Middle Permian Lucaogou Formation Research Articles

Influences of lithofacies on fluid mobility in mixed sedimentary rocks: Insights from NMR analysis of the middle Permian Lucaogou Formation, Junggar Basin

The multi-source mixed sedimentation resulted in a unique series of mixed fine-grained sedimentary rocks evolved within the Permian Lucaogou Formation in the Jimusar Sag, located in the southeastern Junggar Basin, China. The variety of lithofacies within this series resulted in pronounced heterogeneity of pore structures, complicating the analysis of fluid occurrence space and state within reservoirs. As a result, the impact of lithofacies on fluid mobility remains ambiguous. In this study, we employed qualitative methods, such as field emission scanning electron microscopy (FE-SEM) and thin section observation, and quantitative analyses, including X-ray diffraction (XRD), total organic carbon (TOC), vitrinite reflectance (Ro), high-pressure mercury intrusion (HPMI) porosimetry, and nuclear magnetic resonance (NMR), along with linear and grey correlation analyses. This approach helped delineate the effective pore characteristics and principal factors influencing movable fluids in the fine-grained mixed rocks of the Lucaogou Formation in the Jimusar Sag, Junggar Basin. The findings indicate the development of three fundamental lithologies within the Lucaogou Formation: fine sandstone, siltstone, and mudstone. Siltstones exhibit the highest movable fluid saturation (MFS), followed by fine sandstones and mudstones sequentially. Fluid mobility is predominantly governed by the content of brittle minerals, the sorting coefficient (Sc), effective pore connectivity (EPC), and the fractal dimension (D2). High content of brittle minerals favors the preservation of intergranular pores and the generation of microcracks, thus offering more occurrence space for movable fluids. A moderate Sc indicates the presence of larger connecting throats between pores, enhancing fluid mobility. Elevated EPC suggests more interconnected pore throat spaces, facilitating fluid movement. A higher D2 implies a more intricate effective pore structure, increasing the surface area of the rough pores and thereby impeding fluid mobility. Ultimately, this study developed a conceptual model that illustrates fluid distribution patterns across different reservoirs in the Lucaogou Formation, incorporating sedimentary contexts. This model also serves as a theoretical framework for assessing fluid mobility and devising engineering strategies for hydrocarbon exploitation in mixed fine-grained sedimentary rocks.

Hydrocarbon Source Rock Evaluation of the Lucaogou Shale in the Periphery of Bogeda Mountain (SE Junggar Basin, China) and Its Implications for Shale Oil Exploration: Insights from Organic Geochemistry, Petrology, and Kinetics Pyrolysis

Since the discovery of the vast Jimusaer shale oilfield in the southeastern Junggar Basin in 2012, there has been considerable interest in neighboring areas around Bogeda Mountain that have shale oil potential. The primary productive interval in the basin, the Middle Permian Lucaogou Formation (P2l), is well-developed in the areas of Qitai, Mulei, Shiqiantan, Chaiwopu, and Miquan. In this study, we conducted an assessment of the hydrocarbon generation potential of the P2l in these five areas and compared it with that of the P2l in the Jimusaer oilfield, which were determined by GC-MS, total organic carbon (TOC) and vitrinite reflectance (Ro) measurements, Rock-Eval pyrolysis, and organic petrology to investigate the type, origin, thermal maturity, hydrocarbon potential, and oil/gas proneness of organic matter in the P2l. Additionally, we applied open-system pyrolysis of hydrocarbon generation kinetics to explore differences in hydrocarbon generation and expulsion across various P2l mudstone/shale in the southeastern Junggar Basin. The findings of this study revealed that the P2l shale in Qitai and Miquan areas contains more abundant and lower thermally mature organic matter (early mature–mature stage), characterized by primarily Type II1–I kerogen, similar to that found in the P2l shale of the Jimusaer oilfield. Conversely, the P2l shale in Mulei, Shiqiantan, and Chaiwopu contains less abundant and more thermally mature organic matter (mainly mature–highly mature stage), dominated by Type II2–III kerogen. Consequently, shale in these areas is considerably less desirable for oil exploration compared to the Jimusaer shale. The semi-deep to deep lake facies in Miquan and Qitai exhibit the most promising exploration potential. This study can serve as a guide for shale oil exploration in the southeastern Junggar Basin.

Lithological and diagenetic variation of mixed depositional units in the middle permian saline lacustrine deposition, Junggar Basin, NW China

Mixed depositional reservoirs are widely distributed in Junggar Basin, NW China. These reservoirs are featured by complex lithology, ultra-low permeability and extremely heterogeneous pore structures that markedly impact field development efficiency. This study illustrated the distribution of lithological combinations within a single sand bar in the Middle Permian Lucaogou Formation, Jimsar Sag, Junggar Basin and revealed the diagenetic control on the reservoir quality of a single sand bar based on thin sections, scanning electron microscopy (SEM), helium porosity and air permeability measurement, pressure-controlled mercury injection (PMI), rate-controlled mercury injection (RMI), X-ray computed tomography (CT), and collected and published data. The results show that the heterogeneity of microscopic pore structures within a single sandbar is controlled by the distribution of various lithological combinations and different levels of diagenetic alteration. The results show that there are mainly three types of lithology developed in a single sand bar. The reservoir quality of dolomitic siltstone reservoirs is the best, followed by silty dolomite reservoirs. The reservoirs quality of dolomicrite reservoirs is the worst. Three main lithological combinations can be identified within a single sand bar. Controlled by climate, lake level fluctuation and provenance, lithological combination A, characterized by blended mixing of dolomitic siltstone, silty dolomite and dolomicrite, is mainly developed in the middle of single sand bar. Lithological combination B, characterized by saltatory mixing of interbedded dolomitic siltstone and dolomicrite, and lithological combination A developed on the side of the sand bar near the shallow lake, while lithological combination C with blended mixing of interbedded dolomitic siltstone and silty dolomite developed on the side near provenance. Strong compaction is the main factor of the decrease of reservoir quality of sand bar reservoirs. Carbonate cementation promotes the densification of reservoirs. The irregular flaky clay minerals lead to the exponential decline of permeability, and dissolution is the main kind of diagenesis to improve reservoir quality. The dolomitic siltstone reservoirs in the middle of a single sand bar have the best reservoir quality because the overlying dolomicrite layers resist compaction, resulted in a certain amount of primary pores remained. Besides, the dolomitic siltstone reservoirs are far from the sand-mudstone interfaces, which leads to the low carbonate cement content. Furthermore, abundant dissolution pores in dolomitic siltstone reservoirs improve the reservoir quality. These research results are crucial to reservoir evaluation and development in similar mixed depositional tight reservoirs.

Synergistic Evolution of Palaeoenvironment—Bionts and Hydrocarbon Generation of Permian Saline Lacustrine Source Rocks in Jimusar Sag, Junggar Basin

This study focused on Middle Permian Lucaogou Formation saline lake source rocks, utilizing a combination of biomarkers and hydrocarbon generation thermal simulation to analyze their biological compositions, depositional environments, and hydrocarbon generation potential. The Pr/Ph ratio, Ph/nC18 ratio, and Pr/nC17 ratio indicate that the Lucaogou Formation was in a reducing environment during the deposition period, and the lower part of the Lucaogou Formation (P2l1) is more anoxic than the upper part of the Lucaogou Formation (P2l2). The maturity index 20S (%) and ββ (%) reflect that the maturity of organic matter in the P2l1 is slightly higher than that in the P2l2. The G/H index and the ETR index indicate that the stratification of the water column is better during the sedimentary period of Lucaogou Formation and the salinity of the P2l1 is higher than that of the P2l2. The biomarker parameters of nC21−/nC22+, CPI, S/H, and C22T/C21T reflect that the organic matter of the source rocks have a higher abundance of bacteria and algae than higher plants, and the contents of bacteria are more than that of algae. The (7- + 8-MMAs)/Cmax and (C28 + C29 − St)/St parameters indicate that cyanobacteria accounted for a certain proportion of bacteria, and the algae are mainly green algae. The co-evolution of the sedimentary environment and the biological composition reflects the control of the sedimentary paleoenvironment on biological composition. According to the relative content of cyanobacteria, green algae, and Rhodophyta, the source rocks of the upper and lower Lucaogou Formation correspond to the low-salinity type (LS-type) and the high-salinity type (HS-type), respectively. Compared with LS-type source rocks, HS-type source rocks have greater generation potential of oil and weaker gas generation potential. This study is valuable for the accurate assessment of source rocks and holds significant practical implications for the exploration of oil and gas resources.

Effect of Petroleum Generation and Retention on Nanopore Structure Change in Laminated and Massive Shales-Insights from Hydrous Pyrolysis of Lacustrine Source Rocks from the Permian Lucaogou Formation.

The lacustrine shale of the Middle Permian Lucaogou Formation in the Jimusar Sag is the principal prospective unconventional target in the Junggar Basin. The effect of petroleum generation and retention on nanopore structure change during thermal maturity in lacustrine shale is still unclear. In this study, two laminated and two massive shale samples from the Permian Lucaogou Formation were selected to study this change by closed hydrous pyrolysis. The pyrolysis temperatures were 295, 320, 345, 370, and 400 °C, which cover from the mature to the post-mature stage. Total organic carbon (TOC), Rock-Eval, and low-pressure N2 adsorption tests on pyrolyzed shale samples before and after extractable organic matter (EOM) extraction were conducted systematically. The results indicate that (1) the petroleum generation on nanopore structure change is in stages. The peak nanopore volume expanding stage is the late oil window (R o = 0.9-1.35%). At the post-mature stage (R o > 1.35%), the mesopore volume decreased and the majority of the nanopore space is from macropores. (2) The presence of EOM decreased both mesopores and macropores in the peak oil window. (3) The organic-rich laminated shale generated more macropores than massive shale with increasing thermal maturity. The results of this study shed light on the dynamic effect of laminae fabric, petroleum generation, and retention on shale nanopore structure change across the oil window.

Evaluation of the tight oil “sweet spot” in the Middle Permian Lucaogou Formation (Jimusaer Sag, Junggar Basin, NW China): Insights from organic petrology and geochemistry

The Middle Permian Lucaogou Formation in the Jimusaer Sag is a hotspot for exploring and developing lacustrine tight oil in the Junggar Basin, NW China. In this study, we evaluated the tight oil potential from the perspective of source rock hydrocarbon generation and expulsion. Based on a detailed organic, petrological, and geochemical characterization of the target interval within the key well JHBE, kinetic experiments were performed on representative shale samples from the well JHBE and the threshold of hydrocarbon expulsion of the source rock was established. The findings of this study revealed that the Lucaogou Formation shales were deposited in a dysoxic to anoxic and clay-poor lacustrine environment with variable salinity and their organic matter (OM) was contributed from both lamalginite and telalginite with minor vitrinite and inertinite, resulting in good to excellent source rock potential. The lamalginite was deposited in water with low salinity, whereas the telalginite developed in water with relatively higher salinity. Although the greater contribution of lamalginite resulted in a higher OM content than that of telalginite, the former generated a lower amount of hydrocarbons than the latter, because telalginite is capable of generating hydrocarbons earlier than lamalginite, which is indicated by the higher HCI (Hydrocarbon index, S1 × 100/TOC), EOM/TOC (extraction of organic matter/total organic carbon), C29 ββ/(αα + ββ), and C29 ααα20S/(20S + 20R) sterane values for telalginite than for lamalginite and is also evident from the difference in activation energy distributions between lamalginite and telalginite source rocks. As determined from the relationships of sterane maturity parameters C29 ββ/(αα + ββ) and C29 ααα20S/(20S + 20R) vs EOM/TOC ratio and HCI values, the lower C29 ββ/(αα + ββ) and C29 ααα20S/(20S + 20R) limits for active source rocks were approximately 0.28 and 0.44, respectively.

Differences in geochemistry and hydrocarbon generation of source-rock samples dominated by telalginite and lamalginite: A case study on the Permian saline lacustrine source rocks in the Jimusaer Sag, NW China

The Middle Permian Lucaogou Formation in Northwestern China mainly contains typical saline lacustrine oil shale. To study the differences in geochemistry and hydrocarbon generation of source-rock samples in Lucaogou Formation in Jimusaer Sag, 57 core samples from two boreholes were analyzed herein by performing total organic carbon (TOC) analysis, Rock–Eval pyrolysis, and gas chromatographic–mass spectrometry experiments on saturated hydrocarbons. The kinetics of oil generation were studied using two samples comprising typical maceral components. The results showed that the hydrocarbons produced by telalginite are relatively rich in pristane (Pr), phytane (Ph), β -carotane, high-carbon normal alkanes, and C 29 regular steranes. Hydrocarbons produced by lamalginite contain a significantly higher content of C 20 tricyclic terpanes (TT), C 21 TT, C 24 tetracyclic terpanes (TeT), C 29 norhopane, and C 28 regular sterane. Based on the pyrolysis and biomarker compound parameters, telalginite has a higher conversion rate for hydrocarbons than lamalginite in the low-mature to mature stage, which is consistent with their kinetic analysis. Lamalginite source rock display a much narrower distribution of activation energies than telalginite source rocks; Such narrower activation energy distribution effectively narrows the main stage of hydrocarbon generation. In addition, the activation energy distribution of lamalginite is concentrated in the high-value interval, indicating the characteristics of the relatively lagging hydrocarbon generation of lamalginite.

Effect of lithofacies on differential movable fluid behaviors of saline lacustrine fine-grained mixed sedimentary sequences in the Jimusar sag, Junggar Basin, NW China: Forcing mechanisms and multi-scale models

The crude oil movability, acting as one of the controlling parameters for estimating how difficult it is to exploit petroleum resources from lacustrine shale reservoirs, had still not been investigated systematically, especially for the saline lacustrine fine-grained mixed sedimentary series. How to establish an effective assessment criterion which is suitable for deciphering shale oil movability classically, and how to probe the root cause of the differential movable fluid saturation in heterogeneous lithofacies ? In order to unravel the causal linkage and confirm a coupling relationship between lithofacies heterogeneity and hydrocarbon fluid flow activity for the saline lacustrine fine-grained mixed sedimentary series, we present a set of new results from organic geochemistry analysis, FE-SEM imaging, low-pressure N2 adsorption, high-pressure mercury injection intrusion experiments (MIP), and nuclear magnetic resonance (NMR) experiments from the targeted dessert reservoir of the saline lacustrine fine-grained mixed depositional systems of the Middle Permian Lucaogou Formation in the Jimusar Sag, Southern Junggar Basin, NW China. Our results demonstrated a relatively obvious triple-level distribution of the movable fluid-bearing capacity and also reservoir quality for the dominant five identified lithofacies. The sandy dolomitic sandstones and felsic siltstones act as the first-class categories mainly characterized by a highest total pore volume range and also pore volume within pore size range of 50–300 nm, as well as a relatively lowest D2 values defined as the pore-throat fractal dimension of movable fluid. Secondly, both the dolarenites and dolomitic mudstones are considered as the medium quality reservoirs, and particularly noteworthy is a development of the characteristic microfractures in dolomitic mudstones greatly facilitates the formation and widespread distribution of pore-fracture network systems, leading to a relatively better storage capacity for movable fluid as suggested by relatively higher pore volume and lower fractal dimension values compared with those of micritic dolomites suspected as the most unfavorable category. More importantly, a multi-scale detailed model is proposed to illustrate the potential linkage spanning a total of three different scales respectively as lithofacies associations, microscopic reservoir textures and architectures, as well as fluid occurrence behaviors. This work can be considered as an analogue to better understand the spatial distribution patterns of dessert reservoir intervals and differential recoverability of saline lacustrine mixed-source shale oil resources, and to provide further conceptual basis for optimization of development engineering technique schemes.

The Depositional Mechanism of Hydrothermal Chert Nodules in a Lacustrine Environment: A Case Study in the Middle Permian Lucaogou Formation, Junggar Basin, Northwest China

Although chert deposits are limited in geological distribution, their geological and geochemical characteristics can provide important information to reconstruct paleoenvironmental and diagenetic processes. For the Permian period, cherts are utilized to trace global silicon cycles and hydrothermal activities in relation to the Permian Chert Event. In Northwest China, Permian chert nodules have recently been discovered in both the southeastern and northwestern margins of the Junggar Basin. We conducted an analysis of the mineralogy, petrology and geochemistry of chert nodules of the Lucaogou Formation in the southeastern margin of the Junggar Basin to identify silicon sources and determine the precipitation mechanism of chert nodules. As evidenced by petrology, the chert nodules were mainly composed of crypto-microcrystalline silica (94.33% on average), with development of a soft-sediment deformation structure, indicating the synsedimentary deposition of silicon. Proven by trace elements, high Eu/Eu* ratios (average 2.14), low total rare earth element content (average 6.03 ppm), low LaN/YbN ratios (average 0.17) and low Y/Ho ratios (average 25.25) in chert nodules supports the hydrothermal source of silicon. The wide distribution of authigenic metal-bearing minerals and the significant positive Eu anomalies observed suggest that the chert depositions in the Lucaogou Formation intermittently received high-temperature (>250 °C) hydrothermal fluids, likely associated with the initiation of the Bogda Rift in the middle Permian. Following rapid cooling down and differential compaction, siliceous sediments dehydrated and deformed, finally forming chert nodules.

Microcrystalline dolomite in a middle Permian volcanic lake: Insights on primary dolomite formation in a non‐evaporitic environment

AbstractLacustrine dolomite nucleation commonly occurs in modern and Neogene evaporitic alkaline lakes. As a result, ancient lacustrine microcrystalline dolomite has been conventionally interpreted to be formed in evaporitic environments. This study, however, suggests a non‐evaporitic origin of dolomite precipitated in a volcanic–hydrothermal lake, where hydrothermal and volcanic processes interacted. The dolomite occurs in lacustrine fine‐grained sedimentary rocks in the middle Permian Lucaogou Formation in the Santanghu intracontinental rift basin, north‐west China. Dolostones are composed mainly of nano‐sized to micron‐sized dolomite with a euhedral to subhedral shape and a low degree of cation ordering, and are interlaminated and intercalated with tuffaceous shale. Non‐dolomite minerals, including quartz, alkaline feldspars, smectite and magnesite mix with the dolomite in various proportions. The 87Sr/86Sr ratios (0.704528 to 0.705372, average = 0.705004) and δ26Mg values (−0.89 to −0.24‰, average = −0.55‰) of dolostones are similar to those of mantle rocks, indicating that the precipitates mainly originated from fluids that migrated upward from the mantle and were subject to water–rock reactions at a great depth. The δ18O values (−3.1 to −22.7‰, average = −14.0‰) of the dolostones indicate hydrothermal influence. The trace and rare earth element concentrations suggest a saline, anoxic and volcanic–hydrothermally‐influenced subaqueous environment. In this subaqueous environment of Lucaogou lake, locally high temperatures and a supply of abundant Mg2+ from a deep source induced by volcanic–hydrothermal activity formed favourable chemical conditions for direct precipitation of primary dolomite. This study's findings deepen the understanding of the origin and processes of lacustrine primary dolomite formation and provide an alternative possibility for environmental interpretations of ancient dolostones.

Paleo-Environment Reconstruction of the Middle Permian Lucaogou Formation, Southeastern Junggar Basin, NW China: Implications for the Mechanism of Organic Matter Enrichment in Ancient Lake

Paleo-Environment Reconstruction of the Middle Permian Lucaogou Formation, Southeastern Junggar Basin, NW China: Implications for the Mechanism of Organic Matter Enrichment in Ancient Lake

Differences in Imbibition Efficiency Between Bedding and Tectonic Fractures in the Lucaogou Formation in the Jimusar Sag: Evidence From Simulation Experiments

In addition to tectonic fractures, the ubiquitous and important bedding fractures are often overlooked in the imbibition processes. Based on the identification of different fracture types in tight sandstone from the middle Permian Lucaogou Formation in the Jimusar Sag, NW China, spontaneous imbibition of water experiments was carried out, aiming to investigate the effect of tectonic and bedding fractures on hydrocarbon recovery efficiency in tight reservoir rocks. The results show that the difference in connectivity between the vertical (tectonic fractures) and horizontal (bedding fractures) directions of pores and pore throats is the main reason for the distinction in microscale permeability and, hence, fluid imbibition. The difference in the time required for imbibition between bedding and tectonic fractures is subtle, but the former is slightly lower than the latter in terms of imbibition depth and efficiency. Higher temperatures can shorten the reaction time of imbibition. On the basis that the tectonic fractures are opened, opening the same scale of bedding can increase the imbibition effectiveness by 8.7% at reservoir conditions (27 MPa, 80°C), and the closer to the fracture, the higher the imbibition efficiency in the matrix. The samples with higher porosity and permeability per unit volume have high imbibition oil recovery. However, the imbibition efficiency of the samples with lower porosity and permeability is higher, i.e., the final imbibition efficiency is inversely proportional to porosity and permeability.

Geochemistry of Middle Permian lacustrine shales in the Jimusar Sag, Junggar Basin, NW China: Implications for hydrothermal activity and organic matter enrichment

Organic matter enrichment in black shales has frequently been linked with hydrothermal activity. However, it is poorly understood how hydrothermal activity affected paleo-environments and the enrichment of organic matter in ancient lacustrine sediments. The hydrothermal activity recorded in the Middle Permian Lucaogou Formation (LCGF) in the Jimusar Sag, Junggar Basin, Northwest China, provides an ideal sedimentary archive to explore this issue. The mineralogical composition, major and trace element, total organic carbon, and sulfur content of forty-three shale samples of the LCGF were investigated to better understand how hydrothermal activity influences organic matter enrichment in lacustrine shales. The terrigenous influx, redox conditions, paleoproductivity, hydrothermal effects, and organic matter enrichment mechanisms are discussed. The results show that the terrigenous influx is insufficient overall, and the lower section of the LCGF has less detrital materials than the upper section. The redox conditions of the bottom water of the lake in the Jimusar Sag during deposition of the LCGF were suboxic to anoxic, which, combined with relatively high paleoproductivity, is favorable for organic matter enrichment. Hydrothermal activity occurred more frequently during deposition of the lower section compared with the upper section of LCGF. Hydrothermal fluids provided sufficient nutrients to enhance organic matter production during deposition of the LCGF and resulted in organic matter enrichment.

Coarse calcite grains in hydrocarbon-rich tuffaceous mudstone in a Permian volcanic lake—An indicator for subaqueous volcanic deposition and related hydrocarbon generation

Sediments originated from volcanic-hydrothermal activities show clear relationship with hydrocarbon generation and accumulation in fine-grained sedimentary rocks at a regional scale. However, the fundamental processes and mechanisms of this relationship are still poorly understood at a microscopic scale to hamper efficient hydrocarbon exploration. In addition, coarse grains common in volcanic lacustrine fine-grained sediments have been traditionally interpreted as diagenetic products during early and late burial. Our study, however, suggests that coarse calcite grains are not diagenetic, but formed by subaqueous explosion. The calcite grains occur in lacustrine fine-grained sedimentary rocks in the middle Permian Lucaogou Formation in the Santanghu intracontinental rift basin, NW China. They are sand sized, angular, mono- and polycrystalline, and discretely scatter in silt to clay-sized non-calcite pyroclastic matrix. The calcite grains and the mud matrix are interpreted as a pyroclastic debris flow deposit. In-situ 87Sr/86Sr ratios (0.704932–0.705840, and 0.705137 on average) of the calcite grains are similar to those of mantle rocks, indicating an origin closely related to mantle-derived fluids. The in-situ δ13C values (4.24‰ to 5.06‰, and 4.57‰ on average) and extremly negative δ18O values (−21.70‰ to −21.18‰, and − 21.49‰ on average) indicate that extensive hydrothermal alteration and microbial activities influenced the formation of the calcite. Element composition and homogenizaiton temperature of fluid incusions of calcite and biomarkers of the mudstone all suggest a relatively low-temperature hydrothermal condition in a saline and reducing environment. Furthermore, hydrocarbon-rich inclusions are rich in the calcite grains and have two modes of occurrence and maturity. They indicate that the organic matter was generated from algae bloom, which was caused by volcanic-hydrothermal activities. Alternatively, the hydrocarbon may have an inorganic origin originated from a mantle source. Our findings provide an alternative interpretation on the origin of coarse calcite grains in fine-grained sedimentary rocks and demonstrate a microscopic relationship between volcanic-hydrothermal activities and hydrocarbon generation and accumulation in a tectonic active rift basin.

Geochemical Characteristics and Process of Hydrocarbon Generation Evolution of the Lucaogou Formation Shale, Jimsar Depression, Junggar Basin

Lacustrine shale, represented by the Middle Permian Lucaogou Formation in the Jimsar Depression in the eastern Junggar Basin, has become one of the main areas of shale oil exploration in China. In this study, we used 137 samples of shale from the Lucaogou Formation, drawn from 14 wells in the Jimsar Depression, to investigate their characteristics of pyrolysis, organic carbon and soluble organic matter content, biomarkers, organic microscopic composition, and vitrinite reflectance. Basin simulation and hydrocarbon generation thermal simulation experiments were also conducted in a closed system. The results of this study indicate that the input of an algae source was dominant in the source rocks of the Lucaogou Formation, that the water in which the rocks were deposited had high salinity and strong reducibility, and that the source rocks were oil-prone. The Lucaogou source rocks generally had good hydrocarbon generation capability, but showed significant heterogeneity. At the end of the Cretaceous period, the shales in the Lucaogou Formation entered the oil-generation window as a whole. Currently, the shales of the Lucaogou Formation are generally in the high-maturity stage in the deep part of the depression, producing a large amount of high-maturity oil and condensate gas, while those in the shallow part have relatively low maturity and can only produce a large amount of conventional crude oil. The maximum crude oil generation rate of the Lucaogou Formation shale obtained from the thermal simulation results was 220.2 mg/g of the total organic carbon (TOC), and the maximum hydrocarbon expulsion efficiency was estimated to be 59.3–76.4%.

Mechanism for the formation of natural fractures and their effects on shale oil accumulation in Junggar Basin, NW China

Despite the importance of natural fractures in shales, few techniques are currently available to examine their formation mechanism and effects on shale oil accumulation directly. The Middle Permian Lucaogou Formation in the Jimsar Sag of the Junggar Basin is a typical fractured shale. The Jimsar Sag has experienced multiple phases of tectonic movement since the Paleozoic Era, forming numerous natural fractures. However, controversy still surrounds the timing of formation, genetic mechanisms, and effects on shale oil accumulation of these fractures. Based on C-O-Nd isotopes, rare earth elements, fluid inclusions, SmNd isochron dating of fracture cements, rock acoustic emission, and triaxial rheological experiment, this study precisely define the fracture formation time, trace the provenances of fluids filling the fractures, elucidate the genetic mechanism of fractures and their effects on shale oil accumulation. The results suggest that these fractures were mainly formed in the Late Permian, Late Jurassic, and Early Cretaceous. Palaeotectonic stress field simulations further show that the tectonic movements in the Late Permian formed nearly N-S-trending fractures due to severe north-south extrusion. High-temperature fluids originating from the post-collision mantle intruded the Lucaogou Formation between ~258.1 Ma and ~ 257.6 Ma. Subsequently, tectonic movements in the Tianshan Mountains during the Late Jurassic resulted in extrusion from southwest to northeast, forming NEE-trending fractures. During this tectonic event, basin fluids intruded the Lucaogou Formation between ~150.2 Ma and ~ 146.5 Ma. The persistent fold rollback that occurred in the Bogda piedmont foreland sag in the Early Cretaceous subjected the Jimsar Sag to severe extrusion, forming NNW-trending fractures. This tectonic event influenced the intrusion of basin fluids into the Lucaogou Formation between ~123.0 Ma and ~ 120.3 Ma. The Late Jurassic and Early Cretaceous fractures coincided with the peak of oil generation and expulsion in the Lucaogou shale and created conduits and reservoir volumes for the migration and accumulation of shale oil. Our study introduces a new approach that integrates geophysical and geochemical data to investigate the formation processes of fracture and their impact on shale oil accumulation.

Petrological and organic geochemical characteristics of the Permian Lucaogou Formation in the Jimsar Sag, Junggar Basin, NW China: Implications on the relationship between hydrocarbon accumulation and volcanic-hydrothermal activities

The coupling relationship between volcanic-hydrothermal activities and hydrocarbon generation has been a common understanding in recent years. However, there is a lack of theoretical and practical guidance in the application hydrocarbon production. The fine-grained sedimentary rocks of the middle Permian Lucaogou Formation in the Jimsar Sag in the Junggar Basin, NW China, are critical source rocks and have been simplify regarded as a mudrocks for target for tight oil exploration. Recently studies show that rocks were deposited in an intracontinental lacustrine environment with multiple volcanic-hydrothermal activities. Their complex micro-scale lithological characteristics restrict the understanding and further exploration and development of tight oil. Hence, this study focuses on the detailed lithological and their organic geochemical characteristics of the Lucaogou Formation using microscopic thin-section observation, scanning electron microscopy, X-ray diffraction, Rock-Eval, saturated hydrocarbon chromatography and mass spectrometry. The following key conclusions were obtained. First, the massive mudrocks are sub-divided into lithofacies for hydrocarbon generation, including tuffaceous shale, calcareous shale and dolomitic shale, and, microinterval reservoirs, including dolostone and sedimentary tuff. Second, the origins of organic matter in Lucaogou Formation were mainly plankton, bacteria with minor of land plants. The depositional environment is high salinity and anoxic conditions. Materials derived from volcanic-hydrothermal activities, lacustrine precipitation, and terrestrial transportation mixed with various proportions to form this formation. Third, a mixture of carbonate minerals, felsic minerals and clay minerals is the best source rock, and the higher the mixture, the better. This research not only provides insight into the influence of volcanic-hydrothermal activities on lithology and organic matter enrichment, but also providing a potential scheme for the tight oil exploration and development in similar rifted geological settings. • A mixture lithology of carbonate minerals, felsic minerals and clay minerals is the best source rock. • The better the mixture of carbonate minerals, feldspar minerals and clay minerals, the better the quality of hydrocarbon source rock. • Volcanic-hydrothermal activities are favorable to the enrichment and preservation of organic matter. • The Lucaogou formation was formed in a high salinity and anoxic condition, and the biological source was mainly plankton.

Absence of β-carotane as proxies of hydrothermal activity in brackish lacustrine sediments

β -Carotene, the precursor of β -carotane, is commonly rich in brackish sediments and would become unstable theoretically under the exposure to high temperatures, bright light conditions, and acidic or oxdizing environments. However, the specific process and mechanism of the absence of such compounds are still not clear sufficiently. To fill the knowledge gap, in this study we explore the origin of the absence of β -carotane in a 50-m-thick section of the Middle Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin, northwest China, based on a combined mineralogical, petrographic, organic and inorganic geochemical study. Biomarker analyses indicate that the β -carotane-free section was deposited in a brackish, reducing, and semi-deep lacustrine environment. Hydrothermal fluid effects during sediment deposition are inferred to have caused the breakdown of β -carotene. The hydrothermal (<100 °C) effects are evidenced by mineralogy and geochemistry, e.g., the hydrothermal-indicative minerals (reedmergnerit, tuff and pyrite), high Fe/Ti and low Al/(Al + Fe) ratios, rare earth elements (REEs) distribution. Associated biomarker variations were found, e.g., low i -alkanes/ n -alkanes, high C 24 Tetracyclic/C 26 Tricyclic terpanes, high (C 19 + C 20 )/(C 21 + C 23 ) tricyclic terpanes, high C 29 /C 30 hopanes, and very low gammacerane/C 30 hopane ratios. Our data suggest that hydrothermal fluids can prevent the preservation of β -carotene, and the absence of β -carotane indicates a period of hydrothermal activity during deposition of the Lucaogou Formation in a 50-meter-thick sequence. • β -Carotanes were found to be absent in a brackish lacustrine sequence. • Hydrothermal fluids have caused the breakdown of β -carotene. • Anomalously low abundance of isomerization alkanes and high C 24 Tet/C 26 TT ratios were also found. • Hydrothermal activity was present in the Permian Lucaogou Formation, Junggar Basin. • Absence of β -carotane can be used as a proxy for hydrothermal activity.

Geochemical evaluation of produced petroleum from the Middle Permian Lucaogou reservoirs Junggar Basin and its implication for the unconventional shale oil play

The shale of the Middle Permian Lucaogou Formation in the Jimusaer Sag has become one of the most important prospects for unconventional shale oil production in China. The Lucaogou Formation develops two sweet spots, the upper (P2l22) and lower (P2l12) sweet spots, which are not more than 150 m apart. Interestingly, oils produced from the lower sweet spots have higher densities and viscosities than those in the upper sweet spots. Meanwhile, these high-viscosity oils have brought technical challenges to shale oil production. In this paper, geochemical data from 78 source rock solvent extracts and 17 oil samples from various depths were employed to elucidate unconventional petroleum systems and the reason for high viscosities. The source rocks of the Lucaogou Formation are good to excellent, primarily type II and II-III kerogens, and are now in the early-peak oil window. Oils produced from the P2l12 interval are characterized by abundant polar compounds, which are derived from the P2l12 and P2l11 intervals source rocks with relatively high marine sources contribution. Oils in the P2l22 interval are generated from the P2l22 interval source rocks with relatively high terrigenous-marine sources contribution. Terpane biomarker ratios indicate all oil samples are derived from lacustrine source rocks. The distributions of terpanes and steranes in these samples are similar, with high quantities of C29 steranes and low amounts of diasteranes. Migration from sources to reservoirs in the Lucaogou shale oil systems is short-distance given thermal maturity considerations. All oil samples have experienced slight biodegradation (PM level 1–2) at most. The sources and thermal maturities are the primary reasons for the high viscosities of the crude oils produced from the P2l12 interval. This research also indicates the Lucaogou shale petroleum system could be divided into two subsystems, and the production of different unconventional petroleum subsystems requires different strategies.

Pore-throat structure characteristics of tight reservoirs of the Middle Permian Lucaogou formation in the Jimsar Sag, Junggar Basin, northwest China

The Middle Permian Lucaogou Formation in the Junggar Basin, China is a typical tight oil reservoir. Thin sections, helium porosity and permeability, X-ray diffraction, X-ray computed tomography, and focused ion beam scanning electron microscopy were used to analyze 11 core samples with different oiliness from the Lucaogou Formation. Muddy siltstone, tuffaceous siltstone, dolomitic siltstone, and sandy dolarenite are the primary rock types in the Lucaogou tight oil reservoirs with different oil saturations. The dolomite content of the Lucaogou tight reservoir is positively correlated with porosity and permeability, suggesting that dolomite has an important influence on reservoir quality. Sandy dolarenite and dolomitic siltstone are characterized by a high dolomite content with the largest pore and throat radii and have a higher pore-throat network connection ratio with higher permeability and better oiliness. Thus, the quality of the Lucaogou tight reservoir is primarily controlled by the dolomite content.