Permian Changxing Formation Research Articles

Types, Evolution of Fractures, and Their Relationship with Oil and Gas Migration of Permian Changxing Formation in Yuanba Gas Field of Sichuan Basin, China

Yuanba gas field of Sichuan Basin is characterized by weak-tectonic deformation, and the fractures have had a significant effect on oil and gas migration. This paper investigates the types, evolution of fractures, and their relationship with oil and gas migration of the Permian Changxing Formation of the Yuanba gas field according to geological and geochemical analyses. Four genetic types of fractures with different occurrences were distinguished: intercrystal fractures, pressure solution fractures (stylolites), structural fractures, and overpressure fractures. Intercrystal fractures present multiple straight edges between the dolomite particles and no fixed direction. Pressure solution fractures are in the form of zigzags. Structural fractures have straight edges and a certain direction and are usually distributed in groups. Overpressure fractures have irregular shapes and irregular directions and occur in multiple quantity. According to the comprehensive analyses, the intercrystal fractures, pressure so...

Distribution of platform edge reefs and beach as well as their major controlling factors over the Changxing-Feixianguan formations in northeast Sichuan Basin, China

The platform facie, platform edge facie, slope, and basin facie were developed in the Chengkou-Western Hubei Trough zones during the Upper Permian Changxing Formation's Early Triassic Feixianguan period; during this time the platform edge had the most favorable sedimentary facie. The Changxing Formation was composed of three sections in which the second section was rich in framework reef, whereas the third one was rich in micro-reefs at the bottom. Recently, it has been discovered that abundant platform edge shoals are developed in the third member and that oolitic shoals with large thickness are widely distributed. This paper systematically investigated the influence of the changes in reef-building organism system on the development of organic reefs. The results show that the distribution of bank belts is determined through paleogeography framework. The rapid change of relative sea level affects the formation and movement of bank belts. In addition, the regional tectonic activity controls the distribution and transformation of the bank belts. Finally, the prospective zone in the Chengkou-Western Hubei area was evaluated and divided into four favorable zones, in which the Wanyuan-Yunyang area was deemed to be most advantageous.

稳定碳、氧同位素地球化学特征与成岩环境分析——以川东北地区上二叠统长兴组白云岩为例

稳定碳、氧同位素地球化学特征与成岩环境分析——以川东北地区上二叠统长兴组白云岩为例

Dolomite origin and its implication for porosity development of the carbonate gas reservoirs in the Upper Permian Changxing Formation of the eastern Sichuan Basin, Southwest China

High-quality carbonate gas reservoirs of the Upper Permian Changxing Formation (P2ch) in the eastern Sichuan Basin are mainly dolostones. Subsurface core samples from the Longhui-Tieshan area were studied to determine the origins of dolomitizing fluids and interpret porosity evolution during dolomitization. Petrographically, two dolomite types were identified, including micritic to fine-sized dolomites (type 1) and fine-to coarse-sized sucrosic dolomites (type 2). The type 1 dolomites, characterized by micritic crystals (<10 μm) and low Mg/Ca order degrees (mean value of 0.557), selectively replace micritic matrices in carbonates and preferentially occur at intervals rich in micrite, indicating their open, near-surface origin. The type 2 dolomites, including four subtypes, i.e., types 2a, 2b, 2c, and 2d, are characterized by coarse crystals (50 μm ∼ >500 μm) and high Mg/Ca order degrees (mean value of 0.787). Low Fe and Mn concentrations (163 ppm and 69 ppm, respectively), high homogenization temperatures (60 °C–200 °C), depleted δ18O values (mean value of −5.14‰), and Sr isotope compositions (mean value of 0.707643), collectively suggest that the dolomitizing fluid of type 2 dolomites was likely derived from buried, imprisoned brine water closely associated with Early Triassic seawater but unrelated to meteoric water. During the first stage of porosity evolution (i.e., stage A), carbonate porosity was slightly decreased by dolomitizing fluids preferentially replacing micritic components and CO32− ions involvement. During the second stage (i.e., stage B), carbonate porosity was slightly increased because molecular replacement occurred under an environment with insufficient CO32− ions. During the advanced stage (i.e., stage C), dolomitization was intensively weakened because of CO32− ions depletion, yet the acidic dissolution and structural movements produced considerable pores and fractures, forming high-quality reservoirs. Type 1 dolomites are volumetrically minor and have no reservoir potential, effective reservoirs are closely associated with type 2 dolomites. A burial dolomitization model of type 2 dolomites is proposed based on the previous analyses. Dolomitizing fluids moved downward through open faults and unconformities into P2ch carbonates. Potentially high-quality reservoirs likely occur at locations close to dolomitizing fluid conduits.

Origin and distribution of hydrogen sulfide in the Yuanba gas field, Sichuan Basin, Southwest China

The Yuanba gas field in the Permian Changxing Formation (P2c), which exhibits wide variations in its hydrogen sulfide (H2S) concentration (1.20–12.16%), is a typical sour gas field in the northern Sichuan Basin. The sulfur-rich reservoir's solid bitumen (atomic S/C ratios are 0.032–0.142), and late calcite cement δ13C values, which are smaller than the δ13C values of the host dolostone, indicate that the H2S originated from thermal sulfate reduction (TSR) and oil was involved in TSR. The gas souring index (GSI) of P2c's gases is generally lower than 0.1. The ethane δ13C values increase as the GSI increases, although no obvious increase was observed in the methane δ13C values. The calcite cements' δ13C values (−15.36 to +4.56‰) in dolostone are heavier than the typical reported values, which implies that only limited heavy hydrocarbon gases were involved in TSR. No anhydrites developed in P2c's reservoirs, and dissolved sulfate anions (SO42−) were mainly enriched during dolomitization. Insufficient dissolved SO42− most likely caused the lower H2S concentrations in the Permian to Triassic reservoirs in the northeastern Sichuan Basin compared to the Permian Khuff Formation in Saudi Arabia and the Jurassic Smackover Formation in Mississippi. Except for the SO42− in residual water in paleo-oil zones, SO42− from bottom water may also be involved in TSR; therefore, oil reservoirs with bottom water have more SO42− and can produce more H2S than pure oil reservoirs. This phenomenon may be the main cause of the great difference in the H2S concentrations between reservoirs, while gravitational differentiation during late uplift most likely creates differences in H2S concentrations in a single reservoir. Carbon dioxide (CO2), which has a relatively heavy δ13C value (−3.9 to −0.3‰), may be the combined result of TSR, the balance between CO2 and inorganic fluid systems, and carbonate decomposition.

Fluid inclusion and geochemical evidence for the origin of sparry calcite cements in Upper Permian Changxing reefal limestones, eastern Sichuan Basin (SW China)

Gas reservoirs in the Upper Permian Changxing Formation in eastern Sichuan Basin are found mostly in dolostones, rather than the contemporaneous reefal limestones that had most of their porosity occluded by pervasive calcite cementation. The sparry calcite, which was the latest but most abundant type of cement, was responsible for major porosity loss. Hence, this study focuses attention on the origin of sparry calcite by employing petrography (e.g. microscope, cathodoluminescence), geochemistry (carbon, oxygen and strontium isotopes, trace elements, and clumped isotope thermometry) and fluid inclusion analysis. The sparry calcite cements are interpreted to be of burial origin as shown by their enclosement of radiaxial calcite and host rock fragments, the presence of two-phase (liquid and vapor) fluid inclusions with higher homogenization temperatures, and relatively lower oxygen isotopic values. Sparry calcites show none to very dull cathodoluminescence, preserve similar carbon and strontium isotopic signatures to other chemical components of host rocks, and have low concentrations of Fe and Mn. These facts indicate that the source for precipitation of sparry calcites was most probably derived from chemical compaction of host rocks and/or adjacent limestone strata. The contrast in porosity and sparry calcite abundance between the reefal limestones and dolostones imply that pervasive dolomitization of carbonates in the Changxing Formation probably occurred prior to chemical compaction, rather than deep-burial dolomitization proposed in previous studies. Early dolomitization enhanced preservation of porosity because the dolostones were more resistant to chemical compaction than the limestones. Reefal limestones in the Changxing Formation gradually lost their porosity during progressive burial because of chemical compaction and the resultant sparry calcite cementation. This study emphasizes the importance of chemical compaction in carbonate cement precipitation and the resultant porosity occlusion in deep burial carbonate successions.

Lithofacies palaeogeography as a guide to petroleum exploration

Lithofacies palaeogeography as a guide to petroleum exploration is a very important topic. By using the following five exploration examples, this paper discusses the guide of lithofacies palaeogeography or of sedimentary facies to petroleum exploration. These examples include the dolostone of the Lower Ordovician Majiagou Formation 5 in the Ordos area, the Donghe Sandstone of the Upper Devonian and Lower Carboniferous in the Tarim Basin, the reef of the Upper Permian Changxing Formation in the Sichuan Basin, the oolitic bank of the Lower Triassic Feixianguan Formation in the Sichuan Basin, and the lacustrine delta sediments and gravity flow sediments of the Middle and Upper Triassic Yanchang Formation in the Ordos Basin.

New seismic attribute technology for predicting dissolved pore-fracture of deeply buried platform margin reef-beach system in Northeast Sichuan Basin, China

The large reef complexes of the Upper Permian Changxing Formation, with a significant breakthrough for petroleum exploration, are an important target for petroleum exploration in the Yuanba area of the Sichuan Basin in SW China. The storage space types of reef complexes are dominated by the dissolved pore-fracture (DPF). However, using only single geophysical methods, it is difficult to predict effective distribution of DPF. Based on a combination of geological models and geophysics technology, this study proposes two new geophysical methods, including anisotropy coherence technique (ACT) and fracture intensity inversion (FII), to research the characteristics of DPF by facies-controlling in Changxing Formation in Yuanba area. Two major findings are presented as follows: (1) the characteristics of DPF varying with facies are the result of different diagenetic and petrophysical property. The intensity of DPF decreases from reef and bioclastic bank to interbank sea and slope; (2) ACT can qualitatively identify the distribution of DPF with no-directional and dispersed distribution, while FII can quantitatively characterize the intensity of DPF development within various sedimentary facies. When integrated into the geological study, ACT and FII can provide an effective way to predict the distribution of DPF in similar geological settings and the predicted DPF have been supported by the historical well data.

Dolomitization of Gas Reservoirs: The Upper Permian Changxing and Lower Triassic Feixianguan Formations, Northeast Sichuan Basin, China

Abstract The Lower Triassic Feixianguan Formation, and to a lesser extent the Upper Permian Changxing Formation, contain more than ten giant, recently discovered dolomite gas fields on either side of a paleo-embayment (Kaijiang–Liangping Bay), Sichuan Basin, China. This study documents the origin of the dolomite using a combination of petrology, fluid-inclusion, trace-element geochemistry, and stable-isotope and radiogenic-isotope analysis of samples from outcrop and core samples from producing gas fields. Three stages of dolomitization are revealed. Dolomicrite from the lagoon facies in the Feixianguan Formation and minor fabric-retentive dolomite from reef facies in the Changxing Formation on the northeast side of Kaijiang–Liangping Bay represent the initial dolomite phase, caused by reflux of Feixianguan brine, with salinity close to gypsum saturation (∼ 18 wt % salinity), during the first stage of diagenesis at 30°C. The second and dominant dolomite phase (fabric-retentive dolomite), in both the Feixianguan and Changxing formations on the both sides of Kaijiang–Liangping Bay, is the result of reflux of diagenetic fluids with salinity ranging from normal Feixianguan seawater (3.5% salinity) up to mesohaline water (6.5% salinity) during early burial diagenesis at about 35–40°C, as shown by the dominance of single-phase aqueous inclusions, shallow-formed stylolites that cut the second phase of dolomite, and the Early Triassic marine carbon, oxygen, and strontium-isotope, and rare-earth-element and yttrium (REE + Y) signals. The subordinate third dolomite phase (fabric-obliterative dolomite and dolomite cement) was due to burial diagenetic recrystallization between 80 and 140°C in the Feixianguan and Changxing formations on both sides of Kaijiang–Liangping Bay. The third phase of dolomitization involved water expelled from the slightly younger Lower Triassic Jialingjiang Formation marine evaporites and carbonates on the northeast side of Kaijiang–Liangping Bay, revealed by the high-salinity of the aqueous inclusions that homogenize at 80 to 140°C and elevated strontium-isotope data. The carbon-isotope and REE + Y signals of the third phase of dolomite are similar to earlier dolomite cements confirming the lack of exotic input. Future exploration should be focused on dolomitized, high-energy sediments (e.g., reef and oolitic shoal and bank) in similar Triassic Tethyan-paleogeographic settings that are associated with restricted facies, but underlying dolomitized high-energy Permian strata may also be good exploration targets.

Origin of high H2S concentrations in the Upper Permian Changxing reservoirs of the Northeast Sichuan Basin, China

Intense thermochemical sulfate reduction (TSR) and up to 18% H2S are found in the Upper Permian Changxing Formation (P3ch) in the northeast (NE) Sichuan Basin, China, despite that rare gypsum or anhydrite was found in this formation. Here, we present new concentration data of carbonate-associated sulfate (CAS) from carbonate host rocks, C, O, and Sr isotope data for TSR-related calcites, and S isotope data for sulfur compounds obtained during this study. These data along with spatial-temporal changes in palaeogeopressure conditions, hydraulic conductivity and the physical capacity indicate that the H2S was generated locally from TSR within the P3ch reservoirs. We propose that the reactive sulfates were derived from CAS released during dolomitization and recrystallization of earlier dolomite within the P3ch Fm. and from the cross-formational migration of evaporative brines from the Lower Triassic Feixianguan Formation (T1f) to P3ch Fm. Our calculation shows that the two sources could provide enough SO42− for the generation of H2S within the P3ch reservoirs. Early downward migration of sulfate-rich evaporative brines from the T1f formation occurred in near-surface and shallow burial diagenetic settings (mainly <1000 m). The evaporative brines seeped into porous grainstones and displaced preexisting seawater, causing pervasive dolomitization within the P3ch Fm. Subsequently, TSR calcites precipitated from the pore water have high Sr concentrations (up to 7767 ppm), close to the T1f TSR calcites, and 87Sr/86Sr ratios mainly from 0.7074 to 0.7078, which are significantly higher than those of Late Permian seawater but within the range of early Triassic seawater.

Origin of Marine Sour Natural Gas and Gas-Filling Model in the Puguang Giant Gas Field, Sichuan Basin, China

Taking the geology and tectonic evolution characteristics of the Sichuan Basin into account, the chemical and stable isotopic compositions of natural gas, and biomarker compounds in the reservoir bitumen in the Puguang giant gas field, are investigated to identify the genetic type of marine sour natural gas, take the gas-source correlation, and set up the gas-filling model of the Puguang giant gas field in the Sichuan Basin. The alkane gases in the field are dominated by methane, ranging from 22.06% to 99.64% with an average value of 76.52%, and the low content of heavy hydrocarbon gases are dominantly ethane and little propane. The H2S contents occur among the marine carbonate gas reservoirs, ranging from 0 to 62.17%, wherein the H2S contents in the Upper Permian Changxing Formation and Lower Triassic Feixianguan Formation range from 6.9% to 34.72% (average value=15.27%) and from 0% to 62.17% (average value= 13.4%), respectively, indicating that both are H2S-enriched reservoirs. The chemical and carbon isotopic compositions of marine natural gases show that the alkane gas in the Puguang giant gas field is dominantly oil-cracking gas at high maturity stage, and the biomarker characteristics of reservoir bitumen indicate that the major source rocks are the Upper Permian Longtan Formation sapropelic matters. Moreover, various levels of thermochemical sulfate reduction (TSR) were present in the process of oil-gas transformation, not only increasing the content of non-hydrocarbon gas components (CO2 and H2S) and decreasing the content of heavy hydrocarbon gases, but also causing the reversal of carbon isotope compositions of methane and ethane and the heavier carbon isotope of methane. The recovery of structural configurations over geological time investigates that the gas-filling history of Puguang giant gas field can be divided into three stages: formation of paleo-oil accumulation from the middle-late Indosinian period to the early Yanshanian period, thermal cracking of paleo-oil and TSR alteration from the early to the middle Yanshanian period, and adjustment of gas accumulation from the late Yanshanian to the early Himalayan period. The gypsum of the Lower Triassic Jianglingjiang Formation and the Middle Triassic Leikoupo Formation plays the most important role as effective seal to the gas preservation in different periods.

Reflux dolomitization of the Upper Permian Changxing Formation and the Lower Triassic Feixianguan Formation, NE Sichuan Basin, China

AbstractNatural gas is found in Upper Permian and Lower Triassic dolomite reservoirs of the NE Sichuan Basin, China. Studying the mechanisms for porosity‐modifying dolomitization and predicting its spatial distribution is of great importance for exploration and field appraisal. Subsurface core samples from the oolitic Lower Triassic Feixianguan Formation and outcrop samples reef carbonate Upper Permian Changxing Formation were studied using cathodoluminescence (CL), electron microprobe, fluid inclusion and isotope studies (δ18O, δ13C and 87Sr/86Sr), in an attempt to determine the origin of the dolomitizing fluids. Trace element and isotope data show that Lower Triassic seawater was most likely responsible for dolomitization of both the Permian and Triassic rocks. Sr/Ca molar ratios, calculated for the diagenetic fluid, suggest seawater was involved in dolomitization. Strontium isotope data suggest that seawater 87Sr/86Sr during the Jialingjiang (Lower Triassic) reached 0.7078; this ratio is recorded ubiquitously in both Lower Triassic oolitic dolomite and the underlying Lower Permian reef dolomite. In the Permian Changxing Formation, elevated Triassic 87Sr/86Sr values have overprinted the significantly lower 87Sr/86Sr ratio characteristic of Upper Permian seawater. Fluid inclusion analysis led to the conclusion that dolomitization must have commenced at a temperature lower than about 50°C. Integrating the sedimentology, petrology and geochemistry data, we conclude that coeval Lower Triassic (Feixianguan or slightly younger) seawater caused dolomitization by a reflux‐seepage process for both the Lower Triassic and Upper Permian units. There is no evidence for deep burial or meteoric‐marine mixing‐zone dolomitization. It is likely that evaporating seawater initially flowed into the highly porous oolitic Feixianguan Formation in a sabkha/lagoonal setting and precipitated anhydrite nodules and beds. Concomitant dolomitization, in the underlying Permian reef carbonates, continued during seepage even after the supply of sulphate was exhausted, leading to an absence of anhydrite in Permian Changxing Formation.

Geochemical characteristics of the Permian Changxing Formation reef dolomites, northeastern Sichuan Basin, China

The recent discovery of deep and ultra-deep gas reservoirs in the Permian Changxing Formation reefs, northeastern Sichuan Basin is a significant development in marine carbonate oil & gas exploration in China. Reef dolomites and their origins have been major research topics for sedimentologists and oil & gas geologists. The petrography, trace element and isotope geochemistry of the reef dolomites indicated that the dolomites are characterized by low Sr and Mn contents, relatively low Fe contents, very similar δ13C and δ18O values and very different 87Sr/86Sr ratios. Although the calculated results of the fluid mixing suggested that a mixture with 85%–95% meteoric water and 5%–15% seawater seemed to be the dolomitizing fluids of the reef dolomites, the low Mn contents, relatively low Fe contents, high δ13C values and high homogenization temperatures of the dolomites did not support that there were large proportions of meteoric water in the dolomitization process, and the 87Sr/86Sr ratios which were close to coeval seawater also did not support the possibility of the mixture of deep-burial circulated fluids from clastic rocks. High temperature deep-burial circulated seawater with low Mn and Fe contents, high Sr content and high δ13C values from the dissolution of widely distributed Triassic evaporites during the burial diagenetic processes (including dehydration of water-bearing evaporites) could have been the dolomitizing fluids of the reef dolomites.

Special Elemental Sulfur in the Dolomite Reservoirs of the Giant Puguang Gas Field: Proof of Crude Oil Involved Thermochemical Sulfate Reduction

Special elemental sulfurs formed by thermochemical sulfate reduction (TSR) are reported for the first time in the Upper Permian Changxing Formation and the Lower Triassic Feixianguan Formation, Northeastern Sichuan Basin, Southwest China. They occur mainly as single pellets with round morphology, and partly in semi or ¼ pellets of 10 to 100μm in diameter, or form interpenetrating grains. Most of them precipitate at the inner wall of the vugs, oomoldic or inter-granular pores of the dolomitic reservoirs. Petrogenetic sequence analysis demonstrates that they postdate the dolomite formation and cementation, while are prior to the late calcite and quartz cementation The sulfur develops restrictively in the section where pyrobitumen is found, namely in the paleo-oil window, which strongly supports that the crude oil/liquid hydrocarbon had been involved in the TSR.

High quality source rocks in trough facies of Upper Permian Dalong Formation, Sichuan Basin

Source rocks in trough facies of the Upper Permian Dalong Formation of the Sichuan Basin were assessed by field observations and measurements in outcrop areas as well as from logging and core/cutting analysis from drilling sites. The results show that in the Dalong Formation, the black shale and sapropelite facies have an average TOC of 5.86%, with the maximum value up to 24.31%, making them high quality hydrocarbon source rocks. Dark cryptite, with a TOC of 0.77% (on average) is also a good hydrocarbon source rock. A dark silicate, with TOC of only 0.13% on average, has no potential to generate hydrocarbons. Since the source rocks are much evolved, the content of chloroform bitumen “A” extracts tend not to be high. The (S1+S2) value of source rocks in the Dalong Formation are low, with only 4.36 mg/g on average. The major macerals of Kerogen of the Dalong Formation are sapropels, averaging 71.5%. Kerogen is mainly Type II1, and a little belongs to II2. The flourishing of pelagic organisms during deposition of the Late Permian Changxing Formation, slow deposition rate of the Dalong Formation and an anoxic and calm deep water depositional environment are the key controlling factors for accumulation, preservation and generation of organic matter in this part of the basin. ： 通过四川盆地上二叠统海槽相大隆组露头区野外观察测量和钻井区测井及岩心、岩屑分析，对大隆组烃源岩进行评价。研究表明，大隆组黑色泥质岩、腐泥岩TOC平均值为5.86%，最高可达24.31%，是一套优质烃源岩；大隆组暗色泥晶灰岩TOC平均值可达0.77%，也是很好的烃源岩；大隆组暗色硅质岩TOC平均值仅为0.13%，不具备生烃能力。由于烃源岩演化程度较高，氯仿沥青“A”含量总体不高。大隆组烃源岩（S1＋S2）值总体也比较低，平均值仅4.36 mg/g。烃源岩干酪根显微组分以腐泥组为主，干酪根主要为Ⅱ1型，少量为Ⅱ2型。晚二叠世长兴组沉积期浮游类生物繁盛、大隆组沉积速率缓慢以及缺氧、安静的深水沉积环境是有机质富集、保存、形成优质烃源岩的主要控制因素。图6表2参12

TSR origin of sulfur in Permian and Triassic reservoir bitumen, East Sichuan Basin, China

Solid bitumen, H 2S, pyrite and anhydrite samples from the Lower Triassic Feixianguan Formation and Upper Permian Changxing Formation, and source rock kerogen samples from the Cambrian, Ordovician, Silurian and Permian in the East Sichuan Basin were analyzed for their sulfur content and δ 34S values, to determine if the measurements were consistent with a thermochemical sulfate reduction (TSR) origin of the produced H 2S and solid bitumen. The results show that the solid bitumen samples have sulfur contents from 2.6 to 12.5 wt.% and δ 34S values from +13.4‰ to +22.1‰. The δ 34S values are significantly heavier than those of potential Upper Permian source rock kerogens that average +1.3‰ ( n = 3), suggesting that most of the sulfur in the solid bitumen is derived from isotopically heavy H 2S of TSR origin (from +12.0‰ to +13.7‰). Interestingly, two gases produced from gas to water transition zones during drill stem testing (DST) show significantly lighter δ 34S values (+3.3‰ and +4.1‰) than the H 2S from gas-producing intervals. The isotopically light H 2S may be genetically linked to fracture-filling isotopically heavy anhydrite with δ 34S values from +38.5‰ to +39.6‰, which are significantly heavier than those of coeval seawater sulfate (+11.8‰ to +21.7‰) as measured for bedded anhydrite. The link may have resulted from reduction of part of the dissolved sulfate by hydrocarbons under sulfate-limited conditions. Source rock kerogens show a secular change in δ 34S values relative to source rock age. A dramatic decrease and then increase in δ 34S kerogen was found toward the Permian/Triassic boundary for the first time in the East Sichuan Basin.

Preliminary Mo isotope data of Phanerozoic clastic sediments from the northern margin of the Yangtze block and its implication for paleoenvironmental conditions

Mo isotopes along with Nd isotopes and other geochemical characteristics of the Phanerozoic clastic sediments from the northern margin of the Yangtze block were analyzed by MC-ICP-MS and some other techniques, spanning the period from Cambrian to Jurassic. The δ98Mo values and eNd(t) in these sedimets were observed to exhibit a large range of variation (−0.65‰2+1.87‰, −1.46–−10.90, respectively). Specifically, the sample from Late Permian Maokou Formation has relatively positive values in both δ98Mo and eNd(t) (+0.97‰, −1.46, respectively). These elevated values are proposed to relate to the input of a large quantity of H2S and HS− into the contemporaneous seawater by the volcanic eruption of the Emeishan mantle plume. Introduction of these sulfide gases would cause MoO4 2− to be transferred into MoS4 2−, favoring the preferential deposition of the heavy Mo isotope as MoS4 2− and thus leaving a positive Mo isotope value. The sample from Late Permian Changxing Formation has a significantly elevated δ98Mo value of +1.87‰, in association with the relatively negative eNd(t) (−10.90) and remain the relatively higher content of both TOC and total sulfur (TS) (1.52%, 2.02%, respectively). This infers the presence of the enhanced primary productivity and high organic burial during the Changxing period. The production of a huge quantity of HS− and H2S by the sulfate-reducing reaction led to the existence of the elevated seawater δ98Mo values during the Changxing period. The preliminary Phanerozoic data reported here allow us to propose that the Mo isotope signature preserved in reductive sediments is related to the regional tectonic settings and the associated environmental conditions, and thus the measurement of δ98Mo promotes our understanding on the evolution of the paleoenvironmental event in the Earth history.

Three Episodes of Hydrocarbon Generation and Accumulation of Marine Carbonate Strata in Eastern Sichuan Basin, China

Abstract: It is concluded that there are three hydrocarbon generation and accumulation processes in northeastern Sichuan on the basis of the characteristics of solid bitumen, gas‐light oils‐heavy oils, homogenization temperature of fluid inclusions and diagenesis for beach‐ and reef‐facies dolomite gas‐bearing reservoirs in the Puguang Gas Field, northeastern Sichuan Basin, southern China. The first hydrocarbon generation and accumulation episode occurred in the Indosinian movement (late Middle Triassic). The sapropelic source rocks of the O3w (Upper Ordovician Wufeng Formation)‐S1l (Lower Silurian Longmaxi Formation) were buried at depths of 2500 m to 3000 m with the paleogeothermal temperature ranging from 70°C to 95°C, which yielded heavy oil with lower maturity. At the same time, intercrystalline pores, framework pores and corrosion caused by organic acid were formed within the organic reef facies of P2ch (Upper Permian Changxing Formation). And the first stage of hydrocarbon reservoir occurred, the level of surface porosity of residual solid bitumen {solid bitumen/ (solid bitumen + residual porosity)} was higher than 60%. The second episode occurred during the Middle Yanshanian movement (late Middle Jurassic). During that period, the mixed organic source rocks were deposited in an intra‐platform sag during the Permian and sapropelic source rocks of O3w‐S1l experienced a peak stage of crude oil or light oil and gas generation because they were buried at depths of 3500 m to 6800 m with paleogeothermal temperatures of 96–168°C. At that time, the level of surface porosity of residual solid bitumen of the T1f shoal facies reservoirs was between 25% and 35%, and the homogenization temperatures of the first and second stages of fluid inclusions varied from 100°C to 150°C. The third episode occurred during the Late Yanshanian (Late Cretaceous) to the Himalayan movement. The hydrocarbon reservoirs formed during the T1f and P2ch had the deepest burial of 7700 m to 8700 m and paleogeotemperatures of 177°C to 220°C. They could be cracked into dry gas (methane), and the same with the source rocks of the Permian and O3w‐S1l because they all reached the pyrolysis stage under such conditions. Consequently, the present natural gas (methane) reservoirs were developed.

Changxing Formation biohermal gas pools and natural gas exploration, Sichuan Basin, China

Abstract Plain fluvial facies, paralic facies, shallow gentle slope facies on carbonate platforms, platform marginal reef facies, Kailiang trench facies, and north Sichuan western Hubei ocean basin facies are developed in the Permian Changxing Formation and have favorable source–reservoir–seal conditions. To improve the seismic prediction reliability of biohermal anomalies, a recognition mode of biohermal seismic anomalies was established in the late 1990s based on the geological features of known reefs and the corresponding seismic reflection features, and 182 biohermal anomalies were found in eastern Sichuan. Researches on sedimentary facies reveal a platform marginal reef facies from both sides of Kailiang trench to western Hubei, and the seismic anomalies of 36 marginal reefs have been found there. These marginal reefs have large areas and high drill reserves. The facies belt extends to the western margin of the basin. Many point reefs are developed on the slope platform facies, and 146 seismic anomalies of point reefs are found in eastern basin. Point reefs are developed widely in central basin.

Reef-Bank Features and Their Constraint to Reservoirs of Natural Gas, from Permian Changxing Formation to Triassic Feixianguan Formation in Daxian-Xuanhan Area of Sichuan Province, South China

Detail studies of the seismic section and certain wells, such as the Puguang-2, the Puguang-6, and the Maoba Wells, demonstrate that the reef facies from the Permian Changxing Formation to the Triassic Feixianguan Formation are expressed as the lens with medium-strong changing swings and chaotic images in the seismic section. The reef facies is made up of gray limestones and dolomites of both baffling sponge reef and the frame sponge reef. Moreover, the bank facies from the Permian Changxing Formation to the Triassic Feixianguan Formation is represented as the lens with medium-strong changing swings and discontinuity images in the seismic section, which is made up of several petrologic types, that is, the French gray thick-bedded to massive sparitic oolitic dolomite, the sparitic gravel-oolitic dolomite, the sparitic pisolitic-oolitic dolomite, the sparitic bioclastic dolomite, and the sparitic dolarenite. Both the reef facies and the bank facies concomitant are developed and distributed in space along the platform margin, which form a special reef-bank facies zone. This facies zone controls the petrological features, the reservoir qualities, and the spatial distribution of the reservoirs of natural gas in the Daxian-Xuanhan area.