Middle Triassic Leikoupo Formation Research Articles

New directions in an established gas play: Promising dolomite reservoirs in the Middle Triassic Leikoupo Formation of the Sichuan Basin, China

The discovery of carbonate gas fields in the Middle Triassic Leikoupo Formation of the Sichuan Basin has a complex history. In recent years, a series of structural fields have been discovered in the western Sichuan Basin. Their discovery confirms the immense exploration potential of the Leikoupo Formation. In this study, we analyze the characteristics of Leikoupo Formation exploration plays using exploration wells and test data, aiming to provide a reference for further discoveries. The Leikoupo Formation represents the uppermost unit in the Sichuan marine carbonate platform succession. During its deposition, the whole basin was characterized by a restricted and evaporitic platform. Two classes of reservoirs developed. One is pore–fracture reservoirs, in marginal platform and intraplatform shoals, and another is fracture–vug reservoirs in the karstic weathering crust of the formation-capping unconformity. Three hydrocarbon accumulation models were established for the Leikoupo Formation based on the spatial and temporal relationship among the source, reservoir, and cap rocks. Two types of exploration plays are present in the Leikoupo Formation, that is, shoal (including intraplatform shoal and marginal platform shoal) dolomite plays and karstic dolomite weathering crust plays (including intraplatform shoal karst and marginal platform shoal karst). The western Sichuan depression in the karstic slope belt presents immense exploration potential because of a proximal hydrocarbon supply, charging via an extensive fracture network, shoals and karstic reservoir, a good seal rock of terrestrial mudstone, and potential composite hydrocarbon accumulations in stratigraphic traps, making it a promising area for future exploration.

Diagenesis and its impact on a microbially derived carbonate reservoir from the Middle Triassic Leikoupo Formation, Sichuan Basin, China

The uppermost Middle Triassic Leikoupo Formation in the western Sichuan Basin of China has recently been shown to host as much as 5.3 tcf (1.5 × 1012 m3) of natural gas resources. The reservoir rocks, composed mainly of microbially derived dolomudstone (e.g., thrombolites and stromatolites), are characterized by low porosity (<8%) and permeability (<0.001 to 10 md). The limestone is commonly tight and not of reservoir quality because of abundant meteoric calcite cementation, whereas the dolostone has various types of pores dominated by solution-enlarged pores and vugs, microbial framework pores, and micropores. Breccias are well developed in places, probably because of dissolution of underlying evaporites (e.g., anhydrite) by an influx of low-salinity fluids (e.g., freshwater and seawater) during an early burial stage. Early dolomitization created micropores in the dolomudstone, and subsequent diagenetic events were dominated by calcite, dolomite, quartz cementation, pyrite replacement, compaction, fracturing, and development of stylolites. Localized hydrothermal activity has been evidenced by high homogenization temperatures (∼160°C–200°C) obtained from fluid inclusions in fracture-filling cements. Bacterial sulfate reduction probably resulted in H2S generation, pyrite precipitation, and solution-enlarged pore and vug formation, whereas part of the current H2S in these reservoirs may have been sourced from thermochemical sulfate reduction or an underlying formation (e.g., the Feixiangguan Formation). Development of microfractures and associated micropores was probably the final diagenetic event, which improved pore interconnectivity. This study confirms the effect of diagenesis on the development of a microbial dolomudstone reservoir, which may be applicable to other similar microbial carbonate reservoirs elsewhere, for example, Middle Triassic sections of the Tethys region and offshore Brazil.

Genetic types of carbonate shoal reservoirs in the Middle Triassic of the Sichuan Basin (SW China)

Abstract Carbonate shoal reservoirs of the Middle Triassic Leikoupo Formation in the Sichuan Basin were investigated for potential gas resources. The carbonate shoal reservoirs can be divided on the basis of their formation mechanism into residual intergranular pore, karst (further divided into syndepositional-, burial-, and epigenetic-karstification types), dolomitized, and composite reservoirs. Residual intergranular pores and dissolved pores are the main reservoir spaces in the residual intergranular pore reservoirs. Isolated intragranular corroded pores and moldic pores are the main reservoir spaces in syndepositional-karstification reservoirs, the main spaces in burial-karstification reservoirs are dissolved pores and holes, and pores and caves on various scales are the main reservoir spaces of epigenetic-karstification reservoirs. Intercrystal pores and intercrystal solution pores are the main reservoir spaces of dolomitized reservoirs. Composite reservoirs are characterized by a variety of reservoir spaces with pores, holes, and fissures. High-quality shoal reservoirs were formed by superposition of carbonate shoals and constructive diagenetic modification. Preservation of residual intergranular pores was crucial for development of residual intergranular pore reservoirs. Platform-interior micro-highlands, relative declines in sea level, and rainfall controlled the development of syndepositional-karstification reservoirs. Dissolution by unsaturated acidic fluids, pre-existing pores and pore throats, and the direction of corrosive fluid flow were important for burial-karstification reservoirs. Epigenetic-karstification reservoirs formed as a result of significant sea-level drops or tectonic movement. Dolomitized reservoirs formed in shoals affected by dolomitization caused by evaporation of seawater or that occurred during burial. Composite shoal reservoirs were formed by two or more processes.

Origins and Geochemistry of Dolomites and Their Dissolution in the Middle Triassic Leikoupo Formation, Western Sichuan Basin, China

Triassic dolomites occur pervasively in the Western Sichuan Basin. Although these strata have been deeply buried and affected by multiple phases of dolomitization and dissolution, some intervals in the upper part of the Leikoupo Formation have developed high porosity. Based on their petrographic and geochemical features, three major types of dolomite fabrics are recognized, namely, dolomicrite, fabric-retentive dolomite, and fabric-destructive dolomite. Geochemical evidence indicates that the dolomicrite formed following the Sabkha model in a low-temperature hypersaline environment, as these rocks exhibit abnormally high Sr and Na contents, lower Fe and Mn contents, δ18O values generally ranging from −1.70‰ to −1.67‰ (with an average value of −1.69‰), and higher Mg/Ca ratios. The fabric-retentive dolomite formed following the seepage-reflux model in a shallow burial environment, and these rocks exhibit the highest 87Sr/86Sr ratios, δ18O values generally ranging from −6.10‰ to −2.50‰ (with an average value of −3.98‰), and a wide range of Fe and Mn contents, indicating that they may have been altered by meteoric water. The fabric-destructive dolomite formed following the burial model at elevated temperatures; these rocks exhibit the lowest Sr and Na contents, δ18O values generally ranging from −7.01‰ to −6.62‰ (with an average value of −6.79‰), relatively higher Mg/Ca values, and lower 87Sr/86Sr ratios. The early Sabkha, seepage-reflux dolomitization and penecontemporaneous periodic meteoric freshwater selective dissolution processes formed multi-period, overlapping moldic pores, algal framework pores, and intragranular dissolution pores. The superposition of organic acid dissolution during the burial period is the main controlling factor of the formation of deeply buried, high-quality dolomite reservoirs in the Leikoupo Formation.

Geochemical Characteristics and Genetic Types of Natural Gas in the Xinchang Gas Field, Sichuan Basin, SW China

AbstractThe molecular compositions and stable carbon and hydrogen isotopic compositions of natural gas from the Xinchang gas field in the Sichuan Basin were investigated to determine the genetic types. The natural gas is mainly composed of methane (88.99%–98.01%), and the dryness coefficient varies between 0.908 and 0.997. The gas generally displays positive alkane carbon and hydrogen isotopic series. The geochemical characteristics and gas‐source correlation indicate that the gases stored in the 5th member of the Upper Triassic Xujiahe Formation are coal‐type gases which are derived from source rocks in the stratum itself. The gases reservoired in the 4th member of the Xujiahe Formation and Jurassic strata in the Xinchang gas field are also coal‐type gases that are derived from source rocks in the 3rd and 4th members of the Xujiahe Formation. The gases reservoired in the 2nd member of the Upper Triassic Xujiahe Formation are mainly coal‐type gases with small amounts of oil‐type gas that is derived from source rocks in the stratum itself. This is accompanied by a small amount of contribution brought by source rocks in the Upper Triassic Ma'antang and Xiaotangzi formations. The gases reservoired in the 4th member of the Middle Triassic Leikoupo Formation are oil‐type gases and are believed to be derived from the secondary cracking of oil which is most likely to be generated from the Upper Permian source rocks.

Paleogeomorphological reconstruction and geological implications of the weathered-crust karst on the top of the Middle Triassic Leikoupo Formation in the Longgang area, Sichuan Basin, China

Weathered-crust karst carbonate is developed in the third submember of the fourth member of the Middle Triassic Leikoupo Formation (Lei43 submember) in the Longgang area of Sichuan Basin, China, and acts as an important oil and gas reservoir. To reconstruct the paleogeomorphology of the weathered-crust karst, we analyzed the seismic thickness of the interval from the erosional unconformity surface on the top of the Leikoupo Formation to the karst at the bottom of the third member of the Xujiahe Formation (Xu3 member), based on existing three-dimensional seismic and drilling data, and the sedimentary characteristics of the Leikoupo and Xujiahe formations. There are three secondary types of geomorphological unit, namely the karst highlands, the karst transitional zone, and the karst basin, and these have distinct karst water development patterns and hydrological conditions that determine the intensity of karstification and reservoir quality. Among them, the karst highlands have a poor reservoir capacity due to their long-term exposure above the water table, which resulted in severe denudation. The karst transitional zone features the superimposition of multiple periods of strong karstification, and this has resulted in high-quality reservoir conditions in the karst monadnocks but poor reservoir conditions in the karren. The karst basin represents a drainage area that experienced weak karstification, and the reservoir capacity is generally poor, although some good reservoirs were developed in shoals. Paleogeomorphological maps provide excellent guides to finding karst monadnocks in karst transitional zones, and monadnocks should serve as the main targets of exploration in the weathered-crust karst reservoir on top of the Lei43 submember.

Construction risks of Huaying mount tunnel and countermeasures

The Chongqing-Guang’an motorway is planned to cross Huaying mount at Jingguan town of Chongqing city. The whole mount is a colossal anticline whose core is consisted of coal measure strata (upper Permian Longtan formation P2l) and the limbs are limestone strata (middle Triassic Leikoupo formation T2l and lower Triassic Jialingjiang formation T1j). The tunneling is full of risks of collapse, gas explosion or gas outburst, water (mud) inrush, gas inrush because of existence of faults, high pressure gas, karst tectonics and coal goafs around the tunnel. In order to cope with the high risk, two main countermeasures were taken to ensure security of construction. One is geology prediction, and the other is automatic wireless real-time monitoring system, which contains monitoring of video, wind speed, poisonous gas (CH4, CO, H2S, SO2), people location, and automatic power-off equipment while gas contents being more than warning threshold. These ascertained the engineering safety effectively.

Geochemical characteristics and origin of natural gas reservoired in the 4th Member of the Middle Triassic Leikoupo Formation in the Western Sichuan Depression, Sichuan Basin, China

The gas exploration in the 4th Member of the Middle Triassic Leikoupo Formation (T2l4) in the Western Sichuan Depression has achieved a continuous breakthrough in the recent years. However, the gas origin and source remain controversial. The study on the geochemical characteristics indicates that the T2l4 gas in the Western Sichuan Depression is typically dry. Its dryness coefficient is generally higher than 0.99. The δ13C1 and δ13C2 values range from −35.1‰ to −29.3‰ and −34.8‰ to −31.9‰, respectively, with the exception of one gas sample from Well PZ1 with the δ13C2 value of −26.4‰. The δDC1 value ranges from −164‰ to −136‰. The gas souring index is positively correlated with the δ13C2 value in comparison to the δ13C1. The T2l4 gas has experienced heavy alkane-dominated TSR instead of the methane-dominated TSR. The T2l4 gas in the Western Sichuan Depression generally displays a GSI value lower than 0.01 with the exception of two gas samples from Well PZ1 (0.036, 0.04); they indicate extremely low TSR alteration extent. Gas origin identification points out that the T2l4 gas in the Western Sichuan Depression is mainly oil-type gas that has reached the secondary gas cracking stage. The CO2 in the T2l4 gas that has high δ13C values are mainly inorganic. They are mainly derived from the interaction between acidic fluids and carbonate reservoirs.

The three different types of paleokarstification and reservoir distribution of Leikoupo Formation, Middle Triassic in the northern Sichuan Basin, China

The Middle Triassic Leikoupo Formation in the northern Sichuan Basin is a new discovery sequence for strategy replacement in the gas exploration field. The carbonate paleokarst reservoirs of the Middle Triassic Leikoupo Formation in the northern Sichuan Basin provide a case study of gas exploration in carbonate paleokarst reservoirs. By conducting a comprehensive study of well drilling, drill cores, well loggings, and test data, combined with the analysis of geological background of the Sichuan Basin, it is considered that syndepositional, supergene, and burial paleokarstification can be identified in the Leikoupo Formation, and these types of karstification were vital for reservoirs formation. Syndepositional karstification controlled the formation and distribution of early reservoirs. This type of karstification was limited to high-energy grain shoals, and therefore its horizontal extent was limited by the distribution of this facies. During temporary episodes of relative sea level fall, carbonate shoals were exposed subaerially or within the fresh water lens, and significant selective dissolution occurs, forming moldic pores and intragranular dissolved pores. The reservoirs related to supergene karstification mainly occurred in the area of the buried hill containing carbonate rock at the top of the Leikoupo Formation, and are vertically limited within a depth of 70–90 m under the carbonate corrosion unconformity plane of the Leikoupo Formation. The reservoirs related to supergene karstification formed mainly in limestone breccia and vuggy limestone in surface karst zone, and the high-angle dissolved fracture and medium- and small-scale corrosion holes in vertical vadose zone, and medium and small corrosion holes, dissolved fractures, roof fracture-caves in horizontal karst zone. The karst monadnocks within karst slope are the most favorable microgeomorphic units for horizontal distribution of reservoirs. Burial karstification was related to dissolution of carbonate minerals by organic acids, and was the key factor in optimal transformation of reservoirs. Burial karstification corroded and expanded the pore system left after infilling of the earlier pore system, and mainly was to optimize the transformation the reservoir space redistribution of earlier pore system. The compact carbonate layers related to burial karstification only can be dissolved along the early cracks in the channel and formation cracked reservoirs, but development of this type of reservoir was limited. The reservoirs were influenced by syndepositional, supergene, and burial paleokarstification in the Leikoupo Formation, and carbonate paleokarst reservoirs were formed finally.

Differences between the platform-margin shoal reservoirs and the platform-interior shoal reservoirs of the Middle Triassic Leikoupo Formation, Sichuan Basin, China

The carbonate shoal grainstone reservoirs of the Middle Triassic Leikoupo Formation in the Sichuan Basin provide a typical example of gas exploration in shoal carbonate reservoirs. The carbonate shoal reservoirs in Leikoupo Formation are of two types: platform-margin shoals and platform-interior shoals. Based on a comprehensive data analysis, and through analysis and correlation of grain shoal reservoir characteristics, it has been shown that there are significant differences between platform-margin and platform-interior shoal reservoirs in terms of the types of reservoir space, the relationship between porosity and permeability, and the pore throat structures. The pores in the platform-margin shoal reservoirs are mainly relict intergranular pores and intergranular dissolution pores, and their porosity and permeability show a good positive correlation. The throats are thick and the pore throats have a good distribution. Isolated intragranular solution pores and moldic pores are mainly developed in the platform-interior shoal reservoirs. There is a weak positive correlation between porosity and permeability, pore throats are less developed, and the pore throats have poor configurations. Differences in sedimentary environment and diagenetic evolution are the primary causes of the different reservoir characteristics of the two shoal facies. The water body in the platform-margin shoal deposits had a higher energy than that in the platform-interior shoal deposits, and its wave base was deeper that of the platform-interior shoals. The shoal bodies at the platform margin are much thicker than those in the platform interior. The grainstones of platform-margin shoals are much better sorted. The rocks have undergone compaction during most post-depositional stages. The thicker grainstones of the platform-marginal shoals preserved their original intergranular pores better than the lower-thickness grainstones in the platform-interior shoals. This was favorable for formation of intergranular dissolution pores during later burial. In contrast, the grainstones of the platform-interior shoals are less thick and early intergranular cements are well developed, resulting in large-scale loss of the original intergranular pores. However, as a result of selective dissolution by early fresh water in the meteoric diagenetic environment, intragranular solution pores and moldic pores were readily formed. Thus, the platform-margin shoal reservoirs show preservation of the original intergranular pores enhanced by burial dissolution, but the platform-interior shoal reservoirs are mainly the result of early selective dissolution by meteoric fresh water.

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.

Quantitative Evaluation Model of Carbonate Reservoirs Based on Multi-Level Fuzzy Theory

The quantitative evaluation of carbonate reservoir is one of the key issues of oil and gas exploitation. However, due to the characteristics of carbonate reservoir, such as diverse physical distribution, complex pore-throat structure and strong heterogeneity, its quantitative evaluation is relatively difficult. Take the Lei 1 Member carbonate reservoir of Middle Triassic Leikoupo Formation in the Moxi gas field of Sichuan Basin as the example. The improved analytic hierarchy process is adopted based on 9 prioritized parameters which have direct influence on the quality of carbonate reservoir, namely porosity, permeability, water saturation, displacement pressure, average pore-throat radius, effective reservoir thickness, permeability variation coefficient, permeability rush coefficient and permeability range, to build a multi-level hierarchical structure model and quantitatively determine the relative importance quantitative indicators of all the parameters for reservoir quality. And fuzzy comprehensive evaluation method is used to build a quantitative evaluation model of carbonate reservoir. The evaluation results are well consistent with the actual gas capacity, which indicates that the mathematical model is available for quantitatively evaluating the carbonate reservoir and has relatively good feasibility.

Origins of evaporites in the Middle Triassic Leikoupo Formation of the Sichuan Basin, southwest China and their geological implications

The widespread occurrence of evaporites is an important feature of the Middle Triassic Leikoupo Formation in the Sichuan Basin, southwest China. Their origins and geological implications, however, have not been well addressed so far. The issues were preliminarily discussed in this paper. The evaporites include two genetic types. The first is found predominantly in the first to third members of the formation, exhibits a uniform lateral distribution, and presents fairly thin (<5 m) individual layers with a cumulative thickness of <50 m. These evaporites show predominantly laminar bedding with massive beds locally, and their origin is inferred to be from lagoon deposition. The second genetic type was formed later, and occurs from the middle part of the third member to the middle part of the fourth member of the formation. The lithology includes massive gypsum rocks and halites. There is marked variation in both the thickness and lateral distribution of the second type of deposits, and they are inferred to have been deposited in evaporitic depressions whose depositional centers were located in the western and northern basins. These results have implications for the understanding of the region’s tectonics. The tectonic stress field of the Sichuan Basin during the Middle Triassic was subject to a transition from extensional to compressive stress. The time interval represented by the first member to the lower part of the third member of the formation encompassed a stable tectonic stage during which the transition was initiated. In contrast, the time interval represented by the middle part of the third member to the middle part of the fourth member was characterized by an active tectonic phase. During that time, the NW–SE and NE–SW trending stresses became activated, resulting in the formation of new depositional centers. Strong evaporation led to the rapid deposition of evaporites in structural lows. The findings provide a new understanding of the geology of the Lower Triassic Leikoupo Formation in the Sichuan Basin.

Mega-shoaling in carbonate platform of the Middle Triassic Leikoupo Formation, Sichuan Basin, southwest China

Shoaling is a common type of sedimentation in the evolution of carbonate platform, and commonly has poor continuity. This paper presents a newly discovered and rare type of shoaling, i.e., mega-shoaling in nearly basin scale, which is developed in the Middle Triassic Leikoupo Formation of the Sichuan Basin, southwest China. During the Leikoupo time, the studied Sichuan Basin experienced hot and dry climate conditions and developed a carbonate platform within a restricted epicontinental sea. In B sub-layer of the Lei-1-1 sub-member of the Leikoupo Formation a series of grainstones of shoal facies accumulated throughout almost the entire basin, thereby generating features associated with basin-scale mega-shoaling. By detailed core examination and microscopic observation of thin sections, it is shown that the lithology of this set of grainstones is dominated by doloarenite (calcarenite) followed by oolitic dolomite (limestone). In addition, it contains three types of sedimentary sequences characterized by upward-coarsening and upward-shallowing as the followings: restricted lagoon to platform interior beach; restricted lagoon to platform interior beach and to platform flat; and tidal flat to peritidal beach. Subsequently, a multicyclic stratigraphic division and correlation revealed that this set of grainstones can be well traced and compared horizontally, and is generally isochronous. In addition, a template for logging facies, established based on core calibrations and logging data, was employed to analyze the 235 wells in the basin. The results demonstrate the shoal grainstones to be 10–40 m thick with a 15×104 km2 continuous distribution area. These findings indicate that the carbonate platform developed mega-shoals within a short period of time. The genesis of such a mega-shoaling was investigated by focusing on various shoaling conditions, such as paleo-tectonics, paleo-geomorphology, paleo-climate, sea-level changes, and palaeo-hydrodynamics. A specific combination of independent geological factors creates beneficial geomorphologic conditions for the mega-shoaling including a quiescent paleo-tectonic environment, relatively flat paleo-geomorphology and evaporites filling up and leveling off. In addition, a stably settling carbonate platform underwent sea-level fluctuations through swift transgressions and protracted regressions, which is not only conducive to continuous, multicyclic and superimposed vertical development of grain beaches but also beneficial for the horizontal migration, coalescence and superimposition of individual grain beaches. As a consequence, large-scale and continuously-distributed grain beach sedimentation emerges and mega-shoals develop.

Gas genetic type and origin of hydrogen sulfide in the Zhongba gas field of the western Sichuan Basin, China

Abstract Natural gases and associated condensate oils from the Zhongba gas field in the western Sichuan Basin, China were investigated for gas genetic types and origin of H 2 S by integrating gaseous and light hydrocarbon geochemistry, formation water compositions, S isotopes (δ 34 S) and geological data. There are two types of natural gas accumulations in the studied area. Gases from the third member of the Middle Triassic Leikoupo Formation (T 2 l 3 ) are reservoired in a marine carbonate sequence and are characterized by high gas dryness, high H 2 S and CO 2 contents, slightly heavy C isotopic values of CH 4 and widely variable C isotopic values of wet gases. They are highly mature thermogenic gases mainly derived from the Permian type II kerogens mixed with a small proportion of the Triassic coal-type gases. Gases from the second member of the Upper Triassic Xujiahe Formation (T 3 x 2 ) are reservoired in continental sandstones and characterized by low gas dryness, free of H 2 S, slightly light C isotopic values of CH 4 , and heavy and less variable C isotopic values of wet gases. They are coal-type gases derived from coal in the Triassic Xujiahe Formation. The H 2 S from the Leikoupo Formation is most likely formed by thermochemical SO 4 reduction (TSR) even though other possibilities cannot be fully ruled out. The proposed TSR origin of H 2 S is supported by geochemical compositions and geological interpretations. The reservoir in the Leikoupo Formation is dolomite dominated carbonate that contains gypsum and anhydrite. Petroleum compounds dissolved in water react with aqueous SO 4 species, which are derived from the dissolution of anhydrite. Burial history analysis reveals that from the temperature at which TSR occurred it was in the Late Jurassic to Early Cretaceous and TSR ceased due to uplift and cooling thereafter. TSR alteration is incomplete and mainly occurs in wet gas components as indicated by near constant CH 4 δ 13 C values, wide range variations of ethane, propane and butane δ 13 C values, and moderately high gas dryness. The δ 34 S values in SO 4 , elemental S and H 2 S fall within the fractionation scope of TSR-derived H 2 S. High organo-S compound concentrations together with the occurrence of 2-thiaadamantanes in the T 2 l reservoir provide supplementary evidence for TSR related alteration.

Transfer zones within the Longmen Mountains thrust belt, SW China

The Longmen Mountains thrust belt contains complex structures resulting from multi-phase tectonic movements. Field investigations and interpretations of seismic profiles reveal two important transfer zones in the thrust belt: the Anxian and Guanxian transfer zones. Many thrust and strike-slip faults are found in the shallow part of the transfer zones, and duplexes and detachment structures in the deep part. The Anxian transfer zone is located between the Jiaoziding and Jiudingshan nappes, consisting mainly of the Lower Triassic Feixianguan Formation (T1f) and Jianglingjiang Formation (T1j), and Middle Triassic Leikoupo Formation (T2l). The Guanxian transfer zone is located between the Baoxing and Jiudingshan nappes, consisting mainly of Devonian, Carboniferous, Triassic, Jurassic, and Presinian rocks. This zone contains many thrust faults, strike-slip faults, and duplexes in its deeper part (where early structures have been almost completely overprinted), and many klippens are found at the surface.

Remagnetization history of Middle Triassic Leikoupo Formation on Wangcang section in Sichuan Province, China

An intensive paleomagnetic investigation has been conducted on the Middle Triassic Leikoupo Formation on the Wangcang section (32.14°N, 103. 17°E). The results indicate the magnetic minerals are dominant by multidomain magnetite or maghemite, and the characteristic remnant magnetization revealed by stepwise thermal/alternating field demagnetization is close to the present-day geomagnetic direction of the sampling site. This suggests that dolomitization/thermal viscous magnetization is responsible for the remagnetization of this kind of rocks.

Geochemical characteristics of deep-seated potassium-rich brine in the Middle Triassic Leikoupo Formation, Sichuan, China

Brine extremely rich in potassium, boron and bromine has been discovered from the Middle Triassic Leikoupo Formation at a depth of 4300 m in Sichuan Province. It contains ∼50 g/L of K+, >92 g/L of Na+, >12 g/L of B2O3, >2.36 g/L of Br− and ∼0.030 g/L of I+. The solid precipitates during evaporation at 25°C include KB5O8·4H2O, K2B4O7·3H2O, MgCl2·6H2O and KMgCl3·6H2O. The brine ranges from 2.2‰ to 2.8‰ (SMOW) in δ18O, − 38‰ – − 53‰ (SMOW) in δD, 15.6‰ in δ34S, and 13.5‰–15.1‰ in δ11B. These data, particularly the isotopic composition of boron, indicate that the brine has a composite derivation from marine and nonmarine brines and dissolved marine evaporites in the Triassic system.