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A large continuous conglomerate reservoir has been recently discovered in the Upper Permian Upper Wuerhe Formation on the southern slope of Mahu Sag, Junggar Basin. In order to timely summarize, enrich and develop the theory of continuous oil and gas geology, and to provide a basis for deepening regional exploration, it was studied in this paper the reservoir forming elements and reservoir characteristics based on the fine structure interpretation, sedimentary system study and reservoir-cap combination analysis of these reservoirs. The results showed that the source rocks of Fengcheng Formation on the southern slope were widely distributed, with a large thickness, good type and high maturity. In four major material source systems including Zhongguai, Karamay, Baijiantan and Dabasong fans, the sand conglomerate reservoirs in the first and second members of Upper Wuerhe Formation overlapped continuously, and formed a stable reservoir-caprock assemblage with the widely developed mudstone caprock of the third member of Upper Wuerhe Formation. Near EW-oriented deep strike-slip faults and unconformities widespread in the sag provided pathways for hydrocarbon migration and accumulation. The Upper Wuerhe Formation is a "foreland-depression" replacement-type deposit. Under the background of large formation pinch-out zone, a large area of continuous lithologic traps was developed, forming stratigraphic, lithologic and fault-lithologic oil reservoirs. The Zhongguai and Karamay fans are found to have a high potential for exploration and still have many remaining oil and gas points. The Baijiantan and Dabasong fans have similar sedimentary background with the Zhongguai fan, and have a lower exploration degree, which are key area for future exploration.

Reservoirs with overpressure are of great importance for petroleum exploration where commercial production can be usually obtained. Studying the distribution characteristics and controlling factors of reservoir overpressure is crucial for further petroleum exploration. In this study, we investigate the vertical and planar distribution of reservoir overpressure in the Mahu sag, northwestern Junggar basin by analyzing measured pressure data from well testing of oil reservoirs. In the Baikouquan-Jiamuhe Formation, reservoir overpressure is widely distributed, and the pressure coefficient increases from the margin to the center of the sag and generally increases with the increasing altitude. Crossplot analysis of density and velocity in the Triassic strata and Permian Fengcheng Formation is conducted to further investigate the influence of undercompaction in the Mahu sag for the first time. The result suggests that undercompaction has little influence on reservoir overpressure, whereas fluid charging may play a vital role in the development of overpressure. Our research further conducted the analyses of distribution of source rocks and oil-source correlation. The results further confirmed that hydrocarbon generates from the Fengcheng Formation and charges into other reservoirs, suggesting that hydrocarbon generation and fluid charging are the main mechanism of reservoir overpressure.

The characteristics and main controls of continental shale oil reservoirs in the Permian Lucaogou Formation in the Jimsar Sag of Junggar Basin were studied based on seismic, core and logging data. Oil was found in a large area in the Lucaogou Formation, mainly in micro and nano pore throats or bedding fractures in free or adsorbed states. There are two sweet spot layers separated by mudstones. The formation pressure and maturity of shale reservoirs gradually decrease from the sag area to the slope area, while the crude oil density gradually increases. The quality and maturity of source rocks controlled the distribution and properties of shale oil. The sedimentary microfacies controlled the distribution of shale oil. The lithology and physical properties of shale controlled the enrichment of shale oil. Hydrocarbon generation pressurization was the main driving force for shale oil accumulation.

The shale oil layers of the Lucaogou Formation in the Jimsar Sag of Junggar Basin have complex lithology and strong heterogeneity. The formation of "sweet spots" is a key problem restricting exploration and development. Through core observation, thin section analysis and oil-bearing capacity analysis, the geological features of the "sweet spots" were characterized, the geological factors affecting productivity were analyzed, and the formation mechanism of movable oil pores was determined. The lithology of "sweet spots" of the Lucaogou Formation in the Jimsar Sag is mainly lithic feldspar fine sandstone, dolomitic sandstone and gypsum dolomite, with a porosity of 5.75%-11.9% and a permeability of (0.02-1.26)×10-3 μm2, of which the lithic feldspar fine sandstone made the largest contribution to pore throats. Intergranular (intragranular) pores and fractures provide storage space for "sweet spots". Shale oil yield is controlled by the pore distribution for movable oil and the contents of argillaceous, quartz and dolomite. The movable oil pores are formed under the combined action of syngenetic and quasi-syngenetic leaching and acid fluid burial dissolution. The key factor to control permeability is the connection of dissolved pores by bedding cracks. In the process of reservoir diagenesis, the coupling of hydrocarbon generation and expulsion in the reservoir space will eventually form a "sweet spot", mainly in environments such as dolomitic flat, mixed flat, lakeside beach sand and carbonate shoal which are strongly influenced by waves. This discovery can provide guidance for the exploration and development of shale oil of the same genetic type.