Northeast Sichuan Research Articles

Process and mechanism for oil and gas accumulation, adjustment and reconstruction in Puguang Gas Field, Northeast Sichuan Basin, China

With the discoveries of a series of large gas fields in the northeast of Sichuan Basin, such as Puguang and Longgang gas fields, the formation mechanism of the gas reservoir containing high H2S in the ancient marine carbonate formation in superposition-basin becomes a hot topic in the field of petroleum geology. Based on the structure inversion, numerical simulation, and geochemical research, we show at least two intervals of fluid transfer in Puguang paleo-oil reservoir, one in the forepart of late Indo-Chinese epoch to early Yanshan epoch and the other in the metaphase of early Yanshan epoch. Oil and gas accumulation occurred at Puguang structure through Puguang-Dongyuezhai faults and dolomite beds in reef and shoal facies in Changxing Formation (P2ch) - Feixianguan Formation (T1f) in the northwest and southwest directions along three main migration pathways, to form Puguang paleo-oil reservoir. Since crude oil is pyrolysised in the early stage of middle Yanshan epoch, Puguang gas reservoir has experienced fluid adjusting process controlled by tectonic movement and geochemical reconstruction process controlled by thermochemical sulfate reduction (TSR). Middle Yanshan epoch is the main period during which the Puguang gas reservoir experienced the geochemical reaction of TSR. On one hand, TSR can recreate the fluid in gas reservoir, which makes the gas drying index larger and carbon isotope heavier. On the other hand, the reciprocity between fluid regarding TSR (hydrocarbon, H2S, and water) and reservoir rock induces erosion of the reservoir rocks and anhydrite alteration, which improves reservoir petrophysical properties. Superimposed by later tectonic movement, the fluid in Puguang reservoir has twice experienced adjustment, one in the late Yanshan epoch to the early Himalayan epoch and the other time in late Himalayan epoch, after which Puguang gas reservoir is finally developed.

Automatic discrimination of sedimentary facies and lithologies in reef-bank reservoirs using borehole image logs

Reef-bank reservoirs are an important target for petroleum exploration in marine carbonates and also an essential supplemental area for oil and gas production in China. Due to the diversity of reservoirs and the extreme heterogeneity of reef-banks, it is very difficult to discriminate the sedimentary facies and lithologies in reef-bank reservoirs using conventional well logs. The borehole image log provides clear identification of sedimentary structures and textures and is an ideal tool for discriminating sedimentary facies and lithologies. After examining a large number of borehole images and cores, we propose nine typical patterns for borehole image interpretation and a method that uses these patterns to discriminate sedimentary facies and lithologies in reef-bank reservoirs automatically. We also develop software with user-friendly interface. The results of applications in reef-bank reservoirs in the middle Tarim Basin and northeast Sichuan have proved that the proposed method and the corresponding software are quite effective.

Accumulation Mechanisms and Evolution History of the Giant Puguang Gas Field, Sichuan Basin, China

Abstract:Solid bitumens were found throughout the carbonate reservoirs in the Puguang gas field, the largest gas field so far found in marine carbonates in China, confirming that the Puguang gas field evolved from a paleo‐oil reservoir. The fluid conduit system at the time of intensive oil accumulation in the field was reconstructed, and petroleum migration pathways were modeled using a 3‐D model and traced by geochemical parameters. The forward modeling and inversion tracing coincided with each other and both indicated that oils accumulated in the Puguang‐Dongyuezhai structure originated from a generative kitchen to the northwest of the Puguang gas field. The deposition of organic‐rich Upper Permian source rocks dominated by sapropelic organic matter in the Northeast Sichuan Basin, the development of fluid conduit system that was vertically near‐source rock and laterally near‐generative kitchen, and the focusing of oils originated from a large area of the generative kitchen, were the three requirements for the formation of the giant paleo‐oil reservoir from which the giant Puguang gas field evolved. The Puguang gas field had experienced a three‐stage evolution. The post‐accumulation processes, especially the organic‐inorganic interaction in the hydrocarbon‐water‐rock system, had not only profoundly altered the composition and characteristics of the petroleum fluids, but also obviously changed the physicochemical conditions in the reservoir and resulted in complicated precipitation and solution of carbonate minerals.

Coincident negative shifts in sulfur and carbon isotope compositions prior to the end-Permian mass extinction at Shangsi Section of Guangyuan, South China

Sulfur isotope composition of carbonate-associated sulfate (δ34SCAS) and carbon isotope composition of carbonate (δ13Ccarb) were jointly investigated on the Late Permian rocks at Shangsi Section, Guanyuan, Northeast Sichuan, South China. Both δ34SCAS and δ13Ccarb show gradual decline trends in Late Permian strata, inferring the occurrence of the long-term variation of marine environmental conditions. Associated with the long-term variation are the two coincident negative shifts in δ34SCAS and δ13Ccarb, with one occurring at the boundary between Middle Permian Maokou Formation and Late Permian Wujiaping Formation and another at Middle Dalong Formation. Of significance is the second shift which clearly predates the regression and the biotic crisis at the end of Permian at Shangsi Section, providing evidence that a catastrophic event occurred prior to the biotic crisis. The frequent volcanisms indicated by the volcanic rocks or fragments, and the upwelling are proposed to cause the second negative excursion. An abrupt extreme negative δ34SCAS (ca. −20‰) associated with a low relative concentration of CAS and total organic carbon without large change in δ13Ccarb is found at the end of the second shift, which might arise from the short-term oxygenation of bottom waters and sediments that resulted from the abrupt sea level drop.

Subdivision of Permian Fossil Communities and Habitat Types in Northeast Sichuan, South China

Recent achievement in hydrocarbon exploration in Northeast Sichuan demonstrated that Permian calcareous and argillaceous deposits are the major contributing source rocks. Reevaluation on the hydrocarbon potential of the Permian strata over the whole Yangtze region is thus to be desired. A comprehensive corroboration was carried out at the Shangsi Section, Guangyuan, Northeast Sichuan, which is believed to be another promising area in South China. This article deals with the ecological and depositional conditions of the Permian strata in the section, including the Chihsia, Maokou, Wujiaping, and Dalong formations, which share some similarities with those occurring in the broad Yangtze carbonate platforms. Five fossil communities of Mizzia-Permocalculus, Hayasakaia, Inozoan , Ostracod, and Crinoid were identified in the Yangtze region and described in detail including their components and occurrence. On the basis of the fossil communities and depositional features, 23 habitat types, mainly occurring from inner to outer shelves, were recognized at the Shangsi Section in order to reconstruct the depositional conditions and accordingly to evaluate the paleoproductivity. In addition, the subdivision of Chihsia Formation in the section was revised, which would be of significance for the reconstruction of the Chihsian paleogeography in the study area.

Molecular Records of Primary Producers and Sedimentary Environmental Conditions of Late Permian Rocks in Northeast Sichuan, China

A series of biomarkers were identified in the aliphatic and aromatic fractions of the extracts from Late Permian Dalong and Wujiaping formations in Shangsi Section, Northeast Sichuan, South China, on the basis of the analysis of gas chromatography-mass spectrometry (GC-MC). The dominance of lower-molecular-weight n -alkanes throughout the profile suggests the dominant contribution of algae and bacteria to the organics preserved in the marine section. Wujiaping Formation is characterized by the elevated contribution from algae as well as other photoautotrophs such as photosynthetic bacteria as shown by the molecular ratios of hopanes to steranes or tricyclic terpanes as well as the ratio of pristane (Pr) and phytane (Ph) to C 17 and C 18 n -alkanes. This is in accord with the data from the microscopic measurement on the calcareous algae. In contrast, Dalong Formation is featured by enhanced contribution from bacteria and probably terrestrial organics indicated by the enhanced C 24 tetracyclic terpanes relative to tricyclic terpanes. The two formations also show a distinct discrimination in sedimentary environmental conditions including redox condition and salinity. The anoxic condition was only found in the middle of the Dalong Formation as shown by the ratios of Pr/Ph and dibenzothiophene to phenanthrene, consistent with the reported data of Mo and U. An enhanced salinity indicated by the homohopane index is observed at the shallow Wujiaping Formation. On the basis of the composition of primary productivity and the redox condition, Dalong Formation is proposed, herein, to be potential hydrocarbon source rocks in the study site. It is notable that the topmost end-Permian is characterized by a large perturbance in both the redox condition and salinity, with oxic conditions being frequently interrupted by short-term anoxia, likely showing a causal relationship with the episodic biotic crisis across the Permian—Triassic boundary.

Carbon Isotope Records Indicative of Paleoceanographical Events at the Latest Permian Dalong Formation at Shangsi, Northeast Sichuan, China

Paired organic and carbonate carbon isotope compositions of Late Permian Wujiaping and Dalong formations at Shangsi, Northeast Sichuan were analyzed by MAT 251. An abrupt negative excursion in the two isotope records was observed in the middle part of Dalong Formation, in association with a drop in the carbon isotope difference of the two records and an increase of total organic carbon (TOC) content. The negative drop of the paired carbon isotope records is suggestive of the input of 12 C-enriched CO 2 . The molecular ratios of pristane to phytane and dibenzothiophene to phenanthrene indicate the anoxic condition in this interval. The enhanced TOC content is indicative of the elevated preservation of organic matter due to the anoxic condition. These isotopic and organic geochemical data probably infer the occurrence of the upwelling in this interval. The additional contribution of volcanism activity observed in South China cannot be excluded to the input of 12 C-enriched CO 2 and the negative shifts in carbon isotope composition of bulk organic matter and carbonate.

Main Controlling Factors of Organic Matter Richness in a Permian Section of Guangyuan, Northeast Sichuan

A complete Permian section in Guangyuan, Northeast Sichuan, has been investigated, to explore the variation in organic matter richness and its main controlling factors. The research results of the detailed lithological description and organic/inorganic geochemical analysis of about 325 samples indicate that a high content of total organic carbon (TOC) occurs in calcareous mudstones and laminated marls or thin-bedded limestones. In carbonate rocks, the TOC content is negatively related to the thickness of the massive beds; the thinner the bed is, the higher the TOC content is. Marine organic matter is enriched in the outer shelf and relatively shallow basin floor environments with the maximum TOC contents of 5.07% and 14.6%, respectively. The main factors that affect the quantity and quality of marine organic matter include primary productivity, depositional processes, and redox conditions during deposition and the early diagenesis stage. Three intervals of marine good quality source rocks are identified in this section; they are lower Chihsia Formation, topmost Maokou Formation, and the middle segment of the Dalong Formation. They are formed in anoxic environments, in association with high primary productivity. Among these, high productivity in the lower Chihsia interval may originate from an upwelling flow area, whereas, in the rest of the intervals, it is inferred to be related to hot fluid activity due to volcanic eruption. The results of this study suggest that good quality marine source rocks in the Permian strata offer significant hydrocarbon potential.

Assessment on Redox Conditions and Organic Burial of Siliciferous Sediments at the Latest Permian Dalong Formation in Shangsi, Sichuan, South China

The redox sensitive elements, molybdenum (Mo) and uranium (U), in marine sediments from the latest Permian Dalong Formation at the Shangsi Section, Northeast Sichuan, South China, were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) to determine their response to a range of redox conditions, and to estimate the organic carbon burial rate. On the basis of the correlation between authigenic Mo abundance and organic carbon content in modern oceans, the organic carbon burial rates were calculated for the rocks at Dalong Formation, ranging from 0.48–125.83 mmol/(m 2 ·d), which shows a larger range than the mineralization rate of organic carbon at the continental margins (1.6–4.23 mmol/(m 2 ·d)). The Zr-normalized Mo and U abundances show large fluctuations in the entire section. The maxima of Zr-normalized Mo abundance and thus the maxima of the organic carbon burial rates were observed at the interval between the 155th and 156th beds (404–407 m above the base of Middle Permian). A decrease (the minimum) in U/Mo ratios is present in this interval. It is speculated that the oxygen-limited conditions and ultimately anoxia or euxinia may develop within this depth interval. In contrast, an enhanced enrichment of Zr-normalized U abundance is found, in association with less enrichment in Zr-normalized Mo abundance in the interval from the 151st to 154th beds (395–404 m above the base of Middle Permian), inferring the dominance of a suboxic/anoxic depositional condition (denitrifying condition), or without free H 2 S. The presence of small quantities of dissolved oxygen may have caused the solubilization and loss of Mo from sediments. It is proposed that the multiple cycles of abrupt oxidation and reduction due to the upwelling at this interval lead to the enhanced accumulation of authigenic U, but less enrichment of Mo. A decrease in the contents of U, Mo, and TOC is found above the 157th bed (407 m above the base of Middle Permian), in association with the enhanced U/Mo ratio, suggesting the overall oxic conditions at the end of the Dalong Formation.

Depositional and Ecological Features of Permian Oxygen Deficient Deposits at Shangsi Section, Northeast Sichuan, China

Because oxygen deficient conditions enhance the preservation of depositional organic matter, analysis on paleooxygenation conditions of depositional environments becomes a routine work in evaluations of potential hydrocarbon source rocks. The article focuses on depositional and ecological features relating to oxygen deficient shelfal environments at the Shangsi Section, as a part of multidiscipline collaboration to reevaluate the hydrocarbon potential of the Middle and Upper Permian, Guangyuan, Northeast Sichuan Province. Ichnofabric Zoophycos , sepiolite-bearing limestones (SBL) were interpreted as indicators of dysaerobic environments. Laminated calcareous and/or siliceous mudstones with pelagic ammonites and radiolarians were believed to be the deposits of anaerobic environments. When rhythmic succession was considered, average strategy was adopted for the oxygenation explanation of a given interval. The anaerobic condition in this Permian section was observed in the upper part of the latest Permian Dalong Formation, in which siliceous mudstones with ammonite, radiolarians develop in association with lower U/Mo ratio, lower biomarker ratio of Pr/Ph, and the highest TOC content. The topmost Maokou Formation, featured by thin-bedded calcareous and siliceous mudstones with ammonite, thin-shelled bivalves, and laminations would be deposits of the quasianaerobic condition. The middle part of the Members I and III of the Chihsia Formation is proposed to be dysaerobic condition as indicated by occurrences of SBL and ichnofabric features, with the Member III being the severe dysaerobic condition.

Guadalupian (Middle Permian) Conodont Faunas at Shangsi Section, Northeast Sichuan Province

ABSTRACT This study recovered the Guadalupian conodont faunas from Shangsi Section in Northeast Sichuan. Four genera and nine species were identified, and three conodont zones were recognized and established; they include Jinogondolella nankingensis Zone, J. aserrata Zone, and J. postserrata Zone. The Roadian and Wordian boundary is set in the interval 5 m from the top of Bed 86 by the first appearance of the conodont J. aserrata . The Wordian and Capitanian boundary is set in the interval 2.3 m from the top of the Bed 95 by the first appearance of J. postserrata . Most of the specimens demonstrated low color alteration index (CAI) as 1.5–3, indicating that most part of the Maokou Formation may have the suitable thermal conditions for the formation of oil source rocks.

Biogeochemical Records at Shangsi Section, Northeast Sichuan in China: The Permian Paleoproductivity Proxies

The marine primary producers assimilate the atmospheric CO2 to form the organic carbon in surface water. The organic carbon then settles down through the water column and is removed from the oceans by final preservation in sediments in the form of petroleum or nature gases. The reconstruction of paleoproductivity will thus improve our understanding of the biological processes in the formation of fossil energy resource and help to locate new sites for future exploration. In this study, biorelated elements P, Cd, Al, Ba, as well as redox sensitive element Mo, were analyzed in the 448 rock samples collected from Permian strata at the Shangsi Section, Guangyuan, Northeast Sichuan in China. On the basis of the Ti content, the nondetrital contents of P, Ca, and Al, denoted as Pxs, Cdxs, and Alxs, were calculated and found to coincide with the TOC content throughout the whole section, with some enrichment intervals being found in the middle part of Chihsia Formation, topmost Maokou Formation, and Dalong Formation. This suggests that the biorelated elements could be used as proxies for the paleoproductivity here in this section. Baxs, a paleoproductivity indicator widely used in the paleoceanography, shows insignificant correlation with TOC, Pxs, Cdxs, and Alxs, probably arising from the loss of biological barium in anoxic conditions. Compiled with the data of TOC content and Pxs, Cdxs, and Alxs, three episodes of enhanced paleoproductivity were identified in Permian strata including the middle part of Chihsia Formation, topmost Maokou Formation, and Dalong Formation.

Rare earth elements and yttrium in a stalagmite from Central China and potential paleoclimatic implications

In a pilot study, the rare earth elements and yttrium (REE when discussed without Y or REY with Y) were measured by inductively-coupled plasma mass spectrometry (ICP-MS) on a stalagmite (SJ3) from northeast Sichuan, Central China. This stalagmite was dated by the thermal ionization mass spectrometry (TIMS) 230Th method to between 38 and 10 thousand years before present (ka BP). The REY geochemical indices, including the concentrations, Lanthanum, Cerium and Yttrium anomalies and the fractionation between light and heavy REE (LREE and HREE), display significant stadial-interstadial changes corresponding to variations in past climates and environments. The North American Shale Composite (NASC)-normalized REY patterns for sub-samples of SJ3 display obvious negative Ce anomalies and positive Y anomalies. For most sub-samples, the LREE are relatively depleted relative to the HREE. La anomalies during relatively cold and dry climatic phases (including the last glacial maximum (LGM) and the period synchronous with Heinrich event one identified in the North Atlantic region) are usually positive, while the Ce and Y anomalies and the LREE-HREE fractionation are more pronounced. During relatively warm and humid phases the REY pattern displays an opposite signature. The REY sources for SJ3 include the overlying soil layer, which consists mainly of aeolian sediments, and the host rock of the Late Permian limestone. Source change may be the primary mechanism controlling the variations of the REY pattern for SJ3. The REY pattern for the youngest sub-sample from the top dirty section of SJ3, which is porous and rich in detrital materials, seems to be dominated by the overlying soil layer. The REY in sub-samples deposited during relatively cold and dry phases may come mainly from the host limestone and thus show patterns more similar to a marine signal. For the rest of the sub-samples, REY from both sources may be present. In addition to the sources, the REY pattern for SJ3 may also be influenced by REY mobilization from sources and transportation in groundwater, and prior carbonate deposition, all of which should be closely related with changes in climate and environment. Thus the REY pattern for speleothems might be a potential proxy for past climate and environment.

Sedimentary charateristics and hydrocarbon accumulation in Northeast Sichuan basin

Northeast Sichuan basin is a Mesozoic-Cenozoic basin in the northern margin of the Yangtze plate which has a complex tectonic background and hydrocarbon accumulation history. By means of the analysis method of wave processes, major cycles of sedimentary wave process in this basin have been studied. The results show that there are five-order cycles corresponding to 760, 220, 100, 35 and 20 Ma respectively, two first-order sedimentary cycles (220 Ma) and four secondorder sedimentary cycles (100 Ma). The authors find that the second-order sedimentary cycles are well matched with reservoir formation cycles through studying the relationship between second-order sedimentary cycles and reservoir formation cycles, and divide three reservoir formation cycles in this basin.

Control of coupling among three major factors for formation of high-efficiency gas reservoir—A case study on the oolitic beach gas reservoir in Feixianguan Formation in the northeast Sichuan Basin

Through a case study of the high-efficiency gas reservoir in Feixianguan Formation in the northeast Sichuan Basin, quantitative and semi-quantitative analyses of key elements such as hydrocarbon generation, migration and accumulation, and reservoir evolution as well as their interplay in the critical moment of reservoir formation controlled by the energy field were carried out, by means of numerical modeling of the energy field. It was found that the climax time for Permian hydrocarbon generation was Late Triassic-Early Jurassic and accumulation of oil and gas has resulted in large-scale paleoreservoirs in paleostructural traps in Feixianguan Formation, a process facilitated by fractures connecting the sources. The paleoreservoirs have been turned into high-efficiency gas kitchens due to pyrolysis, which resulted from deep burial at a temperature of 170–210°C as induced by tremendously thick sedimentation in the foreland basin of Daba Mountain in Late Jurassic-Cretaceous period. Meanwhile, abundant acid gas like H2S produced from thermo-chemical sulfate reduction (TSR) at high temperatures leads to extensive dissolution of dolostone in the paleoreservoirs, which may in turn result in modification of the reservoirs and preservation of the reservoir rock porosity. The present distribution of gas reservoirs was ultimately determined in the processes of adjustment, cooling and decompression of the paleoreservoirs resulting from intense deformation in the front of Daba Mountain during the Himalayan orogeny.

The Order‐Depression Solution for Large‐Scale Integral Equation and Its Application in the Reduction of Gravity Data to a Horizontal Plane

AbstractIn the equivalent source method used for continuation of potential field from an arbitrary surface, the kernel function of Fredholm equation shows self‐similarity and redundancy. According to these characteristics, we adopted the wavelet compression algorithm with non‐linear cosine threshold value to accomplish the order depression of the Fredholm equation, thus making the equivalent source method, put forward by Bhattacharyya in 1977, can deal with data from large area and with mass volume, which is hard to achieve before. Theoretical modeling indicates that, the hard threshold method obtains great accuracy when the compression ratio is 40.5%, while its accuracy decreases when the compression ratio is 81.1%. At this time, the nonlinear cosine threshold can improve the continuation accuracy obviously. At last, the hard threshold, soft threshold, and nonlinear threshold are used in the compression of line MT‐1 data of northeast Sichuan gas field. Results indicate that, when the compression ratio is 79%, all the three methods can acquire good results. But the nonlinear threshold has the highest fidelity, which can provide important evidence in searching for local structures related with petroleum reservoir.

RESEARCH OF JURASSIC HYDROCARBONIC SAND-STON HORIZONTAL PREDICTION IN THE NORTH-EAST SICHUAN BASIN

In the north-east of Siehuan Basin, the Jurassic stratum is a set of very great thick continental deposition. In recent year, several industrial gas wells have been gotten from this stratum as drilling work was developed in the shallow stratum of Sichuan Basin. So, it shows: the key for increasing a success rate of drilling well is to find out the sandstone and supply about the hydrocarbonic information of the sandstone in the Jurassic stratum of the north-east Sichuan Basin. But, it is difficult to predict the distribution of the channel Sandstone and it's hydroearbonic ease because the main drilling target for Jurassic's J2s is a interbedding constitution with a great horizontal change of the sandstone and shale. In this paper, following a synthetic research view from multiple scientific subjects, first of all, to research carefully the J2s depositional situation in the Wu Baochang-Du Kouhe area of the north-east Sichuan Basin, and then, to combine the research of the geology and logging with the technology of the geophysical information analysis, the plane distribution of J2s2 channel sandstone and it's hydrocarbonic case were predicted by means of the synthetic interpretation of the velocity inversion (strata) and energy absorption analysis (weaa) for the seismic data for the drilling well but also to explore a new way for the reservoir prediction of the shallow continental deposition in Sichuan Basin.

Evidence for Archean Continental Crust in Northern Vietnam and its Implications for Tectonic Evolution in SE Asia

subsided under load. Closure of the Mianlue suture marks the complete collision orogeny in the Qinling region. During Late Jurassic and Early Cretaceous, the northern Yangtze foreland belt thrusted southwards and converged with Wuling-Mufu belt in the Middle Yangtze in Late Jurassic and in Upper Yangtze in Early Cretaceous. The strata in the Yangtze foreland basin was gradually deformed from the east to the west, and depositional centers westward migrated to the northeast Sichuan basin in Late Jurassic and the northwest Sichuan basin in Early Cretaceous. Due to great compression, the Dabieshan arc-shaped thrust belt in the Middle Yangtze and the Dabashan arc-shaped thrust belt in the Upper Yangtze were formed. From the above-geological data, it can be inferred that the Mianlue suture underwent processes of rifting from west to east and closing from east to west. The Yangtze plate subducted northwards and first collided with Qinling-Dabie plate at Dabieshan so that the ultrahigh pressure metamorphic rocks were formed (Li and Sun, 1996). During the process of collision, the Yangtze plate rotated clockwise relative to the Huabei plate. During Middle Jurassic the two plates collided straight. Subsequently, the Yangtze plate continued to press northwestwards, so that the northern Yangtze foreland belt continuously converged with Wuling-Mufu belt from east to west. Dabieshan represents a major thrust, which overlapped or transformed continental marginal basin and foreland basin belts.