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The late Paleozoic Fengcheng Formation shale (LPF shale) in the Junggar Basin, NW China, is the oldest alkaline source rock discovered in the world, providing a unique perspective with which to explore organic matter (OM) enrichment in alkaline lake environments. Combined with the organic carbon isotope profile and paleoenvironmental proxies, this study reveals that the LPF shale was deposited in an arid climate with high salinity and a strong reducing environment, accompanied by frequent volcanic activity. High TOC values are concentrated in two intervals with frequent fluctuations in OM types. A negative excursion due to changes in sedimentary OM source is found in the δ13Corg profile. The excursion corresponds to the OM enrichment interval and is accompanied by abnormally high values of Sr/Ba and Sr/Cu. This implies that the extreme arid climate has led to high salinity, resulting in strong reducibility and changes in paleontological assemblages, which in turn controlled the differential enrichment of OM. The Fengcheng Fm. high‐quality source rocks are the result of the combined action of climatic events, volcanism, high‐salinity water environment and superior hydrocarbon‐generating organisms. The results provide new insights into the formation conditions of terrestrial alkaline high‐quality source rocks and the factors controlling alkaline OM enrichment. [ABSTRACT FROM AUTHOR]

Shale oil and gas are currently the major fields of unconventional hydrocarbon exploration and development. The Fengcheng Formation (FF) shale in the Mahu Sag of the Junggar Basin is an alkaline lacustrine organic-rich shale with an extremely prospective shale oil potential. However, its strong heterogeneity and complex pore structure greatly influence the development of shale oil. It is significant to investigate the pore and geochemical characteristics of shale reservoirs for shale oil extraction. In this study, the pore structure and geochemical characteristics of FF have been investigated using core analysis, Rock-Eval pyrolysis, X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), mercury injection capillary pressure (MICP), low-temperature gas adsorption (LTGA), and X-ray computed tomography (X-ray CT). The results show that the shale of FF has moderate organic matter abundance, and the kerogen is mainly of Type II, which is now at the peak of oil generation. Shale minerals are mainly composed of carbonate (dolomite and calcite) and siliceous (quartz and feldspar) minerals, with extremely low clay mineral content. The pore types are mainly intergranular pores (inter-P), intragranular pores (intra-P), and microfractures associated with mineral particles. The pore space is contributed predominantly by micropores of 0.5–1.2 nm and mesopores of 10–50 nm, whereas macropores are underdeveloped. The pores are mostly ink bottle- and slit-shaped, and the pore connectivity is relatively poor. The pore development of shale in the FF is influenced by organic matter abundance, thermal maturity, mineral composition, etc. Organic matter content (TOC), thermal maturity (Ro), and carbonate minerals have a positive effect on pore development, and the pore volume (PV) increases with TOC, Ro, and carbonate minerals. While clay minerals show a negative effect, the PV decreases with clay minerals. Additionally, the influence of the clay mineral content on the pore morphology of shale should not be ignored. This study investigates the pore structure and geochemical characteristics of the alkaline lacustrine shale of FF in Mahu Sag, which is significant to deepen the understanding of alkaline lacustrine shale and to improve the production of shale oil. [ABSTRACT FROM AUTHOR]

The Bozhong Sag is the biggest hydrocarbon rich sag in the Bohai Sea area. However, hydrocarbon resource potential and its distribution are not clear, which restricts petroleum exploration. According to the material balance principle and the hydrocarbon accumulation threshold theory, the resource potential and distribution characteristics of hydrocarbons were evaluated quantitatively with hydrocarbon accumulation systems as evaluation units. The upper and lower plays in the Bozhong Sag and surrounding areas each can be divided into six hydrocarbon accumulation systems. The total afforded accumulation hydrocarbon quantity in the Bozhong Sag and surrounding areas is 60.265×108 t of oil (43.185×108 t in the upper play, 17.080×108 t in the lower play) and 27.03×1011 m3 of gas (17.76×1011 m3 in the upper play and 9.27×1011 m3 in the lower play). The Shijiutuo (I) and Bodong (II) accumulation systems are the further exploration areas with the greatest afforded accumulation hydrocarbon quantity. Key words: Bozhong Sag, hydrocarbon accumulation system, hydrocarbon accumulation threshold, hydrocarbon accumulation quantity, resource evaluation

Jurassic strata along the southern margin of Junggar Basin are important petroleum system elements for exploration in northwest China. The Lower and Middle Jurassic source rock effectiveness has been questioned as exploration progresses deeper into the basin. These source rocks are very thick and are distributed widely. They contain a high total organic carbon composed predominantly of Type III kerogen, with some Type II kerogen. Our evaluation of source rock petroleum generation characteristics and expulsion history, including one-dimensional basin modeling, indicates that Jurassic source rocks are gas prone at deeper depths. They reached peak oil generation during the Early Cretaceous and began to generate gas in the Late Cretaceous. Gas generation peaked in the Paleogene-Neogene. Source rock shales and coals reached petroleum expulsion thresholds at thermal maturities of 0.8% and 0.75% vitrinite reflectance, respectively, when the petroleum expulsion efficiency was ~40%. The petroleum generated and expelled from these source rocks are 3788.75 x [10.sup.8] and 1507.55 x [10.sup.8] t, respectively, with a residual 2281.20 x [10.sup.8] t retained in the source rocks. In these tight reservoirs, a favorable stratigraphic relationship (where tight sandstone reservoirs directly overlie the source rocks) indicates short vertical and horizontal migration distances. This indicates the potential for a large, continuous, tight-sand gas resource in the Lower and Middle Jurassic strata. The in-place natural gas resources in the Jurassic reservoirs are up to 5.68 x [10.sup.12]-15.14 x [10.sup.12] [m.sup.3]. Jurassic Badaowan and Xishanyao coals have geological characteristics that are favorable for coal-bed methane resources, which have an in-place resource potential between 3.60 x [10.sup.12] and 11.67 x [10.sup.12] [m.sup.3]. These Lower and Middle Jurassic strata have good shale gas potential compared with active US shale gas, and the inferred in-place shale gas resources in Junggar Basin are between 20.73 x [10.sup.12] and 113.89 x [10.sup.12] [m.sup.3]. This rich inferred conventional and unconventional petroleum resource in tight-sand, coal-bed, and shale gas reservoirs makes the deeper Jurassic strata along the southern margin of Junggar Basin a prospective target for future exploration. Les strates jurassiques le long de la bordure sud du bassin de Junggar constituent des elements importants d'un systeme petrolier exploitable dans le nord-ouest de la Chine. L'efficacite de la roche source (Jurassique inferieur et moyen) a ete mise en question a mesure que l'exploration progressait plus profondement dans le bassin. Ces roches sources sont tres epaisses et couvent de grandes etendues. Leur teneur en carbone organique total est elevee et il est principalement compose de kerogene de type III avec un peu de kerogene de type II. Selon notre evaluation des caracteristiques de generation de petrole a partir de roches souches et de l'historique d'expulsion, incluant une modelisation du bassin a une dimension, ces roches sources du Jurassique devraient contenir du gaz a de grandes profondeurs. Ces roches ont atteint un sommet de generation de petrole durant le Cretace precoce et elles ont commence a produire du gaz au Cretace tardif. La generation de gaz a atteint un sommet au Paleogene-Neogene. Les roches sources de shale et les charbons ont atteint des seuils d'expulsion du petrole a des maturites thermiques respectives de reflectance de la vitrinite de 0,8% et de 0,75%, alors que le taux d'efficacite d'expulsion du petrole est d'environ 40%. Le petrole genere par ces roches est de 3788.75 x [10.sup.8] t et celui expulse est de 1507.55 x [10.sup.8] t; 2281.20 x [10.sup.8] t residuelles sont retenues dans les roches sources. Dans ces reservoirs peu permeables, une relation stratigraphique favorable (lorsque des reservoirs de gres peu permeables reposent directement sur les roches sources) signale de courtes distances, verticales et horizontales, de migration, ce qui en retour indique un potentiel pour une vaste ressource ininterrompue de gaz dans un sable peu permeable de strates du Jurassique inferieur et moyen. Les ressources de gaz naturel in situ dans les reservoirs datant du Jurassique atteignent de 5.68 x [10.sup.12] a 15.14 x [10.sup.12] [m.sup.3]. Les charbons Badaowan et Xishanyao (Jurassique) possedent des caracteristiques geologiques favorables aux ressources de methane de houille, lesquelles ont un potentiel in situ entre 3.60 x [10.sup.12] et 11.67 x [10.sup.12] [m.sup.3]. Ces strates du Jurassique inferieur et moyen ont de bons potentiels de gaz de shale comparativement aux gaz de shale americains exploites, et les ressources in situ inferees dans le bassin de Junggar sont de 20.73 x [10.sup.12] a 113.89 x [10.sup.12] [m.sup.3]. Cette riche ressource de petrole inferee, conventionnelle et non conventionnelle, dans des reservoirs de sables peu permeables, de lits de charbon et de gaz de shale, font des strates jurassiques tres profondes le long de la bordure sud du bassin de Junggar une cible prometteuse pour de l'exploration future. [Traduit par la Redaction], Introduction Junggar Basin is one of the most petroliferous basins in China (Ulmishek 1984; Wang 2000b; Li et al. 2003; Kong 2007; Cao et al. 2012; Shao 2013). By 2010, [...]