Search

The Lower Ordovician Tongzi Formation containing abundant shoal sediments is the most promising stratum for the petroleum exploration in the Sichuan Basin. However, the current studies mainly focus on the central part of the Basin, the systematic analysis of the southeastern part with well-developed shoal facies is lacking. This paper aims to clarify the characteristics and genesis of the Tongzi Formation reservoir in Southeastern Sichuan Basin, following an analysis of sedimentary facies within the sequence stratigraphic framework. The research shows that the main types of reservoir rocks are oolitic, intraclastic and bioclastic dolostones. And the reservoir spaces consist mainly of intergranular (dissolved) pores, inter-crystalline (dissolved) pores, intragranular dissolved pores and fractures. Among them, intergranular pores account for the highest proportion, which is followed by intragranular dissolved pores. In addition, most throats are necking and flaky which mainly connect intergranular and inter-crystalline pores, respectively. The diagenetic sequence shows that penecontemporaneous karst well improved the porosity of the reservoir in the early-stage although the cementation and compaction reduced parts of pores. The reservoir formation is associated with the tectonic-depositional settings, diageneses and terrigenous contamination. Paleohighlands and submerged uplifts, forming in the early Ordovician amalgamation between Yangtze and Cathaysia blocks, accumulated shoal sediments as the material basis for the reservoir formation. Penecontemporaneous karst forming intragranular dissolved pores and the dolomitization aiding grainstones to resist the pressure solution are the key to increasing porosity and preserving pores. The absence of terrigenous contamination prevented intergranular pores from being strongly cemented, which resulted in the reservoir difference between the central and southeastern Sichuan Basin. The study can be used as a reference for the further exploration of Ordovician petroleum in the Sichuan Basin and other regions owning similar geological settings. [ABSTRACT FROM AUTHOR]

The Cambrian pre-salt dolomite sequence in the Tarim Basin is a target zone of great strategic significance for hydrocarbon exploration in the basin. Using the results of the interpretation of 3D seismic data from Lunnan and 2D seismic data covering the whole basin, and based on a synthesis of outcrop data, drilling data, well logs, core data, and thin-section data, and the findings from previous studies, this paper studies the characteristics of the facies of the Middle–Lower Cambrian in the platform area in the Tarim Basin, the formation and evolution of platform margins, and the sedimentary characteristics, lithofacies, and paleogeographic characteristics of the Middle–Lower Cambrian series. Based on the types of lithofacies and the seismic facies analysis, the sedimentary facies of the Middle–Lower Cambrian strata can be classified into the five types. From bottom to top, the Middle–Lower Cambrian have experienced several development stages, including the deposition of continental shelf sediments of the Yuertusi Formation, gently sloping non-rimmed platform margin sediments of the Xiaoerbulake Formation, weakly rimmed platform margin sediments of the Wusongger and Shayilike formations, and strongly rimmed platform margin sediments of the Awatage Formation. In each stage, the platform margin shifted further east relative to its location in the previous stage. Based on these results and previous studies, the lithofacies and paleogeography maps of the Middle–Lower Cambrian in the Tarim Basin are modified, and the implications for hydrocarbon exploration in this area are discussed. [ABSTRACT FROM AUTHOR]

Siliceous minerals of the Dengying Formation in the Gaoshiti–Moxi area in the central Sichuan Basin exhibit four types of quartz crystals (cryptocrystalline quartz, chalcedony, microcrystalline quartz, and megacrystalline quartz) and three structural types: cryptocrystalline, microcrystalline, and mosaic (laminated mosaic, window-hole interrupted mosaic, and arc-laminated mosaic). Siliceous minerals have a great influence on the storage performance of the reservoirs in the Dengying Formation. According to the petrophysical parameters of the Dengying Formation and porosity intersection diagrams, the siliceous dolomite and the reservoirs have low impedance characteristics, which makes it difficult to distinguish between them and leads to difficulties in the characterization and prediction of the reservoirs. The transverse wave velocity is favorable for reservoir characterization. Currently, the main method used to estimate the transverse wave velocity is petrophysical modeling, which establishes a relationship between the elastic and physical parameters of the reservoir. In this paper, the siliceous minerals in the dolomite in the study area are regarded as solid inclusions, and the calculation method of the rock matrix modulus is improved by using solid replacement. Then, an improved petrophysical model is constructed by combining the KT (Kuster–Toksöz) model, the DEM (Discrete Element Method) model, the Gassmann equation, and the Wood equation. The transverse wave velocity is estimated using the improved model under the constraint of the longitudinal wave velocity. The shapes of the transverse wave velocity curves obtained by the improved model and the deviations from the measured velocities are significantly better than those of the Xu–Payne model and other models. The results show that the improved model can effectively estimate the transverse wave velocity of the reservoir in this area, which provides a basis for future reservoir predictions in this area. [ABSTRACT FROM AUTHOR]

It is beneficial in terms of the theoretical significance and application prospects to define the structure and reservoir development model of the lower Paleozoic unconformity in the Jiyang Depression of Bohai Bay Basin, China, for oil and gas exploration of unconformity in carbonate strata. Geological and geochemical evidence shows that a regional unconformity formed during the Huaiyuan Movement in the lower Paleozoic strata of the Jiyang Depression. Along the top of the regional unconformity between the Yeli Liangjiashan Formation and Fengshan Formation, various types of karst breccia have developed, showing prominent characteristics of development and vertical karst zonation. The paleokarst zone can be divided into the vadose zone and the underflow zone, and there are apparent differences between the two zones in terms of the mode of karst activity and type of reservoir space. Primitive sedimentary microfacies, dolomitization, and supergene karstification controlled the reservoirs of the Fengshan Formation and Yeli-Liangjiashan Formation. There are significant differences in the original physical properties due to the differences in the original sedimentary microfacies. The pore development of granular dolomite of high-energy beach facies has the best reservoir performance. In the later period, the superposition of dolomitization and supergene karstification resulted in apparent differences in karst development mode, development intensity, reservoir type, and reservoir physical properties. Among them, the granular dolomite reservoir has the best physical properties and has developed a cavity-type reservoir that has a planar distribution along an unconformity surface. [ABSTRACT FROM AUTHOR]

The Zhe-Min Uplift (ZMU) has undergone tectonic subsidence, which has been associated with a notable transgression in the northern marginal seas of China since the middle Pleistocene. However, its uplift history during the late Quaternary is still inadequate. The Old Red Sand (ORS) is a late Pleistocene deposit that is widely distributed along the southeastern coast of China. The ORS consists of partially-cemented fine- to medium-grained sand. In this paper, we focus on mud-rich ORS deposits of the Liushui area, Pingtan Island, Fujian Province, and report optically stimulated luminescence (OSL) ages, and new datasets of grain size, mineralogy (detrital zircon, clay and detrital minerals) and organic carbon and nitrogen isotope profiles to improve understanding of chronology, provenance, palaeoenvironmental evolution and tectonic infuence. Based on the OSL data, we hypothesize that the development of the ORS was associated with lower sea levels, with most deposition occurring within MIS4. Based on comparison of detrital zircon U Pb ages, the coarse fraction of the Liushui ORS was mainly derived from the Minjiang River, with modest input from weathering of coastal granites. The East Asian Winter Monsoon contributed some materials from the East China Sea shelf during the last glacial period, as indicated by the fine-grained nature of the Liushui ORS, low K-feldspar/plagioclase ratios, high dolomite content and the frequency of Neoproterozoic and Palaeoproterozoic detrital zircons. The deposition of the ORS was influenced by both the East Asian Winter Monsoon and sea level changes. The clay minerals in the mud layers of the Liushui ORS were dominated by kaolinite and illite, with little chlorite and no smectite, indicating that they originated from the Minjiang River. The TOC/TN ratios of the mud layers, as well as their isotope characteristics, indicate the organic matter was derived from marine sources, presumably during the transgression event in MIS5. The average uplift rate of the ORS was about 0.03 mm/yr. The uplift of the ORS provided new insights into the geological records of the ZMU, which differed from the subsidence observed in the northern region of the Minjiang River during the late Pleistocene. • The formation of the ORS was association with lower sea level, and the MIS4 was primarily the deposition stage. • During the last glacial, the more sediments from the East China Sea shelf were transported to the ORS by EAWM. • The mud layer was the transgression records during MIS5. • The distribution of the ORS was influenced by tectonic activity. [ABSTRACT FROM AUTHOR]

Sandy Dolomite is a kind of widely distributed rock. The uniaxial compressive strength (UCS) of Sandy Dolomite is an important metric in the application in civil engineering, geotechnical engineering, and underground engineering. Direct measurement of UCS is costly, time-consuming, and even infeasible in some cases. To address this problem, we establish an indirect measuring method based on the convolutional neural network (CNN) and regression analysis (RA). The new method is straightforward and effective for UCS prediction, and has significant practical implications. To evaluate the performance of the new method, 158 dolomite samples of different sandification grades are collected for testing their UCS along and near the Yuxi section of the Central Yunnan Water Diversion (CYWD) Project in Yunnan Province, Southwest of China. Two regression equations with high correlation coefficients are established according to the RA results, to predict the UCS of Sandy Dolomites. Moreover, the minimum thickness of Sandy Dolomite was determined by the Schmidt hammer rebound test. Results show that CNN outperforms RA in terms of prediction the precision of Sandy Dolomite UCS. In addition, CNN can effectively deal with uncertainty in test results, making it one of the most effective tools for predicting the UCS of Sandy Dolomite. [ABSTRACT FROM AUTHOR]

The organic-rich mudstones and dolostones of the Permian Fengcheng Formation (Fm.) are typically alkaline lacustrine source rocks, which are typified by impressively abundant β-carotane. Abundant bcarotane has been well acknowledged as an effective indicator of biological sources or depositional environments. However, the specific biological sources of β-carotane and the coupling control of biological sources and environmental factors on the enrichment of β-carotane in the Fengcheng Fm. remains obscure. Based on a comprehensive investigation of the bulk, molecular geochemistry, and organic petrology of sedimentary rocks and the biochemistry of phytoplankton in modern alkaline lakes, we proposed a new understanding of the biological precursors of β-carotane and elucidated the enrichment mechanism of β-carotane in the Fengcheng Fm. The results show that the biological precursors crucially control the enrichment of β-carotane in the Fengcheng Fm. The haloalkaliphilic cyanobacteria are the primary biological sources of β-carotane, which is suggested by a good positive correlation between the 2-methylhopane index, 7- + 8-methyl heptadecanes/Cmax, C29%, and β-carotane/Cmax in sedimentary rocks and the predominance of cyanobacteria with abundant β-carotene in modern alkaline lakes. The enrichment of β-carotane requires the reducing condition, and the paleoredox state that affects the enrichment of β-carotane appears to have a threshold. The paleoclimate conditions do not considerably impact the enrichment of β-carotane, but they have some influence on the water's paleosalinity by affecting evaporation and precipitation. While it does not directly affect the enrichment of β-carotane in the Fengcheng Fm., paleosalinity does have an impact on the cyanobacterial precursor supply and the preservation conditions. [ABSTRACT FROM AUTHOR]

A total of 18 samples of shallow groundwater around the Hancheng mining area in the Guanzhong Basin were collected from 1–4 May 2018. According to the analysis of hydrochemical data, the Gibbs semi-logarithmic diagram and Piper diagram were used to research the hydrogeochemical behavior of shallow groundwater around the Hancheng mining area in the Guanzhong Basin. The results of the groundwater hydrochemical analyses shown on the Gibbs and Piper plots are as follows: The chemical composition analysis showed that the main cation components were Mg2+, Ca2+, Na+, and K+, the anion components were HCO3−, Cl−, and SO42−. A measure of 89% of the groundwater samples in this area were freshwater, the HCO3− were mainly dolomite, calcite, and gypsum dissolved precipitation resulted. Na+ and Cl− came from the dissolution of halite. Most of the groundwater was of the SO4·Cl-Ca·Mg type, accounting for 61.1%. The main ion chemistry of the shallow groundwater in this area is controlled by rock weathering, and pyrite oxidation is a significant factor affecting the SO42− concentration. These research results will help analyze the formation mechanism of chemical components and provide some basic data for the evolution of mine water in this area in the future. [ABSTRACT FROM AUTHOR]

Xianxian geothermal exploitation area is one of the areas with the best occurrence conditions of geothermal resource in North China, rich in medium and low temperature geothermal resource, and is a typical carbonate geothermal field. However, its hydrogeochemical evolution process is not clear, and no complete conceptual model of geothermal genesis has been established to guide the exploration, development and utilization of geothermal resource in the area. It restricts the sustainable development of regional geothermal resource and the realization of carbon emission reduction targets. In this study, geothermal fluids in the area were collected for testing and analysis in terms of hydrochemical characteristics and stable isotopes. Based on systematic geochemical research, the formation mechanism of geothermal fluids was analyzed, and a conceptual model of geothermal genetic in the study area was established. The main understanding is as follows: the hydrochemical type of geothermal water in the study area is Cl-Na type, and the water-rock interaction is not balanced. The formation of geothermal water in the area is affected by the infiltration of atmospheric precipitation, and it is terrestrial leached water. The flow path of geothermal water is long, the water circulation is slow, and the metamorphism is deep. Dolomite, calcite and gypsum dissolution are the main sources of Mg2+, Ca2+, , and in geothermal water, besides, other minerals containing calcium and magnesium are also part of their sources. The source of geothermal water recharge is atmospheric precipitation, and the recharge elevation of geothermal water is about 619–1069 m. The age of the geothermal water is between 16 000 and 18 000 years. The sulfur in geothermal water mainly comes from the leaching of sedimentary layer. [ABSTRACT FROM AUTHOR]