Lower Permian Fengcheng Formation Research Articles

Natural gas exploration potential and favorable targets of Permian Fengcheng Formation in the western Central Depression of Junggar Basin, NW China

Based on the organic geochemical data and the molecular and stable carbon isotopic compositions of natural gas of the Lower Permian Fengcheng Formation in the western Central Depression of Junggar Basin, combined with sedimentary environment analysis and hydrocarbon-generating simulation, the gas-generating potential of the Fengcheng source rock is evaluated, the distribution of large-scale effective source kitchen is described, the genetic types of natural gas are clarified, and four types of favorable exploration targets are selected. The results show that: (1) The Fengcheng Formation is a set of oil-prone source rocks, and the retained liquid hydrocarbon is conducive to late cracking into gas, with characteristics of high gas-generating potential and late accumulation; (2) The maximum thickness of Fengcheng source rock reaches 900 m. The source rock has entered the main gas-generating stage in Penyijingxi and Shawan sags, and the area with gas-generating intensity greater than 20×108 m3/km2 is approximately 6 500 km2. (3) Around the western Central Depression, highly mature oil-type gas with light carbon isotope composition was identified to be derived from the Fengcheng source rocks mainly, while the rest was coal-derived gas from the Carboniferous source rock; (4) Four types of favorable exploration targets with exploration potential were developed in the western Central Depression which are structural traps neighboring to the source, stratigraphic traps neighboring to the source, shale-gas type within the source, and structural traps within the source. Great attention should be paid to these targets.

Tracing Oils and Gas Accumulation in Complex Petroleum Systems Based on Biomarkers, Light Hydrocarbons, and Diamondoids from Concomitant Crude Oils: A Case Study in the Shawan Sag of the Junggar Basin, NW China.

The Shawan Sag is one of the most promising blocks in the western Junggar Basin for natural gas and oil exploration. To date, the research and exploration degree here is still low, and there is limited understanding of the origin and migration of natural gas and oils, which seriously restricts future exploration and development. At least three oil charging episodes and one gas charging episode could be confirmed in the reservoirs on the western slope of the Shawan Sag based on the analysis of concentrations and ratios of hopanes and steranes, diamondoids, and light hydrocarbons. The first and second charging oils originated from the peak and late oil generation window stages of Lower Permian Fengcheng Formation (P1f) source rocks, respectively. The third charging oil originated from the late oil generation window stage of source rocks of the Middle Permian lower Wuerhe Formation (P2w). Combining the carbon isotopic compositions and the gas composition, the fourth charging gas was derived from the postmature source rocks of Carboniferous(C) and Lower Permian Jiamuhe Formation (P1j) and the high-postmature source rock of Lower Permian Fengcheng Formation (P1f). The preservation conditions of nature gas in the Mesozoic (T-K) cap rocks in the Shawan Sag, as indicated by methyladamantanes (MAs)/methyldiamantanes (MDs) ratio, are better than those in the Mahu Sag. In this study, the methyladamantane maturity parameters of oils in the reservoirs were proposed to evaluate the maturity and further determine the migration direction of late-charging natural gas. The results suggest that late postmatured natural gases were charged from the southern part to the northwest of the Shawan Sag. Therefore, the southeastern direction of Well SP1 is a favorable area for natural gas exploration in the Shawan Sag. Meanwhile the northern part of the sag is favorable for the exploration of high-mature oil generated from source rocks within the Middle Permian Lower Wuerhe Formation.

Heterogeneity and Sedimentary Characteristics of Shale Laminae of Fine-Grained Sediments in Alkaline Lacustrine Strata in the Permian Fengcheng Formation, Mahu Sag, NW China

The heterogeneity, sedimentary characteristics, and distribution of fine-grained sediments of millimeter-scale laminae were studied in alkaline lacustrine strata in the lower Permian Fengcheng Formation (P1f2) in the Mahu Sag to provide a sound theoretical basis for shale oil exploration and development. They were investigated by using core and thin-section observation, geochemical and elemental analysis, mineral identification, and pore examination. Five types of laminae were distinguished, with different mineral compositions, pore characteristics, and distributions. These laminae occur in four distinct combinations. The pore systems predominantly consist of intercrystalline pores, intragranular pores, and dissolution pores of feldspar and dolomite and also contain imporous alkaline mineral particles. Influenced by variations in salinity, the influx of volcanic–hydrothermal material, and the participation of both endogenous and exogenous materials, the formation has gone through five stages of sedimentary evolution. Furthermore, felsic laminae (FQL), dolomite laminae (DOL)/reedmergnerite laminae (RL)/shortite laminae (SL), and chert laminae (CL) developed in single-terrestrial-source still water deposition, hybrid-source still water deposition, and single-intrasource deposition, respectively. Vertically, the FQL-DOL combination shows favorable reservoir characteristics and is the most extensively developed lamina combination in the Fengcheng Formation that primarily developed in the period of the late Feng 1 Member (P1f1), the early Feng 2 Member (P1f2), the late Feng 2 Member (P1f2), and the early Feng 3 Member (P1f3). The FQL-RL/SL combination primarily developed in the period of the middle P1f2, and the DOL-CL combination is the counterpart in the period of the early P1f2 and the late P1f2 stages. Considering this in conjunction with the longitudinal distribution of lamina combinations, a model is proposed for the distribution of fine-grained sediments in alkaline lacustrine strata.

Identification and evaluation of shale oil micro-migration and its petroleum geological significance

Taking the Lower Permian Fengcheng Formation shale in Mahu Sag of Junggar Basin, NW China, as an example, core observation, test analysis, geological analysis and numerical simulation were applied to identify the shale oil micro-migration phenomenon. The hydrocarbon micro-migration in shale oil was quantitatively evaluated and verified by a self-created hydrocarbon expulsion potential method, and the petroleum geological significance of shale oil micro-migration evaluation was determined. Results show that significant micro-migration can be recognized between the organic-rich lamina and organic-poor lamina. The organic-rich lamina has strong hydrocarbon generation ability. The heavy components of hydrocarbon preferentially retained by kerogen swelling or adsorption, while the light components of hydrocarbon were migrated and accumulated to the interbedded felsic or carbonate organic-poor laminae as free oil. About 69% of the Fengcheng Formation shale samples in Well MY1 exhibit hydrocarbon charging phenomenon, while 31% of those exhibit hydrocarbon expulsion phenomenon. The reliability of the micro-migration evaluation results was verified by combining the group components based on the geochromatography effect, two-dimension nuclear magnetic resonance analysis, and the geochemical behavior of inorganic manganese elements in the process of hydrocarbon migration. Micro-migration is a bridge connecting the hydrocarbon accumulation elements in shale formations, which reflects the whole process of shale oil generation, expulsion and accumulation, and controls the content and composition of shale oil. The identification and evaluation of shale oil micro-migration will provide new perspectives for dynamically differential enrichment mechanism of shale oil and establishing a “multi-peak model in oil generation” of shale.

Constraining multi-stage and protracted oil generation of alkaline lacustrine source rocks by heteroatomic compounds

It is unclear whether source rocks deposited in ancient alkaline lakes undergo multiple stages of hydrocarbon generation, particularly of oils. In this study, the lower Permian Fengcheng Formation in the Junggar Basin, China, was investigated using the geochemistry of heteroatomic species, with a focus on N-containing compounds, by negative-ion electrospray high-resolution Fourier transform ion cyclotron resonance mass spectrometry. The maturity parameters based on N-containing compounds (e.g., C0–5/C15–35-DBE 12 N1 and C0–5/C15–35-DBE 15 N1) and aromatic hydrocarbons (TMNr) reveal progressive generation of mature to highly mature crude oils. The polymerization index P1 ([DBE 18 + DBE 15]/DBE 12_N1), alkylation index R1 (RC6−35/RC0−5), and paleo-salinity index (β carotane/nCmax) of the Fengcheng Formation source rocks suggest that the high salinity inhibited molecular polymerization of organic matter and prolonged the oil generation peak, which produced highly mature crude oils in the late stages of hydrocarbon generation. Therefore, alkaline lacustrine source rocks undergo multiple stages of protracted oil generation, which results in a longer oil window and higher threshold for gas generation than in the case of the classic Tissot model. The results highlight the oil generation potential of deep basins containing alkaline lacustrine source rocks and highly mature organic matter, which also have a lower gas-generation potential than previously thought.

Petroleum geochemistry and origin of shallow-buried saline lacustrine oils in the slope zone of the Mahu sag, Junggar Basin, NW China

Recently, significant oil discoveries have been made in the shallower pay zones of the Jurassic Badaowan Formation (J1b) in the Mahu Sag, Junggar Basin, Northwest China. However, little work has been done on the geochemical characteristics and origins of the oil in the J1b reservoir. This study analyzes 44 oil and 14 source rock samples from the area in order to reveal their organic geochemical characteristics and the origins of the oils. The J1b oils are characterized by a low Pr/Ph ratio and high β-carotene and gammacerane indices, which indicate that they were mainly generated from source rocks deposited in a hypersaline environment. The oils are also extremely enhanced in C29 regular steranes, possibly derived from halophilic algae. Oil-source correlation shows that the oils were derived from the Lower Permian Fengcheng Formation (P1f) source rocks, which were deposited in a strongly stratified and highly saline water column with a predominance of algal/bacterial input in the organic matter. The source rocks of the Middle Permian lower-Wuerhe Formation (P2w), which were deposited in fresh to slightly saline water conditions with a greater input of terrigenous organic matter, make only a minor contribution to the J1b oils. The reconstruction of the oil accumulation process shows that the J1b oil reservoir may have been twice charged during Late Jurassic–Early Cretaceous and the Paleogene–Neogene, respectively. A large amount volume of hydrocarbons generated in the P1f source rock and leaked from T1b oil reservoirs migrated along faults connecting source beds and shallow-buried secondary faults into Jurassic traps, resulting in large-scale accumulations in J1b. These results are crucial for understanding the petroleum system of the Mahu Sag and will provide valuable guidance for petroleum exploration in the shallower formations in the slope area of the sag.

Paleo-Sedimentary Environments and Controlling Factors for Enrichment of Organic Matter in Alkaline Lake Sediments: A Case Study of the Lower Permian Fengcheng Formation in Well F7 at the Western Slope of Mahu Sag, Junggar Basin

The Lower Permian Fengcheng formation is a significant source of rocks and a reservoir for the Mahu Sag in the Junggar Basin. Recently, the paleo-environment deposition factors of the P1f formation have become a popular research topic. This research was conducted using data from the F7 well of Mahu Sag, based on the geochemical analysis results of TOC (total organic carbon), REE (rare earth elements), and major and trace elements of 53 samples from P1f (Lower Permian Fengcheng formation), and some deposition factors are discussed. The P1f deposition process was classified into four stages based on paleo-environment elemental indicators. This research describes the deposition process of the evolution of alkaline lakes. The early and preliminary stages of alkali lake evolution are considered as late P1f1 to middle P1f2; the paleoclimate of this process was dry, the reduction conditions increased, and the paleo-productivity and lake salinity were enhanced. The terminal stage of alkali lake evolution is considered as late P1f2 to middle P1f3; in this period, the paleoclimate changed with seasonal cycles, resulting in a decrease in water salinity and an increase in oxidation; the paleo-productivity of the alkaline lake was at a medium level. Until the end of P1f3, the salinity of the lake decreased, and the water body became anoxic and weakly alkaline. Furthermore, the research on TOC and sedimentary parameters confirmed that the deposition of P1f organic matter is affected by multiple types of factors. A relatively warm climate, lack of oxygen, fresh water–brackish water, suitable debris flow, and high primary productivity conditions promoted organic matter deposition.

Controlling factors and models of shale oil enrichment in Lower Permian Fengcheng Formation, Mahu Sag, Junggar Basin, NW China

Based on the combination of core observation, experimental analysis and testingand geological analysis, the main controlling factors of shale oil enrichment in the Lower Permian Fengcheng Formation in the Mahu Sag of the Junggar Basin are clarified, and a shale oil enrichment model is established. The results show that the enrichment of shale oil in the Fengcheng Formation in the Mahu Sag is controlled by the organic abundance, organic type, reservoir capacity and the amount of migration hydrocarbon in shale. The abundance of organic matter provides the material basis for shale oil enrichment, and the shales containing types I and II organic matters have good oil content. The reservoir capacity controls shale oil enrichment. Macropores are the main space for shale oil enrichment in the Fengcheng Formation, and pore size and fracture scale directly control the degree of shale oil enrichment. The migration of hydrocarbons in shale affects shale oil enrichment. The shale that has expelled hydrocarbons has poor oil content, while the shale that has received hydrocarbons migrated from other strata has good oil content. Lithofacies reflect the hydrocarbon generation and storage capacity comprehensively. The laminated felsic shale, laminated lime-dolomitic shale and thick-layered felsic shale have good oil content, and they are favorable lithofacies for shale oil enrichment. Under the control of these factors, relative migration of hydrocarbons occurred within the Fengcheng shale, which leads to the the difference in the enrichment process of shale oil. Accordingly, the enrichment mode of shale oil in Fengcheng Formation is established as “in-situ enrichment” and “migration enrichment”. By superimposing favorable lithofacies and main controlling factors of enrichment, the sweet spot of shale oil in the Fengcheng Formation can be selected which has great significance for the exploration and development of shale oil.

Cyclostratigraphy of Lower Permian alkaline lacustrine deposits in the Mahu Sag, Junggar basin and its stratigraphic implication

The Lower Permian Fengcheng Formation of the Mahu Sag is one of the most potentially petroliferous sequences in China, and its unique alkaline lacustrine deposits provide important information on the paleoclimate and paleoenvironment of the early Permian. However, because of the complexity of the heterogeneous lithology and sedimentary facies in lacustrine deposits, the lateral correlation of lithofacies becomes challenging. Using cyclostratigraphy, we conducted a detailed astronomical cycle analysis of the Lower Permian Fengcheng Formation in the northern Mahu Sag, established an astronomical time scale, and constructed an isochronous sedimentary framework by collating the cycles of the different wells. Nine 405-kyr long-eccentricity cycles in the Fengcheng Formation were identified, and absolute astronomical time scales were established with the anchored point at ∼300 Ma in the Lower member of the Fengcheng Formation. Based on the identification of lithofacies, the spatio-temporal variation in the lithofacies within the Fengcheng Formation was reconstructed. The astronomical time scale has proven to be reliable, and the lithofacies distribution within the isochronal framework is effective for investigating the spatial variation of lithofacies in alkaline lacustrine deposits. Favorable dolomitic mudstones developed in the central and transitional zones, corresponding to the three long-eccentricity cycles in the middle member of the Fengcheng Formation. Tuffaceous mudstones with high potential mainly developed in the lower member of the Fengcheng Formation in the marginal zone of the Mahu Sag. This study demonstrates an approach that can be used to study lithofacies in lacustrine deposits.

Revealing continuous hydrocarbon generation and shale oil accumulation of saline lacustrine source rocks by sequential extraction

Saline lacustrine fine-grained rocks are important hydrocarbon source rocks and host large shale oil accumulations worldwide. However, the specific process and mechanism of hydrocarbon generation and accumulation in such rocks are not clear. Sequential extraction is a method to chemically separate different hydrocarbon fractions in sedimentary rocks, commonly as free, adsorbed and retained hydrocarbons, and thus can theoretically record the entire process of hydrocarbon generation and accumulation. To test this hypothesis, here sequential extraction was carried out on source rock samples with varying maturities from the alkaline lacustrine strata of the Lower Permian Fengcheng Formation, Mahu Sag, Junggar Basin, northwest China. The 17β-ethylandrostane/C29-sterane ααα20R and tricyclic terpane/pentacyclic triterpane ratios vary with maturity. In contrast, some biomarker parameters are not significantly affected by thermal maturity, including pristane/phytane, β-carotane index (β-carotane/main peak n-alkane), sterane/hopane, 7-+8-methyl index (7-+8-monomethyl heptadecane/main peak n-alkane), and C28/C29 regular sterane values, and can thus be used for oil–source rock correlations. The low-maturity source rocks have undergone one stage of hydrocarbon generation and associated shale oil accumulation, whereas source rocks with relatively high maturity have experienced multi-stage hydrocarbon generation and associated shale oil accumulation. In the early stages, cyanobacteria were the main bioprecursor to hydrocarbons and, with increasing maturity, Dunaliella gradually became the more important bioprecursor. Hydrocarbon generation by these alkaline lacustrine source rocks involved multiple sources and stages, which accounts for the abundant shale oil resources in the Mahu Sag with one-billion-ton reserves. This might be general for saline lacustrine source rocks worldwide, which can generate both conventional and unconventional hydrocarbons in large scales. Compared with conventional thermal modeling to constrain the generation history of hydrocarbon source rocks, a unique advantage of sequential extraction is its ability to characterize the entire history of hydrocarbon generation under geological conditions.

Distribution pattern of natural fractures in lacustrine shales: a case study of the Fengcheng formation in the Mahu Sag of the Junggar Basin, China

The Lower Permian Fengcheng Formation in the Mahu Sag develops a set of organic-rich alkaline lacustrine shale strata, which is a key area for shale oil exploration and development. As an important storage space and seepage channel for shale reservoirs, natural fractures have an impact on shale oil enrichment, production and development effect. In this study, the types and characteristics of natural fractures were first analyzed using core, thin section and imaging logging data. On this basis, combined with the distribution of fractures in single wells, the vertical distribution law of fractures is discussed. Finally, the planar distribution of fractures is evaluated using different seismic attributes such as coherence, curvature, likelihood, and AVAz. The results showed that three types of fractures are existed, including transformational shear fractures, intraformational open fractures and bed-parallel shear fractures, with intraformational open fractures being the most developed. The development degree of fractures in different layers has obvious differences, mainly controlled by lithology and brittle mineral content. The basalt and tuff are developed in the Feng 1 Member, with low carbonate mineral content, resulting in a relatively low degree of fracture development. The dolomite and argillaceous dolomite are developed in the Feng 2 Member and the Feng 3 Member, with high carbonate mineral content and brittleness, resulting in a high degree of fracture development. Additionally, the closer to the fault, the higher the degree of fracture development. On the plane, the fracture zone develops near the main and secondary faults, with the trend mainly oriented in the E-W direction and approximately parallel to the direction of the faults. The width of the fracture zone is largest in the central and southern part of the study area. These fractures are fault-related and are caused by regional stress fields resulting from the activity of the main-secondary faults.

Geneses of multi-stage carbonate minerals and their control on reservoir physical properties of dolomitic shales

The formation and distribution of carbonate minerals have important influences on reservoir development. The dissolution-precipitation effect plays an important role in controlling the reformation of reservoir physical properties. High-quality source rocks with abundant reserves exist in the Lower Permian Fengcheng Formation, Junggar Basin, China, and the basin reservoir contains a large amount of carbonate minerals. Multi-stage cementation and dissolution of these carbonate minerals have important influences on diagenesis-porosity evolution of the reservoirs. Because of the coupled influences of deep fluid transformation and volcanic activity in the diagenetic evolution of the Fengcheng Formation, it has experienced a complex evolution of carbonate minerals. In this study, through core observation, thin section, X-ray diffraction mineral composition, carbon and oxygen isotope, fluid inclusion homogenization temperature, and laser Raman and cathodoluminescence analysis, the material sources, development period, dissolution-precipitation reactions, and transformation mechanisms of carbonate minerals in the diagenetic evolution of the Fengcheng Formation reservoir were evaluated in detail. Finally, the controlling effect of multi-stage cementation of carbonate minerals on the reservoir physical properties was determined. The results showed that there are three stages of carbonate minerals in the Fengcheng Formation. The material source of the early carbonate was mainly terrigenous debris, and the formation temperature range was 30–70 °C. The material source of middle-late carbonate minerals was mainly decarboxylation of organic acid, and the formation temperature was 60–110 °C. In addition, some were generated through hydrothermal metasomatism at high ambient temperatures, up to 120 °C. In areas where early carbonate minerals are well developed, a large number of microcracks was more likely to form under the influence of tectonic movements, resulting in high-quality reservoirs. The formation of late carbonate minerals, which is the main reason for reservoir densification in the later stage of diagenesis, resulted in poor reservoir physical properties. This study provides information useful for predicting the distribution areas of favorable reservoirs in the Fengcheng Formation, and a theoretical basis and guidance for improving the accuracy of shale oil exploration and development.

Characterization on Structure and Fractal of Shale Nanopore: A Case Study of Fengcheng Formation in Hashan Area, Junggar Basin, China

The Lower Permian Fengcheng Formation in Halaalate Mountain in the Junggar Basin has enormous potential for shale oil, while few investigations on quantifying pore structure heterogeneity have been conducted. Thus, total organic carbon (TOC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and low-temperature N2 adsorption tests were conducted on the shales collected from the HSX1 well in the Hashan region to disclose the microscopic pore structure and its heterogeneity. Results show that the selected shales mainly consist of quartz, plagioclase, calcite, and clay minerals. The primary pore types are intergranular pores in quartz and carbonate and intragranular pores in clays, while organic matter (OM) pores are rare. Typical types of H2 and mixed H2-H3 were observed. Type H3 shale pore size distributions (PSD) are unimodal, with a peak at about 70 nm, while Type H2-H3 shales are bimodal, with peaks at about 70 nm and 3 nm, respectively. Type H3 shales have lower D2 than Type H2-H3 shale, corresponding to weaker pore structure heterogeneity. Multifractal analyses indicate that macropores in Type H3 shales have stronger heterogeneity with large D10−D0 ranges, while minor D−10−D0 ranges mean weaker heterogeneity of micro- and mesopores, and so do Types H2-H3 shales. The higher the contents of plagioclase and clay minerals, the more heterogeneous the micro- and mesopores are; a larger content of quartz leads to more heterogeneous macropores. Specific surface area, micro-, and mesopores contents positively correlate to D2, while average pore diameter and macropores are on the contrary, thus the higher the content of micro- and mesopores and the specific surface area, the lower the content of macropores and average pore diameter, the more complex the microscopic pore structure of shale. Micro- and mesopores control the heterogeneity of shale pore development with a great correlation of D−10−D0 and D−10−D10, and D2 can effectively characterize the heterogeneity of a high porosity area with a strong correlation of D2 and D0−D10.

Fluid inclusion evidence for overpressure-induced shale oil accumulation

Abstract Shale oil is becoming increasingly important in the global energy market, but its accumulation mechanism is not fully understood. We present novel and direct fluid inclusion data from the Lower Permian Fengcheng Formation, Mahu Sag, Junggar Basin, northwest China. Shortite veins in this source rock contain abundant two-phase gas-liquid hydrocarbon inclusions and coeval aqueous inclusions. The inclusions have highly variable degrees of bubble filling (5–80 vol% vapor) and homogenization temperature differences between oil and aqueous inclusions (~50 °C), which demonstrate that fluid (oil-gas-water) immiscibility occurred at high pressures. The hydrocarbon inclusions record different levels of fluid over-pressure (32.9–43.0 MPa), with a paleopressure coefficient of 1.3–1.7. Episodic fluid over-pressure release resulted in shale oil accumulation, with faults/fractures acting as important migration pathways. Oil from deeper and more mature source rocks within the Fengcheng Formation was expelled upward to the shale oil reservoir. These processes are common and important in shale oil systems. These results show that the accumulation of unconventional hydrocarbons occurs pervasively within the reservoirs, and fluid displacement is critical in exploration and exploitation.

Occurrence state and oil content evaluation of Permian Fengcheng Formation in the Hashan area as constrained by NMR and multistage Rock-Eval

The Hashan area, neighboring the Mahu Sag that is rich in the shale oil resources, showed commercial oil flow in the corresponding lacustrine shales of the Lower Permian Fengcheng Formation (P1f) with reserve scale approximately 789 million tons, presenting great potential for oil exploration. Despite their geographical proximity, the hydrocarbon occurrence and oil-bearing capacity of shale in the Hashan area and Mahu Sag greatly differ owing to the complex tectonic evolution. Therefore, understanding the occurrence state and oil content of the P1f in the Hashan area is crucial for ongoing shale oil exploration activities and the development of the northwestern margin of the Junggar Basin. In this study, an integrated investigation, including petrological observations, scanning electron microscopy (SEM) observation, analysis of nuclear magnetic resonance (NMR) T1–T2 spectra, and conventional and multistage Rock-Eval pyrolysis methods were conducted to evaluate the occurrence state and oil content of the P1f shale in the Hashan area. The results indicate that plagioclase (average 30.7%) and quartz (24.1%) dominate the mineral compositions of the P1f shale samples. A method involving quartz–plagioclase–carbonate minerals is proposed to conduct lithofacies classification. In the Hashan area, the organic matter abundance in the P1f shale is scaled in fair to good range, the thermal maturity ranges from immature to early mature stage, and the primary organic matter types are Types I and II1. Intergranular and dissolution pores are the two most common pore types. The free oil is mostly found in the pores and microfractures of the mineral matrix, whereas the adsorbed oil is mostly adsorbed on the surfaces of kerogen and clay minerals. The high organic matter abundance, quartz content, and porosity account for substantial increase in the oil content, the area rich in shale oil resources coincides with that rich in free oil. The most favorable lithofacies in the Hashan area is the calcareous mudstone/shale, which hosts the highest free oil content (average 2.49 mg), total oil content (15.02 mg/g), organic matter abundance (TOC = 1.88% and S1+S2 = 20.54 mg/g) and porosity (5.97%).

Heterogeneity of alkaline lacustrine source rocks of the lower Permian Fengcheng Formation in the Mahu Sag, Junggar Basin, northwestern China

The alkaline lacustrine source rocks in the lower Permian Fengcheng Formation in the Mahu Sag, Junggar Basin, China, exhibit significant heterogeneity. However, most previous studies have focused on the vertical variability of the source rocks, and the lateral variability is poorly understood. This study investigated recently drilled samples of the Fengcheng Formation in the Manan area to assess the lateral variability of the source rocks. The geochemical characteristics of the Fengcheng Formation source rocks in the Manan area are variable, with low–high abundances of organic matter, which comprises types I–III kerogen that is mature to high-mature. These features differ from the typical Fengcheng Formation source rocks in the Wuxia–Fengcheng area, which have a high abundance of organic matter with substantial hydrocarbon and shale oil generation potential. Biomarker data show that the organic matter in the Fengcheng Formation source rocks in the Manan area is a mixture of higher plants, algae, and bacteria, which differs from the Wuxia–Fengcheng area where algae and bacteria are dominant. The Fengcheng Formation source rocks in the Manan area were deposited in an oxic to anoxic environment characterized by large variations in water salinity and stratification. Mudstones were deposited rather than carbonate rocks. This differs from the strongly reducing, hypersaline, and water-stratified depositional environment of the Fengcheng Formation source rocks in the Wuxia–Fengcheng area. These two areas are separated by the Dazhuluogou Fault, and there are no alkaline lacustrine deposits in the Manan area. The distribution of the alkaline lacustrine source rocks was controlled by the paleogeography and sedimentary environment, which was characterized by significant spatial heterogeneity.

Shale Lithofacies and Its Effect on Reservoir Formation in Lower Permian Alkaline Lacustrine Fengcheng Formation, Junggar Basin, NW China

Lower Permian Fengcheng Formation is considered to be a high-quality alkaline lacustrine shale oil resource in the Junggar Basin, NW China. Based on core and thin section observation, X-ray diffraction, scanning electron microscope, low-pressure N2 adsorption, and high-pressure mercury intrusion porosimetry, different shale lithofacies, and pore structures were examined. According to the mineral composition, shales in well My 1 are divided into five types: dolomitic mudstone, calcareous mudstone, siliceous mudstone, tuffaceous mudstone, and argillaceous mudstone, each of which shows its pore structure distribution. Intragranular pores, inter-crystalline pores associated with clays and pyrites, dissolution pores, and microfractures were commonly observed. There are three segments of pore structures including <50 nm, 50 nm-4 μm, and >4 μm. Clay minerals mainly contribute to mesopores, especially in argillaceous mudstones. The dissolution of carbonate minerals and feldspars is significant for macropores predominantly in dolomitic mudstones and tuffaceous mudstones, respectively. Micron-scale microfractures associated with laminae dominate in dolomitic mudstones. Therefore, the dolomitic mudstones, especially with lamination, and tuffaceous mudstones are proposed to be favored shale lithofacies with great exploration potential in the Mahu Sag.

Source-to-sink evolution of syn-rift alkaline lake sediments in the Lower Permian Fengcheng Formation, Junggar Basin, NW China: Evidence from petrology, detrital zircon geochronology and geochemistry

• Chemical weathering intensity increased upwards in the ∼ 280 Ma Fengcheng Formation. • Detrital material of the FC Formation was supplied by the West Junggar, due to active normal faults. • Syn-rift sediments were deposited in an alkaline lake under arid climatic conditions. Source-to-sink evolution of intracontinental rift basins is an effective tool to evaluate syn -rifting tectonic processes and sediment dispersal. Provenance analysis, using petrological and geochemical analysis and detrital zircons U-Pb dating of the Lower Permian Fengcheng Formation, provides new insights into the provenance of alkaline lake deposits in the Mahu sag, Junggar Basin. Volcanic detritus and detrital feldspar grains are abundant, indicative of proximal sources. The rare earth and trace element characteristics of three sandstone samples are consistent with a dominant source from the Carboniferous – early Permian mafic and felsic rocks in West Junggar. Sandstone geochemistry suggests that chemical weathering intensity increased upwards, probably in response to reduced aridity. The detrital zircon U-Pb age patterns show a prominent Carboniferous peak (340–300 Ma), minor early Permian (∼280 Ma) and Silurian peaks (440–420 Ma). Quantitative unmixing modelling suggests that most detrital zircons were derived laterally from the Central West Junggar (CWJ), and a few zircons were derived axially from the Zharma-Saur and Boshchekul-Chingiz arcs in Northern and Southern West Junggar. The lateral source was more important in the Middle Member than in the Lower Member of the Fengcheng Formation. The average age of the youngest five detrital zircons and the low weathering indices indicate that the maximum slip of the bounding normal fault occurred at 284–?279 Ma. Seismic and hydrothermal activity caused by the normal fault displacement, as well as the increase in the supply of volcanic composition from the lateral CWJ, played an important role in the alkaline lake deposition.

Origin of Shallow Jurassic Heavy Oils in the Northwestern Margin of the Junggar Basin, NW China: Constraints from Molecular, Isotopic and Elemental Geochemistry

AbstractOil group separation, gas chromatography–mass spectrometry analysis of saturated hydrocarbons, carbon isotope analysis of fractions and tests on trace elements were all carried out to determine the origin of shallow Jurassic heavy oils in the northwestern margin of the Junggar Basin, northwestern China. Results showed that all the crude oils had been subjected to different degrees of biodegradation, on an order ranging from PM 6 to 9, which yielded many unresolved complex mixtures (UCM) and formed a huge spike in the mass chromatogram (M/Z = 85). Two heavy oils from the Karamay area underwent slight biodegradation, characterized by the consistent ratios of biomarker parameters. C21/C23 and C23/H of the two samples were 0.81 and 0.85, while G/H, C27/C29 and C28/C29 were 0.38 and 0.40, 0.16 and 0.27, 0.87 and 0.86, respectively. The isomerization parameters of terpane and steranes were 0.50–0.53, and 0.48–0.49, respectively. The above geochemical indices indicated that the crude oils in the study area were in the marginally mature stage. The parent materials were a mixture, consisting of bacteria, algae and some higher plants, formed under reducing depositional conditions, which is in agreement with the source rocks of the Fengcheng Formation in the Mahu depression. The carbon isotopic compositions of saturated hydrocarbon, aromatic hydrocarbon, NSO and asphaltene were –31‰– to –30.3‰, –29.5‰ to –29.03‰, –29.4‰ to –28.78‰ and –28.62‰ to –28.61‰, respectively. These findings are in agreement with the light carbon isotope of kerogen from the lower Permian Fengcheng Formation. Furthermore, V/Ni and Cr/Mo of all the crude oils were 0.01 to 0.032, 0.837 to 10.649, which is in good agreement with the ratios of the corresponding elements of the extracts from the Fengcheng Fm. carbonate source rock. As a result, a two–stage formation model was established: (1) the oil generated from the carbonate source rocks of the Fengcheng Formation migrated to the Carboniferous, Permian and Triassic traps during the Late Triassic, forming the primary oil reservoirs; (2) during the Late Jurassic period, the intense tectonic activity of Yanshan Episode II resulted in the readjustment of early deep primary reservoirs, the escaped oils gradually migrating to the shallow Jurassic reservoir through cross‐cutting faults, unconformities and sand body layers. The oils then finally formed secondary heavy oil reservoirs, due to long–term biodegradation in the later stage. Therefore, joint methods of organic, isotopic and element geochemistry should be extensively applied in order to confirm the source of biodegradation oils.

Assessing source and maturity of oils in the Mahu sag, Junggar Basin: Molecular concentrations, compositions and carbon isotopes

Molecular parameters are both facies and maturity dependent. Source and maturity assessment remains a difficult task for oils which were derived from multiple source rocks at a wide range of maturities from the early to late oil generation window. In this paper, a new approach was presented to assess oil source on the basis of molecular facies parameters under maturity constraint using crossplots of one source parameter versus one maturity parameter. Concentrations of ΣC29 steranes and C30 hopane were tentatively used as maturity indicators. Quantitative GC, GC-MS and GC-IRMS analyses were performed on 52 oils from the southern and central Mahu sag of northwestern Junggar Basin, NW China. The 52 oils are first classified into three groups, i.e., groups I, II and III which were derived from source beds within the Lower Permian Fengcheng Formation (P1f), Middle Permian Lower Wuerhe Formation (P2w) and both, respectively on the basis of carbon isotopes of individual n-alkanes in combination with reservoir geological settings. These three oil groups, in particular groups I and II from different sources were further distinguished sufficiently using crossplots of ratios of pristane (Pr)/n-C17 and phytane (Ph)/n-C18, β-carotane/n-C21 and concentrations of n-alkanes, Pr, Ph, β-carotane, Ts, C29 Ts, C30 diahopane, C20, C21 and C23 tricyclic terpanes versus concentrations of ΣC29 steranes and C30 hopane, respectively. Variation trends of isotopic reversal index (RI) and δ13C value of n-C25, the lowest among those of n-alkanes (n-C12–34), with decreasing concentrations of ΣC29 steranes and C30 hopane demonstrate that carbon isotopes of n-alkanes were unlikely influenced significantly by oil maturities within oil generation window. Concentration of n-alkanes increases while concentrations of Pr, Ph, crotanes, terpanes and steranes generally decrease with decreasing concentrations of ΣC29 steranes and C30 hopane. Application of this approach provides numerous choices and opportunities to distinguish oil groups which were derived from different sources in combination with isotopic data and reservoir geological settings.