Xiucheng Tan

The Cretaceous Itapema and Barra Velha Formations in Brazil’s Santos Basin represent a rare giant rift lacustrine carbonate factory with considerable potential for oil and gas exploration. Through a comprehensive petrological and petrophysical analysis of field A in Santos Basin, this study clarifies the reservoir properties and distribution of this lacustrine carbonate facies and further investigates the controlling factors and differential evolution of lacustrine carbonate reservoirs. The main research results are as follows: (1) The Itapema Formation is characterized by bioclastic limestones that predominantly feature primary interparticle pores. As the alkalinity and salinity of the lake water gradually increased, the reservoir rocks of the Barra Velha Formation evolved into shrubstones and intraclastic limestones, with reservoir spaces comprising growth-framework pores and interparticle pores. (2) High-quality reservoirs are distributed regularly and cyclically below the exposure surfaces of high-frequency cycles. The quality of the reservoir exhibits an improvement from the bottom to the top of upward-shallowing sequences. These sequences, which are generally stacked vertically, enhance the heterogeneity of the reservoir. (3) Fluctuations in lake levels have facilitated the process of penecontemporaneous karstification, resulting in the formation of numerous dissolved pores and vugs. Generally, karstification enhances the quality of reservoirs; however, excessive karstification at the tops of cycles may result in the infilling of clay within breccias, consequently diminishing reservoir quality. (4) Multiple magmatism during rift evolution exert a dual control on reservoir space. Intense silicification can lead to the complete filling of pores and fractures with siliceous cement, thereby compromising reservoir spaces. However, rim-like siliceous cements formed during syn-depositional magmatism effectively isolate pore spaces, inhibiting material exchange during shallow burial. This process achieves a balance between dissolution and precipitation that ultimately preserves porosity. In general, reservoirs within the Itapema Formation exhibited a developmental trajectory that transitioned from superior primary porosity to karst-enhanced intraparticle porosity, ultimately resulting in rim-like siliceous cements that preserved the pores. In contrast, the reservoirs of the Barra Velha Formation experienced a different evolutionary process, commencing with the differentiation of matrix porosity, which was subsequently increased by karstification. During the penecontemporaneous and burial stages, the pores infilled with silica and dolomite cements. This study presents a systematic analysis of the reservoir properties and diagenetic evolution of this set of lacustrine carbonate rocks. It contributes to the theoretical understanding of lacustrine carbonate deposition and provides valuable references for carbonate reservoir exploration in rift lake basins.

Interactions between mountain building, weathering and climate draw significant attention within the geoscience community. In this study, the mixed rocks from the Eocene Lower Ganchaigou Formation (LGCG) of the Qaidam Basin are employed for weathering intensity evaluation by petrology, mineralogy, and geochemistry. Lithological characteristics indicate high‐frequency sedimentary cycles (<10 m) of the LGCG mixed rocks, which are identified by a single cycle with four lithofacies from bottom to top: massive sandstones, laminated silty shales, laminated silty limestones, and bedded dolomites. Provenance analysis suggests a predominant felsic volcanic source within a continental island arc background for these mixed sediments. Chemical weathering intensity indices (CIA, chemical index of weathering, and plagioclase index of alteration) show a high coherence with sedimentary cycles, initially increasing and gradually decreasing in a single cycle. Chemical index of alteration values ranged from 51.1% to 67.2%, indicating weak to moderate weathering intensity. Additionally, correlation analysis reveals that the cyclical variations of chemical weathering intensity correlate strongly with regular paleoclimate evolution and terrigenous input changes, which were controlled by the shift of the westerlies. During the westerly strengthening periods, stronger precipitation results in a warm and humid paleoclimate conditions, leading to higher terrigenous input dominated by fine‐grained clay minerals with moderate chemical weathering intensity. During the westerly weakening periods, less rainfall leads to a cold and dry paleoclimate conditions, resulting in lower terrigenous input dominated by coarser clastic sediments and weak weathering intensity. Our study provides new evidence and insights into the westerly shift and paleoclimate evolution in the Eocene from the perspective of high‐resolution chemical weathering records.

The potential for oil and gas exploration within inter-salt reservoirs is substantial, primarily due to their significant heterogeneity, which complicates accurate predictions. This study focuses on the inter-salt reservoirs of the sixth sub-member of the fifth member of the Majiagou Formation (hereafter referred to as O₂m₅⁶) in the Ordos Basin, North China. Utilizing core samples, thin sections, and petrophysical data, we investigated the differential evolution and primary controlling factors of the inter-salt carbonate reservoirs. The key findings are as follows: (1) During the sedimentary phase of O₂m₅⁶, high-energy sediments, such as shoals and microbial mounds, were deposited in highlands, while low-energy sediments, including dolomitic lagoons and gypsiferous lagoons, emerged in depressions from west to east. (2) In a restricted–evaporative environment, highlands are prone to karstification, which significantly enhances the development of inter-salt reservoirs and generates a variety of reservoir spaces, including interparticle dissolved pores, growth-framework dissolved pores, and micropores between vadose silts. (3) The presence of alternating highlands and depressions obstructs seawater flow, leading to a progressive increase in salinity from west to east. This process ultimately facilitates the infilling of reservoir spaces with calcite, anhydrite, and halite cements in the same direction. (4) The three components—reservoir rocks, karstification, and infilling features—exert varying effects in the region and collectively govern the north–south distribution of inter-salt reservoirs. Overall, this study examines the characteristics and controlling factors of carbonate reservoirs within a restricted–evaporative platform environment and provides pertinent research cases for the exploration of inter-salt reservoirs.

Based on the research results of stratigraphic sedimentary facies, plane maps of sedimentary facies for four periods of the Neogene terrestrial foreland basin in Southwest Tarim were compiled. Research results conclude that the Neogene Miocene developed alluvial fan facies, fan delta facies, and lacustrine facies successively from the piedmont belt to the Maigaiti Slope. During the sedimentation of the Miocene Anju’an Formation, the lake range expanded to its maximum. According to the new division scheme for five-order sequences in prototype sedimentary basin, the stratigraphic sequence of a terrestrial foreland basin was divided and compared, and a sequence development and evolution model was established. Research results show that: (1) the Neogene–Quaternary is a typical type I sedimentary sequence with huge thickness; (2) the Neogene is a period of tectonic compression and flexural subsidence sedimentation in the foreland basin; with the maximum lake flooding surface at the top of the Miocene Anju’an Formation as the boundary, the lower part is a second-order super-sequence of the lacustrine transgression system tracts (TST), the upper part is a second-order super-sequence of the highstand system tracts (HST), and the Quaternary is a second-order super-sequence of the lowstand system tracts (LST) in forced lacustrine regression caused by the Late Himalayan tectonic movement; (3) the classic sequence stratigraphy made an error in establishing the theoretical model of marine type I sequences, as it placed the LST in the wrong position. The corrected type I sequences are in the order of TST, HST, and LST. (4) Finally, two examples were applied to demonstrate the development position of the LST, indicating that whether it is a marine or terrestrial prototype sedimentary basin, the LST was developed during the late stage of tectonic compression and uplift in a basin, and is the sedimentary sequence at the top of the prototype sedimentary basin.

Lacustrine sedimentary processes exhibit high sensitivity to paleoenvironmental changes, often manifesting as high-frequency sedimentary cycles that control the complex variations in sedimentary structure, mineral composition, and element distribution. However, the intricate co-variation mechanism among paleoclimate and paleowater properties at a high precision level (centimeter to meter scale) is still controversial. This study focuses on conducting a high-frequency cycle analysis of lacustrine mixed rocks from the Eocene Lower Ganchaigou Formation (LGCG) in the Qaidam Basin, employing petrology, mineralogy, organic geochemistry, and elemental geochemistry techniques. The lithological variation was characterized by the superposition of three lithofacies types from the bottom to the top with a single sequence: massive sandstone, laminated silty shale, and bedded calcareous dolostone. Geochemical data revealed cyclical variations in the paleoenvironment in the vertical profile, which conformed to the high-frequency lithofacies cycles. Based on the reconstruction of the lake level and paleowater properties, a synthesized paleoclimate–sedimentary model that comprised three consecutive periods within an individual sequence was established. From the bottom to the top of each cycle, the Eocene paleoenvironment varied from reduction and desalination to oxidation and salinization, which was controlled by a decline in the lake level resulting from a transformation of the paleoclimate from warm and humid to cold and arid. The variations in petrology and geochemistry observed in the Eocene Qaidam Basin play a crucial role in comprehending the sedimentary response to paleoenvironmental changes at high precision levels within lacustrine settings.

Due to the present tectonic and stratigraphic distribution characteristics in the northwest margin of the Sichuan Basin are complex, which restricts the understanding of sequence lithofacies paleogeography of the Middle Permian Maokou Formation. To investigate the process of tectonic–sedimentary evolution, basin-margin outcrop and intra-basin well and seismic data were used. The results show two structural sequences, namely, SSQ1 and SSQ2, and five third-order sequences, namely, SQ1–SQ5, in the Middle Permian Maokou Formation. SSQ1, with stable formation thickness, shows a slow transgression–fast regression sequence structure. SSQ2 shows a fast transgression–slow regression sequence structure in the area with large thickness and a sustained transgression–fast regression sequence structure in the area with small thickness owing to thickness differentiation caused by tectonic subsidence. Sedimentological analysis shows that sequences SQ1–SQ2 mainly consist of carbonate-ramp sediments, the sedimentary facies of which change from intermediate ramp to outer ramp as the area depressed topographically from south to north. Sequences SQ3–SQ5 mainly consist of rimmed carbonate platform sediments; the platform margin turned up along Shuangyushi–Jian’ge–Yuanba, and the region to the north subsided rapidly to form slope-basin facies. Lithofacies paleogeography in the Middle Permian epoch shows that the northwest margin of the Sichuan Basin was dominated by extensional stresses in northeast and northwest directions. Tectonic activities have a dominant control on the distribution of two structural sequences and the evolution of carbonate platform types in the Maokou Formation, and the sea-level change has a dominant control on third-order sequence boundaries and sedimentary facies migration. Thus, the Middle Permian tectono-sedimentary evolution model of the northwest margin of the Sichuan Basin is established. Research findings may offer new ideas and theoretical support to promising facies exploration in the Maokou Formation in the northwest margin of the Sichuan Basin.

Based on the identification of visual kerogen assessment, a subdivision method of organic matter types was proposed. Combined with GR curve, the distribution was further studied, the relationship between shale reservoir parameters and organic matter types was analyzed, and the significance was discussed. The results show that the kerogen types of organic rich shale in the lower part of Wufeng-Longmaxi Formation in the south of the Sichuan Basin are mainly of type I, II₁a and II₁b, while type II₂ and type III are not developed. In Wufeng Formation, the shale organic matter is mainly of type II₁a and II₁b. From Long1₁¹ to Long1₁⁴, the shale displays a trend of “Type I-II₁b-II₁a-II₁b”. Type I was characterized by higher TOC, porosity and gas bearing capacity of shale reservoirs than the other organic matter. The reservoir parameter of type II₁a was better than type II₁b, indicating that the organic matter type control on the quality of shale reservoirs. Type difference is closely related to the bio-precursors component, among which type I and type II₁a are related to the content of planktonic algae, while type II₁b is related to the content of benthic algae. This study provides a good reference for the subdivision of marine shale organic matter types and the evaluation and prediction of gas bearing properties.

Pyrite, as a characteristic mineral in organic-rich marine shale, is a significant index for the interpretation of paleoredox conditions. In this study, based on drilling cores and focused ion beam-scanning electron microscopy (FIB-SEM), the occurrence, diameter and particle size distribution of pyrites from 32 samples obtained from the Wufeng–Longmaxi Formation in the southeast Sichuan Basin were analyzed. The results show that pyrite displays various occurrences at the macro-scale and micro-scale. At the macro-scale (mm–cm), pyrite laminations, nodular pyrites and lenticular pyrites can be found from drilling cores. At the micro-scale (nm–µm), the common occurrences of pyrite are pyrite framboids, euhedral pyrites and infilled pyrite framboids. According to the formation mechanism of pyrites, pyrites can be divided into syngenetic pyrites and diagenetic pyrites. The infilled pyrite framboids are categorized as diagenetic pyrites. The mean pyrite framboid diameters (Mean, D) range from 2.94 µm to 5.33 µm (average of 4.26 µm), with most samples showing pyrite framboid diameters from 3.5 μm to 4.8 μm. Most of the diameters of the framboid microcrystals (Mean, d) are less than 0.4 µm. Therefore, according to the (Mean, D) and the (Mean, d), the pyrite framboids can be divided into three sizes: syngenetic framboids (SF, D < 4.8 µm, d ≤ 0.4 µm), early diagenetic framboids (EDF, D > 4.8 µm, d > 0.4 µm) and late diagenetic framboids (LDF, D < 4.8 µm, d > 0.4 µm). Additionally, box-and-whisker charts of the diameter, standard deviation/skewness value of the mean diameter of pyrite framboids (Mean, D) and the ratio of trace elements indicate that the sedimentary water body was a euxinic–dysoxic environment. Euxinic conditions dominated the Wufeng Formation to the lower part of the Long11-3 section, which is beneficial for the preservation of organic matter. However, the middle-upper part of the Long13-Long12 sub-member is a dysoxic sedimentary environment.