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Article

Self-Sourced Unconventional Tight Marlstone Reservoir Potential from Evaporative Lagoon of Triassic Leikoupo Formation in the Central Sichuan Basin

by
Jianyong Zhang
1,2,3,
Yongguang Xin
1,3,
Hao Zhang
1,3,
Han Tian
4,
Wei Chen
1,2 and
Xinjian Zhu
1,2,*
1
PetroChina Hangzhou Research Institute of Geology, Hangzhou 310023, China
2
Key Laboratory of Carbonate Reservoirs, China National Petroleum Corporation, Hangzhou 310023, China
3
PetroChina Research Center of Sichuan Basin, Chengdu 610094, China
4
Research Institute of Petroleum Exploration & Development, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Energies 2023, 16(13), 5086; https://0-doi-org.brum.beds.ac.uk/10.3390/en16135086
Submission received: 22 May 2023 / Revised: 20 June 2023 / Accepted: 21 June 2023 / Published: 30 June 2023
(This article belongs to the Special Issue Unconventional Oil and Gas Resources: Exploitation and Development)
Browse Figures
Versions Notes

Abstract

:
A breakthrough was made in tight marlstone reservoirs from an evaporative lagoon in the second sub-member of the third member of the Leikoupo Formation (Lei3-2) in the Central Sichuan Basin. The source rock characteristics, reservoir characteristics of the marlstone and geochemical parameters of the oil and natural gas were investigated to evaluate the unconventional hydrocarbon exploration potential of the tight marlstone from the evaporative lagoon. The results revealed that the source rocks were deposited in evaporative lagoon environments, and the average total organic carbon (TOC) content value for the samples was 0.75 wt%. The thermal maturity of the organic matter was relatively high, with a calculated vitrinite reflectance (Rc) of 1.7%. The characteristics of marlstones suggest that the potential source rocks had fair to good hydrocarbon generative potential. The condensate samples had low densities, low viscosities and high thermal maturity, with a Rc value of 1.7%. For the natural gas, the dry coefficient was around 0.90, and the carbon isotopic compositions of methane and ethane was −41.3‰ and −28.4‰, respectively. According to the carbon isotopic compositions, thermal maturity and geological background, the oil and natural gas from Lei3-2 are comparable with the marlstone of Lei3-2. Thus, the oil and natural gas is self-sourced and originates from the marlstone in the Lei3-2. Micropores and microfractures are often detected in the marlstone from Lei3-2, and a gypsum layer is conducive to the hydrocarbon preservation. These results suggest that the evaporative lagoon facies in Lei3-2 have large, self-sourced, unconventional, tight marlstone reservoir potential. This study also enhances the prospects for further oil and gas exploration of evaporative lagoon facies in other basins.

1. Introduction

Recently, the interest in unconventional hydrocarbon resources has increased significantly [1,2,3], such as tight sandstone gas [4,5,6,7], shale oil and shale gas [8,9,10]. Unconventional hydrocarbon reservoirs are generally characterized by low porosity and low permeability and cannot be explored by conventional methodology and techniques [2,11,12]. In certain area, organic-rich mudstone can be both source and reservoir rocks [13,14,15]. Thus, mudstones are considered as the main self-sourced, unconventional hydrocarbon resources. However, self-sourced, unconventional, tight carbonate reservoirs are less reported.
Carbonate reservoirs generally include dolomite and limestone reservoirs, and dolomites generally have high porosity and permeability and can be served as high-quality reservoirs in petroliferous basins globally [16,17,18,19,20]. However, the reservoir potentials of limestone and marlstone often get overlooked due to their small pore sizes [21,22,23]. In some basins, marlstone has a high abundance of organic matter and acts as the source rock [24,25,26,27]. These results suggest that marlstone has the conditions to form self-sourced unconventional reservoirs. Several previous studies adopted organic-rich marlstone as an excellent self-sourced, unconventional reservoir [28,29,30,31]. However, these studies mainly involved marine or lacustrine marlstone. Whether marlstone from an evaporative lagoon has self-sourced, unconventional hydrocarbon potential is unclear.
Nowadays, a breakthrough has been made in tight marlstone reservoirs from an evaporative lagoon in the second sub-member of the third member of the Leikoupo Formation (referred to as Lei3-2) in the Central Sichuan Basin [32,33]. The underlying and overlying rocks of this interval detected with abundant hydrocarbons are tight gypsum rocks [34]. Furthermore, the geochemical characteristics of the oil and gas from the Lei3-2 sub-member are different from those previously produced in the Leikoupo Formation, and the natural gas and oil previously produced mainly originate from Permian and Triassic Xujiahe source rock [35,36,37,38]. The results suggest that the tight marlstone of Lei3-2 may be a new unconventional reservoir type, and the oil and gas from Lei3-2 might be trapped in situ in the tight marlstone. Detailed analyses require further study.
Here, we investigate the source rock characteristics and reservoir characteristics of the marlstone in Lei2-3. The geochemical parameters of the oil and natural gas are also provided. The present work aims to (1) estimate the hydrocarbon generation potential of marlstone, (2) investigate the origin of the oil and natural gas from Lei3-2, (3) describe the reservoir characteristics of marlstone and (4) evaluate the self-sourced, unconventional hydrocarbon exploration potential of the tight marlstone from an evaporative lagoon. These findings could be a basis for future oil and gas exploration in the Sichuan Basin and provide new insights for the hydrocarbon exploration in tight carbonate reservoirs from evaporative lagoons in other basins.

2. Geological Setting

The Sichuan Basin is located in the northwestern part of the Upper Yangtze Platform and is an important petroleum-bearing basin, which covers 1.8 × 105 km2. It can be divided into the Western Sichuan Basin, Northern Sichuan Basin, Eastern Sichuan Basin, Southern Sichuan Basin and Central Sichuan Basin (Figure 1), and the breakthrough of an unconventional tight marlstone reservoir from an evaporative lagoon of the Triassic Leikoupo Formation is from the CT1 well located in the Central Sichuan Basin. Previous oil and gas fields in the Leikoupo Formation were mainly found in other sub-members, including the PZ, XC, ZB, YB, LG, MX and AY fields, as shown in Figure 1.
During the deposition of the Leikoupo Formation, the sedimentary environment in the Sichuan Basin is characterized by shallow water columns, high salinity and a wide distribution of evaporate rock (Figure 1). Due to the tectonic movement and eustatic sea level change, the Leikoupo Formation in the Central Sichuan Basin can be divided into four members, including Lei-1, Lei-2, Lei-3 and Lei-4 members from bottom to top. The Lei-1 and Lei-2 members are mainly composed of micritic dolomite and gypsum. The bottom of Lei-1 is characterized by green bean rock (a potassium-rich felsic tuff), which is in contact with the Jialingjiang Formation of the Lower Triassic [39]. The lower and upper parts of the Lei-3 member are predominated by argillaceous micritic limestone, while the middle part is mostly interbedding between argillaceous micritic limestone and gypsum. For the Lei-4 member, the middle and lower parts mainly consist of micritic dolomite and gypsum, while, in the upper part, granular dolomite is locally developed. The top of the Lei-4 member is in unconformable contact with the Upper Triassic Xujiahe Formation (Figure 2).
During the deposition of Lei3-2, the lithofacies paleogeographic framework was controlled by the Kaijiang-Luzhou paleo-uplift, which was characterized by a NE-trending distribution [34,38]. The evaporative lagoon was mainly deposited in the Central Sichuan Basin (mainly developing marlstone), and outward were, successively, the limy flat (mainly developing limestone) and dolomitic flat (mainly developing dolomite) (Figure 1). During this period, the sedimentary environment and sea level change frequently [40]. Under such a geological background, the lower and upper parts of the Lei-3 member are limestone and marlstone, while the middle part is interbedded with gypsum rock and marlstone. According to the lithological characteristics, the Lei-3 member can be divided into three sub-members, including the Lei3-1, Lei3-2 and Lei3-3 sub-members (Figure 2). The thickness of Lei3-2 is between 100 m and 200 m. The marlstone containing organic matter can amount to over 40% of the total thickness of the Lei3-2 sub-member, and the marlstone thickness is larger when the interval thickness is higher.

3. Samples and Analytical Methods

Marlstone samples were collected from a drilling well and ground into fine powder using a disc mill to a size smaller than 200 μm to conduct TOC content, rock pyrolysis, carbon isotopic composition and extraction experiments. The marlstone samples were treated with hydrochloric acid for 24 h, which aimed to remove the carbonate minerals in the marlstone. In order to investigate the marlstone samples’ hydrocarbon generation potential, the TOC content of the studied marlstone samples was measured using an Eltra CS-i apparatus, and the organic carbon of the samples was completely oxidized into carbon dioxide by being heated to 900 °C. Another part of the samples after the acid treatment was used to determine the stable carbon isotope (δ13C) values of the organic carbon to correlate the source rock and oil. We used a Thermo Fisher Flash 2000 EA–Mat 253 isotope-ratio mass spectrometer (IRMS). The treated samples were combusted into CO2 at 980 °C using a Flash HT copper oxide reaction furnace, and the δ13C values were calculated according to Pee Dee Belemnite (PDB) standard.
The crushed samples were pyrolyzed using YQ-Ⅳ Rock-Eval equipment to evaluate the source rocks. The free hydrocarbon (S1) content value was analyzed at 300 °C, which was held for 3 min. Then, the equipment was programmed from 300 °C to 800 °C at 25 °C/min and analyzed the remaining hydrocarbon (S2) content value. The temperature of the maximum S2 pyrolysis yield (Tmax) was determined as proposed by Espitalié [41].
We obtained extractable organic matter (EOM) from the treated samples using the Soxhlet extraction method for 72 h, and the method was the same as previous literature [42]; the saturated, aromatic and resins fractions were separated from the EOM. Then, the GC–MS analysis of the saturated fractions was performed on an Agilent 6890 GC that was coupled to an Agilent 5975i mass selective detector and equipped with a HP-5MS elastic quartz capillary column (column length was 60 m, internal diameter was 0.25 mm and film thickness was 0.25 μm). The GC program was the same as the previous literature [42].
In order to investigate the reservoir characteristics, the porosities and permeabilities of the samples were measured from 2.5 cm diameter core plugs by using a helium permeameter. The dry and clean core samples were placed in the permeameter and injected with helium gas. Thin sections were stained with blue or red epoxy resin to aid in identifying the pores, and scanning electron microscopy (SEM) was employed to characterize the pore geometry, cement morphology and the textural relationships between the minerals. The samples were gold-coated and examined with a scanning electron microscope.

4. Results

4.1. Bulk Organic Geochemical Characterization of the Marlstone

Detailed organic geochemical characteristics were investigated in the marlstone samples, which were collected from Lei3-2 in the Central Sichuan Basin, and the source rock geochemical results are shown in Table 1. The TOC content values for the analyzed samples from Lei3-2 ranged from 0.49 wt% to 1.08 wt%, with an average value of 0.75 wt%.
According to the Rock-Eval pyrolysis results of the samples, the free hydrocarbon (S1) content values and remaining hydrocarbon (S2) content values ranged from 0.05 to 0.28 mg HC/g rock and 0.12 to 0.30 mg HC/g rock, respectively. The hydrogen index (HI) values of the marlstone samples ranged from 20.8 to 30.0 mg HC/g TOC. The extractable organic matter (EOM) yields from the marlstone samples are also shown in Table 1. The samples had relatively low values of the EOM yield ranging from 49.88 to 147.80 ppm, with an average value of 77.18 ppm. The organic carbon isotopic compositions of the studied samples were also measured, and the values ranged from −25.6‰ to −24.1‰, with an average value of −25.0‰.
As important organic geochemical parameters, the biomarker distributions were also analyzed in this study. For terpane, the studied samples from Lei3-2 in the CT1 were characterized by the dominance of 18α(H)-trisnorneohopane (Ts), C29-norhopane and C30-hopane, as shown in Figure 3. Thus, the Ts/Tm ratios were generally high, ranging from 1.16 to 1.33, with an average of 1.23. As shown in Table 2, the C29-norhopane/C30-hopane (C29/C30H) ratios and gammacerane/C30-hopane (Ga/C30H) ratios of the samples ranged from 0.42 to 0.61 and from 0.10 to 0.15, respectively. The sterane distribution showed dominance over C27 sterane (Figure 3). The samples had a relatively higher C27 sterane concentration (from 0.38 to 0.44, with an average of 0.40) compared to the C28 sterane concentration (from 0.28 to 0.33, with an average of 0.31) and C29 sterane concentration (from 0.28 to 0.30, with an average of 0.29).
The aromatic compounds were also investigated, which could be used to evaluate the thermal maturity [43,44,45], and several aromatic ratios for the studied samples were determined, as shown in Table 2, including the methylphenanthrene index (MPI), methyldibenzothiophene distribution index (MDBI) and trimethylnaphthalene ratio (TNR). The MPI, MDBI and TNR values ranged from 0.86 to 0.97, from 0.59 to 0.63 and from 1.23 to 1.31, respectively. The calculated vitrinite reflectance (Rc) based on MPI was about 1.7%, which was consistent with previous studies [33]. However, Rc based on TNR and MDBI were relatively low, mainly ranging from 1.1%, to 1.3%, generally because TNR and MDBI are not appropriate for thermal maturity higher than 1.4% [44].

4.2. Geochemical Characterization of Oil and Natural Gas

In order to investigate the origin of the oil and natural gas from Lei3-2 in CT1, a large amount of geochemical data of the oil and natural gas were collected [36,46,47]. The initial gas/oil ratio (GOR) of CT1 was 2312 m3/m3, suggesting a high thermal maturity. The condensate sample densities ranged from 0.73 to 0.75 g/cm3, and the viscosities ranged from 0.67 to 0.91mPas. Isotopically, the δ13C values of the saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes were −28.6‰, −28.0‰, −28.3‰ and −28.2‰, respectively. MPI and diamondoid hydrocarbons were analyzed to assess thermal maturity [48]. The Rc values based on MPI ranged from 1.76% to 1.86% and that of the diamondoid was 1.65%.
For the natural gas from Lei3-2 in CT1, methane was the dominant component and ranged from 85.83% to 88.15%. The ethane and propane contents ranged from 5.51% to 7.76% and 1.79% to 3.20%, respectively. The dry coefficient (C1/C1+) ranged from 0.871 to 0.924, with an average value of 0.90. Non-hydrocarbon gases, including carbon dioxide and nitrogen, were generally lower than 0.6%, and hydrogen sulfide was not detected. The δ13C values of methane and ethane were −41.3‰ and −28.4‰, respectively.
The geochemical properties of the natural gas from the Leikoupo Formation in other areas are also shown in the Table 3. Methane was the dominant component in all samples and ranged from 63.06% to 99.57%. The ethane content was very low, ranging from 0.10% to 4.96%, and propane was lower than 1%. Obviously, the dry coefficients for the natural gases previously produced in the Leikoupo Formation have a narrow range (from 0.945 to 0.999). This is different from the Lei3-2 in CT1. Non-hydrocarbon gases are mainly composed of carbon dioxide, nitrogen and hydrogen sulfide. The hydrogen sulfide contents in the MX, ZB and XC areas were relatively high, ranging from 1.28% to 8.34%, and generally not detected in other areas. The δ13C values of methane ranged from −38.4‰ to −31.2‰, obviously heavier than Lei3-2 in CT1. The δ13C values of ethane and propane ranged from −36.6‰ to −25.1‰ and −33.9‰ to −23.8‰, respectively, suggesting a mixed origin.

4.3. Characteristics of the Carbonate Rock Reservoir

The lithologies of Lei3-2 in the Central Sichuan Basin are characterized by marlstone and gypsum. As known, gypsum is generally considered a cap rock with low porosity and permeability. The porosities of the investigated marlstones ranged from 0.96% to 4.93%, with an average value of 2.04%, and the permeabilities of the samples ranged from 0.00105 mD to 1.68 mD (Table 4). The thin section and SEM results suggest that marlstones have micropores and microfractures; micropores include organic pores, interparticle pores and intergranular pores (Figure 4). According to the SEM results, the micropore diameters mainly ranged from 0.5 μm to 2 μm.

5. Discussion

5.1. Hydrocarbon Generation Potential of the Marlstone from Lei3-2 in the Central Sichuan Basin

Generally, the higher the total organic matter richness, the greater the hydrocarbon generation potential of the source rock [49,50,51]. The organic matter richness of the marlstone in this study was evaluated based on the TOC content, pyrolysis and bitumen extraction analysis generally. According to the plots of S2 versus TOC (Figure 5) and EOM content versus TOC (Figure 6), the results suggest that the S2 and EOM values are in agreement with the TOC contents, and these marlstones mainly have fair source rock richness.
The type and origin of the organic matter of the marlstone were also studied to evaluate the hydrocarbon generation potential. Biomarker characteristics can provide reliable evidence to evaluate the origin of organic matter [52,53,54]. The Ga/C30 values suggest low salinity for the studied marlstones. The sterane distributions (sterane C27–sterane C28–sterane C29) are characterized by high abundances of C27 regular steranes compared to C28 and C29 regular steranes, suggesting the dominance of plankton or algal-derived organic matter for the marlstone of Lei3-2 in the Central Sichuan Basin, and it should be oil-prone source rock. The HI and Tmax values can also be used to assess the organic matter type in source rocks generally at a low maturity level [41,55]. However, pyrolysis data are not suitable for the study area because of the high thermal maturity.
The organic matter maturity is also required for an assessment. Vitrinite is rare in limestone. The pyrolysis data of the marlstones indicate a high thermal maturity. According to molecular geochemistry, the calculated vitrinite reflectance (Rc) in the present study based on MPI is about 1.7%. In addition, the Rc based on TNR and MDBI is relatively low. However, previous studies suggest that TNR and MDBI are not suitable for the high thermal maturity, with a vitrinite reflectance greater than 1.4% [43,44,45]. Furthermore, in the Central Sichuan Basin, vitrinite reflectance of the source rocks from the Upper Triassic and Permain range from 1.0% to 1.3% and 2.0% to 2.5%, respectively [35,56]. The results suggest that the marlstone source rock from Lei3-2 in the Central Sichuan Basin should be 1.7%.
The Lei3-2 sub-member in the Central Sichuan Basin is mainly deposited in a sedimentary sequence consisting of interbedded lagoonal evaporative carbonates and marlstones. The biomarker parameters suggest that the marlstones of Lei3-2 in the study area are developed by reducing depositional environments with a little bit of salt. Thus, the organic matter preservation condition is great. On the other hand, the marlstones source rocks have fair TOC, HI and EOM content values, showing available organic matter production generation conditions. Marlstones also show high thermal maturities, beneficial to hydrocarbon generation. In addition, gypsum salt rock has high thermal conductivity, which also expands the hydrocarbon generation window of the marlstone source rocks in Lei3-2. Thus, the marlstones from an evaporative lagoon in Lei3-2 show a good hydrocarbon generation potential, and the results are consistent with many marlstones in other basins [29,57]. Furthermore, the marlstone thickness from the Lei3-2 sub-member in the Central Sichuan generally range from 25 to 80 m, and it covers an area of about 12,000 km2, showing great exploration potential.

5.2. Origin of Oil and Gas from Lei3-2 in the Central Sichuan Basin

The natural gas of Lei3 and Lei4 in West Sichuan is typically dry gas with high δ13C values, and previous studies proposed that the gas of the Leikoupo Formation in West Sichuan is mainly sourced from the underlying source rock or the overlying Xujiahe Formation source rock [38,58]. For the Central Sichuan Basin area, the natural gas from Lei1 and Jialingjiang in the MX area is characterized by a high methane content, low heavy hydrocarbon content and low H2S. Previous studies suggested natural gas originates from the underlying Permian source rock [35,37,46]. However, the natural gas of Lei3-2 is wet gas. It features a low methane content, no H2S and low δ13C value, indicating a different origin (Figure 7).
Firstly, the oil and natural gas of Lei3-2 should not originate from overlying Xujiahe Formation source rock. The pressure coefficient of Lei3-2 in the Central Sichuan Basin ranges from 1.81 to 1.96, while that of the Xujiahe Formation is about 1.3 to 1.5 [33,35]. Furthermore, thick gypsum rock is developed in the Leikoupo Formation. It is almost impossible to migrate downward to the Leikoupo Formation and has a greater pressure. On the other hand, source rocks from the Xujiahe Formation are generally gas-prone, and the oil origin from the Xujiahe Formation has not been found.
For the underlying Permian Formation source rock, the thermal maturity is relatively high, and the gas origin from the Permian source rocks in the Central Sichuan Basin is typically dry gas with a high δ13C value of methane and has H2S in general, which is different from that of Lei3-2. The results suggest that the oil and gas should not originate from the underlying Permian Formation source rock.
According to the carbon isotopic compositions of oil, ethane in natural gas and Lei3-2 marlstone, they are comparable. In general, whole oil carbon isotopes are similar to ethane carbon isotopes, but both are lighter than those of the source rocks [59,60,61]. The oil has the characteristics of a light oil with no evidence of evaporative fractionation or biodegradation. For the thermal maturity, the Rc of oil is about 1.7%, which is consistent with the Lei3-2 marlstone. Oil cracking can occur at a maturity of 1.7%, and the maximum reservoir temperature of Lei3-2 is above 170 °C [33,35]. The initial GOR (2312 m3/m3) and overpressure (pressure coefficient of 1.81-1.96) of CT1 coincide with oil cracking. The δ13C1 (carbon isotopic composition of methane) value is generally controlled by the thermal maturity [62,63,64], and according to the empirical equation of Rc-δ13C1, the δ13C1 value of −41.3‰ corresponds to the maturity of 1.0% to 1.3% [65,66]. However, the natural gas from crude oil cracking at the early stage also has a light δ13C1 value. Previous simulation experiments suggested that the δ13C1 value of methane is much lighter than that from kerogen cracking gas at similar temperatures, due to the greater carbon isotope fractionation of the oil cracking methane precursor [67,68]. Thus, the oil and natural gas in CT1 are self-sourced and originate from the marlstone in Lei3-2.

5.3. Self-Sourced Unconventional Tight Marlstone Reservoir Potential from Evaporative Lagoon

The marlstone from the evaporative lagoon facies of Lei3-2 in the Central Sichuan Basin have hydrocarbon generation potential, with a thickness of 25–80 m and an area of 12,000 km2. It also has a large number of micropores, which is conducive to oil and gas accumulation, though the marlstone is tight, with low porosity and permeability. In addition, the top and bottom of the marlstone are thick and have tight gypsum beds. As a result, it is difficult for hydrocarbons to migrate and diffuse, forming overpressures with pressure coefficients of 1.81–1.96. Thus, the evaporative lagoon facies in Lei3-2 have large self-sourced, unconventional, tight marlstone reservoir potential, and the CT1 well has a gas production of 10.87 × 104 m3 and oil production of 47.04 m3 per day.
Evaporative lagoon facies are rarely considered as exploration targets and are regarded as cap beds because of the development of gypsum. However, periodic climatic and biological variations result in changes of the lithofacies in lagoon facies [21,69,70]. In dry conditions, evaporitic minerals deposit and form gypsum beds, while wet conditions result in microbial mats that contribute to organic matter enrichment, and the salinity is relatively low (Figure 8). These sedimentary sequences contain evaporative carbonates and marlstone with organic matter. Marlstone has hydrocarbon generation potential, and a gypsum bed is conducive to oil and gas preservation.
Evaporative lagoon facies are commonly developed globally. It can be proposed that self-sourced, unconventional, tight carbonate reservoirs from evaporative lagoons have great exploration potential, and the exploration of lagoon facies can be expanded.

6. Conclusions

In this study, the depositional environment, organic matter richness, type and thermal maturity were evaluated to determine the hydrocarbon generation potential of the marlstone from Lei3-2 in the Central Sichuan Basin. The marlstone source rocks are deposited within anaerobic reductive evaporative lagoons and have fair–good organic matter abundance, with an average TOC content value of 0.75 wt%. The Tmax and biomarker parameters results suggest that the organic matter from Lei3-2 in the Central Sichuan Basin has a high thermal maturity (Rc is about 1.7%), and the marlstone in the study area has fair to good hydrocarbon generative potential.
The geochemical characteristics of the oil and natural gas from Lei3-2 were also investigated to analyze their origins. They are obviously different from the oil and gas derived from the overlying Xujiahe and underlying Permian source rocks. According to the carbon isotopic compositions, thermal maturity and geological background, the oil and natural gas from Lei3-2 are comparable with the marlstone of Lei3-2. Thus, the oil and natural gas are self-sourced and originate from the marlstone in Lei3-2.
The marlstone from Lei3-2 is tight but develops micropores and microfractures. The porosities of the investigated marlstones range from 0.96% to 4.93%, with an average value of 2.04%, and the permeabilities of the samples range from 0.00105 mD to 1.68 mD. However, hydrocarbons stored in Lei3-2 are hard to migrate and diffuse and have excellent preservation conditions because of the interbedded gypsums and marlstones. Thus, the evaporative lagoon facies in Lei3-2 have large self-sourced, unconventional, tight marlstone reservoir potential.
Evaporative lagoon facies are generally developed globally but rarely considered as exploration targets. This study proposes that the self-sourced, unconventional, tight carbonate reservoirs from evaporative lagoons have great exploration potential. The conclusions enhance the prospects for further oil and gas explorations of evaporative lagoon facies.

Author Contributions

Conceptualization, J.Z. and Y.X.; Methodology, Y.X., H.Z., H.T. and W.C.; Software, H.T. and W.C.; Investigation, YX., H.Z., H.T., W.C. and X.Z.; Data curation, H.Z., H.T. and X.Z.; Writing—original draft, J.Z.; Writing—review & editing, Y.X. and X.Z.; Visualization, W.C. and X.Z.; Project administration, J.Z.; Funding acquisition, J.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Science and Technology Major Project of China (Grant No. 2017ZX05008-005) and the PetroChina Science and Technology Specific Projects (Grant No. 2018A-0105).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are thankful to the Petro China Southwest Oil & Gasfield Company for supplying the studied samples and providing the original geological data. We also appreciate the editors and reviewers for providing constructive and careful comments that significantly improved the manuscript.

Conflicts of Interest

The authors declare no competing financial interest.

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Figure 1. Sketch map showing geographical positions of tectonic units, sedimentary facies and gas fields of Triassic Leikoupo Formation in the Central Sichuan Basin (modified from [32]).
Figure 1. Sketch map showing geographical positions of tectonic units, sedimentary facies and gas fields of Triassic Leikoupo Formation in the Central Sichuan Basin (modified from [32]).
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Figure 2. Generalized stratigraphic diagram in the Central Sichuan Basin (modified from [32]).
Figure 2. Generalized stratigraphic diagram in the Central Sichuan Basin (modified from [32]).
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Figure 3. Mass chromatograms 191 m/z and 217 m/z of saturated hydrocarbon fractions showing the distribution of hopanes and steranes.
Figure 3. Mass chromatograms 191 m/z and 217 m/z of saturated hydrocarbon fractions showing the distribution of hopanes and steranes.
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Figure 4. Thin section and SEM photomicrographs displaying different types of marlstones in Lei3-2 of the Central Sichuan Basin. (a) Marlstone sample with a microfracture and dissolved pore, well CT1, depth 3506 m. (b) Marlstone sample with a microfracture and dissolved pore, well CT1, Lei3-2, depth 3550 m. (c) Marlstone sample showing pyrites and different types of pores, including intergranular pores, interparticle pore and organic pores, well JY1, Lei3-2, depth 2641.1 m. (d) Marlstone sample showing pyrites and different types of pores, including intergranular pores, interparticle pore and organic pores, well JY1, Lei3-2, depth 2641.1 m. (e) Marlstone sample showing an interparticle pore, well JY1, Lei3-2, depth 2641.1 m. (f) Marlstone sample showing an interparticle pore, well JY1, Lei3-2, depth 2641.1 m.
Figure 4. Thin section and SEM photomicrographs displaying different types of marlstones in Lei3-2 of the Central Sichuan Basin. (a) Marlstone sample with a microfracture and dissolved pore, well CT1, depth 3506 m. (b) Marlstone sample with a microfracture and dissolved pore, well CT1, Lei3-2, depth 3550 m. (c) Marlstone sample showing pyrites and different types of pores, including intergranular pores, interparticle pore and organic pores, well JY1, Lei3-2, depth 2641.1 m. (d) Marlstone sample showing pyrites and different types of pores, including intergranular pores, interparticle pore and organic pores, well JY1, Lei3-2, depth 2641.1 m. (e) Marlstone sample showing an interparticle pore, well JY1, Lei3-2, depth 2641.1 m. (f) Marlstone sample showing an interparticle pore, well JY1, Lei3-2, depth 2641.1 m.
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Figure 5. The plot of S2 versus TOC showing these studied marlstones mainly have fair organic matter richness (adapted from [49]).
Figure 5. The plot of S2 versus TOC showing these studied marlstones mainly have fair organic matter richness (adapted from [49]).
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Figure 6. The plot of the bitumen content versus TOC showing these studied marlstones mainly have fair organic matter richness (adapted from [49]).
Figure 6. The plot of the bitumen content versus TOC showing these studied marlstones mainly have fair organic matter richness (adapted from [49]).
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Figure 7. The plot of the dry coefficient versus δ13C1 value showing the difference between the gases from CT1 and other areas.
Figure 7. The plot of the dry coefficient versus δ13C1 value showing the difference between the gases from CT1 and other areas.
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Figure 8. Sedimentary model of the Lei3-2 evaporative lagoon facies in the Central Sichuan Basin.
Figure 8. Sedimentary model of the Lei3-2 evaporative lagoon facies in the Central Sichuan Basin.
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Table
Table 1. The TOC content, Rock-Eval pyrolysis analysis and organic carbon isotopic composition of the studied samples.
Table 1. The TOC content, Rock-Eval pyrolysis analysis and organic carbon isotopic composition of the studied samples.
WellFormationDepth (m)LithologyTOC (%)S1 (mg/g)S2 (mg/g)Tmax (°C) *HI (mg/g)EOM (ppm)δ13C (‰)
CT1Lei3-23560.4marlstone 0.81 0.140.2152925.8147.80-
CT1Lei3-23560.75marlstone 0.80 0.130.1846122.449.88 -
CT1Lei3-23561.3marlstone 0.49 0.060.1450528.660.39 -
CT1Lei3-23561.6marlstone 0.57 0.050.1550926.2--
CT1Lei3-23560.2marlstone 0.60 0.13 0.18 49830.063.18 −25.4
CT1Lei3-23565.81marlstone 0.96 0.26 0.24 47425.172.94 −24.1
CT1Lei3-23566.41marlstone 0.59 0.13 0.12 -20.878.64 -
CT1Lei3-23568.25marlstone 1.08 0.28 0.30 48927.645.09 -
CT1Lei3-23567.4marlstone 0.83 0.24 0.24 -28.951.49 −25.6
* For reference.
Table
Table 2. Molecular parameters of the studied samples.
Table 2. Molecular parameters of the studied samples.
WellFormationDepth (m)Ts/TmC29/C30HGa/C30C27steraneC28steraneC29steraneC29
S/S+R		C29
ββ/αα+ββ		MPI MDBITNRRo-
MPI (%)		Ro-MDBI (%)Ro-
TNR (%)
CT1Lei3-23560.41.160.420.150.440.280.280.410.460.880.631.231.771.311.14
CT1Lei3-23560.81.220.610.100.390.330.280.400.460.970.611.311.721.291.18
CT1Lei3-23561.31.330.570.130.380.320.300.410.460.860.591.271.781.271.16
Ts: C27 18α(H)-22,29,30-trisnorneohopane, Tm: C27 17α(H)-22,29,30-trisnorhopane, C29H: C29 17α(H), 21β(H)-norhopane, C30H: C30 17α(H), 21β(H)-hopane, Ga: gammacerane, MPI (methylphenanthrene index) = (3-MP + 2-MP)/(P + 9-MP + 1-MP) [43], MDBI (methyldibenzothiophenes distribution index) = 4-MDBT/DBT + 1-MDBT + 4-MDBT + 2- + 3-MDBT [44] and TNR (trimethylnaphthalenes ratio) = (1,3,7-TNR + 2,3,6-TNR)/(1,3,5-TNR + 1,3,7-TNR + 1,4,6-TNR) [45].
Table
Table 3. Chemical and isotopic compositions of the natural gases from the Leikoupo Formation, Sichuan Basin.
Table 3. Chemical and isotopic compositions of the natural gases from the Leikoupo Formation, Sichuan Basin.
AreaWellFormationChemical Composition (%)Carbon Isotopic
Composition (‰)
CH4C2H6C3H8iC4H10nC4H10N2CO2H2SC1/C1-5δ13C1δ13C2δ13C3
LGLG3T2L492.814.280.830.160.121.390.2400.945−36.3−25.1−23.8
LGLG7T2L476.590.900.100.040.030.4721.6500.986−37.2−32.2−24.3
LGLG12T2L488.102.200.370.060.040.318.6500.971−35.5−26.2−23.8
LGLG17T2L463.060.510.0200.010.5135.6100.992−35.8−35.3-
LGLG18T2L494.340.790.070.010.010.124.590.010.991−36.5−35.5−30.5
LGLG20T2L483.462.230.380.090.090.0113.360.010.968−38.4−29.0−25.5
LGLG22T2L495.781.720.190.040.030.391.7600.980−37.7−30.8−27.2
LGLG160T2L494.182.140.320.060.061.271.8400.973−35.3−26.6−24.3
LGLG172T2L494.922.310.350.060.050.81.370.010.972−36.3−25.3−24.4
LGLG176T2L495.161.710.230.020.020.342.4200.980−37.8−32.5−30.6
LGLG173T2L492.012.960.650.140.152.870.15-0.959−37.9−28.5−24.8
MXM140T2L199.540.17---0.230.05-0.998−35.0−32.4-
MXM144T2L198.900.18---0.750.16-0.998−34.9−32.1-
MXMS4T1J96.710.280.01--2.310.46-0.997−33.1−34.0−33.9
MXM150T1J99.500.21---0.210.08-0.998−34.7−33.7-
MXM5T1J98.850.17---0.80.18-0.998−34.6−33.2-
MXMS2T2L195.550.20---1.720.831.660.998−33.6--
MXM17T2L194.462.48--- 0.101.280.974−33.7−28.6-
MXM108T2L194.830.14---1.610.952.410.999−33.6−28.5-
MXM114T2L193.560.48---2.011.222.950.995−33.6−28.6-
YBYB2T2L495.390.58---3.280.7500.994−34.2−36.5−39.1
YBYB4T2L496.460.68---2.070.780.000040.99335.3−36.0-
YBYB9T2L491.951.31---5.041.700.00370.986−33.6−29.6−29.4
YBYB12T2L494.521.24---3.380.840.00670.987−31.7−27.7-
YBYB16T2L488.440.80---9.31.5500.991−32.1−32.8−33.4
YBYB221T2L497.511.08---0.41.01-0.989−31.5−30.0-
YBYB223T2L499.100.90---0000.991−35.6−36.2-
YBYL1T2L499.300.70---0000.993−35.4−36.6−24.5
YBYL5T2L492.100.65---5.51.7500.993−35.5--
XCXS1T2L483.720.28---10.620.644.680.997−33.6−32.5−27.0
XCCK1T2L494.470.38---4.460.390.260.996−31.2−34.8−32.6
PZPZ1T2L490.210.10---4.561.513.520.999−31.6−26.4−22.8
ZBZH18T2L385.182.54---2.121.438.340.971−36.9−27.7−22.1
ZBZH18T2L486.882.58---1.694.863.30.971−35.1−28.7−26.1
ZBZH21T2L487.922.75---1.783.651.780.970−35.4−31.1−30.3
ZBZH21T2L487.384.96------0.946−35.1−29.3-
ZBZH21 2T2L481.722.52------0.970−33.7−28.0−26.5
Data were collected from [46,47].
Table
Table 4. Reservoir physical properties of the studied samples.
Table 4. Reservoir physical properties of the studied samples.
WellFormationDepth (m)LithologyPorosity (%)Permeability (mD)
CT1Lei3-23560.49marlstone1.180.00125
CT1Lei3-23560.82marlstone1.480.00105
CT1Lei3-23561.39marlstone1.720.00326
CT1Lei3-23561.53marlstone2.030.004
CT1Lei3-23565.54marlstone0.960.00183
CT1Lei3-23565.81marlstone3.050.00443
CT1Lei3-23566.41marlstone4.931.68
CT1Lei3-23566.8marlstone2.030.0204
CT1Lei3-23567.4marlstone2.610.00157
CT1Lei3-23567.87marlstone1.100.00202
CT1Lei3-23568.45marlstone1.350.00126
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Zhang, J.; Xin, Y.; Zhang, H.; Tian, H.; Chen, W.; Zhu, X. Self-Sourced Unconventional Tight Marlstone Reservoir Potential from Evaporative Lagoon of Triassic Leikoupo Formation in the Central Sichuan Basin. Energies 2023, 16, 5086. https://0-doi-org.brum.beds.ac.uk/10.3390/en16135086

AMA Style

Zhang J, Xin Y, Zhang H, Tian H, Chen W, Zhu X. Self-Sourced Unconventional Tight Marlstone Reservoir Potential from Evaporative Lagoon of Triassic Leikoupo Formation in the Central Sichuan Basin. Energies. 2023; 16(13):5086. https://0-doi-org.brum.beds.ac.uk/10.3390/en16135086

Chicago/Turabian Style

Zhang, Jianyong, Yongguang Xin, Hao Zhang, Han Tian, Wei Chen, and Xinjian Zhu. 2023. "Self-Sourced Unconventional Tight Marlstone Reservoir Potential from Evaporative Lagoon of Triassic Leikoupo Formation in the Central Sichuan Basin" Energies 16, no. 13: 5086. https://0-doi-org.brum.beds.ac.uk/10.3390/en16135086

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