Introduction
Evaporites are regarded as a good seal for oil and gas traps by most geologists, and their role in the formation of reservoir has drawn increasing attention. The development of evaporites is indicative of arid climate and relatively restricted sedimentary environment favorable for the development of seepage-reflux dolomitization and capillary concentration dolomitization[1,2], which can make it likely for the calcite to turn into dolomite due to metasomatism of high-salinity dolomitizing fluid, with the quality of reservoir improved[3]. Evaporites are more soluble than carbonate, so evaporites associated with carbonate can be easily dissolved to form secondary pores[4]. When entering reservoir, high maturity hydrocarbons can have sulfate thermochemical reactions (TSR) with components of evaporite to generate H2S under high temperature, which is of great significance for the formation of high-quality reservoirs[5,6].
Carbonate reservoirs often undergo a complex and long diagenesis process. Highly sensitive to diagenesis[7], they could lose a part of primary pores, so secondary pores take a major proportion in them[8]. Diagenetic processes are complex and diverse in type, however, they can be classified into two main groups i.e. “constructive” and “destructive”. The constructive ones include dolomitization, dissolution, and degypsification etc[9,10,11,12,13]; whereas the destructive ones include mechanical compaction, cementation, and filling[7,14]. In recent years, extensive research has been conducted on the diagenesis of carbonate reservoirs and some non-weathering crust karst reservoirs have been discovered[3,15], making researchers realize the importance of “constructive” diagenesis of carbonate in pore development. Moreover, the development of related theory has provided new basis for the prediction of effective reservoir distribution.
Dolomite reservoirs overseas, that are associated with evaporites, have been proven to be commonly rich in gas resources, for example, the Paleocene of the Wafra oil field in Abu Dhabi[16], the Ordovician in the Texas[17], the Upper Jurassic in the Gulf of Mexico[9], the Lower Triassic in the Persian gulf[10], and the Cretaceous in the Santos Basin[18]. In China, pre-salt dolomite reservoirs have been found in the Triassic Leikoupo Formation in the Sichuan Basin[11], the Cambrian in the Tarim Basin [19] and the Ordovician Majiagou Formation in the Ordos Basin[20]. The Lower Paleozoic Ordovician marine carbonates are an important area for gas exploration in the Ordos Basin[21]. In 1989, Jingbian gas field, was discovered in the weathering crust at the top of the Majiagou Formation of the Ordovician, indicating that the carbonates of the Majiagou Formation may potentially have gas reservoirs that are yet to be discovered[22]. In recent years, as the study on gas accumulation and enrichment law of marine carbonate reservoir deepened, major discoveries have been made in the Middle Assemblage dolomite on the eastern side of the Central Paleo Uplift of the Ordos Basin, demonstrating the promising exploration prospects of the Majiagou Formation carbonates again. Moreover, researchers proposed the theory that Upper Paleozoic coal-bearing source rocks provided oil and gas, which then migrated laterally and accumulated in the Majiagou Formation[23]. In the mid-eastern Ordos Basin, the Majiagou Formation contains multiple sets of evaporite deposits, among which, the evaporites in the M56 sub-member are most extensively distributed and thickest. Thus, the M56 sub-member is often taken as the boundary between the post-salt strata and the pre-salt strata in the Ordos Basin[24], and the M57 to M510 sub-members are referred to simply as pre-salt section[20]. Researchers are interested in understanding the geological conditions of pre-salt strata because a number of exploration wells tested high gas flows in the M57 and M59 sub-members[25]. Although extensive studies have been conducted on the Majiagou Formation before, they primarily focused only on the reservoirs in the weathering crust on top of M5 member[12,13] or discussed the characteristics of carbonate in the whole Majiagou Formation[25]. Although some researchers examined the sedimentary characteristics, reservoir features, accumulation conditions and natural gas geochemical features of dolomite below Ma5 Member[8,23,26-27], there is little understanding on the origins of pores, reservoir forming mechanisms, and the impact of evaporite on reservoir.
In this study, taking the pre-salt dolomite reservoir in the Majiagou Formation as research object, we investigated its petrologic, mineralogic, pore, diagenetic and geochemical characteristics, diagenetic process of the reservoir and its impact on pore development, and summed up the formation and evolution of the dolomite reservoir, in the hope to provide geological basis for exploration of similar carbonate reservoirs in the Ordos Basin and other areas.
1. Geological setting
Ordos Basin is a typical multi-cycle intracratonic basin with an area of 25 000 km2[28]. It has six tectonic units i.e. the Yimeng Uplift in the north, Yishaan Slope in the central, Jinxi Fold Belt in the east, Weibei Uplift in the south and the Tianhuan Depression and Western Margin Thrust Belt in the west[29,30]. The study area is located in the mid-east of the Ordos Basin, covering the Yishaan Slope and the Jinxi Fold Belt, with an area of 5×104 km2 (Fig. 1).
Fig. 1.
(a) The division of tectonic units in the Ordos Basin; (b) Composite stratigraphic column of M5 Member in mid-east of the basin.
During the depositional period of the Ordovician Majiagou Formation, the “L” type paleo-uplift formed by subduction of Qingling-Qilian paleo-ocean crust controlled the sedimentary environment of the Ordos Basin[8,26]. West of paleo-uplift was the intracratonic epeiric sea, where tidal flat, platform margin and shelf sedimentary facies developed. Impacted by the North China sea, tidal flat, restricted platform and evaporite lake sedimentary facies developed east of the “L” paleo- uplift[31,32]. The 5th member of Majiagou Formation (M5) can be divided into 10 sub-members (M51, M52…M510) based on petrology and sedimentary cycles[33] (Fig. 1b). After the deposition of Majiagou Formation, the entire Ordos Basin was uplifted due to the Caledonian and Hercynian orogeny and experience a deposition hiatus of about 150 Ma[29,32]. The top of the Majiagou Formation had long exposed and subjected to leaching by atmospheric water, giving rise to large-scale karstic reservoirs in the M51 to M54 sub-members[34]. But the M56 to M510 sub-members were not greatly impacted by atmospheric water and are characterized by dolomite and evaporite, among them, the M57 and M59 are primarily dolomite and the M56, M58 and M510 are composed of largely thick layers of evaporites[35].
2. The reservoirs
2.1. Petrologic features
Three types of dolomite were identified in the study area i.e. dolomicrite, fine-crystalline dolomite and dolarenite based on observation of core samples and petrographic study of thin sections.
The dolomicrite has crystal size of less than 50 μm in general, mud, small amount of pyrite and gypsum, and horizontal bedding (Fig. 2a). Under cathodoluminescence microscope, the dolomicrite is dark red, with brighter red points in local parts indicative of increase in the Mn concentration[14] (Fig. 2b); whereas gypsum is not luminous. It has a pseudo-breccia structure, average pore development degree, and a small amount of moldic pores. This type of dolomite accounts for 20%-25% of all dolomites in the study area.
The fine-crystalline dolomite is composed of subhedral and euhedral crystals between 50 and 200 μm in point-line contact (Fig. 2c). Its cathodoluminescence was dark red to orange (Fig. 2d). Intercrystalline pores (Fig. 2e), intercrystalline dissolved pores (Fig. 2f) and fractures were observed in the fine-crystalline dolomite, providing ample storage space. The fine-crystalline dolomite is the most common type of dolomite and accounts for 60% to 70% of the dolomites in the study area.
The carbonate grains of dolarenite are mainly algae and a small amount of oolitic dolomite. This kind of dolomite has a grain content range of 60%-80%. The grains come in various sizes, up to 0.5×0.5 mm, and are irregular in shape, mostly rounded to elliptical (Fig. 2g). Their cathodoluminescence is stronger and dark red, while that of the matrix is weaker taupe (Fig. 2h). Very fine-crystalline dolomite was observed between the grains, where dissolved pores may develop (Fig. 2i). This type of dolomite accounts for 10% of all dolomites in the study area.
2.2. Pores and physical properties
The pores in the pre-salt reservoirs include intercrystalline pores, intercrystalline dissolved pores, intergranular dissolved pores, moldic pores, dissolved vugs and fractures.
Intercrystalline pores, occurring in coarse-fine crystalline dolomite, are one of the most common type of pores in the study area. They are in polygon shape (Fig. 2e) with diameters ranging between 5 and 200 μm.
Pores formed by dissolution are another main type of storage space in the study area, including both dissolved vugs in irregular shapes of different sizes able to be seen by naked eyes (Fig. 3a) and intercrystalline dissolved pores (Fig. 2f), intergranular dissolved pores (Fig. 2i) and moldic pores (Fig. 2i) visible only under the microscope. Intercrystalline dissolved pores are possibly formed through further dissolution of intercrystalline pores, hence they have irregular shapes. Intergranular dissolved pores are formed by dissolution of cement or matrix. Moldic pores in the study area are possibly the result of partial or complete dissolution of anhydrite, and these pores are mostly round to ellipse in shape with pore diameters between 20 and 200 μm.
Fig. 2.
Petrologic characteristics of pre-salt dolomite samples from the mid-eastern Ordos Basin. (a) Well J-6, 3 552.50 m, M58 sub- member, dolomicrite with anhydrite patches, ordinary thin section; (b) Well J-6, 3 552.50 m, M58 sub-member, dolomicrite is dark red, anhydrite is not luminous, photo of cathodoluminescence; (c) Well J-12, 3 674.10 m, M59 sub-member, very fine to fine crystalline dolomite, ordinary thin section; (d) Well J-12, 3 674.10 m, M59 sub-member, dolomite is dark red to orange, photo of cathodoluminescence; (e) Well JT-1, 3 633.81 m, M56 sub-member, fine crystalline dolomite with intercrystalline pores, cast thin section; (f) Well T-38, 3 630.10 m, M510 sub-member, intercrystalline dissolved pores are partially filled with coarse dolomite and organic matter, cast thin section; (g) Well J-12, 3 647.32 m, M57 sub-member, dolarenite, cast thin section; (h) Well J-12, 3 647.32 m, M57 sub-member, carbonate grains are dark red, photo of cathodoluminescence; (i) Well T-38, 3 612.00 m, M59 sub-member, intergranular dissolved pores are partially filled with quartz and organic matter, cast thin section.
Fractures are widespread in the study area and are either partially or totally filled with mud or evaporite on the core surface (Fig. 3c). From microscope observation only, it is hard to tell if the fractures are of dissolution or tectonic origin, one thing that the fractures are closely related to dissolution is sure. In addition, dissolved pores are often seen along fractures (Fig. 3d). The fractures are between 10 and 100 μm wide.
Fig. 3.
Pore types and diagenesis of pre-salt dolomite in the mid-eastern Ordos Basin. (a) Well J-12,3 668.45 m, M58 sub-member, dissolution pores on the surface of core; (b) Well J-6, 3 583.10 m, M510 sub-member, gypsum moldic pores, cast thin section; (c) Well J-12, 3 647.45 m, M58 sub-member, dissolution vugs on core surface filled with salt, core photo; (d) Well J-12, 3 647.32 m, M58 sub-member, dissolved pores occurring along the fractures in band shape, cast thin section; (e) Well J-6, 3 556.90 m, M59 sub-member, dolomite formed by recrystallization cut by expending fracture, cast thin section; (f) Well J-2, 3 590.10 m, M57 sub-member, gypsum between grains is dissolved, the fractures and pores are totally or partially filled with organic matter, cast thin section; (g) Well J-6, 3 583.10 m, M57 sub-member, anhydrite is replaced by calcite, with pyrite developing around, ordinary thin section; (h) Well J-6, 3 615.84 m, M510 sub-member, fracture totally filled with anhydrite, cast thin section; (i) Well J-12, 3 642.41 m, M57 sub-member, recrystallization giving rise to big dolomite patches, resulting in the loss of fractures and pores, casting thin section.
Statistics on 53 samples show the reservoirs have a porosity between 2% and 10%, 5.72% on average (Fig. 4a), and a permeability between 0.001×10-3 μm2 and 5.000×10-3 μm2 (Fig. 4b) and mostly below 0.5×10-3 μm2. The results suggest that the pre-salt dolomite reservoirs are low porosity and permeability type. From the rock type, the dolarenite has higher porosity (5.86% on average) and permeability (0.47×10-3 μm2 on average). The fine-crystalline dolomite comes second, with an average porosity of 3.47% and average permeability of 0.31×10-3 μm2.
Fig. 4.
The frequency distribution of porosity (a) and permeability (b) of pre-salt dolomite in the fifth member of Majiagou Formation in Ordos Basin.
3. Diagenetic characteristics
3.1. Diagenesis types
After the deposition of the Majiagou Formation, the study area experienced a series of tectonic activities primarily due to orogeny of the late Caledonian, early Hercynian, Indosinian and Yanshan, so the pre-salt dolomite underwent complex diagenetic processes, including dolomitization, dissolution, filling and recrystallization, pressure solution, cementation, metasomatism and silicification etc.
3.1.1. Dolomitization
The intergranular pores formed by dolomitization comprise part of the reservoir space of the pre-salt dolomite reservoir, and the extensive and large-scale dolomitization can not only generate effective pores, but also contribute to pore preservation[1]. The process of dolomitization in the study area can be divided into three stages: (1) capillary concentration dolomitization in the syngenetic to quasi-syngenetic period; (2) seepage reflux dolomitization in the quasi-syngenetic to shallow burial period; and (3) burial dolomitization during the burial period.
The geochemical analysis results show that the pre-salt dolomite samples have average contents of SiO2 and Al2O3 of 2.09% and 0.46%, respectively. Some of the samples have an SiO2 content of more than 3.00%, which may be related to the presence of terrigenous material from the western paleo-uplift during the depositional stage[36] or the precipitation of authigenic quartz (Fig. 2i). The contents of CaO and MgO can reflect the degree of dolomitization, and the linear correlation between CaO and MgO can be used to tell if the dolomite is sedimentary origin or metasomatic origin[37]. The dolomite in the study area is characterized by high CaO content and low MgO content, which suggests partial dolomitization (Table 1 and Fig. 5). The CaO and MgO contents of the dolomicrite have a positive linear correlation, indicating sedimentary origin; the CaO and MgO contents of fine-crystalline dolomite and dolarenite are in negative linear correlation, indicating metasomatic origin or recrystallization origin. The different types of dolomite all have an isotopic signature of δ13C between -0.57‰ and 1.86‰, and 0.75‰ on average (Table 1 and Fig. 6), equivalent to the δ13C value of global seawater at the same time[38]. The results suggest that the dolomitization fluid was derived from the contemporaneous seawater. The dolomicrite samples have isotopic values of δ18O from -6.18% to -5.05‰, and -5.48‰ on average. The δ18O values of dolarenite samples range between -6.05‰ and -4.12‰, with an average of -5.28‰, equivalent to the δ18O value of the contemporaneous seawater[38] (Fig. 6). The results also suggest that the dolomitization fluid was mainly derived from seawater. The δ18O values of the fine-crystalline dolomite samples range from -7.31‰ to -5.30‰, with an average of -6.11‰. Although most of samples have δ18O values in the range of the values of contemporaneous seawater, some samples have negative δ18O values (less than -7.00‰), which suggest that the dolomitization fluid is closely related to the contemporaneous seawater, however, changes in temperature, salinity and pH value of the later diagenetic fluid may have caused the negative changes of the δ18O of some of the samples. Dolomite formed under high-temperature and pressure subsurface environment often has lower δ18O value. Moreover, the crystalline dolomite has δ18O value of no less than -10.0‰, suggesting that it is not affected by hydrothermal action and has characteristics of buried dolomitization. The 87Sr/86Sr values provide similar evidences. The dolarenite samples have 87Sr/86Sr values between 0.708 323-0.709 602, with an average of 0.709 016; the dolomicrite samples between 0.709 601 - 0.709 844, with an average of 0.709 686; and the crystalline dolomite samples between 0.708 892- 0.710 717, with an average of 0.709 636. The 87Sr/86Sr values of most of the dolarenite and some of the dolomicrite samples are in the range of contemporaneous seawater[38] as shown in Fig. 7, which suggests that the dolomitization fluid is mainly seawater, but the diagenetic fluid might come from or flow through stratum rich in 87Sr. The 87Sr/86Sr values of dolomicrite samples are higher than those of contemporaneous seawater, which could be the result of an increase in the concentration of 87Sr of the seawater possibly due to high evaporation environment[39]; or the result of atmospheric fresh water leading to an increase in the concentration of the strontium isotope[40]. The fine-crystalline dolomite has larger 87Sr/86 Sr value, suggesting that it might be affected by the aluminosilicate fluid from the overburden carboniferous material during burial[41]. Therefore, dolomicrite and dolarenite are the products of penecontemporaneous evaporation dolomitization, whereas the fine-crystalline dolomite is more of a product of seepage-reflux dolomitization and affected by fluid action at the burial stage.
Table 1 Geochemical analysis data of the pre-salt dolomite samples.
| Well | Depth/m | Formation | Lithology | SiO2 content/% | Al2O3 content/% | TFe2O3 content/% | CaO content/% | MgO content/% | MnO content/% | 87Sr/86Sr | δ13C | δ18O |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| J-12 | 3 660.17 | M57 | Dolomicrite | 1.39 | 0.22 | 0.26 | 32.01 | 19.05 | 0.022 | 0.709 647 | 1.2 | -5.17 |
| J-6 | 3 552.50 | M57 | 1.42 | 0.43 | 0.21 | 31.80 | 18.73 | 0.007 | 0.709 601 | 0.47 | -5.55 | |
| J-6 | 3 573.37 | M57 | 2.76 | 0.63 | 0.36 | 31.14 | 18.46 | 0.009 | 0.709 844 | 1.55 | -5.05 | |
| Y-9 | 2 444.30 | M56 | 3.38 | 0.61 | 0.39 | 27.11 | 14.60 | 0.005 | 0.709 651 | 0.76 | -6.18 | |
| J-12 | 3 632.18 | M57 | Crystalline dolomite | 3.67 | 0.81 | 0.30 | 30.40 | 17.88 | 0.008 | 0.710 153 | 0.32 | -5.30 |
| J-12 | 3 653.80 | M57 | 3.24 | 0.71 | 0.37 | 30.74 | 18.02 | 0.012 | 0.710 054 | 1.52 | -6.43 | |
| J-12 | 3 674.10 | M59 | 1.10 | 0.15 | 0.15 | 32.62 | 18.64 | 0.007 | 0.710 045 | 1.20 | -5.96 | |
| J-6 | 3 520.74 | M56 | 0.86 | 0.23 | 0.18 | 32.18 | 19.05 | 0.007 | 0.709 420 | 0.34 | -6.02 | |
| J-6 | 3 576.59 | M57 | 1.29 | 0.28 | 0.22 | 33.00 | 18.04 | 0.007 | 0.708 892 | 1.86 | -5.61 | |
| J-6 | 3 578.40 | M57 | 0.73 | 0.18 | 0.16 | 32.71 | 18.43 | 0.008 | 0.709 237 | 1.56 | -5.66 | |
| J-6 | 3 613.17 | M510 | 1.95 | 0.43 | 0.29 | 32.55 | 17.55 | 0.007 | 0.708 981 | 1.38 | -5.65 | |
| LT-2 | 2 652.70 | M57 | 3.87 | 0.90 | 0.45 | 32.10 | 16.06 | 0.010 | 0.709 786 | -0.08 | -7.05 | |
| YT-1 | 2 530.60 | M57 | 1.59 | 0.30 | 0.30 | 32.04 | 19.14 | 0.010 | 0.710 717 | -0.57 | -7.31 | |
| J-12 | 3 647.32 | M57 | Dolarenite | 0.94 | 0.26 | 0.16 | 29.44 | 17.11 | 0.007 | 0.709 253 | 0.43 | -5.65 |
| J-12 | 3 662.13 | M58 | 2.67 | 0.52 | 0.31 | 31.64 | 17.55 | 0.008 | 0.708 815 | 0.35 | -5.27 | |
| J-12 | 3 672.43 | M58 | 0.71 | 0.16 | 0.12 | 27.65 | 15.36 | 0.006 | 0.708 323 | -0.30 | -5.74 | |
| J-6 | 3 562.17 | M57 | 0.70 | 0.17 | 0.24 | 31.76 | 16.47 | 0.016 | 0.709 602 | 0.17 | -6.05 | |
| J-6 | 3 585.07 | M58 | 0.76 | 0.16 | 0.14 | 29.73 | 16.76 | 0.008 | 0.709 476 | 1.06 | -4.12 | |
| J-6 | 3 622.38 | M510 | 1.19 | 0.21 | 0.21 | 33.86 | 17.21 | 0.009 | 0.708 903 | 0.83 | -4.89 |
Fig. 5.
Curves of MgO value versus CaO value of the pre-salt dolomites.
Fig. 6.
δ13C values and δ18O values of the pre-salt dolomites.
Fig. 7.
87Sr/86Sr ratios of the pre-salt dolomites in the Ordos Basin.
3.1.2. Dissolution
Dissolution is one of the main causes for the development of secondary pores in the pre-salt reservoir. Core observation and microscopic identification of cast thin sections show that the main dissolved pores include intercrystalline dissolved pores (Fig. 2f), intergranular dissolved pores (Fig. 2i), moldic pores (Fig. 3b) and solution fractures (Fig. 3d). The porosity and permeability analysis of Well J-2 show (Fig. 8) that there are some intervals with high porosity and permeability. From microscopic identification of cast thin sections, it is found that the development of high-porosity intervals is closely related to the original grain framework on one hand, and the secondary pores produced by dissolution on the other hand. The dissolved pores are partially filled with organic matter or have organic matter rim along the edges of the pores (Fig. 9).
Fig. 8.
Stratigraphic column of M5 Member in Well J-2.
Fig. 9.
Micrographs of cast thin sections of Ma5 Member dolomites from Well J-2 in Ordos Basin. (a) M57 sub-member, 3 558.10 m, with partially dissolved gypsum between grains, the dissolved pores occur along fractures (red arrow), photo No. ①; (b) M57 sub-member, 3 589.10 m, dissolved anhydrite in the fractue (red arrow), photo No. ②; (c) M57 sub-member, 3 592.01 m, dissolved fracture filled with organic matter (red arrow), photo No. ③; (d) M59 sub-member, 3 608.50 m, intergranular dissolved pores filled with organic matter (red arrow), photo No. ④; (e) M59 sub-member, 3 608.50m, intergranular dissolved pores filled with organic matter (red arrow), photo No. ⑤; (f) M510 sub-member, 3 613.17 m, dissolved fracture, photo No. ⑥.
The geochemical characteristics of the dolomite and characteristics of pores filling suggest that the whole pre-salt dolomite layer series is mainly affected by two stages of dissolution, i.e. Stage 1 and stage 2. Stage 1 is the penecontemporaneous dissolution of atmospheric fresh water caused by transient changes in sea level, and Stage 2 is the dissolution of the dolomite reservoir by multiple fluids during burial. During the sedimentation of the pre-salt dolomite, there were many successive stages of secondary transgression-regressive cycles, when the frequent sea level fluctuation led to temporary exposure of the sediment, and the soluble components were dissolved to create secondary pores. The dissolution vugs on the core surface of the dolomicrite (Fig. 3a) are the result of penecontemporaneous dissolution. The vugs are usually 1-5 cm in size and can reach 10 cm individually. Some of the vugs are filled with evaporite. Therefore, the higher 87Sr/86Sr value in dolomicrite can be understood as the result of atmospheric freshwater dissolution. After the Late Caledonian-Early Hercynian sedimentary stage, the M5 Member dolomite entered the burial stage, and the dissolution processes then mainly consisted of organic acid dissolution and TSR corrosion modification. The organic acids and CO2 produced during the maturation of organic matter were the main sources of acidic fluids in the formation[40]. In Late Indonian-Early Yenshan Stage, Upper Paleozoic coal hydrocarbon source rock entered the peak of hydrocarbon generation[15], the pre-salt dolomite in direct contact with this set of hydrocarbon source rock laterally accepted a large amount of acidic hydrocarbon fluid. The dolomite reservoir was rich in micro-fractures, so during the burial process, the acidic fluid would easily get into the reservoir along the micro-fractures and dissolve the dolomite around the fractures to form enlarged dissolution fractures. Residual organic matter is found filling in some of the dissolved fractures (Fig. 3e). In the intergranular and intercrystalline dissolution pores formed by this kind of dissolution, organic matter filling in pores or organic matter rim along pore edge are commonly seen (Fig. 2f, 2i). The TSR's corrosion modification of the reservoir is reflected in two aspects: (1) During the early reaction process, the hydrocarbons react with evaporite components to form secondary pores (Fig. 3f). (2) The main product of the reaction, H2S, dissolves in water to form strongly acidic hydrosulfuric acid, which has a strong solubility to the dolomite reservoir[6, 43]. Exploratory wells T-38 and JT-1 tested higher H2S content in the pre-salt reservoir, suggesting the pre-salt reservoir is high-sulfur gas reservoir. Microscopic observation reveals that the anhydrite in local part is replaced by secondary calcite, with pyrite developing around the calcite (Fig. 3g), which are typical signs of TSR. The improvement in porosity and permeability of the reservoir in Well J-2 is the result of the corrosion of gypsum in the TSR reaction.
3.1.3. Filling
Filling is the main reason leading to the decrease of reservoir porosity. Common pore fillers include calcite, coarse- grained dolomite, salt, anhydrite and authigenic quartz, besides the organic matter mentioned in the dissolution (Fig. 10), which fill the pores and fissures completely or partially. From the relationship between the characteristics of the fillers and pores, two stages of filling have been identified. Stage 1 happened in the penecontemporaneous stage, when salt precipitant produced by high salinity brine filled the large amount of dissolution pores formed by relative sea level eustacy (Fig. 3a, 3c). Stage 2 took place during the burial stage, and it is further subdivided into three types: Type 1 is the filling by coarse-grained dolomite. Previous studies showed that the coarse-grained dolomite in the Majiagou Formation was related to the reformation caused by hydrothermal fluids[41], and the coarse-grained dolomite edges are surrounded by organic matter, indicating that this type of dolomite was formed before the organic matter (Fig. 2f). Type 2 is the filling when hydrocarbon acidic fluids entered the reservoir: the authigenic quartz associated organic matter in the study area filling pores (Fig. 2i) indicates that the formation of such quartz is closely related to the action of hydrocarbon acidic fluid. Type 3 is the gypsum filling pores caused by backflow of underground brine stored in the eastern salt flats, when flowing through micro-fissures in the reservoir, the brine caused dolomitization, and precipitation of the late stage gypsum (Fig. 3h).
3.1.4. Recrystallization
Recrystallization can lead to the transformation of crystal structure of dolomite. Moderate recrystallization will make the enlarged grains support each other and thus intercrystalline pores enlarge. But strong recrystallization often causes the original pores to disappear or to exist in isolation[14]. The recrystallization in the study area is strong, and has more destruction to the reservoir. The dolomite crystals occur as mosaics or aggregate into large dolomite patches (Fig. 3i), making the pores and fractures formed in the early stages disappear or reduce in size. Dissolution-enlarged micro-fractures filled with organic matter that cut through the dolomite formed by recrystallization (Fig. 3e) were observed, suggesting that recrystallization occurred before the hydrocarbon acid fluid entered the reservoir.
Fig. 10.
The filling frequency of different types of pores in pre-salt Majiagou Formation.
3.2. Diagenetic sequence
Based on petrological characteristics and geochemical characteristics, the diagenetic evolution of the study area is divided into two stages: early and late (Fig. 11).
Fig. 11.
Diagenetic evolution of the pre-salt dolomite reservoir in middle-eastern Ordos Basin.
The early diagenetic evolution occurred in the syngenetic-penecontemporaneous stage, mainly including early dolomitization, atmospheric freshwater dissolution, and filling. In strong evaporation environment and limited seawater conditions of this stage, the concentrated seawater replaced the previously formed carbonate sediments to make the carbonate dolomitize. The dolomite in this stage often comes in dolomicrite and dolarenite. Subsequently, the frequent rise and fall of sea level made the sediment expose to atmospheric fresh water for short periods of time, giving rise to dissolution pores often filled by evaporite formed early.
The late diagenetic evolution happened at the burial stage. In the Late Carboniferous, the study area once again accepted sediment, and the pre-salt strata entered the burial environment. At this time, the diagenetic processes were dominated by later dolomitization and dissolution associated with hydrocarbon components, and recrystallization, filling and compaction and pressure dissolution also began. According to the contact relationships between different fillers from different diageneses and residual organic matter in the pores, it is considered that the destructive diageneses such as recrystallization and filling all took place before the hydrocarbon fluid entered the reservoir, indicating that the entry of hydrocarbon fluid has a protective effect on the reservoir space to a certain extent.
4. Formation of pre-salt dolomite reservoir
Previous studies suggested that the upper assembly of dolomite reservoir of the Ordovician Majiagou Formation in the Ordos Basin was mainly controlled by karst paleogeomorphology[12], and the middle combination of dolomite reservoirs on the eastern side of the Central Paleo uplift was mainly controlled by sedimentary micro-facies and dolomitization[45], while because of little or no paleo-karst, the pre-salt strata of the M5 Member in the central and eastern parts of the basin were paid little attention. Existing studies show that during the depositional stage of M56 to M510, the climate was dry and hot, and the barrier effect of the Central Paleo uplift led to limited water circulation. From the paleo-uplift to the east, the dolomitic flat in platform, shoal, gypsum-bearing dolomitic flat and lagoon facies deposit developed in turn[35]. The shoal micro-facies in the shallow water high energy facies zones could be a favorable reservoir development zone[26, 28]. Through this study, we think that diagenesis has a more powerful control on the reservoir. Dolomitization and dissolution related to hydrocarbon fluid in the burial stage are of great importance for the development of effective reservoir. The formation of reservoir can be roughly divided into four stages, i.e. the penecontemporaneous-shallow burial stage, the Late Caledonian-Early Hercynian tectonic uplifting stage, the Middle Hercynian-Early Yanshanian burial stage, and the stage after Late Yanshanian movement.
Penecontemporaneous-shallow burial stage. Several types of dolomitization laid the foundation for the development of dolomite in the study area. The multi-stages of secondary transgression-regressive cycles caused brief exposures of the sediment and formation of early dissolution pores. The crystalline dolomite formed by seepage-reflux dolomitization is conducive to the development of intercrystalline pores (Fig. 12a), and this process could last to the shallow burial stage.
Late Caledonian-Early Hercynian tectonic uplifting stage. The M51 to M54 sub-member of the Majiagou Formation experienced the reformation by weathering crust karstification. As the pre-salt dolomite reservoir is far from the weathering crust and the evaporite has a barrier effect, this set of dolomite mass didn’t suffer obvious karstification modification in the Late Caledonian-Early Hercynian stage[46] (Fig. 12b).
Middle Hercynian-Early Yanshanian burial stage. The pre-salt strata entered the burial stage. Under the action of overlying strata, the pre-salt layer series increased in temperature and pressure, and strong recrystallization destructed the early pores, and a large amount of magnesium-rich fluid encapsulated in the salt flats in the eastern part of the study area was released, leading to strong burial dolomitization of the dolomite formed in early stage. In the western part of the study area near the paleo-uplift, the dolomite retains the characteristics of the seepage - reflux genesis, with some indicators showing weaker burial genesis[47] (Fig. 12c).
The stage after the Late Yanshanian. In the Ordos Basin, a western-dipping monoclinic tectonic pattern was formed, which promoted the migration of hydrocarbons and associated organic acids formed by carboniferous source rocks to the pre-salt dolomite reservoir. The acidic organic fluids seeping along the reservoir micro-fractures dissolved and enlarged the fractures, so organic matter filling is generally seen in the dissolution fractures. Compared with the dolomite reservoir near the hydrocarbon supply window, the pores formed by organic acid dissolution in the study area feature thinner diameter and small number, and there are no massive intergranular dissolved pores and intergranular dissolved pores, indicating that the organic acid was diluted in the course of migration to the east, resulting in reduced dissolution effect (Fig. 12d). Compared with the Majiagou Formation in other parts of the basin, the TSR effect is a unique dissolution phenomenon in the study area. TSR and its associated hydrosulphuric acid can dissolve the reservoir strongly and continuously to create high-quality reservoir (Fig. 12d). Therefore, the content of hydrosulphuric acid can be used to predict the distribution of favorable reservoirs in future exploration.
Fig. 12.
Origins and foramation stages of pre-salt dolomite reservoirs in the Ordos Basin (Location of the section is shown in
5. Conclusions
The Ordovician pre-salt dolomite reservoirs in the central and eastern Ordos Basin are composed of mainly dolomicrite, fine-crystalline dolomite and dolarenite. The reservoirs have primarily intercrystalline pores, intercrystalline dissolved pores, intergranular dissolved pores, moldic pores, dissolved vugs and fractures as main storage space, and also some intercrystalline pores. They are fracture-pore type reservoirs.
Multi-stages of diagenetic processes control the development of effective reservoirs, among which, dolomitization and dissolution are constructive for reservoir development, and filling and recrystallization are destructive to reservoir.
Dolomitization is one of the key factors for reservoir development, and capillary concentration dolomitization in the syngenetic to quasi-syngenetic stage, seepage-reflux dolomitization in penecontemporaneous - shallow burial stage and late burial dolomitization play important roles in the formation of secondary pores and the preservation of primary pores.
The multi-stages of dissolution have an important influence on the development of secondary pores. The multi-stages of secondary transgression - regressive cycles led to the dissolution of the soluble components in the early sediments and thus the formation of secondary pores. The organic acid dissolution in the burial stage further improved the reservoir space. Due to the presence of evaporites, the unique TSR and hydrosulphuric acid generated by its product H2S in the study area may become an important basis for the next step exploration.
Reference
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Chile is one of several countries that recently implemented a fiscal policy to reduce soft drink (SD) intake and obesity. In 2014 the government increased the existing ad-valorem tax on high-sugar SD by 5% and decreased by 3% the tax on low-sugar SD, based on a 6.25gr/100 ml sugar threshold. This study aims to evaluate the tax modification passed-on to consumers through prices, and to calculate changes in affordability of SDs. We analysed nationally representative consumer price index data of 41 soft drinks within 6 beverage categories between 2009 and 2016. Price change post-tax implementation was estimated for different categories (carbonates, juices, concentrates, waters and energy-sport drinks), using time-series analyses. In addition, changes in affordability were evaluated by estimating the changes in prices relative to wages. The price of carbonates increased by 5.60% (CI 95% 3.18-8.03%) immediately after the tax was implemented. A sustained increase in the prices of concentrates was observed after the implementation. Unexpectedly, a smaller increase was also seen for the price of bottled water - a category that saw no tax change. There were no effects for juices and energy-sports drinks. There was a reduction in affordability for carbonates, concentrates and waters. Overall, the fiscal policy was effective in increasing prices and there are some signs of reduced affordability. Results varied substantially among categories directly affected by the tax policy. While for carbonates the price increase exceeded the tax change ('over-shifting'), in other categories subject to a tax cut, a price reduction was expected but the opposite occurred. As the effect of the tax on prices differed between categories, the effects of the tax policy on consumption patterns are likely to be mixed. Our findings underline the need to better understand and anticipate price setting behaviour of firms in response to a tax.
Origin of Triassic Leikoupo Dolostone Reservoirs in Sichuan Basin
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Vertical zonation of weathered crust ancient karst and the reservoir evaluation and perdiction: A case study of M5 5-M5 1 sub-menbers of Majiagou Formation in gas fields, central Ordos Basin, NW China
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Depositional facies, diagenetic terranes, and porosity development in lower Ordovician Ellenburger Dolomite, Puckett Field, West Texas
A geochemical model for the formation of the pre-salt reservoirs, Santos Basin, Brazil: Implications for understanding reservoir distribution
Characteristics and forming mechanisms of evaporate platform dolomite reservoir in Middle Cambrian of Xiaoerbulake section, Tarim Basin
There was a high hydrocarbon potential in the evaporite platform dolomite of Middle Cambrian in Tarim Basin.Based on the outcrop observation and the comprehensive analyzes of common thin section,scanning electron microscope(SEM),cathodeluminescence(CL) technique and the geochemical data,we carried out the research on the characteristics of a dolomite reservoir which had deposited in the section of X in Tarim Basin during the Middle Cambrian.The results show that the Middle Cambrian rocks in the study area are of various types,and the reservoir′s space mainly includes the dissolved pores and the collapsed caverns (the collapsed shadowing hole).We also studied the reservoir′s forming mechanism through the assay of accompanying minerals and the analyses of the pore characteristics and geochemical characteristics.It indicates that the mechanisms in the study area include the atmospheric water denudation in syngenetic stage,the collapsing rock′s stacking interaction in middle burial stage and the hydrothermal dissolution in deep burial stage.The former two mechanisms were controlled by the fluctuation of sea level,so they distributed widely.However,the distribution of the third mechanism is quite limited,because it was vertically accompanied by the faults and horizontally distributed along the formation,and it may alter the early generated dissolved pores.
Sedimentary environment and sea level change of the subsalt interval of Member 5 of Ordovician Majiagou Formation in central Ordos Basin
Natural gas exploration and development in the PetroChina Changqing and its prospect in the 13th Five-Year Plan
Discovery of the natural gas in lower Palacozoci in Shanganning Basin
This paper bricfly describes the exploration history and synthetical exploration method of the natural gas in Lower Palaeozoic in Shanganning Basin,and demonstrats the basic geological conditions of accumulating natural gas in Lower Palaeozoic and the main exploration direction combining with the structure evolution,sedimentary charactcristics and pool-formed conditions of this basin.
Reservoir-forming by later supply of hydrocarbon: A new understanding of the formation of Ordovician gas reservoirs under gypsolyte in the Ordos Basin
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There are thick gypsumsalt strata developed in the Lower Paleozoic Ordovician in the Ordos Basin. Especially in the 6th segment of the 5th member of the Majiagou Fm (OEm56), the gypsumsalt is most widely distributed, providing good regional capping conditions. Recent drilling and relevant studies have proven that the hydrocarbongenerating capacity of the marinefacies hydrocarbon source rocks under the Ordovician salt layers in the eastern Basin is poor on the whole. The "selfgenerating and selfpreserving type" commercial gas reservoirs are difficult to form under the salt layers of the Mizhi salt depression center. However, the further research of reservoirforming conditions under the gypsolyte on the west side of the gypsalt depression indicates that the OEm57 - OEm510 strata near the east side region of the palaeouplift contact directly with the Upper Paleozoic strata which contain the hydrocarbon source rocks of coal measures strata, forming a "hydrocarbonsupply window"; the Yanshan Movement caused the tectonic inversion of the basin proper. The “easthigh while westlow” tectonic framework is conductive to the further migration of natural gas that was generated by the Upper Paleozoic coal measure HCsource rocks to the updip high position of the east side along the OEm57- OEm510 carrier beds, after entering the dolomite reservoir under the gypsolyte through the "hydrocarbonsupply window". In addition, the presence of the facies transitions in the dolomite rocks under the gypsum salt provides a favorable barrier condition for the regional gathering of natural gas. Therefore, it is considered by a comprehensive analysis that beneath the Ordovician gypsolith in the central region of the Basin on the west side of the gypsalt depression has a potential of forming a reservoir by lateral hydrocarbon accumulation derived from the Upper Paleozoic source rocks of coal measures, which is expected to open a new situation of natural gas exploration under the Ordovician gypsum salt layers in the Ordos basin.
Exploration of Ordovician subsalt natural gas reservoirs in Ordos Basin
Distribution and exploration potential of Ordovician reservoir in Ordos Basin
DOI:10.1016/j.foodchem.2019.125957
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PMID:31864191
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The main aim of this work was to characterize the volatile profile of virgin pistachio oils produced from eight cultivars (Aegina, Avdat, Kastel, Kerman, Larnaka, Mateur, Napoletana and Sirora), under different technological conditions (temperature, roasting, use of whole nuts, screw speed and nozzle diameter), and compare it with those of commercial pistachio oils. Terpenes (15.57-41.05 mg/kg), accounting for ~97% of total volatiles, were associated with appreciated sensory properties, with α-pinene as the main volatile (14.47-37.09 mg/kg). Other terpene compounds such as limonene (0.11-3.58 mg/kg), terpinolene (0.00-1.61 mg/kg), β-pinene (0.12-1.20 mg/kg) and α-terpineol (0.00-1.17 mg/kg) were quantified at lower concentrations. Acids, alcohols, aldehydes, esters and hydrocarbons only summed to ~3% of the total volatile compounds. The volatiles content greatly depended on the pistachio cultivar employed. The influence of extraction conditions was also very relevant; in particular, terpenes doubled (28.38-53.84 mg/kg) using whole pistachios for oil extraction, also being incremented by mild processing conditions. On the contrary, higher temperature or roasting decreased the terpene content (~50-25% respectively), and pyrazines appeared (up to 3.12 mg/kg).
Main factors controlling formation of dolomite reservoir underlying gypsum-salt layer in the 5th member of Ordovician Majiagou Formation, Ordos Basin
DOI:10.1016/j.foodchem.2019.125957
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PMID:31864191
[Cited within: 3]
The main aim of this work was to characterize the volatile profile of virgin pistachio oils produced from eight cultivars (Aegina, Avdat, Kastel, Kerman, Larnaka, Mateur, Napoletana and Sirora), under different technological conditions (temperature, roasting, use of whole nuts, screw speed and nozzle diameter), and compare it with those of commercial pistachio oils. Terpenes (15.57-41.05 mg/kg), accounting for ~97% of total volatiles, were associated with appreciated sensory properties, with α-pinene as the main volatile (14.47-37.09 mg/kg). Other terpene compounds such as limonene (0.11-3.58 mg/kg), terpinolene (0.00-1.61 mg/kg), β-pinene (0.12-1.20 mg/kg) and α-terpineol (0.00-1.17 mg/kg) were quantified at lower concentrations. Acids, alcohols, aldehydes, esters and hydrocarbons only summed to ~3% of the total volatile compounds. The volatiles content greatly depended on the pistachio cultivar employed. The influence of extraction conditions was also very relevant; in particular, terpenes doubled (28.38-53.84 mg/kg) using whole pistachios for oil extraction, also being incremented by mild processing conditions. On the contrary, higher temperature or roasting decreased the terpene content (~50-25% respectively), and pyrazines appeared (up to 3.12 mg/kg).
Geochemical characteristics of Ordovician subsalt gas reservoir and their significance for reunder standing the gas source of Jingbian Gasfield, east Ordos Basin
Low yield gas was discovered in the subsalt Mawu 7 reservoir at Longtan 1 well in the east of Ordos basin. As revealed by the test data, the gas contains 96.871% of methane, only 2.396% of C2+ heavy hydrocarbon characteristic of dry gas of high maturity, and very low contents of non-hydrocarbon components, indicating that the gas is a normal type in a typical reservoir. The carbon isotope of methane is light type, with δ13C1 being -39.26‰, indicating that the gas is of primitive oilfield type at overmature stage in the Ordovician. The hydrocarbon\|specific carbon isotope of C2+ heavy hydrocarbon components is obviously heavy type, withδ13C1varying between -19.27‰ and -23.78‰, suggesting that the gas is similar to coal-generated gas of high maturity. Based on the spatial distribution of the gas reservoir, coal-generated gas in upper Paleozoic is less likely to be involved in the formation of the salt gas, as a result, secondary reactions like TSR reaction could be the dominating affecting element for formation of the salt gas. When Mawu 7 gas from Longtan 1 well is taken as an end member of oilfield type gas and the carbon isotope of methane as a major index, the major source for the Ordovician gas in the Jingbian gasfield can be identified, and a conclusion can be drawn that the major source for natural gas in the Ordovician weathering crust in Jingbian gasfield is the Upper Paleozoic coal measure.
Sedimentary model of marine evaporites and implications for potash deposits exploration in China
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Chile is one of several countries that recently implemented a fiscal policy to reduce soft drink (SD) intake and obesity. In 2014 the government increased the existing ad-valorem tax on high-sugar SD by 5% and decreased by 3% the tax on low-sugar SD, based on a 6.25gr/100 ml sugar threshold. This study aims to evaluate the tax modification passed-on to consumers through prices, and to calculate changes in affordability of SDs. We analysed nationally representative consumer price index data of 41 soft drinks within 6 beverage categories between 2009 and 2016. Price change post-tax implementation was estimated for different categories (carbonates, juices, concentrates, waters and energy-sport drinks), using time-series analyses. In addition, changes in affordability were evaluated by estimating the changes in prices relative to wages. The price of carbonates increased by 5.60% (CI 95% 3.18-8.03%) immediately after the tax was implemented. A sustained increase in the prices of concentrates was observed after the implementation. Unexpectedly, a smaller increase was also seen for the price of bottled water - a category that saw no tax change. There were no effects for juices and energy-sports drinks. There was a reduction in affordability for carbonates, concentrates and waters. Overall, the fiscal policy was effective in increasing prices and there are some signs of reduced affordability. Results varied substantially among categories directly affected by the tax policy. While for carbonates the price increase exceeded the tax change ('over-shifting'), in other categories subject to a tax cut, a price reduction was expected but the opposite occurred. As the effect of the tax on prices differed between categories, the effects of the tax policy on consumption patterns are likely to be mixed. Our findings underline the need to better understand and anticipate price setting behaviour of firms in response to a tax.
Evolution between North China sea and Qilian sea of the Ordovician and the characteristics of lithofacies palaeogeography in Ordos Basin
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To assess validity of a low-intensity measure of fitness (X) in a population of older adults as a proxy measure for the original, high-intensity measure (Y), we used ordinary least square regression with the new, potential proxy measure (X) as the sole explanatory variable for Y. A perfect proxy measure would be unbiased (i.e., result in a regression line with a y-intercept of zero and a slope of one) with no error (variance equal to zero). We evaluated the properties of potential biases of proxy measures. A two degree-of-freedom approach using a contrast matrix in the setting of simple linear ordinary least squares regression was compared to a one degree-of-freedom paired t test alternative approach. We found that substantial improvements in power could be gained through use of the two degree-of-freedom approach in many settings, while scenarios where no linear bias was present there could be modest gains from the paired t test approach. In general, the advantages of the two degree-of-freedom approach outweighed the benefits of the one degree-of-freedom approach. Using the two degree-of-freedom approach, we assessed the data from our motivating example and found that the low-intensity fitness measure was biased, and thus was not a good proxy for the original, high-intensity measure of fitness in older adults.
Palaeotectonics of Ordovician in Ordos
Types and characteristics of carbonate platform margins of marine carbonate rock in three major basins in western China
Based on classic carbonate depositional mode studies and research by Chinese scholars on the characteristics of the carbonate platform margin, previous research into the characteristics of the carbonate platform margin is summarized according to the particularity of carbonate rocks of the three marine basins in western China, taking available classic mode into consideration. In light of the developmental characteristics of carbonate rocks and the actual geological situation of three basins in western China, five kinds of carbonate depositional modes and 12 subclass types are systematically established. Based on the carbonate sedimentary features developed in the three basins at different periods, this paper gives a detailed analysis on the characteristics of the various types of carbonate platform margins and their depositional modes in terms of tectonic evolution, Ethological characteristics, sedimentary facies changes, and sedimentary mode development. The different types of carbonate platform margins and their evolutionary characteristics of depositional patterns are of critical significance in guiding the analysis of time-space matching relationship of elements such as hydrocarbon generation, reservation, and sealing.
Basin evolution and sediments accumulation during Eopaleozoic in Ordos continental block
Natural gas exploration domains in Ordos Basin
Study of Ordovician weathered crust karst reservoir in Ordos Basin
Characteristic and evolution of sedimentary microfacies in the middle Ordovician assemblages in the Ordos Basin
Geochemical characteristics and origin of dolomite: A case study from the middle assemblage of Majiagou Formation Member 5 of the west of Jingbian Gas Field, Ordos Basin, North China
Multi-parameter comprehensive identification of the genesis of Lower Paleozoic dolomite in Tarim Basin, China
87Sr/ 86Sr, δ 13C and δ 18O evolution of Phanerozoic seawater
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Fault- controlled dolomitization at Swan Hills Simonette oil field (Devonian), deep basin west-central Alberta, Canada
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Origin of dolomite in the Middle Devonian Guanwushan Formation of the western Sichuan Basin, western China. Palaeogeography, Palaeoclimatology,
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Geochemistry and dolomitization mechanism of Majiagou dolomites in Ordovician, Ordos, China
The types and genetic mechanism of the Majiagou dolomite in Ordos basin are investigated using petrological analysis and geochemical methods. The results show that dolostones in Majigou Formation mainly consist of dolomicrite and crystalline dolostone. Dolomicrite often associates with gypsum or halite, with medium jacinth light in cathoduluinescence observation, lower degree of order, and high content of Sr and Na. It provides similar delta(13)C and REE distribution pattern with micrite. The value of Sr isotope is close to that of sea water in the same period, and fluid-inclusion homogenization tempetrature approximate to earth's surface. These suggest that dolomitization fluid is high salinity fluid that closely related with sea water, considered as penecontemporaneous dolomitization which can be explained by evaporation pump mode. Crystalline dolostone contains little associated minerals, only pyrite observed occasionally. This crystal structure is usually xenomorphic curve. Crystalline dolostone shows medium-weak jacinth light in cathoduluinescence, whith high degree of order and low content of Sr, while Na and Mn content is close to that of dolomicrite. REE distribution pattern is similar with dolomicrite and close to micrite. The distribution of delta(13)C is also corresponding to micrite, and fluid-inclusion homogenization tempetrature is bight. These indicate that dolomitization fluid of crystalline dolostone is similar with that of dolomicrite. Crystalline dolostone is buried dolomitisation, where continuous compaction from overlying deposits urges the sealed brine to flow laterally, thus supplies unfailing Mg(2+) for dolomitisation.
Bitumen and hydrocarbon generation potential of paleo-reservoirs in the Ordovician, south Ordos Basin
DOI:10.1016/S1876-3804(11)60003-7
URL
Abstract
Bitumen is distributed extensively but not even in the Ordovician, south Ordos Basin. Core observation and slice identification indicate that it is distributed horizontally in the Yongcan 1-Xuntan 1-Chuntan 1-Yaocan 1 wellblock and concentrated vertically in the Majiagou Formation Ma6 Member top-Pingliang Formation bottom and the Ma3 Member top-Ma4 Member bottom. It exists largely in fractures, intercrystalline pores, and solution pores or stylolites. Optical characteristics and bituminous reflectivity reveal that bitumen charging has two phases in this area. According to the solid bitumen amount, abundance degree and thermal simulation experiment, paleo-reservoirs exist in the studied area. They are divided into three types, Ma6 Member pore filling, Ma3-Ma4 Member pore filling and Liangjiashan Formantion (?). Their source rock was developed in a deepwater slope depositional environment of the middle Ordovician period. The hydrocarbon potential was evaluated by solid bitumen content and geological features. The amount of cracking gas is probably 3.6×1012−7.7×1012 m3, providing sufficient gas source for hydrocarbon accumulation in the Ordovician, south Ordos Basin.
摘要
： 鄂尔多斯盆地南部地区奥陶系固体储集层沥青分布较为普遍但不均匀。岩心观察和镜下薄片鉴定表明，奥陶系固体沥青平面上主要分布在永参1井—旬探1井—淳探1井—耀参1井区，剖面上主要集中在马家沟组马六段顶—平凉组底部和马三段顶—马四段底部；其赋存形式常见有3种：赋存于裂缝、晶间孔和溶蚀孔隙/缝合线中。结合光性特征和沥青反射率分析，确定该区发育两期充注的沥青。固体沥青的数量、富集程度和热模拟实验资料证实了研究区古油藏的存在，其可分为3类：马六段孔隙充填型、马三段—马四段孔隙充填型和亮甲山组古油藏(?)。沉积环境分析表明古油藏烃源岩（平凉组泥页岩）发育于中奥陶世的深水斜坡沉积环境。依据固体沥青含量和古油藏的地质特征评价古油藏生烃潜力，总裂解生气量约为3.6×1012～7.7×1012 m3，为南部地区奥陶系成藏提供了充足的气源。图9表2参13
Gas souring by thermochemical sulfate reduction at 140 °C
Genesis and source of the Ordovician mid-assemblage natural gas in the east side of the central paleo-uplift, Ordos Basin
Characteristics and exploration inspiration of the Ordovician assemblage Formation, Ordos Basin
DOI:10.3787/j.issn.1000-0976.2011.12.003
URL
[Cited within: 1]
The Lower Paleozoic carbonates are important targets of gas exploration in the Ordos Basin. The Jingbian Gas Field discovered in the Lower Paleozoic in this central basin in the end of last century is an ancient weathering crust gas reservoir developed on the top Ordovician, and has become one of the major gas fields in this basin up till now. Through a deep investigation into the hydrocarbon pooling mechanism of carbonate reservoirs and continuous exploration practices, the PetroChina Changqing Oilfield Company discovered lithologic carbonate gas reservoirs in the middle Ordovician assemblages in the eastern part of the central paleohighs. These lithologic carbonate gas reservoirs are significantly different from the ancient weathering crust gas reservoirs in the Jingbian area in terms of reservoir development features, trapping mechanism, and gas distribution patterns. Their reservoir spaces are dominated by intercrystalline pores of dolomite, the development of which is controlled by sedimentary microfacies of grain bank and the specific conditions of dolomitization in the eastern paleohighs. Lithologic traps are predominant and their development is controlled by regional lithological change resulted from shortterm transgression and uplifting tectonic setting in the eastern part of the basin during the Yanshan Epoch. The gas reservoirs are in regional distribution around the central paleohighs, forming a play fairway consisting of lithologic traps superimposing vertically and in zoned distribution laterally. Deepening our understanding of hydrocarbon accumulation in the middle Ordovician assemblages is of great significance for future exploration in the Ordos Basin.
Ordovician deep dolomite reservoirs in the intracratonic Ordos Basin, China: Depositional model and Diagenetic evolution
DOI:10.1016/j.envres.2019.109018
URL
PMID:31863943
[Cited within: 1]
Early nutritional management including fortified human breastmilk is currently recommended to fulfil the energy demands and counterbalance risks associated to preterm birth. However, little is known about the potential adverse effects of exposure to persistent organic pollutants (POPs) carried in human milk on preterm infant growth. We conducted a pilot study proving the application of an integrative analytical approach based on mass spectrometry (MS) coupled to advanced statistical models, favouring the comprehensive molecular profiling to support the identification of multiple biomarkers. We applied this workflow in the frame of a preterm infants' cohort to explore environmental determinants of growth. The combination of high resolution gas and liquid chromatography MS platforms generated a large molecular profile, including 102 pollutants and nutrients (targeted analysis) and 784 metabolites (non-targeted analysis). Data analysis consisted in a preliminary examination of associations between the signatures of POPs and the normalised growth of preterm infants, using multivariate linear regression adjusting for known confounding variables. A second analysis aimed to identify multidimensional biomarkers using a multiblock algorithm allowing the integration of multiple datasets in the growth model of preterm infants. The preliminary results did not suggest an impairment of preterm growth associated to the milk concentrations of POPs. The multiblock approach however revealed complex interrelated molecular networks of POPs, lipids, metabolites and amino acids in breastmilk associated to preterm infant growth, supporting the high potential of biomarkers exploration of this proposed workflow. Whereas the present study intended to identify simultaneously pollutant and nutrient exposure profiles associated to early preterm infant growth, this workflow may be easily adapted and applied to other matrices (e.g. serum) and research settings, favouring the functional exploration of environmental determinants of complex and multifactorial diseases.
Geological conditions for hydrocarbon accumulation in middle reservoir-source rock combination of the Ordovician Majiagou Formation on the east side of the paleo-uplift in Ordos Basin
DOI:10.1016/j.foodchem.2019.125957
URL
PMID:31864191
[Cited within: 1]
The main aim of this work was to characterize the volatile profile of virgin pistachio oils produced from eight cultivars (Aegina, Avdat, Kastel, Kerman, Larnaka, Mateur, Napoletana and Sirora), under different technological conditions (temperature, roasting, use of whole nuts, screw speed and nozzle diameter), and compare it with those of commercial pistachio oils. Terpenes (15.57-41.05 mg/kg), accounting for ~97% of total volatiles, were associated with appreciated sensory properties, with α-pinene as the main volatile (14.47-37.09 mg/kg). Other terpene compounds such as limonene (0.11-3.58 mg/kg), terpinolene (0.00-1.61 mg/kg), β-pinene (0.12-1.20 mg/kg) and α-terpineol (0.00-1.17 mg/kg) were quantified at lower concentrations. Acids, alcohols, aldehydes, esters and hydrocarbons only summed to ~3% of the total volatile compounds. The volatiles content greatly depended on the pistachio cultivar employed. The influence of extraction conditions was also very relevant; in particular, terpenes doubled (28.38-53.84 mg/kg) using whole pistachios for oil extraction, also being incremented by mild processing conditions. On the contrary, higher temperature or roasting decreased the terpene content (~50-25% respectively), and pyrazines appeared (up to 3.12 mg/kg).
