Introduction
The hydrocarbon types of in-source oil and gas system of the Lower Permian Fengcheng Formation in the Mahu Sag (P1f), Junggar Basin are mainly tight oil, which is stored in fine dolomitic rocks. For example, high-yield industrial oil flow has been found in thick fine-grained dolomitic rocks with large intervals of hydrocarbon show in Mahu 28, Mahu 33 and other drilling wells. In particular, the accumulated thickness of dolomitic rocks with oil and gas show in Well Maye 1 in the north of the Mahu Sag approximates 235 m, and the daily oil production is 23.33 t [1]. The above exploration results show that the dolomitic rocks are the most important effective tight oil reservoir in the Mahu Sag [2]. Unclear formation mechanism of dolomitic reservoirs in the Permian Fengcheng Formation of the Mahu Sag restricts effective prediction and further exploration of such reservoirs [3]. One of the key constraints is the unclear origin of dolomites. Therefore, the study on the origin of dolomites in the dolomitic rocks of the Fengcheng Formation in the Mahu Sag is helpful to clarify the formation mechanisms of tight oil reservoirs [3], and is of great significance to guide the exploration and development of tight oil.
According to the differences in chemical compositions of lake water, continental salt lakes (with salinity greater than 3.5%) can be divided into three types: carbonate type (alkali lake), sulfate type and chloride type [4]. Continental dolomites (or dolostones) are mainly developed in saline-lake environment [5]. A large number of halophilic bacteria and algae are developed in the salt lake [4], so many scholars use the microbial dolomitization model to explain origin of salt lake dolomites [6-7]. Based on the roles of microorganisms in the formation of dolomite, some scholars regarded it as a mediate rather than an independent dolomitization model [4,8 -9]. Shale (a typical representative of fine-grained sedimentary rock) could keep porosity more than 30% at a burial depth of 1000-1500 m [10], and the brine rich in Mg2+ formed by evaporation flowing back along the lake slope by gravity result in dolomitization of underlying sediments. Thus, formation of shallow burial lacustrine dolomite can be explained by the seepage-reflux dolomitization model [11]. In the study of the origin of lacustrine dolomites, the penecontemporaneous dolomitization is often considered as a major mechanism[5,7,11⇓ -13]. The burial dolomitization is usually regarded as a process of strengthening or adjusting to early dolomitization [8]. The Fengcheng Formation in the Mahu Sag is a typical continental alkaline lake deposit [3]. Alkaline minerals (including shortite, wegscheiderite, etc.) are relatively well developed, and dolomitization is widespread, which produces dolomitic rocks rich in dolomites. At present, there are three different viewpoints on the major origins of dolomites in the Permian Fengcheng Formation in the Mahu Sag: (1) penecontemporaneous dolomitization and burial dolomitization [14-15]; (2) dolomites mainly originated from seepage-reflux dolomitization and modified by further burial dolomitization [2,16]; (3) dolomites mainly related to volcanic hydrothermal solution [17-18]. The above three viewpoints are mainly based on the common saline lake (sulfate and chloride types) and marine dolomite formation model to explain the origin of dolomite in the Fengcheng Formation of the Mahu Sag, without considering the particularity of dolomites formation in alkaline lake sedimentary-diagenetic environment.
In general, the pH value of the alkaline lake is 9-11, the total salinity is 100-350 g/L, and the main cations are K+, Na+, Ca2+and Mg2+ [4]. The special fluid geochemical conditions make the alkaline lake sedimentary rocks have different diagenetic evolution characteristics from the common salt lakes [19], and the formation of alkaline lake deposits in the Fengcheng Formation of the Mahu Sag is closely related to surrounding volcanic rocks and synsedimentary volcanic activities [20]. Thus, the formation models of common saline lake dolomites are difficult to directly explain the origin of alkaline lake dolomites. Geochemical analysis methods such as isotopes, trace and rare earth elements have improved the interpretation accuracy of dolomite genesis. However, if there are no effective constraints in the analysis of dolomite genesis, the interpretation results will be ambiguous. Therefore, based on drilling cores and thin sections, petromineralogy and diagenetic characteristics of dolomitic rocks were analyzed, and combined with scanning electron microscopy, isotopes, fluid inclusions, X-ray diffraction, elements, electron microprobe, energy spectrum, the origin of authigenic dolomites in dolomitic reservoirs of the Lower Permian Fengcheng Formation in the Mahu Sag of the Junggar Basin were further studied, in order to provide theoretical support for exploration of high-quality dolomitic reservoirs.
1. Geological setting
The Mahu Sag locates in the northwest margin of the Junggar Basin, NW China (Fig. 1 a), with an area of about 6800 km2, which is an important hydrocarbon-rich sag. Since exploration beginning in the 1980s, several areas with reserves exceeding hundred million tons have been found, and it is the main area for reserve and production increment in the Junggar Basin [21]. During the sedimentary period of the Fengcheng Formation in the Early Permian, the Mahu Sag was a closed lake with alkaline lake sediments widely developed under an arid-semi-arid climate in general [22], in which dolomitic rocks (referring to the rocks with dolomite content more than 10% [2,14]) were widely distributed. The alkaline lake environment was kept until the late period of the Xiazijie Formation of the Middle Permian [23], and the Mahu Sag basically disappeared in the late Triassic [2]. The thickness of the Lower Permian Fengcheng Formation in the study area is 200-1400 m, and the cumulative thickness of dolomitic rocks is 100-300 m. The Fengcheng Formation is divided into 3 members from bottom to top (Fig. 1 b): (1) The first member (P1f1), 30-370 m thick. The lower and middle parts are mainly comprised of volcanic rocks and tuffs, and the upper part is mainly comprised of mudstones rich in organic matters and dolomitic rocks interbedded with salt rocks, with the accumulated thickness of dolomitic rocks as 10-235 m. (2) The second member (P1f2), 30-650 m thick, is mainly composed of dolomitic tuffs and dolomitic mudstones, and interbeds with several sets of salt rocks, with the accumulated thickness of dolomitic rocks as 5-236 m. (3) The third member (P1f3), 30-560 m. The lower and middle parts are dominated by dolomitic mudstones, and the upper part is dominated by dolomitic siltstones and mudstones, with the accumulated thickness of dolomitic rocks as 10-128 m. Dolomitic rocks are mainly distributed in the upper part of P1f1 and P1f2, and in the lower part of P1f3 [14]. The Fengcheng Formation is the main source rock layer and producing pay of the Mahu Sag [2,21].
Fig. 1. Structural location map of the Mahu Sag (a) and lithologic column of the Fengcheng Formation (b). |
2. Samples and methods
According to the oil and gas show of dolomitic rocks in several drilling wells, 164 thin sections were prepared from 14 wells in the north, middle and south of the Mahu Sag respectively. The optical microscope (Leica DM2500) equipped with fluorescence emission device was used for lithology and mineral identification, and analysis of diagenesis and hydrocarbon charging characteristics. In order to accurately determine the sequence of various diagenetic phenomena, 30 samples were selected for scanning electron microscopy (SEM) using scanning electron microscope CARL ZEISS EVO MA15/LS15, and the minerals that can not be identified under optical microscope were analyzed by energy dispersive spectroscopy (EDS), using electric refrigeration spectrometer TEAMTM XLT EDS. In view of the different occurrence modes and formation stages of dolomites in the Fengcheng Formation, as well as the difficulties to identify some small dolomite crystals, and 29 thin slices were selected for cathodoluminescence (CL) analysis to observe the luminescence characteristics.
Due to small size of dolomitic rocks and salt minerals with high dissolubility in the process of grinding thin slices, combining the microscopic section identification results, 48 dolomite samples with different occurrence states were selected for mineral composition analysis (X-ray diffraction) [24] using X-ray diffraction analyzer D/max-2500pc. Based on the above results, 27 samples were selected from 6 wells like MY1, K207 and FN052 for major and trace elements detection [25-26] using inductively coupled plasma mass spectrometer NexION 350X and X-ray fluorescence spectrometer (XRF) PW2404.
Eighteen samples from 5 wells like MY1, K207 and FN14 were selected to determine the order degrees of dolomites using diffractometer D/MAX-ⅢC. In order to obtain the formation temperatures and fluid characteristics during the formation period of authigenic dolomites, 8 multi-purpose slices (fluid inclusion + micro carbon and oxygen isotopes) from 6 wells such as MY1, MH28 and K207 were made. First, the homogenization and ice point temperatures were measured by cold and hot stage Linkam THMS-600 [27], and then the same dolomite crystal sample was taken by laser sampler for mass spectrometer analysis [28]. At the same time, other 13 samples from 7 wells like MY1, MY2 and FN14 with single compositions of carbonate minerals, were selected for whole rock carbon and oxygen isotopic composition analysis.
Strontium isotope is a good chronological indicator. To better constrain the diagenetic fluid information reflected by carbon and oxygen isotopes, major and trace elements, 10 dolomitic rock samples with single carbonate mineral compositions were selected for micro drilling sampling, and 87Sr/86Sr values were measured by thermal ionization isotope mass spectrometer Triton Plus[29]. In order to obtain the fluid chemical characteristics in the precipitation process of these dolomites, 8 samples were selected for electron microprobe analysis at a total of 30 points. Dolomites with zoned growth structures were continuously sampled and analyzed from the center to edge of the same dolomite crystal, and 9 micro-area chemical components, such as Al2O3, MgO and FeO were determined.
Major and trace elements, in-situ micro carbon and oxygen isotopes, fluid inclusion temperatures, and scanning electron microscope (energy spectrum) experimental analysis were completed at CNOOC Experimental Center. Whole rock carbon and oxygen isotopes test was completed by the Key Laboratory of Natural Gas Accumulation and Development of CNPC. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation of Chengdu University of Technology completed thin section preparation and observation, whole rock mineral X-ray diffraction, dolomite order degree, strontium isotopes, cathodoluminescence and electron probe analysis.
3. Petrological characteristics of dolomitic rocks and microscopic characteristics of dolomites
3.1. Petrological characteristics of dolomitic rocks
The lithology of the Fengcheng Formation in the Mahu Sag is complex, mainly including terrigenous clastic rocks, volcanic rocks, pyroclastic rocks and dolomitic rocks, and dolomitic rock is the most developed one. The dolomitic rocks mainly consist of dolomitic pyroclastic rocks in which the dolomitic tuffs account for the highest proportion, dolomitic sandstones (or dolomitic conglomerates), dolomitic molten lavas and dolomitic mudstone-siltstones. The dolomitic rocks are mainly composed of quartz, K-feldspar, plagioclase, dolomite, salt minerals and volcanic glass. The authigenic minerals are mainly dolomites, calcites and other salt minerals (including shortite, wegscheiderite, trona, nahcolite, reedmergnerite, etc.) are also widely distributed. Silicoides (such as opal and quartz) and aluminosilicates (such as authigenic feldspar and zeolite) being produced by devitrification and dissolution are also common.
3.2. Microscopic characteristics of authigenic dolomites
Subhedral and subhedral-euhedral dolomites are most abundant in the dolomitic rocks of the Fengcheng Formation in the Mahu Sag (Fig. 2 ). For the convenience of comparative analysis, transitional dolomite crystals such as anhedral-subhedral, subhedral-euhedral are classified as subhedral, so dolomite crystals are divided into three types: euhedral, subhedral and anhedral. Dolomites are mainly microcrystalline, silt-crystalline and fine-crystalline, among which silt-sized crystals have the highest content. Dolomite contents variy greatly among different types of rocks. On the whole, finer grained dolomitic tuffs and terrigenous clastic rocks have higher dolomite content. Dolomites in dolomitic tuffs are mainly silt-crystalline and fine-crystalline and subhedral, which exhibit like porphyritic, lumpy and striped forms (Fig. 2 c-2e). In the relatively coarse terrigenous clastic rocks, dolomites are mainly anhedral, occurring in forms of cementation and replacement of clastic particles, and most in poikilitic form (Fig. 2 f).
Fig. 2. Micro characteristics of the dolomites in dolomitic rocks of the Fengcheng Formation in the Mahu Sag. (a) Well FN14, 4162.77 m, P1f2, fine crystalline-silt-crystalline and subhedral-euhedral dolomites are distributed in tuffs in a floating form, under cross-polarized light; (b) Cathodoluminescence of the dolomites is orange red, with the same view field as (a), cathodoluminescence photomicrograph; (c) Well MY1, 4669.85 m, P1f2, silt-crystalline subhedral dolomites are distributed in a striped form, with order degree of 0.49, under plane-polarized light; (d) Cathodoluminescence of the dolomites with zonary structure is orange red-orange yellow, with the same view field as (c), cathodoluminescence photomicrograph; (e) Well MY1, 4595.34 m, P1f3, fine crystalline-silt-crystalline and subhedral dolomites are distributed in a striped form, with orange red cathodoluminescence and zonary structure, cathodoluminescence photomicrograph; (f) Well K204, 4348.05 m, P1f3, dolomites replace volcaniclastics (white arrow), with order degree of 0.71, under cross-polarized light; (g) Dolomites have non-cathodoluminescence, cathodoluminescence of calcites is orange red, with the same view field as (f), cathodoluminescence photomicrograph; (h) Well MY1, 4814.13 m, P1f2, fine crystalline and subhedral-euhedral dolomites (white arrow) filled in the fracture, with order degree of 0.97, under cross-polarized light; (i) Dolomites filling in the fracture have non-cathodoluminescence, in the same view field as (h), cathodoluminescence photomicrograph. |
By comparing the crystal form differences and physical properties of the main dolomites in dolomitic rocks, it can be seen that the dolomitic rocks with relatively good physical properties mainly develop subhedral dolomites, while the dolomitic rocks dominated by euhedral dolomites have lower porosity. Therefore, the origin analysis of authigenic dolomites in the Fengcheng Formation of the Mahu Sag is focused on the subhedral dolomites.
4. Formation period of the dolomites
Dolomites originate from migration and recombination of elements and isotopes, migration characteristics of which change to varying degrees in different diagenetic stages or under different diagenetic conditions [30]. Understanding the formation period of dolomites can constrain the physical and chemical conditions of dolomite formation environment, so as to better explain the material source, evolution and formation mechanism of dolomites in different diagenetic stages [30]. Therefore, firstly, occurrence characteristics of dolomites with various crystalline forms are identified through microscope, and formation sequence is clarified. Then, formation periods of dolomites are determined by inclusion temperatures, in-situ oxygen isotopic geothermometer, fluorescence characteristics and source rock evolution characteristics.
4.1. Identification of dolomite formation sequence based on microscopic occurrence characteristics
Micro- and silt-crystalline subhedral dolomites in the Fengcheng Formation of the Mahu Sag show orange red, orange red-orange yellow cathodoluminescence (Fig. 2 b, 2d, 2e), which often gather in shrinkage joints and bedding planes (Fig. 2 a, 2c). The shrinkage joints formed by volcanic lava cooling and dry cracking of mudstone mainly occurred during syndiagenetic-early diagenetic stage, indicating that the subhedral dolomites were mainly precipitated during penecontemporaneous-shallow burial. Anhedral dolomites in filling and metasomatic forms with dark red cathodoluminescence or nonluminescence (Fig. 2 g) mainly formed in middle burial period. Fine-crystalline dolomites (Fig. 2 h) filled in structural fractures and solution pores with nonluminescence (Fig. 2 i) and the homogenization temperatures of fluid inclusion more than 120 °C, were mostly formed in the middle-deep burial period.
4.2. Temperature measurements of fluid inclusion and oxygen isotopic composition
The fluid oxygen isotope compositions during mineral formation was inversed from 8 samples with homogenization temperatures and in-situ oxygen isotopes [31-32]. According to δ18O values of calcite and dolomite sample points in four shrinkage joints, δ18O values of 3.86‰-5.55‰ of diagenetic fluid are calculated, significantly higher than the Permian average seawater [33], which locate in the δ18O values range of oilfield brine [33]. The mean value of δ18O of 4.69‰ was taken as the oxygen isotope of diagenetic fluid to inverse the formation temperature of burial dolomites and other early carbonate minerals. The δ18O values of 14.46‰-15.78‰ are calculated from the coarse crystalline dolomite sample points filled in four late-stage fractures and caves, which are obviously positive and close to that of volcanic hydrothermal solution [33]. The coarse crystalline dolomites are mainly affected by deep thermal fluid, and the average value of 15.06‰ was used to calculate the formation temperatures of hydrothermal dolomites in the Fengcheng Formation. The formation temperatures of the early salt minerals like shortite and northupite are 17-28°C (average 24 °C), which are in well agreement with the dry and hot climate conditions of the early Permian in the study area and that the modern global warm-phase salt minerals are mainly distributed in the temperature zone of 16-25 °C [4], indicating that the calculated oxygen isotopic values of diagenetic fluid are reasonable.
Based on the homogenization temperatures of fluid inclusions and the calculated temperatures of oxygen isotopes, the dolomites are mainly formed in three temperature ranges of 60-90 °C, 90-120 °C and 120-150 °C (Fig. 3 ), and the main formation temperature ranges of subhedral dolomites are 60-90 °C and 90-120 °C respectively. According to burial environment definitions based on depth by some scholars [34-35] and the burial history of the Fengcheng Formation in the Mahu Sag [36], the three phases of dolomites were formed in shallow burial environment of the early Middle Permian, middle burial environment of the middle of the Middle Permian and middle-deep burial environment of the late Middle Permian. Subhedral dolomites mainly formed in the early-middle of the Middle Permian, which belong to the sedimentary minerals of the early diagenetic stage.
Fig. 3. Distribution of formation temperatures of dolomites and other carbonate minerals in the Fengcheng Formation of the Mahu Sag. |
4.3. Precipitation temperatures of dolomites deduced by microscopic fluorescence
Based on the fluorescence and temperatures of fluid inclusions in the subhedral dolomites, the precipitation temperatures of dolomites without hydrocarbon inclusions are 58.07-69.96 °C, and their paleodepths are mostly less than 1000 m (Fig. 4 a). The precipitation temperatures of hydrocarbon inclusions with orange yellow and yellow fluorescence (Fig. 4 b-4c) are 86.06- 97.40°C, corresponding with the first stage of hydrocarbon charging, and the precipitation temperatures of hydrocarbon inclusions with green and yellow green fluorescence (Fig. 4 d-4e) are 120.9-122.8 °C, corresponding with the second stage of hydrocarbon charging, indicating that the yellow fluorescence inclusions were formed earlier and the green fluorescence inclusions were formed later. Some subhedral dolomites have zonary fluorescence. From the middle to edge of the dolomite crystals, the fluorescence changes from yellow to green (Fig. 4 f), indicating that the source rocks were in a low maturity stage at the initial stage of precipitation, while the source rocks entered the maturity stage when the crystal grows outside. Based on the orange yellow and yellow fluorescence of the subhedral dolomites, it can be indirectly inferred that they mainly formed in the early diagenetic stage during shallow burial.
Fig. 4. Fluid inclusions and fluorescence characteristics of dolomites and their associated minerals in dolomitic rocks of the Fengcheng Formation in the Mahu Sag. (a) Well MY1, 4899.86 m, P1f1, salt water inclusions in the fine crystalline and subhedral dolomite, homogenization temperature of 68 °C, δ13C=2.07‰, δ18O=-0.86‰ (laser isotope measurement), under plane-polarized light; (b) Well FN14, 4165.90 m, P1f2, hydrocarbon inclusions in the subhedral dolomite, homogenization temperature of 91 °C, under plane-polarized light; (c) The subhedral dolomite inclusion with orange yellow fluorescence, in the same view field as (b), fluorescence photomicrograph; (d) Well K207, 4854.30 m, P1f2, hydrocarbon inclusions in the coarse-crystalline dolomite in the dissolved pore, homogenization temperature of 121 °C, δ13C=3.65‰, δ18O=2.29‰ (laser isotope measurement), under plane-polarized light; (e) The hydrocarbon inclusions in dolomite with green fluorescence, in the same view field as (d), fluorescence photomicrograph; (f) Well K204, 4333.05 m, P1f3, the subhedral dolomites with zonary fluorescence, fluorescence photomicrograph. |
4.4. Formation period deduced by order degrees of dolomites
Order degrees of subhedral and anhedral dolomites are 0.49-0.76 (average 0.59, Fig. 2 c) and 0.64-0.79 (average 0.70, Fig. 2 f) respectively. There are two kinds of order degrees of euhedral dolomites: (1) low order degree (less than 0.7), with an order degree of 0.43; (2) high order degree (greater than or equal to 0.7), 0.71-0.97, with an average value of 0.79. According to the correlation between order degree and dolomitization mechanism [12,31], the low order degrees of euhedral dolomites and subhedral dolomites of the Fengcheng Formation correspond with penecontemporaneous period, and anhedral and euhedral dolomites with high order degrees correspond with the middle-deep burial period.
To sum up, dolomites in the Fengcheng Formation were mainly formed in the shallow burial environment of the early Middle Permian, followed by the medium burial environment of the middle Middle Permian.
5. Material sources and fluid evolution during dolomites precipitation
5.1. Material sources
5.1.1. Tracers of chemical compositions of rocks
Dolomites in the dolomitic rocks of the Fengcheng Formation being observed formed by precipitation, and no terrigenous clastic dolomites develop.
The major element distribution pattern of the dolomitic rocks of the Fengcheng Formation in the Mahu Sag is similar to the average neutral and intermediate-acid rocks in China, indicating that the clastic materials in the dolomitic rocks are mainly from neutral and intermediate-acid volcanic rocks (Fig. 5 ). The major and trace element distribution patterns of the dolomitic rocks are similar to the saline-lake clay and marine mudstone (Fig. 5 ), and the dolomitic rocks are relatively rich in Mg, Ca, Na, K and Li, Ni, Cu, Sr, Mo, U. The main reason is that these elements have strong migration ability during weathering and are easy to accumulate in alkaline lake water [37]. Therefore, the formation of dolomites in the Fengcheng Formation is genetically related to alkaline lake fluid.
Fig. 5. Distribution pattern of major elements (a) and trace elements (b) in dolomitic rocks in the Fengcheng Formation of the Mahu Sag (data of igneous rocks (China) quoted from references [38-39]; data of average salt lake clay quoted from references [40-41]; data of shale and marine mudstone quoted from Reference [37]). |
5.1.2. Isotopic tracers
The δ13C values of the authigenic dolomites are roughly equivalent with the Permian marine (salt water) carbonates[42] and ancient marine dolomites [43], which are mainly composed of heavy carbon isotopes, about 87% of which are greater than 0. About 68% of the δ18O values range −8.03‰-2.38‰, which are also similar to the Permian marine (salt water) carbonates [42] and ancient marine dolomites [43].
Paleosalinity indexes (Z) of dolomites was calculated by the formula established by Keith and Weber based on δ18O and δ13C values [42], which are 118-142, and about 95% of which are greater than 120, indicating a saline environment.
Solution concentration of the Fengcheng Formation in the Mahu Sag during dolomite precipitation calculated based on ice point is 13.99%-20.27% [27], which also indicates that the dolomitization fluid is brine with high salinity.
To sum up, the fluid during dolomitization in the Fengcheng Formation is mainly saline water with high salinity.
The distribution ranges of carbon and oxygen isotopes of dolomites in the Fengcheng Formation are basically consistent with modern and ancient alkaline lakes significantly affected by volcanic activity in China and abroad [6,31]. The δ18O values of most dolomites in the Fengcheng Formation are negative, while the δ13C values are obviously positive, so it is difficult to reasonably explain the origin according to traditional burial dolomitization. The δ13C values of authigenic dolomites in the Fengcheng Formation are 2.07‰-5.74‰, with an average of 3.69‰, which are close to the inorganic carbon [33] (−9.00‰-2.70‰), and much higher than the saline-lake sysgenetic dolomites (1.31‰) [43] (Fig. 6 a) and the organic carbon (35.00‰-−19.40‰) [33]. The δ13C values of dolomites in phases I, II and III have not changed basically (Fig. 6 a), indicating that CO2 generated by thermal evolution of organic matters during burial has little influence. Except alkaline lake water, CO2 rich in 13C of volcanic materials is another important carbon source for the authigenic dolomites.
Fig. 6. Distribution histograms of average δ13C values (a) and average δ18O values (b) of the dolomites in dolomitic rocks of the Fengcheng Formation in the Mahu Sag (the δ13C and δ18O values of moderm salt lake syngenetic dolomite data from Reference [43], N represents sample number). |
The δ18O values of authigenic dolomites in the Fengcheng Formation are −10.54‰-6.32‰, with an average of −2.21‰, which are significantly higher than yhe modern saline lake sysgenetic dolomites (−4.65‰) [43] (Fig. 6 b). The δ13C values of the dolomites in Phase II are much lower than the dolomites in Phase I (Fig. 6 b), reflecting 18O depletion caused by temperature increase during burial. The main range of δ18O values of the dolomites in Phase I is −4.53‰-0.63‰, representing the oxygen isotope compositions of the alkaline lake water body under shallow burial and low temperature. Part of the positive δ18O values are mainly due to influence of volcanic hydrothermal solution. The range of δ18O values of the dolomites in Phase II are −8.03‰ to −5.05‰, with an average of −6.60‰, which are mainly related to burial temperature increase. The δ18O values of the dolomites in Phase III fluctuate greatly in −10.54‰-2.38‰, with an average of −2.03‰, and the negative δ18O values are mainly related to burial temperature increase, while the positive ones are mainly related to volcanic materials rich in 18O.
According to the strontium isotopic composition analysis data of dolomites in the Fengcheng Formation, except one sample, 87Sr/86Sr values are lower than the global Permian seawater and marine carbonates [37], far lower than the modern rivers [32], and close to the current mantle source strontium [32] (Fig. 7 ). The authigenic dolomites in the dolomitic rocks generally replace pyroclasts and volcanic glass, and wholly inherit the strontium isotopic compositions of volcanic materials, which is one of the important reasons why 87Sr/86Sr values approximate to the mantle source strontium.
Fig. 7. Distribution of the 87Sr/86Sr values of dolomites in the dolomitic rocks of the Fengcheng Formation in the Mahu Sag and other strontium sources. |
5.1.3. Cathodoluminescence tracer of authigenic dolomites
The correlation is weak between cathodoluminescence of dolomites and compositions of dolomitic rocks of the Fengcheng Formation in the Mahu Sag. For example, cathodoluminescence of the dolomites in basic and acidic rocks is similar, indicating that the cathodoluminescence characteristics are mainly controlled by diagenetic fluid. The most developed subhedral dolomites in occurrence forms of filling, striping and floating mainly have orange red and orange red-orange yellow cathodoluminescence (Fig. 2 d-2e), with low order degree, which mainly formed in the penecontemporaneous-shallow burial period. This type of dolomite often has obvious zonary luminescence, reflecting that the dolomitization fluid in the early stage of crystal growth is closely related to alkaline lake water, but in the late stage of its growth, it is more closely related to burial diagenetic fluid. However, the replacive and fracture-filling dolomites have dark red cathodoluminescence or nonluminescence (Fig. 2 g, 2i), and it is the reason that the replacive dolomites inherit the high Fe content of igneous rock, and the fracture-filling dolomites are related to Fe enrichment during burial diagenesis [32] or volcanic hydrothermal solution rich in Fe.
Based on the oxygen, carbon, strontium isotopic compositions, cathodoluminescence and chemical compositions of the dolomites, it is inferred that the material sources of dolomites are mainly related to alteration of volcanic eruptive materials during the penecontemporaneous to shallow burial periods, and the early dolomites are also affected by alkaline lake water.
5.2. Fluid evolution
The contents of MgO, MnO and SrO gradually decrease from the edge to middle of some subhedral dolomites with zonary growth structure (Fig. 8 a-8d) in the Fengcheng Formation, while the contents of SiO2, FeO and Na2O gradually increase (Table 1 ). The edge of dolomite crystals have nonluminescence or very dark brown yellow cathodoluminescence due to high content of Fe which results in luminescence quenching [32] (Fig. 8 b). The increases of SiO2 and Na2O contents are related to release of Si4+and Na+ from devitrification of volcanic glass, while the significant increase of FeO is related to the burial alteration of volcanic materials. The replacive dolomites partially inherit the chemical compositions of the metasomatized clasts, but show the similar characteristics along the crystal growth direction (from the edge to middle) with the dolomites with zonary growth structure (Fig. 8 e, Table 1 ). Therefore, at the initial stage of dolomite precipitation, its fluid source was mainly related to alkaline lake water. Subsequently, in the burial process, large-scale devitrification and alteration of aluminosilicate minerals released Na+ and Al3+, which results in Na2O decrease and Al2O3 increase. The most significant feature of fracture-filling dolomite is that Fe content is significantly higher than other dolomites (Fig. 8 f, Table 1 ), which is related to the deep burial environment.
Fig. 8. Electron microprobe sample point location and microscopic characteristics of dolomites from the Fengcheng Formation in the Mahu Sag. (a) MY1, 4610.86 m, P1f2, sample point location (white dot, number is sample point number) of electron microprobe of the dolomites with zonary cathodoluminescence, scanning electron micrograph image; (b) Dolomites with zonary cathodoluminescence, the same sample as (a), cathodoluminescence photomicrograph; (c) MY1, 4669.85 m, P1f2, sample point location of electron probe of striped dolomites, scanning electron micrograph image; (d) Dolomites with zonary cathodoluminescence, the same sample as (c), cathodoluminescence photomicrograph; (e) MY1, 4852.18 m, P1f1, sample point location of electron probe of metasomatic dolomites, backscattered electron image; (f) MY1, 4814.13 m, P1f1, dolomites filling in the fracture, scanning electron micrograph image. |
Table 1. Electron microprobe data of dolomites with different occurrences in the dolomitic rocks of the Fengcheng Formation of Well MY1 in the Mahu Sag |
| Depth/ m | Point number | Dolomite occurrence | Mass fraction/% | Fe/Mn | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| MgO | SiO2 | MnO | SrO | Na2O | CaO | FeO | Al2O3 | K2O | ||||
| 4610.86 | 1 | Zonary form | 36.54 | 4.68 | 0.58 | 0.37 | 0.50 | 56.60 | 0.16 | 0.42 | 0.15 | 0.27 |
| 2 | 34.67 | 5.47 | 0.50 | 0.31 | 0.60 | 57.44 | 0.37 | 0.37 | 0.27 | 0.75 | ||
| 3 | 30.80 | 5.96 | 0.43 | 0.17 | 0.19 | 56.84 | 4.14 | 1.22 | 0.25 | 9.82 | ||
| 4 | 30.84 | 3.95 | 0.40 | 0 | 0.18 | 59.46 | 4.45 | 0.37 | 0.35 | 11.42 | ||
| 5 | 33.38 | 2.62 | 0.37 | 0.38 | 0.26 | 62.40 | 0.51 | 0.02 | 0.06 | 1.39 | ||
| 4669.85 | 1 | Striped form | 29.26 | 0.88 | 6.78 | 0.41 | 0.25 | 62.19 | 0.15 | 0.05 | 0.04 | 0.02 |
| 2 | 33.17 | 1.38 | 0.27 | 0.40 | 0.29 | 64.00 | 0.05 | 0.26 | 0.18 | 0.18 | ||
| 3 | 30.49 | 4.15 | 0.04 | 0.65 | 0.73 | 62.67 | 0.16 | 1.02 | 0.09 | 4.56 | ||
| 4852.18 | 1 | Metasomatic form | 22.95 | 20.19 | 0.49 | 0.00 | 0.28 | 43.98 | 1.63 | 5.71 | 4.77 | 3.36 |
| 2 | 33.92 | 4.16 | 0.40 | 0.51 | 0.78 | 59.77 | 0.08 | 0.15 | 0.23 | 0.20 | ||
| 3 | 32.52 | 5.31 | 0.52 | 0.47 | 0.94 | 59.41 | 0.28 | 0.23 | 0.32 | 0.54 | ||
| 4814.13 | 1 | Fracture-filling | 26.96 | 1.51 | 0.15 | 0.16 | 0.17 | 64.21 | 6.46 | 0.02 | 0.36 | 42.61 |
Note: The data in the table are normalized; sample point number locations are in |
6. Dolomite origin
Dolomitization is common in the dolomitic rocks of the Fengcheng Formation in the Mahu Sag. The overall dolomite content is low, with an average of 19.42%, reflecting that the material sources are limited during dolomitization process. One of the important limiting conditions is the burial origin of dolomites. As the subhedral dolomites are the most developed and the dolomitic rocks with subhedral dolomites have the best physical properties, the formation mechanisms of subhedral dolomites are mainly discussed in the study. In order to more accurately explain the origin of dolomites in the Fengcheng Formation of the Mahu Sag, the correlation between microscopic characteristics and geochemical indicators of dolomites in different periods (Table 2 ) were established, so as to overcome the ambiguity of interpretation by using a single indicator. According to the nature and source of dolomitization fluid, and dolomitization mechanisms of dolomites in different periods, the formation models of subhedral dolomites in dolomitic rocks of the Fengcheng Formation in the Mahu Sag were established (Fig. 9 ).
Table 2. Characteristics of dolomites in different periods in the dolomitic rocks of the Fengcheng Formation in the Mahu Sag |
| Formation period | Sample temperature/°C | Crystal features | Main occurrence | Fluorescent color | Cathodoluminescence | Order degree | δ18O/ ‰ | Source of Mg2+ | Formation mechanism |
|---|---|---|---|---|---|---|---|---|---|
| Penecontemporaneous- shallow burial | 55-89 | Euhedral, subhedral; microcrystalline, silt-crystalline | Striping, floating | Non-fluorescence, brown yellow | Orange red, orange yellow | 0.43- 0.66 | −4.53- 0.63 | Alkali lake saline water | Penecontemporaneous dolomitization induced by microorganism, seepage-reflux dolomitization |
| Middle burial | 91-118 | Subhedral; silt-crystalline, fine crystalline | Intergranular pore-filling, metasomatic | Orange yellow | Dark red, nonluminous | 0.64- 0.79 | −8.03- 5.05 | Devitrification of volcanic glass, acidic dissolution of rock matrix | Burial dolomitization |
| Medium- deep burial | 121-144 | Euhedral, anhedral; fine- crystalline | Metasomatic, fracture-filling, dissolved pore | Green | Orange red, nonluminous | 0.71- 0.97 | −10.54- 2.38 | Dissolution of rock matrix, volcanic hydrothermal fluid | Burial dolomitization, hydrothermal metasomatic dolomitization |
Fig. 9. Formation mechanism of suhedral dolomites in dolomitic rocks of the Fengcheng Formation in the Mahu Sag. |
During the sedimentary period of the Fengcheng Formation, the Mahu Sag was a closed alkaline lake, and microorganisms in the sedimentary water body were extremely prosperous [4]. In the syngenetic period, pyroclasts released Na+, K+, Ca2+, Mg2+, Fe2+, etc., due to hydrolization. Salt minerals such as shortite, natronite and aragonite, high-magnesium calcite and calcite [4,44 -45] were first precipitated, so that Na+, K+, Ca2+ in the sedimentary water were reduced and Mg2+ was relatively enriched. Under microbial induction, a large amount of micritic-microcrystalline dolomites were formed through primary precipitation and replacing aragonites, high-magnesium calcites and calcites in the sygenetic period [6,9], which are mainly distributed in the matrix in a dispersed form (Fig. 9 a).
During penecontemporaneous-shallow burial stage, the sediments retained a large number of pores (porosity greater than 30%) [10], and the high-salinity brine continuously reacted with the surrounding rock during the downward infiltration process. A large amount of Na+, Ca2+, Mg2+, Fe2+, etc., were released by the large-scale devitrification of volcanic glass, and salt minerals continued to precipitate. Due to decrease of hydration degree of Mg2+ under high temperature and pressure, continuous supply of Mg2+ enabled continuous growth of micritic-microcrystalline dolomites formed in the syngenetic period, and further replaced high-magnesium calcites, calcites and tuffs to form silt-crystalline dolomites in patchy and lamellar forms (Fig. 9 b).
In the middle burial period, rock porosity continued to decrease, the formation fluid decreased significantly, and the clay minerals released a large amount of interlayer water [10], which is an important supplement to the dolomitization fluid. A large amount of acid was discharged from the source rock [10], rock particles were dissolved, and devitrification was also strengthened. Mg2+ mainly originated from clay mineral transformation, devitrification of volcanic glasses and dissolution of magnesian silicate minerals, and Na+ and Ca2+ were removed in form of salt minerals and authigenic feldspar. At the late stage of this period, porosity and formation fluid were significantly reduced, and the migration of formation fluid was more difficult, so dolomites formed at this period was limited. Because the early dolomites continued growing and further replaced tuffs (Fig. 9 c), dolomite of this period is mainly composed of silt- and fine-sized crystals.
In the middle-deep burial period, as a result of dolomitic rocks becoming very tight and continuous consumption of Mg2+ in the shallow and middle burial stages, large-scale dolomitization was difficult to occur, which is mainly manifested by local cementation and replacement by fine crystalline and subhedral dolomites.
7. Conclusions
Dolomites in dolomitic rocks of the Fengcheng Formation are mainly fine-crystalline and silt-crystalline subhedral dolomites, which are distributed in forms of floating, clumping and striping. The tight oil reservoirs of the Fengcheng Formation are mainly distributed in dolomitic rocks rich in subhedral dolomites.
Dolomites were mainly formed in the penecontemporaneous-shallow burial environment in the early Middle Permian, followed by the middle burial environment in the middle Middle Permian, and a small amount were formed in the middle-deep burial environment in the late Middle Permian.
In the dolomitic rocks, dolomitization fluid is saline water with high salinity under the background of alkaline lake, and Mg2+ mainly comes from alkaline lake water and alteration of volcanic materials.
The subhedral dolomites often have zonary cathodoluminescence, and the chemical composition change is related to the alkaline lake fluid, and the devitrification and diagenetic alteration of volcanic materials of the Fengcheng Formation during burial.
In the penecontemporaneous period, Mg2+ mainly originated from alkaline lake water. Micritic-microcrystalline dolomites were formed through primary precipitation and replacing aragonite, high-magnesium calcite and so on. During the shallow burial period, a large amount of Mg2+ was released by devitrification of volcanic glasses. The silt-crystalline dolomites were mainly originated from continuous growth of micritic-microcrystalline dolomites of the quasi-contemporaneous period and replacement of calcite and tuff. In the middle burial period, Mg2+ mainly came from clay mineral transformation, devitrification of volcanic glass and dissolution of aluminum silicate such as feldspar, and burial metasomatism produced silt- and fine-crystalline dolomites.
The tight dolomitic reservoirs of the Fengcheng Formation in Mahu Sag are mainly developed in the dolomitic rocks rich in subhedral dolomites. Future exploration for tight dolomitic reservoirs should focus on such reservoirs.