The understanding of diagenesis and porosity evolution is significant to the study of reservoir formation and prediction of high-quality reservoirs and oil and gas enriched areas. Many researches have been made ^{[1⇓⇓⇓⇓⇓⇓⇓-9]}. Liu et al. studied the different processes of porosity evolution and preservation in gas reservoirs in craton, foreland, and rift basins ^[1]. Zhou et al. investigated the diageneses and porosity evolution in the Sinian Dengying Formation and Cambrian Longwangmiao Formation in the Sichuan Basin, SW China ^[2-3]. Qin et al. focused on the burial history and porosity evolution in the Ordovician System in the middle-eastern Ordos Basin ^[4]. Zhang et al. studied the diageneses and porosity evolution in the Ordovician Yingshan Formation carbonate rocks in the Gucheng area, Tarim Basin, NW China ^[5]. Xu et al. restored the porosity evolution based on the sequence of cementation and corrosion or bitumen occurrence and organic matter evolution ^[6]. She et al. studied the pore structure and porosity evolution in carbonate rocks through corrosion simulation ^[7]. In summary, porosity evolution may be restored using the following methods: (1) diagenesis and diagenetic sequence for qualitative restoration; (2) diagenesis combined with burial history; (3) diagenesis combined with thermal evolution of organic matter; (4) comprehensive study of diagenesis, burial history, tectonic history and thermal evolution of organic matter for quantitative restoration. However, these methods, both qualitative and quantitative restoration, have the problem of determining the geologic time.

In-situ laser U-Pb dating for carbonate rocks has been developed at CNPC Key Laboratory of Carbonate Reservoirs since 2018 to accurately date carbonate cements and thus determine the important time points in the process of diagenesis and porosity evolution, and restore the porosity evolution with high precision. In this paper, we focus on supra-salt weathering-crust karstic reservoirs and sub-salt mound-shoal reservoirs in the Ordovician Majiagou Formation, Ordos Basin. Based on petrologic study, we use in-situ laser U-Pb dating combined with carbon and oxygen isotopic composition test to restore the diagenesis and porosity evolution for these two reservoir types. Our study offers supports to the prediction of reservoirs and promising oil and gas provinces.

U-Pb isotopic dating is a mature technique widely used for dating the minerals with high U content, e.g. zircon, monazite, xenotime, sphene, rutile, apatite, and garnet. Although it is one of the most commonly used dating methods in geochronology, the conventional U-Pb isotopic dating has not been extensively applied to date carbonate minerals. At present, there are few reports on the solution method, which was used to date the cements in Mesozoic and Cenozoic pores and caverns by measuring the U-Pb isotopes ^{[10⇓⇓⇓-14]}. It is hard to promote its practical application owing to strict requirements of samples and low success rate of testing.

Thanks to the progress in laser ablation, the in-situ laser U-Pb isotopic dating technology has been applied to date high-U minerals. For low-U minerals, particularly carbonate minerals, studies have been done and some achievements have been made ^{[15⇓⇓⇓-19]}. But there are some issues to be addressed, e.g. lack of standard matrix samples and challenge of precise isotopic measurement due to low U content. In 2018, a standard sample of (209.18±1.20) Ma for lab tests was successfully established at CNPC Key Laboratory of Carbonate Reservoirs to fix the problems of ASH15E sample of 3.001 Ma for too young ^[20⇓-22] or data instability caused by WC-1 sample with the age varying in 250.27-254.40 Ma ^{[15⇓⇓-18]}. Laser ablation (LA) was combined with high-resolution single-collector inductively coupled plasma mass spectrometry (ICP-MS) for U-Pb isotopic measurement with high resolution, high precision and high accuracy, and consequently the detection limit of U content could be as low as 10 μg/L. This solved the problem of U detection in age-old marine carbonate rocks due to its low content and thus made it possible to establish in-situ laser ablation U-Pb isotopic dating for age-old marine carbonate rocks ^[23-24]. This technology has been applied to age-old carbonate rock samples acquired from the Sichuan, Tarim, and Ordos Basins. And the results have been compared with the analogues from the parallel tests made at the Radioisotope Laboratory at the University of Queensland, Australia. It is demonstrated that the technology is feasible and easy to operate, and can obtain reliable and comparable data. It is a powerful tool to obtain absolute age and accurate time interval with porosity change in porosity evolution. This is important to the understanding of reservoirs and hydrocarbon accumulation history.

The study samples were collected from the supra-salt weathering-crust karstic dolomite reservoirs and sub-salt mound-shoal dolomite reservoirs in the Ordovician Majiagou Formation, Ordos Basin. Considering that the samples will be used to (1) investigate diagenesis, especially the sequence of pore filling, (2) measure carbon and oxygen isotopic composition in micro-area, and (3) perform U-Pb isotopic dating, the candidates should have primary pores or secondary dissolved pores and cavities, which are completely or partially filled with cements. To eliminate age differences related to geologic positions, all the samples were acquired from the Ma5 Member (Fifth Member of the Majiagou Formation). In accordance with the international chronostratigraphic chart ^[25], the Majiagou Formation deposited in the Middle Ordovician which spans the Dapingian Stage and Darriwilian Stage, lasting for 11.6 Ma from 458.4 Ma to 470.0 Ma. If the Majiagou Formation is equally divided into 6 members, each member is 2 Ma in time span. This means that the age-old Ma5 Member at over 450 Ma could be regarded as isochronal.

In view of above requirements, we acquired 2 supra-salt weathering-crust karstic dolomite reservoir samples and 6 mound-shoal dolomite reservoir samples. Table 1 lists the basic data of the samples. Fig. 1 shows the locations of the samples.

Two supra-salt weathering-crust karstic dolomite reservoir samples, formed in a gypsiferous dolomitic tide flat environment, were taken from the Ma5₁ in Well Yu113 in the Mizhi sag and the Ma5₂ in Well Tong67 in the Hengshan salient, respectively. The reservoir space is constituted by gypsum mold pores, but the fillings and filling types are different in two samples. Sample 1 is filled with dolomitic silts and calcites (Fig. 2a, 2b), while Sample 2 is filled with dolomitic silts (Fig. 2c, 2d). Such lithofacies and filling assemblages represent the rock types and major filling sequences in supra-salt weathering-crust reservoirs, so the samples could be used for porosity evolution restoration.

Six sub-salt mound-shoal dolomite reservoir samples include 2 thrombolite dolomite samples, 3 granular dolomite samples, and 1 powder crystal dolomite sample. Samples 3 and 4 are thrombolite dolomites. The former was acquired from the mound-shoal unit developed in a gypseous dolomitic flat in the Ma5₆ in Well Jin4 in the Taolimiao depression, in which dissolved pores were partially filled with calcites (Fig. 2e, 2f). The latter was taken from the mound-shoal unit in the Ma5₈ in Well Tao112 in the Hengshan salient, in which dissolved pores were filled with two-phase blade-shaped dolomites and crystalline dolomites (Fig. 2g, 2h). Samples 5 and 6 are dolarenite/dolorudite coming from the Ma5₆ in Well Tao112 in the Hengshan salient, in which dissolved pores and cavities were partially filled with crystalline calcites (Fig. 2i, 2j). Sample 7 is oolitic dolomites filled with dogtooth-shaped fine powder crystal dolomites taken from the Ma5₆ in Well Tao86 in the Taolimiao depression, in which intragranular pores, intergranular pores, and intergranular dissolved pores were partially filled with crystalline calcites (Fig. 2k, 2l). Sample 8 is powder crystal dolomites with microcracks and crystalline calcite fillings taken from the Ma5₆ in Well Tao112 in the Hengshan salient (Fig. 2m). Six samples represent the main rock types of sub-salt mound-shoal dolomite reservoirs.

Each sample was cut into 3 slices of 2-3 mm thick for in-situ laser U-Pb isotopic dating, in-situ laser carbon and oxygen isotopic composition testing, and cathodoluminescence microscopy, respectively. Two types of sample, i.e. target and slice, were prepared for dating minerals in accordance with their macroscopic and microscopic fabric characteristics. For the samples with simple composition of wall rocks and large filling minerals, target samples were prepared with spot beam diameter of 160 μm to ensure signal intensity and credible sampling. For the sample with wall rocks with complicated composition and filling materials with complicated structures, slice samples were prepared. The diameter of laser spot beam, which may be 60, 90, or 120 μm, depends on mineral crystal size and micro-area distribution. The smaller the diameter, the more the points to be tested, and the higher the data credibility is.

In-situ laser U-Pb isotopic dating, in-situ laser carbon and oxygen isotopic composition testing, and cathodoluminescence microscopy were performed at CNPC Key Laboratory of Carbonate Reservoirs.

In-situ laser U-Pb isotopic dating was accomplished using LA-ICP-MS (Element XR) at the temperature of 20°C and relative humidity of 50%. Laser carbon and oxygen isotopic test was made using Delta V carbon-oxygen isotope ratio mass spectrograph with laser spot beam diameter of 500 μm at the temperature of 24 °C and relative humidity of 48%. See Fig. 2 for details.

We obtained 14 ages from in-situ laser U-Pb isotopic dating, as shown in Table 2 and Fig. 3. As per wall rocks and the types, phases and geneses of filling minerals in reservoir pores, 14 ages were classified into 5 groups. Group 1 of 444.0-494.0 Ma, represents the matrix dolomite samples of wall rocks, including powder crystal dolomite with anhydrite concretions from wells Yu113 and Tong67, thrombolite dolomite from wells Jin4 and Tao112, dolarenite from Well Tao112, and oolitic dolomite from Well Tao86. Group 2, 440.0-467.0 Ma, represents the forming period of Phase 1 blade- or dogtooth-shaped filling materials of chemical genesis. Group 3, 316.5-381.0 Ma, corresponding to the forming period of Phase 2 interstitial dolomitic silts from weathering crust and transported mechanically. Group 4, 354 Ma, is related to the forming period of Phase 3 interstitial crystalline dolomites of chemical genesis. Group 5, 292.7-319.0 Ma, represents the age of Phase 4 interstitial crystalline calcites of chemical genesis.

The accuracy of 4 groups of test data and influential factors were assessed from the perspectives of U-Pb isotopic age contour map, depositional setting and diagenetic reconstruction before practical application. As shown in the isotopic age contour map (Fig. 3), the closer the data points to the abscissa and the smaller the circle, the more reliable the age is under the same testing environment. This means that among 14 ages, 10 ages may be of high credibility, except for 4 ages with medium to high accuracy error for powder crystal dolomite wall rocks in Well Jin4 (Fig. 3c), blade-shaped dolomitic cements in Well Tao112 (Fig. 3h), interstitial crystalline dolomites in Well Tao112 (Fig. 3k), and calcites filled in gypsum mold pores in Well Yu113 (Fig. 3l).

As for depositional setting, background Pb content may vary with redox conditions in different sedimentary facies and zones. Pb tends to enter calcite or dolomite lattices in reducing conditions to boost background Pb content. As a result, the tested age will be older than the actual age. The depositional setting had a great impact on the six samples in Group 1 with deposition fabrics such as micrite, thrombolite, sand cuttings and oolites. Well Yu113 was in the gypseous salt lake of Mizhi sag, and wells Tong67, Jin4, Tao112, and Tao86 were in a gypseous dolomitic flat or intra-platform mound-shoal environment at the depositional stage of the Ordovician Ma5 Member in a reducing environment with high salinity caused by marine regression. The consequent high background Pb content in sediments led to older tested ages. The tested age of the wall rock taken in Well Yu113 is (492.0±36) Ma (Fig. 3a), that of the wall rock taken in Well Tao112 is (493.0±42) Ma (Fig. 3d), and that of the dolomitic oolitic wall rock in Well Tao86 is (494.0±26) Ma (Fig. 3e), all of which are close to the age of the bottom of the Ordovician System.

Diagenesis and diagenetic reconstruction also have some impacts on U-Pb isotopic age. Normally, large Pb content in the diagenetic process leads to older age and large U content leads to smaller age, and Pb effect is greater than U effect. The ages in groups 2-4 are related to filling materials formed in the diagenetic process and thus they may be affected by diagenetic fluids. The ages of the subsea cements in Group 2 indicate the time of cementation or cements transformation into dolomites. Mineral precipitation was mainly affected by sedimentary environment, and fluids for transformation were also relevant to the seawater in that period. This means that the age of the cements in that period is more affected by the sedimentary environment. A typical example is the Phase 1 cements of (467.0±16) Ma in Well Tao86. The ages of the dolomitic silts in the gypsum mold pores in Group 3 are closely related to weathering-crust karstification, in which meteoric water is a major control. A minute amount of Pb in meteoric water may result in older tested age. For example, the age of the dolomitic silts in gypsum mold pores in Well Tong67 is tested to be (381±27) Ma (Fig. 3i). Fortunately, the tested age of (316.5±6) Ma for the dolomitic silts in gypsum mold pores in Well Yu113 is of high accuracy and could be used to indicate the time period of diagenesis. The age in Group 4 is related to the calcites filled in dissolved pores or cracks. The diagenetic fluid is a mixture which may be consisted of formation water, meteoric water and released water caused by overburden pressure drop. Different proportions of water in different areas or time periods will lead to varied U and Pb contents in different wells or different parts of the same calcite crystal. Consequently, the tested age may vary in a wide range.

As per preceding studies, there are two reservoir types in the Majiagou Formation, i.e. supra-salt karstic dolomite reservoirs and sub-salt mound-shoal dolomite reservoirs bounded by thick gypseous salt rocks in the Ma5₆ Member. The former is mainly micritic and powder crystal dolomites with gypsum mold pores (concretions), and the latter includes granular dolomites or powder crystal and fine crystalline dolomites with residual granular phantom and microbial dolomites. The mechanisms of dolomitization, including penecontemporaneous dolomitization, infiltration backflow dolomitization, burial dolomitization and hydrothermal dolomitization, have been investigated for these two reservoir types ^{[26⇓⇓⇓⇓⇓⇓⇓-34]}. However, there are still controversies caused by non-unique isotopic interpretations of carbon, oxygen and strontium, etc., which result in different diagnoses of environment and time period of dolomitization. Our study is decisive to date the dolomite and deepen the understanding of dolomitization and dolomite evolution. As shown in Table 2, 4 groups of data obtained from this study indicate the ages of rock matrix, cements in intergranular pores, and two-phase interstitial minerals in dissolved pores, respectively. Particularly, Group 1 and Group 2 data are significant to the study of dolomitization.

Six ages of 444.0-494.0 Ma in Group 1 indicate the time period of syndepositional or penecontemporaneous dolomitization. The age of (492.0±36) Ma for Ma5₁ powder crystal dolomite wall rocks with anhydrite concretions in Well Yu113 and the age of (453.0±32) Ma for Ma5₂ powder crystal dolomite wall rock with anhydrite concretions in Well Tong67 represent the time period in which matrix dolomites came into being in supra-salt karstic dolomite reservoirs. The age of (444.0±31) Ma for the coarse powder crystal dolomite wall rocks around Ma5₆ thrombolite dolomites in Well Jin4, the age of (476.0±36) Ma for the sand cuttings in Ma5₆ dolarenite in Well Tao112, the age of (493.0±42) Ma for Ma5₈ thrombolite dolomite wall rocks in Well Tao112, and the age of (494.0±26) Ma for the oolites in Ma5₆ oolitic dolomites in Well Tao86 represent the time period in which matrix dolomite came into being in the sub-salt mound-shoal dolomite reservoirs. Except for the age with large errors of coarse powder crystal dolomite wall rocks in Well Jin4, other 5 ages are of high credibility. Thus, the matrix dolomites, in both supra-salt karstic reservoirs and sub-salt mound-shoal reservoirs, are dated to be 444.0-494.0 Ma. Three ages correspond to the Middle and Late Ordovician, and the other three are close to the beginning of the Ordovician. In view of the influence of diagenetic reconstruction and the detection error of 26-42 Ma, it is concluded that the matrix dolomites were mainly formed during the Early and Middle Ordovician. Carbon and oxygen isotopic composition analysis showed δ¹³C values are -1.51‰ to 1.21‰ and δ¹⁸O is -8.25‰ to -5.81‰ for the matrix dolomites. Compared with δ¹⁸O between -6.5‰ and -4.5‰ and δ¹³C between -2.0‰ and 0.5‰ for seawater cements during the Ordovician, the matrix dolomites have heavier carbon isotopic composition and lighter oxygen isotopic composition, which indicates the features of evaporated seawater. The dolomites have a low degree of order between 0.45 and 0.85, and a high Mg-Ca ratio of 0.9-1.0 ^[26]. In accordance with comprehensive diagnoses, the matrix dolomites are the product of syndepositional-penecontemporaneous dolomitization.

With respect to two age data of (440.0±47) Ma and (467.0±16) Ma in Group 2, the latter was evaluated to be of high accuracy to indicate an age of this group, which is the Middle Ordovician Epoch. The host minerals are the blade-shaped cements in the Ma5₈ thrombolite dolomite in Well Tao112 and the dogtooth-shaped subsea cements in the Ma5₆ oolitic dolomite in Well Tao86. From the petrological perspective, the two types of cements were developed in a subsea environment and supposed to be calcites or aragonites during the precipitation process. The cements and surrounding matrix dolomites may form in a similar time period. As mentioned above, the Ma5₈ thrombolite dolomite wall rock in Well Tao112 and the Ma5₆ dolomitic oolitic wall rock in Well Tao86 were dated at (493.0±42) Ma and (494.0±26) Ma, respectively. Their cements were dated at (440.0±47) Ma and (467.0±16) Ma. Thus, these ages may represent the time period when cements transformed into dolomites. In view of δ¹³C from 0.78‰ to 1.35‰ and δ¹⁸O from -6.95‰ to -6.68‰, which are similar to contemporaneous seawater isotopic composition, cement dolomitization was supposed to be dolomitization caused by infiltration backflow-early burial.

In view of the great differences in diagenetic sequence and porosity evolution, supra-salt karstic dolomite reservoirs and sub-salt mound-shoal dolomite reservoirs are discussed separately.

Supra-salt karstic dolomite reservoirs went through several diagenetic processes and diagenetic sequences including penecontemporaneous dolomitization, compaction, weathering-crust karstification, filling and fracturing in succession, among which karstification and filling are crucial to porosity evolution ^{[35⇓⇓⇓-39]}. For the origin of the gypsum mold pores which constitute major pore space, is diversely attributed to penecontemporaneous karstification and weathering-crust karstification. In contrast, filling is uniformly ascribed to 12 major types of interstitial materials, i.e. dolomitic silt, (ferriferous) calcite, (ferriferous) dolomite, quartz, fluorites, pyrite, kaolinite, anhydrite, residual shale and organic matter, etc. They constitute 8 types of filling assemblages, of which dolomitic silt + calcite is the most important one. Two samples in our study belong to this assemblage. As shown in Fig. 4, Phase 1 interstitial materials, i.e. dolomitic silts, in gypsum mold pores were dated at (381.0±27) Ma and (316.5±6) Ma. The latter was assessed to be more credible to reveal that the dolomitic silts were formed in the Late Carboniferous Epoch. Dolomitic silts were developed to be simultaneous with or slightly later than gypsum mold pores. Thus, it is inferred that gypsum mold pores were formed in the period of epigenic karstification and weathering from the Late Devonian Epoch to the Late Carboniferous Epoch, which agrees with the regional tectonic evolution. Phase 2 interstitial materials, i.e. calcites, in gypsum mold pores were dated at (303.0±23) Ma, corresponding to the Late Permian Epoch. This means that the interstitial calcites turned up in a deep burial environment after Permian deposition. δ¹³C of -4.53‰ and δ¹⁸O of -11.43‰ are lighter in isotopic compositions than the contemporaneous seawater, and the inclusion homogenization temperature of 95.7- 108.2°C indicates organic carbon going into fluids at high temperature. In summary, the gypsum mold pores in supra-salt weathering-crust dolomite reservoirs were shaped in the period with weathering and denudation due to regional tectonic uplift, i.e. from the Late Devonian Epoch to the Late Carboniferous Epoch. Thanks to the weathering-crust karstification, reservoir porosity reached 10%-40%. At the same time or after that, dolomitic silts entered the gypsum mold pores, making the porosity drop to 6%-30%. Then in the subsequent burial process, the reservoir pores were filled with calcites and some additional minerals including quartz, crystalline dolomite, fluorite and gypsum in the Late Permian Epoch. Finally, the porosity further declined to 0-20%, mainly 3%-8%. As the overlying formations were compacted and tightened, the reservoirs kept being in a pressure compartment for a long period until gas injection and accumulation to form gas reservoirs.

Compared with the supra-salt karstic reservoirs, the sub-salt mound-shoal dolomite reservoirs underwent more complicated diagenetic evolution, including penecontemporaneous dolomitization, compaction, subsea cementation, penecontemporaneous corrosion, infiltration backflow dolomitization, filling and fracturing. The reservoir space is mainly composed of residual intergranular (dissolved) pores, microbial growth framework (dissolved) pores and small dissolved cavities ^{[40⇓⇓-43]}. Take Ma5₆ microbial dolomite reservoirs in Well Tao112 for example, the pore space is mainly composed of microbial growth framework (dissolved) pores and small dissolved cavities, which originated from microbial growth framework and penecontemporaneous corrosion. Three phases of interstitial materials have been identified: Phase 1 blade- or dogtooth-shaped subsea cements, Phase 2 crystalline dolomites, and Phase 3 coarse crystalline calcites. As shown in Fig. 5, Phase 1 blade-shaped dolomitic cements and dogtooth-shaped fine powder crystal dolomitic cements were dated at (440.0±47) Ma and (467.0±16) Ma, respectively, and the latter was assessed to be more accurate to represent the time period of penecontemporaneous-infiltration backflow dolomitization or cements forming stage. Phase 2 interstitial crystalline dolomites were dated at (354.0±46) Ma. Compared with the Ordovician seawater isotopic composition, the lighter δ¹⁸O of -7.48‰ and the obviously heavier δ¹³C of 1.38‰ indicate the fluid origins from evaporated brine water and fresh water. During the long-period weathering and denudation of the Ordovician System, meteoric water, mixed with evaporated brine water, flowed downward through denudation windows or cracks into the sub-salt reservoirs to precipitate crystalline dolomites. Phase 3 interstitial coarse crystalline calcites dated at (292.7±3) Ma mainly occurred in dissolved cavities. The δ¹³C and δ¹⁸O (-5.09‰ and -9.08‰, respectively) show negative anomalies. The interstitial materials in Phase 3 are consistent with the calcites in supra-salt gypsum mold pores and the calcites in cracks in terms of petrologic and geochemical features and forming time. Therefore, they are supposed to be the products of fluid precipitation in the same period. On the other hand, these interstitial materials were presented in chronological order, i.e. the calcites in cracks are the earliest at (319.0±18) Ma, followed by the calcites filled in gypsum mold pores in supra-salt karstic reservoirs at (303.0±23) Ma and the latest calcites filled in the pores and cavities in sub-salt mound-shoal reservoirs at (292.7±3) Ma. This means that during the Carboniferous-Permian deposition, released water caused by overburden pressure drop in coal measures first went into cracks and then filtered into supra-salt karstic reservoirs and sub-salt mound-shoal dolomite reservoirs in succession. Thus, the primary pores in sub-salt mound-shoal dolomite reservoirs turned up at the depositional stage, with the primary porosity of 10%-30%. Then affected by seawater cementation, the porosity declined to 0-6% and then rose back to 5%-15% due to penecontemporaneous corrosion. In the burial process, some pores were filled with crystalline dolomites, which had little effect on the porosity, then more filled with calcites, which made the porosity decrease quickly to 0-12%, mainly 2%-6%.

Fourteen ages in 5 groups were obtained by dating of 8 reservoir samples from the Majiagou Formation, Ordovician. Group 1, 444.0-494.0 Ma, represents surrounding matrix dolomites. Group 2, 440.0-467.0 Ma, represents the interstitial fibrous or dogtooth-shaped dolomites. Group 3, 316.5-381.0 Ma, represents interstitial dolomitic silts. Group 4, 354.0 Ma, corresponds to interstitial crystalline dolomites. Group 5, 292.7-319.0 Ma, corresponds to Phase 4 interstitial crystalline calcites.

In accordance with the dating data and other testing results including carbon and oxygen isotopic compositions, the psammitic, oolitic and powder crystal dolomite wall rocks of the dolomite reservoirs in the Majiagou Formation are related to depositional or penecontemporaneous dolomitization, while the cements are the product of infiltration backflow-early burial dolomitization.

The supra-salt weathering-crust karstic dolomite reservoirs went through several diagenetic processes including penecontemporaneous dolomitization, compaction, weathering-crust karstification, filling and fracturing in succession. Gypsum mold pores were developed by epigenic karstification, and filled with dolomitic silts and calcites. The porosity declined from initial 10%-40% to 3%-8%.

The sub-salt mound-shoal dolomite reservoirs went through several diagenetic processes including penecontemporaneous dolomitization, compaction, subsea cementation, penecontemporaneous corrosion, infiltration backflow dolomitization, filling and fracturing. The reservoir primary porosity declined from the initial 10%- 30% to 0-6% caused by seawater cementation, then rose back to 5%-15% owing to penecontemporaneous corrosion, and finally declined to 2%-6% after filled with crystalline dolomites and calcites.

The methodology of in-situ laser U-Pb dating technology combined with in-situ laser carbon and oxygen isotopic composition testing was used to restore multi-phase dolomitization and other diagenetic processes in different time periods in the coordinate system of absolute age for dolomite reservoirs in the Majiagou Formation. These powerful tools can eliminate the issue of non-unique interpretation, and provide more accurate restoration of dolomite reservoir forming for promising reservoir prediction. This method is effective and easy to use, and may be helpful to the restoration of porosity evolution in other basins or series of strata.