J. Cent. South Univ. (2017) 24: 1450-1464

DOI: 10.1007/s11771-017-3549-y

Origin of deep carbonate reservoir in northeastern Sichuan Basin: New insights from in-situ hydrothermal diamond anvil cell experiments

ZHANG Shan-ming(张单明)^{1, 2}, LIU Bo(刘波)², QIN Shan(秦善)¹, ZHANG Xue-feng(张学丰)²,

TIAN Yong-jing(田永净)³, GUO Rong-tao(郭荣涛)^{1, 2}, LIU Jian-qiang(刘建强)^{1, 2}

1. School of Earth and Space Sciences, Peking University, Beijing 100871, China;

2. Institute of Oil and Gas, Peking University, Beijing 100871, China;

3. China United Coalbed Methane Corporation Ltd, Beijing 100011, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2017

Abstract: In situ Raman analysis on the segregated near-equilibrium carbonate-fluid interaction at elevated temperatures (room temperature-260 °C) and pressures (13-812 MPa) in a hydrothermal diamond anvil cell (HDAC) reveals the preservation mechanism of porosity in deep carbonate reservoirs in the northeastern Sichuan Basin. The carbonate-fluid interaction was investigated by separately heating carbonate minerals and rocks with four different acid solutions (saturated CO₂ and H₂S solutions, HCl, CH₃COOH) in a sealed sample chamber. A minor continuous precipitation with increasing temperatures and pressures was observed during the experiments which caused minor sample volume change. The closed system is a preservation of pores and burial dissolution may not be the dominant diagenesis in the origin of porosity. Thin section photomicrographs observations in Changxing and Feixianguan Formations demonstrate that eogenetic pores such as moldic or intragranular pores with late small euhedral minerals, intergranular, intercrystal and biological cavity pores are the main pore types for the reservoirs. Early fast deep burial makes the porous carbonate sediments get into the closed system as soon as possible and preserves the pores created in the early diagenetic stage to make significant contribution to the deep reservoir quality. The anomalous high porosity at a given depth may come from the inheritance of primary pores and eogenetic porosity is fundamental to carbonate reservoir development. The favorable factors for deep reservoir origin include durable meteoric leaching, early fast deep burial, early dolomitization, etc. This deep pores preservation mechanism may be of great importance to the further exploration in deep carbonate reservoirs in the northeastern Sichuan Basin.

Key words: hydrothermal diamond anvil cell (HDAC); closed system; early fast deep burial; porosity preservation; Sichuan Basin

1 Introduction

Sichuan Basin is a rhombic basin covering an area of about 2.3×10⁵ km² (88780 mi²). Results of the recent petroleum exploration in the northeastern Sichuan Basin indicate significant gas accumulations in Changxing (P₂c) and Feixianguan (T₁f) Formations in the deep buried carbonate reservoirs [1-3]. Puguang gas field, one of the largest gas fields so far found in China, exhibits relatively high porosity up to 29% with burial depth greater than 5.0×10³ m (16404 ft) [1]. The proven geological reserves of natural gas in Yuanba gas field during the first exploration stage reached 1.592×10¹¹ m³ (5.62 TCFG) with gas reservoir depth ranging from 6240 to 6950 m (20472-22801 ft), making it the domestically deepest marine gas field [4]. Although deeply buried, the P₂c and T₁f reservoirs display relatively high porosity and permeability, with measured core porosity between 0.9% and 30% and permeability between 0.01 and 9664 mD [5, 6].

The controlling factors on the creation of deep- burial carbonate reservoir pores have been studied for many years [7-9]. Some authors proposed that significant volumes of porosity should be created by burial dissolution [10-14]. However, more recent studies demonstrated that major dissolution pores were formed mainly in subaerial meteoric environments, commonly tied to subaerial exposure and freshwater dissolution [15-21]. To understand the contribution of burial dissolution, a series of analogous laboratory experiments were conducted to simulate the carbonate-fluid interaction under various temperatures and pressures where high pressure vessels [22], rotating disk apparatus [23-26] or titanium high pressure mixed-flow reactors [27, 28] were extensively applied. The dissolution/ precipitation trend was determined by the weight loss of the solid samples or reaction fluid analysis at ex-situ conditions. This will inevitably cause some errors in explaining the results [29]. In addition, open systems designed in the published experiments mainly represent the subaerial environment. Near the surface, the reaction is far from equilibrium and significant new secondary porosity may develop because of the flow of meteoric water [30, 31]. At greater depth, the pore-water flow rate is extremely low, and burial diagenesis may represent nearly closed system where the pore water will quickly come to chemical equilibrium with surrounding carbonate rocks [16, 18, 27]. The reaction gradually changes from an open system into a closed environment with increasing depth.

Owing to the transparency, durability and inertness of diamond, hydrothermal diamond anvil cell (HDAC) is designed for the studies of solid-liquid interaction in the crust [32, 33]. Combined with Raman spectroscopy, direct visual observation and in situ measurement can be conducted to record the whole reaction process [34]. This allows the HDAC to be an excellent tool to simulate the carbonate-fluid interaction in a closed system at elevated temperatures and pressures [35]. The purpose of this work is to simulate the near-equilibrium carbonate-fluid interaction and reveal the dissolution/precipitation tendency of carbonate at elevated temperatures and pressures. This will be beneficial to understand the porosity preservation mechanism and speculate on its potential control on deep carbonate reservoir origin in the northeastern Sichuan Basin.

2 Geological setting

2.1 Facies and reservoir distribution

The study area is located in the northeastern of Sichuan Province, China (Fig. 1). The construction of this area is gentle in general and the tectonic deformation is weak [36]. A rapid basement subsidence happened in the Kaijiang and Liangping area during the Permian period led to the development of apparent depositional differentiation between the deep trough and the shallow platform facies, including open platform, marginal reef and shoal, platform slope and open shelf (Fig. 1(a)). The high quality reservoirs mainly developed in marginal reef and shoal facie. It has been confirmed that the NE Sichuan area has the main gas production interval in the dolostone reservoir of Changxing formation (P₂c) and oolitic limestone of Feixianguan Formation (T₁f)(Fig. 1(b)) [1, 36, 37].

2.2 Pore types and reservoir diagenesis contribution

2.2.1 Pore types

There are two types of reservoir spaces in P₂c and T₁f in the northeastern Sichuan Basin, namely porous and fractured reservoir spaces. The carrier system of porous type can be distinguished into three categories: the first type is the intergranular pores (Fig. 2(a)) that developed in favorable facies. Moldic pores (Fig. 2(b)), intragranular pores (Fig. 2(c)), biological cavity pores (Fig. 2(d)) and dissolution-enlarged pores (Fig. 2(e)) make up the second type that are mainly related to dissolution. Intercrystal pores (Fig. 2(f)) as the last type, are caused by early dolomitization. In contrast, the fractured reservoir spaces such as fractures (Fig. 2(g)) and stylolites are relatively less developed here.

2.2.2 Reservoir diagenesis contribution

Combined with core and thin section photomicrographs observations, Adobe Photoshop Quantification can be used to quantifying the rock textural data and porosities [38]. Proportion of different pore types was especially quantified to speculate the reservoir diagenesis contribution (Fig. 3).

Fig. 1 Distribution of sedimentary facies (a) and lithologic section diagram (b) of Changxing and Feixianguan Formation in the northeastern Sichuan Basin (modified from Ref.[39])

Fig. 2 Reservoir pore type and diagenetic features in northeastern Sichuan Basin:

Intercrystal pore, as a main pore type in the study area, is closely related to the early dolomitization. Moldic pores, intragranular pores and biological cavity pores which occupy a considerable proportion were mainly formed by the meteoric selective dissolution. Statistics showed that the porosity created by the eogenetic diagenesis account for about half and 90% in Puguang and Yuanba gas field respectively. Burial dissolution and tectonic action have a minor influence here especially in Yuanba gas field. In summary, eogenetic diagenesis exert a fundamental control on reservoir properties and eogenetic pores is the foundation for reservoir formation. Eogenetic diagenesis induced pores are very important for reservoir development. But how can the eogenetic pores could have survived burial and diagenesis for millions of years is another problem that merits further investigation.

3 Samples and experimental method

3.1 Sample description

In order to simulate the actual reservoir carbonate- fluid interaction and better reflect the dissolution difference between calcite and dolomite, four types of samples were used in our experiments: natural calcite/ dolomite minerals, limestone and dolostone. All samples were obtained from the drilling cuttings of the P₂c and T₁f formations in Yuanba gas field, NE Sichuan Basin, China. These samples were characterized structurally by X-ray (l=0.6199 ) powder diffraction (Fig. 4) and chemically by JXA-8100 electron microprobe (Table 1).

Saturated CO₂ (0.033 mol/L with pH=5.6) and H₂S (0.1 mol/L with pH=3.9) solutions at ambient temperature and pressure were prepared by the following reactions:0.1 mol/L HCl (pH=1) and 0.2 mol/L CH₃COOH (pH=2.5) acid solutions were also prepared for the carbonate-fluid reaction.

Fig. 3 Reservoir diagenesis contribution in northeastern Sichuan Basin (modified from Ref. [40]):

Fig. 4 XRD patterns of calcite/dolomite minerals:

Table 1 Chemical composition of samples from Yuanba gas field, NE Sichuan Basin, China

BaCO₃+H₂SO₄=BaSO₄+H₂O+CO₂

Na₂S+2HCl=2NaCl+H₂S

3.2 Experimental device

HDAC is a remarkable device for making observations in situ on carbonate-fluid interaction under various temperatures and pressures [33, 35]. The general setup of HDAC is described in Fig. 5(a), two opposing 1/4 carat diamonds were mounted on tungsten carbide seats surrounded by a ceramic heater. A Rhenium gasket with initial thickness of ~0.25 mm (9.84 μinch) was placed between two diamonds with culet size of 1.5 mm (59.06 μinch) (Fig. 5(b)). At the center of the gasket, a ~0.5 mm hole was drilled by laser perforating device, serving as the sample chamber. K-type thermocouple directly attached to the anvils was used to determine the temperature of the chamber with accuracy of ±0.5 ^oC.

The spectra were acquired by a Renishaw 1000 Raman spectrometer with 30 s long working objective,with a 100 mW 514.5 nm Ar⁺-ion laser detecting the samples. The accuracy of the spectrometer is ±1 cm^-1.

Fig. 5 Schematic diagram of hydrothermal diamond anvil cell and experimental set-up (a), parameters of diamonds and magnified picture of sample chamber (b) and a real picture of a conventional DAC (c)

3.3 Experimental procedures

Before loading the sample, a ceramic heater was cemented around the diamond anvils. A small piece of quartz was first loaded into the chamber as the pressure calibration, and then a piece of sample was placed in the hole of the gasket. The size of the sample was ~150 μm (5.91 μinch) in diameter and ~100 μm (3.94 μinch) in thickness. These samples were rectangular fragments selected from crushed plates of calcite and dolomite single crystals. A drop of the acid solution was attached to the hole, and bubbles were specially excluded to avoid air. The two diamonds were drawn together by four screws and guided by four rods. Turning the screws immediately after the injection of the acid solution drove the diamond anvils together, creating a segregate system for the carbonate-fluid interaction. The experiments could be observed along the pressure axis through the transparent diamond anvils. Unlike conventional DACs (Fig. 5(c)), pressure was generated by increasing temperature in HDAC.

The pressure in HDAC was calculated by changes in frequency of the 464 cm^-1 Raman peak of the quartz using the following equations [41]:

P=0.36079×[(△V_P)₄₆₄]²+110.86×(△V_P)₄₆₄                 (1)

(△V_P)_464,P=0.1MPa=2.50136×10^-11×T⁴+1.46454×

10^-8×T³-1.801×10^-5×T²-0.01216×T+0.29,

0≤(△V_P)₄₆₄≤20                            (2)

The calculations are valid for -196 ^oC≤T≤560 ^oC and P<2.0 GPa with accuracy of ±50 MPa.

Before the experiments, the sample chamber was sealed by applying a small initial pressure to the gasket. Then, pressure was allowed to increase as the sample was heated [42]. Given a normal geothermal gradient of 30 ^oC/km in the northeastern Sichuan Basin [43], the sample chamber was heated from ambient temperature to 260 ^oC, representing burial depth from the near surface to maximum of 8 km (Table 2).

Table 2 Temperatures and corresponding burial depth at different experimental points

In the experiments, the HCO₃^-/HS^- content in aqueous solutions is proportional to the dissolution content of carbonate as described by the overall reaction:

CaCO₃+CO₂+H₂OCa²⁺+2HCO₃^-                 (3)

CaMg(CO₃)₂+2CO₂+2H₂OCa²⁺+Mg²⁺+4HCO₃^-   (4)

CaCO₃+H₂SCa²⁺+HCO₃^-+HS^-                (5)

CaMg(CO₃)₂+2H₂SCa²⁺+Mg²⁺+2HCO₃^-+2HS^-  (6)

The dissolution/precipitation tendency of carbonate could be determined by the change of HCO₃^-/HS^- concentration in aqueous solution. Many experiments have proved that the solute concentrations of solutions can be determined by Raman intensity radio [44, 45]. For aqueous solutions, it is reasonable to treat the Raman OH stretching band of water as an internal standard [46]. In this work, the intensity radio and which were proportional to the dissolution content of carbonate were used to determine the change of HCO₃^- and HS^- concentrations in solutions. and are the intensity of the band and HS^- band, respectively, and I_Wis the sum of intensities of the two used OH stretching sub-bands. This semi-quantification method was used to determine the dissolution/ precipitation tendency of carbonate based on the relative intensity ratio of and .

4 Results

In situ microscopic images of carbonate-CO₂-H₂O interaction (Fig. 6), carbonate-H₂S-H₂O interaction (Fig. 7), carbonate-HCl-H₂O interaction (Fig. 8) and carbonate-CH₃COOH-H₂O interaction (Fig. 9) were taken under different temperatures and pressures. By comparison of images, a minor continuous precipitation with increasing temperatures and pressures was observed in arrow positions for both minerals and rocks especially for CO₂ (Fig. 6) and H₂S (Fig. 7) systems. While the precipitation phenomenon of the samples in HCl and CH₃COOH systems is indistinct through naked eyes. For CO₂ and H₂S systems, carbonate-fluid interaction has a greater impact on minerals than rocks.

At each experimental point, the sample chamber temperature was maintained for over half an hour to make sure the carbonate-fluid reaction reached chemical equilibrium. In situ microscopic images of each experiment present a closed chemical equilibrium system. The observed precipitation content caused by the combined influence of temperature and pressure is minor. Semi-quantification Raman spectra data (Fig. 10) of each experimental point demonstrates a decreasing intensity ratio of and for each acid solution (Fig. 11), representing a reduced concentration with the increase of temperature and pressure. The diminution of concentration means carbonate trends to precipitate rather than dissolution during the experiment.

The carbonate-fluid reaction was sealed in a closed system under the combined influence of temperature and pressure. Retrograde solubility of carbonate makes temperature be an adverse factor for carbonate dissolution. On the other hand, increasing pressure results in more acid gas dissolving in solution, making the carbonate dissolution be easier. At last, a minor continuous precipitation with increasing temperatures and pressures was revealed for both minerals and rocks, carbonate tends to precipitate rather than dissolution. Considering the higher experimental pressure gradient (30-80 MPa/km) relative to normal geothermal gradient in the northeastern Sichuan Basin (about 10 MPa/km) [47], the precipitation tendency of carbonate as a result of the increase of burial depth should be more obvious in real geological conditions. Compared with the sample volume, the precipitation content is minor, the volume change due to the precipitation can be negligible because the supply of the reaction material is very limited in a closed system.

The closed system designed in the in-situ simulation experiments is a significant premise for this work. In the underground, the depth limits of regional groundwater flow systems change within 2-3 km according to the different geological conditions [48, 49]. After burying to this depth when mechanical compaction has mainly ceased, burial diagenesis may represent geochemically closed systems just like the experiments where mineral dissolution and cementation must be balanced [50]. Regardless whether it is saturated or unsaturated with carbonate, the pore water will quickly come to chemical equilibrium with surrounding carbonate rocks. Only when the burial depth (pressure and temperature) has changed, the chemical equilibrium of the reaction can move slightly. Because of the limited flow rate of the saturated pore water, minor dissolution/precipitation may happen, which has a net implication on the porosity. In other words, carbonate tends to precipitate with the increase of the burial depth, but the amount of precipitation is negligible comparing to the host rock. The closed system is a preservation of the early-formed pores.

Fig. 6 In situ microscopic images taken at different experimental points (a) and a sketch of precipitation phenomenon in carbonate-CO₂-H₂O system (C-calcite, L-limestone, D-dolomite/dolostone, Q-quartz) (b)

Fig. 7 In situ microscopic images taken at different experimental points (a) and a sketch of precipitation phenomenon in carbonate-H₂S-H₂O system (C-calcite, L-limestone, D-dolomite/dolostone, Q-quartz) (b)

Fig. 8 In situ microscopic images taken at different experimental points in carbonate-HCl-H₂O system (C-calcite, L-limestone, D-dolomite/dolostone, Q-quartz)

Fig. 9 In situ microscopic images taken at different experimental points in carbonate-CH₃COOH-H₂O system (C—calcite, L—limestone, D—dolomite/dolostone, Q-quartz)

Fig. 10 Typical Raman spectrum in different carbonate-fluid systems

5 Constructive diagenesis and origin of deep carbonate reservoir in northeastern Sichuan Basin

The experimental results illustrate the pores preservation in a closed system. But the development and preservation process of the reservoir space is very complicated that major constructive diagenetic processes must be identified to understand it.

Fig. 11 Variation tendency of HCO₃^-/HS^- and H₂O ratios in fluid at different experimental points (Note: bimodal fitting of Raman spectra of H₂O):

5.1 Constructive diagenesis

5.1.1 Near surface to shallow diagenesis

Favorable sedimentary facies belts not only control the distribution of primary pores, but also affect the diagenesis after deposition. Although primary pores suffered great loss due to the compaction and cementation, the residual primary pores can be well developed in shallow high-energy sedimentary environment [37]. In the early diagenesis, due to the relative sea level fluctuations, early meteoric dissolution may change the sediments, resulting in increased porosity. Many eogenetic dissolution pores were created in the open system and became a foundation for reservoir development. Moldic pores with geopetal structure (Fig. 2(h)) were also observed as a characterization of meteoric dissolution. A large scale of early dolomitization also occurred before the hydrocarbon emplacement (Fig. 2(i)) [39, 51] which improved the rock compressive resistance [52], so the pores formed before dolomitization were well preserved.

5.1.2 Early fast deep burial and hydrocarbon filling

A basic conclusion in the in-situ simulation experiments is presented by the preservation of the pores in a closed system. That need the reservoirs change from the shallow open system into the deep closed environment as soon as possible. Based on previous research on tectonic and thermal history in the northeastern Sichuan Basin, burial history and hydrocarbon filling time of P₂c and T₁f in the study area was reconstructed (Fig. 12). Reservoirs experienced a rapid subsidence to a depth of about 2500 m (8202 ft) in a very short time, which made the reservoirs remain partially or completely closed shortly after the deposition. Primary and eogenetic secondary pores were well preserved due to the early fast deep burial, minimizing the time for compaction and cementation.

Because of the early fast deep burial, the reservoirs were buried to about 3000 m with 90 ^oC at the late Triassic. At this moment, the source rocks reached the hydrocarbon generation threshold [37] and the closed system was broken. Two periods of hydrocarbon emplacement happened from the late Triassic to the early Jurassic and in the middle-late Jurassic respectively [53], slightly after the end of the early rapid subsidence. The preserved eogenetic pores provided the main reservoir space for hydrocarbon emplacement. And a good matching between the end of the fast deep burial and the hydrocarbon emplacement is vital to oil and gas accumulation.

Fig. 12 Burial history and hydrocarbon filling time of P₂c and T₁f formations in northeastern Sichuan Basin (modified from Ref. [54])

5.1.3 Deep burial diagenesis

Organic acid and thermochemical sulfate reduction induced corrosion has long been recognized as a process of geologic significance in deep burial stage. However, the effect of burial dissolution on P₂c and T₁f porosity has not been well documented and remains controversial [21]. Great attention has been paid to believe that burial dissolution is an important process leading to intensive carbonate dissolution and significant porosity creation [54]. Others hold the idea that TSR-related calcite cementation reduced the porosity and TSR has an overall destructive effect on P₂c and T₁f [21]. Wells with little H₂S gas indicating weak TSR were also drilled in the study area [55]. Research on diagenesis contribution show that TSR has a negligible influence on the total porosity despite local minor dissolution happened here.

5.2 Preservation mechanism of eogenetic pores in northeastern Sichuan Basin

5.2.1 Pore evolution

In order to reflect the fundamental control of eogenetic diagenesis on the deep reservoir origin, intragranular pore which is a significant pore type is especially analyzed to see the pore evolution (Fig. 13). In the syngenetic stage, the porous carbonate sediments just deposited and the original porosity of oolitic limestone was about 40% [56]. The ooids were lithified by the marine cements slightly after deposition. During the exposure time, meteoric dissolution and cementation caused increased secondary dissolution porosity and reduced primary porosity. In the shallow burial stage, compaction and cementation were dominant and generally greatly destructive to reservoir quality. Accompanying this has been the largely decrease of the primary porosity. Meanwhile the continuous meteoric dissolution resulted in the continuous increase of early secondary dissolution porosity. Then, the early fast deep burial made the reservoir reach the deep burial stage swiftly and the reservoir gradually changed from the shallow open system into a closed environment. Then, a late precipitation happened in the intragranular pores, which was too minor to cause a porosity change, just like the experimental results. The minerals precipitated in the closed system were not only small but also euhedral in appearance, which represented that the minerals suffered no burial dissolution. Finally, the preserved primary and early secondary dissolution pores contributed the main reservoir space and provided the foundation for reservoir performance.

Fig. 13 Sketch picture of pore evolution and origin of deep carbonate reservoirs

5.2.2 Origins of deep carbonate reservoirs

Using the porosity-depth data, a typical porosity evolution curve and burial stages of the dolomite reservoirs was given (Fig. 14). The porosity was quantified by Adobe Photoshop Analysis and an initial porosity of 40% of was assumed for the dolomite reservoir in the northeastern Sichuan Basin [57]. Deep carbonate reservoirs properties were characterized by porosities of 276 dolomite samples with burial depth from 6200-7100 m (20341-23294 ft). And average porosity variation with depth for carbonate reservoirs all over the world was also presented [19].

Normally porosity decreases rapidly with depth in the near-surface condition for both curves. This porosity reduction is mainly caused by the combined influence of compaction and cementation. After burying to about 2500 m (8202 ft), the carbonates get into a closed system and the early-formed porosity is preserved to make the reservoirs in the northeastern Sichuan Basin display a higher porosity. The difference between the two porosity evolution curves is due to the porosity preservation in closed conditions. The shaded part is a focus of deep oil and gas exploration. During the deep burial stage, porosity will remain nearly constant and there is a threshold depth of porosity. Deeper than the porosity keep line, few pores will be created or cemented. If the carbonate reservoir is regarded as a large closed system, it is possible that a part of solid will be dissolved. At the same time, the by-products will precipitate in some other places and occupy the same volume as the dissolved part. The pore space is not increased, but just a redistribution of pores [58]. Closed-system dissolution simply exchanges primary for secondary porosity, but gives no direct improvement in reservoir quality [59]. Early fast deep burial facilitates the process of reservoir getting into a closed system and the reservoir reaches the porosity keep line in a short time. Then, the pores created in the early diagenetic stages can be preserved well and make great contribution to the porosity generation. From the near-surface to the reservoir reach the porosity keep line is the main pore formation period, while below the line is the main period for porosity preservation and redistribution.

Fig. 14 Average porosity variation with depth for worldwide carbonate reservoirs [19] and typical porosity evolution curve and main diagenesis aspects of dolomite reservoirs in northeastern Sichuan Basin (The difference between the two porosity evolution curves in deep burial stage is a result of porosity preservation in a closed system. The circles represent the measured porosities in P₂c and T₁f formations)

In general, closed system may preserve the pores created in the early diagenetic stages and be of great significance for oil accumulation. The high quality carbonate reservoirs in P₂c and T₁f formations are integrate results controlled by several factors including durable meteoric leaching, early fast deep burial, early dolomitization, etc. This deep pores preservation model can be helpful in assessing risk on reservoir quality evaluation and provide a different idea to the exploration of deep carbonate reservoirs in the northeastern Sichuan Basin.

6 Conclusions

1) The results of the in-situ simulation experiments display a minor continuous precipitation with increasing temperatures and pressures in a closed system. The precipitation content is too minor to cause a porosity change and the closed system is a preservation of the pores. Porosity enhancement as a result of burial dissolution may only be possible under the influence of fractures and hydrothermal activities.

2) Eogenetic moldic and dissolution pores are the principal pore types for the Changxing and Feixianguan Formations. All these pores are eogenetic pores formed in near-surface to shallow burial stage before hydrocarbon emplacement. The diagenetic process of P₂c and T₁f formations was characterized by the early fast deep burial and the reservoirs were in a closed system shortly after the deposition. The high quality reservoirs might originate from the eogenetic pores preservation.

3) In the northeastern Sichuan Basin, favorable sedimentary facies belts and durable meteoric leaching resulted in the development of abundant primary and eogenetic secondary pores. In addition, early fast deep burial, weak compaction and cementation and early dolomitization preserved the early-formed porosity well, which played an extremely important role in porosity evolution. These pores can be open again during the migration of oil and gas filling and the pore fluid can be replaced by natural gas. So, the gas reservoirs in the northeastern Sichuan Basin are not considered to be the results of burial dissolution at least in the reservoirs which develop massive primary pores.

References

[1] MA Yong-sheng, GUO Xu-sheng, GUO Tong-lou, HUANG Rui, CAI Xun-yu, LI Guo-xiong. The Puguang gas field. New giant discovery in the mature Sichuan Basin, southwest China [J]. AAPG Bulletin, 2007, 91(5): 627-643.

[2] MA Yong-sheng, ZHANG Shui-chang, GUO Tong-lou, ZHU Guang-you, CAI Xun-yu, LI Mao-wen. Petroleum geology of the Puguang sour gas field in the Sichuan basin, SW China [J]. Marine and Petroleum Geology, 2008, 25(4): 357-370.

[3] HU Ming-yi, HU Zhong-gui, QIU Xiao-song, ZHAO En-zhang, WANG Dan. Platform edge reef and bank structure and depositional model of Changxing formation in Panlongdong section, Xuanhan, northeastern Sichuan [J]. Journal of Earth Science, 2012, 23(4): 431-441.

[4] CHEN Lei, LU Yong-chao, GUO Tong-lou, XING Feng-cun, JIAO Yang-quan. Seismic sedimentology study in the high-resolution sequence framework—A case study of platform margin reef-beach system of Changxing formation, Upper Permian, Yuanba Area, Northeast Sichuan Basin, China [J]. Journal of Earth Science, 2012, 23(4): 612-626.

[5] HAO Fang, GUO Tong-lou, ZHU Yang-ming, CAI Xun-yu, ZOU Hua-yao, LI Ping-ping. Evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the Puguang gas field, Sichuan Basin, China [J]. AAPG Bulletin, 2008, 92(5): 611-637.

[6] LI Ping-ping, HAO Fang, GUO Xu-sheng, ZOU Hua-yao, YU Xin-ya, WANG Guang-wei. Processes involved in the origin and accumulation of hydrocarbon gases in the Yuanba gas field, Sichuan Basin, southwest China [J]. Marine and Petroleum Geology, 2015, 59: 150-165.

[7] JAMES N P, CHOQUETTE P W. Diagenesis 9. Limestones-the meteoric diagenetic environment [J]. Geoscience Canada, 1984, 11: 161-194.

[8] EHRENBERG S N. Factors controlling porosity in Upper Carboniferous-Lower Permian carbonate strata of the Barents Sea [J]. AAPG Bulletin, 2004, 88(12): 1653-1676.

[9] AJDUKJEWICZ J M, NICHOLSON P H, ESCH W L. Prediction of deep reservoir quality using early diagenetic process models in the Jurassic Norphlet Formation, Gulf of Mexico [J]. AAPG Bulletin, 2010, 94(8): 1189-1227.

[10] MAZZULLO S J, HARRIS P M. Mesogenetic dissolution: its role in porosity development in carbonate reservoirs (1) [J]. AAPG Bulletin, 1992, 76(5): 607-620.

[11] LAMBERT L, DURLET C, LOREAU J P, MARNIER G. Burial dissolution of micrite in Middle East carbonate reservoirs (Jurassic–Cretaceous): Keys for recognition and timing [J]. Marine and Petroleum Geology, 2006, 23(1): 79-92.

[12] WIERZBICKI R, DRAVIS J J, AL-AASM I, HARLAND N. Burial dolomitization and dissolution of Upper Jurassic Abenaki platform carbonates, Deep Panuke reservoir, Nova Scotia, Canada [J]. AAPG Bulletin, 2006, 90(11): 1843-1861.

[13] DUTTON S P, LOUCKS R G. Reprint of: Diagenetic controls on evolution of porosity and permeability in lower Tertiary Wilcox sandstones from shallow to ultradeep (200–6700 m) burial, Gulf of Mexico Basin, USA [J]. Marine and Petroleum Geology, 2010, 27(8): 1775-1787.

[14] ZAMPETTI V. Controlling factors of a Miocene carbonate platform: implications for platform architecture and off-platform reservoirs (Luconia Province, Malaysia). Cenozoic Carbonate Systems of Australasia [J]. SEPM, Special Publication, 2010, 95: 129-145.

[15] TAYLOR T R, GILES M R, HATHON L A, DIGGS T N, BRAUNSDORF N R, BIRBIGLIA G V, KITTRIDGE M G, MACAULAY C I, ESPEJO I S. Sandstone diagenesis and reservoir quality prediction: Models, myths, and reality [J]. AAPG Bulletin, 2010, 94(8): 1093-1132.

[16] K. Open-system chemical behavior of Wilcox Group mudstones. How is large scale mass transfer at great burial depth in sedimentary basins possible? A discussion [J]. Marine and Petroleum Geology, 2011, 28(7): 1381-1382.

[17] K. Relationships between depositional environments, burial history and rock properties. Some principal aspects of diagenetic process in sedimentary basins [J]. Sedimentary Geology, 2014, 301: 1-14.

[18] K, JAHREN J. Open or closed geochemical systems during diagenesis in sedimentary basins: Constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs [J]. AAPG Bulletin, 2012, 96(12): 2193-2214.

[19] EHRENBERG S N, WALDERHAUG O, K. Carbonate porosity creation by mesogenetic dissolution: Reality or illusion? [J]. AAPG Bulletin, 2012, 96(2): 217-233.

[20] EHRENBERG S N, WALDERHAUG O, K. Carbonate porosity creation by mesogenetic dissolution: Reality or illusion? Reply [J]. AAPG Bulletin, 2013, 97(2): 347-349.

[21] HAO Fang, ZHANG Xue-feng, WANG Cun-wu, LI Ping-Ping, GUO Tong-lou, ZOU Hua-yao, ZHU Yang-ming, LIU Jian-zhang, CAI Zhong-xian. The fate of CO₂ derived from thermochemical sulfate reduction (TSR) and effect of TSR on carbonate porosity and permeability, Sichuan Basin, China [J]. Earth-Science Reviews, 2015, 141: 154-177.

[22] TESTEMALE D, DUFAUD F, MARTINEZ I, P, HAZEMANN J L, SCHOTT J, GUYOT F. An X-ray absorption study of the dissolution of siderite at 300 bar between 50 °C and 100 °C [J]. Chemical Geology, 2009, 259(1): 8-16.

[23] ALKATTAN M, OELKERS E H, DANDURAND J L, SCHOTT J. An experimental study of calcite and limestone dissolution rates as a function of pH from -1 to 3 and temperature from 25 to 80 °C [J]. Chemical Geology, 1998, 151(1): 199-214.

[24] ALKATTAN M, OELKERS E H, DANDURAND J L, SCHOTT J. An experimental study of calcite dissolution rates at acidic conditions and 25 °C in the presence of NaPO₃ and MgCl₂ [J]. Chemical Geology, 2002, 190(1): 291-302.

[25] GAUTELIER M, OELKERS E H, SCHOTT J. An experimental study of dolomite dissolution rates as a function of pH from -0.5 to 5 and temperature from 25 to 80 °C [J]. Chemical Geology, 1999, 157(1): 13-26.

[26] SHIH S M, LIN J P, SHIAU G Y. Dissolution rates of limestones of different sources [J]. Journal of Hazardous Materials, 2000, 79(1): 159-171.

[27] POKROVSKY O S, GOLUBEV S V, SCHOTT J. Dissolution kinetics of calcite, dolomite and magnesite at 25 °C and 0 to 50 atm PCO₂ [J]. Chemical Geology, 2005, 217(3): 239-255.

[28] POKROVSKY O S, GOLUBEV S V, SCHOTT J, CASTILLO A. Calcite, dolomite and magnesite dissolution kinetics in aqueous solutions at acid to circumneutral pH, 25 to 150 °C and 1 to 55 atm PCO₂: New constraints on CO₂ sequestration in sedimentary basins [J]. Chemical Geology, 2009, 265(1): 20-32.

[29] STERPENICH J, SAUSSE J, PIRONON J, A, HUBERT G, PERFETTI E, GRGIC D. Experimental ageing of oolitic limestones under CO₂ storage conditions: Petrographical and chemical evidence [J]. Chemical Geology, 2009, 265(1): 99-112.

[30] SCHMOKER J W, HALLEY R B. Carbonate porosity versus depth: a predictable relation for south Florida [J]. AAPG Bulletin, 1982, 66(12): 2561-2570.

[31] JIN Zhu-jun, ZHU Dong-ya, HU Wen-xuan, ZHANG Xue-feng, ZHANG Jun-tao, SONG Yu-cai. Mesogenetic dissolution of the middle Ordovician limestone in the Tahe oilfield of Tarim basin, NW China [J]. Marine and Petroleum Geology, 2009, 26(6): 753-763.

[32] BASSETT W A, SHEN A H, BUCKNUM M, CHOU I M. A new diamond anvil cell for hydrothermal studies to 2.5 GPa and from -190 to 1200 °C [J]. Review of Scientific Instruments, 1993, 64(8): 2340-2345.

[33] CHOU I, ANDERSON A J. Diamond dissolution and the production of methane and other carbon-bearing species in hydrothermal diamond-anvil cells [J]. Geochimica et Cosmochimica Acta, 2009, 73(20): 6360-6366.

[34] SOWERBY J R, KEPPLER H. The effect of fluorine, boron and excess sodium on the critical curve in the albite-H₂O system [J]. Contributions to Mineralogy and Petrology, 2002, 143(1): 32-37.

[35] PRESSER V, HEIβ M, NICKEL K G. EOS calculations for hydrothermal diamond anvil cell operation [J]. Review of Scientific Instruments, 2008, 79: 085104.

[36] GUO Tong-lou. Reservoir characteristics and its controlling factors of the Changxing Formation reservoir in the Yuanba gas field, Sichuan basin, China [J]. Acta Petrologica Sinica, 2011, 27(8): 2381-2391.

[37] MA Yong-sheng, GUO Tong-lou, ZHAO Xue-feng, CAI Xun-yu. The formation mechanism of high-quality dolomite reservoir in the deep of Puguang Gas Field [J]. Science in China Series D: Earth Sciences, 2008, 51(1): 53-64. (in Chinese)

[38] ZHANG Xue-feng, LIU Bo, WANG Jie-qiong, ZHANG Zhe, SHI Kai-bo, WU Shuang-lin. Adobe photoshop quantification (PSQ) rather than point-counting: A rapid and precise method for quantifying rock textural data and porosities [J]. Computers & Geosciences, 2014, 69: 62-71.

[39] TIAN Yong-jing, MA Yong-sheng, LIU Bo, ZHANG Xue-feng, LIU Jian-qiang, SHI Kai-bo, WU Shuang-lin. Dolomitization of the Upper Permian Changxing Formation in Yuanba gas field，NE Sichuan Basin, China [J]. Acta Petrologica Sinica, 2014, 30(9): 2766-2776. (in Chinese)

[40] MA Yong-sheng, CAI Xun-yu, ZHAO Pei-rong. Characteristics and formation mechanism of reef-shoal carbonate reservoirs of Changxing-Feixianguan formations, Yuanba gas field [J]. Acta Petrolei Sinica, 2014, 35: 1001-1011.

[41] SCHMIDT C, ZIEMANN M A. In-situ Raman spectroscopy of quartz: a pressure sensor for hydrothermal diamond-anvil cell experiments at elevated temperatures [J]. American Mineralogist, 2000, 85(11, 12): 1725-1734.

[42] BASSETT W A, ANDERSON A J, MAYANOVIC R A, CHOU I M. Hydrothermal diamond anvil cell for XAFS studies of first-row transition elements in aqueous solution up to supercritical conditions [J]. Chemical Geology, 2000, 167(1): 3-10.

[43] WANG Y F, XIAO X M. An investigation of paleogeothermal gradients in the northeastern part of Sichuan Basin [J]. Marine Origin Petroleum Geology, 2010, 15: 57-61 (in Chinese).

[44] THOMAS R. Determination of the H₃BO₃ concentration in fluid and melt inclusions in granite pegmatites by laser Raman microprobe spectroscopy [J]. American Mineralogist, 2002, 87(1): 56-68.

[45] AZBEJ T, SEVERS M J, RUSK B G, BODNAR R J. In situ quantitative analysis of individual H₂O-CO₂ fluid inclusions by laser Raman spectroscopy [J]. Chemical Geology, 2007, 237(3): 255-263.

[46] SUN Q, QIN C J. Raman OH stretching band of water as an internal standard to determine carbonate concentrations [J]. Chemical Geology, 2011, 283(3): 274-278.

[47] ZHAO Wen-zhi, WANG Ze-cheng, WANG Yi-gang. Formation Mechanism of Highly Effective Gas Pools in the Feixianguan Formation in the NE Sichuan Basin [J]. Geological Review, 2006, 52(5): 708-718. (in Chinese)

[48] FORSTER C, SMITH L. The influence of groundwater flow on thermal regimes in mountainous terrain: A model study [J]. Journal of Geophysical Research. Solid Earth, 1989, 94(7): 9439-9451.

[49] MACHEL H G. Effects of groundwater flow on mineral diagenesis, with emphasis on carbonate aquifers [J]. Hydrogeology Journal, 1999, 7(1): 94-107.

[50] SAFARICZ M, DAVISON I. Pressure solution in chalk [J]. AAPG Bulletin, 2005, 89(3): 383-401.

[51] CHEN Pei-yuan, TAN Xiu-cheng, LIU Hong, MA Teng, LUO Bing, JIANG Xing-fu, YU Yang, JIN Xiu-ju. Formation mechanism of reservoir oolitic dolomite in Lower Triassic Feixianguan formation, northeastern Sichuan Basin, southwest China [J]. Journal of Central South University, 2014, 21(8): 3263-3274.

[52] TAN Xiu-cheng, ZHAO Lu-zi, LUO Bing, JIANG Xing-fu, CAO Jian, LIU Hong, LI Ling, WU Xing-bo, NIE Yong. Comparison of basic features and origins of oolitic shoal reservoirs between carbonate platform interior and platform margin locations in the Lower Triassic Feixianguan Formation of the Sichuan Basin, southwest China [J]. Petroleum Science, 2012, 9(4): 417-428.

[53] LI Jian, XIE Zeng-ye, DAI Jin-xing, ZHANG Shui-chang, ZHU Guang-you, LIU Zhao-lu. Geochemistry and origin of sour gas accumulations in the northeastern Sichuan Basin, SW China [J]. Organic Geochemistry, 2005, 36(12): 1703-1716.

[54] CAI Chun-fang, HE Wen-xian, JIANG Lei, LI Kai-kai, XIANG Lei, JIA Lian-qi. Petrological and geochemical constraints on porosity difference between Lower Triassic sour- and sweet-gas carbonate reservoirs in the Sichuan Basin [J]. Marine and Petroleum Geology, 2014, 56: 34-50.

[55] XIA Ming-jun, DENG Rui-jian, JIANG Yi-wei, BI Jian-xia, JIN Xiu-ju, GUO Hai-xia, WAN Xiao-wei. Exposition on material base and preservation causes of oolitic beach reservoir in Puguang Gas Field [J]. Fault-Block Oil & Gas Field, 2009, 6: 004. (in Chinese)

[56] HEYDAR E. Porosity loss, fluid flow, and mass transfer in Limestone Reservoirs: Application to the Upper Jurassic Smackover Formation, Mississippi1 [J]. AAPG Bulletin, 2000, 84(1): 100-118.

[57] ZHANG Xue-feng, GUO Tong-lou, LIU Bo, FU Xiao-yue, WU Shuang-lin. Porosity formation and evolution of the deeply buried lower triassic feixianguan formation, Puguang Gas Field, NE Sichuan Basin, China [J]. Open Journal of Geology, 2013, 3: 300-312.

[58] GILES M R, de BOER R B. Origin and significance of redistributional secondary porosity [J]. Marine and Petroleum Geology, 1990, 7(4): 378-397.

[59] GILES M R. Mass transfer and problems of secondary porosity creation in deeply buried hydrocarbon reservoirs [J]. Marine and Petroleum Geology, 1987, 4(3): 188-204.

(Edited by DENG Lü-xiang)

Cite this article as: ZHANG Shan-ming, LIU Bo, QIN Shan, ZHANG Xue-feng, TIAN Yong-jing, GUO Rong-tao, LIU Jian-qiang. Origin of deep carbonate reservoir in northeastern Sichuan Basin: New insights from in-situ hydrothermal diamond anvil cell experiments [J]. Journal of Central South University, 2017, 24(6): 1450-1464. DOI: 10.1007/s11771-017-3549-y.

Foundation item: Project(2011ZX05005-003-010HZ) supported by the National Science and Technology Major Project, China; Projects(41272137, 41002029) supported by the National Natural Science Foundation of China

Received date: 2015-11-09; Accepted date: 2016-03-17

Corresponding author: QIN Shan, Professor; Tel: +86-10-62751166; E-mail: sqin@pku.edu.cn