Introduction
Carbonate formations associated with evaporite are important for the increase of global oil and gas reserves and production. Domestic and international exploration has confirmed that the discovery of many large-scale oil and gas fields is related to carbonate-evaporite assemblages [1⇓⇓⇓⇓⇓-7]. In China, oil and gas discoveries in the Lower Paleozoic of the three major marine basins, namely Tarim, Ordos and Sichuan Basin, have confirmed that carbonate reservoirs are also associated with gypsum-salt rock [8⇓-10]. Recently, exploration practice of the Cambrian subevaporite Xiaoerbulake Formation in the Tarim Basin and the fourth member of the Ordovician Majiagou Formation (Ma-4 in brief) in the Ordos Basin further proved the exploration potential of this assemblage [11⇓⇓⇓-15]. At present, a large number of research results have been achieved by the predecessors in exploring the influence of evaporite on hydrocarbon accumulation in carbonates from the perspective of hydrocarbon generation, reservoir formation and hydrocarbon accumulation. On hydrocarbon generation, more attention has been paid to sediments and depositional environments. One viewpoint states that undercompensated basins, evaporative lagoons, platform-edge slopes and semi-occluded and occluded undercompensated bays are favorable environments for the high-abundance source rocks development [16-17]. Another viewpoint believes that depositional environment plays an important role in the formation of marine hydrocarbon source rocks, and factors associated with biological productivity, depositional rate, and preservation conditions of the sedimentary environment can determine the type and abundance of organic matter in the hydrocarbon source rocks[18-19]. Water stratification results in the enrichment of organic matter in the saline environment to form high-quality source rocks, and different types of source rocks are developed in different depositional environments with different depositional patterns [20-21]. Some scholars have also focused on how gypsum-salt rocks affect the maturation of organic matter under overpressure and other environments, as well as the relationship between overpressure and hydrocarbon generation, which in turn affects hydrocarbon migration and accumulation [22⇓-24]. In terms of reservoir formation, early studies focused on the influence of gypsum-salt rocks on dolomite diagenesis, and concluded that the influence of evaporite on dolomite reservoir formation is reflected in two aspects: (1) Mg2+-rich reflux promotes the formation of intergranular pores by dolomitization; and (2) the dissolution of gypsum nodules forms gypsum- mold pores [25⇓⇓⇓-29]. In terms of reservoir formation, the mainstream view is that gypsum-salt rocks mainly serve as a cap layer [30]. Gypsum-salt rocks serving as a cap layer depends on their brittle and plastic properties which are controlled by temperature and pressure. Shallow buried gypsum-salt rocks are mainly brittle, and less effective for sealing hydrocarbon because they are easy to be destructed by fractures and faults [31⇓-33]. Deep buried gypsum-salt rocks are plastic, and easy to flow and not easy to fracture, and even make the existing fractures and faults disappear[34-35]. In addition, in recent years, some scholars have paid attention to the intrinsic connection of carbonate-evaporite combination from a comprehensive point of view, and have elaborated on the integrality and research connotation of dolomite-evaporite system (assemblage) [36⇓⇓-39].
Generally speaking, previous researches mainly focused on single-discipline analysis which lacks of systematicity. The previous research has analyzed the characteristics of sedimentology, reservoir, and hydrocarbon accumulation of this system, but rarely explored the connotation of carbonate-evaporite assemblages from the perspective of facies successions. Therefore, based on a detailed dissection of rock types and combination characteristics of ancient carbonate-evaporite facies at typical basins within China and other countries, this paper clarifies the concept and definition of carbonate-evaporite assemblage, analyzes the characteristics of the facies and their internal relationship within the carbonate-evaporite assemblage, divides the carbonate-evaporite assemblage into various types of hydrocarbon accumulation modes, and discusses the favorable exploration areas of carbonate-evaporite assemblages in the Cambrian in the Tarim Basin, the Cambrian in the Sichuan Basin, and the Ordovician in the Ordos Basin. The findings are expected to provide supports for future exploration of oil and gas in deep ancient carbonate rocks.
1. Main rock types and combinations of carbonate-evaporite assemblages
1.1. Main rock types of ancient carbonate-evaporite assemblages in typical basins in China and other countries
This study focuses on analyzing the rock types developed in the Cambrian-Ordovician strata in three major marine basins in China. In the Tarim Basin, the rock types of the Cambrian Shayilik and Awatag formations are mainly medium- to thick-bedded gypsum-salt rocks interbedded with gypsum-bearing dolomite (or mud-bearing dolomite), mainly gypseous dolomite flat and saline lake deposits [40-41]. Gypsum-salt rocks of the Cambrian Longwangmiao and Gaotai formations in the Sichuan Basin are commonly developed. Drilling operation in the eastern Sichuan Basin has shown that the thickest gypsum-salt layer may be up to 1 000 m [9]. Some scholars have reported that gypsum-salt rocks are also developed in the Ediacaran and Ordovician [42-43]. The Ordovician Majiagou Formation in the Ordos Basin mainly develops carbonate-evaporite deposits, dominated by carbonate-evaporite interbeds. The gypsum-salt rocks may develop for a few hundred meters, as interlayers in thick carbonate rock. The evaporite-bearing layers appear in various forms such as nodular and breccia gypsum [44] (Fig. 1a ).
Fig. 1 Neoproterozoic-Cambrian carbonate and evaporite assemblages in typical basins in China and abroad. |
In foreign basins such as in East Siberia, Oman and Officer in Australia, the Lower Paleozoic gypsum-salt rocks are mainly distributed in the Newfoundland Series, which are slightly earlier than the gypsum-bearing formations in the three major marine basins in China. The Lower Paleozoic gypsum-salt rocks in the Tarim, Sichuan and Ordos Basins are mainly developed in the Miaolingian. In addition, gypsum-salt rocks are also developed in the Neogene in the Tarim Basin. On the whole, except for the East Siberian Basin, Oman Basin and Sichuan Basin, the Ediacaran gypsum-salt rocks are relatively undeveloped in other basins. The typical foreign basins are basically similar to the three major Chinese basins in terms of rock type assemblages (Fig. 1b ).
In the East Siberian Basin, the Ediacaran-Terreneuvian Series has multiple sets of widely distributed evaporites, mainly interbedded with carbonate rocks. The average thickness of evaporites exceeds 200 m, including gypsum rock, gypseous dolomite, and halite. Several oil and gas fields have been discovered there [45-46]. The Ara Group deposited in the Oman Basin is a combination of Cambrian Terreneuvian carbonate and gypsum-salt rocks which contain interbedded and intercalated forms. The gypsum-salt rocks are dominated by gypsum rock, gypseous dolomite, and halite. The gypsum-salt layer deposited in the Ara Group can reach up to 1 000 m in thickness, which is dominated by gypsum rock and contains a large amount of halite. The 20-250 m thick carbonate deposited during the subsequent transgression is separated into a number of isolated platforms where commercial oil and gas fields have been built [3,47 -48]. Evaporite deposits are also developed at the base of the Hawker Group in Officer of Australia, dominated by gypsum (anhydrite) rock and gypseous dolomite, with the upper part dominated by limestone deposits and the lower part by dolomite deposits, which are mainly interbedded assemblages [49-50].
In general, carbonate rocks include gypseous (gypsum-bearing) dolomite, grain dolomite, crystalline dolomite, dolomitic limestone, and limestone, and evaporites include gypsum rock, dolomite (mud)-bearing gypsum-salt rock, dolomitic (argillaceous) gypsum-salt rock and halite. Among them, the gypsum-bearing dolomite includes (anhydrite) gypseous dolomite and (anhydrite) gypsum-bearing dolomite. The gypsum rock includes pure (anhydrite) gypsum rock, dolomitic (anhydrite) gypsum rock, mud-bearing gypsum-salt rock and argillaceous gypsum-salt rock. The gypsum-bearing mudstone includes gypsum-bearing mudstone and gypseous mudstone. The halite type is mainly referred to salt rock.
1.2. Carbonate-evaporite assemblages and their characteristics
Based on a detailed analysis of the development environment and main rock types of carbonate-evaporite assemblages in typical marine cratonic basins at home and abroad, this study proposes three models of carbonate-evaporite assemblages mainly developed in marine sedimentary basins based on the analysis of facies successions (Fig. 2 ). Model I refers to intercalated carbonate and gypsum-salt rocks. The intercalated layers are mainly lamellar, breccia and nodular, distributed in the form of isolated thick layers along bedding, including a thick gypsum-salt layer sandwiched between carbonate, and thick carbonate layer sandwiched between gypsum-salt rocks. The thickest single gypsum-salt layer may be more than 1 000 m. This model is mainly distributed in the Cambrian Awatag Formation in Tarim Basin, the Cambrian Gaotai Formation in Sichuan Basin, the Ordovician Majiagou Formation in Ordos Basin, the Cambrian Ara Group in Oman Basin, and the Cambrian Angarskaya Formation in East Siberian Basin. Model Ⅱ refers to the interbedded carbonate and gypsum-salt rocks. The difference in thickness of the two types of rocks is relatively small, with high frequency of interbanding and strong lateral heterogeneity. The thickness of a single gypsum-salt layer is usually less than 10 m. This model is mainly found in the Majiagou Formation in Ordos Basin, the Wusonger Formation in Tarim Basin, the Cambrian Litvintsevo Formation in East Siberian Basin, and the Cambrian Hanseran Group of Rajasthan Basin in India. Model III refers to the combination of carbonate and gypsum-salt rocks intercalated or interlayered with clastic rocks. It is distributed in a hybrid environment with evaporite-bearing facies in sedimentary basins, such as the Gaotai Formation in Sichuan Basin, the Ara Group in Oman Basin, and the Wusonger Formation in Tarim Basin.
Fig. 2 Models of carbonate-evaporite assemblages in marine sedimentary basins. |
On the basis of analyzing the rock types and their combinations in carbonate and evaporite assemblages, and from the perspective of facies successions, it is further clarified that the carbonate-evaporite assemblage refers to the combination of carbonate and gypsum-salt rocks that deposited in specific period of geologic history. Spatially, these two types of rocks are in close contact with each other, and they can be presented in a variety of types, such as thick-layered contact, interlayer, brecciform, and nodule mosaic, etc. And there is also a symbiotic assemblage with clastic rocks deposited in the same or similar period of geologic history.
2. Source rock and hydrocarbon generation potential related to carbonate-evaporite assemblages
Oil and gas reservoirs in the carbonate-evaporite assemblages of the three major marine cratonic basins in China usually have two major types of source rocks. One is shale outside the assemblage, which is mainly developed in the ramp-basin depositional background during the transgressive period and located at the bottom of the carbonate-evaporite assemblage, such as the Cambrian Qiongzhusi Formation in Sichuan Basin and the Cambrian Yurtus Formation in Tarim Basin. Since many studies have been conducted on it, we won’t describe in detail in this paper. The other type is a layered system dominated by argillaceous carbonate rocks (argillaceous limestone and argillaceous dolomite) within the assemblage, which is mainly developed in a saline restricted platform depositional environment affected by frequent sea level fluctuations. The carbonate rocks and gypsum-salt rocks are distributed in an intercalated or interbedded manner, such as the Ordovician Majiagou Formation in Ordos Basin and the Triassic Leikoupo Formation in Sichuan Basin.
2.1. Development of source rocks in platform saline environment and controlling factors
In terms of depositional environment, the depositional facies belts related to the formation of evaporites mainly include Sabkha tidal flats, evaporative lagoons and gypsum salt lakes in restricted platforms, and different types of source rocks are developed in different depositional environments. In Sabkha tidal flats, stenohaline and euryhaline organisms are rarely developed, but evaporation and oxidation are strong. With less sources of organic matter, the original production capacity of organic carbon is weak, and newly formed organic matter is easily oxidized and destroyed. The preservation conditions of organic matter are poor, which is not conducive to the formation of high-quality source rocks. Under hot and arid climate conditions, continuous injection of seawater into the lagoon makes the salinity of the water increase continuously to form a saline lagoon, which is usually a favorable area for the development of marine source rocks [51]. The salinization of the lagoon water first begins with surface seawater, and the water gradually increases in density and sinks to the bottom of the lagoon during the salinization process. When the salinity increases to a certain degree, most of the organisms die and sink to the bottom of the lake, providing a large amount of material for the formation of organic matter. Due to the weakened vertical circulation of the lagoon water, the bottom is in an anoxic state, which is conducive to the preservation of organic matter. The hydrocarbon-generating parent material is mainly phytoplankton algae.
Taking the Ordovician Majiagou Formation in the Ordos Basin as a case, three times of transgression and regression occurred alternately during the depositional period. The sixth, fourth and second members of the Majiagou Formation were deposited during the transgressive period, and the fifth, third and first members of the Majiagou Formation were deposited during the regressive period [17,52]. During the regressive period, the evaporated tidal flat-lagoon environment was more widely distributed, and favorable for the formation of high-quality source rocks. Especially in the early stage of regression, high-abundance high-quality source rocks were developed. Through the study on the depositional microfacies and organic carbon in the cuttings of the Ordovician Majiagou Formation in Well CC1 and others, it is found that the source rocks of the Majiagou Formation are dominated by dark dolomitic mudstone concentrated in the third and first members (Fig. 3 ). The single layer is thin, and the cumulative thickness is 10 m to 20 m, accounting for 30%-50% of the whole formation. The organic matter abundance is high, with TOC (total organic carbon content) ranging from 0.3% to 5.1%, and 1.3% on average. For example, Well CC1 is located in a secondary low-lying area developed in a circum-salt depression in terms of paleogeographic location, where high-abundance source rocks are mainly distributed in the third and first members of the Majiagou Formation, and they are argillaceous dolomite and gypsum-bearing dolomite. The third member has the highest TOC (4.3%). The interval with TOC higher than 1% is more than 10 m in thickness, which is a high-quality source rock interval. The highest TOC of the first member is 1.8%, and most of the intervals with TOC higher than 0.6% are good source rock intervals. During the depositional period of the third and the first members, the sea level dropped, the seawater was evaporated and salinized, the water flow was relatively blocked, and the salinity was high, resulting in a reducing environment which was conducive to the formation and preservation of source rocks, and thus high-quality source rocks formed (Fig. 3 ).
Fig. 3 Comprehensive histogram of source rocks of the carbonate-evaporite assemblage in the Majiagou Formation in Well CC1 in the Ordos Basin (GR—natural gamma ray). |
Generally speaking, the preservation of source rocks is affected by both depth and salinity stratification. In a restricted platform where water is shallow, it is difficult to produce an anoxic environment with depth stratification, but easy to produce salinity stratification. In the early stage of regression, some low-lying areas are in a restricted environment with a certain deep-water environment and high microbial productivity. Evaporation and concentration salinize the seawater and deposit halite, resulting in an anoxic environment at the bottom of the water, where organic matter can be well preserved. at the late stage of regression, the water becomes shallow, the salinity is too high, the biological productivity is low, and the halite is not enough to create an anoxic environment so that no source rocks are developed. It can be concluded that the depositional period in the early stage of regression controls the development of high-quality source rocks. During transgression, the water is deeper, but generally turbulent, so source rocks only develop in low-lying areas.
2.2. Influence of gypsum-salt rocks on hydrocarbon generation potential of source rocks
The evaluation on the hydrocarbon generation potential of source rocks is mainly to explore the influence of gypsum-salt rocks on the hydrocarbon generation potential of source rocks. Zhao et al. carried out thermal simulation experiments on the kerogen of the Ordovician Pingliang Formation marine source rock in the Ordos Basin and found that the addition of gypsum-salt rock increased the amount of hydrocarbon generated from kerogen pyrolysis by 50%, showed a strong catalytic effect, and changed the composition of hydrocarbons generated from organic matter pyrolysis [52-53]. Wang et al. also demonstrated that the addition of sulfate and chloride salts increased the gaseous hydrocarbon yield from source rocks pyrolysis through hydrocarbon generation simulation experiments at high temperature (450-600 °C) and increased the hydrocarbon gas yield by about 77% under the action of sulfate salts at 550 °C, and changed the percentage composition of different alkanes.
In this paper, gold tube simulation experiments on source rocks in gypsum rock-kerogen and halite-kerogen assemblages were carried out under different medium conditions. The kerogen samples were obtained from the shale of the Qiongzhusi Formation. The initial temperature was set to 325 °C, the heating rate was set to 2 °C/h and 20 °C/h respectively, the pressure was set to 35 MPa, and data were recorded every 25 °C. The results show that at appropriate temperature, gypsum rock and halite had important impacts on kerogen cracking and gas generation. They inhibited hydrocarbon generation in low, medium and over maturity stages, and promoted hydrocarbon generation in the high maturity stage. The experiment was carried out in three groups, gypsum rock + kerogen, halite + kerogen and pure kerogen. The heating rate was 2 °C/h. The simulation results show that when temperature was less than 450 °C (equivalent to Ro<2.0%), gypsum rock and halite weakly inhibited the generation of gaseous hydrocarbons; when temperature was between 450 °C and 575 °C (equivalent to 2.0%<Ro<3.8%), gypsum rock promoted the generation of gaseous hydrocarbons; when temperature was between 450 °C and 525 °C (equivalent to 2.0%<Ro<3.0%), halite promoted the generation of gaseous hydrocarbons; and when temperature was higher than 575 °C (Ro>3.8%), gypsum rock and halite inhibited the generation of gaseous hydrocarbons (Fig. 4a ). The production rate of ethane generally followed the law of total alkane production rate, and the law of methane generation rate was slightly different. When temperature was higher than 425 °C, the methane production rate changed; when temperature was between 425 °C to 525 °C, gypsum rock and halite acted as inhibitors; and when temperature was higher than 525 °C, gypsum rock and halite played a promoting role. From the above experiments, it can be concluded that gypsum-salt rocks have obvious catalytic effects on the hydrocarbon generation from source rocks, and the catalytic effect of gypsum rock is obviously larger than that of halite. With the increase of temperature, the catalytic effect of gypsum rock and halite on hydrocarbon generation from source rocks was enhanced, which accelerated the process of thermal evolution of organic matter and improved the efficiency of hydrocarbon generation. The yields of whole oil, ethane and methane were all increased to different degrees under the catalytic effect of gypsum at 475, 500 and 525 °C. The experimental results can better support the hydrocarbon generation potential of deep ancient carbonate-evaporite assemblages.
Fig. 4 Results of simulation on the effect of gypsum-salt rocks on hydrocarbon production potential. (a) Variation of methane, ethane and whole oil yields at different temperatures; (b) Yields of Mg2+-catalyzed methane, ethane, propane and total gaseous hydrocarbons under different experimental conditions; (c) Production of H2S in MgSO4 system. |
In addition, experiments were carried out on the hydrocarbon generation potential of source rocks and crude oil cracking under different medium conditions for evaporite-kerogen assemblage. The argillaceous limestone samples taken from the Cambrian Longwangmiao Formation in Shixihe of Chengkou, Sichuan Basin, were heated from 0 to 168 h at 365 °C and 50 MPa. The experimental results show that gypsum-salt rocks had an obvious catalytic effect on the gas generation of source rocks, and the catalytic effect of gypsum rocks was obviously larger than that of halite. In the medium to low maturity stages, the gypsum-salt rocks inhibited gas generation, in the high to over maturity stages, they promoted gas generation, in the medium to low maturity and the early high maturity stages, they inhibited oil production, and in the late high maturity to over-maturity stages, they promoted oil production. Crude oil cracking experiments under different media conditions show that the yields of methane, ethane, propane, and total gaseous hydrocarbons in the MgSO4 system were higher than those in other systems, and the yield of gaseous hydrocarbon was obviously increased (Fig. 4b ). Simulation on the MgSO4 system (which is similar to the formation fluid in the assemblage of gypsum rock and dolomite) shows that thermochemical sulfate reduction (TSR) occurred obviously, forming high-sulfur natural gas (Fig. 4c ). This suggests that gypsum rock-dolomite assemblage was catalyzed by Mg2+ in the formation fluid, which promoted the cracking of crude oil and the formation of high-sulfur natural gas and played an important role in later reservoir modification. Therefore, the gypsum-salt rocks in the ancient marine carbonate-evaporite assemblage have a positive influence on the hydrocarbon generation from organic matter.
3. Characteristics and controlling factors of dolomite reservoir related to gypsum-salt rock
3.1. Types and characteristics of dolomite reservoirs related to gypsum-salt rocks
Two types of dolomite reservoirs are developed in carbonate-evaporite assemblages. One is gypseous dolomite flats which are directly related to gypsum-salt rocks, and the other is sub/supra-evaporite microbial mound and grain shoal which are indirectly related to evaporite.
3.1.1. Gypseous dolomite flat
A gypseous dolomite flat is mainly developed in the upper part of an intertidal zone or supratidal zone, and mostly associated with evaporite minerals such as gypsum rock and halite, or interbedded with evaporite. The reservoir is mainly composed of gypsum (-bearing), micritic and powder crystal dolomite, and breccia formed by gypsum dissolution can be seen locally. The pores are gypsum-mold pores, residual breccia pores and dissolution vugs and cracks.
The gypseous dolomite flat is mainly composed of gypsum-bearing micritic or powder crystal dolomite and residual grain dolomite (Fig. 5a ), the anhydrite is mainly produced in the following three forms: (1) striped, needled-shaped or granular anhydrite dispersed in the micritic dolomite (Fig. 5b, 5c and 5d ); (2) porphyritic, granular or fibrous anhydrite filled in the micritic dolomite or powder crystal dolomite, and some anhydrite has been dissolved into gypsum-mold pores in some samples (Fig. 5c, 5e and 5f ); and (3) slabby or regularly striped anhydrite filled in dissolution vugs (Fig. 5g-5i ). The first two types of anhydrites are direct precipitation from evaporated seawater under arid climatic conditions, and the third may be transformed from earlier gypsum as buried. The reservoir space is dominated by gypsum- mold pores, intergranular (gypsum) dissolution pores, intercrystalline pores, and gypsum-dissolution-based interbreccia pores (Fig. 5 ). Most of the gypsum-mold pores are selective fabric pores from dissolution of the gypsum or aragonite/magnesium-rich calcite that hasn’t been dolomitized. The gypsum-dissolution-based interbreccia pores are formed by the collapse and brecciation of dolomite layers caused by the dissolution of gypsum-salt rocks. The diameters of the gypsum-mold pores are generally 0.1-2.0 mm, mostly isolated and poorly connected. The gypsum-dissolution-based interbreccia pores are irregular and variable in size, and moderately connected.
Fig. 5 Microscopic characteristics of gypseous dolomite flat. (a) Gypsum-mold pores; gypseous dolomite; Jinshayankong section; Xixiangchi Formation; (b) Inter-crystalline pores; gypseous and powder crystal dolomite; Well MT1; Ma-2 Member; 3 048.0 m; (c) Inter-crystalline pores; gypseous micritic dolomite; Well S364; Ma-5 Member; 4 025.0 m; (d) Inter-crystalline pore; gypseous micritic dolomite; Well MT1; Ma-5 Member; 2 617.0 m; (e) Moldic pores; gypseous micritic dolomite; Well L36; Ma-4 Member; 3 744.0 m; (f) Gypsum-mold pores; fine powder crystal dolomite; dolomite and a small amount of quartz semi-filled; Ma-5 Member; 3 898.0 m; (g) Gypsum-mold pores; gypseous micritic dolomite; Well H4; Wusonger Formation; 4 813.0 m; (h) Karst interbreccia pores; gypseous dolomite; Well ZS5; Wusonggeer Formation; 6 561.3 m; orthogonal light; (i) Residual intergranular pores and micro-cracks in gypsum mold pores; micritic dolomite; Well S136; Ma-4 Member; 3 922.0 m. |
Gypseous dolomite flat reservoirs mainly occur in the middle and upper parts of the evaporative platform or evaporative tidal flat where the content of gypsum rock gradually increases, and provides dissolution pores for later reservoir formation. In addition, the frequent exposure of intertidal sediments out of water in the evaporitic environment facilitates the dissolution of unstable sulphate minerals by atmospheric water. Therefore, the reservoirs tend to be developed in the middle and upper parts of the upward shallowing succession of the evaporitic tidal flats, whereas it is difficult to develop effective pores in the lower gypsum-free micritic dolomite.
3.1.2. Sub/supra-evaporite microbial mound and grain shoal dolomite reservoirs
Microbial mounds and grain shoal dolomite reservoirs are developed both above and below the gypsum-salt rocks, and are usually located at the margin of the platform or the structural highs in the platform. There are various rock types in the reservoirs, including microbial dolomite that maintain the original sedimentary fabrics, grain dolomite and crystal dolomite with destroyed fabric (Fig. 6 ). The microbial dolomite reservoirs are mainly composed of foam cotton layers and thrombolite stone. The grain dolomite reservoirs with more developed pores are mainly residual oolitic dolomite and dolarenite. The crystal dolomite reservoirs with most developed pores are fine- and medium-crystalline dolomite composed of authigenic/semi-authigenic dolomite.
Fig. 6 Microscopic characteristics of sub-evaporite and supra-evaporite microbial mound and grain shoal dolomite reservoirs. (a) Intergranular pores; grain dolomite; Well YM36; Xiaoerbulak Formation; 5 596.9 m; plane-polarized image; (b) Intergranular (dissolution) pores; thrombolite dolomite; Well MX39; 4 685.0 m; Xixiangchi Formation; plane-polarized image; (c) Framework pores and dissolution pores; foam cotton dolomite; Well F1; 4 604.4 m; Xiaoerbulak Formation; plane-polarized image; (d) Intercrystalline pore and solution pore, containing stromatolite powder crystal dolomite; Well MT1; 2 419.0 m; Ma-5 Member; plane-polarized image; (e) Residual intergranular pores and intergranular dissolution pores; residual clastic dolomite; Well LG1, 4 424.0 m; Xixiangchi Formation; plane-polarized image; (f) Intergranular pores and dissolution pores; thrombolite dolomite; Xishuihoutan profile; Xixiangchi Formation; plane-polarized image; (g) Intergranular pores; oolitic dolomite; Well H4, 4 817.0 m; Xiaoerbulak Formation; plane-polarized image; (h) Intergranular pores; Xixiangchi Formation; Xishuihoutan profile; plane-polarized image; (i) Intercrystalline pores; fine-crystal dolomite; Well MT1, 2 472.0 m; Ma-4 Member; plane-polarized image. |
In terms of reservoir space, the pore types of the microbial mound and grain shoal dolomite reservoirs are closely related to the original rock structure, usually with obvious selective fabrics, and also modified by later dissolution. There are mainly three types, including intergranular (dissolution) pores, intercrystalline (dissolution) pores, and dissolution caves. Thrombolite dolomite mainly develops inter-clot dissolution pores and caves (Fig. 6a, 6b ). Microbial mound dolomite reservoirs mainly develop framework (dissolution) pores related to the foam cotton layers (Fig. 6c ). The pore development in stromatolite dolomite is relatively poor (Fig. 6d ). Grain dolomite has intergranular, intergranular dissolution, and intragranular pores (Fig. 6e-6h ), and some of them have been filled with dolomite, quartz, etc. Crystalline dolomite mainly develops intercrystalline pores, intercrystalline dissolution pores, and other types of pores (Fig. 6i ), and some of the pores have been filled with late dolomite or calcite, etc.
3.2. Genesis and developmental models of dolomite reservoirs associated with gypsum-salt rocks
Gypseous dolomite flat is formed in a dry and hot climate. Affected by strong evaporation, tidal flat micritic aragonite or magnesium-rich calcite is transformed into micritic or powder crystal dolomite. High-salinity seawater tends to have heavy oxygen isotopic compositions, so that the oxygen isotope values of the dolomite are characterized by a clear obvious positive drift (Fig. 7a ). In addition, this type of dolomite is often associated by a certain amount of gypsum or anhydrite which is distributed in the forms of plaques and irregular strips or interbedded with micritic dolomite. This lays the foundation for the development of gypseous dolomite flat. Gypsum is unstable near surface, and very easy to suffer atmospheric leaching. When gypsum is dissolved to different degrees, different types of gypseous dolomite flat are developed. As a result, the 87Sr/86Sr, δ18O and δ13C values in the gypseous dolomite flat are usually slightly higher than those in the coeval seawater, which also reflects the influence of atmospheric freshwater with rich 87Sr in the terrestrial source (Fig. 7b ).
Fig. 7 δ18O, δ13C and 87Sr/86Sr values in the dolomite reservoir. |
Short-term exposure and dissolution and late dissolution are the main factors on the formation of grain shoal and microbial mound reservoirs [53]. In addition, gypsum- salt is an important contributor to TSR reaction and reservoir modification in the burial process. Gypsum-bearing intervals provide a rich source of sulfate ions, and TSR reaction can occur once hydrocarbons are charged and at appropriate temperature. Strongly corrosive acid gas can modify the adjacent carbonates along fractures and create a large number of secondary pores [54]. The deeper the burial, and the higher the temperature, the stronger the TSR reaction, and the more obvious the modification of the reservoir is. It plays an important role in increasing the early pores in the carbonate-evaporite assemblages. Taking the Cambrian Xixiangchi Formation in the Sichuan Basin as an example, the lowest δ13C value of the coarse calcite filled in the dissolution pores is less than -20‰, indicating that the reservoir was dissolved by multiple stages of fluid when buried (Fig. 7a ).
The development of the gypseous dolomite flat is mainly controlled by two factors. A large amount of gypseous dolomite in carbonate-evaporite assemblage provides the material basis for the reservoir development; and the atmospheric leaching in the penecontemporaneous or epigenic period controls the large-scale development of reservoir. Dissolution simulation experiments have also confirmed that the porosity of halite-bearing or gypsum-bearing dolomite can be increased by 10.3-19.8 percentage points after epigenic dissolution, and calcium sulfate has an important role on promoting dolomite dissolution [39]. At present, industrial discoveries have been made in the gypseous dolomite flat in the Majiagou Formation of the Ordos Basin. A large amount of powder crystal dolomite containing anhydrite nodules was found in the fifth member of the Majiagou Formation. Affected by tectonic uplift in the Caledonian period, the anhydrite-bearing dolomite was exposed to accept the transformation of dissolution and erosion through atmospheric leaching and filtration, forming a large area of layered weathered crustal karst reservoirs, which is one of the primary pay zones in Jingbian Gas Field [51]. In addition, there are a large number of gypsum-mold pores caused by penecontemporaneous dissolution in the Middle and Lower Cambrian gypseous dolomite flat around Well Yaha in the Tarim Basin. To this end, the development model of dolomite reservoirs associated with gypsum-salt rocks is established (Fig. 8 ).
Fig. 8 Development model of dolomite reservoirs associated with gypsum-salt rocks. |
4. Hydrocarbon accumulation models and exploration significance of carbonate-evaporite assemblages
The brittleness and plasticity of gypsum-salt rocks depend on temperature and pressure. When temperature is lower than 130 °C and pressure is lower than 50 MPa, the gypsum-salt rocks in carbonate-evaporite assemblages are brittle, and may fracture. However, when temperature is higher than 130 °C and pressure is higher than 50 MPa, the gypsum-salt rocks in carbonate-evaporite assemblages change from brittle to plastic, which making the assemblages present a better sealing property. In shallow burial area, temperature and pressure are low, so the gypsum-salt rocks are brittle and easy to fracture, resulting in poor sealing ability. In deep formations, with temperature and pressure increase, the gypsum-salt rocks become plastic and easy to flow rather than fracture, and even the existing fractures may disappear [36]. After analyzing the influence of the types and thickness of gypsum-salt rocks on carbonate-evaporite assemblages [55⇓-57], and the typical traps and hydrocarbon reservoirs discovered in the carbonate-evaporite assemblages in the Tarim Basin, Sichuan Basin and Ordos Basin, the geologic conditions and accumulation models of the typical Paleozoic oil and gas reservoirs in the three major marine basins were analyzed. According to the configurational relationship between gypsum-salt rocks and hydrocarbon source, transport path and reservoir assemblage, hydrocarbon transport and accumulation characteristics and the possibility of gypsum-salt rocks changing from brittle to plastic, three primary reservoir models are built: sub-evaporite accumulation model, supra-evaporite accumulation model, and inter-evaporite accumulation model, and each includes several secondary models (Fig. 9 ).
Fig. 9 Typical hydrocarbon accumulation models in carbonate-evaporite assemblages. |
4.1. Sub-evaporite hydrocarbon accumulation model
The sub-evaporite accumulation model refers to the oil and gas reservoirs that are located under gypsum-salt layer. The gypsum-salt rock plays the role of a cap layer and decollement fold in the formation of the reservoir, and it is usually developed in the thrust fold belt in a foreland basin. In the fault-fold belt with intense deformation, an imbricated thrust faulted anticlinal hydrocarbon accumulation model is often developed. In the thrust fold transition belt and the front deformation belt with weak deformation, a broad, gentle and low-relief (faulted) anticlinal accumulation model is generally developed. These secondary models are mainly found in assemblage models I and III. In a severely deformed zone, the gypsum-salt layer between two carbonate layers suffers from intense compression and resulting in strong slippage folding. The upper and lower sets of carbonate rocks are severely deformed in an uncoordinated manner, and creating sub-evaporite, imbricated, thrust and faulted- anticline trap groups. They may develop into effective reservoirs when the supply of hydrocarbon source is sufficient. In a weakly deformed zone, short-fold deformation occurs after the gypsum-salt layer is compressed, and broad, gentle and low-relief (faulted) anticlinal reservoirs may appear. In this model, the source rock is usually shale outside the assemblage.
The Precambrian back thrust and uplifted structure and double-layered structure are developed in the Dabashan Mountain in the Sichuan Basin, constituting the Sinian-Lower Paleozoic Cambrian oil- and gas-bearing system, and forming multistage primary and secondary oil and gas reservoirs. Some large faults cut through multiple gypsum-salt layers and mudstone detachment layers, and the deep source rock of the Lower Cambrian Qiongzhusi Formation, and act as the dominant transport channels communicating with the Lower Cambrian source rock and controlling hydrocarbon accumulation. So, there may be imbricated, thrust and faulted-anticline reservoirs in the deep Cambrian Longwangmiao Formation and the Sinian Dengying Formation, with good accumulation conditions in North Slope of central Sichuan Uplift. In the Tarim Basin, broad, gentle and low-relief anticlines are developed on Tazhong North Slope and the Maigaiti Slope which are weakly deformed; grain shoal and microbial mound reservoirs are found in the Xiaoerbrak Formation; and evaporite caps are developed in the Middle Cambrian Shayilik and Awatag formations. These favorable structures provide best conditions for sub-evaporite hydrocarbon accumulation.
4.2. Supra-evaporite hydrocarbon accumulation model
The supra-evaporite accumulation model refers to the oil and gas reservoir located above gypsum-salt layer formed by the effective migration and accumulation of oil and gas discharged from the source rock. In the hydrocarbon accumulation process, the decollement folding of gypsum-salt rocks creates various traps, and lithologic reservoirs caused by reservoir heterogeneity mainly appear in the strata of assemblage types I or III. The controlling factors on hydrocarbon accumulation in supra-evaporite reservoirs are sufficient gas sources and good preservation conditions. The possibility of gypsum-salt rocks changing from brittle to plastic also plays an important role. A variety of trap types are found in supra-evaporite formations, including faulted-anticline, faulted-block, buried-hill or unconformity and salt-welded, which are affected by structures, stress properties and strength. In the supra-evaporite reservoir model, the brittle-to-plastic transition of gypsum-salt rocks plays a key role in hydrocarbon migration.
Under the influence of brittle-to-plastic transition of gypsum-salt rocks and compression, different types of supra-evaporite reservoir are developed in carbonate-evaporite assemblages in different tectonic sites. For example, faulted-anticline, faulted-block, buried-hill or unconformity oil and gas reservoirs were found in the Cambrian and Ordovician formations in the eastern and southern Sichuan Basin and the Mazatag front uplift in the Tarim Basin. Under the effect of intense compression, supra- and sub-gypsum- salt layers are severely deformed in foreland thrust belts, resulting in a series of salt-welded structures, such as the Cambrian and Ordovician in the Maigaiti Slope. With sufficient supply of hydrocarbon sources, faulted-anticline, salt-welded, superimposed supra-evaporite reservoir can be developed, such as the Cambrian Xixiangchi Formation in the eastern and southern Sichuan Basin.
4.3. Inter-evaporite hydrocarbon accumulation model
The inter-evaporite accumulation model refers to the interbedded reservoir and evaporite in the longitudinal contact relationship. Carbonate reservoir and gypsum-salt cap rock are basically in the same interval. Argillaceous carbonate and gypseous carbonate are important source rocks. The reservoir is of strong heterogeneity. In the process of accumulation, gypsum-salt rock acts as the lateral barrier or local cap. If the shale below or on the side of the assemblage acts as the source rock, the time when hydrocarbon migrates and trap develops decides the effectiveness of hydrocarbon accumulation. The inter-evaporite reservoir model is almost found in assemblage type II. Controlled by reservoir quality, hydrocarbon supply and trap effectiveness, most inter-evaporite reservoirs are tectonic-lithologic reservoirs. Influenced by tectonic and sedimentary background, inter-evaporite reservoirs are divided into two types: updip pinchout and lithologic barrier type and faulted-anticline type.
In the area where compression is intense, thin shoal dolomite reservoirs may be developed between two gypsum-salt layers in the carbonate-evaporite assemblage. In the case of sufficient supply of hydrocarbon, faulted-anticline reservoirs may be accumulated, such as the Cambrian gas reservoirs in Well ZS1 in the central Tarim Basin, and the gas reservoirs of the Majiagou Formation in the Ordos Basin. In addition, the Gaotai Formation in the Sichuan Basin develops gypseous dolomite flat facies along the Huaying Mountain Fault, which has good conditions for reservoir development and favorable exploration condition in Weiyuan-Hechuan and Guang'an-Yilong- Yingshan areas. On the large stable slope where structural deformation is relatively weak, updip pinchout and lithologic barrier reservoirs are common around the gypsum-salt lake. Typical lithological reservoirs have been found in the Cambrian Wusonger Formation in the Tarim Basin, and in the third and fourth members of the Ordovician Majiagou Formation in the Ordos Basin, and the south of Wushenqi, the periphery of Jingbian and the south of Yulin are favorable exploration zones.
5. Conclusions
A carbonate-evaporite assemblage refers to the combination of carbonate and gypsum-salt rocks deposited in certain period of geological history. These two types of rocks are in close contact with each other as thick-layered, interbedded, brecciated, and nodule mosaic, etc. This symbiotic combination of clastic rocks deposited in the same or similar geological history can be divided into three types: intercalated, interbedded, and symbiotic. Carbonate- evaporite assemblages usually have two types of source rocks: shale and argillaceous carbonate rock. The development of the source rock in a salinized platform environment is controlled by sedimentary cycles. Experiments have proved that gypsum-salt rock has a catalytic effect on hydrocarbon generation, and the catalytic effect is enhanced with the increase of temperature. Two types of dolomite reservoirs are found in carbonate-evaporite assemblages: gypseous dolomite flat, and grain shoal and microbial mound. The gypseous dolomite flat has the δ18O and δ13C values slightly higher than those of coeval seawater. The 87Sr/86Sr value confirms the occurrence of atmospheric leaching, and penecontemporaneous or epibiotic atmospheric leaching is the major factor controlling of the reservoir development. Late burial and dissolution (TSR and hydrothermal action) may reconstruct the reservoir.
Three primary hydrocarbon accumulation models are founded in carbonate-evaporite assemblages: sub-evaporite, supra-evaporite and inter-evaporite. They are further divided into eight secondary models. The sub-evaporite model includes imbricated, thrust and faulted-anticline reservoir, and broad, gentle and low-relief anticline reservoir. The supra-evaporite model has anticline reservoir, faulted-block reservoir, buried-hill or stratigraphic reservoir. The inter-evaporite model has faulted-anticline, salt-welded and superimposed reservoir, fault-anticline reservoir, updip pinchout and lithologic barrier reservoir. After analyzing the ancient carbonate-evaporite assemblages, and considering the characteristics of hydrocarbon source rocks, reservoirs and hydrocarbon accumulation models in the Lower Paleozoic in three major marine basins, it is concluded that the Cambrian sub-evaporite formation in the Tarim Basin is the primary exploration target at present, where there are favorable zones such as the Maigaiti Slope and Tazhong North Slope, etc.; in the Sichuan Basin, the Cambrian supra-evaporite Xixiangchi Formation and the sub-evaporite Canglangpu Formation may become hotspots of gas exploration, and the Lower Paleozoic inter-evaporite Gaotai Formation is likely to become an important backup target in the future, where there are favorable exploration zones such as the northern slope of the Central Sichuan Paleo-uplift, and Weiyuan-Hechuan and Guang'an-Yilong-Yingshan areas; in the Ordos Basin, the Ordovician Ma-4 member has favorable conditions for large-scale exploration, and the south of Wushenqi, the periphery of Jingbian and the south of Yulin are favorable exploration zones.