Geochemical characteristics of temperature and pressure of Paleozoic reservoir fluid inclusions in Xuanjing region,Lower Yangtze area

DONG Min, LIANG Minliang, DONG Hui, FENG Xingqiang,ZHANG Linyan and WANG Zongxiu

1. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China;

2. Key Lab of Shale Oil and Gas Geological Survey, Chinese Academy of Geological Sciences, Beijing 100081,China;

3. Key Laboratory of Petroleum Geomechanics, China Geological Survey, Beijing 100081, China;

4. Xi’an Center of China Geological Survey, Xi’an 710054, China

Abstract: In order to understand the geochemical characteristics of Paleozoic reservoir fluids in Xuanjing region, Lower Yangtze area, drilling core samples from Y and D wells were tested and analyzed to study the fluid inclusion types and composition.Pressure correction was undertaken to determine the temperature and pressure environment for inclusion formation, and the influence of fluid characteristics of the Upper Permian and Lower Triassic reservoirs on the preservation of shale gas was investigated.According to petrographic observations, fluid inclusions are mainly brine and bitumen inclusions.Bitumen inclusions are mainly distributed in holes and fractures, and with smaller individuals.No visible fluorescence was observed, and the vitrinite reflectance is 3.39%–3.92%.This indicates that there had been oil and gas accumulation in the early stage of diagenesis in the study area, but due to the influence of magmatic hydrothermal solution, oil and gas underwent thermal metamorphism in the early stage, making liquid petroleum into solid bitumen.At present, oil and gas in the reservoir were largely formed in the late stage.During the continuous process in which shale was buried, light oil and gas were generated.Light oil and gas underwent magmatic and tectonic hydrothermal processes in some areas, resulting in high-temperature metamorphic cracking that formed dry gas.Moreover, nitrogen inclusions are found in fluid inclusions, forming metamorphic fluids caused by magmatic hydrothermal activities.The study shows that Paleozoic reservoirs in Xuanjing area are characterized by self-generation and self-storage.Furthermore, the mechanism of shale gas accumulation is not only related to the buried hydrocarbon generation process of shale itself, but is also related to later magmatic activity and tectonic hydrothermal transformation.Therefore, preservation conditions are generally key factors of shale gas accumulation in this area.

Keywords: fluid inclusions; geochemical characteristics; Paleozoic; Xuanjing area

Introduction

Shale gas is a kind of clean energy.As an important source of energy, it has great potential in repacing other energy types (Zouet al., 2010; Donget al., 2016).Shale gas exploration in Lower Yangtze area has attracted attention for years, of which the Upper Permian and Lower Triassic reservoirs in Xuanjing area are one of the key horizons for shale gas exploration at present (Caoet al., 2016).In recent years, many exploration wells deployed in the China Geological Survey in Xuanjing area for Paleozoic shale have obtained good indications of oil and gas(Songet al., 2017, 2019; Shiet al., 2019).The results show that Mesozoic shale in Xuanjing area has a large distribution area and high organic matter content,which therefore has the potential to form shale gas (Duet al., 2010; Panet al., 2011; Chenet al., 2013; Huanget al., 2013; Baiet al., 2021).Systematic study of the geochemical characteristics of Paleozoic reservoirs in Xuanjing area is needed to lay the foundation for subsequent shale gas exploration.At present, there is a lack of research on the temperature and pressure characteristics of shale reservoirs.Therefore, in this study,diagenetic minerals in micro-fractures of Paleozoic reservoirs in Y and D drilling cores in Xuanjing area were systematically collected for analysis and the temperature pressure geochemical characteristics of Paleozoic reservoirs fluid were studied.The shale gas reservoir accumulation and preservation conditions are discussed so as to provide a geological basis for shale gas exploration and development in this area.

1 Geological setting

Lower Yangtze area experienced a long geological evolution, with extremely thick sedimentary strata developed, including Paleozoic–Lower Mesozoic marine strata and Mesozoic continental sediments.The Paleozoic marine strata were mainly developed in a deep-water shelf environment, while the Mesozoic and Cenozoic strata were mainly formed in lacustrine,fluvial and coastal environment.Multi-stage tectonic movements have taken place in this area since Sinian,resulting in the development of a series of regional thrust nappe structures in Lower Yangtze area and the severe erosion of in some area (Chenet al., 1999;Yaoet al., 1999; Guoet al., 2002; Baiet al., 2021).Regional stratigraphy of Xuanjing area in the south of Lower Yangtze is from old to new, and consists of the Silurian to Quaternary (Fig.1).The strata in the region are relatively complete except for the missing of Jurassic and Paleogene successions.

Xuanjing area is located between Nanling- and Xuancheng basins.Its structural position is in front of Maoxi fault zone, with the west side near the NEtrending Jiangnan fault and the north side near the EW-trending Zhouwang fault.Both wells are in Jingxian County (Fig.1).Well Y is located in the west wing of Gufeng syncline in Jingchuan Town,Jingxian County, with the axis of Gufeng syncline at 70°–75° and with a total length of about 17 km.The core area is mainly composed of Triassic Biandanshan Formation limestone, whereas the two wings consist of Permian Dalong- and Longtan formations.The surface is largely an oblique syncline, with slow in northwest and steep in southeast, and the dip angle of its surface outcrop strata is generally between 40°–45°.The Permian strata intersected by this well are pleated and steep, with high-angle faults.Some faults are nearly vertical.Fractures are well-developed, of which some have been filled.The total drilling depth of the well is 1 247.3 m, the intersected strata range from Lower Triassic Biandanshan Formation to Upper Permian Longtan Formation, in normal order.Well D is located in the southeast wing of NE-trending Caicun syncline in Caicun Town, Jingxian County.The core area is mainly composed of Triassic Zhouchongcun Formation limestone and the two wings comprise Triassic and Permian strata.The Caicun syncline is steep in the northwest wing.It is gently tilted and horizontally folded in the southeast wing.The strata encountered by this well are gentle, with dip angle of 10°–25°.The sequence of strata drilled is in normal order, and the target intervals are Permian Dalong-, Longtan- and Gufeng formations, with a total depth of 2 130.7 m.Well D intersects the Triassic Yinkeng Formation at 1 537 m and the Permian Dalong Formation at 1 836 m.The well terminates in the Permian Qixia Formation (Fig.2).

2 Materials and methods

To evaluate the shale gas exploration potential of Permian marine shale in the study area, the characteristics of organic matters, including total organic carbon, vitrinite reflectance and bitumen reflectance were analyzed using core data from Y and D wells in Xuanjing area.Fluid inclusion is the geological fluids that were collected and sealed in the diagenetic process.It is the most direct and reliable means of studying the formation of geological fluids and has been widely used in conventional and unconventional oil and gas exploration and development (Mclimans,1987; Luet al., 1990).At present, the application of fluid inclusions in shale research is relatively lacking, mainly because shale mineral particles are generally small and poor in crystallinity and it is difficult for hydrothermal fluids to enter these minerals to form inclusions (Wenet al., 2004; Liuet al., 2005, 2009; Chenet al., 2013; Zhanget al.,2017; Donget al., 2019).However, under certain geological conditions, a certain number of fluid inclusions may develop in micro-cracks formed at different stages, and the interbedding of sandstone,siltstone and silty mudstone in shale will also form inclusions by collecting geological fluids.These diagenetic mineral fluid inclusions associated with shale gas record geochemical information such as temperature, pressure, composition and formation time, which is closely related to the process of shale gas accumulation.

In order to study the characteristics of fluid inclusions across different stages, calcite vein samples filled in fractures in Paleozoic reservoirs were systematically collected.The sampling positions are shown in Fig.2.Leica microscopes (D2700P) were used to observe the micro-petrography of fluid inclusions.Furthermore,spectroscopy studies of fluid inclusions were carried out on LabRa-010 laser Raman spectrometer, and representative samples with relatively large individuals were selected.Microscopic cold and hot temperature measurements were carried out with THSG-600 hot and cold stage and pressure simulation were completed.

Fig.2 Geologic columns of wells Y (a) and D (b) in Xuanjing area

3 Results

3.1 Organic geochemical characteristics

The Permian Dalong Formation in Well Y is deposited in a deep-water basin, with kerogen type II as the main type and a small amount of kerogen type III(Songet al., 2017).The Triassic source rocks in Well D are mainly kerogen type II, followed by type III.

Abundance and maturity of organic matter are important parameters for evaluating the hydrocarbon generation capacity of source rock (Lianget al., 2008,2009).According to the total organic carbon content(TOC) data, the shale in Well Y has an average TOC of around 0.20%–0.22% (Fig.3a).The average TOC content measured from the Permian Dalong Formation in Well D is around 0.2%–0.4%.The average TOC content measured from the first member of the upper Longtan Formation is around 2.5%, which reaches the medium source rock level.The average TOC content measured from the second member is around 1.5%, which is slightly lower than that of the first member and reaches the medium source rock level.The average TOC content measured from the middle Gufeng Formation is around 11.5%, which indicates an excellent source rock (Fig.3b).

In terms of the maturity of organic matter measured by the analysis of vitrinite reflectance (Ro) of drilling cores, organic matter in this area is in the mature to highly mature stage.From the Triassic Yinkeng Formation to the Permian Longtan Formation in Well Y, the measured Ro values are around 1.27%–2.38% and the average value is 1.59%, in which the Ro values of one sample in Yinkeng Formation at 1 106.3 m are greater than 2%, and those of other samples are less than 2%, as shown in Fig.4a.The maturity Ro value of the Permian Longtan Formation is around 1.27%–1.64% and the average value is about 1.46%, indicating that it has entered the stage of dry gas generation, and the level of thermal evolution belongs to a more mature stage.From Triassic Yinkeng Formation to the Permian Longtan Formation in Well D, the measured Ro is distributed at 1.59,and the vitrinite reflectance Ro of Triassic Yinkeng Formation is around 1.2%–1.4%, as shown in Fig.4b.This indicates that it is mature-high mature stage.The maturity of crude oil adamantane shows condensate oil and the Permian shale Ro is greater than 1.3% overall.

3.2 Characteristics of fluid inclusions

The fluid inclusions in the Upper Permian and Lower Triassic reservoirs in Xuanjing area were observed in detail using petrography and microscopes.As shown in Fig.5, the formation order of cements and authigenic minerals in the Paleozoic reservoirs of the study area was as follows: early fine-grained calcite and dolomite → calcite filled in cracks → bitumen filled in intergranular pores of fine-grained calcite,pores and cracks filled with calcite and bitumen, and the surrounding rock contains bitumen → coarsegrained calcite and quartz filled in late stage cracks.

Fig.3 Relationship between buried depth and TOC of wells of Y(a) and D (b)

Fig.4 Relationship between buried depth and Ro of Y (a) and D wells (b)

Two diagenetic stages are identified as the early diagenetic stage and late diagenetic stage respectively.The types of inclusions in the early diagenetic stage are brine inclusions and bitumen inclusions, and those in the late diagenetic stage are mainly brine inclusions(Fig.5).In the early diagenetic stage, I is early matrix calcite (d-f, l-o), fine grained calcite particles (b, c, g-j)and fine grained dolomite particles (a); II are holes and cracks filled with bitumen (g), holes and cracks filled with calcite (m-o), veinlets filled with calcite (e), fibrous calcite, recrystallized quartz (d, e) and fluorapatite growing at the edge of cracks(l); III shows bitumen filled in the intergranular pores of microcrystal calcite(a-c) and surrounding rocks containing bitumen(i).In the late diagenetic stage, II shows veins filled with calcite (a-c, h-j), III shows cavities and fractures filled with calcite (g, n, o), and coarse-grained calcite growing in fractures or quartz (e) and IV shows coarse-grained calcite formed in fractures (f).

As shown in Table 1, in the early diagenetic stage, the gas-liquid ratio of gas-liquid two-phase brine inclusions is less than 10%, mostly in 8%–9%, and its shape is nearly elliptical.The host minerals are calcite,quartz and fluorapatite, with an inclusion length of 2–4 μm.Inclusions are rare, and their volumes are small,with individual lengths reaching 17 μm.The wall of the inclusion is thick, most of the minerals are isolated and some are distributed in cracks or pores.The gas/liquid ratio of gas-liquid two-phase brine inclusions in the late diagenetic stage is less than or equal to 30%,mostly 8%–20%, with irregular and nearly elliptical shapes, and some rectangles, which are generally colorless under the microscope.Distributed in holes,fractures or otherwise isolate, the host minerals are calcite and quartz, and the length of most inclusions is 2–8 μm, with maximum length reaching 12 μm.

The fluid inclusions are mainly gas-liquid twophase brine inclusions.Moreover, the intergranular pores and surrounding rocks are filled with bitumen organic inclusions and the genetic types of inclusions are mainly primary inclusions.As shown in Fig.6, bitumen has filled in the intergranular pores of microcrystalline calcite, or many bitumen veins are developed in the surrounding rocks, which are dark brown under the transmitted light and have no fluorescence.This indicates that there had been oil and gas accumulation in the study area.However, influenced by magmatic hydrothermal solution, N2inclusions are found in fluid inclusions, forming metamorphic fluids caused by magmatic hydrothermal activity.

Fig.5 Distribution characteristics of diagenetic minerals in reservoir fluid inclusions

Table 1 Raman analysis and homogenization temperature of fluid inclusions in Xuanjing area (The data come from Chinese University of Petroleum, East China)

Fig.6 Microscopic characteristics of fluid inclusion and Raman spectra in Xuanjing area

Fig.7 Histograms showing distribution of homogenization of fluid inclusions samples from Triassic (a) and Permian (b) in Xuanjing area

3.3 Homogenization temperature and pressure of fluid inclusions

Wells Y and D were selected for microscopic temperature measurement of brine inclusions in the Triassic and Permian reservoirs.It was found that the homogenization temperature of gas-liquid twophase brine inclusions is mainly concentrated between 80–100 ℃ and 140–150 ℃ in the Triassic reservoir and 70–100℃ in Permian reservoir (Fig.7).Compared with the homogenization temperature of the Triassic and Permian in the sample, the Triassic has an obvious peak at 140–150 ℃, while the Permian is mainly concentrated at 70–110 ℃, indicating that the fluid inclusions of Permian are more developed in the medium and low temperature area.The homogeneous temperature of some samples exceeds 180 ℃, indicating that the thermal action of late tectonic magma influenced the drilled reservoir, resulting in a significant increase in fluid temperatures.

4 Discussion

4.1 Laser Raman spectrum characteristics of bitumen inclusions

The laser Raman spectrum characteristics of bitumen inclusions show there are many calcite veins in the sample and the surrounding rocks contain bitumen.The bitumen has a granular texture and is filled in the primary pores of the reservoir in an amorphous state.This indicates strong anisotropy and constitutes a micro-petroleum reservoir.In the Raman spectrum of reservoir inclusions in the sample, the laser Raman spectrum of bitumen has two first-order characteristic peaks.Carbonaceous bitumen in the reservoirs of the study area is mainly distributed in holes, crevices and surrounding rocks.It is opaque under transmitted light(Xu, 1991; Songet al., 2015).There are many highcrevice components such as bitumencarb in petroleum components.In the Raman spectrum of carbonaceous bitumen, both D and G peaks are prominent and sharp.The G peak is sharper than D peak, showing highcarbon components, and the intensity of the D peak is largely consistent with the G peak, indicating that the carbonization degree of reservoir bitumen is high.As shown in Table 2, the thermal variation parameters of carbon pitch show that the hydrogen-carbon ratio of the Permian Dalong Formation is 0.52, the reflectance(Ro) is 3.39%; the hydrogen-carbon ratio of the Longtan Formation is 0.39–0.46, and the reflectance(Ro) is 3.51%–3.90%, with an average of 3.71%.These figures show that early reservoir bitumen formed a residue with a higher thermal stability.The subsequent high temperature and high pressure caused by deep burial led to the thermal cracking of early crude oil, forming natural gas and reservoir bitumen.Because of the poor fluidity of bitumen, it can be regarded as an in-situ occurrence.The carbonization of bitumen in this area is significant, which reveals that the carbonaceous bitumen formed by early oil and gas accumulation is highly mature.

As shown in Fig.8, the D peak of Y09 is between 1 320–1 350 cm-1and the G peaks of Y01 and Y09 are between 1 590–1 650 cm-1.Referring to related literature (Kelemenet al., 2001; Ouirioet al., 2005;Ouet al., 2006; Sonibareet al., 2010; Chenet al.,2010; Kostovaet al., 2012; Wanget al., 2015; Jianget al., 2020), the following indexes can be used to quantitatively characterize the changes of D and G peaks of bitumen during thermal changes: Peak position: WD and WG, Peak position difference:RBS=WG-WD; Width at half maximum: FWHM-D and FWHM-G; Half-width ratio: FWHM-D/FWHM-G; Peak intensity ratio: ID/IC; Peak area ratio:AD/AG; Bitumen reflectance: BRo.These parameters correlate to some extent with bitumen reflectance (BRo)and can be considered to reflect the maturity level fairly well.

Table 2 Bitumen reflectance values and main laser Raman parameters in Y well Xuanjing area (The data come from Chinese University of Petroleum, East China)

Fig.8 Raman spectra of different types of bitumen samples

Carbonaceous bitumen is widely exposed in Xuanjing area.With the increase of burial depth, the geological age, thermal evolution and bitumen reflectivity increase.The upper Paleozoic source rocks are generally in the mature stage , Meanwhile, the lower Paleozoic is mainly in the “high mature” to “over mature” stages,in which carbonaceous bitumen is the product of high thermal activity.The range of atomic ratio of hydrogen to carbon (H/C) and bitumen reflectance in the study area is 0.39–0.52 and 3.39%–3.92%, respectively.This indicates the late stage of moisture to the thermal evolution of dry gas.Carbonaceous bitumen has experienced the dissipation of early generated oil and gas and experienced a long-time thermal evolution process.The vitrinite reflectance (Ro) is quite different from that of the Permian, but the Ro of Lower Cambrian shale in southern Anhui is as high as 4.0%on average (Wang, 2021).Therefore, it is speculated that bitumen in the surrounding rock may not be the product of crude oil cracking in this layer but instead may be residual bitumen generated by oil and gas accumulation in the early diagenetic stage.This would indicate that the solid bitumen from the Permian in this area is the evidence of the contribution by Lower Cambrian source rock.

4.2 Reconstruction of paleo-temperature and pressure of aqueous inclusions

Bitumen inclusions and brine inclusions are coexisted in calcite and quartz veins, brine inclusions were used to reconstruct the temperatures and pressure in Xuanjing area.As shown in Table 3, by comparing the results of fluid inclusion homogenization with the simulation results, it was found that the capture temperature (Ttrap) of Triassic strata is around 85.16 –177.25 ℃, the range of capture pressure (Ptrap) is between 9.6–61.4 MPa, whereas the capture temperature of Permian strata is around 70.21–189.5 ℃ and capture pressure is around 5.1–48.6 MPa.The test results show that the capture pressure of fluid inclusions in different horizons is related to the structural position.Well Y is located in the west flank of the Gufeng syncline, where the stratum experienced erosion.With oil and gas accumulating in the early stage underwent strong structural activity, and crude oil cracked into bitumen.For hydrocarbon generation occurred in the later stage, the upper strata of Permian reservoirs had higher pressure and a better seal-reservoir assemblage,because the capture pressure of Triassic is relatively high, which preserved hydrocarbon generated in late stage in the high-quality reservoirs.

In the early diagenetic stage of Well D’s Longtan Formation, the capture temperature of recrystallized quartz is 163.5 ℃, the capture pressure is 44.7 MPa,while the capture temperature of recrystallized fluorapatite is 104.5 ℃, and the capture pressure is 19.4 bar.In the late diagenetic stage of the Longtan Formation, the capture temperature of calcite filled in fractures is 97.7–143.8 ℃, the capture pressure is 17.6–33.6 MPa, with average capture pressure is 28.9 MPa.The fracture in the late diagenetic stage of the Gufeng Formation is filled with calcite with capture temperature of 120.9–140.9 ℃, capture pressure of 34.4–48.6 MPa and an average capture pressure of 41.2 MPa.Therefore, the pressure in the early diagenetic stage of the Longtan Formation is greater than that of the late diagenetic stage, and the formation pressure in the Longtan Formation was a slow decompression process.In the late diagenetic stage, the formation pressure of the Gufeng Formation is greater than that of the Longtan Formation.The deeper the formation, the greater the pressure.The homogeneous temperature is 70-100 ℃,and the homogenization pressure of brine inclusions is 10-20 MPa (Fig.9).

Table 3 PVTx simulation parameters and results of fluid inclusion in Xuanjing area (The data come from Chinese University of Petroleum, East China)

Fig.9 Relationship between temperature and pressure of brine inclusions in Xuanjing area

4.3 Influence on shale gas preservation conditions

The drilling revealed that the Permian Gufeng,Longtan, Dalong and Triassic Yinkeng formations were source rock.Before the Paleozoic period, the strata subsided slowly, but there was rapid deposition during the Triassic period.The formations then was uplifted and eroded to a great extent.There was oil and gas accumulation in the early stage of the study area.The strata were then uplifted and denuded in the later period and the oil and gas accumulation in the reservoir was destroyed, leaving a large amount of solid bitumen in the reservoir pores.The second oil and gas accumulation stage was from early Neogene to the present, which produced the light oil and gas in the present shale.Influenced by the stratum burial and geothermal gradient, the source rock of the Middle Paleozoic gradually matured in the Early Triassic rapid burial stage.The degree of thermal evolution of the Permian was set in the Yanshanian, and the uplifting and denudation in the later stage gradually reduced the stratum temperature, hydrocarbon generation stopped after reaching the present thermal stage.After the Permian deposition, the main oil and gas accumulation period was in the Late Cretaceous–Paleocene, in which the Permian source rocks were in the late stage of oil and wet gas generation, and the Triassic source rock was at the peak and late stage of oil generation.

To sum up, hydrocarbon accumulation in the reservoirs intersected by Well Y and Well D are typical syncline structural controlled accumulations, and the hydrocarbon accumulation is related to the structural setting.In the early stage, oil and gas accumulation were destroyed and crude oil was cracked to generate bitumen.In the late hydrocarbon generation stage, the structure was relatively stable and syncline structures were formed by tectonic compression, and some local syncline strata were eroded.The overlying strata of the Permian reservoir were good seals with high pressure,which formed a good reservoir-cap combination for the storage and preservation of shale gas in the later stage (Baoet al., 2016).Therefore, the Paleozoic reservoirs in Xuanjing area have good materials for source rock, large reservoir thickness and high organic carbon content.These may have created favorable horizons for shale gas formation and storage in Lower Yangtze area.Reservoir geochemical analysis shows that oil and gas currently in the Upper Permian and Lower Triassic reservoirs in Xuanjing area were mainly formed in the late stage, and light oil and gas was produced through the continuous burial process.With the increase of ground temperatures, some light oil and gas cracked to form bitumen.Since the late Paleogene period, this area has experienced continuous deposition, with stable tectonic activity and good conditions for preserving shale gas.However, due to magmatic and tectonic hydrothermal effects in some areas, some light oil and gas underwent high temperature metamorphic cracking and formed dry gas, and various metamorphic fluids were formed with mixing of large amount of N2inorganic gas.The Permian strata in the study area are buried deeply,and no major tectonic movement has damaged the late hydrocarbon accumulation.However, due to magmatic/hydrothermal action, many metamorphic fluids (mainly N2inorganic gas) were formed and enriched in some areas.The oil and gas accumulation in the shale is not only related to the hydrocarbon generation process of shale itself, but also related to the later magmatic activity and tectonic hydrothermal transformation.Therefore, in evaluating shale gas prospects, effective geophysical and geochemical methods should be first applied to avoid the areas with active magma and tectonic hydrothermal activities.Through the analysis of inclusion temperature and pressure, it is possible to locate oil and gas exploration prospects with relatively low local thermal evolution in the Paleozoic area of Xuanjing area.

5 Conclusions

(1) Fluid inclusions in Xuanjing area are mainly gas-liquid two-phase brine inclusions and bitumen organic inclusions, and the genetic types are mainly primary inclusions.The homogeneous temperature peaks of gas-liquid two-phase brine inclusions are 70–80 ℃ and 90–100 ℃ respectively, with homogeneous temperature of some samples exceeds 180 ℃.This indicates that thermal action has affected the reservoir to a certain extent.

(2) Through an analysis of inclusion temperature and pressure, this study of fluid geochemistry of two wells in Xuanjing area shows that the homogeneous temperature is 70–100 ℃, and the homogenization pressure of brine inclusions is 10–20 MPa.

(3) Based on the simulation of capture temperature and capture pressure, it was found that with the deepening of depth, the capture pressure decreases and the upper strata of Permian high-quality reservoirs have higher pressure, better sealing and better reservoircap assemblage, providing good preservation for the hydrocarbon generated in the late stage to be stored in good quality reservoir.The testing of thermal variation parameters of carbon pitch in fluid inclusions showed that the thermal evolution of carbon pitch in this area is high.Moreover, the pitch in the surrounding rock may not be the product of crude oil cracking in this layer but may be the residual pitch produced by oil and gas accumulation in the early diagenetic stage.

Acknowledgments

We appreciate CHEN Yong and NI Rui (Institute of School of Geosciences, Chinese University of Petroleum, East China), for offering experimental data.