The geochemical characteristics, origin, migration and accumulation modes of deep coal-measure gas in the west of Linxing block at the eastern margin of Ordos Basin
“… Figure 1 The tectonic location and structural characteristics of the Yushe-Wuxiang Block, modified from Ref. 3 , 53 . …”
Section: Geological Backgroundmentioning
confidence: 99%
“…Coal-measure gas refers to a set of coalbed methane, coal-measure shale gas (includes shale gas and certain coal-derived gas, abbreviated as shale gas), and tight sandstone gas that are linked through genetic relationship and by vertical superimposition 1 – 3 . It is significant unconventional energy in terms of multi-strata, widely distributed, and resources 4 , 5 .…”
Superimposed accumulation mechanism and model of vertical source rock–reservoir system of coal-measure gas is crucial to evaluate the exploration potential, and also the basis of co-exploration and co-production of coal measure gas. This work investigates the formation mechanism of various types of reservoirs (coalbed methane, shale gas, tight sandstone) in the Taiyuan Formation (Yushe-Wuxiang Block, eastern Qinshui Basin). Source rocks (coal seams and coal-measure mudstones) in the study area are characterized by type III kerogen, organic rich and over-mature, and reach a gas generation peak during the Early to Late Cretaceous. Coalbed methane mainly adsorbs on the surface of micropores, shale gas mainly occurs in micropores, macropores and micro-factures in adsorbed and free states, and tight sandstone gas mainly occurs in macropores in a free state. The combinations of successions are identified, coalbed methane, shale gas, and tight sandstone gas horizons are divided into a mudstone-sandstone reservoir (combination I), a coal-mudstone-sandstone reservoir (combination II), and a coal-mudstone reservoir (combination III). This division occurs from top to bottom in the succession and is identified on the basis of lithology, total organic carbon content (TOC) of mudstones, gas logging, superimposition relationships, and the source rock-reservoir-caprock assemblage. The strata thickness, continuity, and gas logging results of combination III comprise the most favorable conditions for fairly good development potential, followed by combination I. The development potential of combination II is poor due to the small strata thickness and poor continuity. The identification of superimposed reservoirs can provide an engineering reference for the exploration of coal-measure gas.
“… Figure 1 The tectonic location and structural characteristics of the Yushe-Wuxiang Block, modified from Ref. 3 , 53 . …”
Section: Geological Backgroundmentioning
confidence: 99%
“…Coal-measure gas refers to a set of coalbed methane, coal-measure shale gas (includes shale gas and certain coal-derived gas, abbreviated as shale gas), and tight sandstone gas that are linked through genetic relationship and by vertical superimposition 1 – 3 . It is significant unconventional energy in terms of multi-strata, widely distributed, and resources 4 , 5 .…”
Superimposed accumulation mechanism and model of vertical source rock–reservoir system of coal-measure gas is crucial to evaluate the exploration potential, and also the basis of co-exploration and co-production of coal measure gas. This work investigates the formation mechanism of various types of reservoirs (coalbed methane, shale gas, tight sandstone) in the Taiyuan Formation (Yushe-Wuxiang Block, eastern Qinshui Basin). Source rocks (coal seams and coal-measure mudstones) in the study area are characterized by type III kerogen, organic rich and over-mature, and reach a gas generation peak during the Early to Late Cretaceous. Coalbed methane mainly adsorbs on the surface of micropores, shale gas mainly occurs in micropores, macropores and micro-factures in adsorbed and free states, and tight sandstone gas mainly occurs in macropores in a free state. The combinations of successions are identified, coalbed methane, shale gas, and tight sandstone gas horizons are divided into a mudstone-sandstone reservoir (combination I), a coal-mudstone-sandstone reservoir (combination II), and a coal-mudstone reservoir (combination III). This division occurs from top to bottom in the succession and is identified on the basis of lithology, total organic carbon content (TOC) of mudstones, gas logging, superimposition relationships, and the source rock-reservoir-caprock assemblage. The strata thickness, continuity, and gas logging results of combination III comprise the most favorable conditions for fairly good development potential, followed by combination I. The development potential of combination II is poor due to the small strata thickness and poor continuity. The identification of superimposed reservoirs can provide an engineering reference for the exploration of coal-measure gas.
“…China’s unconventional natural gas exploration has made breakthroughs. − At present, the exploration of unconventional gas has gradually changed from the early single type to the composite type, with the focus being put on the exploration of unconventional natural gas in the transitional facies coal measure strata. − For Carboniferous-Permian coal measure strata in Ordos Basin, the sedimentary environments, lithology types, reservoir-cap combinations, and accumulation processes are diverse, and it is rich in natural gas resources, thus enabling its great exploitation potential. , …”
In this study, the pore-throat structure and lithological and fractal characteristics of fine-grained sandstone reservoirs of Permian Shanxi Formation in Ordos Basin are investigated using various experiments, including casting thin section, scanning electron microscopy (SEM), high-pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR). The relation between rock type difference and pore structure difference is explored, and the controlling effect of porethroat structure on physical properties of fine-grained sandstone reservoir is studied. The results show that the sandstone sample can be categorized into quartz arenites, sublitharenite, and lithic sandstone. The average porosity and permeability are, respectively, 4.46% and 0.129 mD. The pores are mainly of intergranular and intercrystalline types, and the throats are lamellar and tubular types. Furthermore, the fine-grained sandstone is typically characterized by a binary pore structure. Micropore-throat, as the main reservoir space, is the dominant factor of the reservoir physical property difference. In addition, the correlation between fractal dimension and rock composition is studied with pore-throat morphology being comprehensively analyzed. The results indicate that the quartz arenite is dominated by quartz intergranular lamellar throats, and there are numerous intergranular pores and tubular throats connecting them in litharenite and sublitharenite. Moreover, the content of pores and throats play major roles in the change of fractal dimension. The characteristics, causes, and main controlling factors of micropore and throat types should be emphatically analyzed to predict the physical properties of fine-grained sandstone.
“…China is rich in CBM resources, with a geological resource of 30.05 × 10 12 m 3 , and the development and utilization of CBM can optimize China’s energy structure and reduce greenhouse gas emissions. , China has made great progress in the CBM accumulation mechanism and development methods, such as in the Ordos Basin, − Qinshui Basin, , and Jungar Basin. , However, to achieve China’s goal of carbon neutrality by 2060, the scale of CBM development in China still needs to be urgently upgraded. Some areas with a low priority for exploration become the current focus of the investigation, such as the Zhuozishan coalfield at the western margin of the Ordos Basin.…”
Section: Introductionmentioning
confidence: 99%
“…10.5−20.78 m), various coal burial depths (133.1−1232.1 m), medium coal rank (R o , max , 1.04− 1.35%), relatively high vitrinite contents (59.1−77.3%), and relatively high ash yields in coal (14.21−57.93%). The coal structure is dominated by primary and cataclastic coals, and the coal seam has low permeability and normal reservoir pressure.…”
The Zhuozishan coalfield
at the western margin of the
Ordos Basin
is one of the main coal-mining areas in China, and recent explorations
have revealed the great potential for coalbed methane (CBM) resources
in its Carboniferous and Permian strata. In this paper, the controlling
factors of CBM enrichment of the major coals are studied in this coalfield
and the CBM resources are estimated based on the analysis of the coal
petrology and compilation of literature data on the gas content. The
result of the coal petrology analysis of 10 samples shows that the
vitrinite content of No. 16 coal (71.9–77.3%) is higher than
that of No. 9 coal (59.1–65.1%), and the inertinite content
of No. 16 coal (18.9–23.5%) is lower than that of No. 9 coal
(30.1–34.9%). The R
o,max value
of No. 16 coal (1.18–1.35%) is higher than that of No. 9 coal
(1.04–1.13%), and both coals are of medium rank. Due to greater
thickness, deeper burial depth, and better coal petrology characteristics,
the No. 16 coal seam of the Taiyuan Formation is selected as the major
coal seam for CBM resource estimation, which has a thickness of 1–6
m and a present-day burial depth of 200–1100 m. The gas content
of this coal seam varies mostly between 4 and 10 m3/t.
Positive correlation between the coal seam thickness as well as present-day
burial depth and the gas content suggests that the thick and deeply
buried coal seams are favorable for CBM preservation. The ash yield
shows an insignificant negative correlation with the gas content,
indicating that ash yield is not an important factor for CBM enrichment.
The syncline hinges located below the thrust zones show higher gas
content due to greater burial depths. In contrast, the anticline hinges
at shallower depths tend to have lower gas contents. Based on the
combined information about sedimentary environments, structural patterns,
and hydrogeology, two CBM accumulation models are put forward in the
study area that include synclinehydraulic plugging below thrust
nappe and faultconfined aquifer plugging. The volumetric method
is used to estimate the CBM resources, and results indicate that the
CBM resource in the whole coalfield is 428.78 × 108 m3, and the total resource abundance is 0.74 × 108 m3/km2. Two favorable areas for the
CBM exploration are optimized based on the resource amount and resource
abundance. One of the favorable areas is the Kabuqi area in the northern
part of the coalfield, and another is the Baiyunwusu area in the central
part of the coalfield. These two areas will be the CBM priority exploration
areas at the western margin of the Ordos Basin.
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