Northwestern Sichuan Basin Research Articles

Paleoenvironmental conditions of organic-rich Upper Permian Dalong Formation shale in the Sichuan Basin, southwestern China

Organic geochemical and trace element analysis of a suite of 33 samples collected from the Shangsi section of the Upper Permian Dalong Formation, located near Guangyuan in the northwestern Sichuan Basin, have been used to better understand the marine depositional environment of its shale lithofacies. A 15 m-thick black shale interval near the middle of the formation is enriched in total organic carbon (TOC = 1.1–14.4%, mean = 6.4%). The n-alkane, terpane and sterane biomarker distributions of the shale suggest that its organic matter is of predominantly marine algal origin. Pr/Ph, Pr/n-C17, Ph/n-C18 and gammacerane/C30 hopane ratios demonstrate that reducing conditions prevailed during its deposition. This interpretation is supported by the redox-sensitive abundance of Mo and the trace element redox ratios Ni/Co, V/Cr, U/Th and V/(V + Ni), all of which indicate deposition in a euxinic/anoxic setting. The Mo-TOC relationship in the Dalong shales plots mid-way between those of sediments in the present-day Saanich Inlet and Cariaco Basin, suggesting a moderately restricted depocentre. However, the δ13Corg values of these shales reveal that their organic matter is isotopically much lighter than that found in modern euxinic sediments, implying extremely anoxic bottom waters and a slow-down of oceanic circulation during the Late Permian.

Discovery of a shoal-controlled karst dolomite reservoir in the Middle Permian Qixia Formation, northwestern Sichuan Basin, Southwest China

The dolomites of the Middle Permian Qixia Formation have been important targets of natural gas exploration in the Sichuan Basin for decades. However, more and more exploration and research indicate that the formation of the reservoir might be related to karstification. To testify this hypothesis, we conduct comprehensive outcrop, core, and logging analyses based on a case study in the representative northwestern Sichuan Basin, which has obtained exploration breakthroughs recently. Results show that the Qixia dolomite reservoirs are mainly developed within fine-crystalline dolomites formed by a series of diagenetic modifications, which can be further divided into three types according to the macro- and micro-occurrences of dolomites: euhedral-subhedral crystalline dolomites in the quasi-stratiform karst system (mean porosity and permeability is 3.51% and 3.11 mD, respectively), euhedral-subhedral crystalline dolomites in the leopard porphyritic karst system (mean porosity and permeability is 3.36% and 1.22 mD, respectively), and allotriomorphic mosaic crystalline dolomites with residual parent rock fabrics (mean porosity and permeability is 0.94% and 0.92 mD, respectively). Their reservoir qualities decrease along the order. The formation mechanism of the reservoir is shoal-controlled karst. The preservation of residual intergranular pores within the thin-layer grainstones of shoal facies provides favorable channels for karst water. In the vadose zone, the heterogeneous dissolution within grainstones leads to the formation of leopard porphyritic dissolution features. In the phreatic zone, the karst water flowing along the stratiform grainstones results in the formation of quasi-stratiform dissolution features. The karst system is filled with loose carbonate sands and gravels, whose reservoir properties are far superior to parent rocks, and they can provide migration channels for the hydrothermal fluids with rich Mg2+ in the burial stage. The replacement of hydrothermal fluid results in the redistribution of pores and vugs of inter-fillings within karst system and the formation of intercrystalline pores and residual vugs, but the reservoir space of parent rocks keeps the same as the original condition. Therefore, the exploration of the Qixia dolomite reservoir should be changed to shoal-controlled karst.

The geochemistry and origin of lower Permian gas in the northwestern Sichuan basin, SW China

The lower Permian series in northwestern Sichuan basin of China is a favorable region for natural gas accumulation with several potential source rocks and high-quality reservoirs. However, the tectonic evolution in this area has gone through long-term and complicated activities and the origin of gases in the lower Permian series is complex. For the wise exploration strategies planning, the origin and accumulation of natural gas in lower Permian reservoir have been addressed. And the results reveal that: (1) Three sets of potential source rocks, including the mudstone of Lower Cambrian Qiongzhusi formation, Lower Silurian Longmaxi formation and the marine carbonate source rocks of Lower Permian, have been identified as the gas sources in Lower Permian reservoirs. (2) The gases mainly contain by hydrocarbon gases, with the ratio of C1/C2+ > 0.99, and the characteristics of carbon isotopes are various and several wells’ are reversed or close to reversed. The δ13C2 values of Hewanchang and Wujiaba gas fields are ranging from −33.35‰ to −36.34‰(PDB), similar to the production of sapropelic organic matter. The Shuangyushi and Kuangshanliang gas fields, with the δ13C2 values ranging from −28‰ to −30‰(PDB), are source mixed gas. And the δ13C2 values of Jiulongshan gas field are the heaviest with the value range from −25.23‰ to −28.4‰(PDB). (3) The biomarkers of reservoir extracts are consistent with those of the lower Permian source rock, but the bitumen scales in lower Permian are not as much as those in the lower Cambrian or Lower Silurian series, which reveals that the crude oil creaked gas formed in lower Permian series falls a great deal than that in Cambrian and Silurian series. Based on the comprehensive researches of natural gas composition, carbon isotope, biomarkers of extract and other parameters, the natural gases in lower Permian series are mixed by the crude oil creaked gas from deep formation and the lower Permian source rock, and the scale of fracture determines the amount of creaked gas mixed in the lower Permian reservoir.

Influence of natural fractures on gas accumulation in the Upper Triassic tight gas sandstones in the northwestern Sichuan Basin, China

The Upper Triassic Xujiahe Formation in the northwestern Sichuan Basin, China, is a typical tight gas sandstone reservoir that contains natural fractures and has an average porosity of 1.10% and air permeability less than 0.1 md because of compaction and cementation. According to outcrops, cores and image logs, three types of natural fractures, namely, tectonic, diagenetic and overpressure-related fractures, have developed in the tight gas sandstones. The tectonic fractures include small faults, intraformational shear fractures and horizontal shear fractures, whereas the diagenetic fractures mainly include bed-parallel fractures. According to thin sections, the microfractures also include tectonic, diagenetic and overpressure-related microfractures. The diagenetic microfractures consist of transgranular, intragranular and grain-boundary fractures. Among these fractures, intraformational shear fractures, horizontal shear fractures and small faults are predominant and significant for fluid movement. Based on the Monte Carlo method, these intraformational shear fractures and horizontal shear fractures improve the reservoir porosity and permeability, thus serving as an important storage space and primary fluid-flow channels in the tight sandstones. The small faults may provide seepage channels in adjacent layers by cutting through layers. In addition, these intragranular and grain-boundary fractures increase the connectivity of the tight gas sandstones by linking tiny pores. The tectonic microfractures improve the seepage capability of the tight gas sandstones to some extent. Low-dip angle fractures are more abundant in the T3X3 member than in the T3X2 and T3X4 members. The fracture intensities of the sandstones in the T3X3 member are greater than those in the T3X2 and T3X4 members. The fracture intensities do not always decrease with increasing bed thickness for the tight sandstones. When the bed thickness of the tight sandstones is less than 1.0 m, the fracture intensities increase with increasing bed thickness in the T3X3 member. Fluid inclusion evidence and burial history analysis indicate that the tectonic fractures developed over three periods. The first period was at the end of the Triassic to the Early Jurassic. The tectonic fractures developed during oil generation but before the matrix's porosity and permeability reduced, which suggests that these tectonic fractures could provide seepage channels for oil migration and accumulation. The second period was at the end of the Cretaceous after the matrix's porosity and permeability reduced but during peak gas generation, which indicates that gas mainly migrated and accumulated in the tectonic fractures. The third period was at the end of the Eogene to the Early Neogene. The tectonic fractures could provide seepage channels for secondary gas migration and accumulation from the Upper Triassic Xujiahe Formation into the overlying Jurassic Formation.

Restoration of paleokarst landform and its geological significance: A case from Middle Permian Maokou Formation in Northwestern Sichuan Basin

Based on the analysis of deposition and tectonism, “residual thickness method” is used to restore the paleokarst landform of Middle Permian Maokou Formation in northwestern Sichuan Basin. With the feature of plain with karst hilllock, the paleokarst landform in this area can be classified into three secondary geomorphic units: karst platform, karst slope and karst groove, in which the karst hillocks and monadnocks on karst platform and karst slope are the favorable zones for the development of karst reservoirs, and favorable exploration zones in the next step. Furthermore, in the karst grooves, the Maokou Formation are often denuded into Members Mao 3 or Mao 2, and the seismic profiles show the top of Maokou Formation in karst groove is missing due to erosion. Members Mao 4 and Mao 3 are generally preserved in the karst platform. The seismic profiles across the karst platform and karst groove show that the NE and NW striking erosion grooves were the result of differential uplift and erosion caused by basement faulting at the end of Middle Permian, which then successively developed and formed the NW striking Guangyuan-Wangcang and the NE striking Jiangyou-Guangyuan troughs in Changxing Period. It is suggested to pay more attention to the geological research and exploration of the shallow carbonate platform areas adjacent to the syneclise and trough in fairly deep water.

Detrital zircon U–Pb geochronology and provenance of Lower Cretaceous sediments: Constraints for the northwestern Sichuan pro-foreland basin

Detrital zircons were recovered from several units within the Lower Cretaceous sedimentary cover of the northwestern Sichuan Basin, and dated using U–Pb geochronology to assess their provenance and depositional setting. Five major age populations were identified in the U–Pb age spectra: a Neoarchean to late Palaeoproterozoic group (composed of two distinct age populations at 2754–2151 and 2102–1627Ma), a Neoproterozoic group (979–621Ma), an Early Ordovician to Early Carboniferous group (478–326Ma), an Early Permian to Late Triassic group (297–205Ma), and an Early Jurassic to earliest Cretaceous group (188–138Ma). Each of these age populations corresponds with one or several potential sources around the NWSB; the Neoarchean to late Palaeoproterozoic zircons are primarily derived from the South Qinling Belt, the northern Yangtze Craton, the Bikou terrane, and the Songpan–Ganzi terrane, with an additional contribution from the southwestern Yangtze craton in the Neoproterozoic. Recycling of sediments in the Songpan-Ganzi terrane and the South Qinling Belt may be the source of the Early Ordovician to Early Carboniferous zircon population. Early Permian to Late Triassic zircons are predominantly sourced from Triassic granitoids in the South Qinling Belt, as well as the Late Triassic Songpan–Ganzi turbidites during their eastward transportation. Recycling of Upper Jurassic to Lower Cretaceous sediments in the Qinling orogen most likely supplied the observed population of Jurassic to Early Cretaceous age zircons. Detrital zircon age distributions in Lower Cretaceous sediments record a pro-foreland basin setting characterized by intracontinental subduction. Oblique or tangential compression arising from clockwise rotation of the South China Block (SCB) served as an additional driver for the formation of the Cretaceous pro-foreland basin, and reactivation and inheritance may produce differentiation.

Tectonic evolution from Late Sinian to Early Paleozoic and natural gas exploration in northwestern Sichuan Basin, SW China

Abstract Based on the field outcrops, drilling and seismic data of northwestern Sichuan Basin, this paper systematically discusses tectonic evolution characteristics from the Late Sinian to the Early Paleozoic, analyzes the petroleum geological significance, and points out the recent favorable exploration directions in this area. The area experienced mainly three times of tectonic evolution during the Late Sinian to the Early Paleozoic: (1) During the Late Sinian to the early period of Early Cambrian, this area was affected by tension action and formed two sets of nearly north-south trending continental rifts, and two sets of platform margins in the Sinian Dengying Formation. (2) During the middle period of the Early Cambrian to the Middle Ordovician, this area was eroded strongly (stronger in the northwest than in the southeast) because of multi-period uplifting of the northwestern Bikou paleo-land. (3) During the Late Ordovician to the Late Silurian, this area was eroded strongly again because of the compression of the Kwangsian Orogeny, and the erosion was stronger in the southwest than in the northeast. During the Late Sinian to the Early Cambrian, the formation of continental rifts controlled the development of bioherm beach facies reservoirs on the platform margins in the Dengying Formation and the high quality source rock in the Lower Cambrian series. During the middle of the Early Cambrian, the uplifting of the Bikou paleo-land controlled the distribution of the Kongmingdong Formation oolitic beach facies reservoirs around the paleo-land. It is suggested that, vertically, the fourth Member of the Dengying Formation and the Kongmingdong Formation be the main exploration targets, and, horizontally, the Jiulongshan structural trap be the most beneficial exploration belts recently due to its matching with petroleum migration and accumulation.

Significance of new breakthrough in and favorable targets of gas exploration in the Middle Permian system, Sichuan Basin

The Middle Permian system in the Sichuan Basin represents favorable conditions for the formation of natural gas reservoirs. However, only fracture-vuggy limestone gas reservoirs were discovered in early exploration, whose scales and results were limited. In 2014, a high-yield industrial gas flow was produced there in Well Shuangtan 1, the major risk-exploration well, which means a great breakthrough in natural gas exploration in the Middle Permian system, Sichuan Basin. This plays a strategically important and guiding role in oil and gas exploration in ultra-deep marine formations. This paper illustrates the deployment background of Well Shuangtan 1, major exploration achievements and its significance in the gas exploration in the Middle Permian system, Sichuan Basin. Moreover, exploration potential and subsequent exploration targets of Middle Permian are analyzed in terms of its hydrocarbon source conditions, sedimentary characteristics, reservoir characteristics and areal extension, resources and favorable exploration zones. It is concluded that the Middle Permian system in the Sichuan Basin is a continent-connected carbonate platform with carbonate deposits being dominant. There are three types of reservoirs, including pore dolomite, karst fracture-vuggy, and fracture, which present considerable natural gas resources and huge exploration potential. Favorable exploration zones classification of Middle Permian in the Sichuan Basin indicates that the most favorable exploration zones are mainly distributed in the central-western Sichuan Basin. In particular, the northwestern Sichuan Basin is the most favorable exploration area which is characterized by the most excellent hydrocarbon source conditions, developed dolomite reservoirs and zonal distribution of structures.

Recognition of a novel Precambrian petroleum system based on isotopic and biomarker evidence in Yangtze platform, South China

The Precambrian Doushantuo Formation is regarded as a possible petroleum source rock horizon with TOC values of 0.85–1.85% and type I kerogen in northwestern Sichuan Basin, Yangtze platform, South China, while showing high-to post-mature thermal maturity. A set of outcropping bitumen and oil sands from various reservoirs display overall depleted values of δ13C from −36 to −32‰ (average −35‰), typical of a Precambrian source. In addition, the sterane distributions in these bitumens and oils are unusual compared to marine Phanerozoic oils with a predominance of C29 members, as well as their triaromatic counterparts. High amounts of 20-n-alkylpregnane with very low diasteranes indicate restricted, sulfur-rich conditions, typical of an anoxic carbonate source rock. This interpretation is supported by some other specific biomarkers such as abundant 30-norhopane and C24-tetracyclicterpane, and elevated C35 hopane. Furthermore, the presence of gammacerane indicates a saline, stratified water column. Hydrocarbons extracted from the solid bitumen and oil sands have many compositional characteristics in common with other oils derived from Precambrian carbonate source rocks from elsewhere. This implies the existence of a Precambrian marine petroleum system in the Yangtze platform, with the Doushantuo carbonate as the likely source. It is supposed that the reduction of temperature and pressure following basin uplift resulted in petroleum fractionation (deasphalting) in Lower Paleozoic reservoirs with the fractionated oils having migrated along the faults becoming trapped in stratigraphically younger reservoirs, where the petroleum underwent heavy biodegradation.

The dust provenance and transport mechanism for the Chengdu Clay in the Sichuan Basin, China

The Chengdu Clay in the Sichuan Basin, China, was named by Thorp and Dye in 1936. The genesis, dust provenance and transport mechanism for the Chengdu Clay, however, are still debated. In the present work, the Chengdu Clay and adjacent loess from the eastern Tibetan Plateau, the Chinese Loess Plateau and the Qinling Mountains were investigated. The content and grain size distribution of quartz, quartz δ18O, Sm–Nd isotopic system, rare earth elements and other trace element concentrations for the Chengdu Clay and its adjacent loess were analyzed. Based on comparison and spatial variation analysis, the results confirm that the Chengdu Clay is of aeolian origin. The dust provenance of the Chengdu Clay differs from those of the adjacent loess. The Chengdu Clay is possibly of local origin and is transported by an ancient katabatic wind over a short distance during glacial and stadial periods. The alluvial sediments in the northwestern Sichuan Basin are possibly the major sources of the Chengdu Clay. There is little possibility of an effective aeolian transport bringing dust from the Tibetan Plateau, the arid area in northwestern China or Inner Mongolia into the Sichuan Basin, where it may be deposited to form the Chengdu Clay. Thus, long-range dust transport by the East Asian winter monsoon, the westerly jet or the Tibetan Plateau winter monsoon possibly plays a minor role in controlling the magnitude of the dust flux of the Chengdu Clay in the Sichuan Basin.

Journal of Petroleum and Gas Exploration Research

Oilsand in China is very rich, especially in northwestern Sichuan basin. Houba oilsand is one of the famous oilsand belts, it consists of medium-coarse grained sandstone with high porosity and permeability. The genesis of those pores is very important in the research of Houba oilsand reservoir. Based on thin section analyses, SEM observation and X-ray diffraction (XRD) analysis, the diagenesis and pore evolution of oilsand reservoir have been studied. It shows oilsand reservoir in Shaximiao Formation had experienced the early diagenesis and the late diagenesis including early mechanical compaction, four times cementation and two times dissolution. Diagenetic alterations affect the pore evolution in the long geology history. At last, porosity evolution model of the oilsand reservoir have been established.

A comparison of results from two different flash pyrolysis methods on a solid bitumen sample

A Cambrian solid bitumen from northwestern Sichuan Basin, southern China was analyzed using two different flash pyrolysis methods coupled with gas chromatography–mass spectrometry analysis, including Pyroprobe® and analytical laser micropyrolysis. Results show that pyrolysis products from a Pyroprobe® (model 5000) analysis are dominated by mono-, di- and tricyclic aromatic hydrocarbons, whilst those from laser micropyrolysis are dominated by aliphatic hydrocarbons (n-alk-1-ene/n-alkane doublets), which is consistent with the results from an FT-IR spectrum of the solid bitumen. According to the molecular compositions of the pyrolysates from two types of pyrolysis, results from a 532nm continuous wave laser may be more suitable for pyrolysis research of bitumen/asphaltene. Thus, differences in pyrolysis techniques must be considered when used to characterize oil asphaltenes or source rock kerogens.

Reservoir characteristics and main controlling factors of oil sand in Houba area, northwestern Sichuan Basin, China

Houba oil sand in frontier Longmenshan Mountain is one of the most typically important unconventional resources. The basic reservoir characteristics of oil sand and the main factors affecting reservoir quality were examined in this article based on porosity, permeability, and mercury porosimetry measurements; thin section analyses; SEM observation; and X-ray diffraction analysis. This study shows that the oil-bearing sandstone reservoir is mainly medium−coarse-grained sublitharenite and litharenite. The main pore type is intergranular pores, including residual primary intergranular pores, dissolved intergranular pores, and dissolved intragranular pores; fractures are common in this study area. The quality of sandstone reservoir is of high porosity and high permeability with a high oil saturation of 89.84 %. It is indicated that the main controlling factors of the reservoir in the study area include deposition, diagenesis, and tectonism. Deposition laid a foundation to porosity evolution, and channel sand is the most favorable depositional facies for the reservoir. Diagenetic alterations are the keys to reservoir evolution; dissolution and chlorite coatings cementation play an effective role in the generation and preservation of pores. Compaction, carbonate cementation, and quartz overgrowth cause many damages to the reservoir porosity. Fractures caused by structural breakages can improve the reservoir permeability and they also can provide fluid migration pathways to the late corrosion, which formed a lot of corroded fissures as reservoir and percolation spaces.

Effects of diagenesis on the acoustic velocity of the Triassic oolitic shoals in the Yudongzi outcrop of Erlangmiao area, Northwest Sichuan Basin

The oolitic shoals of the Triassic carbonate platform margin in the Yudongzi (鱼洞子) outcrop of Erlangmiao (二郎庙) area in the northwestern Sichuan (四川) basin present a scarce oppor- tunity to quantitatively describe their diagenesis and its effects on the acoustic velocity. Using a detailed field geologic survey, profiles illustration of typical depositional system, and systematic testing, five types of diagenesis have been identified in the oolitic shoals: micritization, cementation, compaction and pressolution, dissolution, and dolomitisation. The cementation is composed of four subtypes (micrite cements, fibrous calcite cements, granular calcite cements, and blocky calcite cements). The dissolution is formed from three subtypes (freshwater selective dissolution, burial non-selective dissolution, and burial selective dissolution). The dolomitisation is composed of three subtypes (fine-crystalline dolo- mites, microcrystalline dolomites, and medium-crystalline dolomites). In order to quantitatively de- scribe the diagenetic fabric of oolitic shoals, the micritic grain content, calcite cement content, mean pore diameter, pore types, dolomite content, and dolomite types have been evaluated. Based on these data, the relationship between the acoustic velocity and diagenesis of oolitic shoals has

A complete series of C31–C34 25-norbenzohopanes in the Devonian and Jurassic bitumen sands, NW Sichuan Basin

A complete series of C32–C35 benzohopanes and corresponding C31–C34 25-norbenzohopanes were tentatively identified in Devonian and Jurassic bitumen sands from the northwestern Sichuan Basin in southwestern China. Although other origins cannot be excluded, the relative distributions of C32–C35 benzohopanes and C31–C34 25-norbenzohopanes, along with other biomarker parameters, suggest that the 25-norbenzohopanes formed by demethylation of the corresponding benzohopanes.

Distribution and formation mechanism of lime mudstone in Upper Triassic in northwestern Sichuan, China

The lime mudstone in the oolites of the Upper Triassic Maantang formation has not been examined in detail by many geologists though the intraclast-bearing lime mudstone is obviously different from the underlying and overlying gray oolitic layers in northwestern Sichuan Basin, China. This study presents an interpretation of four newly measured stratigraphic sections for investigating the distribution and thickness of the lime mudstone. The distribution of the lime mudstone is discontinuous, and the deposits vary in thickness. The lime mudstone occurs as lentoids or bands of approximately 0.2–2 m in thickness and ranges in length from about 5 m to more than 30 m in length in the outcrops. Similar complexes occur in other places in the world although there are regional and site-specific variances. However, the phenomenon in the Maantang mudstone formation seems most like those of the modern Bahama bank. The lime mudstone of Maantang is assumed to have formed by sequential historic storms of great strength, much as did the similar Bahama deposits.

Magnetic fabric investigation in the northwestern Sichuan Basin and its regional inference

An investigation of the anisotropy of magnetic susceptibility (AMS) in 39 sites has been carried out in the northwestern Sichuan Basin which is adjacent to the northeastern Tibetan plateau, discussing the deformation pattern of the basin and the orogenic process of the corresponding orogens. Stepwise demagnetization of three orthogonal isothermal remanent magnetization suggests that hematite is the main magnetic carrier mineral. Combined with the low value of magnetic susceptibility, it is considered that the AMS is some combination of hematite and paramagnetic matrix minerals. Different types of magnetic fabrics from sedimentary to pencil structure magnetic fabric and sharps of susceptibility ellipsoids from oblate to prolate were observed. The existence of sedimentary magnetic fabric and the accordance of magnetic lineation and bedding strike indicate the magnetic fabrics along the front of Longmen Shan belt were the result of tectonic deformation. Thermochronology researches in the orogens and our apatite fission track analysis in the northwestern Sichuan Basin suggest that magnetic fabric mainly reflects the Cenozoic deformation. The magnetic lineation which is consistent with the fault and fold strikes in the Longmen Shan was induced by Mesozoic and Cenozoic deformation, and the latter is the primary one. Besides the typical weak deformation magnetic fabrics, atypical magnetic fabrics characterized by a magnetic lineation that is oblique to the bedding strike are shown in 53% sites along the front of Micang Shan. It is proposed that such a type of atypical magnetic fabric is the result of two competing sub-fabrics in the weakly deformed tectonic superposition region.

How was the Triassic Songpan‐Ganzi basin filled? A provenance study

The Triassic Songpan‐Ganzi complex comprises >200,000 km2of 5–15 km thick turbiditic sediments. Although surrounded by several magmatic and orogenic belts, the Triassic high‐ and ultrahigh‐pressure Qinling‐Tongbai‐Hong'an‐Dabie (QTHD) orogen, located several hundred kilometers to the east, was proposed as its major source. Middle to Late Triassic samples from the northern and southern Songpan‐Ganzi complex, studied using detrital white mica40Ar/39Ar ages, Si‐in‐white mica content, and detrital zircon U/Pb ages, suggest that the northern Songpan‐Ganzi deposystem obtained detritus from the north: the north China block, east Kunlun, northern Qaidam, Qilian, and western Qinling; the southern Songpan‐Ganzi deposystem was supplied from the northeasterly located Paleozoic QTHD area throughout the Ladinian and received detritus from the Triassic Hong'an‐Dabie orogen during the Carnian, indicative of exhumation of the orogen at that time. The QTHD orogen fed the Norian samples in the southeastern southern Songpan‐Ganzi deposystem, signifying long drainage channels along the western margin of the south China block. An additional supply from the Emeishan magmatic province and/or the Yidun arc is suggested by the paucity of white mica in the southern Songpan‐Ganzi deposystem. Mica ages of Rhaetian sediments from the northwestern Sichuan basin best correlate with those of the Triassic QTHD orogen. Our Si‐in‐white mica data demonstrate that the high‐ and ultrahigh‐pressure rocks of the Hong'an–Dabie Shan were not exposed in the Middle to Late Triassic.

Kinetic-simulating experiment combined with GC-IRMS analysis: application to identification and assessment of coal-derived methane from Zhongba Gas Field (Sichuan Basin, China)

The main purpose of this study is to model the δ 13C values of methane derived from coal by combining kinetic-simulating experiment with the gas chromatography–isotope ratio mass spectrum (GC-IRMS) analysis. The stable carbon isotopic variation of methane in pyrolysates with heating temperature indicates that the assumptions for both a constant kinetic isotope effect ( α) and a uniform initial isotopic composition ( δ 13C o) are impractical for explaining the carbon isotopic fractionation during coalification. For purposes of simplification, two approaches are used in this paper to deal with the heterogeneity of terrestrial organic matter. One is that, assuming a uniform initial isotopic composition (i.e., δ 13C i, o= δ 13C o) for all methane-generating precursors in coal, the isotopic variation of methane is fitted by adjusting Δ E a, i ( E a 13C, i − E a 12C, i ) for each hypothetical reaction. The other is that, assuming a constant kinetic isotope effect during the whole gas formation, that is all Δ E a, i values are identical, the modeling of methane isotopic composition is achieved by changing the 13CH 4 generation potential of each reaction ( f i, 13C ), namely, by adjusting the initial δ 13C value ( δ 13C i, o) for each methane-generating precursor. Results of the kinetic calculation shows that the two simulating methods can yield a similar result at a geological heating rate of 2 °C/My, which further demonstrates that those natural gases with methane δ 13C value being approximately −36‰ are possibly sourced from the upper Triassic coal measure strata in the Northwestern Sichuan Basin.

A Study of Forest Hydrologic Effect in the Sichuan Basin Region of the Upper Reaches of the Yangtze River

The Sichuan Basin Region is important composition part of the upper reaches of the Yangtze River. Eco-environment change in this region exerts important effect to the middle and lower reaches of the Yangtze River. The forest cover had hugely been reduced from the 1950s to the mid-1970s, and it has obviously been increased since the end of the 1970s, because that the reforestation project has been implemented. The water source conservation capacity in the studied watersheds has been getting better through study and analysis of the methods for assessment of the forest hydrologic effect. It is found that the method of index of the water source conservation capacity can be used to assessment of the forest water source conservation in the big scale watershed in the studied region. In addition, the retention water capacity of the different vegetation vertical zones in the typical mountain in the north-western Sichuan Basin was studied, its result provides the base for protection and building of the forest ecosystem in the mountain area in the studied region.