The carbonate-gypsum salt rock combination is widely distributed globally^[1,2,3,4], and the carbonate-gypsum salt rock combinations in China are mainly developed in the Cambrian- Ordovician, Carboniferous-Permian, Triassic, Paleogene^[5,6,7,8]. The carbonate-gypsum salt rock combination plays an important role in oil and gas exploration. According to the statistics of 206 major carbonate rock fields in the world, 63 oil and gas fields are in carbonate-gypsum salt rock combinations, which account for 30.6% of the total oil and gas fields, and their reserves account for about 46% of the total reserves of carbonate rocks^[9,10]. The Middle East is the area with the most oil and gas fields in the paragenesis combination of carbonate rock and salt rock, and the reserves in these oil and gas fields account for 40% of the total reserves of carbonate oil and gas fields in the Middle East. The Ghawar oil and gas field in the Saudi Arabia, the world’s largest oil and gas field, has recoverable oil reserves of 90.11×10⁸ t and natural gas reserves of 5.27×10¹² m³. The world’s largest gas field-the North-Pars gas field in the Qatar-Iran has recoverable natural gas reserves of 36.73×10¹² m³. Both of them have carbonate reservoir and gypsum salt caprock. 70% of oil and gas fields and 80% of the reserves in Central Asia-Russia are in the carbonate-gypsum- salt paragenesis combinations. The Karachaganak Oilfield, with recoverable oil reserves of 41.10×10⁸ t^[11], also has carbonate reservoir and gypsum salt caprock. The carbonate-gypsum salt combinations in the Sichuan Basin are mainly distributed in the Lower Triassic Feixianguan Formation, the Leikoupo Formation and the Jialingjiang Formation, in all of which gas reservoirs of various sizes have been discovered. The carbonate-gypsum salt combinations in the Ordos Basin mostly occur in the Ordovician Majiagou Formation and the Cambrian, and the large Jingbian Gas Field is in the Majiagou Formation carbonate-gypsum salt combination. The carbonate-gypsum salt rock combinations in the Tarim Basin are in the Middle and Lower Cambrian, which are important exploration formations. Therefore, the study of reservoir type, genesis and reservoir-caprock combination types of carbonate-gypsum salt paragenesis systems is not only of important theoretical significance, but also important practical significance for marine carbonate oil and gas exploration in China.

A lot of basic petroleum geological research on the carbonate-gypsum salt rock paragenesis system has been done before^{[11,12,13,14,15,16,17,18,19,20]}. With regard to the reservoir and reservoir-caprock combination in the carbonate-gypsum salt system, recent studies have revealed that there are microbial dolomite reservoirs, microbial limestone/bioclastic limestone reservoirs besides gypsum dolomite reservoir below gypsum salt^[15,16], and the caprock of gypsum salt rock is conducive to hydrocarbon accumulation^{[17,18,19,20]}. However, the type and genesis of dolomite in this system, the contribution of early low-temperature dolomitization to the development of reservoirs remain unclear. The inevitability or contingency of the paragenesis relationship between microbial dolomite and bioclastic limestone reservoirs and gypsum rocks has been unclear. The phenomenon that there is no dolomite reservoir under the salt rock in some combinations has been difficult to explain, and the understanding on the types of reservoir and caprock combinations needs to be deepened further.

In this study, based on the investigation of global carbonate reservoirs, four carbonate rock - gypsum salt combination sections in China and abroad were dissected, to set up lithological paragenesis combination sequences and explored the coupling relationship between paleoclimate, paleo-ocean geochemical characteristics and carbonate rock - gypsum salt rock lithological combination. Moreover, the type and genesis of dolomite in the carbonate rock - gypsum salt rock system were examined through geological and experimental analysis, and the genesis of the reservoir. Finally, the characteristics and types of reservoir-caprock combination in the carbonate rock - gypsum salt paragenesis system were analyzed to provide a basis for the evaluation of the exploration field according to the analysis of the known oil and gas reservoirs.

Through the study on four sections, the Lower Ordovician Majiagou Formation in the Well Jin 2 of the Ordos Basin, Majiagou Formation on the Jinsushan outcrop of the Ordos Basin, the Ariri-Barra Velha Formation in the Well B of the A Basin in Brazil, and the Leikoupo Formation in the Well Yashen 1 of the Sichuan Basin, the complete lithological combination sequence of the carbonate rock - gypsum salt sedimentary system was established to reveal the relationship between paleoclimate change and lithological combination characteristics.

The Ma 5⁶ - Ma 5¹⁰ Submembers of the Lower Ordovician Majiagou Formation in Well Jin 2 is a typical combination of carbonate rock - gypsum salt rock (Fig. 1), which is mainly composed of algae-laminated dolomite, algal stromatolitic dolomite, algal psammitic dolomite, gypsum dolomite, and gypsum, generally reflecting the lithological combination sequence characteristics of microbial dolomite → gypsum dolomite → gypsum salt rock → micrite.

Upwardly, the content of gypsum salt rock gradually increases, reflecting that the climate became arid gradually. The lower part of the cycle is characterized by interbeddings or intercalations of algae lamination/algal stromatolite/algal psammitic dolomite and micrite dolomite. In the middle of the cycle, the algae lamination/algae stromatolite/algae psammitic dolomite significantly decrease, and gypsum dolomite takes dominance, with algae lamination/algal stromatolitic dolomite lenticular bodies or thin layers and thin-layered gypsum. The upper part of the cycle is a large set of gypsum rock, intercalated with thin-layered gypsum dolomite, algae lamination/ algae stromatolite dolomite. The Ma 5⁵ submember transforms into micrite limestone, which represents the commencement of another climate cycle from humid to arid. Every lithology in the large cycle is composed of several small cycles, for example, the dominance of microbial dolomite, but it also has a thin-layered of gypsum dolomite. For instance, dominant gypsum dolomite may contain thin-layered microbial dolomite and gypsum rock; the dominant gypsum rock may interbed with thin-layered gypsum dolomite and microbial dolomite. The large cycle is the product of regional paleoclimatic changes, whereas the small cycle may be related to the differences in paleomorphology.

The lower part of the sixth member of the Majiagou Formation on the section of the Jinsushan outcrop is mainly composed of algal limestone and bioclastic limestone. The algal limestone includes algal lamination limestone, algal stromatolite limestone, and algal psammitic limestone. The upper part is mainly made up of algae laminated dolomite, algae stromatolitic dolomite, and algal psammitic dolomite, and the content of microbial carbonate rock gradually increases upwardly (Fig. 2). On the outcrop section, there is no gypsum dolomite and gypsum salt rock discovered in Well Jin 2. There are two possibilities; one is that the strata are not completely exposed, and the salt rock especially can be easily dissolved, weathered and covered. The second is that there was no extreme drought in the paleoclimatic cycle in this area, so gypsum dolomite and gypsum salt rock weren’t created. However, bioclastic/algae lamination/algae stromatolite/algal psammitic limestones not appearing in Well Jin 2 occur in the lower part, showing two possibilities as well. One is that Well Jin 2 didn’t reach bioclastic/algae lamination/algae stromatolite/ algal psammitic limestones. The other is that it hasn’t experienced the stage of humid climate, so bioclastic/algae lamination/algae stromatolite/ algal psammitic limestones weren’t formed.

From the algae lamination/algae stromatolite/algal psammitic dolomite in both Well Jin 2 and on the section of Jinsushan outcrop, a complete lithological combination sequence from humid to arid climate can be established, namely the lithological combination of microbial limestone/bioclastic limestone → microbial dolomite → gypsum dolomite → gypsum salt rock, which is an inevitable response to climate change. The abrupt climatic change or the incomplete sequence of the cycle will lead to the incompleteness of the lithological combination sequence.

The Lower Cretaceous Ariri-Barra Velha Formation in the Well B of A Basin in Brazil provides a case of an abrupt variation of lithological combination sequence when humid climate changed suddenly to drought climate (Fig. 3). The Barra Velha Formation is a combination of micrite, bioclastic limestone, and algae limestone, and the overlying Ariri Formation suddenly transforms into the gypsum salt rock, lacking transitional lithologies like microbial dolomite and gypsum dolomite.

From comparative analysis, it can be seen that the Lower Cretaceous in the Well B of Basin A in Brazil and Ma 5⁶ - Ma 5¹⁰ Submembers of the Lower Ordovician Majiagou Formation in the Well Jin 2 record two opposite abrupt climate variations. The Lower Cretaceous in the Well B of Basin A in Brazil records the lithological combination of abrupt climate variation from wet to dry, where the micrite, bioclastic lime-stone, and algae limestone in the Barra Velha Formation change abruptly into gypsum salt rock in the Ariri Formation. The Ma 5⁶ - Ma 5¹⁰ Submember in the Well Jin 2 records the lithological combination of opposite abrupt climate variations from the salt rock in extreme arid climate to micrite in the humid climate. This further demonstrates that the abrupt climate variation or the incomplete sequence of the cycle would lead to the incompleteness of the lithological combination sequence, and also explains the lack of microbial dolomite and gypsum reservoirs in the carbonate rock - gypsum salt rock combination.

The Middle Triassic Leikoupo Formation section of the Well Yashen-1 in the Sichuan Basin provides a case of reversed lithological combination when the climate changed from drought to humid (Fig. 4). In the transgressive system tract, as the climate varied from drought to humid, the combination sequence of gypsum salt rock → gypsum dolomite → algae dolomite (mainly including algal laminated dolomite, algal stromatolite dolomite, and algal psammitic dolomite) → algal limestone appeared in turn.

Form the comparative analysis of the climate variation recorded in the Leikoupo Formation of the Well Yashen 1 and the Majiagou Formation of the Well Jin 2, it can be concluded that the climate turns from drought to humid in two forms. One is the abrupt climate variation represented by the Majiagou Formation in the Well Jin 2, where the gypsum salt rock in M₅⁶ changed abruptly to micrite interval in the Ma₅⁵, reflecting abrupt climate change from extreme dry to humid. The other is the gradual migration represented by the Leikoupo Formation in the Well Yashen 1, where there are transitional lithologies such as gypsum dolomite and microbial dolomite between gypsum salt rock and limestone. This further reveals the inevitable transformation of the combination sequence from limestone to microbial dolomite, gypsum dolomite and gypsum salt rock for the carbonate rock-gypsum salt rock with the variation from humid to dry or from dry to humid climate.

In general, the paleoclimate change during the deposition of the carbonate rock and gypsum rock sedimentary system is closely related to the lithological combination sequence. The humid climate background is characterized by the deposition of normal marine limestone (micrite, bioclastic/algae lamination/algae stromatolite/algae psammitic limestone). As the climate gradually becomes arid and the salinity increases, the halophilic archaea or sulfate-reducing bacteria and methanogenic archaea begin to flourish, which is conducive to the development of microbial dolomite. The salinity of the Great Salt Lake in Utah, USA is 15-25%, and 58 kinds of archaea and 42 kinds of bacterial microbial mats propagate rapidly^[21]. As the climate goes even drier and the salinity increases, the archaea or bacteria die out, and the gypsum nuclei begin to come up, forming a gypsum dolomite. When the salinity is more than 350‰, gypsum or rock salt deposition^[22] begins to form a layered gypsum salt rock. Therefore, the changes in the combination sequence of limestone, microbial dolomite, gypsum dolomite, and gypsum salt rock are the responses to paleoclimatic variations, which can be gradual or abrupt, either the positive cycle of humid → dry climate or reversed cycle of dry → humid climate. Through the study of the abovementioned lithological combination sequences of carbonate rock - gypsum salt rock, it is found that two types of dolomite, namely microbial dolomite and gypsum dolomite mainly occur in this sedimentary system. The genesis of these two types of dolomite are to be elaborated below.

Two types of dolomite are mainly developed in the carbonate rock-gypsum salt rock paragenesis system. One is the microbial dolomite which retains the original rock structure such as algal lamination/algal stromatolite, and the other is gypsum-bearing plaque or convoluted micrite dolomite (gypsum dolomite), which is formed under an arid climate background and is belonged to an early low temperature dolomite^[23]. It was further confirmed by the halophilic archaea-induced dolomite precipitation experiment and investigation on modern lacustrine that the microbial dolomite is originated from deposition; however the gypsum-bearing plaque or convoluted micrite dolomite (gypsum dolomite) is originated from replacement.

Land^[24] pointed out that under the conditions of surface temperature and pressure (less than 50 °C, pressure at several meters deep), dolomite hadn’t been produced through pure inorganic route after 32 years of geological effects. Therefore, people turned the study of dolomite genesis to organic genesis and carried out experimental research on microbe induced dolomitization. Although microorganisms are ubiquitous, the presence of microbial limestone in the lower part of the carbonate rock - gypsum salt rock sequence is enough to show that not all microorganisms can induce dolomite precipitation, and dolomite precipitation may be associated with specific types of microorganisms. Through experiment, Vasconcelos et al.^[25] found that sulfate-reducing bacteria could induce the precipitation of dolomite; Warthmann et al.^[26] concluded that methanogens could induce the precipitation of dolomite, and Kenward et al.^[27] compared dolomites precipitated in the experiment with that in the salinized coast of Lagoa Vermeia, and found they have similar spherical and characteristics of low order degree, therefore concluded that special types of microorganisms (sulfate-reducing bacteria, methanogens) were the conditions for the precipitation of original dolomite.

Since the microbial dolomite in the carbonate-gypsum salt rock combination is formed in a relatively arid climate background, experimental study on the precipitation of dolomite induced by halophilic archaea was conducted in this study. The experiments were done the National Key Laboratory of Biogeology and Environmental Geology of China University of Geosciences. The experimental results show halophilic archaea can induce precipitation of the original dolomite. The original dolomite were precipitated under the action of Natrinemas sp. (extremely halophilic archaea) for over 72 hours (Fig. 5a), and under the action of Haloferax volcanii (Walk’s salt-rich bacteria) for 72 h too (Fig. 5b, 5c). ). The dolomite produced has similar spherical characteristics with that precipitated in the experiments by Vasconcelos et al.^[25] and Warthmann et al.^[26]. The experimental results also show that higher cell concentration is conducive to the precipitation of the original dolomite (Fig. 6a), no original dolomite exists when the salinity is low, increase of salinity can increase the carboxyl density on cell surface and promote the precipitation of the original dolomite (Fig. 6b, 6c), and short period evaporation process doesn’t significantly affect the microorganism-induced precipitation of original dolomite. The carboxyl functional group on the surface of halophilic archaea cells in high salinity environment plays an important role in dolomite precipitation.

The experimental results above are sufficient to prove the inevitability of microbial dolomite development in the carbonate-gypsum salt rock combination. As long as the salinity is high enough to suit the reproduction of halophilic archaea, halophilic archaea would proliferate and induce the precipitation of original dolomite, forming microbial dolomite. The Mg²⁺ and Ca²⁺ in seawater are in the form of water compounds or complexes, and the electrostatic attraction between Mg²⁺ and water is greater than that between Ca²⁺ and water and between Mg²⁺ and CO₃^2-, so under low temperature, the Mg²⁺ is low in concentration and not easy to make the entry into the calcium carbonate crystal lattice to form dolomite. Higher salinity and temperature will lead to the increase of Mg²⁺ concentration in seawater. The microbial action can overcome the electrostatic attraction between Mg²⁺ and water, and increase the electrostatic attraction between Mg²⁺ and CO₃^2-, making it easier for Mg²⁺ to enter the calcium carbonate crystal lattice to produce dolomite.

The success of the original dolomite precipitation experiment led to the prevalence of “original dolomite precipitation induced by microbe”, and the dolomite retaining the structure of original rock during geological history was all attributed to the microbial induced sedimentation. But in fact, the micrite dolomite and gypsum dolomite associated with the gypsum rock in geological history have no evident microbial structure, and they are widely distributed, for example, the micrite dolomite and gypsum dolomite in the Majiagou Formation of the Ordos Basin^[28], the Jialingjiang and Leikoupo Formations^[29] in the Sichuan Basin, and the Cambrian subsalt^[30] in the Tarim Basin. It is difficult to explain the genesis by the theory of “original dolomite precipitation induced by microbe”.

The authors investigated the three salt lakes of Jibulangtunuoer, Gabujintuohugeke, and Dulanyouniquan and six non- salt lakes of Tarigen, Tarigenhuoer, Dabusannuoer, Dundenuoer, Buridenuoer, and Hujirinuoer. The salinities of the three salt lakes were 57.4 g/L, 120‰ and 100‰, respectively. The analysis of loosened sediments revealed no traces of microorganisms. The original dolomite with X-ray diffraction peak of 30.95° in the sediments of the salt lakes was discovered from the X-ray diffraction analysis. Microscope observation reveals that the content of dolomite can reach 30%-40% (Fig. 5d), and there are no dolomite in the sediments around the salt lake. However, there were no dolomites in the sediments of non-salt lake regardless in the lake bottom or lake margin. This indicates that microbial induction is not required in a high salinity environment, and dolomite can also be formed. Since the experiment has confirmed that the original dolomite can’t be precipitated under the surface temperature and pressure conditions (less than 50 °C, pressure at several meters deep)^[24], and these dolomites were originated from early replacement, rather than directly precipitated from the lake water. The dolomite is mainly in the form of micrite dolomite, with anhydrite plaque or convolution and has no microbial structure such as algae lamination and algae stromatolite.

This not only gives good explanation to genesis of most dolomicrite and gypsum dolomite associated with arid climate in geological history, but also reveals the inevitability of overlapping sequence of microbial dolomite and gypsum dolomite. When the salinity is in the range suitable for the proliferation of halophilic archaea (35‰-100‰), the proliferation of halophilic archaea would induce the precipitation of original dolomite and the extensive development of microbial dolomite. When the climate gets even drier and the salinity increases beyond the salinity range suitable for halophilic archaea, the halophilic archaea would die out and the microbial dolomite would be replaced by the gypsum dolomite. The appearance of large quantity of gypsum nodules and plaques suggests that the salinity is beyond the range for the proliferation of halophilic archaea. When the climate is extremely dry and the salinity increases further (greater than 350‰), the gypsum dolomite is replaced by a salt rock, unless there is abrupt change and reversion in the climate.

There are three main types of reservoirs in the carbonate- gypsum salt paragenesis system.

(1) Microbial limestone/bioclastic limestone reservoir formed under the background of humid climate, which is dominated by microbial limestone and bioclastic limestone, with minor psammitic bioclastic limestone. The storage space in this kind of reservoir is mostly intergranular pore and framework pore. For example, the microbial limestone from the Lower Cretaceous Ariri-Barra Velha Formation in the Santos Basin, Brazil, and algal limestone from the Upper Ganchaigou Formation of the Paleogene in the Qaidam Basin (Fig. 5e). This kind of reservoir is controlled by sedimentary facies, and the shoal reef is the basis for the development of such reservoir^[31].

(2) The microbial dolomite reservoir formed under the transitional climate background, with largely algae framework pores, for example, the fourth member of the Dengying Formation in the Sichuan Basin, and the microbial dolomite in the Wumishan Formation of the Jixian System in Renqiu City of North China. The microbial dolomite has excellent potential to become high-quality reservoir and is honeycomb- shaped in core and outcrop samples (Fig. 5f). Calcite cement can hardly been found, which is in sharp contrast with granular limestone.

(3) The gypsum dolomite reservoir formed under the background of arid climate with mainly gypsum molded pores, for example, the upper combination of the Majiagou Formation in the Ordos Basin^[28]. The gypsum dolomite reservoir is also controlled by sedimentary facies and is in ring-shaped distribution in the gypsum dolomite facies belt in the periphery of gypsum salt lake. It is vertically adjacent to the salt rock layer. The early dolomitization is important for the preservation of the gypsum mold pores^[32,33]. The dolomite not easily soluble in the epidiagenetic environment constitutes the framework of gypsum mold pore (Fig. 5g).

Among the abovementioned three types of reservoirs, the genesis of microbial limestone/bioclastic limestone and gypsum dolomite reservoir has been described in many literatures^[31,32,33]. This paper focuses on the main factors controlling the development of microbial dolomite reservoir.

Microbial carbonate rocks are the material basis for the development of such reservoirs and are the carriers of primary pores. Investigations of modern microbial carbonate rocks in the Bahamas Platform revealed that the porosity of primary pores can be as high as 54%, whether it is a microbial carbonate rock at the bottom of the lagoon or the non-petrochemical microbial mat at the edge of the lagoon (Fig. 7). In addition, there are the developments of two sets of microbial dolomite reservoirs in the Neoproterozoic in the Eastern Siberia region, respectively in the Late Liffeian and the Late Wende periods. The two are mainly composed of primary pores, and the porosity of the microbial dolomite reservoir in the Late Liffei period exceeded 10%. The porosity of microbial dolomite in the Late Wende period is 7%-10%, with recoverable reserves of oil and gas of 22×10⁸ t^[34,35]. The microbial dolomite reservoir of the Wumishan Formation in Renqiu City of North China has a porosity of 15% after superimposition of Phanerozoic karstification alteration on the primary pores (Fig. 5g).

The CO₂ gas formed by the early degradation of microorganisms is beneficial to the development and preservation of pore space. The CO₂ gas formed in the early low temperature degradation by microbial anaerobic respiration, fermentation and nitrate reduction makes the pore water in carbonate formation acidic^[36]. This may be a reason for the lack of calcite or dolomite cement in the microbial dolomite, (another reason is low abundance of calcium carbonate in the arid climate background), which is conducive to the formation of secondary pores and the preservation of pre-existing pores.

The CO₂ gas and organic acid formed by microbial carbonate rock thermal degradation are beneficial to the development and preservation of pores. Three sets of microbial carbonate rock samples were selected to carry out simulation experiments on hydrocarbon generation. The first group of samples were taken from the Paleogene in the Qaidam Basin, with TOC of 0.30%, S₁ of 0.04 mg/g, S₂ of 0.18 mg/g, HI of 60 mg/g, and R_o of 0.42%, and are in the immature-low maturity stage currently and have not experienced the peak of hydrocarbon generation in geological history. The second group of samples are gray mudstone samples taken from the Paleogene of Biyang Sag, with a TOC of 2.64%, S₂ of 15.83 mg/g, HI of 600 mg/g, and R_o of 0.38%. The third group of samples were marl samples taken from the Middle Permian of the Maoshan Section in Luquan, with a TOC of 3.33%, S₁ of 1.11 mg/g, S₂ of 13.9 mg/g, HI of 403 mg/g, and R_o of 0.42%. The simulation experiment was completed by Sinopec Wuxi Petroleum Geology Research Institute. The experimental equipment was a formation pore thermal pressure simulation experiment instrument developed by Research Institute of Wuxi Petroleum Geology. The model was DK-III and the experimental conditions are shown in reference [37].

The experimental results (Fig. 8) show that the microbial carbonate rock is comparable with the gray mudstone and marl in oil and gas yields and has the potential to generate hydrocarbons. Although it is not a good source rock compared with black mudstone, it is large in scale and can be realistic source rocks. The simulation experiments reveal also that CO₂ gas and organic acid are formed in the hydrocarbon generation process of microbial carbonate rocks. The significance is that they can create acidic environment of pore water in the microbial carbonate rock formation, which can prevent the precipitation of calcite or dolomite in the burial period, and is conducive to the formation of secondary pores and preservation of pre-existing pores.

The early dolomitization of microbial dolomite is conducive to the preservation of pores. As mentioned above, microbial dolomite is resulted by precipitation. It has already been dolomite before burial, which is dominated by algae framework pores. The compaction and pressure dissolution of this type of dolomite in the burial environment is completely different from that of limestone. First of all, the compaction resistance of dolomite is greater than that of limestone due to increased density (greater than limestone) caused by early dolomitization and early consolidation (consolidation of modern microbial rocks in the Bahamian Platform), which is one of the reasons for the preservation of more primary sedimentary pore in the microbial dolomite. Secondly, the microbial dolomite was more resistant to pressure-dissolution than limestone. The pressure-dissolution stylolite which is commonly observed in limestone is almost invisible in dolomite. This is also one of the reasons that the pre-existing pores of microbial dolomite are more substantially preserved. A typical case is the core at the depth interval of 6085-6090 m of the Feixianguan Formation in the Longgang area of ​​the Sichuan Basin, and the boundary between dolomite (6085-6088 m) and limestone (6088-6090 m) was 6088 m. There was no stylolite in the dolomite interval and were almost no fillings of calcite or dolomite. The oolitic molded pores were well preserved (Fig. 5h), and the stylolites were well developed in the limestone interval. Intergranular pore, body vug, and molded pore were all filled by sparry calcite (Fig. 5i), and the product of pressure dissolution provides a source of calcite cement.

The hydrocarbon source of the carbonate-gypsum salt rock combination is diverse, and this paper focuses on the discussion of reservoir-caprock combination types. As mentioned above, there are three types of reservoirs developed in the carbonate-gypsum salt rock system: microbial limestone/bioclastic limestone, microbial dolomite and gypsum dolomite, which can be divided into two backgrounds of subsalt and suprasalt. Theoretically, there should be seven typles of reservoir-caprock combinations in three categories subsalt and seven types of reservoir-caprock combinations in three categories suprasalt, a total of fourteen types of reservoir-caprock combinations in 6 categories (Table 1). Also, they are the responses to the paleoclimate changes from humid to arid and from arid to humid, from a gradual change to abrupt change.

Among the abovementioned reservoir-caprock combinations, microbial limestone/bioclastic limestone and microbial dolomite can develop into high-quality reservoirs, which, with largely matrix pores and good pore connectivity, have high porosity and high permeability. The gypsum dolomite containing largely gypsum molded pores has high porosity and low permeability. The sealing and plasticity characteristics of gypsum salt rock are favorable for the formation of good reservoir-caprock combinations under subsalt and suprasalt conditions^[13]. This article introduces the following four kinds of reservoir-caprock combinations.

Gypsum salt rocks are extremely good regional caprock, and there are three sets of carbonate rock reservoirs underlying, namely microbial limestone/bioclastic limestone, microbial dolomite, gypsum dolomite, and gypsum salt rock. The hydrocarbon may source from outside of the carbonate rock- gypsum salt rock combination system or partly from pyrolysis of microbial rock within the system.

The North-Pars gas field, the world’s largest gas field in Qatar-Iran, belongs to this category^[38] (Fig. 9). The gas field is located in the shallow sea of ​​the Persian Gulf in the northeastern Qatar Peninsula, with the proven natural gas reserves of 36.73×10¹² m³. The main producing layers of the gas field are the dolomite and microbial limestone in the Permian Khuff Formation. The main caprock is the anhydrite and shale in the Sudair Formation in the middle of the Permian. The main source rocks are the Cambrian and Ordovician neritic shale. The African Paleogene Zeit Bay and Ras Fanar oil and gas fields also fall into this category^[11].

Gypsum salt rock is extremely good regional caprock, and there are microbial limestone/bioclastic limestone reservoirs underlying. The hydrocarbon may source from outside of the carbonate rock-gypsum salt rock combination system or partly from pyrolysis of microbial rock within the system.

The world’s largest oil and gas field, the Saudi Ghawar oil and gas field, belongs to this type^[11] (Fig. 10), with proven recoverable oil reserves of 90.11×10⁸ t and natural gas reserves of 5.27×10¹² m³. The main producing layers are the microbial limestone, bioclastic limestone, psammitic bioclastic limestone in the Jurassic Arab Formation. The source rock is mainly the marl in the Tuwaiq Formation below and the gypsum salt rock acts as caprock. The Middle Permian Tengiz, Korofevskoy, Zhanazhol, Urikhtau, and Karachaganak oil and gas fields in middle Asia and Russia also belong to this type^[39].

Gypsum salt rocks are extremely good regional caprock, and there are two sets of microbial dolomite and gypsum dolomite reservoirs underlying. The two sets dolomite reservoirs can occur both or only one set occur. The hydrocarbon source may be derived from a carbonate rock-gypsum salt rock combination system or partly from pyrolysis of microbial rocks within the system.

The Middle Triassic Wuolonghe Oilfield and the Lower Ordovician Jingbian oil and gas field in the Asia-Pacific oil and gas zone belong to this type^[38]. The gas reservoirs in the Jialingjiang Formation and the Leikoupo Formation in the Sichuan Basin^[40] and the gas reservoirs in the middle combination (Ma 5⁶ Submember-Ma 5¹⁰ Submember) of the Majiagou Formation in the Ordos Basin are also this type^[6] (Fig. 11). The Cambrian subsalt dolomite in the Tarim Basin belongs to this exploration domain. The proven natural gas geological reserves of the Majiagou Formation in the Ordos Basin are 2 007×10⁸ m³.

The first three types of reservoir-caprock combinations are the ones formed under the background from humid to arid. In the lithological combination sequence formed by the variation from drought to humid paleoclimate, oil and gas reservoirs have been discovered in the reservoir-caprock combination of gypsum salt rock → gypsum dolomite → microbial dolomite and the overlying tight carbonate rock or clastic rock. In this kind of combination, the reservoirs are gypsum dolomite and microbial dolomite, and the caprock is the tight carbonate rock or fine clastic rock rather than gypsum salt rock layer. Due to the sealing of underlying gypsum salt rock, the hydrocarbon source is mainly from the overlying new strata.

The upper combination of the Majiagou Formation in the Ordos Basin (Ma 5¹ - Ma 5⁴ Submembers) belongs to the gas reservoir in the combination of gypsum salt rock → gypsum dolomite → microbial dolomite → fine clastic rock^[41] (Fig. 12), in which the main reservoir is gypsum dolomite with gypsum molded pores as the main storage space, the gas source is mainly from the overlying Carboniferous deposits of swamp facies, the caprock is the Carboniferous fine clastic rock, and the geological reserves of proven natural gas in this reservoir are 6547×10⁸ m³.

Judging from the current data available and oil and gas discovery statistics, the vast majority of oil and gas reservoirs in the carbonate reservoir-gypsum rock system are in the abovementioned four kinds of reservoir-caprock combinations, which are obviously related to the inherent laws of paleoclimatic changes in geological history. Through the above research, microbial limestone/bioclastic limestone → microbial dolomite → gypsum dolomite → gypsum salt rock combination, microbial limestone/ bioclastic limestone → gypsum salt rock combination, microbial dolomite → gypsum dolomite → gypsum salt rock combination and gypsum dolomite → microbial dolomite → tight carbonate rock or clastic rock combination are realistic reservoir-caprock combinations, with high possibility to fine oil and gas. But it can’t be ruled out that there are oil and gas reservoirs in other types of reservoir-caprock combinations. That is just the significance of the research on reservoir-caprock combination in this sedimentary system.

Based on the investigation on global carbonate reservoirs, this paper conducted analysis on four carbonate rock- gypsum salt profiles in China and abroad, and has gained the following three aspects of understandings.

The climate change from humid to arid determines the lithological sequence of carbonate rock-gypsum rock system, the inevitable variation trend of microbial limestone/bioclastic limestone → microbial dolomite → gypsum dolomite → gypsum salt rock, and vice versa. The abrupt variation of climate can result in the loss of some lithologies.

In the carbonate rock-gypsum salt paragenesis system, there are the developments of two types of dolomite respectively from precipitation and replacement and three types (microbial dolomite/bioclastic limestone, microbial dolomite and gypsum dolomite) of reservoirs. The reasons why microbial dolomite can become high- quality reservoir are CO₂ and organic acids formed in the early degradation of microorganisms and the late pyrolysis of microbial rock have dissolution effect conducive to the development of pores, and the early dolomitization is good for the preservation of pores.

Fourteen types of reservoir-caprock combinations in six categories subsalt and suprasalt have been sorted out. The current oil and gas discoveries are mainly found in four reservoir-caprock combinations, microbial limestone/bioclastic limestone → microbial dolomite → gypsum dolomite → gypsum salt rock combination, microbial limestone/bioclastic limestone → gypsum salt rock combination, microbial dolomite → gypsum dolomite → gypsum salt rock combination, and gypsum dolomite → microbial dolomite→ tight carbonate rock or clastic rock combination, which are related to the inherent laws of paleoclimatic changes in geological history. Through the analysis of reservoir-caprock combinations with oil and gas reservoirs discovered, it is found that the abovementioned four combinations in the carbonate rock-gypsum salt rock system are realistic combinations for oil and gas exploration, with high possibility to find oil and gas. But the possibility that there are oil and gas reservoirs in other reservoir-caprock combinations can’t be ruled out. That is definitely the significance of the research on reservoir and reservoir combination in sedimentary systems.