Lower Triassic Jialingjiang Formation Research Articles

Fine Detection and Analysis of Hidden Karst in Wellsite with Quasi-Three-Dimensional TDEM Based on Lateral Constraint

Considering that karst caves, underground rivers, and dissolution fractures in shallow carbonate formation in the Sichuan Basin are extremely developed, leakage, failure and plugging difficulties are easy to occur in the drilling process. The TDEM was used to carry out the exploration of hidden karst geological bodies in well QM2, and the quasi-three-dimensional inversion based on lateral constrain was used to invert the TDEM data. Three NW trending anomalous bands were identified in the lower Triassic Jialingjiang Formation within the range of drilling, consisting of seven relatively low-resistivity anomalous zones. Under the guidance of TDEM quasi-three-dimensional inversion resistivity data, the low-resistivity karst development area is avoided, and the specific drilling location of well QM2 is determined. No karst cave and underground river were drilled in the later drilling process of well QM2, as well as no instability phenomenon occurred. It indicates that the TDEM detection results are consistent with the actual drilling, and the quasi-three-dimensional TDEM inversion interpretation data based on lateral constraints is reliable and can accurately detect the buried karst in the wellsite.

Hydrocarbon Generation and Expulsion Histories of the Upper Permian Longtan Formation in the Eastern Sichuan Basin, Southwest China.

The Upper Permian Longtan Formation is the main source rock of the Lower Triassic Jialingjiang Formation in the Eastern Sichuan Basin, Southwest China. However, studies of its maturity evolution and oil generation and expulsion histories are lacking, which are not conducive to the accumulation dynamics of the Jialingjiang Formation in the Eastern Sichuan Basin. Based on the study of tectono-thermal history and geochemical parameter data of the source rock, this paper uses basin modeling technology to simulate the maturity evolution and hydrocarbon generation and expulsion histories of the Upper Permian Longtan Formation in the Eastern Sichuan Basin. The results show that the Longtan Formation source rock in the Eastern Sichuan Basin entered the oil generation threshold at the middle stage of the Early Jurassic and reached the high-maturity stage in the north and central regions at the late stage of the Early Jurassic, and the maturity did not increase after the late stage of the Middle Jurassic. The source rock had the characteristic of "one-stage oil generation and one-stage oil expulsion"; the corresponding period of high oil expulsion was 182-174 Ma (the late stage of the Early Jurassic), which was later than the formation time of the trap of the Jialingjiang Formation, possibly providing oil sources for the paleo-oil reservoirs of the Jialingjiang Formation. The results are of great significance to the gas accumulation process and exploration decision-making in the Eastern Sichuan Basin.

Analysis of geothermal fluid chemical characteristics and genetic model—A case study from the urban area of Jingmen China

Over the years, scholars both at home and abroad have been committed to the research of geothermal fluid chemistry and genetic model, and geochemical and isotopic methods are usually the first choice for evaluation. In this paper, the characteristics of geothermal fluids and their genesis patterns in the Jingmen urban area are analyzed and studied by comprehensive utilization of techniques such as hydrochemistry, trace elements, and isotope analysis. The results show that the water chemical type of surface water and groundwater is Ca-HCO3 type, while the geothermal fluid belongs to the Ca-SO4 type. The recharge source of the geothermal fluid is the infiltration recharge of meteoric precipitation, and the recharge altitude is 395–435 m, and the geothermal fluid is modern water (less than 5–10 years old). The water-rock reaction has not reached equilibrium, and the dissolution is still in progress. The estimated mixing ratio of cold-water ranges from 30%–40%. The thermal reservoirs are mainly calcareous dolomite and dolomite of the Lower Triassic Jialingjiang Formation (T1j). The red sandstone and mudstone of the Upper Cretaceous Paomagang Formation (K2p) distributed in the area are good caprocks of the geothermal field. The geothermal energy in the urban area of Jingmen is a system of lateral “seepage.” Thermal water surges up along the Jingmen fracture and fracture zone, forming a relatively high-temperature center, and a part of its “seeps” laterally into the shallow pore aquifer or pore-fracture aquifer, which mixed with the cold water in it to form medium and low-temperature hot water around the center of the geothermal area.

Natural gas characteristics and gas-source comparisons of the lower triassic Jialingjiang formation, Eastern Sichuan basin

There are abundant natural gas resources in the Lower Triassic Jialingjiang Formation in the Eastern Sichuan Basin (ESB). Important progress has been made in the study of reservoir characteristics and sedimentary facies of marine reef-beach facies in the Jialingjiang Formation. However, there are still differences in the understanding of its natural gas origin and gas source rocks, which greatly restricts the understanding of the reservoir forming process and formation mechanism of the gas reservoirs, so further study is urgently needed. Based on experimental tests of natural gas composition, stable isotopes, and noble gas isotopes with gas chromatography (GC) and mass spectrometry (MS) studies of source rocks and reservoir asphalt saturated hydrocarbons, the natural gas geochemical characteristics, genetic identification and gas-source comparison of the Jialingjiang Formation were studied. The results showed that (1) the natural gas of the Jialingjiang Formation was mainly hydrocarbon gas, and methane was dominant. The average drying coefficient was 99.266%, which is typical of high maturity dry gas reservoirs. The δ13C1, δ13C2 and δ13C3 values ranged from −36.00‰ to 24.80‰, −37.80‰–27.60‰ and −39.20‰–23.20‰, respectively. The δD1, δD2 and δD3 values were between −164.00‰ and −124.00‰, −162.00‰ and −111.00‰ and −173.21‰ and −71.00‰, respectively. (2) The natural gas has a crustal source origin, which was mainly oil-type thermogenic gas generated by secondary cracking of crude oil. The gas-generating parent material is mainly partial sapropelic kerogen and was in the sedimentary environment of salt to brackish water. (3) The natural gas was mainly generated by argillaceous source rocks of the Upper Permian Longtan Formation. (4) The natural gas accumulation in the Jialingjiang Formation has experienced three stages, including paleo-oil reservoir, paleo-gas reservoir and current gas reservoir. The results provide a basis for the exploration decision-making of natural gas in the Jialingjiang Formation in the ESB.

Prediction of Fracture Opening Pressure in a Reservoir Based on Finite Element Numerical Simulation: A Case Study of the Second Member of the Lower Triassic Jialingjiang Formation in Puguang Area, Sichuan Basin, China

All stages of oil and gas exploration and development involve the study of in-situ stress. Since the traditional two-dimensional and three-dimensional homogeneous models can no longer fulfil the requirements of research and production, numerical simulation of the stress field has become an effective study method. In this study, we took the Jia 2 member in Puguang area as a case to establish a geological model and a mechanical model based on the tectonic framework and the distribution characteristics of the rock mechanical parameters, respectively, and loaded the model with the present-day in-situ stress state calculated from the logging data as the boundary conditions. The simulation results show that 1) the orientation of the maximum horizontal principal stress in the study area is near E-W, and the in-situ stress orientation is locally deflected due to the influence of faults; and 2) the magnitude of in-situ stress is predominantly affected by the burial depth and lithology, and the minimum horizontal principal stress, maximum horizontal principal stress, and differential stress are mainly concentrated in the ranges of 30–60, 50–80, and 10–40 MPa, respectively. We also analysed the opening sequence of the multiple fracture systems during development, using the present-day stress field model. The analysis revealed that the E-W fractures will open first, and the continuously increasing operating pressure will lead to formation breakdown, producing a fracture network.

Diagenetic evolution and its impact on reservoir porosity in Lower Triassic Jialingjiang Formation in eastern Sichuan Basin, SW China: Evidence from petrography, trace elements, and Isotopic Studies

The Lower Triassic Jialingjiang Formation, characterised by a carbonate–evaporite succession, is one of the important pay layers in the eastern Sichuan Basin. Several diagenetic events were identified in this study, including meteoric dissolution, dolomitisation, anhydrite/gypsum replacement/cementation, TSR, BSR, and multiple calcite cementation (C1–C3). Dolomitisation and anhydrite/gypsum replacement/cementation are the major diagenetic events affecting reservoir porosity. The dolostone is composed of D1, D2, and D3 dolomite. D1 dolomite is very finely crystalline, dominating in the low-energy, restricted depositional facies. D2 dolomite is finely crystalline, constituting a major portion of the dolostones. D3 dolomite is depositional fabric retentive dolomite, including dolo-wacke-/packstone, dolo-grainstone, and dolo-boundstone. Four types of anhydrites (A1–A4) were identified in the core and thin sections. A1 is stratabound anhydrite. A2 is the anhydrite in dolostones, filling in the interparticle spaces in dolo-grainstone and the intraparticle spaces in dolo-wackestone. A3 is the anhydrite in limestones, which occurs non-fabric-selectively. A4 is the anhydrite filling in the fractures. The diagenetic anhydrites (A2–A4) are interpreted to postdate to major dolomitisation. D1 and D2 dolomites are characterised by δ13CVPDB values of −3.4–+7.3 ‰ and δ18OVPDB values of −4.7−-2.4 ‰. The REE patterns of D1–D3 dolomites are characterised by light rare earth element (LREE) depletion relative to heavy rare earth elements (HREE) ((Pr/Tm)N of 0.54–1.16, 0.81 ± 0.20), middle rare earth elements (MREE) enrichment (BSI values of 1.06–1.38, 1.26 ± 0.10), and negative Ce anomalies (Ce/Ce* ratios of 0.74–0.94, 0.83 ± 0.07). The diagenetic anhydrites (A2–A4) show δ34SV-CDT values of +25.9–+45.7 ‰, similar to those of the stratabound anhydrite (+29.6–+43.0 ‰). The large variations in the δ13CVPDB values of D1 and D2 dolomites are attributed to methanogenesis and BSR. The δ18OVPDB values and REE characteristics of D1–D3 dolomites suggest that the carbonates are dolomitised by the mesohaline fluid. The δ34SV-CDT values of A1–A4 indicate that the diagenetic anhydrites are formed by redistribution in a closed system. The petrographical and geochemical data suggest that the Jialingjiang carbonate evolved in a closed system during burial diagenesis. Primary pores dominate in the Jialingjiang carbonate reservoirs, and the boundstone and grainstone show the best porosities based on the porosityversus lithofacies data. Therefore, we propose that the lithofacies, instead of the diagenetic events, are the key factors of the Jialingjiang carbonate. The resisting compaction of dolomitisation helps preserve the reservoir porosity, and anhydrite is the major diagenetic event destroying the reservoir porosity.

Natural Gas Characteristics and Gas-Source Comparisons of the Lower Triassic Jialingjiang Formation, Eastern Sichuan Basin

Natural Gas Characteristics and Gas-Source Comparisons of the Lower Triassic Jialingjiang Formation, Eastern Sichuan Basin

A target-oriented integrated workflow for seismic delineation of thin tight reservoirs in the eastern Sichuan Basin, China

Complex lithology association along vertical direction results in thin layers with different interbed patterns of the Lower Triassic Jialingjiang Formation (T 1 j), Sichuan Basin. The prediction of thin tight reservoirs in the Jialingjiang Formation is important for further oil and gas exploration. However, seismic data in the eastern Sichuan Basin has poor lateral continuity of seismic events because of strong interference noises, which makes it difficult to pick horizons. Moreover, low dominant frequency (<25 Hz), low seismic resolution, and weak response characteristics of strong transverse heterogeneity on logging parameters make it challenging for the thin reservoirs prediction. To tackle these issues, in this study, we propose a target-oriented workflow by integrating seismic data processing with interpretation to delineate the thin reservoirs and corresponding hydrocarbon production capacity. In the processing workflow, we first remove the noise interferences using the structure-oriented filtering (SOF), which can improve the effective information of seismic events and also preserve the features of fault structures. Second, we propose a novel method based on spectral shaping and sparse inversion to enhance the seismic resolution. Tests on filed data demonstrate that the effective information and resolution of processed seismic data are significantly enhanced. Then, in the seismic interpretation, fluid mobility and spectral decomposition are first used to delineate the spatial distribution of favorable reservoirs. Furthermore, multi-attribute integrated interpretation involving impedance analysis, reservoir thickness estimation and porosity inversion is used to evaluate the storage capacity and productivity of the thin reservoirs. The estimated impedance, reservoir thickness and porosity show good agreement with well information, demonstrating the feasibility of the proposed method for accurately predicting thin reservoirs in this area. Multi-attribute integrated interpretation revealed that the favorable thin reservoirs are mainly developed from the northeast to the southwest in the study area, which can provide effective guidelines for future production well drilling and distribution delineations of a favorable reservoir in the Eastern Sichuan Basin. • Structure-oriented filtering (SOF) can remove noises and enhance the effective continuities and structural features. • A new resolution enhancement (RE) method improves the seismic resolution significantly and reduces the artifacts. • The target-oriented processing workflow can markedly improve the quality of the seismic data. • Multi-attribute analyses indicating the favorable thin reservoirs is developed for the target zone.

Early Triassic initial collision between the North China and South China blocks in the eastern Qinling Orogenic Belt

The closure of Mianlue Ocean lead to the collisional assembly between the North China Block (NCB) and the South China Block (SCB). But the timing of initial continental collision remains unclear, with the proposed ages ranging from the Late Permian to the Late Triassic. This study presents stratigraphic, sedimentological, tectonic subsidence, and provenance data from the Triassic strata in the Zigui and Dangyang basins lying along the eastern Qinling Orogenic Belt. These data document a tectonic transition in the central northern-SCB, and locally provide constraints on the timing of initial SCB–NCB collision. The Lower Triassic Daye Formation was deposited in a marginal basin to slope environment, and consists of mudrocks containing detrital zircons with major age peaks at ca. 1900 and 850 Ma. The overlying Lower Triassic Jialingjiang Formation comprises a shallowing carbonate sequence, with intercalated evaporites at the top of the formation. It contains mudrocks that are rich in detrital zircons with of ca. 270‐–245 Ma. The mixed carbonate and siliciclastic sequences of the Middle–Upper Triassic Badong Formation records tectonic uplift at its base, and contains detrital zircons with UPb age peaks at ca. 2500, 1850, 920–800, 450 and 350–220 Ma. Combined with petrographical observations and tectonic subsidence calculation, these detrital zircon UPb ages record a significant provenance change from recycled material derived from the SCB to newly generated detrital material from the NCB. This provenance change records the initial SCB–NCB collision in the eastern Qinling Orogenic Belt during the Early Triassic.

Multi-Level Detachment Deformation of the West Segment of the South Dabashan Fold-and-Thrust Belt, South China: Insights From Seismic-Reflection Profiling

The South Dabashan arcuate tectonic belt located at the northern margin of the Yangtze Block in South China, which primarily comprises a series of northwestern (NW)-trending foreland fold-and-thrust belts (FTBs), is useful for determining the intracontinental orogeny processes of the Yangtze Block. In this study, we integrated the latest pre-stack depth migration of three- and two-dimensional seismic profiles, drill hole, and outcrop data to explore the structural geometric and kinematic features of the west segment of the South Dabashan FTB. This belt is characterized by multi-level detachment structures due to the presence of three predominant sets of weak layers: the Lower Triassic Jialingjiang Formation gypsum interval, Silurian mudstone beds, and Cambrian shale beds. The belt is accordingly subdivided vertically into three structural deformation systems. The upper system appears above the Jialingjiang Formation gypsum layer and exhibits Jura-type folds, which were formed by alternating anticlines and synclines that are parallel to each other. The middle system comprises Silurian shale as the base and Jialingjiang Formation gypsum interval as the passive roof and exhibits NW-striking imbricate thrusts. The lower system is bounded by Cambrian and Silurian detachment layers, forming a duplex structure. The Sinian and Proterozoic basements below the Cambrian were not involved in deformation. The west segment of the South Dabashan FTB underwent four periods of tectonic evolution: Late Jurassic to Early Cretaceous, Late Cretaceous, Paleogene, and Neogene to Quaternary. The deformation was propagated southward in imbricate style, resulting in the passive uplifting of the overlying strata. Based on the magnetotelluric and deep seismic profile, the tectonic processes of the west segment of the South Dabashan FTB are inferred to be primarily controlled by the Yangtze Block northward subduction under the Qinling Orogenic Belt and the pro-wedge multi-level thrusting during the Late Jurassic to Cretaceous.

Characteristics and origin of a new type of polyhalite potassium ore in the Lower Triassic Jialingjiang Formation, Puguang area, northeastern Sichuan Basin, SW China

A new type of polyhalite potassium ore (NTPPO) was found in the Lower Triassic Jialingjiang Formation, NE Sichuan Basin, SW China. It is water soluble, therefore can be exploited using the water-solution method, and is of great potential of economic value and research significance. Based on cores, thin sections, energy spectrum and SEM analyses, its microfeatures, macrofeatures and origin are discussed, and a genetic model is established to provide a scientific basis for future evaluation, prediction and exploration of potassium ore in the Sichuan Basin. It is proposed that the NTPPO was caused by storm activities: (1) the storm broke the original sedimentary polyhalite–gypsum beds, whose fragments were transported into the salt basin with high content of K+ and Mg2+; (2) in the basin, the polyhalite continued to be formed from gypsum by metasomatism with K- and Mg-rich brine; (3) during diagenesis, under high temperature and high pressure, K–Mg-rich brine from halite continued to replace anhydrite (or gypsum) to form polyhalite.

Multiple thrust detachments and their implications for hydrocarbon accumulation in the northeastern Sichuan Basin, southwestern China

The northeastern Sichuan Basin thrust belt located in southwestern China, is a large-scale intracontinental thrust system with multiple detachments represented by a series of subparallel chevron anticlines. We conduct a comprehensive study of the geometry and kinematics of the thrust belt and its implications for the petroleum system based on seismic-reflection profile interpretation, field investigation, analysis of wells, and geochronology. Two major detachments occur within the allochthonous succession: the (1) gypsum-bearing lower to middle Cambrian Longwangmiao and Gaotai Formations and (2) Lower Triassic Jialingjiang Formation. The dark gray shales in the lower Silurian Longmaxi Formation are a favorable source rock that may also act as a third detachment in this study area. The stratigraphic succession is divided by three detachments into three structural intervals: (1) lower Cambrian–Silurian structural interval, (2) middle Silurian–Triassic structural interval, and (3) upper Triassic–Jurassic structural interval. The lower structural interval may be a good candidate for hydrocarbon exploration because of the occurrence of high-quality source rock and its reservoir-trapping evolutionary history.

Engineering properties and slope failure mode of Karst Breccia

Karst Breccia is the secondary action product of carbonate rocks on surface or near surface. In addition to the properties of soil, the Karst Breccia slope material also form sedimentary structure or inherit the original rock structure during diagenesis. The Karst Breccia is widely distributed in the dam site of a power station in Wujiang River, which is produced in the upper part of Member 4 of the Lower Triassic Jialingjiang Formation (T1j4-2), and the maximum height of the slope is about 180m after excavating. By studying the basic characteristics and engineering characteristics of Karst Breccia, the failure modes of the Karst Breccia slope can be summed up as three basic failure modes: plane sliding failure, collapse failure, and circular arc sliding failure. Moreover, the Karst Breccia has both the engineering characteristics and structural characteristics of soil and rockmass. The failure mode of the Karst Breccia slope is also more complicated, and there is a combined failure mode: circular arc - plane failure mixed-mode.

Sedimentary facies and sequence stratigraphy in the Lower Triassic Jialingjiang Formation, Sichuan Basin, China

The Lower Triassic Jialingjiang Formation is an important oil and gas exploration target in the Sichuan Basin. The lithofacies mainly consists of clastic rocks, carbonates, and evaporates, which represents a transition from continental to marine depositional environment. The Jialingjiang Formation of the Sichuan Basin is a typical epeiric carbonate platforms. The study area contains four types of facies, including mixed tidal flat facies, open platform facies, restricted platform facies, and evaporitic platform facies. Based on the analysis of cores and logging data, two types of type II sequence boundaries were identified, which are, respectively, local exposure unconformity boundary and lithology and lithofacies transformation boundary. The Jialingjiang Formation is divided into three third-order sequences and six system tracts. Thus, we establish a connected multi-well sequence stratigraphic framework based on typical single-well sequence stratigraphic division. The sedimentary evolution of the Jialingjiang Formation was divided into three major sedimentary periods and six stages, with rising and falling of the sea level, mixed tidal flat, open platform, and restricted platform prograded first, and then restricted platform and evaporitic platform retrograded. It also shows that the open platform shoal and restricted platform tidal flat have oolitic limestone and algal dolomite with good-quality physical properties and are the favorable reservoir facies belt for hydrocarbon exploration and development.

Lithology mapping of a mixed siliciclastic−carbonate−evaporite system using 3D seismic and well data: Lower Triassic Jialingjiang Formation, Sichuan Basin, southwestern China

Abstract High-quality three-dimensional (3D) seismic data acquired in the central Sichuan Basin, southwestern China, offer an opportunity to map complex lithologies in a mixed siliciclastic–carbonate–evaporite system in the Lower Triassic Jialingjiang (T1j) Formation. The formation consists of siliciclastics, limestone, dolostone, anhydrite, and salt. The lithologies consist several source-reservoir-cap assemblages in the area. Lithologies in the T1j Formation change rapidly in the vertical direction, forming different interbed patterns in thin layers. In the mean time, the lateral extend of each lithology is complex. This vertical and lateral distribution makes it difficult to predict lithology by single seismic attribute. Therefore, principle component analysis (PCA) was applied to tens of seismic attributes to extract useful information. The first three components contain most (83.02–99.85%) of the lithology information preserved in seismic attributes, which were used to correlate with lithology content calculated by core-calibrated wireline logs. Correlation coefficients of the three seismic components with lithologies are significantly higher (0.37–0.79) than those of individual seismic attributes (near zero to 0.58). Different assemblies of end-member lithologies were selected from anhydrite, siliciclastics, tight dolostone, limestone, and salt to perform PCA in different sequences. Lithologic content distribution of individual end members was shown by color-blending method to map the lithology mixture. Sedimentary history in the T1j Formation was reconstructed based on lithology-mixture maps and regional geology background in the study area. Eight 4th-order sequences were interpreted in the T1j Formation. Only the second sequence was influenced by siliciclastic input, forming a complete mixed siliciclastic–carbonate–evaporite system. Open platform and/or restricted evaporative environment dominated the other seven sequences, forming a mixed carbonate–evaporite system.

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.

Applying principal component analysis to seismic attributes for interpretation of evaporite facies: Lower Triassic Jialingjiang Formation, Sichuan Basin, China

In China and elsewhere, it is important to predict different lithologies and lithofacies for hydrocarbon exploration in a mixed evaporite-carbonate-siliciclastic system. The lower section of the second member of the Jialingjiang Formation (T1j2L) is mainly composed of anhydrite, dolostone, limestone, and siliciclastic rocks, providing a rare opportunity to reconstruct detailed facies in a [Formula: see text] 3D seismic survey with 31 wells. Wireline logs (sonic, density, and gamma ray) calibrated by core analysis are essential in distinguishing anhydrite, siliciclastics, and carbonates. Although different lithologies are characterized by different acoustic impedance (AI), with certain overlapping, it is still difficult to predict lithology by any single seismic attribute because of the limited seismic resolution in a thinly interbedded formation of multiple lithologies. In our study, principal component analysis (PCA) was applied to extract lithologic information from selected seismic attributes; the first two principal components were used to predict the content of anhydrite, siliciclastics, and carbonates. Content maps of anhydrite, siliciclastics, and carbonates — created by mixing the represented color — were used to reconstruct lithofacies of the T1j2L submember. It is quite difficult, even with the PCA approach, to uniquely resolve the three lithologies due to the overlapped AI and the limited resolution of the seismic data. However, the workflow that we evaluated dramatically improved the prediction accuracy of lithology and lithofacies. Facies transition during the deposition of the T1j2L submember in the study area was inferred from a paleo-uplift in the southwest to a restricted lagoon and then to an open marine setting in the northeast.

Genetic origin of gas condensate in Permian and Triassic strata in the southern Sichuan Basin, SW China

Gas condensate is produced from Permian and Triassic marine strata in the southern Sichuan Basin, where the Lower Triassic Jialingjiang Formation is one of the major gas condensate reservoirs. Condensate gas pools are mainly distributed laterally in the core area and the flank of the Luzhou paleo-uplift. Abundant reservoir bitumen and high GOI (grains containing oil inclusions/total grains×100%) values have been found in the Permian Maokou Formation and the Triassic Jialingjiang Formation, which indicate that both formations once hosted oil accumulations. Very high methyldiamantane contents and gas to oil ratios (GOR) were found in the condensate pools, and the carbon isotopic values of the saturates and the whole condensates both become increasingly enriched from the core area to the flank of the Luzhou paleo uplift. These lines of evidence indicate that the condensates were generated by intensive oil cracking. Oil reservoirs in the Jialingjiang Formation were destroyed in the Middle Triassic period by uplift and erosion. The Maokou Formation was buried to greater depths and oil reservoirs in it experienced intensive oil cracking in the Middle Jurassic and later Cretaceous periods. These oils were transformed into gas condensates. Much of the gas condensate in the Maokou Formation migrated upward along the faults and accumulated in the Jialingjiang Formation reservoirs during the Himalayan period (65–0Ma).

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.

Geochemical identification of marine and terrigenous condensates—A case study from the Sichuan Basin, SW China

Condensates are extensively produced from the marine Lower Permian Maokou Formation and the Lower Triassic Jialingjiang Formation (the main marine condensate-producing strata) in southern Sichuan Basin, and also in the terrigenous Upper Triassic Xujiahe Formation in the central and southern Sichuan Basin. Based on the analyses of light hydrocarbons, biomarkers and stable carbon isotopic compositions of marine and terrigenous condensates and the light hydrocarbon composition of natural gases in the Jialingjiang Fm and Xujiahe Fm in the Sichuan Basin, the following conclusions are drawn: (a) condensates produced from the carbonate reservoirs of Jialingjiang Fm and the Maokou Fm (marine condensates) belong to a single oil family, and the condensates from the terrigenous Xujiahe Fm (terrigenous condensates) belong to another family; (b) marine condensates, were generated from sapropelic organic matter in the late stage of high maturation, and the biomarker composition indicates that they were derived from the Lower Silurian shale and the Lower Permian carbonate, while the terrigenous condensates were generated from mature humic source rocks; (c) most of the marine condensates and gases encountered in the Lower Permian Maokou Fm and Lower Triassic Jialingjiang Fm were generated from the same source, and a few of the gases in the Jialingjiang Fm contain admixtures of coal-derived gas, while terrigenous condensates and gases were generated from the Xujiahe Fm humic source rocks; (d) effects that could be related to evaporative migration and biodegradation are not evident in the examined condensates.