Sage Journals: Discover world-class research

Abstract

In gypsum–carbonate rock assemblages, multistage and complex fluids control the formation of dolomite reservoirs that are a focus of hydrocarbon exploration. It is difficult to determine the types of dolomite reservoirs and their formation mechanisms due to the diverse rock assemblages and multiple stages of diagenesis. In this study, we investigated the petrology, reservoir physical properties, and geochemistry of the 6th sub-member of member five of the Majiagou Formation (i.e. Ma₅⁶) in the Ordos Basin, China. These data were used to determine the nature and types of gypsum–carbonate rocks, and constrain their reservoir characteristics and diagenetic history, and fluid-related mechanisms that led to dolomite reservoir development and preservation. The Ma₅⁶ was deposited on a restricted evaporatic platform in the North China Craton, and contains three main types of dolomite reservoirs with variable types of reservoir space. Dolomite reservoir formation was closely related to penecontemporaneous dolomitization, karstification, and differential cementation. Early large-scale dolomitization produced dolomitized carbonate sediments that were resistant to compaction and dissolution, which was conducive to the preservation of primary and secondary pores. The intermittent exposure and dissolution of mound–shoal facies sediments, due to high-frequency sea-level fluctuations, was the dominant mechanism for formation of secondary dissolved pores and high-quality reservoirs. During burial, differential cementation occurred due to interaction between fluids and pore size, which determined the extent of reservoir preservation. In general, the studied dolomite reservoirs have undergone multistage diagenesis and alteration, which led to complex and multistage development of the reservoir porosity. However, the reservoir lithology and pore space developed mostly in the depositional to penecontemporaneous stages. Our results provide new insights into the origins of deeply buried dolomite reservoirs in carbonate–evaporite successions.

Keywords

Gypsum–carbonate rocks reservoir genesis pore evolution Majiagou formation Ordos Basin

Introduction

Evaporitic and carbonate rock associations are commonly deposited in restricted evaporatic settings, and are important hydrocarbon reservoirs worldwide (Hu et al., 2019; Liu et al., 2018). The oil and gas reserves in these types of rocks account for 46.2% of the total reserves in all carbonate rocks (Mu, 2017), and include the Gulf of Mexico Basin in the USA, Abyssal Oceanic Basin in Brazil, Persian Gulf in the Middle East, and Pre-Caspian Basin in Central Asia. These basins contain vast oil and gas reserves, which highlight the importance of conducting research on evaporitic and carbonate rock associations. However, such rock assemblages often experience dolomitization and complex diagenetic processes, which hinder the identification of dolomite types and mechanisms of reservoir formation. For example, gypsum (i.e. anhydrite) is dissolved leaving residual marl due to the action of near-surface meteoric waters (Adams and Diamond, 2017; Xiao et al., 2019a; Zhao et al., 2014). Anaerobic activity of microorganisms means that early low-temperature degradation of such rocks is caused by nitrate reduction, and secondary dissolution is induced by pyrolysis-generated CO₂ gas and organic acids during burial (Druckman and Moore, 1985; Hu et al., 2019). Further dissolution can be caused by H₂S generated by thermochemical sulfate reduction reactions (Fu et al., 2019; Kotarba et al., 2020; Machel, 2001; Moore and Heydary, 1997; Torghabeh et al., 2021).

The Ordovician strata in the Ordos Basin contain a number of types of evaporite–carbonate rock assemblages. One type is suprasalt that is characterized by the development of gypsiferous nodular dolostone due to karst weathering. Such rocks host the renowned Jingbian gas field (Yang, 1991). Compared with suprasalt, the presalt units are characterized by rhythmically layered gypsum and carbonate rocks, which are located distal from the unconformable surface of the karst weathering surface. Due to the loading effects of the overburden, the presalt unit undergoes completely different sedimentary, diagenetic, and reservoir formation processes than the suprasalt units. In particular, for the Ma₅⁷ and Ma₅⁹ sub-members, peneconozoic karstification was critical to reservoir formation (Fu et al., 2019; Xiao et al., 2019b). The Ma₅⁶ separates the suprasalt and presalt units, and is considered to be a thick evaporitic rock layer, although it has not been studied in detail. However, several deep wells in the central–eastern part of the Ordos Basin have intersected the Ma₅⁶, and demonstrated its exploration potential. This sub-member only contains limited evaporitic strata, and consists of large amounts of gypsum nodule-bearing, rhythmically interbedded evaporatic–carbonate rocks. This complex gypsum–carbonate rock assemblage is relatively unusual and requires further investigation in order to understand the mechanisms of reservoir formation.

As such, we investigated the petrology, reservoir physical properties, and geochemistry of the Ma₅⁶ in the Ordos Basin. We use our observations and data to classify the complex gypsum–carbonate rock types, constrain the reservoir characteristics and diagenetic history, and assess the mechanisms responsible for dolomite reservoir development and preservation. This paper focuses on reconstructing the relationship between the rock assemblages and pore evolution, and presents a model for reservoir formation in complex evaporite–carbonate rock assemblages. Our model also has implications for regional hydrocarbon exploration.

Geological setting

The Ordos Basin is located in the western North China Craton, and is a rectangular basin that extends across Shaanxi, Gansu, Ningxia,Inner Mongolia, and Shanxi provinces. The basin has an area of 3.7 × 10⁵ km². In the Early Ordovician, a central paleo-uplift formed in the basin due to abrupt subsidence in the adjacent western Helan Rift. The northern Shaanxi depression formed due to the compensatory subsidence in response to the tectonism in the east, while the southwestern depression formed due to extension (Hou et al., 2002; Shao et al., 2019; Zhao and Liu, 1993). The central paleo-uplift controlled the Ordovician paleogeography and distribution of depositional facies (Shao et al., 2019).

The study area is located in the central–eastern Ordos Basin, and structurally, lies across the Yimeng Uplift and Yishan Slope (Figure 1). The study area covers a region that is 1.12 × 10⁵ km² in size. Tectonism in the study area resulted in three transgressions and three regressions during deposition of the Majiagou Formation in the Early Ordovician, which generated multiple, complete transgressive–regressive depositional cycles with alternating development of carbonate and evaporitic rocks (Shao et al., 2019). Based on the depositional cycles and lithological associations, the Majiagou Formation can be divided into six members, with the fifth member corresponding to the depositional period during a regression. In addition, secondary depositional cycles are evident in the fifth member, which is further divided into 10 sub-members. The Ma₅⁶ recorded the largest regression in the member, and was characterized by the deposition of thick gypsum beds along with thin carbonate beds.

Figure 1.

Tectonic units, well locations, and stratigraphy of the central–eastern Ordos Basin, China.

Samples and methods

This study is based on data from 23 cores of the Ma₅⁶ in the central–eastern Ordos Basin. The cores have a total length of about 160 m, and were systematically observed, described, and sampled. The samples were made into thin-sections and stained with alizarin red. Thin-sections were also prepared of samples (n = 150) injected with blue epoxy resin, in order to highlight porosity. Stable C–O isotope and trace element analyses were conducted at the Southwest Petroleum University, Division of Key Laboratory of Carbonate Reservoirs, China National Petroleum Corporation. The whole-rock samples (20 mg in weight) for stable C–O isotope analyses were obtained by microdrilling, and then analyzed in an ultraclean laboratory with a MAT 253 Plus mass spectrometer. The analytical procedure was as follows: (1) a sealed reaction bottle containing the carbonate rock powder was flushed with high purity He gas, injected with phosphoric acid, and held at a constant temperature of 70˚C for 1 h; (2) the released CO₂ gas was moved by He flow into the mass spectrometer. The data were standardized to V-PDB and have an accuracy of < ± 0.01‰ (δ¹³C) and < ± 0.02‰ (δ¹⁸O). For trace element analysis, the samples were embedded in epoxy resin and made into thick-sections. Analytical sites were chosen under a microscope in plane-polarized light and from cathodoluminescence images. The thick-sections were then cleaned by ultrasonication in ultraclean water. Finally, the trace element analyses were undertaken by laser ablation inductively coupled plasma mass spectrometry. The different types of rock fabric were also tested using in situ microbeam analysis technology (Dautriat et al., 2010).

Porosity and permeability analyses were undertaken in the State Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering, Southwest Petroleum University, based on a standard rock porosity and permeability measurement method (SY/T 6385-2016), with an accuracy of ± 0.01% for porosity and ± 0.005 × 10^–3 µm² for permeability.

Results

Petrography of the dolomite reservoirs

Lithofacies types

The Ma₅⁶ consists mainly of four types of rocks: muddy to micritic dolomite, grain dolostone, microbialite, and gypsum. The specific characteristics of each type are as follows:

The muddy to micritic dolomite consists of dolomicrite and micritic dolomite. The former is dark gray in color and dense, horizontally or nonstratified, and contains occasional, isolated pores in gypsum (Figure 2(a)) and salt (Figure 2(b)). The dolomicrite was deposited in a low-energy, restricted setting. The latter is yellowish to gray–brown in color (Figure 2(c)), and has a mosaic texture and ghost particles. The protolith rock was initially doloarenite that was deposited in grain shoal facies above the wave base.

The grain dolostone consists of doloarenite (dolorudite) and “granophyric” doloarenite. The former has a clear structure and good preservation, and is particle-supported (60–75% of 0.1–0.5-mm-sized grains; Figure 2(d)) and contains intergranular and intergranular dissolution pores (Figure 3(e)). The latter has an arenaceous texture and is assumed to have formed from doloarenite or arenaceous micritic dolomite deposited in grain shoal facies under high-energy conditions. The “granophyric” doloarenite contains intergranular and intergranular dissolution pores (Figure 3(f)), and some pores are filled with gypsum and silt.

The microbialites can be divided into two types: thrombolites and stromatolites. The thrombolites are gray in color, layered, and exhibit cluster structures (Figure 3(a)), cemented granular structures (Figure 3(b)), and grid textures (Figure 3(c)) under the microscope. Only a small amount of residual lattice pores are visible, as these have been largely filled with agglutinates (Figure 3(c)). The stromatolites typically exhibit bright and dark layers (Figure 2(i) and (j)), in which the dark layers are organic-rich micritic dolomite. The bright layers are mostly brown–gray, muddy to micritic dolomite, with near-parallel and slightly wavy laminae (Figures 2(i) and 3(d)), and some dome-shaped laminae (Figure 2(j)). Both stromatolites and thrombolites require photosynthesis to grow, and thus formed in a high-energy shallow-water environment.

The gypsum can be classified as lamellar dolomitic gypsum and nodular gypsum. The former typically contains bright and dark layers (Figure 2(k)). The dark layers are gray–brown dolomicrite. The bright layers are gray–white or gray–brown gypsum that formed in a low-energy and strongly evaporatic environment. The layers vary from tens of centimeters to several meters in thickness. The nodular gypsum is light gray and off-white in color, and comprises gypsum nodules of different sizes and shapes set in a dark gray argillaceous dolomite (Figure 2(l)). Nodular gypsum is widely developed in the study area, and formed mostly at the top of the high-frequency cycles in low-energy, shallow-water, and high-salinity conditions where subaerial exposure was common. Each lamina is about ten to several tens of centimeters in thickness.

Figure 2.

Petrological types and features of the Ma₅⁶ sub-member in the central–eastern Ordos Basin, China. (a) Gypsiferous dolomicrite with pores in gypsum (well T62; 3619.92 m). (b) Salt-bearing dolomicrite with pores in salt (well J5; 3545.88 m). (c) Crystalline dolostone (well T112; 3383.92 m). (d) Sparry doloarenite (dolorudite) (well T105; 3668.52 m). (e) Doloarenite (well J5; 3542.08 m). (f) Dolomite (well T85; 3310.69 m). (g) Algal-bound dolostone (well T85; 3310.32 m). (h) Thrombolitic dolostone (well T102; 3603.38 m). (i) Stromatolitic dolostone (well T78; 3765.38 m). (j) Stromatolitic dolostone (well T102; 3604.30 m). (k) Lamellar dolomitic gypsum (well Tg8; 3334.15 m). (l) Nodular gypsum with a mesh texture (well T112; 3351.91 m).

Figure 3.

Types of reservoir space in the Ma₅⁶ sub-member in the central–eastern Ordos Basin, China. (a) Thrombolitic dolostone (well T85; 3310.69 m). (b) Thrombolitic dolostone (well T85; 3310.32 m). (c) Thrombolitic dolostone (well T102; 3603.38 m). (d) Stromatolitic dolomite (well T102; 3604.30 m). (e) Sparry doloarenite (dolorudite) with intergranular pores (blue areas; well T105; 3668.52 m). (f) Grain dolostone (well J5; 3542.08 m). (g) Crystalline dolomite with inter-crystal (i.e., dissolution) pores (well T112; 3383.92 m). (h) Gypsiferous dolomicrite with pores in gypsum (well To-76; 3725.78 m). (i) Salt-bearing dolomicrite with pores in salt (well J5; 3545.88 m).

Depositional sequences

Our petrological observations identified several upward-shallowing sedimentary sequences, and these sequences can be divided into four lithofacies assemblages: dolomicrite–stromatolitic dolostone–doloarenite; dolomicrite–doloarenite–thrombolitic dolostone–stromatolitic dolostone–gypsum breccia; dolomicrite–doloarenite–gypsum breccia; dolomicrite–dolomitic gypsum–stromatolitic dolostone–thrombolitic dolostone (Figure 4).

Figure 4.

Typical upward-shallowing depositional sequences in the Ma₅⁶ sub-member in the central–eastern Ordos Basin, China.

For the dolomicrite–microbialite–grain dolostone–gypsum breccia (Figure 4(a) to (c)), the lithological change from dolomicrite to grain dolostone reflects shallower water and higher energy conditions. This led to the stacking and migration of mound–shoal complexes, the water body becoming more restricted and saline, and sulfate saturation and precipitation of gypsum-bearing tidal deposits. For the dolomicrite–dolomitic gypsum–microbialite (Figure 4(d)), the lithological change from dolomicrite to dolomitic gypsum reflects normal-salinity seawater during early deposition and increasingly saline conditions during microbialite deposition. This indicates that the microbialites formed mainly in a restricted, shallow marine environment.

Therefore, it is proposed that there were four types of upward-shallowing paleo-environmental sequences in the Ma₅⁶ in the study area: dolomite lagoon–microbial mound facies; dolomite lagoon–grain shoal–microbial mound–gypsum-bearing tidal facies; dolomite lagoon–grain shoal–microbial mound facies; dolomite lagoon–gypsum lagoon–microbial mound facies. Good reservoir rocks, such as grain shoal and microbial mound facies deposits, typically developed in the middle and upper parts of the upward-shallowing sequences. In addition, the change from carbonate to gypsiferous rocks suggests the climate gradually became more arid and that sea-level was low, which is consistent with the regressional depositional setting of the Ma₅⁶. In the restricted evaporatic platform environment, high-frequency cycles led to the rhythmically interbedded gypsum–carbonate rocks that were controlled by the microgeomorphology.

Pore characteristics and physical properties

Petrographic observations indicate that the main reservoir rocks in the study area are microbialites, grain dolostone, and muddy to micritic dolomite. The microbialites (47.9% of the studied rocks) have excellent reservoir properties and contain grid (i.e. fenestral), interlayer, and dissolution pores (Figure 3(a) to (d)). The microbialites have an average porosity of 3.93% and an average permeability of 2.09 × 10^–3 μm². The grain dolostone (22% of the studied rocks) has moderate reservoir properties, and contains intergranular and intragranular dissolved pores (Figure 3(e) and (f)), which may be filled with gypsum and vadose silt. The grain dolostone has an average porosity and permeability of 3.5% and 2.06 × 10^–3 μm², respectively. The muddy to micritic dolomite contains mainly inter-crystal (i.e. dissolution), gypsum, and salt pores (Figure 3(g) to (i)), which are closely related to the lithofacies types. The micritic dolomite (13.7% of the studied rocks) contains inter-crystal (i.e. dissolution) pores (Figure 3(g)), and has an average porosity and permeability of 3.04% and 1.78 × 10^–3 μm², respectively (i.e. good reservoir properties). The dolomicrites contain isolated gypsum pores (9.3% of the studied rocks) and salt pores (5.5% of the studied rocks). The dolomicrites with gypsum pores contain pores that are usually sub-rounded in shape (Figure 3(h)), well-preserved, and not infilled, and have an average porosity of 1.69% and an average permeability of 0.22 × 10^–3 μm². The dolomicrites with salt pores contain pores that are relatively regular in shape with straight pore edges (Figure 3(i)), and have an average porosity of 1.12% and an average permeability of 0.55 × 10^–3 μm².

Based on the physical property data, samples with a porosity > 1% account for about 76.5% of the studied samples, and samples with a permeability > 0.01 × 10^–3 μm² account for about 68.9% of the studied samples. Therefore, the reservoir in the M₅⁶ sub-member is of medium–low porosity and permeability.

In Figure 5, it is evident that the reservoir permeability and porosity of the M₅⁶ sub-member are somewhat positively correlated. However, some samples with a similar porosity exhibit 2–4 orders of magnitude variations in permeability, and thus the reservoir permeability was likely improved by microfracturing. In general, the M₅⁶ sub-member contains mainly porous- and some fracture–pore-type reservoirs.

Figure 5.

Plots of porosity versus permeability for the Ma₅⁶ sub-member in the central–eastern Ordos Basin, China. (a) Gypsiferous dolomicrite and salt-bearing dolomicrite. (b) Grain dolostone. (c) Microbialites. (d) Micritic dolomite.

Geochemistry

Carbon and oxygen isotopes

The C–O isotope data are shown in Figure 6. The range of δ¹³C values in the M₅⁶ dolomites is −2.38 to + 0.93‰, with an average of −0.36‰, which is similar to the seawater range (δ¹³C = –2.00‰ to + 0.50‰) at the time of deposition. δ¹⁸O values range from −9.19‰ to −4.87‰, with an average of −7.30‰. Compared with seawater values (δ¹⁸O = –6.60‰ to −4.00‰) at the time of deposition, most of the dolomite data are more negative. The microbialites have the highest δ¹³C values (–1.52‰ to + 0.93‰) and δ¹⁸O = –9.78‰ to −4.88‰. The micritic dolomite has δ¹³C and δ¹⁸O values of −2.38‰ to + 0.89‰ and −7.97‰ to −5.85‰, respectively. The grain dolostone has δ¹³C and δ¹⁸O values of −2.08 to + 0.01‰ and −9.19‰ to −5.83‰, respectively. In summary, data for the muddy to micritic dolomite are similar to those for the grain dolostone.

Figure 6.

Plot of C–O isotopes for different types of dolomite in the M₅⁶ sub-member in the central–eastern Ordos Basin, China.

Trace elements

The microbialites are characterized by low Mn (average = 39.29 ppm), high Sr (average = 67.57 ppm), and high Ba contents (average = 2.00 ppm) (Table 1). The grain dolostone has moderate Mn (average = 47.38 ppm), Sr (average = 48.85 ppm), and Ba (average = 1.32 ppm) contents. The muddy to micritic dolomite has high Mn (average = 48.96 ppm), low Sr (average = 33.53 ppm), and low Ba (average = 0.98 ppm) contents.

Table 1.

Trace element data for the different types of dolomite in the central–eastern Ordos Basin, China.

Lithofacies type	Sample number	Mn/ppm	Sr/ppm	Ba/ppm	Sr/Ba	Mn/Sr
Microbialites	T102-2	35.15	64.05	3.26	19.64	0.55
	T102-3	34.28	70.52	0.85	82.85	0.49
	T102-4	38.86	66.53	1.71	38.94	0.58
	T102-5	35.55	65.69	0.94	69.56	0.54
	T102-6	37.17	61.21	2.20	27.83	0.61
	T102-11	36.19	64.81	2.64	24.59	0.56
	T102-13	38.76	62.89	1.23	51.13	0.62
	T102-16	33.31	71.29	1.15	62.08	0.47
	T102-17	33.31	64.82	1.63	39.76	0.51
	T78-4-1	42.68	82.53	2.34	35.29	0.52
	T78-4-3	41.47	72.41	4.36	16.61	0.57
	T78-4-5	61.72	64.12	4.32	14.83	0.96
	T78-4-6	41.03	63.77	1.43	44.67	0.64
	T78-4-8	41.85	71.56	1.71	41.89	0.59
	T78-4-11	38.73	64.85	1.34	48.52	0.60
	T78-4-12	38.50	70.06	0.94	74.64	0.55
Grain dolostone	T105-3	40.68	44.70	1.22	36.64	0.91
	T105-7	47.44	46.26	2.18	21.24	1.03
	T105-10	46.30	52.89	1.45	36.37	0.88
	T105-15	39.16	44.15	1.36	32.53	0.89
	T105-23	69.50	68.08	1.88	36.16	1.02
	T105-24	47.71	46.10	1.67	27.55	1.04
	T105-30	47.14	47.38	0.84	56.72	1.00
	T105-31	41.31	41.90	0.99	42.33	0.99
	T105-32	47.19	50.37	0.98	51.42	0.94
	T105-34	48.79	47.50	1.06	44.79	1.03
	T105-35	46.02	48.00	0.88	54.46	0.96
Muddy to micritic dolomite	T78-89-5	48.07	29.62	0.42	70.60	1.62
Muddy to micritic dolomite	T78-89-7	49.85	37.44	1.55	24.09	1.33

Discussion

Early dolomitization

During deposition of the M₅⁶ sub-member in the Ordovician in the Ordos Basin, a secondary regression occurred. A hot–arid climate and rifted setting formed a restricted evaporatic environment in the central–eastern Ordos Basin (Xiong et al., 2021). In this setting, strong evaporation led to high-salinity and -Mg/Ca seawater, which promoted large-scale early dolomitization (Hardie, 1987; Machel, 2004; Zhao et al., 2014). The dolomite reservoir rocks in the study area (muddy to micritic dolomite, microbialite, and grain dolostone) all preserve original rock fabrics. The microbialite and grain dolostone consist of dolomicrite and muddy to micritic dolomite (Figure 3), with semiidiomorphic crystals, visible bioclasts (Figure 3(c)), and stromatolitic structures (Figure 3(d)).

The Mn/Sr ratios of the different types of dolostone are all <2 (average = 0.79), which means that diagenetic alteration was relatively limited and geochemical information regarding the original fluid is well-preserved (Kaufman et al., 1993; Liu et al., 2020). The Sr/Ba ratios of the different types of dolostone are all >1 (average = 44.03), which are anomalously high and reflect deposition in highly saline seawater (Wang et al., 2021). δ¹³C values of the different types of dolostone are similar to those of Ordovician seawater (Figure 7), indicating that the dolomitizing fluids were originally derived from seawater (Veizer et al., 1999). δ¹³C values of some muddy to micritic dolomite samples are higher than those of Ordovician seawater, which may be related to ¹³C-rich bottom-waters formed by the localized strongly evaporatic and reducing water conditions (Xiong et al., 2020a, 2020b). In summary, the properties of the dolomitizing fluids inferred from the C–O isotope and trace element data are consistent with those proposed in previous studies (He et al., 2014). Most of the dolomites in the M₅⁶ sub-member experienced relatively low-temperature and oxidizing diagenesis, although some dolomites experienced relatively high-temperature and reducing diagenesis. All the diagenetic fluids were marine in origin. Therefore, it is proposed that the dolomites in the M₅⁶ sub-member formed in a subaerial and shallow burial diagenetic setting, which was closely associated with high-salinity seawater in the penecontemporaneous stage.

Figure 7.

Lithostratigraphy, reservoir physical properties, and depositional cycles of the M₅⁶ sub-member in well J5.

Previous studies have investigated the formation and pore development of deeply buried dolomite reservoirs (Budd, 1997; Fu et al., 2019; Hardie, 1987; Machel, 2004; Warren, 2000; Wu et al., 2016; Zhao et al., 2014). It is commonly thought that the reservoir pores are mainly inherited from primary or secondary pores generated by early dissolution, and that early dolomitization has a key role in the preservation of primary pores. A large number of dissolution experiments have also shown that dolomite is more difficult to dissolve than limestone in subaerial or shallow burial environments (Gautelier et al., 1999; Shen et al., 2016). Therefore, early dolomitization can help preserve primary pores and also change the composition and structure of the initial rock allowing it to resist compaction and pressolution, which further enhances the preservation of primary pores (e.g. intergranular and lattice pores). These pores provide space and migration channels for diagenetic fluids and hydrocarbons (Ehrenberg et al., 2006; Lucia, 2000). In addition, such rocks are prone to (micro-)fracturing because dolomite is more brittle than limestone (Chen and Qian, 2017; Moore, 2001), which enhances the permeability and oil and gas migration.

Diagenetic controls on reservoir quality

Penecontemporaneous karstification and reservoir properties

Although the fifth member of the Majiagou Formation records regressive deposition, high-frequency sea-level fluctuations during deposition of the member occurred (Xie et al., 2020; Xiong et al., 2019). The top of mound–shoal facies deposits in relatively high parts of the depositional setting would have experienced brief exposures to meteoric waters, which formed dissolution pores (i.e. fabric-selective and nonfabric-selective). In the microbialites and grain dolostone, granophyric dissolution is visible (Figure 2(e) and (f)). Grid-like dissolution (Figure 3(a)), intragranular dissolution (Figure 3(b) and (f)), interlayer (internal) dissolution (Figure 3(d)), and intergranular dissolution (Figure 3(e)) pores can be observed. Thus, pores and fractures formed by penecontemporaneous karstification are the main type of reservoir space.

δ¹⁸O values (–9.19‰ to −4.87‰) of the dolomite samples are lower than those of Ordovician seawater (–6.60‰ to −4.00‰; Veizer et al., 1999), which probably reflect O isotopic fractionation during burial and heating (Nielsen et al., 2010; Xiao et al., 2020). The δ¹⁸O values of the various types of dolostone, especially the grain dolostone and microbialites, vary widely, which was possibly caused by variable alteration due to interaction with meteoric waters. δ¹⁸O values of some grain dolostone and microbialite samples are obviously lower than those of the dolomicrite and Ordovician seawater, indicating the grain dolostone and microbialites were more strongly affected by karstification. This is consistent with the petrological characteristics of the dissolution pores in these two types of dolomite.

The mound–shoal facies deposits of grain dolostone and microbialites were mainly developed in the middle and upper parts of a single upward-shallowing sequence. However, for a single depositional sequence, the physical reservoir properties gradually deteriorate from top to bottom, which results in “downward-deteriorating reservoir heterogeneity” (Figure 7). This is consistent with the distribution and physical properties of reservoirs controlled by penecontemporaneous exposure and dissolution in many oil and gas basins in China and worldwide (Ma et al., 2019; Xiao et al., 2016; Xiong et al., 2019). This indicates that the development of high-quality reservoirs in the study area was closely associated with penecontemporaneous karstification due to high-frequency subaerial exposure.

Figure 8.

Plot of the proportion of cemented pores versus the diameters of primary pores in reservoir rocks of the M₅⁶ sub-member.

Cementation controls on the preservation of reservoir space

In the Ma₅⁶ reservoir, the diameters (D) of primary pores are >125 μm, the ratio of the cumulative cemented pore to the total cemented pore is >65%, and the preserved pore is <11% of the total preserved pore. When D < 125 μm, the preserved pore is >75% (Figure 8). This indicates that the cementation strength of the reservoir pores is closely associated with pore size. Given the occurrence of thin carbonate reservoirs between thick gypsum-bearing units in the M₅⁶ sub-member, the study area underwent significant cementation (Figure 9(a) and (b)). This was mainly due to the gradual increase in formation pressure during shallow burial. A large volume of high-salinity fluids was released from the gypsum–salt layer, entered the adjacent carbonate reservoirs with high porosity and permeability, and precipitated minerals in the early-formed pores. When these high-salinity fluids were released from the relatively dense gypsum-bearing rocks into the high-porosity and -permeability carbonate rocks, the sudden expansion of pore space might have reduced the effective solubility of minerals such as gypsum and carbonates, leading to the preferential cementation infilling of larger pores (Emmanuel et al., 2010; Xiong et al., 2019, 2020). As such, the reservoir has infilled large pores and preserved empty small pores. The pore size distribution of preserved pores is closely connected to the primary rock fabric (Figure 9). The preserved pore diameter in the doloarenite is mainly 63–125 μm, and accounts for 46.52% of the total porosity, while in the stromatolitic dolostone is mainly 16–53 μm, and accounts for 67.13% of the total porosity. In addition, cementation during early diagenesis (i.e. prior to compaction) can not only limit compaction and pressure solution, but also form a relatively closed diagenetic system during burial that prevents late diagenetic fluid seepage and transformation, which further preserves the pore space (Liu et al., 2020). Therefore, differential cementation is important in the preservation and distribution of the reservoir space.

Diagenetic and pore evolution

The petrological observations and geochemical data show that, after deposition, the Ma₅⁶ reservoir rocks experienced multiple stages of diagenetic modification. Three different diagenetic stages in variable diagenetic settings (Figure 10) successively modified the reservoir pores:

Diagenesis on the seafloor in the contemporaneous–penecontemporaneous stage: The Ma₅⁶ sub-member was deposited in a restricted evaporatic environment under hot–arid climate conditions. Due to the gradual evaporation, the seawater salinity increased, and gypsification affected the seafloor deposits and gypsum was continuously precipitated(Nasri et al., 2015). The precipitation of CaSO₄ increased the Mg/Ca ratio of the seawater, which was conducive to large-scale, penecontemporaneous dolomitization and dolomite cementation, which developed the lithological framework of the Ma₅⁶ reservoirs. This early dolomitization had an important role in the preservation of primary pores, such as intergranular and lattice pores, and formed secondary pores, such as intercrystalline pores.

Diagenesis by meteoric waters in the penecontemporaneous stage: Due to the high-frequency sea-level changes, the tops of the mound–shoal facies deposits in relatively high parts of the depositional setting were repeatedly exposed for short periods of time and subjected to karst transformation by meteoric waters. This formed a large number of irregular, secondary dissolution pores, such as enlarged, intergranular, intragranular, and intercrystalline dissolution pores (Figures 3(a) and (b) and 3(d) to (g)), which constitute the most important reservoir space in the Ma₅⁶ reservoir. These pores are mainly distributed in the middle and upper parts of the upward-shallowing sequence, and are concentrated in multiple thin layers (Figure 7). The meteoric waters dissolved gypsum nodules and cement, and formed pores in gypsum (Figure 3(h)), as well as other secondary pores.

Diagenesis during shallow burial in the early diagenetic period and deep burial in the middle–late diagenetic period: With increasing burial depth, compaction and pressure solution gradually increased. High-salinity diagenetic fluids derived from the evaporatic rocks flowed into adjacent carbonate reservoir rocks, and caused gypsum, dolomite, and calcite cementation (Jones and Xiao, 2005), which resulted in a decrease in porosity. Given the fluid pressure changes, there was differential cementation, which infilled large pores and preserved small pores, and thus resulted in reservoir heterogeneity. In addition, the regional gypsum rocks generated a closed and highly saline diagenetic environment in the underlying reservoirs (Grunau, 1981; Xiong et al., 2021), which blocked the flow of fluids, allowed the formation fluids to rapidly reach equilibrium, and hindered further dissolution and cementation in the deeply buried dolomite reservoirs. Tectonism formed fractures with different lengths and widths in the laminae, and thus improved the pore connectivity and permeability. However, the basic features of the reservoirs, including the lithologies and type of reservoir space, did not change fundamentally during burial diagenesis.

Figure 9.

Pore diameter distribution features of preserved and cemented pores in different types of reservoir rocks in the Ma₅⁶ sub-member. (a) Muddy to micritic doloarenite with dissolution pores, some of which are cemented by sparry calcite (well T78; 3713.07 m). (b) Pore diameter distribution of preserved.

Figure 10.

Lithostratigraphy, reservoir physical properties, and depositional cycles of the M₅⁶ sub-member in well J5.

Based on the depositional setting of the interbedded gypsum–carbonate rocks in the Ma₅⁶ sub-member, the diagenetic evolution, and the development and preservation of pores, we propose a three-stage pore evolution model for the grain dolostone (Figure 11). The three stages are the contemporaneous–penecontemporaneous, meteoric water dissolution, and burial stages.

Figure 11.

Pore evolution model for a grain dolostone reservoir in the Ma₅⁶ sub-member.

In summary, in the interbedded gypsum–carbonate rocks, porous deposits formed in grain–shoal and microbial mound facies in localized, relatively high-energy environments, in which the primary porosity was >40% (Choquette and Pray, 1970; Fu et al., 2019). During the contemporaneous–penecontemporaneous stage, large-scale early dolomitization favored the preservation of primary pores. However, more than half of the primary porosity was lost due to early compaction and cementation. Subsequently, the reservoir experienced dissolution by meteoric waters, which greatly improved the porosity and permeability of the dolomite reservoirs in the mound–shoal complexes, with an estimated 10–20% increase in porosity. In the burial diagenetic stage, the development of microfractures increased the porosity and permeability of the reservoirs, but the continuous compaction and cementation damaged the reservoir space, leading to a gradual decrease in reservoir porosity to <5% (Figure 10). Therefore, although the Ma₅⁶ reservoir underwent complex diagenetic transformations, the basic framework of the reservoir generally formed in the depositional to contemporaneous–penecontemporaneous stages, and should be categorized as an “early formation reservoir type.”

Implications for exploration

Oil and gas exploration in carbonate–evaporite strata worldwide is based on the conventional viewpoint that the layer between the presalt and suprasalt forms a single and tight, thick seal, which hinders effective oil and gas accumulation. The Ma₅⁶ sub-member occurs between the presalt and suprasalt units in the Majiagou Formation in the Ordos Basin. We have found that the Ma₅⁶ sub-member contains diverse sedimentary rock assemblages in different parts of the Ordos Basin, and consists of gypsum nodule-bearing and rhythmically interbedded evaporate–carbonate rock associations, which differ from the thick evaporitic units in the eastern Ordos Basin. The reservoir was eogenetic karst that was affected by high-frequency sea-level changes. Subaerial exposure and karstification in the early diagenetic stage caused by high-frequency sea-level changes were critical to reservoir formation, irrespective of the rock associations. This observation is of significance to reservoir formation in such carbonate–evaporitic rock associations in the Ordos Basin and worldwide. Given that the Ma₅⁶ sub-member is a typical example of the unit between the presalt and suprasalt units, our results have implication for oil and gas exploration in similar geological settings.

Conclusions

The Ma₅⁶ sub-member consists of gypsum and salt layers interbedded with thin carbonate rocks, which were deposited in a restricted evaporatic platform environment, and are characterized by various types of upward-shallowing sedimentary sequences. The main types of reservoir rocks are muddy to micritic dolomite, grain dolostone, and microbialites. The main reservoir space consists of intercrystalline (i.e. dissolution), intergranular, intragranular (i.e. dissolution), lattice (i.e. dissolution), and fenestral pores, which have formed medium-low porosity and permeability, porous- and fracture–pore-type reservoirs.

The formation of the Ma₅⁶ dolomite reservoir was closely related to penecontemporaneous dolomitization, karstification, and differential cementation. Large-scale early dolomitization formed the rock framework for the reservoir, which was conducive to the preservation of primary (and secondary) pores. The development of high-quality dolomite reservoirs was closely linked to the penecontemporaneous karstification, and differential cementation favored pore preservation and resulted in the heterogeneous distribution of reservoir space.

After deposition of the Ma₅⁶ sub-member, it successively experienced different diagenetic stages, including the contemporaneous–penecontemporaneous dolomitization, meteoric water dissolution, and protracted burial stages. Although the reservoir experienced a complex evolution, the basic framework of the reservoir and pores formed in the depositional to contemporaneous–penecontemporaneous diagenetic stages, indicating it is a type of “early formation reservoir.”

Footnotes

Acknowledgments

We are appreciative for the thorough and valuable comments of Associate Editor Prof. Cao and anonymous reviewers,which improved the manuscript significantly.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: This work was supported by the Science and Technology Cooperation Project of the CNPC-SWPU Innovation Alliance (grant number 2020CX010300).

ORCID iDs

Xiucheng Tan

Di Xiao

References

Adams

Diamond

(2017) Early diagenesis driven by widespread meteoric infiltration of a central European carbonate ramp: A reinterpretation of the upper Muschelkalk. Sedimentary Geology 362: 37–52.

Budd

(1997) Cenozoic dolomite of carbonate islands: Their attributes and origin. Earth-Science Reviews 42(1/2): 1–47.

Chen

Qian

(2017) Deep or super-deep dolostone reservoirs: Opportunities and challenges. Journal of Palaeogeography 19(02): 187–196.

Choquette

Pray

(1970) Geologic nomenclature and classification of porosity in sedimentary carbonates. AAPG Bulletin 54(2): 207–250.

Dautriat

Bornert

Gland

, et al. (2010) In-situ analysis of strain localization related to structural heterogeneities of carbonate rocks. In: 14th International Conference on Experimental Mechanics (ed Brémand

), Poitiers, France, 4–9 July 2010, pp.1–6. Paris: EDP Sciences Publishing.

Druckman

Moore

(1985) Late subsurface secondary porosity in a jurassic grainstone reservoir, smackover formation, Mt. Vernon field, Southern Arkansas. In Roehl PO and Choquette PW (Eds), Carbonate Petroleum Reservoirs (pp. 369–383). New York: Springer.

Ehrenberg

Eberli

Keramati

, et al. (2006) Porosity-permeability relationships in interlayered limestone-dolostone reservoirs. AAPG Bulletin 90(1): 91–114.

Emmanuel

Ague

Walderhaug

(2010) Interfacial energy effects and the evolution of pore size distributions during quartz precipitation in sandstone. Geochimica Et Cosmochimica Acta 74(12): 3539–3552.

Zhang

Chen

, et al. (2019) Characteristics, formation and evolution of pre-salt dolomite reservoirs in the fifth member of the Ordovician Majiagou formation, mid-east Ordos Basin, NW China. Petroleum Exploration and Development 46(06): 1087–1098.

10.

Gautelier

Oelkers

Schott

(1999) An experimental study of dolomite dissolution rates as a function of pH from − 0.5 to 5 and temperature from 25 to 80°C. Chemical Geology 157(1): 13–26.

11.

Grunau

(1981) Worldwide review of seals for major accumulations of natural gas. AAPG Bulletin 65: 933.

12.

Hardie

(1987) Dolomitization :A critical view of some current views. Journal of Sedimentary Research 57(1): 166–183.

13.

Shou

Shen

, et al. (2014) Geochemical characteristics and origin of dolomite: A case study from the middle assemblage of Ordovician Majiagou formation member 5 of the west of Jingbian gas field, Ordos Basin, North China. Petroleum Exploration and Development 41(3): 375–384.

14.

Hou

Fang

Zhao

, et al. (2002) Depositional environment model of middle Ordovician Majiagou formation in Ordos Basin. Marine Origin Petroleun Geology 7(01): 38–46.

15.

Shen

Yang

, et al. (2019) Dolomite genesis and reservoir-cap rock assemblage in carbonate-evaporite paragenesis system. Petroleum Exploration and Development 46(5): 916–928.

16.

Jones

Xiao

(2005) Dolomitization, anhydrite cementation, and porosity evolution in a reflux system: Insights from reactive transport models. AAPG Bulletin 89(5): 577–601.

17.

Kaufman

Jacobsen

Knoll

(1993) The Vendian record of Sr and C isotopic variations in seawater: Implications for tectonics and paleoclimate. Earth and Planetary Science Letters 120(3): 409–430.

18.

Kotarba

Bilkiewicz

Kosakowski

(2020) Origin of hydrocarbon and non-hydrocarbon (H₂S, CO₂ and N₂) components of natural gas accumulated in the Zechstein main dolomite carbonate reservoir of the western part of the Polish sector of the Southern Permian Basin. Chemical Geology 554: 119807.

19.

Liu

Tan

, et al. (2018) Occurrence and conceptual sedimentary model of Cambrian gypsum-bearing evaporites in the Sichuan Basin SW China. Geoence Frontiers 9(4): 1179–1191.

20.

Liu

Xiong

, et al. (2020) Evolution of diagenetic system and its controls on the reservoir quality of pre-salt dolostone: The case of the lower Ordovician Majiagou formation in the central Ordos Basin, China. Marine and Petroleum Geology 122: 104674.

21.

Lucia

(2000) Dolomitization: A porosity-destructive process. AAPG Bulletin 84(11): 32–40.

22.

Zhao

, et al. (2019) A new progress in formation mechanism of deep and ultra-deep carbonate reservoir. Acta Petrolei Sinica 40(12): 1415–1425.

23.

Machel

(2001) Bacterial and thermochemical sulfate reduction in diagenetic settings—old and new insights. Sedimentary Geology 140(1–2): 143–175.

24.

Machel

(2004) Concepts and models of dolomitization: A critical reappraisal. Geological Society, London, Special Publications 235(1): 7–63.

25.

Moore

(2001) Carbonate Reserviors: Porosity Evolution and Diagenesis in a Sequence Stratigraphic Framework. Netherlands: Elsevier Science, 1–444.

26.

Moore

Heydary

(1997) The role of burial diagenesis in hydrocarbon destruction and H₂S accumulation, upper jurassic smackover formation, black creek field, mississipian. AAPG Bulletin 81: 26–45.

27.

(2017) Global Petroleum Exploration and Development Situation and oil Company Dynamics. Beiing: Petroleum Industry Press.

28.

Nasri

Bouhlila

Saaltink

, et al. (2015) Modeling the hydrogeochemical evolution of brine in saline systems: Case study of the Sabkha of Oum El Khialate in South East Tunisia. Applied Geochemistry 55: 160–169.

29.

Nielsen

Swennen

Keppens

(2010) Multiple-step recrystallization within massive ancient dolomite units: An example from the Dinantian of Belgium. Sedimentology 41(3): 567–584.

30.

Shao

Bao

Wei

, et al. (2019) Tectonic palaeogeography evolution and sedimentary filling characteristics of the Ordovician in the Ordos area. Journal of Palaeogeography 21(04): 537–556.

31.

Shen

Zheng

Chen

, et al. (2016) Characteristics, origin and distribution of dolomite reservoirs in lower-middle Cambrian, Tarim Basin, NW China. Petroleum Exploration and Development 43(03): 340–349.

32.

Torghabeh

Kalantariasl

Ghorbani

, et al. (2021) Multivariate thermochemical sulphate reduction (TSR) low temperature origin for H₂S production: A fars provinance gas field. Journal of Natural Gas Science and Engineering 88: 103795.

33.

Veizer

Ala

Azmy

, et al. (1999) ⁸⁷Sr/⁸⁶Sr, Δ¹³c and δ¹⁸O evolution of phanerozoic seawater. Chemical Geology 161(1/3): 59–88.

34.

Wang

Liu

, et al. (2021) The Sr/Ba ratio response to salinity in clastic sediments of the Yangtze river delta. Chemical Geology 559: 1–15.

35.

Warren

(2000) Dolomite: Occurrence, evolution and economically important associations. Earth-Science Reviews 52(1–3): 1–81.

36.

Cao

, et al. (2016) A unique lacustrine mixed dolomitic-clastic sequence for tight oil reservoir within the middle permian lucaogou formation of the junggar basin, NW China: Reservoir characteristics and origin. Marine and Petroleum Geology 76: 115–132.

37.

Xiao

Cao

Luo

, et al. (2020) On the dolomite reservoirs formed by dissolution: Differential eogenetic vs. hydrothermal in the lower permian sichuan basin, SW China. AAPG Bulletin 104(7): 1405–1438.

38.

Xiao

Tan

Fan

, et al. (2019a) Reconstructing large-scale karst paleogeomorphology at the top of the ordovician in the Ordos Basin, China: Control on natural gas accumulation and paleogeographic implications. Energy Science & Engineering 7: 3234–3254.

39.

Xiao

Tan

, et al. (2016) An inland facies-controlled eogenetic karst of the carbonate reservoir in the middle permian maokou formation, southern sichuan basin, SW China. Marine and Petroleum Geology 72: 218–233.

40.

Xiao

Tan

Zhang

, et al. (2019b) Discovery of syngenetic and eogenetic karsts in the middle ordovician gypsum-bearing dolomites of the eastern Ordos Basin (central China) and their heterogeneous impact on reservoir quality. Marine and Petroleum Geology 99: 190–207.

41.

Xie

Tan

Feng

, et al. (2020) Eogenetic karst and its control on reservoirs in the ordovician majiagou formation, eastern sulige gas field, Ordos Basin, NW China. Petroleum Exploration and Development 47(6): 1246–1261.

42.

Xiong

Tan

, et al. (2020) Research advances,geological implication and application in Ordos Basin of the “pore-size controlled precipitation”in diagenesis of carbonate rock reservoir. Journal of Palaeogeography 22(04): 744–760.

43.

Xiong

Tan

Zuo

, et al. (2019) Middle ordovician multi-stage penecontemporaneous karstification in north China: Implications for reservoir genesis and sea level fluctuations. Journal of Asian earth sciences 183(Oct.1): 103969.1–103969.14.

44.

Xiong

Wang

Tan

, et al. (2021) Dolomitization of the ordovician subsalt majiagou formation in the central Ordos Basin, China: Fluid origins and dolomites evolution. Petroleum Science 18: 362–379.

45.

Yang

(1991) Discovery of the natural gas in lower palaeozoic in shanganning basin. Atural Gas Industry 11(02): 1–6.

46.

Zhao

Liu

(1993) Advances in Geology of Petroliferous Basins. Xi’an: Northwestern University Press, 1–448.

47.

Zhao

Shen

Zheng

, et al. (2014) The porosity origin of dolostone reservoirs in the Tarim, sichuan and Ordos Basins and its implication to reservoir prediction. Science China: Earth Sciences 57: 2498–2511.

Genesis and pore evolution of dolomite reservoir in the Majiagou Formation,Ordos Basin,China

Abstract

Keywords

Introduction

Geological setting

Samples and methods

Results

Petrography of the dolomite reservoirs

Lithofacies types

Depositional sequences

Pore characteristics and physical properties

Geochemistry

Carbon and oxygen isotopes

Trace elements

Discussion

Early dolomitization

Diagenetic controls on reservoir quality

Penecontemporaneous karstification and reservoir properties

Cementation controls on the preservation of reservoir space

Diagenetic and pore evolution

Implications for exploration

Conclusions

Footnotes

Acknowledgments

Declaration of conflicting interests

Funding

ORCID iDs

References