Introduction

Sequence stratigraphy is a branch of geology, which attempts to subdivide sedimentary rocks into system tracts based on sequence boundaries. Sequence boundary is maximum flooding surfaces and unconformities primarily, which define the maximum and minimum sea level stands. (Bosence et al. 2000; Barnaby et al. 2007; Nalin et al. 2009; Hemmesch et al. 2014; Kadkhodaie et al. 2017). It is also a surface separating younger strata (above) from older strata (below), along which there are evidences of subaerial erosional truncation and possibly correlative submarine erosion, or subaerial exposure, with a significant hiatus indicated (Slatt 2013). Previous studies have confirmed that there are a large number of sequence boundaries of different orders in carbonate formations developed, and those sequence boundaries play important roles in reservoir development (Overstreet et al. 2003; Aurell et al. 2004; Krause et al. 2004; He et al. 2014). The control function of sequences on high-quality carbonate reservoirs has been verified by many exploration practices all over the word (Bosence et al. 2000; Pan et al. 2011; Guo et al. 2011). Accordingly, the sequence division of carbonate formations is important for hydrocarbon exploration and development (Xu et al. 2011; Hu et al. 2012).

Well logging data are widely used in sequence division of carbonate formations owing to its high resolution (Yi et al. 2011; Feng et al. 2016a, b; Gao et al. 2016). Nowadays, the divisions of carbonate sequences are mainly based on natural gamma data, and the principle is that the increase of natural gamma is response for the increase of mud content, which indicates the deepening of water body (Nazeer et al. 2016; Kadkhodaie et al. 2017; Mansour et al. 2018). Accordingly, the thick mudstone is the sign of water body deepening, which is consistent with the maximum flooding surface (MFS) (Gao et al. 2016). For this, the natural gamma curve has been well applied in the sequence division of terrigenous clastic rocks and deep-water carbonate formations (Fu et al. 2013; Yin et al. 2015; Pang et al. 2020a). However, some challenges of this method exist during the sequence division in shallow platform: (1) the increasing of clay content in the mudflat indicates the shallow of the water body, which is contradictory with the principle mentioned. (2) Some weathered carbonate formations features high natural gamma value, which is not the result of the deepening of water body but the response of the tectonic uplift. (3) The enrichment of uranium in reservoirs can cause the high natural gamma, which is not the response for the deepening of water body as well.

In this paper, with Bozhong 21–22 structure of Bohai Sea as an example, we clarified the origin of high gammas in shallow platform, and synthesized a new curve indicating the sedimentary environment. Based on the new curve, the sequence stratigraphy of the study area was divided, and the relationship between sequences and high-quality reservoir was discussed as well. The results bring important guidance for the future exploration of the study area, and provide a new sequence division method with well logging data for shallow platform, which has reference significance for the sequence division of the same geological conditions all over the world.

Geological backgrounds

Bohai Bay Basin (BBB) is a major continental petroliferous basin located in Eastern China (Huang et al. 2014; Ye et al. 2020a), and Bohai Sea is the offshore part of the Bohai Bay Basin, which covers an areas of about 4 × 104 km2. (Xu et al. 2018; Ye et al. 2019a, b). As the biggest hydrocarbon-generating depression in Bohai Sea, Bozhong depression is characterized by Neogene oil and gas accumulation, but a large amount of hydrocarbon has been explored in buried hills recently (Fig. 1a) (Gong et al. 2007; Hao et al. 2009; Huang et al. 2016a, b; Ye et al. 2020b). Bozhong 21–22 structure located in the southwest of Bozhong depression, which also in the west of TanLu fault zone (TLFZ) (Fig. 1b). The structure is adjacent to Bozhong 28–1 oil field and Penglai 19–3 oil field, which is the biggest carbonate buried hill oil field and the biggest oil filed in the Bozhong depression, respectively (Fig. 1b). The results of oil source correlation show that the hydrocarbon mainly comes from the source rocks of the third member of the Dongying Formation (E3d3) and the first member of Shahejie Formation (E2s1) commonly; they provide the preservation conditions for the oil and gas as well (Zhou et al. 2017) (Fig. 1c). With fractures and dissolved pores as the main reservoir spaces, the reservoirs in the study area are mainly distributed in Majiagou Formation of Ordovician (Fig. 2).

Fig. 1
figure 1

Tectonic location of Bozhong depression and BZ21–22 structure. a The distribution of Bozhong depression, (b) the distribution of Bozhong 21–22 structure and c the gas distribution of Bozhong 21–22 structure

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Fig. 2
figure 2

Stratigraphic column of the Bozhong depression

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Bozhong depression has experienced Paleogene syn-rifting stage and Neogene-Quaternary post-rifting stage (Huang et al. 2016a, b; Ye et al. 2016). The syn-rifting sequence includes Shahejie Formation (E2s) and Dongying Formation (E3d), in which the first member of Shahejie Formation (E2s1) and the third member of Dongying Formation (E3d3) are mainly composed of lacustrine mudstone, and the first and second members of Dongying Formation are mainly composed of fan delta sandstone (Ye et al. 2019a, b). In the post-rifting sequence, Guantao Formation, Minghuazhen Formation and Pingyuan Formation are developed, which are mainly composed of fluvial facies (Ye et al. 2020c). As the basement of Bozhong depression, Majiagou Formation was divided into upper Majiagou Formation and lower Majiagou Formation (Chen et al. 2016). The tectonic movements since the Paleozoic in East Asia have an important influence on the evolution of sedimentary basins and oil and gas geological conditions in Eastern China (Pang et al. 2019, 2020b). Affected by Caledonian movement, Yanshan movement and Himalayan movement, the top of the upper Majiagou Formation is a regional unconformity. The lower Majiagou Formation is mainly composed of inter-bedding of mudstone and argillaceous dolomite, and the upper Majiagou Formation is mainly composed of inter-bedding of mudstone and limestone (Fig. 2).

Data and method

Data

The data from two scientific exploration wells were collected, which include cutting logging data, conventional logging data, electrical imaging logging data and natural gamma spectrometry data. Conventional logging data include natural gamma ray (GR), density (DEN), acoustic time difference (AC), neutron porosity (CNL), deep and shallow resistivity (RD, RS); natural gamma spectrometry data include U, Th and K curves. The data of conventional logging, electrical imaging logging and natural gamma spectrometry logging were completed by China Oilfield Services Limited (COSL). At the same time, data of 3.2 m cores and 49 sidewall cores were collected, and the physical properties of all sidewall cores were analyzed. The casting thin section observation was carried out on 49 samples to determine the characteristics and types of the reservoir, and the electrical imaging logging data were used to interpret reservoir as well. Natural gamma spectrometry data were used to divide the sequences.

Method

To divide the sequence of the study area, we analyze the origin of gamma anomalies (high gamma sections with GR value bigger than 25 API, which is the background value of the pure limestone and dolomite), and separate the types which are contribute to stratigraphic sequences divided. By comparing the well logging data (including the ratio of different curves) with the gamma anomalies of different origins, we found the ratio of Th/K can reflect the changes of sedimentary environment of shallow platform effectively (see below for detailed discussion). To eliminate the high-frequency information of the new curve (Th/K), the Th/K curve is processed by continuous wavelet transform (CWT). The CWT is a signal-processing tool, which can reveal the hidden information of signal, and is widely used in geological interpretations (Kadkhodaie et al. 2017). The basic principle of the wavelet transform is the expansion and translation of functions (Tang 2006). It transforms the independent variable t of the wavelet function ψ (t) with scaling (a) and shifting (b), and then we take the inner product with the function f(t) to get the function WTf (a, b) (Wang et al. 2012; Ren et al. 2013):

$$ WT_{{\text{f}}} (a,b) = \frac{1}{{\sqrt a }}\int_{{{\text{ - }}\infty }}^{{ + \infty }} {f(t)\phi (\frac{{t - b}}{a})} dt,a > 0 $$
(1)

Obtaining the results of WTf (a, b) includes the following five steps:

  1. 1.

    Select a wavelet with (a) as the fixed value, and compare it with the original signal;

  2. 2.

    Calculate WTf (a, b), which represents the correlation degree between this signal and the selected wavelet. The larger WTf (a, b) is, the more similar the two are;

  3. 3.

    Move the wavelet to the right, repeating step (1) and step (2) until the full part of the signal is processed;

  4. 4.

    Increase the (a) of the wavelet (stretch), repeating step (2) and step (3);

  5. 5.

    Repeating step (1) to step (4) for all selected (a) to obtain all WTf (a, b).

In this study, the Gaus wavelet was selected as the initial waveform, and (a) was iteratively calculated from 1 to 128. The calculated results were compared with the lithology and geochemical (carbon and oxygen isotope) data, and the sequence division was carried out with the results with the best sequence correspondence. The study of reservoir characteristics is mainly based on the physical property statistics of the sidewall cores, the casting thin sections and the electrical imaging logging data.

Results

Geological significance of high natural gamma

The high gamma characteristics in the study area can be divided into three types: high gamma caused by weathering, uranium element enrichment in reservoirs and argillaceous sediment.

High gamma caused by weathering: this type of high gamma is usually along with the weathering crust and caused by the enrichment of the radioactive material such as weathered clay during the leaching process (Liu et al. 2009). There exists a weathering interface in the depth of 4862 m, showing the Majiagou Formation is covered by Paleogene (65 Ma), and the silty mudstone can be found near the weathering crust (Fig. 3a). Owing to the weathering intensity decreases as the distance to the weathering interface increase (Fig. 3a), the value of gamma reduced until the value is equal to the background value of the carbonate rocks (mostly less than 40 API), showing rocks did not affect by weathering any more. For that, this kind of high gamma always shows a funnel-shaped curve characteristic, and the DEN, AC and RD always show bell-shaped curve characteristic. The value of U and Th increase and show a funnel-shaped curve characteristic from top to bottom, while the value of K does not change in the natural gamma spectra. In addition, there is no obvious change in the value of Th/U, while Th/K curve shows a similar type as natural gamma curve (Fig. 3a).

Fig. 3
figure 3

The characteristics of conventional logging and natural gamma spectra logging of different genesis of high gamma, (a) high gamma caused by weathering, note the curves of GR, U, Th and Th/K all show a funnel-shaped characteristic from top to bottom, and (b) high gamma caused by uranium enrichment in reservoir, note only U increased. c High gamma caused by sedimentary mudstone, note the Th/K difference of the two origin mudstone

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High gamma caused by the uranium enrichment in reservoir: in conventional logging, it shows approximately “box-shaped” characteristics where the natural gamma value is slightly higher than the background value of limestone. It is worth noting that only U curve presents positive anomaly while the Th and K curves are normal in gamma spectrum curve. In addition, the positive anomaly of U is commensurate with that of natural gamma, which indicates that the high gamma is caused by the uranium enrichment. Such kind of high gamma have been reported in many studies and they are mostly developed in the inner reservoir (Zhang et al. 2006; Peng et al. 2009). The enrichment of uranium is mostly related to the activity of formation water. The formation water rich in (UO2)2+ migrated to the deep reservoirs along with the fault, karst fracture development zone or tectonic fracture zone. Then they were reduced to tetravalent uranium and precipitated under the reduction condition, thus forming the phenomenon that the uranium enrichment and the gamma increased in the reservoir (Peng et al. 2009; Feng et al. 2016a, b). Actually, in the segment of the high gamma, the obviously decreasing of electrical resistivity (RD) and the increasing of interval transit time (AC) all prove the development of the reservoir (Fig. 3b).

High gamma caused by argillaceous sediment: sedimentary mudstones are featured with high GR, high CNL, high AC and low RD value. In natural gamma spectrum, U, Th and K curves all present obvious positive anomaly, while obvious differences can be found from Th/U and Th/K. The grey mudstone co-exist with thick-layer limestone which mostly developed in reducing environment, shows no Th/K anomaly but the Th/U irregular variation, while the purple red mudstone co-exist with dolomites, which mostly developed in oxidation environment, shows the high Th/K without Th/U changes (Fig. 3c).

Three types of high natural gamma in the study area are formed due to different genesis and have different indication significances in sequence division. The high natural gamma segments of the weathering crust are mainly related to the large-scale unconformity surface controlled by tectonic movements, which usually indicate to second-order sequence interface. The gamma anomaly caused by uranium enrichment in reservoir is the response to reservoir which has no environmental indicated meaning. The mudstone formed in reduction environment mostly indicates the deepening of water body, which usually locate near the maximum marine flooding surface, while the mudstone formed in oxidation environment are mostly the response of the water body gets shallow as well as indicate the top and bottom interface of sequences.

Comparing results between the gamma anomalies and their logging response show high gamma caused by weathering and oxidation mudstone have high Th/K, indicating shallow water, while high gamma caused by reduction mudstone presents low Th/K, indicating deep water. More importantly, although the uranium enrichment makes gamma positive anomaly, Th/K has no abnormality, which eliminating the influence of this factor effectively. Therefore, Th/K was selected for sequence divided in this study.

Sequence development characteristics

The ratio of the Th/K in Majiagou Formation of Bozhong 21–22 structure shows obvious cycling vertically. To identify the sequence interface of different levels more directly, the Th/K curves were processed with wavelet transform. By trying different wavelet types and parameters (mainly parameter a), it was found that when the Gaus wavelet was taken as the initial wavelet and a = 64, the processing results are in good agreement with the third-order sequence.

The processed results show that there are five cycles developed in well BZ21-B-1, and four cycles in well BZ22-A-2. Considering depositional stability of the Paleozoic due to Craton background, it is concluded that there are five cycles (third-order sequences) developed in Ordovician, including SQ1, SQ2, SQ3, SQ4 and SQ5 from bottom to top. Among them, there are two developed in lower Majiagou Formation and three developed in upper Majiagou Formation (Fig. 4). The comparative analysis shows that the SQ1–SQ4 in two wells has a good correlation (Fig. 4), while the SQ5 was corroded in BZ22-A-2 during “Caledonian-Yanshan movement”.

Fig. 4
figure 4

Well comparison of three-order sequences between well BZ21-A-2 and well BZ22-1-A. Noting that in low gamma background, high density and high neutron represent dolomite (the yellow fill part between DEN and CNL curve), and conversely, limestone

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SQ1 presents relatively high gamma, positive intersection of the DEN and CNL curves (filling with yellow color), and bow-shape of the RD in well logging, and the lithology of the lower part is composed of dolomite, calcite dolomite and mudstone, while the upper part is dominated by limestone (Fig. 4). The contents of the dolomite support the lithology characteristics, showing higher dolomite content in the lower part than upper part. SQ2 characterized of low gamma, and the DEN overlaps with the CNL in well logging; the lower part of the sequence is composed of limestone and argillaceous limestone, while the upper part was dominated by pure dolomite (Fig. 4). The SQ3 was composed of pure dolomite (the yellow filling part is wider), which was the marker layer for regional comparison. With the high gamma in the lower part of SQ4, the gamma decreases upward, showing the “bell-shaped” characteristics; the lower part of this sequence is composed of limestone and mudstone, while the upper part was dominated by pure limestone (Fig. 4). The SQ5 was dominated by limestone, but the lower part of this sequence characterized with three high gamma spikes, indicating the higher clay content compared with the upper part (Fig. 4).

Reservoir characteristics

Fractures and pores are the main reservoir space in the study area. The fractures can be divided into structural fractures and dissolved fractures, and pores can be divided into dissolved pores and inter-granular pores further.

With the characteristic of straight fracture surface, multiple-phase structural fractures were observed in thin sections. The fractures of the early stage are filled seriously, while the fractures of later stage are still open (Fig. 5a), which are effective fractures for hydrocarbon accumulation. The dissolved fracture’s surface is irregular (Fig. 5b), which always distribute along the unconformity.

Fig. 5
figure 5

Reservoir characteristic and petrology characteristic of Bozhong 21–22 structure. a BZ22-A-2, 4365.15 m, limestone, noting the structural fractures cut each other, and the later-stage fractures are effective; b BZ22-A-2, 4389.5 m, limestone, dissolved fracture with irregular fracture surface; c BZ22-A-2, 4383 m, gravel clastic dolomite, the dissolved pores in gravel clastic; d BZ21-A-1, 4891.1 m, bioclastic limestone, mold pore; e BZ22-A-2, 4401.5 m sandy limestone, inter-granular dissolution pores; f BZ22-A-2, 4417.2 m, granular dolomite with selective dissolution pores; g BZ22-A-2, 4435.1 m, crystal powder dolomite with inter-crystalline pore; h BZ22-A-2, 4423.5 m, crystal powder dolomite with inter-crystalline pore

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Dissolved pores developed near the weathering crust and most of them are related to the dissolution of atmospheric fresh water (Fig. 5c); petro-fabric selected dissolved pores are well developed in the study area as well. The selected dissolution includes dissolution of granules and cement, and the granules dissolution mostly relates to pene-sedimentary karst (Huang et al. 2015; Zhu et al. 2015). Some bio-detritus were dissolved seriously by atmospheric fresh water, which formed mold-pores (Fig. 5d). At the same time, the dissolution of some particles were observed in thin sections as well. Although the dissolved pores were partially filled nowadays (Fig. 5f), storage ability of reservoirs were still preserved. The selected dissolution of cements is mainly developed in granular limestone, which may be related to the burial dissolution (Fig. 5e). The inter-crystalline pores in dolomite are mostly related to dolomitization, with small pore diameter and few fractures developed as well (Fig. 5g, h).

The statistics of physical properties form 49 samples show that the physical properties in the study area are high heterogeneous. The porosity ranges from 0.5 to 16.7%, with an average value of 4.1%. The relationship between physical property and the distance to the weathering surface shows that zonation is present. With dissolved fractures developed, the physical properties of the reservoir within 20 m are generally good, which related to the strong weathering and tectonic movement. High-porosity inner-zone still developed between 50 and 150 m from the weathering surface, which are mainly composed of dissolved pores nearly horizontal distributed (Fig. 6). Both of them control the development of thick reservoirs in the study area.

Fig. 6
figure 6

The relationship between porosity and the distance to the weathering surface

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Discussion

Sequence difference

Regional distribution

Ordovician sequences of BZ21-A-1 well are well compared with surrounding outcrops. There are two third-order sequences developed in the lower Majiagou Formation and three third-order sequences in the Upper Majiagou Formation. Li divided the Wuning profile of Shanxi, Mentougou profile of Beijing and Pingquan profile of Hebei into five third-order sequences as well, (Li 2007). The lithologic combination of the study area is similar with that of the Pingquan of Hebei province mostly, indicating they have similar geological conditions during the deposited stage. With thick argillaceous limestone or limestone deposited, the maximum flooding surface of the Majiagou Formation is located in SQ4 (Fig. 7), which is consistent with previous studies (Han et al. 1997; Tian et al. 1997; Li 2007). All results indicate that the sequence of Majiagou Formation in Bozhong 21–22 structure are well contrast with that of the outcrops of North China Platform. The thickness of the individual third-order sequence in the study area is between 40 and 60 m, and the thickness of different third-order sequences varies in different regions, which may be related to the micro-paleo-geomorphology.

Fig. 7
figure 7

Third-order sequence contrast between surrounding areas and well BZ21-A-1

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Sequence boundary

There are two types of sequence boundaries developed: type-I sequence boundary (SB I) shown as regional unconformity and type-II sequence boundary (SB II) with weak erosion. The bottom interface of SQ1 and the top interface of SQ5 belongs to type-I sequence boundary, and they are important secondary-order sequence interfaces because they represent regional unconformities of “Huaiyuan movement” and “Caledonian-Yanshan movement”, respectively. The other interfaces belong to type-II sequence boundary. Under the influence of “Huaiyuan movement”, a significant unconformity (bottom interface of SQ I) develops between the Liangjiashan Formation and the Majiagou Formation in North China platform, showing a type-I sequence boundary (Tian et al. 1997; Song et al. 2001). The unconformity has different characteristics in different regions: 0.55-m-thick purple mudstone was developed in Quyang area of Hebei province, while the dolomite of lagoon covered on the algal mat of tidal flat in Tangshan (Tian et al. 1997; Li et al. 2007). Meanwhile, under the influence of “Caledonian-Yanshan movement”, there exists a regional unconformity on the top of Ordovician (the top interface of SQ5), which distributed over the whole North China platform, controlling the paleo-geographic and geotectonic changes above and below the interface (Tian et al. 1997). Other sequence boundaries are the sequence transition surface (SB II) between the two unconformities without obvious exposure.

Lithology and lithofacies of different sequence

SQ1–SQ3 are the products of the first transgression after the “Huaiyuan movement” (Han et al. 1997). Those sequences show high content of dolomite with an average value of about 30% (Fig. 8). Among them, the content of the dolomite in SQ3 is the highest with the content of 70% (Fig. 8), which indicates that the sea level has significantly decreased during this time. The lithology in SQ1–SQ3 are dominated by dolomite and calcite dolomite, while grained dolomite and grained limestone are rarely developed, which proves that they are deposited in restricted platform facies with relative high salinity and low hydrodynamic conditions. SQ4–SQ5 forms another cycle. The dolomite content is significantly reduced which is mostly less than 10%, and amount of granular limestone is developed. The lithology is dominated by limestone and granular limestone, with thin dolomitic limestone and mudstone (Fig. 8), which reflects the open platform facies with low salinity. In addition, a large number of studies have shown that there is a good correspondence between δ13C and sea level (Zhao 2015; Chen et al. 2012). The decrease of the δ13C in carbonate rocks corresponds to the regressive period of sea level, which is verified on the well of BZ21-B-1 (Fig. 8).

Fig. 8
figure 8

The lithofacies of the Well BZ21-B-1. TF—tidal facies, LF—lagoon facies, BF—beach facies, BISF—beach interact sea facies. Noting the high dolomite content of SQ1–SQ3, and the low dolomite content of SQ4–SQ5

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Relationship between reservoirs and sequences

The relationship between sequence and the formation of weathered reservoirs

The drilling revealed that the fractured reservoirs were mainly distributed along the unconformity of the top of the Majiagou Formation. This unconformity is the result of the “Caledonian-Yanshan movement” (Jiang et al. 2015; Zhang et al. 2019), which is a long-term weathering interface and an important tectonic transformation surface.

A large number of studies have confirmed that the reservoirs associated with weathering are mainly response for the tectonic movement and the weathering leaching (Huang et al. 2016a, b; Wu et al. 2017; Mao et al. 2018). The traps of the study area are mainly related to the compression fold during the Indosinian period (Fig. 1c) (Ye et al. 2019a, b). The stress field simulation result confirms that the top of the fold is the region where stress is concentrated (Fig. 9), leading a large number of fractures formed alone the top surface (Zu et al. 2013; Ju et al. 2014). The existence of those fractures provides good channels for the atmospheric fresh water flowing, forming fractured reservoirs with strong corrosion. Physical statistics and well drilling have proved that the reservoirs developed along the unconformity have good physical properties. Therefore, the secondary sequence interface controls the distribution of reservoirs associated with weathering.

Fig. 9
figure 9

The distribution of maximum stress in the extrusion fold

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The relationship between sequence and the formation of inner-type reservoirs

The buried depth of the carbonate in the study area is generally bigger than 4000 m (Fig. 1c), which belongs to the deep-ultra deep reservoirs. A large number of studies have shown that the deep carbonate reservoirs, especially inner reservoirs, are mainly developed in dolomite (Ma et al. 2010; He et al. 2019). On the one hand, dolomites have higher original matrix porosity than limestone. The statistical results show that the matrix porosity of fine-powder crystal dolomite is generally between 3 and 7%, which is much higher than that of micrite-powder crystal limestone with an average matrix porosity of 2% in study area. Through the study of Pingfangwang buried hill in Jiyang depression, Wang et al. confirmed that the carbonate inner reservoirs are mainly developed in dolomite, and the physical property of dolomite was far better than that of limestone (Wang et al. 2017). Ma et al. also affirmed this rule based on the physical property statistics of Puguang gas field in Sichuan basin (Ma et al. 2010). On the other hand, the dissolution ability of different lithology under deep burial environment also affected reservoir quality (Yang et al. 1995, 2014). Through the experiment under buried conditions, She et al. presented that the dissolution rate of limestone is higher than that of dolomite when the temperature is below 90 ℃, but the dissolution rate of the dolomite is significantly enhanced when the temperature exceeds 90 ℃ (She et al. 2013). That is, the dolomite is more likely to be dissolved in the deep buried environment with high temperature. The buried temperature in the study area is generally between 140 ℃ and 160 ℃, leading the dolomite has stronger dissolution rate than limestone. It is the high primary porosity and high dissolution rate that make the dolomite being the main lithology for inner reservoir developed.

The relationship between reservoirs and lithology indicates that the inner reservoirs of the study area are mainly developed in dolomite indeed (Fig. 4). This indicates that the inner reservoirs of study area are controlled by lithology and lithofacies obviously, and the restricted platform is the main facies for dolomite deposited and reservoir forming. Controlled by the regional transgression after “Huaiyuan movement”, SQ1–SQ3 deposited are the main sequences where restricted platform developed, also the inner reservoirs distributed.

The reservoirs developed pattern

There developed weathered reservoirs and inner reservoirs in the study area are controlled by the second-order sequence boundary (unconformity of Majiagou Formation) and the third-order sequence, respectively. Weathered reservoirs are distributed along the second-order sequence boundary at the top of the Ordovician. With stable distribution in plane, such reservoirs are within 40 m below the weathering crust and not controlled by lithology. The inner reservoirs are mainly controlled by lithology with less affected by weathering, which are mainly concentrated in SQ1–SQ3 sequences where dolomite flat developed. Such reservoirs are stratiform, and their distributions are more isochronous compared with the weathered reservoir (Fig. 10). These two types of reservoirs constitute the distribution of reservoirs in the study area together; among them, the distribution of weathered reservoir is stable, while the development of the inner reservoirs greatly extends the lower limit of exploration depth of buried hills.

Fig. 10
figure 10

The reservoirs developed pattern of BZ21/22 structures

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Conclusion

  1. 1.

    The origin of high nature gammas includes weathering, uranium enrichment in reservoirs and normal sedimentary mudstone. Among them, the high gamma caused by weathering and sedimentary mudstones indicates significances for sequences; and the ratio of Th/K curve can distinguish high gamma anomaly of different origin effectively.

  2. 2.

    Based on the continuous wavelet transform of the Th/K curve, the Majiagou Formation are divided into five third-order sequences including SQ1, SQ2, SQ3, SQ4 and SQ5 from bottom to top; among them, SQ1–SQ3 belong to the lower Majiagou Formation and SQ4–SQ5 belong to the upper Majiagou Formation, which can be well compared with regional stratigraphy.

  3. 3.

    There are developed weathered reservoirs and inner reservoirs, and the weathered reservoirs are mainly composed of fractures while the inner reservoirs are dominated by dissolved pores. Weathered reservoirs are controlled by the second-order sequence boundary of the top of the Majiagou Formation, and the inner reservoirs are mainly distributed in the SQ1–SQ3 where dolomite developed.