J. Cent. South Univ. (2016) 23: 1439-1448

DOI: 10.1007/s11771-016-3196-8

Sequence of densification and hydrocarbon charging of Xu2 reservoir in Anyue–Hechuan area, Sichuan Basin, China

CHEN Cong(陈聪), XU Guo-sheng(徐国盛), XU Fang-hao(徐昉昊), YUAN Hai-feng(袁海锋), CHEN Fei(陈飞)

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of

Technology), Chengdu 610059, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: The sequence of the densification and hydrocarbon charging of the Xu2 reservoir in the Anyue–Hechuan area of Central Sichuan Basin is discussed. The diagenetic sequence is considered a time line to determine the historical relationship between the densification process and the hydrocarbon charging of the Xu2 reservoir in the study area: Early diagenetic stage B (the first stage of hydrocarbon charging, which was about 200–160 Ma ago, with a porosity of about 20%, consolidated and not tight) → middle diagenetic stage A (the second stage of hydrocarbon charging, which was about 140–120 Ma ago, with a porosity of 10%–20% and relatively tight) → middle diagenetic stage B (the third stage of hydrocarbon charging, which was about 20–5 Ma ago, with a porosity of 6%–10% and tight; However, fractures have developed). The study results prove that large-scale hydrocarbon charging and accumulation completed before the densification of the Xu2 reservoir, showing that the Upper Triassic Xujiahe Fm unconventional tight reservoir in the Sichuan Basin is prospective for exploration.

Key words: Anyue–Hechuan area; Xu2 member; diagenetic sequence; reservoir densification; hydrocarbon charging; inclusion

1 Introduction

Based on the third petroleum resource evaluation of China [1], the total natural gas resources of the Triassic reservoirs in the Sichuan Basin were 25901.5×10⁸ m³, accounting for 48.43% of the whole basin (53477.4×10⁸ m³). The proved geological reserves were 4111.7× 10⁸ m³, including 942.66×10⁸ m³ in the Upper Triassic Xujiahe Fm, with a proved rate of only 10.4%. Compared with the mature basins abroad, the Xujiahe Fm presents low resource discovery and proved rates, suggesting a tremendous and prospective exploration potential. Over the years, issues such as the precedence of densification and large-scale hydrocarbon charging as well as the large-scale gas accumulation of the Upper Triassic Xujiahe Fm reservoir in the Sichuan Basin have puzzled petroleum researchers. Using the Xu2 Member in the Anyue–Hechuan area of the Central Sichuan Basin as an example, a typical tight sandstone gas reservoir with a large potential for resources which has not been explored as often and is underdeveloped, has a strong reservoir heterogeneity, a complicated distribution and a hard prediction of gas-rich zones.

Issues such as densification and hydrocarbon charging of Xujiahe reservoir in the central Sichuan area have been studied respectively. ZHU [2] and ZHANG [3] believed that stages II, III quartz overgrew and other cement was the main reason for the sandstone reservoir densification. Based on the study on fluid inclusion homogenization temperature, ZHAO et al [4], TAO et al [5] and XIE et al [6] concluded that large-scale gas filling of the Xujiahe Fm reservoir in the central Sichuan Basin occured in late Jurassic. However, the research on the coupling relationship between reservoir densification and hydrocarbon filling stage of Xujiahe Fm reservoir in the central Sichuan area is still insufficient. Complicated diagenesis recorded the densification process and hydrocarbon charging stages of reservoirs and the relationship between the two can be analyzed directly through the study on the diagenetic sequence [7-9]. Based on the above theoretical basis, the reservoir’s diagenetic process is used to analyze the reservoir densification process in this work. Combined with the evolutionary features of hydrocarbon inclusion and its relationship with the formation time of diagenetic minerals, a correlation is established among the diagenetic sequence, reservoir densification and hydrocarbon charging to ascertain the historical relationship between the reservoir densification process and the hydrocarbon charging. From this, some questions can be answered. For example, had a great amount of natural gas entered the Xu2 sandstone reservoir in the Anyue–Hechuan area before the reservoir densified? If so, what was the gas migration path? How has the reservoir space changed? Was the palaeo–gas reservoir reformed? The answers to these questions can provide a scientific basis for further exploration of the Xujiahe tight gas reservoir in the Sichuan Basin.

2 Geology

The Anyue–Hechuan area is located in the Central–Southern Sichuan transitional zone, China. In the Anyue area, the Xujiahe reservoir is in the central part of the Zhongxie gentle tectonic zone of the Central Sichuan palaeohigh (Fig. 1), where small shallow highlands and gentle depressions with low closures are developed. In the Hechuan area, the Xujiahe reservoir is contained in the monoclinal structure at the west side of the Huaying Mountain along the Central–Southern Sichuan transitional zone (Fig. 1). The structure is gentle (Fig. 1, Fig. 2) and the fault has not developed.

The Xujiahe Fm, which is composed of alternative continental river–lake terrigenous clastic sediments, with a stable thickness of 448-571 m, can be divided into 6 members from bottom to top. The members Xu1, Xu3 and Xu5 are the major source beds and caprocks of the Xujiahe Fm. The members Xu2, Xu4 and Xu6 are the major reservoir beds of the Xujiahe Fm. In the Anyue–Hechuan area, the Xu2 Member has the best reservoir property and exploration prospects.

Fig. 1 Location of study area

Fig. 2 Well-tie profile of sampling wells in Anyue–Hechuan area (see Fig. 1 for profile position)

3 Diagenetic sequence and densification process of reservoirs

The diagenetic sequence and burial history are used to describe the densification process of the Xu2 reservoir in the study area. Based on the Petroleum Industry Standard of PRC (SY/T5477–2003), the Xu2 reservoir successively completed 5 stages from sedimentation to diagenesis: The penecontemporaneous stage, early diagenetic stage A, early diagenetic B, middle diagenetic stage A and middle diagenetic stage B. The specific diagenesis and physical property variation at each diagenetic stage is identified through the microanalysis and statistical analysis of a cast slice examined by a scanning electron microscope (SEM).

3.1 Penecontemporaneous stage and early diagenetic stage A

At the penecontemporaneous stage—early diagenetic stage A (260-240 Ma ago), the buried depth was 0–1000 m, the paleotemperature was 0-70 °C, the vitrinite reflectance was less than 0.35%, and the porosity was 30%–40%. Stage I calcite cementation occurred and the calcites were embedded in the clastic particles in an intergrowth shape (Fig. 3(a)). SHALABY et al [10] studied the Middle Jurassic Khatatba sandstone of the Shoushan Basin, Egypt, and they believed that this kind of intergrowth calcite was formed at an early diagenetic stage [10], and KHIDIR and CATUNEANU [11] thought that it was the principal cause for the large reduction of porosity at this stage. Based on the estimate of pores occupied by the intergrowth calcites, the porosity was no less than 30% from the penecontemporaneous stage to the early diagenetic stage A. During this period, the source rocks in the study area were immature, i.e., stage I calcite was formed prior to the hydrocarbon discharge of source rocks.

3.2 Early diagenetic stage B

At the early diagenetic stage B (240-210 Ma ago), the buried depth was 1000-2000 m, the paleotemperature was 70-90 °C, and the vitrinite reflectance was 0.35%–0.50%. At this stage, the cementation mainly exhibited as stage I, quartz overgrowth at one side of the quartz grains (Fig. 3(b)). The fluids in the formation were gradually discharged with the increase in buried depth and the particles contacted more compactly. Hence, the rock porosity dropped from 30% to about 20% [12].

3.3 Middle diagenetic stage A

At the middle diagenetic stage A (210-90 Ma ago), the buried depth was about 2000-3000 m, the paleotemperature was 90-150 °C, the vitrinite reflectance was 0.50%-1.30%, and the organic matter matured and began to heavily generate hydrocarbons. At stages II and III, quartz overgrowth filled the intergranular and dissolved pores, and the feldspar became heavily corroded (Fig. 3(c)). The primary and dissolved pore types dominated this period, and the porosity dropped from 20% to about 10% [12].

Superposed growth of stages II and I quartz overgrowth was often seen in the study area, and its outer boundary was usually corroded (Fig. 3(b)). WAZIR et al [13] believed that the priority of the two stages of overgrowth could be observed from the contact relationship with the superposed growth, namely, stage II quartz overgrowth occurred after early diagenetic stage B. ZHU et al [12] analyzed the tight genesis of the Upper Triassic Xujiahe Fm in the Sichuan Basin, with an average experimentally tested homogenization temperature of inclusion at 92.9 °C. They believed the stage II quartz overgrowth was formed at the middle diagenetic stage A. Combined with the above analyses, stage II quartz overgrowth in this study area was believed to be formed during the middle diagenetic stage A. Stage III quartz cementation occurred in the authigenic quartz among pores or inside the dissolved pores, with high idiomorphic levels (Fig. 3(d)). The cementation formed later than the previous two stages of quartz cementation, i.e. during the middle diagenetic stage A–middle diagenetic stage B.

3.4 Middle diagenetic stage B

At the middle diagenetic stage B (90-60 Ma ago), the formation continually subsided from 2000 m to about 4000 m, the paleotemperature ranged in temperature from 150 °C to 175 °C. 60 Ma ago, the formation began to uplift, from 4000 m to about 2000 m. After the formation uplifted, affected by the action of tectonic stress, local fractures began to occur, and the principal diagenesis was the late cementation of quartz and calcite.

Stage II calcite cementation generally occurred in the sparry calcite (ferrocalcite) filled in the clastic particles or inside the dissolved pores (Fig. 3(e), Fig. 3(f)). Referring to Fig. 3(e), the calcite replaced the quartz overgrowth. TAMAR–AGHA [14] believed that the calcite replacing the quartz was formed after the quartz overgrowth. ZHONG et al [15] suggested that the calcite was the product at the middle diagenetic stage. Based on the statistical analyses of about 300 micrographs of 33 cast slices and inclusion slices of the study area, formed prior to stage II calcite cementation, the reservoir porosity was not lower than 18%, and the porosity reduced by stage II calcite cementation was about 5%. LUO et al [16] studied the physical property variation of the Yanchang sandstone reservoir in the Ordos Basin, and discovered that the carbonate cementation at the middle diagenetic stage B reduced the reservoir porosity by about 20%, and it was one of the important causes for the tightness of the reservoir.

Fig. 3 Diagenetic phenomena of Xu2 reservoir in Anyue–Hechuan area:

Both stage III calcite (Fig. 3(g), Fig. 3(h)) and stage IV quartz (Fig. 3(i)) are cements filled in the fractures and vugs at the late stage. A study on their developmental stages can help to further confirm the fracture developmental stages, which are necessary to analyze the hydrocarbon charging stages. In this work, the quartz veins filled in the fractures of the Xu2 reservoir in Well Yue 113 of the Anyue gas field are selected for ESR dating (sampling location is shown in Figs. 1 and 2 and the test method, instrument, conditions and range of error are listed in Table 1). The test results (Table 1) show that fracture development occurred in this area at two stages (20 Ma ago and 6 Ma ago) or more. BAI et al [17-18] used the quartz ESR dating method to study the fracture developmental stages of the Xu2 reservoirs in the Jiulongshan and Southwestern Sichuan Basin areas, and discovered fractures developed in the Xu2 reservoirs about 40 Ma and 66 Ma ago. Therefore, after the Xu2 reservoir had become tight at middle diagenetic stage B, fracture development occurred at least at two stages. During fracture formation, the Xu2 reservoir had experienced or was experiencing large-scale hydrocarbon generation.

Table 1 Test results of ESR dating of tectoclase filler (quartz vein) of Xu2 reservoir in Anyue gas field

4 Inclusion development and hydrocarbon charging stages of Xu2 reservoir

4.1 Test method and study basis

The samples for the hydrocarbon inclusion test were taken from 12 tight sandstone cores of the Xu2 reservoir in 8 single wells from the Anyue–Hechuan area (see Figs. 1 and 2 for sampling locations). The samples were tested and analyzed by the Analytic Test Research Center of Beijing Nuclear Industry Geological Research Institute using the LINKAM THMS 600 model heating and freezing platform and test method and basis in EJ/T 1105-1999 (easurement of mineral fluid inclusion temperature) at a temperature of 25 °C and humidity of 40%.

The developmental stages of hydrocarbon inclusions have been ascertained based on the synchronism of diagenesis [19-20] and hydrocarbon generation evolution in the sedimentary basin and the age when the diagenetic minerals containing the hydrocarbon inclusions were formed [21] and combined with the organic constituent maturity evolution of hydrocarbon inclusions. The stages and time of hydrocarbon charging have been ascertained by considering the stages and homogenization temperatures of the inclusions.

4.2 Inclusion development and hydrocarbon charging stages

The developmental stages of the inclusions were divided by the physical phase of the hydrocarbon, the diagenetic mineral age and the hydrocarbon composition maturity [19-22]. The characteristics of associated organic inclusions would change regularly with the ceaseless conversion of organic matter into hydrocarbon [23]. At least four stages of hydrocarbon inclusions can be marked for the Xu2 reservoir in the Anyue–Hechuan area (Table 2) based on the age of the minerals containing inclusions and the organic matter maturity of inclusions.

The temperatures of inclusions formed at different buried depths and sites due to charging of fluids at different stages may differ according to the hydrocarbon charging stages and give a migration and accumulation system [24]. Well Yue 113, where the developmental stages of inclusions are relatively complete, is selected as a representative site to measure the temperature and recognize the hydrocarbon charging stages. The differences in depth of the measured inclusion samples are less than 5 m (see Figs. 1 and 2 for sampling locations). According to the developmental stages of the hydrocarbon inclusions and the homogenization temperature distribution of hydrocarbon brine inclusions in Well Yue 113, as well as the thermal history of this well (Fig. 4(a), which is prepared based on the Easy-Ro chemical kinetics model, at a geothermal gradient of 3.5 °C/100 m), at least three stages of hydrocarbon charging occurred in the Xu2 reservoir of the Anyue– Hechuan area (Figs. 4(a) and (b)).

① Stage I hydrocarbon charging is divided by the characteristics of stage I hydrocarbon inclusions (Table 2), which were developed during stage I quartz grain overgrowth with a low–moderate abundance (GOI of 2%-5%) and low compositional maturity (Table 2). The homogenization temperatures of the hydrocarbon brine inclusion of Well Yue 113 range from 60 °C to 90 °C. The source rock maturity was not high, presenting early mature stage of source rocks. The hydrocarbon charging volume was relatively small.

Table 2 Hydrocarbon inclusion stages of Xu2 reservoir in Anyue–Hechuan area

Fig. 4 Analysis of hydrocarbon charging stages of Well Yue 113:

② The stage II hydrocarbon charging is divided by the characteristics of stages II and III hydrocarbon inclusions, which were developed after stage II quartz grain overgrowth and during the cementation of sparry calcites. The composition maturity of hydrocarbon inclusions was high at this stage according to the characteristics of hydrocarbon inclusions (Table 2). The homogenization temperatures of the inclusions measured using the hydrocarbon brine inclusion of Well Yue 113 range from 110 °C to 120 °C. The hydrocarbon inclusion abundance GOI is 3%–10%, and the hydrocarbon charging volume is large.

③ The stage III hydrocarbon charging is categorized by the characteristics of stage IV hydrocarbon inclusions (Table 2), which were developed after stage III calcite (late fracture and vug calcite) filling. The developmental abundance is very high (GOI= 10%±). The inclusions include brown and dark brown liquid hydrocarbon inclusions (30%-40%), light yellow and gray gas–liquid hydrocarbon inclusions (about 40%), and dark gray gas hydrocarbon inclusions (20%-30%). The homogenization temperatures of the inclusions measured using the hydrocarbon brine inclusion of Well Yue 113 range from 100 °C to 120 °C, and the organic composition maturity is higher. Although the temperature range of the homogenization is close to that of stage II hydrocarbon charging, the inclusion host mineral is the fracture and vug filling calcite. This was the product after fractures were formed in the late stage. Stage III hydrocarbon charging occurred about 20-5 Ma ago (Neogene), which was the gas pool adjustment period when the Himalayan movement resulted in a certain amount of fractures. In this period, the reservoir had become tight (porosity of 6%-12%), and the tectoclase and microfracture were the main paths for gas migration [25-26].

5 Discussion

The correlation between the diagenetic sequence and densification process of the reservoir and the hydrocarbon charging stages is established through the above analyses. It is known that stage I hydrocarbon charging occurred after reservoir rock consolidation and before it became tight (porosity of 20%-30%). The hydrocarbon charging volume is little during that period when the hydrocarbon came from the source rocks inside the Xujiahe Fm. The primary pores and throat of the sandstone layer were the hydrocarbon migration channels. During the stage II hydrocarbon charging, the porosity reduced to 10%–20% and the reservoir became slightly tight. The hydrocarbon charging volume reached a maximum and was dominated by natural gas. The sandstone layer, possessing relatively good physical properties, was the migration channel. The gas was preserved in favorable traps and palaeo–gas reservoirs were formed [27]. Afterward, the buried depth continually increased; The reservoir became tight during the late Cretaceous; The natural gas could not be released, and superpressure gas reservoirs were formed. Following the Cretaceous period, the formation started to uplift; The tectonic activity was violent, and a certain scale of tectoclases began to be formed in the reservoir. The natural gas migrated toward the relatively high traps where the tectoclases developed via the tectoclases connecting the palaeo–gas reservoirs under the action of buoyancy and formation pressure [27]. The microfractures and the dissolved pores formed the main reservoir spaces. The present gas reservoirs were formed, which was the stage III hydrocarbon adjustment charging.

Figure 5 shows the relationship between the homogenization temperature of hydrocarbon brine inclusions and the salinity of the Xu2 reservoir in the Anyue–Hechuan area. The salinity distribution is wide, but the salinity of each stage of the inclusion is higher than the salinity of sea water (3.5 wt% NaCl) [28]. BARON et al [29], ATIKA et al [25], and YANG et al [30] judged the source of hydrocarbon fluid based on the salinity of the inclusions in the sandstone reservoirs. The high salinity of a hydrocarbon inclusion is exhibited in Fig. 5. It shows the hydrocarbon fluids in the Xu2 reservoir of the Anyue–Hechuan area come from the interior of the Xujiahe Fm where there is hardly any external fluid exchange. In the Sichuan Basin, the Xujiahe Fm gas source mainly came from the formation itself, representing a self generation and self preservation gas reservoir [31]. The Xu3 Member, as the direct caprock, is thick in the Central and Southern Sichuan Basin, providing good preservative conditions [32]. Large–scale hydrocarbon charging and accumulation occurred before the densification of the Xu2 reservoir in the Anyue–Hechuan area (Fig. 6), and the fractures resulted from the Himalayan tectonic uplifting which adjusted and reformed the palaeo–gas reservoirs, leading to the present gas reservoirs (Fig. 7). Due to suitable preservative conditions from the large–scale natural gas charged at an earlier stage and larger resources after adjustment at a later stage, the relatively high quality pore-fracture developmental zone (or partially filled by quartz and calcite vein) in the tight reservoir is the present in the gas-rich zone.

Fig. 5 Scatter diagram of relation between hydrocarbon inclusion salinity and homogenization temperature of Xu2 reservoir in Anyue–Hechuan area

Fig. 6 Composite correlation of densification process and hydrocarbon charging stages of Xu2 reservoir in Anyue–Hechuan area:

Fig. 7 Natural gas accumulation pattern of Xu2 reservoir in Anyue–Hechuan area:

6 Conclusions

1) There are three stages of hydrocarbon charging were experienced in the Xu2 reservoir of Anyue- Hechuan area. Stage I hydrocarbon charging occurred at early diagenetic stage B, 200–160 Ma ago, with a porosity of about 20%. It was consolidated, not tight, and with relatively few hydrocarbon charging volumes. Stage II hydrocarbon charging occurred at the middle diagenetic stage A, 140–120 Ma ago, with a porosity of 10%–20%. It was a relatively tight reservoir with a large hydrocarbon charging volume where the palaeo–gas reservoir was formed. Stage III hydrocarbon charging occurred at the middle diagenetic stage B, 20–5 Ma ago, with a porosity of 6%–10%. A tight reservoir was formed but developed fractures. The palaeo–gas reservoir was readjusted and the present gas reservoir formed with tectoclase and microfracture migration channels.

2) The reservoir was hydrocarbon charged and accumulated intensively before the densification of the Xu2 reservoir in the Anyue–Hechuan area. The hydrocarbon preservative conditions were barely affected since the tectonic movement was relatively gradual in the Central Sichuan Basin. Currently, the natural gas in the Anyue–Hechuan area was not well proven. Therefore, the Xu2 tight gas reservoir has a high potential and prospects for exploration.

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(Edited by YANG Hua)

Foundation item: Project(41372141) supported by the National Natural Science Foundation of China; Project(2008ZX05001–05–01) supported by Special and Significant Project of National Science and Technology, China

Received date: 2015-06-15; Accepted date: 2015-12-10

Corresponding author: XU Guo-sheng, Professor, PhD; Tel: +86-13880020853; E-mail: xgs@cdut.edu.cn