Mixed sedimentation is mainly a phenomenon by which siliciclastic and carbonate are mixed under a similar sedimentary environment. The study of the composition of mixed rock can reflect the sedimentary environment. The mixed sediments of the Canglangpu formation in the eastern Sichuan basin are widely developed and exhibit well exploration prospects. In this study, a classification diagram of mixed sediments suitable for the Canglangpu formation was proposed by combining lithologic characteristics with logging data. Based on the observations of typical outcrop profiles, the identification of thin sections, and drilling and geological logging data, a model was established to identify the electrical responses of the mixed sedimentary rocks in the study area. The results show that (1) the Canglangpu formation can be vertically divided into two complete transgression-regression cycles with the lower part mainly consisting of sedimentary carbonate diamictite and the upper part principally composed of clastic diamictite; (2) mixed sediments in the Canglangpu formation can be divided into six classes, namely carbonate sandstone, argillaceous sandstone, sandy carbonate rock, argillaceous carbonate rock, sandy mudstone, and Carbonate mudstone; (3) the sedimentary environment of the Canglangpu formation be divided into (a) mixed sediments in the shore, which include mixed lagoon and mixed tidal flat, (b) mixed sediments in the shallow marine shelf, which includes mixed beach bar, terrigenous detrital mixed shelf, and carbonate mixed shelf; (4) a mixed shore-mixed shelf sedimentary model has been established; it says that the mixed sediments show obvious characteristics of ring-like development around underwater paleo-heights. The formation mechanism of mixed sedimentation in the eastern Sichuan basin is mainly controlled by the change of sedimentary facies belt, and the development intensity is mainly influenced by early tectonic movement and sea-level change.
Mixed sedimentation is mainly a phenomenon by which siliciclastic and carbonate are mixed under a similar sedimentary environment [1,2,3,4,5,6,7,8]. In recent years, mixed reservoirs have been discovered successively in the Qaidam [9,10,11], Tarim [12,13], and Bohai bay basins [14,15,16,17]. The mixed sediments are widely developed in the lower Cambrian Canglangpu formation of the Sichuan basin [18,19,20,21,22]. Particularly, the Well Jiaotan 1 revealed a gas production rate in October 2020 of 51.62 × 104 m3/day which demonstrated good exploration potential of the Canglangpu formation of the central Sichuan basin .
At present, there is a lack of drilling cores data in the lower Cambrian Canglangpu formation in the eastern Sichuan basin, and the research on there mainly relies on cuttings and logging data. The types of minerals in the Canglangpu formation are complex, and the lithology is mainly mixed between clastic rocks and carbonate rocks [20,24,25]. Compared with pure clastic and pure carbonate rocks, the log curve characteristics of mixed sediments were evident. Since most of the logging curves can comprehensively indicate the physicochemical properties of the layered rocks, the mineral species of the Canglangpu formation can be identified [10,26,27]. Depending on the sensitivity characteristics of various conventional logging curves, such as natural gamma-ray (GR), neutron (NEL), acoustic logging (AC), and neutron-density (DEN), to the composition and structure of mixed sedimentary reservoirs, optimized the composition and structure-sensitive curves of mixed rocks using boxplots to determine the boundaries of each category. To diagnose the sedimentary characteristics and genetic mechanism of the mixed rock in the study area, the Component-Structure Logging Classification Method was used . To analyze the types and characteristics of mixed sedimentary rocks, this study integrated the logging facies identification method and the three-end-member classification scheme of clay-terrigenous detritus-chemical precipitated carbonate lithologic classification method [24,25,30–33]. Then, the vertical and horizontal distribution law of the mixed sedimentary facies belt is analyzed. Finally, the genetic model of mixed sediments in the Canglangpu formation of lower Cambrian in eastern Sichuan is established; furthermore, the genetic mechanism of mixed sedimentation is analyzed. These research results can provide references and ideas for the exploration of mixed reservoirs in the Sichuan basin.
2 Geological setting
Sichuan basin is one of the four major basins in China ; it is located in southwestern China, between latitude and longitude ranges of 103°E–108°E and 28°N–32°N, respectively. The study area is located in the eastern part of the Sichuan basin and is structurally subordinated to the eastern Sichuan basin high-steep fold belt . The northeastern part of the study area is bounded by the Daba mountain fold-thrust belt, while the southeastern part is the Qiyue mountain fault zone, bounded by the western Hunan and Hubei fault-fold zone. The western part is delimited by the Huaying mountain fault zone . The paleogeographic pattern of the Canglangpu formation in the eastern Sichuan basin region is mainly inherited from the three underwater paleo-uplifts formed in the Qiongzhusi formation, the Suining-Guang’an, Dazhu-Kaijiang, Dong’an-Wuxi, and the Chengkou-Kaixian depression. The paleogeographic pattern consists of “three uplifts with one depression” along with the Chengkou-Kaixian depression, forming a semi-restricted shore-shelf environment. Due to the uplift of Hanjiang and Motianling ancient lands in the northern margin of the Sichuan basin, the Dazhou-Kaijiang paleo-uplift was uplifted again from the early Canglangpu period to the Longwangmiao period, when it merged with the Sichuan basin middle paleo-uplift, forming Leshan-Longnvsi paleo-uplift (Figure 1) [28,29,37,38].
The Canglangpu formation in the eastern Sichuan basin area is well developed. Its thickness gradually increases from west to east, within a range value of 180–250 m, and east to the outside the basin, reaching more than 300 m . During the Canglangpu period, the water depth of the sedimentary environment in the study area was relatively limited, paleontological features were not developed, and the shelf environment was the mainstay. The underlying strata of the Canglangpu formation and the Qiongzhusi formation are important source rocks in the Sichuan basin, consisting mainly of clayey deposits with low sandy and bio-fossil contents. The upper member of the Canglangpu formation, the limestone of the Longwangmiao formation, and dolomitic limestone are in conformity contact [19,20]. During the depositional stage of the Canglangpu formation, the supply of terrigenous clastic sequences varied significantly, with strengthening and then weakening trends, which can be divided into upper and lower parts, corresponding to two sedimentary cycles . The lower member consists of shore-mixed shallow marine shelf deposits, developed in the context of rapid transgression conditions and insufficient terrigenous supply, mainly carbonate facies. Whereas the upper member consists of shallow water shelf-mixed shore deposits, where terrigenous debris is relatively abundant and developed under the background of relatively shallow water and sufficient terrigenous source supply (Figure 2).
3 Data and methods
3.1 Data and samples
The data used in this study consist of logging data from 9 wells (Dingshan 1, E’can 1, Gaoshi 16, Gaoshi 21, Guangtan 2, Li 1, Loutan 1, Mashen 1, and Wutan 1). The logging curve was determined by Sichuan Petroleum Logging Company from oil wells using EXCELL2000 series logging tools.
A total of 304 samples, 217 thin sections, and 150 core photos were collected form six wells (Gaoshi 16, Guangtan 2, Li 1, Loutan 1, Mashen 1, and Wutan 1) and a survey of the Yangsilqiao outcrop. Thin sections were prepared from samples recovered from wells and polished at a thickness of 30 μm at the Sedimentary Basin Experimental Center of Yangtze University (Wuhan, China). A research-grade smart transflective polarizing microscope (Leica DM4 P Upright Polarizing Microscope, Leica Microsystems (Shanghai) Trading Co., Ltd.) was used to observe all thin sections for detailed sedimentological and petrological analyses.
Lithology identification and classification are the basic problems that need to be primarily solved in this study.
The rocks and textural characters were described using the Yang CQ (1990) and Wang JQ (2014) classification of the mixed sedimentation [4,31]. According to the standard of three terminal element classification scheme and the characteristics of four logging curves including GR, AC, CNL and DEN, the lithology classication standard was eastalished [10,26].
The accuracy of logging data is reduced, due to the interference of reservoir properties and included fluids. Regarding the mixed sediment classification process in the Canglangpu formation, a low accuracy identification of mixed sediments of the diamictite, with a content of less than 25%, was observed in the Canglangpu formation, suggesting an adjustment of the classification of the mixed sediments in the area. In fact, the argillaceous sandstone containing carbonate components was mainly distributed within the facies boundary, showing a slight difference from the logging curve of argillaceous sandstone. Among the sand-clastic mudstone/silty mudstone, a small amount of carbonate sand (<25%) does not affect the structure of the rock itself and is of little importance for environmental identification; thus, it can be included in the mudstone category. The classification of mixed mudstone in the area was further simplified after adjustment. Mixed rocks can be divided into seven classes: carbonate sandstone, argillaceous sandstone, sandy carbonate rock, argillaceous carbonate rock, sandy mudstone, carbonate mudstone, and mixo-sedimentite (Figure 3). Nevertheless, mixo-sedimentite is not developed in the study area.
Finally, under the guidance of the theory of sedimentology and sequence stratigraphy, the lithologic characteristics and vertical superposition relationships of mixed sediments in the Canglangpu formation were defined, the genetic model of mixed sediments in the Canglangpu formation of lower Cambrian in eastern Sichuan was established. Furthermore, the genetic mechanism of mixed sedimentation was analyzed.
4.1 Types and characteristics of mixed sedimentary rocks
4.1.1 Classification of mixed sediments
The measured data of the Yangsiqiao outcrop section in the Canglangpu formation in the northern part of the study area and the analysis of over 200 drilling thin sections cutting revealed that fossils are not developed in strata. In addition, according to the different contents of exogenous clays and endogenous carbonate rocks, the lithologic type of the Canglangpu formation in the study area can be classified into three classes, namely sandstones, mudstones, and carbonates.
Sandstones: the mixed sediments with an exogenous clastic mass range of 50–75% are classified into five types: argillaceous siltstone, argillaceous fine sandstone, dolomitic siltstone, dolomitic fine sandstone, and calcareous siltstone (Figure 4a and b).
Mudstones: the mixed sediments with exogenous clay mass range of 50–75% are classified as mudstones, characterized by relatively simple composition and structure compared to those of sandstone, and belong to various relatively low-energy sedimentary facies. Only three types of mudstones are distinguished in the study area, namely sand-bearing silty mudstone, calcareous mudstone, and dolomitic mudstone (Figure 4c and d).
Carbonates: the mixed sediments with endogenous carbonate mass range of 50–75% are classified as carbonates, with a dominance of silty dolomite silty limestone in the study area. Three types of argillaceous limestone are distinguished, namely sandy dolomite, silty limestone, and muddy limestone (Figure 4e and f).
4.1.2 Logging facies analysis
Based on the simplified classification and logging, the sensitivity of each logging curve in the Canglangpu formation encountered in the study area and adjacent areas for the structure and composition of the mixed rock was tested and analyzed. The results showed that natural GR, neutron, and AC are sensitive to different compositions and structures of mixed sediments. However, due to the overall large burial depth and high compaction degree of the Canglangpu formation in the eastern Sichuan basin, the density curve was less sensitive to the structure and composition of mixed sediments. Indeed, it was found that the DEN method can only roughly distinguish between mud and non-mud, suggesting poor applicability of the DEN curve. Therefore, the DEN intersection diagram was not used in this study. The rock-electric response relationship in the Canglangpu formation is shown in Table 1.
|Lithologic||Type||GR (API)||AC (μs/m)||CNL (%)|
|Range||Logging curve shape||Range||Logging curve shape|
|Sandstones||Argillaceous sandstone||70–190||Jagged blocky||>60||Jagged||<17|
|Dolomitic sandstone||<70||Jagged blocky||55–60||Jagged blocky||<13|
|Calcareous sandstone||<65||Irregular jagged||45–50||Irregular jagged||1–4|
|Mudstones||Sandy mudstone||>110||Jagged blocky||>60||Jagged blocky||>13|
|Mudstone||>150||Blocky with irregular jagged||>60||Blocky with irregular jagged||>13|
|Sandy dolomite||<65||Jagged blocky||45–50||Jagged blocky||<5|
|Gypsum rocks||Very low||Irregular jagged||45–50||Irregular jagged||Very low|
|Ooid limestone||35–65||Jagged blocky||45–50||Jagged blocky||4–6|
|Sandy limestone||>30||Jagged blocky||45–50||Jagged blocky||3–7|
GR, AC, and CNL are natural gamma-ray logging, acoustic transit time logging, and neutron logging, respectively.
Natural gamma logging is mainly used to determine the formation properties by observing the natural gamma radioactivity of the rock formations. A large part of the exogenous debris in the Canglangpu mixed sediments is from the Hanjiang ancient land in the northern part of the basin, which contains a large amount of basal neutral magmatic rocks and metamorphic rock cuttings. The natural GR value increases with increasing foreign debris and clay component. The natural GR logging curves of the Canglangpu formation in the study area were highly sensitive to the different compositions and structures of mixed rocks. In the composition classification, the three sediments are bounded by the lower quartile in the box plot. Moreover, mudstone-argillaceous rocks exhibited the highest GR value, often exceeding 70 API, while dolomitic rocks showed a lower value than 70 API (Figure 5a). The lowest GR value of limestone-calcareous rock was below 35 API. In terms of structure classification, the highest GR value of the gravel structure was below 63 API, consisting mainly of limestone, while the clay structure exhibited a higher GR value than that the silt-sand structure. The latter was bounded by an upper quartile value of 130 API. In addition, high and low values were found in the clay and silt-sand structures, respectively (Figure 5b).
On the other hand, the neutron-density log of the Canglangpu formation in the study area was significantly sensitive to the different compositions and structures of mixed sediments. In terms of the composition classification, the boundary was defined by the lower quartiles of the three sediments in the boxplots, with the highest curve values observed in mudstone and argillaceous rocks, often exceeding 9.5%, while the dolomitic rocks showed lower values (less than 9.5%). Limestone-calcareous rocks showed the lowest value, below 3.5% (Figure 5c). In terms of structure classification, the grain structure consisted mainly of limestone, within a range value of 4–6%. Whereas the proportion of mud structures was higher than that of sand structures, showing a value of above 10.5% and in the range of 4.5–10%, respectively (Figure 5d).
Acoustic transit time logging is a logging method for determining the formation properties by observing the ability of the formation to transmit acoustic waves. It is only related to the physical properties of the rock formation, and it has a high sensitivity to different components of the mixed sediments. The acoustic time difference of carbonate facies mixed rocks in the Canglangpu formation was generally less than the lower quartile of mudstone and argillite (55 μs/m), while the curve distribution of Limestone-calcareous rocks showed minor variations, ranging from 45 to 50 μs/m combined with the natural gamma curve, the composition of the mixed sediments in the study area can be better identified (Figure 5e). However, according to the results of boxplots, the acoustic transit time curve was unable to distinguish between the muddy and sandy structures, reflecting the sedimentary characteristics of highly mixed sand and mud in the mixed shelf in the study area (Figure 5f).
4.2 Facies types and distribution characteristics of mixed sediments
4.2.1 Mixed sedimentary facies zone classification
Based on the analysis of drilling and logging data of exploration Wells in the Canglangpu formation, the observation of the outcrop of the Yangsiqiao section, the regional geological background, and previous research results, the facies of mixed shore and mixed shallow shelf sediments, as well as several sub-facies and microfacies types, were identified (Table 2).
|Mixed sediments shore||Mixed lagoon||Muddy lagoon|
|Mixed tidal flat||Sandy tidal flat|
|Dolomitic tidal flat|
|Muddy sedimentary tidal flat|
|Mixed beach bar||Oolitic shoal|
|Mixed sand bar|
|Mixed sediments Shallow marine shelf||Terrigenous detrital mixed shelf||Sandy shelf|
|Sandy and muddy shelf|
|Carbonate mixed shelf||Gray shelf|
|Argillaceous gray shelf|
|Sandy dolomitic shelf|
220.127.116.11 Mixed shore sediment facies
The topography of mixed shore sediment facies is relatively wide and slow, developed on the wave base plane. This zone develops, indeed, on the general wave base plane. According to the different geographical locations of development, this facies zone can be classified into three sub-facies: the mixed lagoon isolated from the wide sea, the mixed tidal flat around the lagoon, and the mixed beach bar distributed along the coast. The mixed shore facies in the area are dominated by facies-change mixed sediments (Figure 6).
It is mainly developed on the inner side of the mixed beach and bar and separated from a wide sea, with a low-energy of depositional environment and limited development. The bottom is dominated by mudstone deposits with silty thin layers, while the upward transition consists of thin interbeds of argillaceous-silt-dolomitic components. On the other hand, logging curves showed high GR values, with a box-shaped superposition of fingers of GR curves. In addition, the results revealed low AC values, while the neutron and density curves were twisted, with opposite change directions (Figure 6).
Mixed tidal flat sub-facies
It is developed at the top of the lower member and the top of the upper member of the Canglangpu formation. The sandy tidal flat and dolomitic tidal flat microfacies are dominated by carbonate siltstone and argillaceous siltstone, respectively, while the muddy sedimentary tidal flat microfacies are dominated by mudstone, gypsum rock, and dolomitic mudstone, which showed twisted particle and density curves were in opposite directions (Figure 6).
Mixed beach bar sub-facies
These sub-facies can be classified as oolitic beach and mixed sand bar, with relatively open sedimentary water and closed sedimentary water, suggesting weak and strong terrigenous detrital supply, respectively. The oolitic shoal microfacies are developed in the lower member of the Canglangpu formation. The lithology consists mainly of oolitic limestone. In addition, the GR and AC curves were low-amplitude box-shaped dentate, while the neutron value was very low. The mixed sand bar microfacies are developed in the upper member of the Canglangpu formation, with a high lithological mixing degree, including terrigenous quartz sand, quartz silt, feldspar, rock debris, carbonate sand debris, clayey sand, and gravel. GR and AC curves form a serious toothed box type (Figure 6).
18.104.22.168 Mixed shallow shelf sediment facies
The mixed shallow shelf is developed below the general wave base and above the storm wave base. These facies can be divided into clastic mixed shelf and carbonate mixed shelf sub-facies, depending on the difference between the main sediments. The carbonate mixed shelf sub-facies is far from the shoreline (Figure 6).
Terrigenous detrital mixed shelf sub-facies
Canglangpu formation was widely developed in the study area during the deposition, showing a thicker upper member than the lower member. Moreover, the terrigenous debris supply is more sufficient in the late Canglangpu formation, consisting mainly of sandy mudstone, calcareous mudstone, mudstone, argillaceous fine sandstone, and calcareous siltstone. According to the different main components, this sub-facies can be classified into the sandy shelf, sandy and muddy shelf, and muddy shelf microfacies (Figure 6).
Carbonate mixed shelf sub-facies
Carbonate mixed shelf sub-facies is developed in the entire Canglangpu formation. The thickness of the upper member is less than that of the lower member, indicating that the terrigenous debris supply is more sufficient in the late Canglangpu formation, which consists mainly of mud-silt limestone. In addition, the AC and GR curves were box-shaped with low amplitude, while the neutron and density curves were open-shaped in opposite directions. Based on the main composition and characteristics, these sub-facies can be classified into the gray shelf, argillaceous gray shelf, and sandy dolomitic shelf microfacies (Figure 6).
4.2.2 Mixed sedimentary distribution characteristics
22.214.171.124 Longitudinal distribution of mixed sedimentary characteristics
It can be seen from the continuous well map (Figure 7) that the lower member of the Canglangpu formation in the NW direction is dominated by carbonate miscible shelf sub-facies (Well Mashen 1), while mixed tidal flat sub-facies and clastic-miscible shelf sub-facies are developed in eastward to the paleo-uplift of the submarine (Well Wutan 1). Whereas the carbonate mixed shelf sub-facies were developed in the lower member of clastic mixed shelf deposition (Well Li1). In addition, a large number of clastic shore facies deposits were developed in the upper member of the Canglangpu formation (well Mashen 1), indicating an exogenous debris deposition, while in the well Wutan 1, mixed beach bar sub-facies were developed due to the high terrain. In the upper member of the Canglangpu formation, the mixed tidal flat sub-facies are mainly developed in Mashen 1 and Wutan 1 wells, while carbonate mixed tidal flat sub-facies are developed in Li 1 well, showing the lowest water depth and the maximum retreat.
On the other hand, Gaoshi 16 well and Guangtan 2, in the Canglangpu formation, are located on the slope of the paleo-uplift (Figure 7). In addition, some carbonate miscible shelf sub-facies are developed in the lower member of the Canglangpu formation, while the miscible tidal flat, miscible clastic shelf and miscible tidal flat sub-facies are mainly developed in Wutan 1 well. Whereas the sedimentary facies belt consisted mainly of miscible carbonate shelf, miscible clastic shelf, and miscible thin beach bar sub-facies in the upper member of the Loutan 1 well in the Canglangpu formation; Moreover, the well diagram showed that the water depth first becomes deeper and then becomes shallow. Indeed, Kaijiang and Guang’an areas, where Wuxun 1 Guangxun 2 wells are located, exhibited the highest sea level, followed by Hechuan and Fengjie areas, where Gaoshi 16 Louxun 1 wells are located, respectively. The lithology in the upper member of the Canglangpu formation reflects the upward and downward trends of water depth, reaching the maximum retreat at the upper member, and consisting mainly of mixed tidal flat sub-facies.
The lower member of the Canglangpu formation is a mixed shore-mixed shallow shelf deposit developed during rapid transgression and insufficient terrigenous supply. The mud-silt-crystal dolostone is in abrupt contact with the mud texture layer of the lower Qiongzhusi formation, which is a continuous dolomitic tidal flat sedimentary environment. However, with the gradual deepening of the sea level, the lithologic gradually changed to dolomitic sandstone and argillaceous sandstone, accompanied by a gradual increase in the content of debris content and a maximum transgression in the middle section of the argillaceous shelf. In addition, with the high mud content, the depositional environment gradually changed to more exposed and oxidized conditions, and dolomitic mixed tidal flats developed, resulting in a gradual change of the lithology to silty-fine-crystalline dolostone. The first transgression–regression cycle of the Canglangpu formation in the study area has been terminated.
The upper member of the Canglangpu formation consists mainly of the mixed neritic shelf-shore sediments, developed under the background of relatively shallow water and sufficient terrigenous supply. While only a small fraction of the mixed littoral facies deposits were developed on the upper member of the paleo-uplift. In the early stage of the Canglangpu formation, the mixed neritic shelf-shore sediment development was affected by slow transgression and important terrigenous clasts supply, resulting in the formation of dolomitic siltstone and argillaceous siltstone, with a sandy argillaceous shelf depositional environment, and then continued to retreat, reaching the maximum retreat at the upper member of the Canglangpu formation, forming a set of mixed tidal flat deposits with high terrigenous clastic contents. The second transgression–regression cycle experienced in the Eastern Sichuan basin region during the Canglangpu period has been terminated (Figure 7).
126.96.36.199 Planar distribution of mixed sedimentary characteristics
The depositional range of clastic neritic shelf deposits in the study area can be better revealed using numerous methods, including representative profiles of the study area, exploration wells (e.g., Well Wutan 1, Well Loutan 1), analysis of the plane distribution characteristics of the sedimentary facies of the Canglangpu formation, combined with the contour map of the regional stratigraphic thickness.
The overall thickness of the lower member of the Canglangpu formation is relatively small, with a thickness value of 65 m in the Loutan 1 well area, while in other locations, the thickness value is less than 100 m. From the east to the periphery of the basin, the sedimentary environment gradually changed to the deepwater shelf facies, while sediments changed to pure carbonate rock. The sedimentary thickness gradually exceeded 200 m, with a dominance of local subaqueous fan deposits in the area of Ecan 1 Well in the East direction. A set of mixed shore-clastic shelf and carbonate shelf deposits were developed in the area from West to East. In addition, the mixed oolitic beach deposits containing quartz silty sand were developed locally (Figure 8a and b).
The exogenous debris supply of the upper member of the Canglangpu formation has increased significantly. The sedimentary thickness was less than 120 m overall and might exceed 200 m as it extends out of the basin to NW, NE, and SE. Overall, there was little change in the sedimentary patterns in the north–south direction, while the northwestern and southeastern parts revealed significant sedimentary patterns. The mixed detrital shelf is largely developed, while the mixed sand bar and mixed lagoons are developed in Guang’an-Dazhou and Kaixian-Wuxi regions, respectively. In addition, a series of the mixed shore-clastic mixed shelf and carbonate mixed shelf were mainly observed from NW to SE and from the extremity of the basin to the deepwater shelf, while the mixed shoal-bar and lagoon-facies were developed locally (Figure 8c and d).
Based on the studying the previous research results on mixed sedimentation and combined with the semi-confined shore-shelf environment surrounded by underwater paleo-uplift in the eastern Sichuan basin, a mixed shore-mixed shelf sedimentary model has been established, and three main mixed sedimentary facies in the shore-shelf environment have been revealed, namely the clastic sedimentary area in the northwest limit of the study area, the mixed sedimentary area in the study area, and the deepwater sedimentary area outside the eastern basin.
During the deposition of the Canglangpu formation in the study area, the Kang-Dian ancient and Hanjiang ancient land on the western and northern parts, respectively, were the main source areas of exogenous debris. The Chengkou-Kaixian sag between the three paleo-uplifts of Dazhou-Kaijiang, Dong’an-Wuxi, and Suining-Guang’an is a mixed sedimentary zone. Whereas from the Tongjiang area (outside the study area) to the Dazhou-Kaijiang area, the content of exogenous debris gradually decreased, and the endogenous carbonate gradually increased. The gradual decrease in the topography and the gradual increase in the water depth in the eastern part of Liangping-Kaixian led to a gradual decrease in the exogenous debris in the sedimentary water and a gradual increase in the endogenous carbonate content, resulting in a transition to the deepwater shelf facies (Figure 9).
The sedimentary environment of the Canglangpu formation in the eastern Sichuan basin was inherited from the sedimentary pattern of uplift and depression in the Qiongzhusi formation. Indeed, in the early stage, the vertical lifting movement weakened, and the paleogeomorphology has gradually changed to a gentle slope, with high and low values in the western and eastern parts of the study area, due to an overall transformation of the tectonic setting of the upper Yangtze craton . Although the northwestern part exhibits the characteristics of coastal deposition due to uplift, it remains entirely below sea level, while the sedimentary system mainly changes continuously in the transverse and vertical directions, without uniform and planar distribution, which is the entire area of the exogenous debris and endogenous carbonate interaction, with a fluctuation in the paleogeomorphology of the sedimentary environment.
In the early stage of Canglangpu, the terrigenous detritus supply in the western provenance area was inhibited due to the rapid increase in sea level and sedimentary water energy. The endogenous carbonate deposition was dominant, and the exogenous detritus only appeared on the landward part of the underwater paleo-uplift. Subsequently, under the background of the overall sea level decline, the sea level fluctuates frequently within a small range, strengthening the intermittent supply of terrigenous debris. The mixed tidal flat interbedded with terrigenous debris and endogenous carbonate was widely developed in the region, while the mixed lagoons were developed in small areas, covering Liangping-Guang’an (Figures 8a and 9). Because of the overall shallow water depth, sudden storm events may transport the terrigenous debris from the shallow water to the deepwater sedimentary zones. At the end of the period, the sea level increased again, resulting in a gradual dominance of the endogenous carbonate, while mixed carbonate shelf deposits were gradually transformed into carbonate shallow shelf of the Longwangmiao formation.
In this study, we established the criteria for identifying different lithologies through outcrop observation, thin section identification and morphological analysis of well logs. Then, under the guidance of sedimentological theory, the framework of sequence stratigraphic correlation is established. The rock characteristics and sedimentary facies development law of the Canglangpu formation of the target strata were investigated. In addition, the genetic mechanism of mixed sedimentary was further analyzed. The main conclusions reached are as follows:
A three-end member classification method for mixed rocks, suitable for Canglangpu formation in the eastern Sichuan basin, was established. The sedimentary exogenous clay, exogenous clastic, and endogenous carbonate, with a mass fraction range of 25–50%, were classified into hybrid mixed sediments. The mixed rocks with less than 50% of the three components were classified as normally mixed sediments, providing a reference for mixed deposit studies in areas lacking coring data.
The mixed neritic shelf deposits of the Canglangpu formation are widely developed in the Eastern Sichuan basin area, with slight differences in the north–south direction. In addition, two main facies zones are developed, from West to East, namely clastic sedimentary, mixed sedimentary, and deepwater sedimentary facies. From the vertical view, the Canglangpu formation can be divided into lower and upper members. Indeed, the lower member is composed of the clastic mixed shelf that gradually transforms eastward to the carbonate mixed shelf, while the upper member is mainly composed of the clastic mixed shelf.
The formation mechanism of mixed sedimentation in the eastern Sichuan basin is mainly controlled by the change of sedimentary facies belt, and it develops in a circular belt along the edge of successive underwater paleo-uplift. The development intensity is mainly influenced by early tectonic movement and sea-level change, while climate and storm events are minor factors. In addition, the fractions of argillaceous material showed increases from West to East, while the fractions of sandy material revealed decreases from West to East, with the carbonate rock used as a distribution point for the compensating deposition.
This work was supported by the National Science and Technology Major Project of China (Grant No. 2016ZX05007-002) and The Study on the geological characteristics of the lower Cambrian in Dazhou-Fengjie area, PetroChina Southwest Oil & Gas field Company Chongqing Gas Mine, CNPC (Grant No. XNS02JS2019-0036).
Funding information: The National Science and Technology Major Project (2016ZX05007-002).
Conflict of interest: The authors have no conflicts of interest to declare that are relevant to the content of this article.
Data availability statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.
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