a world-class case study for showing how the shallow plumbing system of a volcanic complex may control the growth of large-scale trapping structures for various fluids, such as drinkable water, geothermal fluids and hydrocarbons.

Main controlling factors evolution on high-quality source rocks development in the Shanan Sag, Bohaibay Basin, NE China: Implication from structure, depositional environment, and organic matter

Lacustrine source rocks have a significant oil potential and contribute to 95 % of oil reserves in China's offshore areas, Nature of these rocks is related to climate and structure and they are commonly heterogeneous. Mechanism of organic matter accumulation in these rocks has been a matter of debate. Based on Rock-Eval pyrolysis, organic petrographic examinations, gas chromatography-mass spectrometry (GC–MS) analysis, and trace elements analysis, this paper illustrates the paleoclimate, tectonic activity, paleo-productivity, and preservation conditions recorded in high-quality source rocks of the Shanan Sag. The aim is to understand the controlling factors on the deposition of these source rocks. The results indicated that the organic matter of source rocks in middle segment of the third member of the Shahejie Formation (Es3m) was mainly from phytoplankton (average content = 52.2 %), the development of source rocks was controlled by high paleo-productivity. The primary productivity was low during the period of the first and second members of the Shahejie Formation (Es1-2), and the high gammacerane index, and low Pr/Ph ratios indicated that the source rocks were mostly controlled by a good reduction environment with high salinity. The source rocks of the third member of the Dongying Formation (Ed3) were characterized by high terrestrial materials (average exinite content = 62 %) and low to moderate Pr/Ph ratios, which indicated the source rocks were affected by both paleo-productivity and preservation environment. A productivity-controlling model in Es3m was established along with a co-controlling model of productivity and preservation in Ed3, which was largely developed under the background of a warm-humid climate and strong depression-controlled fault activity. The development model of source rock in Es1-2 can be concluded as a preservation model under a hot arid climate and weak fault activity. The development model of source rock in Es1-2 can be concluded as a preservation model under a hot arid climate and weak fault activity.

Evaluation of the Geological Characteristics and Exploration Potential of Tight Oil in the Neogene Upper Ganchaigou Formation in the Zhahaquan Area, Qaidam Basin.

The study of the geochemical characteristics of source rocks is an important part of tight oil evaluation. The Zhahaquan area of the Qaidam Basin is a new area for tight oil exploration in China. During the sedimentary period of the Neogene Upper Ganchaigou Formation (N1) in the Zhahaquan area, a set of source rocks of semideep lake and deep lacustrine facies as well as a set of thin, interbedded fine sandstone and argillaceous limestone was deposited, providing favorable conditions for the formation of tight oil. However, the study on the geochemical characteristics of source rocks in this area is relatively weak. The geochemical characteristics of the source rocks in the Zhahaquan area were determined via the experimental analysis of parameters such as vitrinite reflectance (Ro), chloroform bitumen "A", total organic carbon (TOC), group components, kerogen types, rock pyrolysis, and aromatic compounds of crude oil. The following results were obtained: the Zhahaquan area had II1-type hydrocarbon source rock organic matter, and the TOC was 0.32%-1.32%. The type index (TI) was 48.70-72.23, the chloroform bitumen "A" content was 0.0034%-0.1133%, Ro was 0.810%-1.265%, and the cracking hydrocarbon peak temperature (Tmax) was distributed in the temperature range of 362-444 °C. The hydrocarbon generation conversion rate was 0.89%-10.0%. The reservoir mainly had intergranular pores, dissolution pores, and microfractures. The average porosity was 8.0%, and the average permeability was 0.861 × 10-3 μm2. The average oil saturation was 30.0%, and the average water saturation was 21.6%. From a comprehensive analysis of the results, the following inferences were derived. The parent material of the source rocks in the Zhahaquan area mainly originated from algae and other phytoplankton in the lake basin, which was a good source rock for oil exploration. The source rocks of this area have entered the threshold of hydrocarbon generation and are in the peak oil-generation stage. They have potential as industrial oil and gas resources. The oil test results of YD103 and seven other wells showed that the daily oil index per meter ranges from 0.38 to 6.5 m3/d·m, indicating that the source rocks have the ability to form industrial oil. Analysis of the geochemical characteristics of source rocks and study of reservoir geological characteristics will provide theoretical support and reference for tight oil exploration and development in Zhahaquan.

Performance of evolutionary optimized machine learning for modeling total organic carbon in core samples of shale gas fields

Rock samples' TOC content is the best indicator of the organic matter in source rocks. The origin rock samples’ analysis is used to calculate it manually by specialists. This method requires time and resources because it relies on samples from many well intervals in source rocks. Therefore, research has been done to aid this effort. Machine learning algorithms can estimate total organic carbon instead of well logs and stratigraphic studies. In light of these efforts, the current work present a study on automating the total organic carbon estimation using machine learning approaches improved by an evolutionary methodology to give the model flexibility and precision. Genetic algorithms, differential evolution, particle swarm optimization, grey wolf optimization, artificial bee colony, and evolution strategies were used to improve machine learning models to predict TOC. The six metaheuristics were integrated into four machine learning methods: extreme learning machine, elastic net linear model, linear support vector regression, and multivariate adaptive regression splines. Core samples from the YuDong-Nan shale gas field, located in the Sichuan basin, were used to evaluate the hybrid strategy. The findings show that combining machine learning models with an evolutionary algorithms in a hybrid fashion produce flexible models that accurately predict TOC. The results show that, independent of the metaheuristic used to guide the model selection, optimized extreme learning machines attained the best performance scores according to six metrics. Such hybrid models can be used in exploratory geological research, particularly for unconventional oil and gas resources.

Metal elements potential for hydrocarbon–source correlation

Inorganic geochemical approaches (i.e., trace elements) for hydrocarbon–source rock correlations in sedimentary basins are a frontier area of research in petroleum geochemistry. This study used inorganic geochemistry to the over mature petroleum systems in the Sinian (Neoproterozoic) Dengying Formation in the Sichuan Basin of China. Our results show that the BaN–NiN–CuN ternary diagram, VN–NiN–MoN ternary diagram and V/Ni–U/Th diagram (related to paleo-productivity and redox conditions), rare earth element (REE)–Y patterns, Ce/Ce*–La/La* diagram, and Ce/Ce*–La/La*–(LREE/HREE)N ternary diagram (related to fluid interactions) can robustly distinguish different source rocks. In particular, V, Ni, and Mo have a strong relationship to organic matter in source rocks, while their occurrence is most likely related to porphyrins in petroleum. Therefore, the VN–NiN–MoN ternary diagram can be used for hydrocarbon–source rock correlations. In contrast, Ba, Cu, U, Th, and REE–Y have different and complex geochemical behaviors, limiting the application in hydrocarbon–source rock correlations. The application in the Sichuan Basin shows that the hydrocarbons in the Dengying reservoir were mainly derived from the lower Cambrian Qiongzhusi Formation with minor contribution from the lower Cambrian Maidiping and Sinian Doushantuo formations. Compiled data from Lower Paleozoic crude oils in the Tarim Basin show that the VN–NiN–MoN ternary diagram can also be applied to low-maturity crude oil systems. Therefore, V, Ni, and Mo have strong relationship to organic matter in petroleum systems and the potential for hydrocarbon–source correlation, while other elements should be used under given conditions.

The role of Triassic source rocks in the formation of oil accumulation in the Fukang Depression (China)

This article presents the results of geological and geochemical studies of the organic matter of source rocks and oil from reservoirs in the Karamay Formation (Anizian–Ladinian stages) in the eastern part of the Fukang Depression in Junggar petroleum basin. The quantity, quality, and degree of maturity of the organic matter in clayey mudstones in upper part of the Karamay Formation (T2k) were considered and the composition of the original organic matter, the conditions of its accumulation and the degree of maturity were reconstructed using pyrolytic parameters and biomarkers. The dark gray and gray mudstone in upper part of the Karamay Formation accumulated in fresh or low salinity lakes with relatively reducing conditions. At present, the organic matter of the Formation within the depression is in the zone of mesocatagenesis MK1–MK3, and therefore has not fully realized its oil-geological potential. It has been previously established that mudstones in upper part of the Karamay Formation provided the hydrocarbon fl uids for the Karamay reservoirs located in the submerged part of the Fukang Depression.

Genetic Relationship between Mississippi Valley-Type Pb–Zn Mineralization and Hydrocarbon Accumulation in the Wusihe Deposits, Southwestern Margin of the Sichuan Basin, China

The coexistence of numerous Mississippi Valley-type (MVT) Pb–Zn deposits and oil/gas reservoirs in the world suggests a close genetic relationship between mineralization and hydrocarbon accumulation. The Wusihe MVT Pb–Zn deposits are located along the southwestern margin of the Sichuan Basin. Based on the spatiotemporal relation between Pb–Zn deposits and paleo-oil/gas reservoirs, ore material sources, and processes of mineralization and hydrocarbon accumulation, a new genetic relationship between mineralization and hydrocarbon accumulation is suggested for these deposits. The Wusihe Pb–Zn deposits are hosted in the Ediacaran Dengying Formation dolostone, accompanied by a large amount of thermally cracked bitumen in the ore bodies. The Pb–Zn deposits and paleo-oil/gas reservoirs are distributed along the paleokarst interface; they overlap spatially, and the ore body occupies the upper part of the paleo-oil/gas reservoirs. Both the Pb–Zn ore and sphalerite are rich in thermally cracked bitumen, in which µm sized galena and sphalerite may be observed, and the contents of lead and zinc in the bitumen are higher than those required for Pb–Zn mineralization. The paleo-oil/gas reservoirs experienced paleo-oil reservoir formation, paleo-gas reservoir generation, and paleo-gas reservoir destruction. The generation time of the paleo-gas reservoirs is similar to the metallogenic time. The source rocks from the Cambrian Qiongzhusi Formation not only provided oil sources for paleo-oil reservoirs but also provided ore-forming metal elements for mineralization. Liquid oil with abundant ore-forming metals accumulated to form paleo-oil reservoirs with mature organic matter in source rocks. As paleo-oil reservoirs were buried, the oil underwent in situ thermal cracking to form overpressure paleo-gas reservoirs and a large amount of bitumen. Along with the thermal cracking of the oil, the metal elements decoupled from organic matter and H2S formed by thermochemical sulfate reduction (TSR) and minor decomposition of the organic matter dissolved in oilfield brine to form the ore fluid. The large-scale Pb–Zn mineralization is mainly related to the destruction of the overpressured paleo-gas reservoir; the sudden pressure relief caused the ore fluid around the gas–water interface to migrate upward into the paleo-gas reservoirs and induced extensive metal sulfide precipitation in the ore fluid, resulting in special spatiotemporal associated or paragenetic relations of galena, sphalerite, and bitumen.

Evaluation of Hydrocarbon Generation Potential of Laiyang Formation Source Rocks in Ri-Qing-Wei Basin, Eastern Shandong

The Ri-Qing-Wei Basin is a newly discovered Late Mesozoic rift basin on the eastern Shandong coast in recent years. Thick, continuous deposited source rocks are found in the Lingshan Island scientific drilling project. Therefore, it is necessary to evaluate the distribution and hydrocarbon generation potential of source rocks in this area. Organic geochemical experiments were carried out on samples from the core of LK-1 and outcrops in the Lingshan Island, the Laoshan area, and the Jimo Zhougezhuang area to evaluate the maturity, abundance, and type of organic matter in source rocks. The results show that the cumulative thickness of the source rocks in the study area is more than 500 m, and the TOC content is generally greater than 1.0%. The organic matter type is good (mainly type II1 and type II2 kerogen) and the Ro value is more than 2.0. The thermal evolution degree of the organic matter is high with natural gas predominantly generated. We carried out Rock-Eval, PY-GC and gold tube experiments on low-maturity samples of Laikong 2 and established a chemical kinetic model to quantitatively evaluate the study area in combination with sedimentary burial history and thermal history data. The results show that the total resources of the five sags in the study area are about 476 billion cubic meters, and the average resource intensity is about 82.2 million cubic meters/km2. Among them, the resource intensities of the Lingshan Island Sag and Laoshan Sag are 112.6 million cubic meters/km2 and 98.8 million cubic meters/km2, respectively. Studied sites are “small and fertile” and may be used as favorable exploration prospect areas.

Maturity trends for the Late Devonian Duvernay Formation and identification of the Willesden-Green Maturity High

Maturity trends appearing in the literature for the Late Devonian Duvernay Formation within the Western Canada Sedimentary Basin (WCSB) are inconsistent and do not reflect hydrocarbon production. To address these inconsistencies and more accurately delineate Duvernay maturity trends, this study produces solid-state 13C Nuclear Magnetic Resonance (NMR) spectroscopy and Rock-Eval Tmax data from 38 wells throughout the WCSB. The Duvernay is divided into three different assessment areas that include, from east to west, Innisfail, Willesden-Green, and Kaybob. Based upon the proportion of aromatic to aliphatic carbon of the source rock organic matter, the Duvernay maturity generally increases towards the southwest and mimics structural depth, except within the Willesden-Green area. The Innisfail area has the shallowest depths and only the southern portion reaches oil maturity. The Willesden-Green and Kaybob areas range from oil to dry gas maturity. Although Willesden-Green and Kaybob have similar structural depths (P = 0.24), they have a significant difference in solid-state 13C NMR proportion of aromatic to aliphatic (P = 0.05) and Tmax values (P = 0.00). The average residuals from the depth vs solid-state 13C NMR proportion of aromatic carbon and Tmax best fit line show an increase of 0.09 and 10 °C respectively in Willesden-Green compared to Kaybob. Maps of the maturity indicators show the increase in maturity within Willesden-Green is perpendicular to structural strike. This study names this anomalous feature the Willesden-Green Maturity High.

Reconstruction of geochemical characteristics of original organic matter in drilling cuttings contaminated by oil-based mud

Oil-based drilling mud (OBM) contamination, commonly occurring in cuttings during petroleum exploration, has had been a problem seriously hindered source rock identification and evaluation. To eliminate the effect of contamination on “free” bitumen extracted directly from source rock, “enclosed” and “bound” bitumens were released from the mineral matrix by demineralization and from the kerogen structure using an improved catalytic hydropyrolysis technique, respectively. Geochemical and carbon isotopic compositions of the three types of bitumen were used to reconstruct the geochemical characteristics of original organic matter in their source rocks. Results indicate that “free” and “enclosed” bitumens have similar biomarker distributions and n-alkane δ13C values in non-contaminated cuttings, but the former is more susceptible to OBM contamination. “Enclosed” bitumen can thus be used to characterize the organic geochemistry of source rocks contaminated by OBM. The carbon isotopic compositions of individual n-alkanes in “free” and “enclosed” bitumens differ from those in “bound” bitumen, likely because of the effects of diagenesis and other secondary alteration. “Bound” bitumen is more likely to preserve the molecular carbon isotopic characteristics of original organic matter in source rocks, while the “free” and “enclosed” bitumens more represent the generation products. Based on biomarker characteristics and carbon isotopic compositions of “bound” hydrocarbons, Wenchang (WC) Formation (Fm.) source rocks in the Zhu I Depression of the Pearl River Mouth Basin (PRMB) can be divided into three types in normal medium–deep (WC–I), shore–shallow (WC-II), and special medium–deep (with different algal blooms growth rates; WC-III) lacustrine source rocks. The special medium–deep lacustrine source rocks of Wenchang Fm. are characterized by high ratio of C30 4-methylsteranes to C29 steranes (4-Me/C29 > 0.66) and heavy δ13C values (δ13Ckerogen < −26.1‰), which are obviously different from those of normal medium–deep lacustrine source rocks. The combination of “free”, “enclosed”, and “bound” hydrocarbons geochemical and isotopic compositions thus provides a comprehensive understanding of source rocks.

Hydrocarbon potential and sedimentary environment of organic matter in source rocks of Linhe and Guyang Formations in Linhe depression, Hetao Basin

Hydrocarbon potential and sedimentary environment of organic matter in source rocks of Linhe and Guyang Formations in Linhe depression, Hetao Basin

The significance of the macerals identification for understanding and study the transformation processes of organic matter in source rocks

The question of the correct terminology is highly relevant, since researchers should understand one another in communication with specialists in different branches of geology. An overview of the terms already existing in the English-speaking world for describing oil-prone macerals, such as alginite, bituminite, bitumen, solid bitumen, pyrobitumen, oil, was presented, and it is more convenient to combine all zooclasts into bioclasts, since it is not always possible to determine whether the remains belong to the animal or plant world. Particular difficulties emerged with the term “bitumen”, which in Russian has two meanings. Increasingly, the films between mineral grains can be named both bitumen and oil. It was also suggested to divide bituminite in pre-mature bituminite, mature bituminite and post-mature bituminite in order to emphasize the features of those transformations that occurred with the initial organic matter during thermal maturity. Due to the fact that in the process of thermal maturity, the reactive part of kerogen = bituminite will transform into a liquid and gas, its solid part (“residual organic matter”) was suggested to be called solid bitumen or pyrobitumen.

BIOMARKER GEOCHEMISTRY OF PALEOGENE CRUDE OILS AND SOURCE ROCKS FROM THE RAOYANG SAG, BOHAI BAY BASIN, NE CHINA: AN OIL – SOURCE ROCK CORRELATION STUDY

This study presents a systematic geochemical analysis of Paleogene crude oils and source rocks from the Raoyang Sag in the Jizhong sub‐basin of the Bohai Bay Basin (NE China). The geochemical characteristics of fifty‐three oil samples from wells in four sub‐sags were analysed using gas chromatography (GC) and gas chromatography – mass spectrometry (GC‐MS). Twenty core samples of mudstones from Members 1 and 3 of the Eocene‐Oligocene Shahejie Formation were investigated for total organic carbon (TOC) content and by Rock‐Eval pyrolysis and GC‐MS to study their geochemistry and hydrocarbon generation potential. The oils were tentatively correlated to the source rocks.The results show that three groups of crude oils can be identified. Group I oils are characterized by high values of the gammacerane index and low values of the ratios of Pr/Ph, Ts/Tm, 20S/(20S+20R) C29 steranes, ββ/(ββ+αα) C29 steranes, C27 diasteranes/ C27 regular steranes and C27/C29 steranes. These oils have the lowest maturity and are interpreted to have originated from a source rock containing mixed organic matter deposited in an anoxic saline lacustrine environment. The biomarker parameter values of Group III oils are the opposite to those in Group I, and are interpreted to indicate a highly mature, terrigenous organic matter input into source rocks which were deposited in suboxic to anoxic freshwater lacustrine conditions. The parameter values of Group II oils are between those of the oils in Groups I and III, and are interpreted to indicate that the oils were generated from mixed organic matter in source rocks deposited in an anoxic brackish–saline or saline lacustrine environment.The results of the source rock analyses show that samples from Member 1 of the Shahejie Formation were deposited in an anoxic, brackish – saline or saline lacustrine environment with mixed organic matter input and are of low maturity. Source rocks in Member 3 of the Shahejie Formation were deposited in a suboxic to anoxic, brackish – saline or freshwater lacustrine environment with a terrigenous organic matter input and are of higher maturity. Correlation between rock samples and crude oils indicates that Group I oils were probably derived from Member 1 source rocks, while Group III oils were more likely generated by Member 3 source rocks. The Group II oils with transitional characteristics are likely to have a mixed source from both sets of source rocks.

Application of Machine Learning Algorithms in Predicting Pyrolytic Analysis Result

Geochemical studies of organic matter in source rocks play an important role in predicting the oil and gas accumulation of any territory, especially in oil and gas shale. For deep understanding, pyrolytic analyses are often carried out on samples before and after extraction of hydrocarbon with chloroform. However, extraction is a laborious and time-consuming process and the workload of laboratory equipment and time doubles. In this work, machine learning regression algorithms is applied for forecasting S2ex based on the pyrolytic analytic result of non-extracted samples. This study is carried out using more than 300 samples from 3 different wells in Bazhenov formation, Western Siberia. For developing a prediction model, 5 different machine learning regression algorithms including Multiple Linear Regression, Polynomial Regression, Support vector regression, Decision tree and Random forest have been tested and compared. The performance of these algorithms is examined by R-squared coefficient. The data of the X2 well was used for building a model. Simultaneously, this data is divided into 2 parts – 80% for training and 20% for checking. The model also was used for prediction of wells X1 and X3. Then, these predictive results were compared with the real results, which had been obtained from standard experiments. Despite limited amount of data, the result exceeded all expectations. The result of prediction also showcases that the relationship between before and after extraction parameters are complex and non-linear. The proof is R2 value of Multiple Linear Regression and Polynomial Regression is negative, which means the model is broken. However, Random forest and Decision tree give us a good performance. With the same algorithms, we can apply for prediction all geochemical parameters by depth or utilize them for well-logging data.

Compositional evolution of organic matter in Boquillas Shale across a thermal gradient at the single particle level

The molecular composition of petroliferous organic matter and its compositional evolution throughout thermal maturation provides insight for understanding petroleum generation. This information is critical for understanding hydrocarbon resources in unconventional reservoirs, as source rock organic matter is highly dispersed, in contact with the surrounding mineral matrix, and may occur as multiple organic matter maceral types. Here, Raman spectroscopy and optical microscopy approaches were applied to a marginally mature (vitrinite reflectance ~0.5%) sample of the Late Cretaceous Boquillas Shale before and after hydrous pyrolysis (HP) at 300 °C and 330 °C for 72 h. This analytical approach allowed for correlative examination of micro-scale changes in organic matter compositional properties (e.g., aromaticity) for a variety of organic matter macerals across a thermal gradient (from marginally mature into the late oil/wet gas window) at the single particle level. Results indicate that while the examined amorphous organic matter, solid bitumen, and vitrinite particles exhibit different aromatic signatures in the unheated shale, they effectively progress along a similar trend through composition space with thermal maturation. Examined inertinite fragments were generally insensitive to the applied thermal stress, reinforcing the idea that reservoir temperature may be secondary for dictating the molecular composition of inertinite. Additional analysis of Raman spectra for individual organic matter macerals was performed using multivariate curve resolution (MCR) and correlation of standard Raman and reflectance-derived thermal maturity proxies against MCR parameters shows consistent trends. This trend suggests that MCR may be a fast and statistically robust method for extracting compositional information from Raman spectra of sedimentary organic matter, and can be used to construct thermal maturity relationships. These findings inform our understanding of how different petroliferous organic matter maceral types evolve throughout thermal reactions and further demonstrate that Raman spectroscopy combined with petrographic analysis can provide complementary estimates of organic matter composition and thermal maturity.

Source Rock Evaluation and Hydrocarbon Generation Model of a Permian Alkaline Lakes—A Case Study of the Fengcheng Formation in the Mahu Sag, Junggar Basin

The alkaline lake source rocks of the Fengcheng Formation are developed in the Mahu Sag of the Junggar Basin. Different from traditional continental fresh water and saltwater lake source rocks, alkaline lake source rocks lack targeted evaluation criteria, and it is unknown whether their hydrocarbon generation models are consistent with traditional models. Therefore, in the present study, evaluation standards and hydrocarbon generation models of alkaline lake source rocks are discussed based on geological and organic geochemical data and a systematic summary of the geochemical characteristics of the Fengcheng Formation source rocks. The Fengcheng Formation source rocks are mainly diamictite with mixed argillaceous rock and dolomite; most total organic carbon (TOC) values range from 0.2–1.4%; and the kerogen is primarily oil-prone type II, reaching low- to high-maturity stages. Based on the types of organic matter in source rocks and the relationships between organic matter abundance parameters, the evaluation standard of alkaline lake source rocks is proposed. The Fengcheng Formation is mainly composed of good to excellent source rocks (55.5%) with high hydrocarbon generation potential. The single-peak hydrocarbon generation model of the Fengcheng Formation is similar to that of traditional freshwater or saltwater lakes, with a high hydrocarbon generation rate, two to five times that of the traditional model; its main particularity is in the formation of naphthenic crude oil from the kerogen of bacteria and algae. A new understanding of the hydrocarbon generation potential and model of alkaline lake source rocks in the Fengcheng Formation can provide support for tight oil and shale oil exploration in the Mahu Sag.

Petrographic Characterization and Maceral Controls on Porosity in Overmature Marine Shales: Examples from Ordovician-Silurian Shales in China and the U.S.

The pore structure characterization and its controlling factors in overmature shales are keys to understand the shale gas accumulation mechanism. Organic matter in source rocks is a mixture of various macerals that have their own specific evolutionary pathways during thermal maturation. Pores within macerals also evolve following their own path. This study focused on petrographic characterization and maceral controls on porosity in overmature marine shales in China and the United States. Shale from Ordos Basin in China was also selected as an example of overmature transitional shale for maceral comparison. Organic petrology techniques were used to identify maceral types and describe morphological features in detail; scanning electron microscopy techniques were then used to document the abundance and development of pores within macerals. Helium measurement, mercury intrusion capillary pressure, and CO2 adsorption were especially applied to quantify the pore structure of Wufeng-Longmaxi shale from Sichuan Basin in China. The vitrinite reflectance equivalent of the studied overmature samples is ~2.4%. The macerals within the studied marine shales are composed mainly of pyrobitumen and zooclasts. At this maturity, pyrobitumen develops abundant gas-related pores, and their volume positively correlates to gas content. Three types of pyrobitumen and its related pore structure are characterized in Wufeng-Longmaxi shales. Zooclasts contribute to total organic carbon (TOC) content but little to porosity. When the TOC content is above 1.51% in Wufeng-Longmaxi samples, the TOC content positively correlates to quartz content. Organic matter strongly controls micropore development. Pores of diameter ~ 0.5 nm provide a significant amount of micropore volume. Clay mineral and quartz contents control micro- and macropore increments in organic-lean shales. MICP results indicate that pores within 3-12 nm and 900-2500 nm account for a major contribution to pore volume obtained. Determining the proportions of pyrobitumen to zooclasts within the total organic matter in pre-Devonian organic-rich marine shales is important in predicting porosity and gas storage capacity in high-maturity shales.

Geochemical characteristics of marine shale in the Wufeng Formation–Longmaxi Formation in the northern Sichuan Basin, South China and its implications for depositional controls on organic matter

Marine black shales of Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation in the Sichuan Basin are considered as excellent source rocks in South China. According to the geochemical characteristics of source rocks, the formation and preservation conditions of organic matter in source rocks are showed. In order to study the aggregation mechanism of organic matter, geochemical proxies were proposed, including redox proxies (V, V/Al, U, U/Al, Mo, and Mo/Al), paleoproductivity proxies (P, P/Ti, Ba, Ba/Al, and SiXS), paleoclimate proxies (CIA), and terrigenous flux proxies (Al, Zr, and Zr/Al). At present, the average content of TOC in Wufeng Formation is 3.63 wt%, which is in the range of 2.59–4.22 wt%. The average content of TOC in Longmaxi Formation is 1.02 wt%, and ranges from 0.45 to 2.07 wt%. The redox proxies show that the Wufeng shale is mainly accumulated under anoxic conditions. Longmaxi shale is mainly accumulated under suboxic-oxic conditions. The P/Ti and excess silicon (SiXS) values indicated that the whole sedimentary sequence had high paleoproductivity, but it was reduced to some extent in the Longmaxi Formation. The reduced Corg/Ptot ratios implies that the decrease of productivity may be caused by the increase of P (phosphorus) burial. The restricted degree of watermass shows that Wufeng shale deposits under strongly restricted watermass, while Longmaxi shale deposits under weakly restricted watermass. During the sedimentary period of the Wufeng Formation, the strongly restricted of the ocean circulation of the northern Sichuan Basin and the effective stratification of water column is beneficial to the preservation of organic matter. During the Longmaxi shale sedimentary period, tectonic movement and global sea level changes controlled the production and preservation of organic matter, affecting terrestrial clastic material flux, redox conditions and nutrient cycling.

Source rock characteristics and sedimentary environment analysis of Lucaogou Formation in the eastern Jimusaer Sag, Junggar Basin

In recent years, shale oil has gradually become the focus of oil exploration. The source rocks of Lucaogou Formation in the east of Jimusaer Sag of the Junggar basin have great shale oil potential. However, there is still much controversy regarding the sedimentary environment of the source rocks which restrict shale oil exploration. In order to address this problem, 45 mudstone samples of Jx-1 well are selected for testing and analysis. The results show that the Permian Lucaogou Formation in the Jimusaer Sag is developed with high quality source rocks, and the organic matter type is mainly type II, and the source rocks are in mature stage. Both higher terrestrial plants and aquatic algae are important sources of organic matter in source rocks, but the source of organic matter is still dominated by lower aquatic organisms. The source rocks of Lucaogou Formation are mainly deposited in reducing environment, and the water salinity is high, which is conducive to the preservation of organic matter.