Search

The sedimentary cycle of Fengcheng Formation in Junggar Basin changes rapidly and is difficult to identify. The low classification accuracy of high-frequency fine-grained sedimentary cycles restricts the exploration of unconventional shale oil and gas. Full-interval coring of well Maye 1 provides a solid foundation for small-layer research. In this paper, the high-frequency sedimentary cycle of Milan kovitch in Fengcheng Formation is studied by means of spectral analysis and wavelet transform, and the shale sweet spot is identified. The long period of eccentricity of 405ka, short period of eccentricity of 100ka and long period of axis slope of 42736a can be used as semi-quantitative basis for the classification of the fourth, fifth and sixth sequences. The Fengcheng Formation can be divided into 2 quaternary sequence cycles, 9 quaternary sequence cycles and 16 quaternary sequence cycles. A long period of eccentricity of 405ka can identify large-scale lake recessional and recessional, and there is a maximum lake flooding surface in the middle of the second Fengcheng Formation, which corresponds to high abundance of organic matter and high hydrocarbon generation potential. The 100ka short period eccentricity can identify the sweet spot layer, and there are 8 sweet spots in the well Maye 1, which is consistent with the results of nuclear magnetic interpretation. The research results provide a semi-quantitative basis for the classification of high-frequency sedimentary cycles and the identification of sweet spots of Fengcheng Formation in Mahu Sag, Junggar Basin.

The Late Paleozoic Fengcheng Formation within the Mahu Sag of the Junggar Basin (China) harbors the world’s oldest alkaline lake hydrocarbon source rocks. Spectral analysis of the natural gamma-ray (GR) series obtained from four boreholes traversing the Fengcheng Formation, with wavelength ranges of 28.4 m–50 m, 5.9 m–12.6 m, 2.3 m–3.9 m, and 1.2 m–2.7 m. These were controlled by Early Permian astronomical cycles, including 405 kyr long eccentricity, 100 kyr short eccentricity, 34.2 kyr obliquity, and 20.7–17.4 kyr precession. The most significant cycle was notably that of the 405 kyr long eccentricity, which was instrumental for dividing and correlating the high-frequency sedimentary sequences in lacustrine shales. Nine intermediate-term and 36 short-term base-level cycles were identified in the P1f1 and P1f2 members of the Fengcheng Formation. These cycle types are equal to the 405 kyr long eccentricity cycle and ∼100 kyr short eccentricity cycle, respectively. The paleolake-level variations in the Fengcheng Formation were reconstructed using sedimentary noise modeling, revealing that lake levels reached their highest value during the deposition of the P1f2 Member. The spatial distribution patterns of lithofacies in the Fengcheng Formation can be clearly demonstrated within the isochronous cycle framework under the constraints of long eccentricity cycles. The use of astronomical cycles in isochronous stratigraphic correlation offers great potential for characterizing alkaline lacustrine sequences and predicting favorable areas for shale oil exploration with higher accuracy.

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.

In this paper, the lithology, pore type, throat structure, and physical characteristics of the sandstone and conglomerate reservoirs of the Upper Permian Shangwuerhe Formation in the Fukang Sag of the Junggar Basin were analyzed through rock cast thin section, scanning electron microscopy, fluid inclusions, piezometric mercury, and porosity–permeability analysis. In addition, the reservoir densification mechanism and the genesis of deep effective reservoirs were discussed. The results show that the reservoir is dominated by lithic sandstone (or lithic sandstone conglomerate). The lithic fragments primarily comprise tuffaceous volcanic rocks, supported by grains and cemented by clay, carbonate, authigenic quartz, and laumontite. The reservoir properties are characterized by extra-low porosity and permeability, and the pore type is dominated by inter- and intragrain dissolved pores of lithic fragments, feldspar, and quartz. The pore connectivity is poor due to poorly sorted extra-fine throat channels. The tightness of the reservoir.is due to the strong cementation of calcite, chlorite, montmorillonite, illite-montmorillonite mixed layer, authigenic quartz, and laumontite. Furthermore, the large amount of Ca2+ released by the hydration of tuff rock debris and intermediate–basic volcanic rock debris and the CO2-rich thermal fluid from the deep layers cause the development of several calcites. The formation of several montmorillonites is mainly related to the alteration of filled volcanic ash, and the hydration of volcanic tuff material primarily causes the development of laumontite cementation. The dissolution of feldspar and various volcanic lithic fragments by acidic fluids triggers the physical improvement of the reservoir in the local section.

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.

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 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.

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.

The features and exploration potential of the Permian tight oil in the Junggar Basin were analyzed and evaluated using outcrop, core and geochemical data etc. The Junggar Basin in the Early-Mid Permian is a saline lacustrine basin after the residual sea is closed, a set of hybrid sedimentation of deep-lake dark mudstone and dolomitic rock is developed, and the high-quality mudstone source rocks and the dolomite mudstone are alternated. High quality source rocks in mature stage are next to tight dolomitic rock reservoirs closely and provide good conditions for tight oil accumulation of proximal source type. Tight oil reservoirs are mainly distributed in the centre and slope region of the lake basin, and two types sweetspots of “dissolved pore” and “fracture-pore” exist locally. The enrichment of tight oil is controlled by the distribution of effective source rocks and dolomitic rocks, and the tight oil occurs in the entire strata vertically and spreads across large continuous areas horizontally. The Junggar Basin has four Permian tight oil distribution areas, Fencheng Formation in the Mahu sag, Lucaogou Formation in the Jimusaer sag, Pidiquan Formation in the Shazhang-Shishugou sag, and Lucaogou Formation in the Bogeda piedmont. A number of wells obtained oil flow in these areas, suggesting great resource potential and favorable targets for future exploration. Key words: Junggar Basin, Permian, saltwater lake, dolomitic rock, tight oil, resource potential