Grain shoals are one of principal reservoir facies of carbonate rocks in the oil and gas exploration domains. With the discovery of oil and gas in the grain shoal reservoirs of Longwangmiao Formation in central Sichuan paleo-uplift, the characteristics and distribution of grain shoal reservoirs are still unclear in the northern slope of the central Sichuan paleo-uplift. Through integrated analysis of wireline logs and high-quality 3D seismic data, combined with seismic forward modeling and waveform classification, reservoir seismic facies, reservoir classification, and reservoir distribution of different grades are investigated. The results show that: (1) Under the thin thickness (i.e., the thickness is around 80 m) of the Longwangmiao Formation, (i) when reservoirs are not developed, the seismic waveform is manifested as a single peak, and the top interface of the Longwangmiao Formation corresponds to 1/8 λ above the single peak. (ii) When a single set of reservoirs is merely developed in the second member of the Longwangmiao Formation (SMLF), the waveform still displays a single peak, and the top peak moves downward. In addition, when multiple sets of reservoirs are developed in the SMLF, the seismic reflections of multiple reservoirs are consistent with those of a single reservoir with the same cumulative thickness. (iii) When the reservoirs are developed in both the first and second members of the Longwangmiao Formation, the single peak exhibits moving downward, whether the reservoir thickness of the SMLF is greater than that of the first member, or the reservoir velocity of the SMLF is lower than that of the first member. (2) Under the thick thickness (i.e., the thickness is approximately 110 m) of the Longwangmiao Formation, (i) when reservoirs are not developed, the waveform of the Longwangmiao Formation exhibits a double peak, and the top interface of the Longwangmiao Formation corresponds to the extreme value of the upper peak. (ii) When a single set of reservoirs or multiple sets of reservoirs are only developed in the SMLF, the Longwangmiao Formation exhibits a single peak, and the top interface of the Longwangmiao Formation corresponds to the trough. (iii) With the reservoir development in both the first and second members of the Longwangmiao Formation, the waveform of the Longwangmiao Formation shows an asymmetric low-frequency peak, whether the reservoir thickness of the SMLF is greater than that of the first member, or the reservoir velocity of the SMLF is lower than that of the first member. (3) Using the waveform classification method, the seismic waveforms corresponding to the Longwangmiao Formation could be summarized into four types, including two types of reservoir waveforms. Two reservoir waveforms are distributed as two strips spread from north to south in the central study area. This paper recommends that the waveform classification method is more refined than amplitude attribute extraction for carbonate reservoir prediction.
Research Article
Prediction of grain shoal reservoirs via seismic forward modeling and waveform classification: Application to the northern slope of the central Sichuan uplift, Sichuan Basin, SW China
https://doi.org/10.21203/rs.3.rs-3801199/v1
This work is licensed under a CC BY 4.0 License
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Northern Slope Area
Longwangmiao Formation
Seismic Forward Modeling
Waveform Classification Analysis
Grain Shoal Reservoirs
With the exploration and development of marine carbonate reservoirs, identifying carbonate reservoirs is crucial in the early exploration stages (Zoveidavianpoor et al., 2013; Shen et al., 2015; Singh et al., 2020). Carbonate reservoirs mainly exist in reef and grain shoal facies, and these facies are widespread in the Sichuan Basin, Tarim Basin, and Ordos Basin in China (Fan, 2005; Bai, 2006; Jiang et al., 2008; Jia et al., 2011; Zhang et al., 2014; Shen et al., 2017). In the Sichuan Basin, one of the largest marine carbonate gas reservoirs is found in the core of the central Sichuan paleo-uplift, and the reservoir is located within the Cambrian Longwangmiao Formation (Du et al., 2014; Wei et al., 2015a; Yang et al., 2019; Ma et al., 2019a; Li et al., 2021).
Previous research had focused on the Longwangmiao Formation of the central Sichuan paleo-uplift. The Longwangmiao Formation was a restricted platform deposit with grain shoal and intershoal sea sub-facies (Du et al., 2016; Wang et al., 2016; Yang et al., 2018; Liu et al., 2020; Tan et al., 2020; Yang et al., 2021). The Longwangmiao Formation in the core of the paleo-uplift was located in the high terrain, which was affected by frequent sea-level fluctuations and strong hydrodynamic environment. Consequently, the Longwangmiao Formation in the core of the paleo-uplift was thicker (approximately 100 m) with extensive and well-connected distribution of grain shoals (Yang et al., 2018; Yang et al., 2021; Song et al., 2021; Zhang et al., 2021a; Li et al., 2022). Based on this understanding, the previous researchers observed and identified the seismic facies in the core of the paleo-uplift, and utilizing the strong amplitude peaks within the Longwangmiao Formation was advisable to predict the reservoir. The core of the central Sichuan paleo-uplift serves as an early exploration area. To expand the exploration area, the northern slope becomes a promising and substantial hydrocarbon potential area. Recently, studies on the northern slope have caught much attention. Owing to the structural features of the paleo-uplift and the difference of micro-topography, the thickness of the Longwangmiao Formation in the northern slope is thinner than that in the core of the paleo-uplift. For example, a discovery made from the well JT1 show a thinner thickness of the Longwangmiao Formation (approximately 80 m), and stronger heterogeneity of the grain shoal reservoirs, when compared to the core of the central Sichuan paleo-uplift (Tan et al., 2020; Dai et al., 2023). It is uncertain whether the seismic reflection (i.e., the strong peak reflection within the Longwangmiao Formation) of the reservoir in the core area of the paleo-uplift could be applied to the northern slope area (Zhang et al., 2016; Dai et al., 2019; Tan et al., 2020; Wang et al., 2021; Yang et al., 2021). As a result, reservoir distribution urgently needs to be studied in the northern slope area.
This contribution mainly addresses the following key issues to help us better identify the seismic response of the reservoir in the northern slope area: (1) investigating lithofacies relationship and establishing the stratigraphic sequence and cycle characteristics of the Longwangmiao Formation based on the analysis of single well and cross well facies in the northern slope area; (2) utilizing seismic forward modeling method, understanding the different grades of grain shoal reservoirs, including no reservoir, single set reservoir, multiple sets reservoirs, and their corresponding seismic response, respectively; (3) using waveform classification and reservoir classification analysis to study the distribution of the favorable reservoir; (4) through analyzing the tectonic evolution of the Longwangmiao Formation, revealing the main controlling factors of reservoir development.
2.1 Geographical location
The Sichuan Basin covers an area of approximately 2.6×105 km2 and includes the central and eastern of Sichuan Province and part of Chongqing City in China (Figs. 1a and 1b). The Sichuan Basin developed on the Neoproterozoic crystalline basement of the Yangtze Craton, is categorized as a superimposed basin. It can be further classified into six structural belts, named known as the central Sichuan gentle flat belt, the western Sichuan low-steep belt, the northern Sichuan low-flat belt, the eastern Sichuan high-steep fault fold belt, the southern Sichuan low-steep belt and the southwestern Sichuan low-steep fold belt (Fig. 1b) (e.g., Lin et al., 2007; Zhang, 2021b; Li et al., 2022; Zhang et al., 2022; He et al., 2022). The central Sichuan gentle flat belt could be further divided into two secondary structural units, namely the Gaoshiti-Moxi uplift and Penglai-Neijiang sag (Chen et al., 2017; Su et al., 2021). In this context, the study area is located in the northern of Gaoshiti-Moxi uplift within the central Sichuan gentle flat belt (Figs. 1b and 1c).
2.2 Stratigraphic characteristics
In the northern slope area of the central Sichuan Basin, the Cambrian strata consist of the following conformable lithostratigraphic units from the base to the top: the Qiongzhusi, Canglangpu, Longwangmiao, Gaotai, and Xixiangchi Formations (Fig. 2). These strata were formed in the marine environment. The Longwangmiao Formation is the carbonate platform deposits, it could be divided into the first member and the second member (Fig. 2b) (Zhang et al., 2010; Jin et al., 2014). The first member of the Longwangmiao Formation mainly developed relatively low-energy micrite dolomite, whereas the second member of the Longwangmiao Formation (SMLF) as the study interval mainly developed relatively high-energy carbonate rocks including detrital dolomite and oolitic dolomite (Fig. 2b) (Yao et al., 2013; Zhao et al., 2014; Yang et al., 2015; Jin et al., 2015).
3.1 Database
The primary database is used in this paper, including three-dimensional seismic, logging, drilling and reservoir interpretation data in the Longwangmiao Formation. The three-dimensional seismic data covers about 3250 km2 located on the northern slope area referred to as the ‘Shehong survey’. The seismic data of the Shehong survey has been processed to zero phases displayed by polarity under the Society of Exploration Geophysicists standards, and the dominant frequency is 33 Hz. The seismic vertical resolution ranges from 15 m to 20 m. The logging data includes gamma ray (GR), sonic (AC), neutron density (DEN), resistivity (RT and RXO) and other logs data of four wells (namely wells JT1, PS1, PS12 and PS15, see Fig. 1 for well location) on the Shehong survey. Through well seismic calibration, the top and bottom interfaces of the Longwangmiao Formation are found in the three-dimensional seismic data, and the Shehong survey is conducted for 10 × 10 (200 m × 200 m) interpretation grid. The drilling data covers the lithology of the Longwangmiao Formation and the upper and lower strata of the Longwangmiao Formation. According to the lithology and logging curve, the reservoir of Longwangmiao Formation is interpreted, and the reservoir interpretation data comes from logging company.
3.2 Methods
This study carefully analyzes the lithology, stratal thickness, reservoir thickness, and cycle information of four wells in the northern slope. By using the seismic forward method, identify seismic waveforms under different reservoir development conditions and summarize the waveform patterns corresponding to reservoir development. The geometric and petrophysical parameters of the seismic models were obtained from four wells (namely wells JT1, PS1, PS12 and PS15) located in the NSSB, including the geometric shape, stratal thickness (80 m and 110 m), reservoir thickness (0 m ~ 25 m), P-wave velocity of stratum, P-wave velocity of reservoir (Tables 1 and 2), reservoir development intervals (reservoir is only developed in the SMLF, and reservoirs are developed in both the first and second members of the Longwangmiao Formation). The reservoirs developed in the northern slope were subdivided into three grades based on the difference in P-wave velocity, they are high-, medium-, and low-velocity reservoirs (Shang et al., 2023) (Table 2). The reservoir is set up as a wedge-shaped model and the development of reservoirs in both the first and second members of the Longwangmiao Formation.
Based on the understanding of reservoir waveforms, choose waveform classification (a neural network algorithm used in the attribute extraction) to classify and extract waveforms from the Longwangmiao Formation in the study area, and then display favorable waveforms in a plane, representing the distribution of favorable reservoirs in the displayed area. Finally, combined with the amplitude attribute extraction results of the internal peak attribute and the top trough attribute in the Longwangmiao Formation, the most favorable reservoir and distribution area are shown in plane views.
| Stratigraphy | Paleo-environment | P-wave velocity range (m/s) | P-wave average velocity (m/s) | Model stratal thickness (m) |
|---|---|---|---|---|
| Gaotai Formation | Mixed platform | 5280 ~ 6948 | 6096 | 170 ~ 280 |
| Longwangmiao Formation | Restricted platform | 6130 ~ 7038 | 6772 | 0 ~ 110 |
| The Second Member of the Canglangpu Formation | Shallow water shelf | 5737 ~ 4789 | 5255 | 200 |
| Reservoir type | Reservoir thickness of the second member (m) | Reservoir thickness of the first member (m) | P-wave velocity of reservoir (m/s) | Sedimentary facies |
|---|---|---|---|---|
| High-velocity reservoir | 0 ~ 25 m | 0 ~ 25 m | 6255 | grain shoal |
| Medium-velocity reservoir | 0 ~ 25 m | 0 ~ 25 m | 5800 | grain shoal |
| Low-velocity reservoir | 0 ~ 25 m | 5300 | grain shoal |
4.1 Stratigraphic and sedimentary characteristics of the Longwangmiao Formation
The lithology of Longwangmiao Formation is different from that of Canglangpu Formation and Gaotai Formation (Fig. 3). The Canglangpu Formation is a mixed tidal flat environment that mainly comprises dark shale. The Longwangmiao Formation is a restricted platform environment constituted by grain dolomite and crystal dolomite. The Gaotai Formation is also a mixed tidal flat environment with thin-layer mud shale and argillaceous dolomite development. Additionally, the well loggings in the Longwangmiao Formation also have an apparent difference with its surrounding stratus. The Longwangmiao Formation displays a serrated box-shaped, low gamma ray (GR) and high electrical resistivity log. However, the Canglangpu Formation and Gaotai Formation both display tooth-shaped high gamma ray log and low-value electrical resistivity log.
Average thickness of the Longwangmiao Formation in the northern slope ranges from 60 m to 130 m based on the well analysis and seismic interpretation, with the second member being generally thicker than the first member (Figs. 3 and 4). The first member is mainly composed of residual detrital dolomite and powder crystal dolomite. The former reflects a high-energy environment which is considered grain shoal facies, whereas the latter indicates a relatively low-energy environment representing intershoal sea facies. The grain shoal in the first member exhibits a characteristic of thickening toward the northern slope area and continuous distribution. Differently, the second member is constituted of the coarser and thicker oolitic siliceous dolomite and detrital dolomite, indicating well-developed grain shoals (Figs. 3). The grain shoals tend to thin out towards the northern slope area in contrast to the central Sichuan paleo-uplift, with a more limited horizontal distribution (Fig. 4).
4.2 Responses of the seismic forward modeling
4.2.1 Seismic forward responses for no reservoir developed
Canglangpu Formation, Longwangmiao Formation and Gaotai Formation in the model are set to constant velocity, and the velocity value is derived from the average formation velocity of the four wells on the north slope, which are 5255 m/s for the Canglangpu Formation, 6772 m/s for the Longwangmiao Formation, and 6096 m/s for the Gaotai Formation. A wedge-shaped model of the Longwangmiao Formation with undeveloped reservoirs is established to observe seismic waveform varying with its stratal thickness (0 ~ 110 m) (Fig. 5a). The thickness of the Canglangpu Formation and Gaotai Formation in the model is relatively thick, which will not affect the waveform of the Longwangmiao Formation.
Combined with the drilling results of the central Sichuan paleo-uplift and the northern slope, the thickness of the Longwangmiao Formation is between 60 m and 130 m. Therefore, we focus on the waveform features of the Longwangmiao Formation with a thickness greater than 60 m in Fig. 5. The results show that the bottom interface of the Longwangmiao Formation constantly corresponds to the extreme value of the wave trough, while the top interface of the Longwangmiao Formation varies with its stratal thickness (Fig. 5b).
i) When the thickness of the Longwangmiao Formation is between 60 m and 90 m (e.g., the thickness of the Longwangmiao Formation is 80 m), the top interface of the Longwangmiao Formation is located about 1/8λ above the wave peak (Fig. 5b).
ii) When the stratal thickness reaches more than 90 m, the waveform of the upper Longwangmiao Formation changes from single peak to double peak gradually. When the stratal thickness is greater than 110 m, the two peaks of double peak are obviously separated (Fig. 5b).
4.2.2 Seismic forward responses for reservoir developed
According to the above, with the different of the stratal thickness, the upper Longwangmiao Formation will produce two types of waveforms, namely single peak and double peak (Fig. 5). Therefore, when the stratal thickness of the Longwangmiao Formation is thin (60 ~ 90 m) and thick (greater than 90 m) and when reservoirs are developed internally, it is necessary to explore the waveform changes corresponding to the Longwangmiao Formation, in order to achieve the purpose of identifying reservoirs through the seismic facies.
(1) Reservoir seismic reflection under thin formation (60 ~ 90 m) background
Under the background of the stratal thickness of 60 m ~ 90 m in the Longwangmiao Formation, a wedge-shaped geological model is established, in which only the upper reservoir (i.e., the reservoir developed in the SMLF) and both the upper and lower reservoirs (i.e., the reservoir developed in the first and second members of the Longwangmiao Formation) are developed (Figs. 6 and 7). The parameters, such as and formation thickness, formation velocity, reservoir thickness and reservoir velocity of the Longwangmiao Formation, are shown in Table 2. By the seismic forward modeling, the results indicate that:
i) When the reservoir is developed in the SMLF, the seismic waveform of the Longwangmiao Formation is still dominated by the skew-symmetrical waveform from the peak to the trough, but the top peak has the characteristics of downward movement (Model 1 in Fig. 6). The downward movement will become more obvious with the decrease of the reservoir velocity of the SMLF, which will eventually lead to the peak located in the middle of the Longwangmiao Formation (Model 2 in Fig. 6). At the same time, as the reservoir velocity decreases, the peak amplitude inside the Longwangmiao Formation will also increase (compared with Models 1, 2 and 4 in Fig. 6). Meanwhile, the top interface of the Longwangmiao Formation is generally located below the upper trough. The lower the reservoir velocity, the stronger the trough amplitude corresponding to the top interface of the Longwangmiao Formation.
ii) When multiple sets of reservoirs are developed in the SMLF, the velocity of each set of reservoirs is set to be consistent with the velocity of a single set of reservoirs, and the cumulative thickness of multiple sets of reservoirs is also consistent with the thickness of a single set of reservoirs. The forward modeling results show that the waveforms of multiple sets of reservoirs are consistent with those of a single set of reservoirs (compared to Models 2 and 3 in Fig. 6).
iii) When reservoirs are developed in both the first and second members of the Longwangmiao Formation, firstly, the reservoir characteristics of the first and second members are set to be identical (i.e., consistent thickness and velocity). Meanwhile, the waveform of the Longwangmiao Formation does not change significantly (Model 1 in Fig. 7). The actual geological conditions do not lead to this situation. Secondly, if the reservoir thickness of the SMLF is greater than that of the first member of the Longwangmiao Formation (Model 2 in Fig. 7), the wave peak will move downward, and as the reservoir velocity decreases, the wave peak will move downward more significantly and the peak amplitude will also increase (Model 3 in Fig. 7). Thirdly, if the reservoir velocity of the SMLF is lower than that of the SMLF, the wave peak moves downward (Model 4 in Fig. 7).
In summary, when the thickness of the Longwangmiao Formation is 70 m ~ 90 m, the reservoir development in the SMLF can be determined by three characteristics: the upper peak of the Longwangmiao Formation moves down into the formation, the middle peak amplitude increases, and the top trough amplitude increases.
(2) Reservoir seismic reflection under thick formation (over 90 m) background
Under the background that the thickness of the Longwangmiao Formation is more than 90 m, a wedge-shaped geological model was established, in which only the upper reservoir (i.e., the reservoir developed in the SMLF) and both the upper and lower reservoirs (i.e., the reservoir developed in the first and second members of the Longwangmiao Formation) are developed (Figs. 8 and 9). The parameters, such as formation thickness, formation velocity, reservoir thickness and reservoir velocity of the Longwangmiao Formation, are shown in Table 2. Using seismic forward modeling, the results are as follows:
i) When reservoirs are developed in the SMLF, the top peak moves down, and the internal peak of the Longwangmiao Formation undergoes interference and merges into a single peak. Currently, the Longwangmiao Formation exhibits a trough-peak-trough reflection from top to bottom (Fig. 8). As the reservoir velocity of the SMLF decreases, the wave peak moves downward more significantly, and the peak amplitude within the formation increases as well (compared to Models 1 and 2 in Fig. 8) and the trough amplitude corresponding to the top interface of the Longwangmiao Formation increases.
ii) When multiple sets of reservoirs are developed in the SMLF, the velocity of each set of reservoirs is set to be consistent with the velocity of a single set of reservoirs, and the cumulative thickness of multiple sets of reservoirs is also consistent with the thickness of a single set of reservoirs (compared to Model 2 and 3 in Fig. 8). The forward modeling results show that the waveforms of multiple sets of reservoirs are consistent with those of a single set of reservoirs, indicating that the cumulative thickness of the reservoir is one of the critical factors affecting the waveform reflection of the reservoir.
iii) When reservoirs are developed in both the first and second members of the Longwangmiao Formation, firstly, the reservoir characteristics of the first and second members of the Longwangmiao Formation are set to be identical (i.e., consistent thickness and velocity). Meanwhile, the top peak of the Longwangmiao Formation moves slightly downward, and the top peak connects with the middle peak to form a low-frequency peak (Model 1 in Fig. 9). Secondly, if the reservoir thickness of the second member in the Longwangmiao Formation is greater than that of the first member of the Longwangmiao Formation, the double peak in the upper Longwangmiao Formation will interfere and merge into a wave peak, and the top peak will move downward (Model 2 in Fig. 9). It should also be noted that the morphology of single peak is an asymmetric low-frequency peak, and the maximum value of the wave peak is close to the middle Longwangmiao Formation. Thirdly, when the reservoir thickness of the SMLF is greater than that of the first member of the Longwangmiao Formation, and the reservoir velocities of the second and members of the Longwangmiao Formation are lower, there is still only a wave peak within the Longwangmiao Formation, but the wave peak moves down more significantly, and the wave peak shape is still asymmetric. The maximum value of the wave peak is close to the upper Longwangmiao Formation (Model 3 in Fig. 9), which is different from Model 2 in Fig. 9. Fourthly, when the reservoir thickness of the second member is equal to that of the first member in the Longwangmiao Formation, but the reservoir velocity of the second member is lower than that of the first member, the wave peak moves downward and the wave peak shape is similar to that of the Model 3 in Fig. 9 (Model 4 in Fig. 9).
Based on the above results, when the thickness of the Longwangmiao Formation is greater than 90 m with the reservoir development, three characteristics can be used to judge the reservoir development in the SMLF: the double peak is merged into single peak, the asymmetric peak moves down into the formation, and the top interface of the Longwangmiao Formation corresponds to the trough.
4.3 Seismic facies of reservoir in seismic profiles
By comparing the features of forward modeling waveforms with actual cases, four types are summarized (Table 3).
The Waveform-1 has the characteristics of the top peak downward movement sharply, and a high-amplitude peak reflection occurs in the formation. Therefore, the Waveform-1 indicates the following situations: In the thin formation situation, only a single or multiple sets of reservoirs are developed in the SMLF; In the case of thin formation, reservoirs are developed in both the first and second members of the Longwangmiao Formation, but reservoirs with large thickness and low-velocity in the SMLF; In the case of thick formation, only a single or multiple sets of reservoirs developed in the SMLF; In the case of thick formation, reservoirs are developed in both the first and second members of the Longwangmiao Formation, with an overall high-velocity reservoir and a thick reservoir of the SMLF (see Figs. 6, 7 and 8 and Model 2 in Fig. 9). In the seismic profile of the cross-well, the seismic profiles of wells JT1, PS12, and PS1 exhibit the characteristics of Waveform-1 (Figs. 10b, 10c and 10d).
The Waveform-2 has the characteristics of the top peak downward movement, and the asymmetric low-frequency peak reflection occurs in the formation. This waveform could be compared to the forward seismic model of the Longwangmiao Formation with a thickness of 110 m, and reservoirs are developed in the first and second members of the Longwangmiao Formation, but the reservoir thickness of the SMLF is larger when the overall reservoir velocity is relatively low, or the reservoir velocity of the SMLF is lower than that of the first member (see Models 3, and 4 in Fig. 9). In the seismic profile of the cross-well, the seismic profiles of well PS15 exhibit the characteristics of Waveform-2 (Fig. 10a).
The Waveform-3 shows the top interface corresponds to 1/8 λ above the wave peak, which is similar to the forward seismic model of the Longwangmiao Formation with a thickness of 80 m and no reservoir development (Fig. 5).
The Waveform-4 is a double-peak reflection, and the top interface corresponds to the upper wave peak. This waveform could be compared to the forward seismic model of the Longwangmiao Formation with a thickness of 110 m, but the reservoir is not developed. Thus, the Waveform-4 is considered as the non-reservoir development (Fig. 5). In the cross-well seismic profile, the feature of Waveform-4 is found in the seismic profile next to the PS15 well (Fig. 10a).
4.4 Reservoir classification and corresponding planar distribution
Based on the reservoir classification in Table 3, using the automatic waveform classification method, four elementary waveforms are divided within the Longwangmiao Formation in the northern slope area (Fig. 11).
Figure 10 and Table 3 show that Waveform-1 is formed under various reservoir development conditions. The Waveform-1 indicates the reservoir development, located in the central area of the study area and distributed in an east-west striped pattern, accounting for 31.1% of the study area (Fig. 11). The northern area of Waveform-1 (the western area of well JT1) is significantly larger than the northern area (the eastern area of well PS1) (Fig. 11b).
The Waveform-2 indicates that both the upper and lower reservoirs are developed. However, the, the reservoir velocity of the SMLF is lower than that of the first member when the reservoir thickness is consistent, or the reservoir thickness of the SMLF is greater than that of the first member when the reservoir velocity is the same. Waveform-2 is mainly distributed in the southern area of well PS15 (southern area of the study area), closer to the central Sichuan paleo-uplift. The area of Waveform-2 accounts for 13.8% of the study area (Fig. 11). This is consistent with the previous understanding that the Longwangmiao Formation in the central Sichuan paleo-uplift has a large stratal thickness and reservoir thickness (Dai et al.,2019; Ma et al., 2019b).
The Waveform-3 (light blue) indicates that the reservoirs are not developed under the condition of thin formation (the thickness of the Longwangmiao Formation is 60 ~ 90 m) that the Longwangmiao Formation, which is interpreted as intershoal sea sedimentation. The area covered by the Waveform-3 accounts for 46.7% of the total area of the study area (Fig. 11a).
The Waveform-4 (dark blue) indicates that the reservoirs are not developed under the condition of thick formation (the thickness of the Longwangmiao Formation is greater than 90 m) in the Longwangmiao Formation, and the distribution range of Waveform-4 is interpreted as intershoal sea facies. The Waveform-4 covers 46.7% of the total area of the study area (Fig. 11a).
Enlarge and display the waveform classification of the area where the four wells are located. Wells PS12, PS1, and JT1 correspond to Waveform-1, and well PS15 corresponds to Waveform-2, which are consistent with the seismic profile and drilling results (Figs. 10 and 11c).
Table 3 Seismic reflection characteristics and waveform classification of the Longwangmiao Formation in the northern slope
5.1 Favorable attributes for predicting grain shoal reservoirs
Through statistical analysis of reservoir information from substantial drilling wells in the central Sichuan paleo-uplift, using the seismic forward modeling, previous researchers found the downward movement of the top peak was favorable seismic facies for predicting the reservoir of Longwangmiao Formation in the central Sichuan paleo-uplift (Dai et al., 2019; Tan et al., 2020). Based on the above understanding, the predecessors extracted the amplitude attributes of the interlaminar peak and the amplitude attributes of the top trough to predict the reservoir distribution of the Longwangmiao Formation in the central Sichuan paleo-uplift (Dai et al., 2019; Xiao et al., 2014; Tan et al., 2020). In this paper, using the data of the northern slope, the waveform of the high-quality reservoir is obtained by seismic forward modeling, which is similar to the waveform proposed by the predecessors in the central Sichuan paleo-uplift, namely the Waveform-1 corresponding to the high-quality reservoir development in SMLF. Therefore, we identified this type of reservoir by extracting the amplitude attribute of the interlaminar peak and the amplitude attribute of the top trough, and obtained the plane distribution of the reservoir in the Longwangmiao Formation of the northern slope (Fig. 12). The reservoir distribution in the attribute extraction diagram is similar to the reservoir distribution in the waveform classification (Figs. 11c and 12).
Nevertheless, the above two methods of extracting amplitude attributes make it difficult to identify the reservoir represented by Waveform-2. The top peak downward shift of Waveform-2 is not as obvious as that of Waveform-1, and the interlaminar peak amplitude of Waveform-2 is not as strong as that of Waveform-1. When extracting the interlaminar peak amplitude or the top trough amplitude, it is easy to misjudge the poor development of the reservoir. However, the above waveform classification has a great advantage in predicting the reservoir of Waveform-2 (under the background of ‘Longwangmiao Formation thick, and the reservoir scale of SMLF is better than that of the first member of Longwangmiao Formation’).
In addition, for the intershoal sea facies with reservoir non-development, the waveform classification technology could also be divided into two cases of thin and thick strata in the Longwangmiao Formation, namely the distribution range of waveform-3 and waveform-4 (Fig. 11). This is also better than the prediction results of extracting amplitude attributes, so the waveform classification method is more suitable for predicting the reservoir of SMLF on the northern slope (Fig. 12).
The waveform classification method also has a drawback, which is difficult to identify high-quality reservoir distribution ranges in the same type of reservoir. This method cannot ‘select the best from the good’ within the range of reservoir development. In other words, when selecting the high-quality reservoir plane distribution of Waveform-1 or Waveform-2 in the waveform classification, we must combine the prediction results of amplitude attributes.
In summary, the waveform classification method is preferred to predict the plane distribution of grain shoal reservoirs on the northern slope. After the reservoir plane distribution represented by Waveform-1 or Waveform-2 is characterized, amplitude attributes are extracted to select the best from them, ultimately obtaining the highest quality reservoir distribution.
5.2 Reservoir genesis of the Longwangmiao Formation
The genesis of reservoirs in the Longwangmiao Formation was controlled by several factors, such as regional tectonic evolution, paleo-morphology, as well as sea-level fluctuations. During the sedimentary period of the Longwangmiao Formation, the central Sichuan paleo-uplift and the northern slope had the conditions of high terrain, shallow water, and strong energy, prompting the formation of grain shoal sediments composed of oolites and detrital (Zou et al., 2014; Wei et al., 2015b; Wang et al., 2017; Yang et al., 2019; Yang et al., 2020; Li et al., 2021). The grain shoal sediments provided an important lithological foundation for the reservoir. Simultaneously, the frequent sea-level fluctuations resulted in the grain shoals developed near the high terrain were more likely to be exposed, and then were subjected to the leaching of meteoric water and dolomitization (Fig. 13) (Zhang et al., 2021; Xu et al., 2023). This process formed a large number of dissolution pores, ultimately improving the physical properties of the reservoirs. Subsequently, the Caledonian Movement strongly uplifted the central Sichuan paleo-uplift and caused extensive erosion in the study area (Zhou et al., 2015; Hu et al., 2020; Gu et al., 2023). The meteoric water flowed down the slope in the grain shoal, making the Longwangmiao Formation form a large-scale karst reservoir (Fig. 13). Afterward, the Longwangmiao Formation was influenced by multiple tectonic movements, such as the Indosinian movement and the Himalayan movement, and formed a series of structural fractures (Dai et al., 2017; Su et al., 2021; Sun et al., 2021).
Our study demonstrated that the first and second members of the Longwangmiao Formation both experienced complete T-R cycles, and the grain shoals were predominantly formed during the regression stage. The falling sea level in every regression stage results in exposure of grain shoals and further facilitates the syngenetic karstification on the top of the grain shoals. After the sedimentation of the Longwangmiao Formation, the Caledonian Movement led to the Sichuan Basin significantly uplifting from the western to the central part, causing widespread exposure and erosion in the study area. Thus, the grain shoal reservoirs were transformed again by epigenetic karstification. Nevertheless, the second member of the Longwangmiao Formation near the unconformity surface suffered a greater extent of leaching, leaving an overall better reservoir quality than those in the first member of the Longwangmiao Formation.
The integrated results have led to the following interpretations as detailed.
(1) The SMLF is an interlacing of grain shoal with residual oolitic siliceous dolomite and detrital dolomite and intershoal sea with powder crystal dolomite. The grain shoals tend to thin out towards the northern slope area in contrast to the central Sichuan paleo-uplift, and the distribution of single set of grain shoals was limited horizontally.
(2) Through seismic forward modeling, the waveform characteristics of reservoir development in the Longwangmiao Formation are identified. Under the thin thickness of the Longwangmiao Formation, the top peak moves down when the reservoir is developed, and the interlaminar waveform shows a strong peak reflection. Under the thick thickness of the Longwangmiao Formation, the upper peak of the Longwangmiao Formation moves down and superimposes the interlaminar peak when the reservoir is developed. Meanwhile, the interlaminar waveform is the strong amplitude peak or an asymmetric low-frequency peak.
(3) Using the waveform classification method, the seismic waveforms corresponding to the Longwangmiao Formation could be summarized into four types, including two waveforms of reservoir development. The reservoirs are distributed as two strips spread from north to south in the central study area. This paper recommends that the waveform classification method is a more refined method than amplitude attribute extraction for carbonate reservoir prediction.
This research was supported by the Natural Science Foundation of Chongqing Municipality, China [grant number CSTB2023NSCQ-MSX0934], the Young Scholars Program of BaYu Scholars Program [grant number YS2021091], the Science and Technology Innovation Fund for Postgraduates of Chongqing University of Science and Technology [grant number YKJCX2220111], and the Science and Technology Innovation Fund for Postgraduates of Chongqing University of Science and Technology [grant number YKJCX2320112].
All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.
Acknowledgments
Thanks to ‘Central Sichuan Oil and Gas Field of PetroChina Southwest Oil and Gas Field Company’ for supplying the subsurface data and approving the publication of the findings, we would like to convey our gratitude. We sincerely appreciate the significant contributions that Hong Chen, Keliang Lu, Jie Wang, and Runchao Wang (Chongqing University of Science and Technology) have made to the interpretation and mapping of seismic data.
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No competing interests reported.