Response of Paleogene Fine-Grained Clastic Rock Deposits in the South Qiangtang Basin to Environments and Thermal Events on the Qinghai-Tibet Plateau

The occurrence and genesis of grain-coating chlorite were investigated in order to evaluate the impact of graincoating chlorite on preserving porosity in the deep-buried Triassic Karamay volcaniclastic sandstones based on thin sections, scanning electron microscopy, and an electron probe. Grain-coating chlorite was formed during the eogenesis, originating from the precursor of smectite through the solid-state transformation (SST) mechanism. The hydration and dissolution of unstable, intermediately basic volcanic rock fragments provided essential Fe 2+ and Mg 2+ ions for the formation of grain-coating chlorite. Due to relatively high stability and low susceptibility to dissolution, acidic volcanic rock fragments could not promote chlorite formation but resulted in authigenic quartz and clays as pore-filling cements. This process would destroy reservoir properties. Under high hydraulic conditions, medium-to coarse-grained sandstone experienced saltation transport, creating significant velocity differentials and pressure differentials on grain surfaces. Subsequently, clay grains adhere to the surfaces, forming grain-coating chlorite during diagenesis with good continuity. In contrast, pebbly sandstone undergoes rolling transport, resulting in smaller velocity differentials on grain surfaces. This makes relatively ineffective clay adsorption and leads to discontinuous grain-coating chlorite in subsequent stages. Under weak hydraulic conditions, grains and clay particles in fine-grained sandstone undergo suspended transport, lacking mutual movement and velocity differentials. Clay particles cannot effectively cover particles but instead fill the pores between them. Therefore, continuous grain-coating chlorite is more commonly developed in the medium-to coarse-grained sandstones and is crucial for inhibiting quartz cementation with a coverage rate exceeding 80%. Inadequate coatings fail to inhibit quartz cementation effectively, while excessive coatings may block pore throats. Optimal protection of primary porosity could occur only when graincoating chlorite is moderately developed.

Section: Discussionmentioning

confidence: 99%

Origin of Grain-Coating Chlorite and Implications for Reservoir Quality in the Triassic Deep-Buried Volcaniclastic Sandstones, Central Junggar Basin, Northwestern China

Jia,

Zeng,

Guo

et al. 2024

“…CIA and WIP have been extensively used to evaluate the weathering intensity of silicate rocks in the provenance area based on the proportions of alkali and alkaline earth elements in weathered products. − Stronger chemical weathering results in higher CIA values and lower WIP values. , Generally, low CIA values (50–65) represent low chemical weathering intensity; values between 65 and 85 indicate moderate chemical weathering, whereas high CIA values (85–100) suggest a strong chemical weathering rate . Potassium metasomatism alteration may influence CIA values and can be evaluated by the Al 2 O 3 –CaO* + Na 2 O–K 2 O (A–CN–K) diagram .…”

Section: Discussionmentioning

confidence: 99%

Geochemical Characteristics of the Middle Jurassic Coal-Bearing Mudstones in the Dameigou Area (Qaidam Basin, NW China): Implications for Paleoclimate, Paleoenvironment, and Organic Matter Accumulation

Zhou,

Sun,

Yang

et al. 2023

The Qaidam Basin is a prominent oil and gas exploration and production base of NW China’s Jurassic coal-bearing strata. Coal-bearing mudstones are important source rocks for unconventional reservoirs and can record valuable paleoenvironment and paleoclimate information. Here, geochemical analysis including total organic carbon (TOC), total sulfur, organic carbon isotopic composition, rock pyrolysis, X-ray diffraction, and major and trace elements were carried out on mudstone samples from the Middle Jurassic coal-bearing strata of the Dameigou section in the Qaidam Basin to reveal the paleoclimatic and paleoenvironmental conditions during the deposition of the strata and their controls on organic matter accumulation. Results show that the Middle Jurassic Dameigou and Shimengou formations include three significant stages based on their average TOC values of (3.32%, Stage I; 0.87%, Stage II; and 4.42%, Stage III) from the bottom to the top. The organic matter in mudstones in Stages I and II are mainly derived from terrestrial higher plants, while the organic matter has mixed sources of higher plant debris and lower aquatic organisms in Stage III. Paleoclimate parameters indicate that the mudstones in Stage I were deposited under humid and warm conditions, while the climate in Stage II changed to semiarid and warm conditions before turning dry and hot in Stage III. The varying paleoenvironmental characteristics under different paleoclimatic conditions have also been reconstructed. Our results suggest that the accumulation of organic matter in Stages I and II was primarily controlled by redox conditions, while paleoproductivity is the major controlling factor for organic matter accumulation in Stage III.

Geochemical Characteristics and Uranium Mineralization Processes in the Shawan Formation of the Chepaizi Uplift, Junggar Basin, Northwestern China

Chen,

Li,

Mao

et al. 2024

The Chepaizi Uplift, situated on the western edge of the Junggar Basin in northwestern China, has recently become a significant target area for in situ leach sandstone-type uranium exploration. The Neogene Shawan Formation, a newly identified uranium-bearing layer, has gained considerable attention for its potential. This study utilizes scanning electron microscopy (SEM), X-ray powder diffraction (XRD), whole-rock geochemistry, and electron probe microanalysis (EPMA) of uranium minerals. Combined with sedimentological and tectonic background analysis, these methods were applied to investigate geochemical characteristics and uranium mineralization processes. The sandstones in the Shawan Formation are primarily lithic sandstone and subarkose, with the provenance dominated by felsic rocks from the upper crust. Coffinite is the predominant uranium mineral, accompanied by titanium−uranium oxides and minor amounts of pitchblende. Coffinite appears as colloidal coatings around framboidal pyrite, in short-prismatic aggregates corroding albite, and as banded structures within calcite cement. Elemental ratios indicate that the Shawan Formation's paleo-hydrological environment was arid, continental, and brackish, with paleo-redox conditions reflecting a hot, dry climate. Uranium mineralization occurred in two stages: initially, uranium-containing oxygenated waters migrated laterally across slope zones, forming a redox transition zone and resulting in the pre-enrichment of uranium. Subsequently, hydrocarbons migrated along faults and unconformities, leading to secondary reduction of the interlayer oxidation zone and resulting in uranium enrichment and mineralization at the interface of grayish-green and gray sandstone layers.