Influences of burial process on diagenesis and high-quality reservoir development of deep-ultra-deep clastic rocks:A case study of Lower Cretaceous Qingshuihe Formation in southern margin of Junggar Basin,NW China

Taking the Lower Cretaceous Qingshuihe Formation in the southern margin of Junggar Basin as an example,the influences of the burial process in a foreland basin on the diagenesis and the development of high-quality reservoirs of deep and ultra-deep clastic rocks were investigated using thin section,scanning electron microscope,electron probe,stable isotopic composition and fluid inclusion data.The Qingshuihe Formation went through four burial stages of slow shallow burial,tectonic uplift,progressive deep burial and rapid deep burial successively.The stages of slow shallow burial and tectonic uplift not only can alleviate the mechanical compaction of grains,but also can maintain an open diagenetic system in the reservoirs for a long time,which promotes the dissolution of soluble components by meteoric freshwater and inhibits the precipitation of dissolution products in the reservoirs.The late rapid deep burial process contributed to the development of fluid overpressure,which effectively inhibits the destruction of primary pores by compaction and cementation.The fluid overpressure promotes the development of microfractures in the reservoir,which enhances the dissolution effect of organic acids.Based on the quantitative reconstruction of porosity evolution history,it is found that the long-term slow shallow burial and tectonic uplift processes make the greatest contribution to the development of deep-ultra-deep high-quality clastic rock reservoirs,followed by the late rapid deep burial process,and the progressive deep burial process has little contribution.

Restoration of reservoir diagenesis and hydrocarbon accumulation process by calcite in-situ U-Pb dating and fluid inclusion analysis: A case study on Cretaceous Qingshuihe Formation in Gaoquan Structure, southern Junggar Basin, NW China

The complexity of diagenesis and hydrocarbon accumulation in the deep reservoirs in southern Junggar Basin restricts hydrocarbon exploration in the lower reservoir assemblage. The lithofacies and diagenesis of reservoirs in the Cretaceous Qingshuihe Formation in the Gaoquan structure of the Sikeshu Sag, southern Junggar Basin were analyzed. On this basis, the thermal history was calibrated using calcite in-situ U-Pb dating and fluid inclusion analysis to depict the hydrocarbon accumulation process in the Gaoquan structure. The results show that the Qingshuihe reservoir experienced two phases of calcite cementation and three phases of hydrocarbon charging. The calcite cements are dated to be (122.1±6.4) Ma, (14.4±1.0) Ma - (14.2±0.3) Ma. The hydrocarbon charging events occurred at around 14.2-30.0 Ma (low-mature oil), 14.2 Ma (mature oil), and 2 Ma (high-mature gas). The latter two phases of hydrocarbon charging contributed dominantly to the formation of reservoir. Due to the S-N compressive thrust activity during the late Himalayan period since 2 Ma, the traps in the Gaoquan structure were reshaped, especially the effective traps which developed in the main reservoir-forming period were decreased significantly in scale, resulting in weak hydrocarbon shows in the middle-lower part of the structure. This indicates that the effective traps in key reservoir-forming period controlled hydrocarbon enrichment and distribution in the lower reservoir assemblage. Calcite U-Pb dating combined with fluid inclusion analysis can help effectively describe the complex diagenesis and hydrocarbon accumulation process in the central-west part of the basin.

Hydrocarbon generation differences of shales composed of green algal and cyanobacteria: A case study of Mesozoic and Cenozoic saline lacustrine shales in Junggar Basin, NW China

The Mesozoic and Cenozoic strata in the Junggar basin developed two sets of shallow to semi-deep lacustrine shale, namely, the Cretaceous Qingshuihe Formation (K_(1q)) and the Paleogene Anjihaihe Formation (E_(2-3a)). Through organic petrology and scanning electron microscope (SEM) observation, it is found that the primary hydrocarbon-generating organic matter (OM) in the two sets of strata is different. The biological precursor of the E_(2-3a) OM is mainly green algae (Pediastrum), while the precursor of K_(1q) kerogen is mainly cyanobacteria (Oscillatoria). Then, the E_(2-3a) green algae-rich shale and K_(1q) cyanobacteria-rich shale were subjected to hydrous pyrolysis and kinetic analysis, respectively. The results show that the evolution modes of hydrocarbon generation of the typical shales are very different. Green algae have the characteristics of a low oil generation threshold, heavy oil quality, and no prominent oil peak, while cyanobacteria have the characteristics of late oil generation, concentrated hydrocarbon generation, and relatively light oil quality. The characteristics of oil generation can also be well reflected in the composition evolution of the crude oil components. The carbon isotope of gas, kerogen, and extracts of the E_(2-3a) green algae-rich shale are significantly heavier than the K_(1q) cyanobacteria-rich shale, which may be related to the living habits of their biological precursors, carbon source usage, photosynthesis efficiency, and carbon fixation efficiency.

Reformation of deep clastic reservoirs with different diagenetic intensities by microfractures during late rapid deep burial: Implications from diagenetic physical simulation of Cretaceous Qingshuihe Formation in the southern margin of Junggar Basin, NW China

Constrained by the geological burial history of Cretaceous Qingshuihe Formation in the southern margin of Junggar Basin, the diagenetic physical simulation experiment was carried out with the low-mature sandstone samples taken from the outcrop area. Then, coupling with the regional geological data, the reformation of reservoirs with different diagenetic intensities by microfractures and the significance of microfractures for development of high-quality reservoirs were discussed. The results show that the large-scale microfractures were formed in the stage of late rapid deep burial, roughly equivalent to the period when organic acids were filled. The microfractures created good conditions for migration of oil and gas in deep and ultra-deep clastic rocks, and also enabled the transport of organic acids to the reservoirs for ensuing the late continuous dissolution of cements and particles. The existence of matrix pores and microfractures in the reservoirs before the rapid deep burial determined how the microfractures formed during rapid deep burial improved the reservoir quality. If matrix pores and microfractures were more developed and the cementation degree was lower before the rapid deep burial, the microfractures would be more developed and the dissolution degree would be higher during the late rapid deep burial, and so the reservoir quality would be improved more greatly, which can increase the reservoir permeability by up to 55%. If cementation was very strong, but matrix pores were not developed and microfractures existed locally before the rapid deep burial, the microfractures would also be more developed during the late rapid deep burial, which can increase the reservoir permeability by 43%. If cementation was strong, matrix pores were absent, and microfractures were not developed, limited microfractures would be formed during the late rapid deep burial, which can increase the reservoir permeability by only 16%. Formation of large-scale microfractures during late rapid deep burial and promotion of such microfractures to the dissolution of organic acids are considered as key diagenetic factors for the development of deep and ultra-deep high-quality reservoirs.