Low Maturity Stage Research Articles

NMR response of maturity parameters and biodegradation grade of heavy oil

The NMR of the Trinity reservoir indicate that changes in crude oil compositions result in differences in NMR characteristics. On this basis, the chemical properties of crude oil are analyzed, and two criteria affecting the NMR characteristics of crude oil with different physical properties are presented. The research shows that the chromatographic characteristics of crude oils with different maturity and biodegradability correspond well with NMR characteristics. Lateral relaxation time of heavy oil at low maturity stage and experiencing strong biodegradability is shortened. Two parameters, maturity index and biodegradation level, can be reliably applied to the evaluation and production of heavy oil reservoir.

Geochemical characteristics and organic matter accumulation of argillaceous dolomite in a saline lacustrine basin: A case study from the paleogene xingouzui formation, Jianghan Basin, China

Shale oil resources have been discovered in argillaceous dolomite in the Lower Member of Paleogene Xingouzui Formation (LXF) of the Jianghan Basin, opening up a new field of shale oil exploration and development. Previous studies have mainly focused on the evaluation of reservoir quality, but the research on the enrichment mechanisms of organic matter (OM) in argillaceous dolomite is very limited or unknown. On the basis of organic geochemical and mineralogical data, combined with sedimentological analyses, the factors controlling the differential enrichment of OM in the LXF Member II oil Bed argillaceous dolomite are studied. The results show that the average TOC content of the samples is greater than 1.0%, which could be regarded as good source rocks. The kerogen type is mainly type II, with a small amount of type I and type III, and most of the samples are in the low-mature stage. Vertically, the development of the LXF Member II oil Bed argillaceous dolomite can be divided into three stages: the lower section (LS) has TOC content ranging from 0.57 to 4.95%, the middle section (MS) has a higher TOC content, between 0.55 and 9.52%, and the TOC content of the upper section (US) is relatively low, between 0.41 and 3.49%. The distribution of biomarkers showed that both higher plants and aquatic organisms contributed to the OM accumulation. The parameters of biomarkers such as pristane/phytane (Pr/Ph), gammacerane/C30 hopane (G/H), extended tricyclic terpane ratio (ETR) and their degree of dispersion, as well as sedimentological characteristics, indicate that the LS was deposited in a stratified deep waterbody with anoxic bottom water, the MS was deposited in a saline stratified moderate-deep waterbody with anoxic bottom water, and the US was deposited in a disturbed saline waterbody. Different sedimentary environments controlled the source of OM, the effectiveness of nutrients recycling and preservation conditions, which jointly affect the enrichment of OM. The stratified moderate-deep waterbody with anoxic bottom water and no physical disturbances, coupled with low terrigenous organic matter (TOM) inputs and the contribution of algae and other phytoplankton, are the best conditions for the enrichment of OM in argillaceous dolomite.

Lake basin evolution from early to Middle Permian and origin of Triassic Baikouquan oil in the western margin of Mahu Sag, Junggar Basin, China: Evidence from geochemistry

Large-scale oil and gas fields have been discovered after years of petroleum exploration in Mahu Sag, Junggar Basin, confirming the significant resource potential in its western margin. However, the origin of oil in the Triassic Baikouquan (T1b) reservoir and lacustrine basin evolution during the Permian are still poorly understood. In this study, Permian source rocks and T1b crude oil were analyzed using Rock-Eval pyrolysis, vitrinite reflectance, stable carbon isotopes, and biomarker compositions. Results show that the Lower Wuerhe source rock (P2w) was developed in a freshwater lacustrine environment with high oxygen content, higher plant input, and now contains typical humic (type-III) kerogen at the low thermal maturity stage. The Fengcheng source rock (P1f) was developed in an evaporative lake after mixed seawater (sea and lake). Deposition included anoxic conditions, terrigenous input, high salinity, and intense evaporation, and was dominated by sapropelic (type-II) kerogen, which is currently at the thermal maturity stage. The Jiamuhe source rock (P1j) was developed in a transitional marine and terrigenous environment with terrigenous input, moderate salinity, and reducing water, and now contains mainly type-III kerogen at the high thermal maturity stage. The Junggar region's climate was humid–arid–humid for the Early to Middle Permian. Mahu Sag was not completely closed until the end of P1j deposition and did not completely evolve into a lacustrine basin with large-scale freshwater input until the P2w deposition. Crude oil of the T1b formation is very mature and is derived from the source rock of the transitional marine and terrigenous environments deposited in the center of Mahu Sag, with high salt content, weak reducibility, and low evaporite content. Using biomarkers, carbon isotopes, and fine lithologic analysis, the oil-source correlation shows that T1b crude oil came from P1f calcareous and tuffaceous source rocks.

Geochemical Characteristics and Sedimentary Control of Pinghu Formation (Eocene) Coal‐bearing Source Rocks in Xihu Depression, East China Sea Basin

AbstractCoal‐bearing source rocks of the Pinghu Formation in the Xihu Depression comprise an important material basis of oil and gas resources in the East China Sea Basin. Based on drilling core observation results combined with the analysis and test results of macerals, trace/rare earth elements, and rock pyrolysis, the geochemical characteristics and sedimentary control of coal‐bearing source rocks formation are discussed in a high‐frequency sequence framework. The results indicate that the macerals composition of the coal‐bearing source rocks of the Eocene Pinghu Formation in the Xihu Depression is dominated by vitrinite, with low‐medium abundance of exinite and almost no inertinite. The coals and carbonaceous mudstones display higher amounts of total organic carbon (TOC) (14.90%–65.10%), S1 + S2 (39.24–136.52 mg/g), and IH (191–310 HC/g TOC) respectively, as compared to the mudstones. Organic matter is plotted in type III kerogens and partially in type II; it is mainly in the low maturity stage. The trace elements results imply that the samples were deposited in a weakly reducing to weakly oxidizing environment and were occasionally affected by seawater. The coal‐bearing source rocks were deposited in a relatively oxygen‐containing environment. The coal‐bearing source rocks development is jointly controlled by the coal accumulation environment, the water conditions affected by ocean currents in offshore basins in China, oxidation–reduction cycles of aqueous media and paleoclimate evolution in a high‐frequency sequence framework.

Gas generation from Jurassic coal measures at low mature stage and potential gas accumulation in the eastern Junggar Basin, China

The increasing global demand for clean energy has made it imperative to exploit unconventional oil and gas resources. Early thermogenic gas is one of the research target of unconventional gas. The Jurassic coal measures were widely distributed in the eastern Junggar Basin with high abundance of humic organic matter and low thermal degrees, showing potential for early thermogenic gas exploration. The burial depth of Jurassic source rocks ranges between 2200 m and 4500 m, with vitrinite reflectance ranging from 0.5% to 0.9%. To study the potential and possibility of gas generation from Jurassic coal measures at low mature stage in the eastern Junggar Basin, mudstone and coal samples were selected for pyrolysis experiments. The pyrolysed temperatures setting for this study are 330 °C, 350 °C, and 370 °C. The heating times at each desired temperature are 3, 7, 14, 21, and 30 days. Parameters such as vitrinite reflectance and certain biomarker ratios indicate all the samples are still at the low mature stage when the heating process finished. Genetic correlation between δ13C1 and C1/(C2+C3) ratio reveals that the generated gas in this study is mainly sourced from Type III kerogen and at the stage of low maturity. Hydrocarbon gas yield results suggest that 370 °C is the most favorable temperature for early thermogenic gas generation, especially heating for 14 days or longer. Pyrolysis results indicate time effect plays an important role in the formation of early thermogenic gas. The results of fourier transformed infrared spectra suggest aliphatic components in macromolecular structure of kerogen provide gas generating material during low mature stage. The evolution history of the Jurassic source rocks and the pyrolysed results indicate the Jurassic coal measure strata in the eastern Junggar Basin has favorable geothermal condition for accumulation of early thermogenic gas.

Environmental response to volcanic activity and its effect on organic matter enrichment in the Permian Lucaogou Formation of the Malang Sag, Santanghu Basin, Northwest China

Lacustrine organic-rich fine-grained rocks are widespread in the second member of the Permian Lucaogou Formation (P2l2) in the Malang Sag, Santanghu Basin, with complex lithology and significant changes of lithofacies due to frequent volcanic activity. In this study, the mineralogical and petrological characteristics, types of organic matter (OM), and effect of volcanism on the sedimentary environment and OM accumulation were analyzed using organic, carbon, and oxygen isotopes, as well as major and trace element data. The results revealed that the P2l2 was characterized by mixed deposition of volcanic ash and carbonate minerals, with four main rock types: tuff, dolomitic tuff, tuffaceous dolomite, and dolomite. The fine-grained rocks dominantly contain type I and II1 kerogens, which were in a low-mature stage and exhibited excellent original hydrocarbon generative potential. When volcanic activity was strong, large amounts of nutrients were released by the volcanic ash, which then entered the lake, in turn promoting the blooming of phytoplankton, and causing the δ13C values in the tuff to decrease. In addition, the paleotemperature decreased for a short time, and the suboxic-euxinic environment was conducive to the preservation of OM, resulting in high total organic carbon (TOC) content in the tuff. In contrast, during intermittent or weak periods of volcanic activity, the climate was hotter and drier, and evaporation resulted in a sustained increase of salinity. Consequently, the growth of algae was restricted, which brought about an increase in the δ13C values and relatively low TOC content in the dolomite. The relationships among primary productivity, redox conditions, clastic influx proxies, and TOC content illustrate that primary productivity was the most vital control on the accumulation of OM. High primary productivity was mainly driven by volcanic ash falls. Meanwhile, high salinity and suboxic-euxinic conditions were beneficial to the preservation of OM, giving rise to a high degree of OM enrichment.

Influence of palaeoclimate and hydrothermal activity on organic matter accumulation in lacustrine black shales from the Lower Cretaceous Bayingebi Formation of the Yin’e Basin, China

The lacustrine black shales of the Lower Cretaceous Bayingebi Formation are an important hydrocarbon source rock in the Yin’e Basin. It is important to understand the mechanism of organic matter accumulation in these shales to reconstruct the palaeoenvironment and palaeoclimate in the Early Cretaceous in northwestern China and search for potential oil and gas resources. Based on petrological, elemental geochemical and organic geochemistry analyses of typical field profile samples, the influence of palaeoclimate and hydrothermal activity on organic matter accumulation in different sedimentary units (Unit I, II and III) is revealed. Unit II has the highest total organic matter abundance (TOCavg = 5.95%), mainly type I kerogen (lamalginite), which is in the immature to low mature stage. Organic carbon isotope data (δ13Corg), element ratios (Sr/Cu and C-value) and weathering indexes (including CIA, CIW and PIA) together revealed that the region experienced a semihumid to semiarid palaeoclimate in the middle of the Early Cretaceous. The increase in rainfall in Unit II enhanced the abundance of lake algae and surrounding terrestrial vascular vegetation and elevated the lake level, which contributed to adequate sources and favourable preservation conditions for organic matter accumulation. Several lines of evidence, including high sulfur contents, the combination of vein pyrite and micritic dolomite, positive Eu anomalies, and element geochemical correlations, confirm the existence of intermittent low–temperature white smoker–type hydrothermal activity. The decrease of oxygen content in lake bottom water may be related to the input of hydrothermal fluid, which provided necessary nutrient elements for biological productivity. According to palaeontological and radioisotope dating data, it can be inferred that the black shales of the Bayingebi Formation are the product of the combined effect of global palaeotemperature rise in the early Aptian stage of the Early Cretaceous and intermittent low–temperature hydrothermal activity.

Competitive adsorption characteristics of CH4/C2H6 gas mixtures on model substances, coal and shale

Dynamic adsorption experiments were conducted on different samples to investigate the process and mechanism of competitive adsorption between CH4 and C2H6 molecules. Based on adsorption breakthrough curves obtained from adsorption experiments, mathematical models of the adsorption process were established and related adsorption parameters were calculated. The results show that competitive adsorption between CH4 and C2H6 molecules on adsorption materials can occur when adsorption materials have a significantly greater adsorption capacity of C2H6 than that of CH4. Meanwhile, fitted Yoon and Nelson model indicates different adsorption rate constants in CH4 and C2H6. Generally, samples with competitive adsorption characteristics, including 5 Å molecular sieve, 13X molecular sieve, activated carbon and shale, are characterized by obvious higher adsorption rate constant of CH4 than that of C2H6. Whereas, samples with no competitive adsorption characteristics, including 3 Å molecular sieve and coal, are characterized by almost same adsorption rate constants. The significantly greater adsorption capacity of C2H6 than that of CH4 and obvious higher adsorption rate constant of CH4 than that of C2H6 for samples in this study may result from a stronger induced force and dispersion force of C2H6 with adsorbent compared with CH4 in existing of metal ion. During the increasing maturity stage of kerogen, competitive adsorption of methane and ethane can obviously influence the gas composition in free gas at relatively low maturity stage and such influence becomes weaker at higher maturity stage.

Organic geochemistry characterization of Late Jurassic bituminous shales and their organofacies and oil generation potential in the Shabwah depression, southeast Sabatayn, Yemen

Bituminous shales of the Late Jurassic Madbi Formation from Shabwah depression in the south-eastern Sabatayn Basin has been collected and analyzed. The organofacies, paleo-sedimentary environmental conditions and oil generation potential are discussed based on combined geochemistry and petrology investigations. Biomarkers indicate that the bituminous-analyzed shales contain mainly marine phytoplankton algae and minor land plants and deposited under reducing environmental conditions. The rich in lipids from phytoplankton algae and land plants suggest high Type II to mixtures of Types II and Type III kerogen as the original organic facies during deposition. This is consistent with significant amounts of alginite and amorphous organic matter, with minor vitrinite land plants observed under microscope and hydrogen index (HI) values of 210–679 mg HC per g TOC and indicated good to excellent oil-source rocks. The presence of the reducing conditions during deposition consequently enhanced the preservation and subsequently gave rise to enrichment of organic matter in the analyzed bituminous shale as indicated by the relatively high TOC values between 1 and 14 wt%.The geochemical maturity indicators show that the analyzed bituminous shales have reached a low maturity stage, and commercial oils have not yet generated. Therefore, the results presented and discussed in this study suggest that the low maturity bituminous shales can be heated to crack the kerogens and subsequently significant amount of oil can be generated. This will lead to a huge alternative potential of unconventional resources and provide a sense of extending the exploration activities for both unconventional and conventional petroleum resources in the whole Basin.

Changes in physicochemical properties of organic matter by uranium irradiation: A case study from the Ordos Basin in China

Carbonaceous debris (CD) are exclusively disseminated in sandstones of uranium-bearing strata from the Dongsheng uranium ore field of the northern Ordos Basin, northern China. Physicochemical properties of CD were investigated through a series of tests including element analyses, Fourier transform infrared spectroscopy (i.e., FTIR) and organic carbon isotope (i.e., δC13) as well as specific surface area. The results show that δC13 of the samples from uranium-bearing strata range from −22.86‰ to −25.82‰, indicating that the CD is from the same origin. The average values of C, H, O and N are 54.54%, 4.55%, 39.15% and 0.49%, respectively, and H/C as well as O/C atomic ratios vary from 0.77 to 1.26 and 0.36 to 1.72, respectively, implying that CD is mostly composed of C and O, followed by H, and is grouped into Type Ⅲ kerogen at an immature to low mature stage. Compared with the samples without uranium enrichment, C, H, N and δ13C of the uranium-bearing samples increase by 13.66%, 5.32%, 12.50% and 0.98‰, respectively, while decrease by 18.50%, 7.84% and 33.33%, respectively, for O, H/C as well as O/C atomic ratios. As the intensity of uranium enrichment increases, H/C atomic ratios and aliphaticity decrease, and aromaticity increases. Besides, uranium-bearing samples are of smaller specific surface area than those without uranium enrichment. Alpha irradiation energy released by uranium and its daughters could cause the crosslinking in n-alkanes and long-chain polymers through a free-radical mechanism, which is supported by a decrease in O, aliphaticity and an increase in aromaticity. Hence, it results in polycondensation of molecular structure with a decrease in side chain length, function groups as well as specific surface area, and changes in physicochemical properties of organic matter, which is instructive in prospecting.

Experimental constraints on the evolution of organic matter in oil shales during heating: Implications for enhanced in situ oil recovery from oil shales

In situ oil shale conversion experiments have revealed that to efficiently obtain shale oil, factors such as heating temperature, products, and matter thermal evolution of organic matter during heating need to be optimized. In this study, oil shales from China were subjected to retorting experiments involving eight heating steps. During steps I-II (≤300 °C), the expelled product is mainly water, the hydrocarbon generation potential (PP) of the samples was extremely high, and the organic matter was in the immature to low-mature stage. The oil shale produced a large amount of oil in steps III–VI (300–475 °C), and the TOC contents and S2 of the shale showed the greatest decreases during these steps. The analysis data suggest that the organic matter entered the mature stage during these steps. After heating to 400 °C and expelling a large amount of oil and gas, the PP decreased gradually. From 475 to 520 °C (steps VII–VIII), only a small amount of gas was produced and the organic matter had almost no PP in the high- and over-mature stages. Based on our experimental results, in situ conversion of oil shale the final heating temperature should be ~440 °C. These constraints require further field testing to be properly optimized.

Oil generation character of Mesoproterozoic Xiamaling formation during stepwise pyrolysis

The exploration potential of Mesoproterozoic petroleum system has getting increasingly attention and the oil generation character of Mesoproterozoic source rock is still uncertain. To solve the problem, this study choose sample from Mesoproterozoic Xiamaling formation from north China, using the stepwise gold tube pyrolysis method to study the oil generation character and the composition of liquid pyrolysates, and choose sample from Mesozoic Yanchang formation in middle China as a comparison to study their differences in oil generation character. The results showed that oil generation window of Mesoproterozoic Xiamaling sample in this research was earlier and narrower, with an earlier oil generation peak, this maybe the result of the single source of organic matter and its labile chemical bound. Furthermore, mainly source of organic carbon, like cyanobacteria and acritarchs, were lack of lipid compounds, perhaps this could make the relative concentration of saturated hydrocarbon in liquid pyrolysates of Mesoproterozoic Xiamaling sample much lower than the Mesozoic Yanchang sample with the same kerogen type, nonpolar fraction of liquid pyrolysates in the oil generation window were mainly aromatic hydrocarbon. Maybe the results mean that although Mesoproterozoic Xiamaling formation is still in low maturity stage, it may has already started oil generation, and type of oil generated by Mesoproterozoic source rock in oil generation window may have significant difference from the Phanerozoic source rock.

Biomarker signatures of Upper Cretaceous Latrobe Group petroleum source rocks, Gippsland Basin, Australia: Distribution and geological significance of aromatic hydrocarbons

Aromatic hydrocarbons extracted from Upper Cretaceous (Maastrichtian and Campanian) Latrobe Group coaly shales and shales from the Gippsland Basin were investigated by gas chromatography–mass spectrometry. A variety of 2-ring to 6-ring aromatic hydrocarbons were identified, including both higher-plant-derived and combustion-derived polycyclic aromatic hydrocarbons (PAHs). These aromatic compounds indicate that the Upper Cretaceous coaly shales and shales were deposited in a fluvial/deltaic environment with significant higher plant input and are at a relatively low thermal maturity stage. Higher plant parameters and angiosperm/gymnosperm indices based on aromatic components suggest that higher land plants, including both angiosperms and gymnosperms, were an important part of the palaeovegetation in the Upper Cretaceous. The aromatic angiosperm/gymnosperm index that includes chrysene- and picene-type PAHs is not as reliable for evaluation of relative angiosperm and gymnosperm contributions as the aliphatic indices due to input of combustion-derived PAHs and the complexity of burning conditions altering the relative contributions. However, combustion-derived PAHs and aromatic angiosperm/gymnosperm indices supplement the information from aliphatic hydrocarbons and provide further evidence to support the hypothesis that palaeoclimate cooled from the Campanian to the Maastrichtian.

Evolution of water content in organic-rich shales with increasing maturity and its controlling factors: Implications from a pyrolysis experiment on a water-saturated shale core sample

Abstract In the present study, a pyrolysis experiment was performed on an immature water-saturated shale core sample, and a set of artificially matured subsamples (maturity with equivalent vitrinite reflectance values of 0.67–3.20%, i.e., EqVRo values) with initial water phases was obtained. The water content of these shale samples and its influencing factors were investigated. The results show that the water content significantly varies with maturity and is closely related to the hydrocarbon generation and expulsion, clay mineral transformation, and pore structure evolution of shales. According to the changes of water content with maturity and the main influencing factors, the evolution of the water content in shales is approximately divided into three stages. At the low maturity stage (0.67% 1.36%), with increasing maturity, the water content remarkably decreases at the wet gas window (1.36% 1.90%), which is mainly controlled by the displacement drainage of gaseous hydrocarbons. In addition, the pore structure evolution of shale, which changes its water-storage capacity, also has an influence on the water content evolution.

Sedimentary and geochemical characteristics of the Triassic Chang 7 Member shale in the Southeastern Ordos Basin, Central China

The Ordos Basin is the largest petroliferous basin in China, where the Chang 7 Member shale serves as the major source rock in the basin, with an area of more than 100,000 km2. So far, sedimentary and geochemical characterizations have rarely been conducted on the shale in shallow (< 1000 m) areas in the southeastern part of the basin, but such characterizations can help identify the genesis of organic-rich shale and promote the prediction and recovery of shale oil. In this paper, several outcrop sections of the Chang 7 Member in the Tongchuan area were observed and sampled, and sedimentary and geochemical characterizations were conducted for the well-outcropped YSC section. The study results show that the Chang 7 Member shale is widely distributed laterally with variable thickness. The organic-rich shale is 7–25 m thick in total and exhibits obvious horizontal variation in mineral composition. In the eastern sections, the shale contains organic matter of Type II2–III and is low in thermal maturity, with high clay mineral content, low K-feldspar content, and no pyrite. In the western sections, the shale contains Type II1 organic matter and is low in thermal maturity, with high clay mineral, K-feldspar, and pyrite contents. The YSC section reveals three obvious intervals in vertical mineral composition and organic abundance. The Chang 7 Member organic-rich shale (TOC > 10%) contains mainly sapropelite and liptinite, with Type II kerogen. It is generally characterized by a hydrocarbon potential of more than 70 mg/g, low maturity, and shallow–semideep lacustrine facies. In the western sections, the shale, still in a low maturity stage, has a higher hydrocarbon potential and is optional for shale oil recovery. However, the Chang 7 Member shale in the study area is highly heterogeneous and its shale oil recovery is practical only in the organic-rich intervals.

Petroleum accumulation history of Nanbaxian belt — Study of gas generation and fluid phase, northern margin of Qaidam Basin, West of China

To study the phase evolution and the fluid accumulation history of Nanbaxian petroleum field, in this study, a series of anhydrous closed-system gold-tube thermal simulation experiments have been performed on a carbonaceous mudstone (type Ⅱ kerogen) from the northern margin of the Qaidam Basin in the Xiaomeigou Formation (J1x). The hydrocarbon generation kinetics research was used to simulate the gas generation history of Yiliping depression, and the phase characteristics of the evolution were analysed by PVTsim. The influence of kerogen generated CO2 on the fluid phase was discussed that CO2 has a significant influence on the fluid phase at low maturity stage that the fluid phase could be a gas phase under geological condition. As a result, the fluid was migrated to Lower Ganchaigou Formation as gas phase, in the early stage of hydrocarbon accumulation. The fluid phase was gradually transformed into oil phase with the CO2 loss and accumulated as primary reservoir. With the increase of the maturity, the effect of CO2 on the fluid phase gradually decrease. During the Shizigou Formation depression, the tectonic action and the hydrocarbon generated from Yiliping depression led to the evaporation fractionation of the primary reservoir and the gas washing which cause the reconstructed of the primary reservoir in Lower Ganchaigou Formation and form of the secondary reservoir in Shizigou Formation.

Formation mechanisms of calcite cements in tight sandstones of the Jurassic Lianggaoshan Formation, northeastern Central Sichuan Basin

The characteristics and formation mechanism of calcite cements in the tight sandstone of the Jurassic Lianggaoshan Formation in the northeastern Central Sichuan Basin were analysed using petrographic and isotopic techniques. In the tight sandstone of the Lianggaoshan Formation, cements are mostly calcite and occur as poikilitic, pore-filling, fracture-filling and replacement of clastic particles. Contents of Al, Si, Fe and Mn in the poikilitic calcites are significantly less than that in the dissolution pore-filling and metasomatic calcites. Three stages (early, middle and late) of authigenic calcites correspond to temperature ranges of <60, 60–100 and ≥100 °C, respectively, with most calcite cements formed under lower temperature (<100 °C) conditions. The δ18O values of the early–middle authigenic calcites are in equilibrium with connate water, and the δ18O values of late calcites are depleted in 18O indicating equilibrium at higher temperatures. The early authigenic calcites precipitated in a relatively open system associated with CO2 from bacterial fermentation at an immature to low-mature stage, and a Ca2+- and alkaline-rich environment owing to hydration–hydrolysis and dissolution of silicate minerals during phase A of eodiagenesis. The middle–late authigenic calcites precipitated in a relatively closed system with CO2 from decarboxylation of organic acids and Ca2+ from dissolution of silicate minerals and transformation of clay minerals during phase B of eodiagenesis to mesodiagenesis. Calcite cements mainly occur in the medium and fine sandstones of sand flats and beach bars. Authigenic calcite dissolution is extremely weak, and calcite cementation is pore-space destructive.

Organic petrography and pore structure characterization of low-mature and gas-mature marine organic-rich mudstones: Insights into porosity controls in gas shale systems

Analyses of both type II organic-rich low-mature mudstones from the Kimmeridge Clay Formation (Yorkshire, England) and gas-mature mudstones from the Vaca Muerta formation (Argentina) were performed using petrographic and SEM observations to characterize the organic and inorganic components of the rocks and their relationship with porosity. The porosity analyses using nitrogen adsorption measurements and mercury intrusion porosimetry were then evaluated as a function of both composition and thermal maturity. Despite the absence of variation in the pore size distribution and the total pore volume with maturity, which is controlled by total organic carbon content (TOC), this study demonstrates that the pore network of these marine mudstones varies considerably with thermal maturity. Contrary to low-mature samples, whose porosity depends essentially on mineral interparticle pores, the porosity of thermally-mature rocks is mainly influenced by organic-matter-hosted pores located within secondary solid bitumen. Organic matter (OM) porosity, which is rare in the low-mature stage, appears to increase during thermal maturation in response to the thermal cracking of kerogen and oil, to become predominant in gas-mature rocks. It can therefore be considered that thermal maturation is a major process for the development of the OM porosity. However, the OM of high-TOC (>5.5 wt%) gas-mature samples contains smaller pores than gas-mature low-TOC mudstones probably due to a difference in the original OM composition between these high- and low-TOC samples. By analogy with the Kimmeridge Clay formation, which is often considered as time equivalent in terms of OM composition (Uliana et al., 1999), the high-TOC Vaca Muerta samples appear to have originally contained greater proportions of oil-prone amorphous OM and thus a better OM intrinsic quality for oil generation. This difference of OM composition and petroleum potential between Vaca Muerta marine mudstones appears to have dramatically influenced the OM-hosted pore genesis during maturation. Consistent with other studies, these results show that thermal maturity is not the only process that can explain OM-hosted pore genesis in gas shale systems. The OM composition can greatly influence the evolution of this porosity during maturation. Organic composition and OM thermal maturity may therefore have a complementary effect, explaining the diversity of trends observed on porosity in natural formations.

Kinetic analysis of hydrocarbon generation based on saline lacustrine source rock and kerogen samples in the western Qaidam Basin, China

Tertiary carbonate rocks from the Qaidam Basin in China are typical plateau saline lacustrine source rocks. Despite the fact that this set of source rocks has a relatively low abundance of organic matter (major frequency of total organic carbon, 0.4–0.6%), the organic matter has a high hydrocarbon conversion rate and commonly forms low-maturity oil. In this study, a comparative analysis was conducted on Tertiary source rock and kerogen samples from the Qaidam Basin using a kinetic experiment of hydrocarbon generation. The objectives of the study were to investigate the mechanism of hydrocarbon generation from source rock to low-maturity oil and the catalysis of kerogen pyrolysis by minerals. The results showed that both the source rock and kerogen samples reached a maximum liquid hydrocarbon yield during the low-maturity stage. Eighty percent of the hydrocarbons were expulsed before the Easy Ro reached 0.8%. Special algal parent materials in the saline lacustrine basin could be the primary cause that leads to generating low-maturity oil. Compared to the kerogen samples, the source rock was more prone to hydrocarbon generation via pyrolysis. The latter had a higher liquid hydrocarbon yield per unit organic matter and a lower activation energy; thus, it entered the hydrocarbon generation threshold earlier. The key factor for the aforementioned differences might be catalysis by minerals (particularly carbonate minerals) and the presence of soluble organic matter in the source rocks.

Origin of petroleum accumulation in the Chaheji-gaojiapu structural belt of the Baxian Sag, Bohai Bay Basin, China: Insights from biomarker and geological analyses

Abstract The origin of petroleum accumulations in the Chaheji-gaojiapu structural belt of the Baxian Sag, Bohai Bay Basin was evaluated based on the results of Rock-Eval pyrolysis on 135 samples and a biomarker analysis on 26 source rock samples and 33 oil samples. The four possible source rock intervals have different biomarker assemblages and were deposited in different environments. For example, pristane/phytane, C27-diasterane/C27-sterane and C19/C23-tricyclic terpane ratios show an increasing trend, but (C28+C29)-tricyclic terpane/[(C28+C29)-tricyclic terpane + Ts] and steranes/hopanes parameters display a decreasing trend among the lower part of the first member (Es1L) and the middle (Es3M) and lower part (Es3L) of the third member of the Eocene Shahejie Formation. This finding indicates that a decreasing anoxic and alkaline environment with an increasing input of terrigenous organic matter developed during the deposition of the Es1L, Es3M and Es3L source rocks. However, the biomarker parameters of the forth member (Es4) of the Eocene Shahejie Formation display highly similar characteristics with the Es3L source rocks, except for the C29-sterane 20S/(20S + 20R) and C27-diasterane/C27-sterane ratios, which show obviously higher than the other source rock values. The Es3-derived oils are absolutely dominant in the Chaheji-gaojipu structural belt, based on the biomarker analysis of saturated hydrocarbon, with slight or no contributions from the Es1L and Es4 source rocks because of the low maturation for the Es1L source rock and the absence of vertical migrating channels for the Es4 source rock. Faults are vital to petroleum migration and accumulation in this area and demonstrate the following double effects: (1) the deep faults cutting off the Es3 source rocks are the key channels for oil vertical migration; (2) the shallow faults entrap oil and finally form the commercial oil pools. As a result, the fault block reservoir is the major oil-reservoir type in the Chaheji-gaojiapu structural zone, and the lithologic reservoir is another oil-reservoir type in this area. Based on 1D burial and thermal history modelling, the E3 and E4 source rocks reached the oil-generation peak at approximately 5 Ma and 20 Ma, respectively, but the Es1L source rock is still in the low-mature stage. Therefore, the process of the large oil migration and accumulation for the Es3 source rocks occurred during the late Neogene-Quaternary. Through this research, there are two considerable exploration potentials for this area, including condensate reservoirs in the deep strata and low-mature reservoirs in the Es1 stratum interval.