The microstructure of pores strongly influences fluid mobility in tight sandstone reservoirs. The impacts of the pore–throat structure on fluid mobility are restricted due to the limitations of ...existing methods for determining pore–throat structures. Therefore, the methods utilized to obtain more comprehensive and accurate pore–throat information need to be further discussed by combining multiple experiments. In this research, eight samples were selected from the Upper Triassic Yanchang Formation in the Ordos Basin to investigate the microstructure and its impacts on fluid mobility by means of nuclear magnetic resonance (NMR), rate-controlled porosimetry (RCP), scanning electron microscopy, and thin sections. The throat radii in the throat, pore–throat, and pore areas were studied by combining NMR and RCP with an improved method. The throat radius and pore–throat volume affect the movable fluid from connectivity and movable ratio, respectively. The throat and pore–throat (pore) areas belong to unrelated movable fluid networks. Each area contains intragranular and intergranular spaces. Irregularly shaped pores commonly occur in the intragranular space, with strong connectivity. The pores with regular shape in the intergranular space connect a small amount of throat, and a large number of clay minerals divide the space into small throats. Clay minerals reduce fluid mobility by increasing the thickness of the water film in the throat. Therefore, fluid mobility in the intragranular space is greater than that in the intergranular space from the perspective of water film thickness and movable probability in the same area. This research improves the accuracy of the combined method of NMR and RCP and clarifies the distribution mechanism of movable fluids in different spaces.
Carbonate rocks are important targets of hydrocarbon exploration in the Gucheng area, Tarim Basin. In this study, the thin sections, carbon and oxygen isotopes, trace elements and rare earth elements ...of 25 carbonate samples from the Upper Cambrian and the Ordovician are conducted to research the original sedimentary environment of these carbonate rocks. The carbonate rocks have undergone high-temperature hydrothermal activity due to the movement of magmatic water in the Permian based on carbon and oxygen isotope and Eu
anom
data. The Upper Cambrian and Ordovician are divided into five third-order sequences based on carbon isotopes and natural gamma-ray spectral logging combined with previous researches. The SQ1~SQ3 in the Upper Cambrian and Middle-Lower Ordovician predominant in sea-level decline cycle, whereas the SQ4 and SQ5 in the Middle-Upper Ordovician are dominated by sea-level rise cycle. Ce
anom
and Z values are suitable to analyze the redox environment and paleosalinity quantitatively, which suggests an oxidizing and marine origin in this area. The dolomites are formed in a paleoenvironment with a higher salinity, a stronger oxidation and a lower falling sea level than that of the limestones.
Pyrite is the most common authigenic mineral preserved in many ancient sedimentary rocks. Pyrite also widely exists in the Longmaxi and Wufeng marine shales in the middle Yangtze area in South China. ...The Longmaxi and Wufeng shales were mainly discovered with 3 types of pyrites: pyrite framboids, euhedral pyrites and infilled framboids. Euhedral pyrites (Py4) and infilled framboids (Py5) belong to the diagenetic pyrites. Based on the formation mechanism of pyrites, the pyrites could be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. Under a scanning electron microscope (SEM), the syngenetic pyrites are mostly small framboids composed of small microcrystals, but the diagenetic pyrites are variable in shapes and the diagenetic framboids are variable in sizes with large microcrystals. Due to the deep burial stage, the pore space in the sediment was sharply reduced and the diameter of the late diagenetic framboids that formed in the pore space is similar to the diameter of the syngenetic framboids. However, the diameter of the syngenetic framboid microcrystals is suggested to range mainly from 0.3 µm to 0.4 µm, and that of the diagenetic framboid microcrystals is larger than 0.4 µm in the study area. According to the diameter of the pyrite framboids (D) and the diameter of the framboid microcrystals (d), the pyrite framboids could be divided into 3 sizes: syngenetic framboids (Py1, D < 5 µm, d ≤ 0.4 µm), early diagenetic framboids (Py2, D > 5 µm, d > 0.4 µm) and late diagenetic framboids (Py3, D < 5 µm, d > 0.4 µm). Additionally, the mean size and standard deviation/skewness values of the populations of pyrite framboids were used to distinguish the paleoredox conditions during the sedimentary stage. In the study area, most of the pyrite framboids are smaller than 5 µm, indicating the sedimentary water body was a euxinic environment. However, pyrite framboids larger than 5 µm in the shales indicated that the sedimentary water body transformed to an oxic-dysoxic environment with relatively low total organic carbon (TOC: 0.4–0.99%). Furthermore, the size of the framboid microcrystals could be used to estimate the gas content due to thermochemical sulfate reduction (TSR). The process of TSR occurs with oxidation of organic matter (OM) and depletes the H bond of the OM, which will influence the amount of alkane gas produced from the organic matter during the thermal evolution. Thus, syngenetic pyrites (d ranges from 0.35 µm to 0.37 µm) occupy the main proportion of pyrites in the Wufeng shales with high gas content (1.30–2.30 m3/t), but the Longmaxi shales (d ranges from 0.35 µm to 0.72 µm) with a relatively low gas content (0.07–0.93 m3/t) contain diagenetic pyrites. Because of TSR, the increasing size of the microcrystals may result in an increase in the value of δ13C1 and a decrease in the value of δ13C1-δ13C2. Consequently, the size of pyrite framboids and microcrystals could be widely used for rapid evaluation of the paleoredox conditions and the gas content in shales.
Numerous deep oil and gas reservoirs have recently been discovered in the Kelasu-Yiqikelike structural belt of the Kuqa Depression in the northern Tarim Basin, north-western China. The sources and ...accumulation processes of the oil and gas are determined in this study through a series of biomarker geochemical parameter analyses and fluid inclusion analyses, respectively. The geochemical parameters show that the oil of the Kela 2 gas field originated from the Triassic Huangshanjie (T
3
h) Formation lacustrine source rock and that the oil of the Jurassic reservoirs in the Dibei gas field was sourced from Jurassic coal measures, with some contribution from the T
3
h source rock. Furthermore, the gas in the Kela 2, Keshen and Dibei gas fields was derived from the Jurassic coal measures. Based on petrographic and fluid inclusion analyses, combined with a reconstruction of the reservoir burial history, timing estimates and charge models of the hydrocarbon accumulation have been obtained. Two stages of oil charging and one stage of gas charging were identified in the Kela 2 gas field. The two stages of oil charging occurred in the middle–late Miocene and in the early Pliocene. The gas charge occurred in the middle–late Pliocene. Additionally, one stage of gas charging occurred in the Keshen gas field in the late Pliocene to Quaternary. In contrast, two hydrocarbon charging stages were identified in the Dibei gas reservoirs, with the first occurring in the early Miocene and the second occurring in the Pliocene.
► GC–IRMS was performed for crude oils and components in source and reservoir rocks. ► The δ13C values show the marine oils are derived from two types of source rocks. ► The δ13C values for oil ...components in reservoir rocks reveal source variations. ► δ13C values do not correlate with biomarker parameters for mature oils.
Carbon isotopic compositions were determined by GC–IRMS for individual n-alkanes in crude oils and the free, adsorbed and inclusion oils recovered by sequential extraction from reservoir rocks in the Tazhong Uplift and Tahe oilfield in the Tabei Uplift of Tarim Basin as well as extracts of the Cambrian–Ordovician source rocks in the basin. The variations of the δ13C values of individual n-alkanes among the 15 oils from the Tazhong Uplift and among the 15 oils from the Triassic and Carboniferous sandstone reservoirs and the 21 oils from the Ordovician carbonate reservoirs in the Tahe oilfield demonstrate that these marine oils are derived from two end member source rocks. The major proportion of these marine oils is derived from the type A source rocks with low δ13C values while a minor proportion is derived from the type B source rocks with high δ13C values. Type A source rocks are within either the Cambrian–Lower Ordovician or the Middle–Upper Ordovician strata (not drilled so far) while type B source rocks are within the Cambrian–Lower Ordovician strata, as found in boreholes TD2 and Fang 1. In addition, the three oils from the Cretaceous sandstone reservoirs in the Tahe oilfield with exceptionally high Pr/Ph ratio and δ13C values of individual n-alkanes are derived, or mainly derived, from the Triassic–Jurassic terrigenous source rocks located in Quka Depression.
The difference of the δ13C values of individual n-alkanes among the free, adsorbed and inclusion oils in the reservoir rocks and corresponding crude oils reflects source variation during the reservoir filling process. In general, the initial oil charge is derived from the type B source rocks with high δ13C values while the later oil charge is derived from the type A source rocks with low δ13C values.
The δ13C values of individual n-alkanes do not simply correlate with the biomarker parameters for the marine oils in the Tazhong Uplift and Tahe oilfield, suggesting that molecular parameters alone are not adequate for reliable oil-source correlation for high maturity oils with complex mixing.
There are many structural buried hills in the east of Tazhong Low Rise that have significantly valuable for oil/gas exploration and research. In this work, geochemical and geological methods were ...used to research the complicated diagenetic process of Yingshan carbonate. The Yingshan carbonate underwent four diagenetic stages with three types of calcite, which diagenetic environment was complex and variable. During the Early Ordovician of syndiagenetic stage, the high-magnesium C
I
calcite develops along the micro-fracture and has no or very weakly cathodoluminescence color. In the following early diagenetic, the Yingshan carbonate subsided and developed the C
III
calcite along the corroded fissure and pressolved sutures. After the M1 buried hill formed during the Late Silurian, the Yingshan carbonate experience the supergene stage, and suffered the weathering and denudation for a long period. During the Carboniferous karst, two cave horizons developed in the M1 buried hill, corresponding with two sets of Carboniferous carbonate rocks by the stratigraphic correlation. The cement C
II
calcite contains less Sr, Na, Mg, and more Mn and Fe than sea water, showing bright red in the cathodoluminescence. During the following middle and late diagenetic stage, M1 buried hill underwent a process of persistent burial and slight uplifting with C
III
calcite cementing from the formation water. During the Permian, the hydrothermal fluid intensely influenced the buried hill, leaving the hydrothermal minerals.
The Tazhong Low Rise has abundant hydrocarbon resources in the Yingshan Formation of Lower Ordovician, where dolomite has been demonstrated to be a significant target for carbonate hydrocarbon ...exploration. Using optical observation and geochemical tests, three types of dolomite can be used as replacement and cement products. D1 dolomite is silt or fine crystalline, and has the fluid inclusions of low Th (71.4 °C) and corresponding salinity (9.89 wt% NaCl). In addition, geochemical data indicate that D1 dolomite was formed with shallow burial fluids and was intensely affected by weathering karst in the middle Ordovician, which served as a mixing-zone for dolomitisation. D2 dolomite of medium-coarse grain has similar geochemical characteristics as D1 dolomite except that it has higher diagenetic temperature and negative δ
18
O, suggesting that it generally developed in a deep burial environment. The cathodoluminescence showed two zonal structures in D1 dolomite, while at least three zonal structures in D2 dolomite were influenced by multiple burial diagenetic fluids. The D3 dolomite has a saddle structure with medium-coarse crystalline, high contents of Mn, Ba, Fe, and Sr, high Th (140 °C), high salinity (20.36 wt% NaCl), and negative δ
18
O (–10.1 to–7.8‰). Meanwhile, D3 dolomite is distributed near faults and fractures that are accompanied by hydrothermal minerals and is obviously influenced by hydrothermal fluids.
Knowledge of supercritical methane adsorption behavior in shales is crucial for evaluations of total shale gas-in-place (GIP) resources in deep formations. The primary object of this study is to ...establish an optimized model for the adsorption of supercritical methane in shale. An adaptive L-SDR model, which is a modification of the supercritical Dubinin–Radushkevich (SDR) model in combination with the Langmuir model, was introduced to interpret the measured absolute adsorption isotherms. A series of shale gas high pressure methane adsorption isotherms were measured at 303.15 K for Mesozoic shale collected in the Kuqa Depression of the Tarim Basin. In addition, published methane adsorption data on Posidonia shales were also investigated. The absolute adsorption results were fitted using the Langmuir, SDR and L-SDR models, and the accuracy of each model was analyzed and compared using an average relative error analysis. It was found that the L-SDR model can improve predictions of supercritical methane adsorption compared with predictions from the Langmuir and SDR models at both low and high temperatures, and the results indicate that this model can effectively model supercritical methane adsorption on shale. The individual contributions of the Langmuir and SDR models to the overall adsorption capacity were investigated at multiple temperatures, and the results showed that the contribution of the Langmuir model became more significant with increasing temperatures.
•An optimized L-SDR model was established and used to estimate the supercritical methane adsorption of shale.•The L-SDR model is more accurate compared with the Langmuir and supercritical Dubinin–Radushkevich (SDR) models.•The L-SDR model can describe the behavior of methane adsorbed by shales at different temperature.
The Eopalaeozoic was a pivotal transitional period following the Neoproterozoic glaciation that witnessed dramatic changes in global ocean environments. The black rock series were extensively ...sedimented, preserving the original seawater signature. However, due to the scarcity of geological records, the palaeo-environment and organic matter (OM) enrichment in the Tarim Basin, China, are poorly constrained. Herein, Eopalaeozoic shales from typical outcrops in the Kalpin block, northwestern Tarim Basin, have been subjected to integrated organic and inorganic geochemistry research. In the study area, Eopalaeozoic black rock series can be found in the Yuertusi Formation (YUF), Saergan Formation (SAF) and Yingan Formation (YIF) from bottom to top. Although the maturity is moderate (~1.2%), the OM abundance varies greatly vertically. Microscopic observations and biomarkers revealed that the OM source was dominated by diverse planktonic algae. The chemical alteration index indicates a transition from cold and dry climates with low chemical weathering to warm and humid climates with moderate chemical weathering, and subsequently back to cold and dry climates. The stratified water column and euxinic–suboxic conditions are revealed by redox proxies (Mo, enrichment factors, Th/U, MoU covariation pattern, pristane/phytane and gammacerane index). The YUF was deposited in a euxinic environment. Productivity proxies (biogenic Ba, Cu/Al, Ni/Al and carbon isotopes) show that primary productivity was superelevated in the lower YUF and moderate in the other formations. Primary productivity and preservation conditions were heavily influenced by complementary hydrothermal activities and upwelling currents. The hydrothermal activities and upwelling currents provide abundant nutrients and highly reductive environments, creating enormous possibilities for the formation of excellent source rocks with considerable hydrocarbon geological significance.
•Three sets of Eopalaeozoic black rock series, northwestern Tarim Basin were systematically researched.•Integrated organic and inorganic geochemistry analyses were conducted for palaeo-environment interpretation.•Palaeo-environmental evolution and organic matter enrichment differed tremendously vertically.•Hydrothermal activities and upwelling currents favours the formation of high-quality source rocks.
There are many examples of oil and gas reservoirs throughout the world that have tight carbonate rock as the caprock. These oil and gas reservoirs are often vertically overlapped (with multiple oil ...and gas sections) and laterally connected (to many oil and gas reservoirs). The closure of carbonate caprock is affected by many factors, such as the clay content, differential diagenesis and layer thickness. As evaluation parameters, the clay content can be determined by a gamma ray well-logging curve; the differential diagenesis is reflected in filling effects, rupture effects, cement composition and content and can be measured through the displacement pressure; the layer thickness can be obtained from drilling and well-logging data. In different regions or different members, the factors and parameters to be considered or evaluated for the closure are dependent on actual geological conditions. In this study, the clay content, displacement pressure and layer thickness were analysed, determined, calculated and normalized to generate an overall closure evaluation index (λ) for the caprock in wells on Ordovician Liang 3–5 mud-bearing limestone members on the northern slope of the Tazhong uplift. On the plane distribution map, the area with λ ≥ 1 was effectively sealed. The tight carbonate in the inner Yingshan Formation (with high-resistivity, referred to as a high-resistivity layer) was also effectively caprock. Regardless of whether it can seal oil and gas, the sealing capacity mainly depends on the displacement pressure difference between the high-resistivity layer and the underlying carbonate reservoir. Based on drilling results and the comparison of the displacement pressure difference between the two layers (by actual measurements and calculations), we observed that when the pressure difference is more than 1.95 MPa, it is an effective caprock.
•A large scale of tight carbonate rock can seal hydrocarbon.•λ can evaluate sealing capacity of mud-bearing tight carbonate rock. (λ indicates the comprehensive evaluation index).•ΔP can evaluate sealing capacity of pure tight carbonate rock. (ΔP indicates the displacement pressure difference).
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