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The Permian source rocks in the Junggar Basin are widely developed, especially the Fengcheng Formation, which is the most significant source rock in the basin. However, due to insufficient research on the hydrocarbon generation (HG) and hydrocarbon expulsion (HE) characteristics of the source rocks, it is unclear whether a significant amount of retained hydrocarbons remain within shales. In general, the original organic matter abundance and kerogen type control hydrocarbon generation potential (HGP) and HE capacity in lacustrine shales. Therefore, the degradation rate method was used to establish the original organic carbon recovery model for different types of kerogen. Combined with the geologic and geochemical characteristics of the source rock, the HG, HE, and shale oil resource potential of the Fengcheng shale have been evaluated. We have found that the Fengcheng shale is mainly carbonate-type mudstone widely distributed with an average thickness greater than 100 m. The Fengcheng shale is composed of type II kerogen and reached the mature to high-mature thermal maturity stage, with the maximum original organic carbon exceeding 4.0 wt%. Meanwhile, the amount of retained hydrocarbons within shales is abundant according to the HGP model. Monte Carlo simulation finds that the shale oil resources of the Fengcheng shale are 23.30 × 108 t. Free oil resources account for 60%, reaching 13.75 × 108 t, indicating tremendous shale oil exploration potential.

Recently, the exploration and development of the Chang 7 shale oil reserves and production have increased in the Ordos Basin. However, the characteristics of the Chang 7 reservoir vary greatly in different areas, which affect the exploration and development of shale oil. We used various analytical methods such as core observation, casting section, high-pressure mercury injection, nuclear magnetic resonance, X-ray diffraction (XRD), and logging interpretation to study characteristics of the reservoir in the Heshui area. The lithology of the Chang 7 member is mainly feldspathic quartz sandstone suggesting that the content of quartz is higher than that of feldspar, and it has relatively low carbonates. In addition, the kinds of feldspar are mainly plagioclase and potassium feldspar, and the concentration of clay minerals is 80%, mainly comprising illite and chlorite. The reservoir of the Chang 7 member is chiefly comprised of submicron pores, such as feldspar dissolved pores, intergranular pores, dissolved pores, microfractures, and intergranular pores. The porosity ranges from 6% to 12%, whereas permeability is less than 0.2 × 10−3 [Formula: see text]. The Chang 7 reservoir has a strong heterogeneity. Specifically, the heterogeneity of Chang 71 is weaker than that of Chang 72. There is a complex diagenesis such as compaction, dissolution, and cementation, and the compaction and cementation are relatively more and the dissolution is dominated by constructive diagenesis of feldspar dissolution. Sedimentary microfacies are one of the main factors controlling reservoir development. The physical properties of the reservoir in the branch channel are better than the edges of the branch channel and the lacustrine. The diagenesis affects the development and distribution of good reservoirs, and destructive diagenesis such as compaction and cementation reduces porosity and permeability. Conversely, dissolution increases the physical properties of the reservoir. Tectonics has no obvious effects on reservoir but plays a positive role in the migration and accumulation of hydrocarbon when microfractures developed.

The Chang 7 oil-bearing layers from the Ansai area of the Ordos Basin are abundant in oil and gas resources. These layers are chiefly comprised of shales interbedded with tight sandstone. In the regions where the exploration level is relatively low and the structures are gentle in the Ansai area, the distribution of fractures in shale reservoirs and tight sandstone reservoirs is significant for predicting sweet spots and determining the success rate of hydraulic fracturing. In this research, the results acquired from the scanning electron microscope (SEM), cores, thin sections, and image logging determined the developmental characteristics of fractures. Moreover, the Himalayan stress field was modeled using ANSYS, whereas the two-factor method is used to predict the distribution of tectonic fracture by combining stress field and fracture criterion. The results indicate a gradually decreasing trend of the maximum principal stress from east to west. The minimum principal stress in the western part is less than that in the eastern part. The differential stress that can indicate the degree of tectonic deformation ranges from 17 Mpa to 25 MPa. The fracture density ranges from 0 strip/m to 1.5 strip/m. In addition, the areas with more fractures are mainly located in the western and southeastern parts, where shale oil is relatively rich. Similarly, the comparison of different maps illustrates that the distribution of tectonic fracture is related to mechanical parameters and stress fields.

Vanadium in the black rocks has economic and environmental impacts. In sediments, it is broadly disseminated as a multivalent metal element mainly sensitive to redox settings. Globally in petroleum, it is considered an abundant component. Vanadium is an essential tool to determine the relationship of the Earth with extra-terrestrial bodies. In the Yangtze region, the black rocks of the Early Cambrian Niutitang Formation are highly enriched in the concentration of V, Co, Ni and Mo. These sediments are comprised of a high total organic carbon content, and the average concentration of vanadium is over 240 ppm. Here we discuss the mechanisms and conditions that were responsible for the accumulation of vanadium in these black sediments in the Yangtze region. The oxygenated ocean water is favorable for the dissolved vanadate species V(V). Therefore, in oxic ocean-water, it can be reduced by organic matters or by H2S to vanadyl ions V(IV), which can facilely be adsorbed to the tiny particles and finally deposit into the sediments with the settling of the particles. The presence of V2O3 in the Niutitang Formation indicates the isomorphism state of vanadium existence in the clay minerals. Clays and pyrite are the most favorable mineral for vanadium enrichment. However, it is suggested the quartz of non-biogenic origin might be unfavorable for vanadium enrichment. Vanadium is mainly derived from the diagenetic transformation of its precursor (porphyrin pigments and chlorophyll) from the organism. During the Early Cambrian period, the massive transgression in the sea level created a favorable environment for organisms to survive. Additionally, the hydrothermal activities brought massive nutrient supply in the form of vanadium and other metal elements from the deep Earth. These creatures consumed the vanadium-rich nutrients, which became a part of their bodies in the form of hard and soft parts. Later on, when these organisms died and were submerged in the sediments. After the diagenetic actions, this vanadium became a part of these black sediments along with organic carbon. Therefore, these black rocks in the Yangtze region are enriched in vanadium and organic carbon. It is suggested the various processes such as adsorption, complexation, and reductions are the main factors responsible for the precipitation of dissolved vanadium into the organically rich sediments.

In this research, we estimated the total organic carbon and presented a paleo depositional model of the Paleocene Patala Formation from the western Salt Range of Pakistan. Results shows, during the Paleocene period, the Patala Formation was settled in barrier-island or in back-barrier depositional environments. This evidence is on the basis of lithofacies analysis and TOC determination data from measured stratigraphic sections. According to lithofacies analysis and the TOC estimation of the Patala Formation, the carbonaceous shale deposits of Western Salt Range at Khairabad, Chitta Wahan, and the Kalri area were deposited in back-barrier marshes. In the study area, the Patala Formation 16 meter thick. Based on detailed outcrop investigation, we have divided the Patala Formation into five different lithofacies, i.e., grey shale, carbonaceous shale, sandstone, limestone, and a mixture of sandstone and shale facies. The TOC was measured with the help of a wet combustion titration method. Carbonaceous shale and grey shale lithofacies contain an average of 3.64 % and 1.39 % of TOC value, respectively. Results show moderate to very good TOC values in the shales of Patala formation, which indicates that Patala Formation in the Western Salt Range has a good source rock potential to generate hydrocarbon.

Tight sandstone reservoirs are widely distributed worldwide. The Upper Triassic Chang 6 member of the Yanchang Formation is characterized by low permeability and porosity. The facies model offers a unique approach for understanding the characteristics of various environments also heterogeneity, scale, and control of physical processes. The role of subsurface facies features and petrophysical properties was unclear. Notable insufficient research has been conducted based on facies and petrophysical modeling and that demands to refine the role of reservoir properties. To tackle this problem, a reservoir model is to be estimated using various combinations of property modeling algorithms for discrete (facies) and continuous (petrophysical) properties. Chang 6 member consists of three main facies, i.e., channel, lobe main body, and lobe margin facies. The current research is aimed at comparing the applicability and competitiveness of various facies and petrophysical modeling methods. Further, well-log data was utilized to interpret unique facies and petrophysical models to better understand the reservoir architecture. Methods for facies modeling include indicator kriging, multiple-point geostatistics, surface-based method, and sequential indicator simulation. Overall, the indicator kriging method preserved the local variability and accuracy, but some facies are smoothed out. The surface-based method showed far better results by showing the ability to reproduce the geometry, extent, connectivity, and facies association. The multiple-point geostatistics (MPG) model accurately presented the facies profiles, contacts, geometry, and geomorphological features. Sequential indicator simulation (SIS) honored the facies spatial distribution and input statistical parameters. The porosity model built using sequential Gaussian simulation (SGS) showed low porosity (74% values

Organic matter enrichment and preservation are significant in the formation of source rocks. The Lower Paleozoic source rocks of the Qaidam Basin have not been extensively studied. We collected outcrop samples in order to examine the sedimentary environment, evolution, and abundance of the organic matter in the “a" member of the Upper Ordovician Tanjianshan Group from the northern margin of the Qaidam Basin. We applied detailed geochemical and mineralogical techniques (including X-ray diffraction, X-ray fluorescence spectrometry, Rock-Eval pyrolysis and gas chromatography-mass spectrometry) to assess its source rock potential, paleoenvironmental setting, analyze the main controlling factors associated with enrichment of organic matter and present an organic matter enrichment model. The results show that the total organic carbon content in the “a" member is elevated, with an average value of 1.75 wt%. Additionally, the organic matter has reached a mature-overmature stage (Requ>1.8%), and the kerogen was mainly type Ⅱ1. The ratios of Sr/Cu, Rb/Sr, Cvalue and CIA reflect hot-arid paleoclimatic conditions during the deposition of the mudstone. Paleoredox indicators V/(V + Ni), δU, Ceanomaly, and Ce/La reveal the mudstones formed in an anoxic environment. Paleowater depth indicators Mn/Fe and Mn/Ti show that the depth of the sedimentary water was shallower than 250 m. Meanwhile, the Mo/TOC value and the U–Mo co-variation model indicate that it was deposited under strongly restricted conditions. The apparent hot-arid climatic conditions were not conducive for the reproduction of algae, which limited paleoproductivity. The formation of strong restricted water conditions and a stratified water column were caused by the change of paleowater depth that positively affected the organic matter accumulation. The TOC content has a good correlation with paleoredox conditions, but does not correlate with the paleo-bioproductivity index P/Ti and Ba/Al. Therefore, the present research suggests the formation of these rocks was mainly based on preservation conditions, resulting from the restricted water setting, and paleoredox conditions, which played a significant role in the organic matter enrichment.

The diagrams of intergranular volume versus cementation volume brought up by Houseknecht (1987) and Ehrenberg (1989), are extensively used around the globe to assess the relative significance of compaction and cementation to porosity reduction in sandstones. However, Houseknecht (1987) overlooked the fact that the bulk sediment volume is dynamically compressed during burial, which directly leads to deviation in evaluating original porosity loss. The present discussion is based on the theoretical and contrastive analysis. These analyses indicate the compactional porosity loss calculated by Ehrenberg's (1989) method is always greater than Houseknecht's (1987) method with a maximum difference of 12.7%. In contrast, the cementation porosity loss using Ehrenberg's (1989) method is always less than Houseknecht's (1987). Besides, the demonstration based on the data of Wei et al. (2020) also supports Ehrenberg (1989). Therefore, the method proposed by Ehrenberg (1989) should be highly recommended to be granted as a reference far and wide.

： This paper analyzed the geochemical characteristics and the sources of natural gas in the western Qaidam Basin based on isotopes and gas composition. The natural gas in the western Qaidam Basin is mainly comprised of hydrocarbon gases, including sapropelic, humic and mixed gas. In the southwestern Qaidam Basin, the main gas is sapropel gas, with some mixed gas and a small amount of humic gas. In contrast, the northwestern Qaidam Basin contains mainly humic gas, some mixed gas, and a small amount of sapropel gas. The dryness coefficient varies between 0.6 and 0.95. The carbon isotopes of methane, ethane, and propane are distributed between −32.9‰ ~ −51.4‰, −34.2‰ ~ −22.4‰, −30‰ ~ −17.4‰, respectively. The calculated Ro value of the natural gas is between 0.5% and 1.87%. In general, the natural gas shows the characteristics of high dryness coefficient, heavy carbon isotopes, and high thermal maturity. The gas-source correlation makes it clear that the main hydrocarbon source rocks in the western Qaidam Basin are the lower Ganchaigou (E3), the upper Ganchaigou (N1) and the lower Youshashan formations (N21). The gas-producing layers in the northwestern Qaidam Basin are primarily from the lower Ganchaigou (E3), the lower Youshashan (N21), and the upper Youshashan formations (N22). The primary gas source rocks are the lower Ganchaigou (E3), the upper Ganchaigou (N1), and the lower Youshashan formations (N21). The main gas-producing layers in the southwestern Qaidam Basin are the lower Ganchaigou (E3), the upper Ganchaigou (N1), the lower Youshashan (N21) and the upper Youshashan formations (N22), with the main gas source rocks being the lower Ganchaigou Formation (E3).

A decline in conventional petroleum reserves has resulted in the study of unconventional reservoirs. Techniques, such as hydraulic fracturing, permit new types of reservoirs to be developed. Organically rich shales are the prime horizons for the exploration and development of unconventional oil/gas. The Talhar Shale is a unit of the Lower Goru Formation (Cretaceous), mainly composed of dark grey-black shale with minor siltstone and sandstone. In the study area, the Talhar Shale is 70–90 m thick. This research primarily focuses on the geochemical evaluation of the Talhar Shale to evaluate its shale oil and gas potential by using total organic carbon, Rock-Eval pyrolysis, gas chromatography-mass spectrometry (GC-MS), X-ray diffraction (XRD), vitrinite reflectance, and maceral analysis. Talhar Shale has moderate to good hydrocarbon potential (0.43–10.73 mg HC/g rock). The laboratory results of the TOC reveals a good match with the Δlog R technique relative to the CARBOLOG technique. Biomarker data of the studied samples exhibit the dominance of C19 tricyclic terpanes, C30 pentacyclic triterpanes, and C27 steranes. Maturity parameters (Tmax, and vitrinite reflectance) suggest that the Talhar Shale is thermally peak-late mature, consistent with biomarker maturity parameters (steranes and hopanes). Oxic environmental conditions prevailed during the deposition of these Cretaceous sediments, and the organic matter was mainly mixed (primarily derived from terrigenous sources). This interpretation is based on various biomarker criteria, e.g., elevated Pr/Ph values, a large amount of C24TeT comparative to C23 TT, the lower amounts of C29/C30 17α(H)-hopane, etc. Organic petrographic analysis exhibits vitrinite as the dominant maceral group. However, solid bitumen, inertinite, and liptinite macerals were also observed. The van Krevelen diagram and the HI vs. Tmax indicate the presence of mixed type II/III kerogen. Various geochemical parameters, e.g., Tmax, n-alkanes carbon preference index (CPI), sterane parameters (20S/20S + 20R, ββ/ββ+αα), and the hopane Ts/(Ts + Tm) and C31 22S/(22S + 22R) suggest the orgaic matter (OM) in Talhar Shale is thermally mature. Based on XRD, the Cretaceous Talhar Shale contains a high percentage of quartz. Additionally, the crossplot between TOC and quartz content displays a positive association that suggests the quartz content in Talhar Shale might be from the biogenic origin, which supports a brittle nature and could be favorable for hydraulic fracturing. Moreover, it has a lower value of Poisson's ratio and moderates to high Young's modulus, indicating favourability for hydraulic fracking. Cretaceous sediments from the Lower Indus Basin of Pakistan are chiefly considered as oil and gas-prone horizons. Geochemical properties of the Talhar Shale compared with other major unconventional shale reservoirs around the globe (United States, Germany, Canada, and China) exhibits suitable average organic matter abundance (2.19 wt %), mixed type II/III kerogen, an adequate level of thermal maturity, massive shale thickness, burial depth, and high porosity. These characteristics of the Cretaceous Talhar Shale in the Lower Indus Basin compared to other global unconventional reservoirs may indicate its possibility as a favorable target for unconventional energy exploration.

The Triassic Chang 6 Member sandstones are important oil reservoirs in the Ordos Basin of north-western China, and are actually considered to be typical tight sandstone reservoirs. Diagenetic alterations play a prominent role in considerably impacting the reservoir quality. In this study, an integrated approach of: thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), cathodoluminescence (CL), fluid inclusions, laser Raman spectrographic analysis, and stable isotope analysis was applied to examine the diagenetic history recorded in the Chang 6 tight sandstones and investigate the influences of diagenesis on reservoir quality. The sandstones consist of fine-grained, moderated-to-well sorted arkoses (Q48.5F40.9R10.6) and lithic arkoses (Q54.3F29.1R16.6) with an average porosity and permeability of 9.10% and 0.39 mD, respectively. The sandstones have undergone complex diagenetic processes; dominant eogenetic features include: (ⅰ) mechanical compaction, (ⅱ) leaching of feldspar in meteoric water, (ⅲ) chlorite coats originating from the transformation of precursor clays and precipitating directly from interstitial water, (ⅳ) calcites filling intergranular pores as poikilotopic cements, and (ⅴ) quartz and albite overgrowths on surfaces of grains. Subsequent mesogenetic events include: (ⅰ) ankerite cements filling porosity and replacing framework grains, (ⅱ) extensive dissolution of feldspars and precipitation of kaolinites due to emplacement of organic acids, and (ⅲ) secondary quartz overgrowths in the late period of mesogenesis. Early calcite is characterized by 13C (δ13CPDB ranging from −3.45‰ to −0.30‰, mean = −2.00‰) and 18O (δ18OPDB varying from −16.70‰ to −5.80‰, mean = −10.50‰), while late ankerite is rich in 12C (δ13CPDB ranging from −10.71‰ to 0.80‰, mean = −3.58‰) and 16O (δ18OPDB varying from −24.30‰ to −15.80‰, mean = −19.93‰). Moreover, calcite hosts single liquid fluid inclusions, two-phase fluid inclusions are contained in ankerite cements (with mean homogenization temperature (Th) of 96.0 °C and salinity estimates from 11.47 to 18.96 eq. wt.% NaCl). These data indicate that the calcite originated from meteoric water and the ankerite was associated with organic matter and recrystallization of the early calcite cements. Furthermore, the metallic ions such as Fe2+, Mg2+, and Ca2+ required for ankerite precipitation were produced through alterations of volcanic fragments, feldspars, and biotites. In comparison, compaction has more important role than cementation in the loss of original porosity although cementation exerts enormous influence on reducing porosity. Quantitatively, the compactional porosity loss reaches 54.9%, while the cementation porosity loss accounts for up to 42.8%. Carbonate cements and the other authigenic products (quartz and albite overgrowth, and kaolinite) mostly fill intergranular pores, thus reduce porosity. Chlorite coats have effects in inhibiting compaction and restraining the development of quartz overgrowth, thereby protect the intergranular porosity to some extent. Early secondary porosity was enhanced by leaching and dissolution of feldspar in the eogenetic period. However, these processes did not increase the net porosity in the mesogenetic stage.

Early Cambrian organic-rich black rocks are extensively distributed in the southern China. In this study, we have discussed the relation of paleo-sedimentary environments with enrichment of organic matter. Based on TOC data, we divided the Niutitang Formation into three parts (Upper, Middle, and Lower). The amount of total organic carbon in the Lower (1.55 wt %) and the Upper part (1.35 wt %) of Niutitang Formation is significantly smaller than the Middle part (8.0 wt %) which suggest initial transgression, and regression stages of the sea level respectively. The Middle part from both sections (Longbizui and Sancha) contains higher TOC content, which is a clue of intense transgression stage of sea level with high bio-productivity. Kerogen type index (KTI) using different macerals specify the organic matter in these black rock series is mainly type-I with a minor amount of type-II1. Moreover, these black rocks were deposited in weak to moderately restricted hydrographic conditions. The heterogenic mineral composition was also observed in these black rocks, including quartz, clay, gypsum, pyrite, barite, dolomite, and plagioclase. Quartz content in the Middle part is relatively higher than the Upper, and the Lower part displays a strong positive relation with TOC which suggests the main source of quartz during deposition of the Middle part was biogenic quartz. During Early Cambrian period due to extension between the Yangtze and the Cathaysian plate, the hydrothermal fluids rich in uranium, vanadium, molybdenum, zinc, barium, etc. from the deeper part of the earth crust and entered the ocean basin through extensional remnant fissures and cracks, and due to upwelling phenomena entered to the shelf area. These nutrient-rich deep crustal fluids at the surface of the sea enhanced the evolution and breeding of planktons and other marine life; meanwhile, it upsurges the bio-productivity. Additionally, it creates bottom water hypoxia, which is favourable for the preservation of organic matter in the sedimentary rocks. The main factors responsible for the enrichment of organic matter in these Early Cambrian black rock series of Niutitang Formation were a hydrographic restriction, hydrothermal activities, and paleo-redox conditions, respectively.