Lishui Sag Research Articles

Paleoclimatic and paleoenvironmental constraints on organic matter enrichment in the Paleocene strata in the Lishui Sag, East China Sea Shelf Basin

Paleoclimatic and paleoenvironmental constraints on organic matter enrichment in the Paleocene strata in the Lishui Sag, East China Sea Shelf Basin

Characteristics of Weathering Reservoirs and Differences in Fracture Formation in the Weathering Crust of the Pre-Cenozoic Basement of Lishui Sag, East China Sea Basin, China

Characteristics of Weathering Reservoirs and Differences in Fracture Formation in the Weathering Crust of the Pre-Cenozoic Basement of Lishui Sag, East China Sea Basin, China

Tectono-stratigraphic evolution in the Paleocene Lishui Sag, East China Sea Shelf Basin

The tectonic evolution of the Lishui Sag in the East China Sea Shelf Basin significantly influences its sequence stratigraphy. By integrating seismic reflections, well logs and core data, this study examines the quantitative activity of the Lingfeng fault, sequence framework and sequence architecture in the Lishui Sag. The Paleocene succession is divided into one second-order sequence (SSQ1), three third-order sequences (SQ1–SQ3) and nine system tracts, representing a complete syn-rift evolution consisting of initial syn-rift, rift climax and late syn-rift stages. The results indicate that the differential activity of the Lingfeng fault shapes the basin structure and controls the subsidence centre, leading to prominent variations in the spatial distribution and filling of the sequence stratigraphy during different tectonic stages. The initial syn-rift sequence (SQ1), characterised by a low activity of the segmented Lingfeng fault, is primarily found in isolated small sub-sags. The rift climax sequence (SQ2), with increasing displacement and longer segmented fault zones, exhibits a wedge-shaped filling pattern with substantial topographic elevation differences. The late syn-rift sequence (SQ3) shows a gradual decrease in tectonic activity and uniform filling, with all system tracts within the sequences fully developed. The sequence architecture of the hanging wall in the Lishui Sag can be classified into fault-scarp and fault-slope type, whereas the sequence architecture of the hanging wall dip-slope can be classified into sedimentary slope-break and faulted slope-break.

Paleoenvironmental Evolution and Organic Matter Accumulation in a Hydrocarbon-Bearing Depression in the East China Sea

Investigating the paleoenvironment and characteristics of source rocks in sedimentary basins is crucial for understanding organic matter accumulation and guiding hydrocarbon exploration. The Lishui Sag, a significant hydrocarbon-bearing depression in the East China Sea, has experienced extensive marine transgression and increasing salinity in the Paleocene, but the changes in accumulation factors of organic matter during this evolution process remain unclear. Through a comprehensive analysis of total organic carbon (TOC), major and trace elements, and biomarker data, this study investigates the characteristics of source rocks from two lithostratigraphic units, namely the Paleocene Yueguifeng and Lingfeng formations, to gain deep insight into the effects of paleoenvironment on organic matter accumulation and hydrocarbon distribution. Our results indicate that the Lishui Sag transitioned from a closed lake to an open-marine environment in the Paleocene, with a shift from warm-humid to arid climate conditions. The biomarker distribution suggests a change in the origin of organic matter, with a higher input of terrestrial organic matter in the Lingfeng Formation. During the early stage, the lacustrine source rocks in the lower Yueguifeng Formation were formed in a relatively humid and anoxic environment within brackish water, resulting in a substantial influx of terrestrial and lacustrine algae organic matter. In contrast, in the late stage, the marine source rocks in the overlying Lingfeng Formation were developed in an arid and oxidizing environment. The lacustrine source rocks in the Yueguifeng Formation were notably more favorable to developing good-quality source rocks. Compared with the other regions, the western and northeastern parts of the study area have greater hydrocarbon generation potential due to the wider distribution of high maturity and organic-rich source rocks, with higher terrestrial and algal organic matter input. Moreover, considering the practical circumstances in the exploration, the northeastern part of the Lishui Sag is recommended as the next exploration target zone.

Tectonic-sedimentary evolution in a Palaeocene rifted Lishui Sag, East China Sea Shelf Basin

Influenced by episodic tectonic activity and multiple sediment sources, the source-to-sink systems in rifted basins are relatively complex. Based on seismic data, well logging, cores, thin sections, and heavy mineral data, the growth and linkage of the Lingfeng fault were reconstructed and its influence on the sedimentary responses of the Palaeocene-rifted Lishui Sag in the East China Sea Shelf Basin was examined. The researchers conducted a fault throw-distance analysis on the Lingfeng fault and found that it displayed an overall weak-strong-weak evolutionary process. During the early syn-rift stage, the Lingfeng fault exhibited a segmented geometry consisting of seven fault segments with low fault activity along its strike, and small-scale source-to-sink systems developed in both the hanging wall dip-slope and footwall. As the main syn-rift stage progressed, strong fault activity induced large-scale exposure of the source area, resulting in a high sediment-supply rate and large-scale source-to-sink systems. During the late syn-rift stage, the fault weakened and evolved into a connected basin-boundary fault, and proximal source-to-sink systems in the footwall gradually disappeared. Semi-quantitative measurements of the geomorphic parameters of these source-to-sink systems indicated wide and shallow sediment transport pathways in hanging wall dip-slope areas (average width and depth of approximately 6.34 kmand 0.13 km, respectively) and narrow and deep sediment transport pathways in the footwall (average width and depth of approximately 3.75 km and 0.15 km, respectively). The semi-quantitative statistical analysis implies that the cross-sectional area of the transport channels was positively correlated with the corresponding fan/delta complexes in a rift basin.

Integrated method for calculating source rock thickness using drilling and seismic reflection: A case study of the Lishui Sag, China

Integrated method for calculating source rock thickness using drilling and seismic reflection: A case study of the Lishui Sag, China

How to distinguish between marine and lacustrine sedimentary environments? —A case study of Lishui Sag, East China Sea Shelf Basin

The identification and characterization of marine versus lacustrine sedimentary environments can aid in the assessment of source and reservoir rock distribution, which is important for hydrocarbon exploration. Analysis of sedimentary facies, trace fossils, micropaleontology, geochemical characteristics, and relative water-levels was conducted in this study. Analysis of the core revealed typical sedimentary structures, including fourteen lithofacies and eleven lithofacies associations, which are indicative of wave-influenced origins and tidal processes. The sediments of the Yueguifeng Formation and upper Mingyuefeng Formation were deposited in shallow water settings, which later transitioned into the semi-deep water environments of the Lingfeng Formation and lower Mingyuefeng Formation. Sediments of the lower Mingyuefeng Formation and Lingfeng Formation were deposited in semi-brackish water or brackish water environments, whilst from the lower Mingyuefeng Formation to upper Mingyuefeng Formation and lower Lingfeng Formation to Yueguifeng Formation, the environment was semi-brackish to freshwater. The Lishui Sag experienced a large-scale and rapid relative water-level increase from the Yueguifeng Formation to the lower Mingyuefeng Formation and a decrease during the upper Mingyuefeng Formation. The relative water-level of the Lingfeng Formation and the lower Mingyuefeng Formation was consistent with the inferred eustatic global sea-level, and the relative water-level of the Yueguifeng Formation and the upper Mingyuefeng Formation deviated from the inferred global eustatic sea-level. The typical lithofacies associations, micropaleontology, and geochemical characteristics are identified as the key indicators to determine the sedimentary environments. The sedimentary environment analysis results show that the Lingfeng Formation to the lower Mingyuefeng Formation developed in a marine setting, whereas the Yueguifeng Formation and the upper Mingyuefeng Formation developed in a lacustrine sedimentary environment. The sediments deposited within shallow marine settings form good source rocks, the coastal sediments form good reservoirs, sediments of shallow lacustrine can generate source rock, and the deep-lacustrine sediments form good source rocks that produce waxy crude. Therefore, accurately distinguishing the difference between marine and lacustrine basins is important in terms of the overall understanding of the basin and the hydrocarbon play elements contained within it.

Impact of Terrigenous Organic Matter Input on Organic Matter Enrichment of Paleocene Source Rocks, Lishui Sag, East China Sea

To clarify the organic matter (OM) enrichment of the Lishui Sag, the factors influencing the variable abundance of OM in the Lingfeng Formation are studied using organic geochemical data. The source rocks of the Lingfeng Formation have medium–high total organic carbon (TOC) values (0.53–3.56%). The main type of kerogen is II2-III. Compared to the shallow marine subfacies source rocks, the TOC of the delta front subfacies source rocks is higher. The distribution of biomarkers shows that the redox environment of the delta front subfacies source rock is the sub-oxidizing and oxic environment, and the source rock is mainly supplied by terrigenous higher plants; the redox environment of shallow marine subfacies source rocks is a sub-reducing and suboxic environment, and the OM mainly comes from algae. The link between OM input and OM abundance demonstrates that terrigenous OM (TOM) input has a considerable influence on OM abundance. However, there is no obvious relationship between preservation and OM abundance, which suggests that preservation is not the determining element in OM enrichment. The strong sediment flux decreases the amount of time that OM is exposed to oxygen. As a result, delta front subfacies with large TOM input have a huge number of excellent source rocks. This paper proposes a “delta front-OM input model” for excellent source rocks.

Geochemical Characteristics and Environmental Implications of Trace Elements of the Paleocene in the West Lishui Sag, East China Sea Basin

Analysis of the sedimentary environment during the clastic formation process is of great significance for reservoir evaluation and desert prediction. This paper focused on the Paleocene in the West Lishui Sag, East China Sea Basin. XRF fluorescence diffraction and carbon and oxygen isotope tests were carried out on core samples from four wells. Based on the geochemical characteristics of the samples and the changes in the elemental ratios, combined with the lithologic characteristics and sedimentary structure of the samples, the paleoclimate, paleosalinity, paleobathymetric, paleoredox, paleotemperature, and other paleoenvironmental characteristics were analyzed. The results show that the characteristics of major and trace elements were similar in the lower Mingyuefeng Formation (E1m2), Upper Lingfeng Formation (E1l1), Lower Lingfing Formation (E1l2), and Yueguifeng Formation (E1y). The Paleocene in the West Lishui Sag was mainly in the reducing environment of brackish-salt water with weak water stratification. The water depth showed a trend of becoming deeper, then shallower, and then deeper. The paleoclimate in the West Lishui Sag was warm on the whole. However, the content of Sr became smaller after later deposition, so the calculated paleowater temperature was higher. In addition, oxygen isotopes were affected by diagenesis, resulting in a negative oxygen isotope value. The paleoproductivity was low, and the hydrocarbon generation potential was poor. The content of nutrient elements mainly came from terrigenous input rather than biological origin, and terrigenous intrusion characteristics gradually increased from E1y to E1m2. The study also shows that paleoproductivity was affected by the paleoclimate and paleowater depth. Warm and humid climate and deep water body were conducive to the accumulation of paleoproductivity.

Paleosalinity Reconstruction for the Paleocene Sequence of Lishui Sag in the East China Sea Shelf Basin

Paleosalinity Reconstruction for the Paleocene Sequence of Lishui Sag in the East China Sea Shelf Basin

Influence of Provenance and the Sedimentary Environment on the Formation of High-Quality Source Rocks in the Paleocene Lishui Sag, China.

Organic matter (OM) is the material basis for hydrocarbon generation. Its type and abundance determine the hydrocarbon generation ability of source rocks, which is closely related to the provenance and sedimentary environment of source rocks. The tectonic backgrounds of the eastern and western subsags (ESS and WSS) of the Lishui Sag in the East China Sea Shelf Basin are significantly different and their influence on the OM in the source rocks is worthy of attention. This paper comprehensively analyzes the provenance and environmental characteristics and their influence on the features of the OM of Paleocene source rocks in the ESS and WSS. The study finds that the source rocks in the ESS have multiple sources. During the deposition period, as the salinity and paleoproductivity of the water column increased, the proportion of OM in the autochthonous components of the water column continued to increase, but the overall water column was in an oxidizing environment, resulting in a generally low abundance of OM. The provenance of the source rocks in the WSS was relatively simple and terrestrial. Also, the sedimentary environment had little effect on the type of OM. However, the whole water column of the WSS was in an anoxic environment, so the OM was better preserved, resulting in a higher abundance. Due to the influence of provenance and the sedimentary environment in different areas of the sag, the characteristics of OM in the source rocks are different, so relevant exploration strategies need to be adopted in actual exploration.

Improved understanding of the origin and accumulation of hydrocarbons from multiple source rocks in the Lishui Sag: Insights from statistical methods, gold tube pyrolysis and basin modeling

Hydrocarbon source correlations and the determination of hydrocarbon accumulation models in petroliferous basins with multiple potential source rocks are significant for hydrocarbon exploration. In this study, on the basis of gas chromatography and gas chromatography-mass spectrometry experiments, the oil origin was determined by a multiparameter oil-source correlation based on statistical methods. Through gold tube pyrolysis, fluid inclusion analysis, laser Raman analysis of inclusions and basin modeling, this study deeply explores the origin and accumulation of oil and gas in the Lishui Sag, East China Sea Basin. The oil in the west sub-sag is a mixture from source rocks of the Yueguifeng and Lingfeng formations (Danian-Selandian), while the oil in the east sub-sag is derived from source rocks of the Yueguifeng Formation (Danian). The hydrocarbon gas reservoir of the LA well block in the middle of the sag contains mainly oil-associated gas derives from the Yueguifeng Formation (Danian). The 3D basin modeling shows that the oil and gas generated by source rocks of the Yueguifeng and Lingfeng formations (Danian-Selandian) migrated upward into the reservoirs in the Mingyuefeng Formation (Thanetian). Due to the poor preservation conditions in the northern part of the study area, a large amount of oil and gas escaped and failed to accumulate. Controlled by fluid potential and faults, hydrocarbons in the Lishui Sag mainly migrated from the source kitchen to the slope zone. The stable carbon isotope data combined with the 3He∕4He ratios show that the CO2 in the Lishui Sag are mantle-derived inorganic gas related to volcanic activity. The filling of CO2, caused the loss and reaccumulation of early filled oil and gas, which impacted the distribution of oil and gas reservoirs in the Lishui Sag. Therefore, the migration and accumulation mechanism of CO2 should be fully considered in oil and gas exploration in the Lishui Sag.

Comparison of the Pore Structure of Ultralow-Permeability Reservoirs Between the East and West Subsags of the Lishui Sag Using Constant-Rate Mercury Injection

In this study, the differences in reservoir parameters, such as pore radius, throat radius, and pore-throat ratio, between the east and west subsags of the Lishui Sag are analyzed by using data obtained from a constant-rate mercury injection experiment. Furthermore, the quality of the reservoirs in the two subsags is systematically evaluated. Results show that the throat radius of the Lishui west subsag is larger than that of the east subsag, and this parameter has a positive correlation with reservoir quality. However, the pore-throat ratio of the east subsag is larger than that of the west subsag, which has an inverse relationship with reservoir quality. The main reasons for this reservoir difference can be attributed to sedimentation and diagenesis. The sedimentary facies types of the Lishui east subsag are the fan delta, shore lake, shallow lake, and shore shallow lake; their sandstone composition maturity is low; the clay mineral content is high; and the rock has undergone strong diagenesis. Therefore, the physical conditions of the reservoir are poor. However, the sandstones in the Lishui west subsag have weak cementation and compaction, mainly with an intergranular pore structure type, which leads to good connectivity between pores. Therefore, the storage performance and seepage capacity of the Lishui west subsag are better than those of the east subsag; the west subsag is the main area of oil and gas accumulation, as confirmed in the process of exploration and development.

Modeling Hydrocarbon Generation of Deeply Buried Type Ⅲ Kerogen: A Study on Gas and Oil Potential of Lishui Sag, East China Sea Shelf Basin

The hydrocarbon generation model and hydrocarbon potential are investigated in the Lishui Sag, based on gold-tube pyrolysis experiments of deeply buried type Ⅲ kerogen. From this, we discuss the classification of kerogen types of source rocks with mixed organic matter sources. The hydrocarbon generated from the source rocks of the Lingfeng Formation and Yueguifeng Formation is dominated by natural gases with little oil in the West subsag, and the hydrocarbon generation model of the Lingfeng Formation is similar to that of Yueguifeng Formation, but the gas potential of Lingfeng Formation is higher than that of Yueguifeng Formation. The hydrocarbon potential of the Yueguifeng Formation in the East subsag is much higher than the West subsag, and it has considerable oil potential. Macerals diversity of source rocks is responsible for the difference of hydrocarbon generation characteristics for type Ⅲ kerogen in the Lishui Sag. It is not rigorous to evaluate the hydrocarbon potential of kerogen only based on pyrolysis parameters. Application of kerogen type index (KTI) can improve the accuracy of the classification of kerogen types with mixed organic matter sources. According to the classical kerogen classification template, the selected samples belong to type III kerogen. In this article, the selected samples were further subdivided into type III and type II/III based on the KTI value. Type III kerogen (0.5 ≤ KTI < 1.5) mainly produces gas, and type II/III kerogen (1.5 ≤ KTI < 5) mainly produces gas, but its oil potential is higher than that of type III.

Characteristics and deposition models of the paleocene source rocks in the Lishui Sag, east China sea shelf basin: Evidences from organic and inorganic geochemistry

The Lishui Sag, situated at the East China Sea Shelf Basin, is regarded as one of the most prospecting oil exploration zones. Low exploration in this region requires detailed identification of source rocks to enhance further oil and gas exploration. Three potential source rocks, including the Paleocene Yueguifeng (E1y), Lingfeng (E1l) and Mingyuefeng (E1m) Formations, were investigated by integrating organic and inorganic geochemistry to estimate hydrocarbon generation potential, and to document original organic matter input and deposition environments. Two deposition models for source rocks were established considering tectonic subsidence and climate variation. The result shows that the E1y mudstones are dominated by high TOC content, oil/gas-prone kerogen at moderate maturity stage with relatively high hydrocarbon generation potential, whereas E1l and E1m mudstones are dominated by low TOC content, gas-prone kerogen at low to moderate maturity stage with low hydrocarbon generation potential, and coals with high hydrocarbon generation potential. The original organic matter input and deposition environment are obviously different among these three potential source rocks and are controlled by the coupling of tectonic subsidence and climate variation. Insufficient material supply lead mudstone deposition in a hemi-deep/deep lake during the E1y period, resulting in inefficient water mixing and stable water column stratification (high Gammacerane Indexs). The weak water inflow supplied minor oxygen and terrigenous organic matter to the lake due to arid-hot climate, as indicated by low Pr/Ph, C19 T/C23T, C20 T/C23T and C24Tet/C26T ratios, low Oleanane Indexs, and high C35 Homohopane Indexs and V/Ni ratios. Water algae, including pediastrums and chlorellas, which contributed to the development of high-quality source rocks, could be efficiently preserved in dysoxic condition. High subsidence rate together with sufficient sediment supply during the transgression in the E1l and E1m periods developed a coastal marine or paralic environment. The alternated climate brought unstable water inflow against stable water column stratification (low Gammacerane Indexs). Strong water inflow due to warm-humid climate provides abundant terrigenous organic matter and oxygen to the lake as indicated by high Pr/Ph, C19 T/C23T, C20 T/C23T and C24Tet/C26T ratios, high Oleanane Indexs, low C35 Homohopane Indexs and V/Ni ratios, and rich C29 steranes. Algal might be preferentially oxidized and degraded in aerobic conditions. Thus, mudstone is regarded as general-quality source rocks but coal is regarded as high-quality source rocks. Weak water inflow under arid-hot climate provided subtle aquatic and terrigenous organic matter to the lake, resulting in poor-quality source rocks.

Sedimentary Provenance Analysis of Palaeocene in the X Sag, East China Sea Shelf Basin

The analysis of sedimentary provenance characteristics is the premise of sedimentary system analysis. At present, the understanding of sedimentary provenance is not unified in the exploration and development of Lishui Sag. To solve this problem, in this paper, we used the clastic components composition and the major element oxides to analyze the tectonic setting in provenance area, the analysis of paleogeomorphology, heavy minerals, mudstone color index and paleocurrent indicated by seismic reflection characteristics were used to analyze the provenance direction. Through the analysis of clastic components composition, it is considered that the provenance of Paleocene sedimentary system in the X sag mainly comes from the areas of Recycled Orogenic, Dissected Arc and Transitional Arc, and the analysis of major element oxides shows that the provenance tectonic setting and type of Paleocene sedimentary system in the X sag is mainly active continental margin and island arc, these two conclusions are basically consistent. The paleogeomorphology, heavy minerals, mudstone color index and seismic reflection characteristics analysis shows that the provenance direction in the sedimentary period mainly comes from the west, southwest of Minzhe Uplift and East of Yandang Uplift. Different provenance areas provide substantial sediments basis for the development of sedimentary facies in the study area.

Pore-Throat Size Distribution and Classification of the Paleogene Tight Sandstone in Lishui Sag, East China Sea Shelf Basin, China

The characteristics of tight sandstone reservoirs are closely influenced by the complex pore structure and the widths of pore-throat size distributions (P-TSDs). Because of the limitations of vario...

Base level changes based on Basin Filling Modelling: a case study from the Paleocene Lishui Sag, East China Sea Basin

Estimation of base level changes in geological records is an important topic for petroleum geologists. Taking the Paleocene Upper Lingfeng Member of Lishui Sag as an example, this paper conducted a base level reconstruction based on Basin Filling Modelling (BFM). The reconstruction was processed on the ground of a previously interpreted seismic stratigraphic framework with several assumptions and simplification. The BFM is implemented with a nonlinear diffusion equation solver written in R coding that excels in shallow marine stratigraphic simulation. The modeled results fit the original stratigraphy very well. The BFM is a powerful tool for reconstructing the base level, and is an effective way to check the reasonableness of previous interpretations. Although simulation solutions may not be unique, the BFM still provides us a chance to gain some insights into the mechanism and dynamic details of the stratigraphy of sedimentary basins.

Diagenesis and reservoir quality of Paleocene tight sandstones, Lishui Sag, East China Sea Shelf Basin

The Lishui Sag in the East China Sea Shelf Basin (ECSSB) has significant potential hydrocarbon resources, with Paleocene sandstones as its main oil and gas-bearing reservoir. The sandstones in the Paleocene formations are mainly composed of medium-grained, moderately sorted litharenite. In this study, distributions of the porosity and permeability reveal that the Paleocene sandstones have the characteristics of medium porosity and low permeability. The sedimentary microfacies, primary texture, diagenesis, and bioturbation jointly control the reservoir quality. Sandstones with distributary channel and sand bar sedimentary microfacies as well as better sorting tend to exhibit a higher quality. Compaction and carbonate cementation are the major determinants to reductions in porosity and permeability. Grain-coating chlorites within a certain content range may play a positive role in the preservation of porosity in eodiagenesis. Early dissolution has a constructive effect on the reservoir quality. In contrast, the dissolution products such as kaolinite and dawsonite cannot move over a long distance and precipitate nearby to block the intergranular pores when reservoir closure increases. Therefore, dissolution in a relatively closed geochemical reservoir system may contribute little to porosity. Contrastingly, the permeability of the Paleocene reservoir may be reduced by the conversion of the pore network. In the Lishui Sag, the bioturbation plays a role in the deterioration of reservoir quality in a limited scale by decreasing reservoir sorting and increasing compaction.

CH4 accumulation characteristics and relationship with deep CO2 fluid in Lishui sag, East China Sea Basin

Co-accumulation of carbon dioxide and hydrocarbon gas is common and widely existing in oil and gas basins. Such two natural pools are located in Lishui sag (East China Sea Shelf Basin), as a favorable prospecting area for exploration. CO2 gas derived from inorganic mantle sources and CO2 accumulation times have been investigated. In this study, petrographic observation shows pure CH4, CH4-rich, H2O-rich and hybrid CH4-CO2 fluid inclusions exist in healed micro-fractures and overgrowths of quartz mineral in Palaeocene reservoirs of Lishui sag. The pressure–temperature–time–composition (PVT–x) properties of CH4-bearing inclusions were obtained by Raman quantitative analysis and thermodynamic models. These results provide important constraints on the P-T entrapment conditions and predict relative accumulation time of CH4 fluid. Combination of carbon isotope analyses, inclusion petrography, microthermometry, PT entrapment conditions of CH4-bearing inclusions, hydrocarbon generation and simulation of burial history permit to assign periods of CH4 fluid activity and its sequence with CO2 fluid. Methane charging time occurs during two stages in LS-A: low density CH4 fluid (~0.10 g/cm3 or lower) were trapped during late Oligocene (30-20 Ma) and high density (>0.115 g/cm3) during a Pliocene (8-1 Ma) CH4 accumulation event. CO2 fluid accumulation (Miocene, 21-4 Ma) displaced the methane gas trapped in the early stage. Pliocene CH4 fluid partial displaced Miocene CO2 fluid. In well B, only high density (0.144–0.153 g/cm3) recorded a late Oligocene to early Miocene (36-19 Ma) CH4 accumulation event, Pliocene (5-3 Ma) CO2 fluid displaced early CH4 fluid. Micro-thermometry, composition of fluid inclusion, features of reservoir, thermal maturity reflect hydrothermal fluid vertical migration and the existence of Supercritical CO2 extraction.