Journal of Natural Gas Geoscience最新文献

The fault-karst carbonate reservoir in the Shunbei area of the Tarim Basin is controlled by deep strike-slip faults and forms a fault-karst system. The reservoir space primarily includes holes and fractures, and its strong anisotropism aggravates the complexity of the reservoir seismic response characteristics. High-quality reservoirs in the fault-karst body in this area have different burial depths, which is not conducive to the establishment of low-frequency models in traditional inversion. Facies-based Bayesian simultaneous inversion technology combines Bayesian classification with pre-stack simultaneous inversion, divides different facies based on multi-elastic parameters such as P-wave and S-wave velocity and density, and conducts an in-depth trend analysis for each phase to establish the initial model. Compared with traditional inversion technology, this technology not only improves the inversion accuracy but also increases the stability of the density inversion. Taking the carbonate fault-karst body in the northern section of the No.5 fault zone in the Shunbei area as the research object, combined with the actual production situation, two facies, fractured-cavity limestone, and tight limestone, were divided by elastic parameters and then subjected to depth trend analysis and inversion. Through the single fracture-cavity equivalent model test and practical application analysis, the density data obtained by the Facies based Bayesian simultaneous inversion were highly consistent with the reservoirs drilled by Wells W3 and W3C in the northern section of the No.5 fault zone, which verifies the applicability and reliability of the inversion technique in the study area and the reliability of the results.

Clarifying the deformation characteristics and formation mechanisms of the box fold in the eastern Qiulitage structural belt can provide important references for the reconstruction of the evolution process and petroleum exploration in the Qiulitage structural belt. As a result of the deep geological structure displayed by seismic data and characteristics of faults and fractures within the box fold, the mechanical mechanism and structural evolution of the box fold in the eastern Qiulitage structural belt were investigated, along with the genesis and significance of hydrocarbon exploration of faults and fractures within the box fold. The results show that the surface box fold in the Qiulitage structural belt was formed via the conjugate kinking of the supra-salt structural layer, driven by the intensive southward compression during the Middle and Late Himalayan movements. The box fold has experienced three evolution stages, namely, the tectonically-inactive stage before the deposition of the Kuqa Formation, the fold rudiment stage during the early to middle deposition of the Kuqa Formation (Kuqa period), and the stage of fold finalization and uplift-denudation. The front flank of the box fold develops north-dipping thrust faults and network fracture systems formed during the early to middle Kuqa period and cemented by gypsum due to the precipitation of deep, high-salinity formation water. However, later faulting can cut and dislocate the cement. The upper fold core develops north-dipping tensile faults and near EW tensile fractures, while the lower fold core is associated with small back-thrust structures and near NS shear fractures. The neutral plane is expected to be in the middle-lower part of the fold. The back-flank of the box fold develops south-dipping back-thrust faults and near EW interlayer shear fractures caused via interlayer detachment. The core and back flank of the fold were less affected by the high-salinity formation water, leaving faults and fractures with no considerable cementation. The kink zone and its surroundings have high storage and flow capacities and thus the potential to and develop oil and gas reservoirs. Correctly interpreting kink zones in concealed areas can help expand the scale of original oil and gas reservoirs or discover new petroleum exploration domains. In the Qiulitage structural belt, the connection between deep and shallow fault systems leads to the migration of deep hydrocarbons to shallow layers and subsequent accumulation. The structural-lithologic oil and gas reservoir formed in the Paleogene thin sand layers of the upper part of the Lower Cretaceous and the structural oil and gas reservoir formed in supra-salt sandstone layers of the surface box fold are among The potential exploration domains in shallow layers.

With ongoing advancements in natural gas exploration, the second member of the Sinian Dengying Formation (Deng 2 Member) has emerged as a crucial region for providing natural gas reserves in the north of the central Sichuan Basin. The Deng 2 Member has a significant volume of reservoir solid bitumen. Geochemical characteristics and development mechanism of the solid bitumen were determined through measurements and analyses of the Deng 2 Member samples collected from primary exploration wells by using optical microscopy, SEM, gas chromatograph-mass spectrometer, and laser Raman spectrometer. The results show that the Deng 2 Member has a generally high content of solid bitumen, ranging from 2.96% to 5.13% on average in single wells. The solid bitumen mainly occurs as the fillings of dissolved pores (caves) and fractures, followed by intergranular pores, in the shape of spots, balls, plates, and veins dominantly. Diasteranes content and laser Raman spectrograms indicate that the solid bitumen is in the high maturity stage. The bitumen reflectance calculated by laser Raman spectroscopy is distributed between 2.49% and 4.09%, indicating the major source of the thermal cracking of crude oil. Solid bitumen in the Deng 2 Member has different contents of 21α(H)–C₂₉ norhopane, C₃₅ hopane, and C₃₄ hopane, and Ts/Tm values from the Deng 4 Member in the Gaoshiti Moxi area. It is inferred that some solid bitumen is from the source rocks of the Lower Cambrian Maidiping Formation and the Sinian Doushantuo Formation. Two stages of bitumen were developed in the Deng 2 Member, indicating possible two stages of oil filling and thermochemical sulfate reduction (TSR) in the geologic history.

A semi-open system temperature-pressure controlled hydrocarbon generation and expulsion simulation experiment was carried out to explore the hydrocarbon generation and expulsion potential and mechanism of deep environment “coal measure” source rocks in the Qaidam Basin. A WYNN-3 high temperature and high pressure (HTHP) simulator and Middle Jurassic source rocks (III type organic matter, carbonaceous mudstone, and coal, R_O is 0.67% and 0.64%, respectively) of Well DMG1 in the northern margin of the Qaidam Basin were used during the investigation. The results demonstrated that: (1) The maximum total oil yields of carbonaceous mudstone and lignite, which respectively measured at 79.38 mg/g_TOC and 37.30 mg/g_TOC, revealed a “double peaks” evolution law as a whole. (2) In the lower evolution stages (T ≤ 300 °C, P ≤ 42.0 MPa), the expelled/discharged oil yields of the two types of source rocks were lower than those of the residual oil, and the hydrocarbon expulsion efficiencies were low. However, at 400 °C (51.0 MPa), they significantly increased reaching 76.84% and 83.72%, respectively. (3) The main group components of the discharged oil were resin and asphaltene, and the component yields were generally comparable to those of liquid hydrocarbons. The yields of expelled oil group components of carbonaceous mudstone were higher than those of coal. (4) The simulated gas was primarily composed of hydrocarbon gas and non-hydrocarbon gas (CO₂, N₂), and with the elevated thermal evolution, the yields of total hydrocarbon gas/gaseous hydrocarbon increased. The two types of source rocks had maximum hydrocarbon gas yield of 116.46 mL/g_TOC and 36.85 mL/g_TOC, respectively. (5) The vitrinite reflectance (R_O) increased as temperature and pressure conditions enhanced, and it exhibited good temperature consistency. The results of this temperature-pressure controlled simulation experiment showed that temperature was still the dominant factor in the thermal evolution of organic matter, fluid pressure had a “dual” control on the formation of type III organic hydrocarbon products, and “coal measures” source rocks still had a strong potential for hydrocarbon generation in the later stages of evolution. This research provided a certain data reference for the hydrocarbon generation and expulsion law of Jurassic deep “coal measures” source rocks in the northern margin of the Qaidam Basin.

Hydrocarbon accumulation in coal measure strata is consist of several types which includes intra-source accumulation and adjusted accumulation, particularly in basins that had experienced multi-stage tectonic activities. The mechanism governing this accumulation is extremely complex. Using the Paleozoic petroleum system in the Huanghua Depression as an example, the mechanism of hydrocarbon accumulation in deep reservoirs can be revealed by means of core observation, geochemistry testing of coal measure source rock, thin section observation, fluid inclusions testing, and profile analysis of typical reservoirs, combined with the burial and thermal history of strata. Based on the regional tectonic, the mechanism of hydrocarbon accumulation in coal measures is defined and the accumulation model was established. The results show that the Paleozoic petroleum system in the Huanghua Depression has three different types of source-reservoir-cap assemblages: under-source, intra-source, and above-source assemblage. Regional tectonic evolution controlled the hydrocarbon generation process of source rocks, showing the hydrocarbon generation characteristics of “oil in the early stage and gas in the late stage.” The Middle Cretaceous is the first of two periods of hydrocarbon accumulation, during which stage occurred in low-maturity oil and a small amount of natural gas were mainly stored in intergranular pores and dissolution pores. The tectonic uplift in the Late Cretaceous caused the destruction of the ancient reservoir. The second accumulation stage occurred in the Middle Paleogene to the present. A large amount of natural gas and high maturity oil were mostly stored in dissolution pores and structural fractures. The hydrocarbon accumulation in Huanghua Depression was simultaneously regulated by reservoir reformation, tectonic activities, and source rock maturation. Based on the foregoing understanding, the hydrocarbon accumulation model of “oil and gas transfer station” in coal measures is put forward, that is, oil and gas are first accumulated in intra-source or near-source reservoirs, and are then adjusted from this “oil and gas transfer station” to other areas as a result of tectonic activity.

Due to a great increase in the plasticity of deep marine shale reservoirs in southern Sichuan under high-temperature and high-pressure conditions, the single brittleness evaluation method is difficult to effectively characterize its fracability, which significantly limits the selection of sweet spots and fracturing reconstruction in the area. In the case of the deep marine shale reservoir of the Wufeng-Longmaxi formations in the southern Sichuan Basin, through triaxial high-temperature and high-pressure experiments, fracture toughness and X-ray diffraction experiments, the mechanical properties and its influencing factors in the shale reservoir are studied, and the rock fracture morphology under various loading conditions is quantified. According to the morphological characteristics of shale, the analysis of influencing factors and comprehensive quantitative evaluation of the brittleness has been carried out. The deep marine shale resource in the southern Sichuan Basin is likely to be characterized by its high elastic modulus and low I fracture toughness. The mineral composition, temperature, pressure, and degree of bedding development are the primary factors for determining the brittleness; with high quartz mineral content (>50%), low confining pressure (<20 MPa), medium and low temperature (<60 °C) and high density of the shale bedding, the fractal dimension of the sample after the experiment is higher; whereas the geometry of cracks are mainly complex shear cracks, and brittleness is higher. The analytic hierarchy approach establishes a comprehensive evaluation index by analyzing the relationship between the normalized rock mechanical parameters, the stress–strain curve's brittleness index, and the fractal dimension. The brittleness of deep marine shale can be more accurately described by this evaluation index. The primary target layer for future shale gas exploitation, the 3¹ sublayer of the first member of the Longmaxi Formation, is shown to have a high brittleness index.

Recently found substantial discoveries in the Lower Jurassic Sangonghe Formation in Taibei Sag, Turpan-Hami Basin was made possible through exploration wells Qintan1 and Ji7H. The two wells represented a major breakthrough in lithologic oil and gas reservoir exploration in the depression area and offered promising results for the investigation of the “lower depression source” in the Turpan-Hami Basin. The two favorable conditions of lithologic reservoir formation in the sag area are systematically summarized based on the two wells, and specific suggestions for further exploration are specified. The results show that: (1)Three sets of source rocks, namely Badaowan Formation, Sangonghe Formation, and Xishanyao Formation, are developed in Shuixigou Group of Jurassic in Taipei Sag. The new drillings reveal that Sangonghe Formation serves as both a regional caprock and a high-quality source rock. According to the source rock re-evaluation, the Shuixigou Group in Taipei Sag has excellent hydrocarbon generation potential. (2)Sangonghe Formation of Lower Jurassic is the main target layer for exploration in the depression area. Braided river delta sediments are developed. Sandbodies are developed in the depression area. At a depth of more than 5000 m, effective reservoirs are still developed as a result of secondary dissolution and structural fractures. (3)There are five favorable zones of rock reservoirs in the Lower Jurassic of Taibei Sag, namely, Pudong, Hongbei, Lingbei, Qiudong, and Gebei. These zones have excellent exploration potential and represent an important direction for future exploration in the Turpan-Hami Basin.

Helium, a coordinated resource in natural gas, is an essential and strategic resource, but research on its enrichment mechanism is relatively weak. This study investigated helium enrichment in major reservoirs in China using geochemical methods to address the challenging topic of how helium accumulates in helium-rich gas reservoirs. It concludes that helium is enriched in a few reservoirs which capture additional helium exsolution from ancient groundwater. Released dissolved helium from ancient groundwater into the reservoir is the main mechanism of helium accumulation in helium-rich gas reservoirs. This was the first time that the concept of “multiple sources of helium supply and primary sources of helium enrichment” was applied to guide the formation of helium-rich gas reservoirs. Moreover, the main sources of helium comes from the radioactive decay of uranium (U) and thorium (Th) in hydrocarbon rocks, reservoirs, and the decay of U and Th dissolved in groundwater from other rocks. When dissolved helium in ancient groundwater crosses free gas or gas reservoirs, the partial pressure of helium in the water is significantly higher than the partial pressure of helium in the free gas or gas reservoirs, proving Henry's Law which states that the helium in the water is almost completely exsolution into the gas reservoir, forming a helium-rich gas reservoir.