Sequence Stratigraphy and Microfacies of the Middle Ordovician Yijianfang Formation in the Tarim Basin, NW China Open Access

As one of the crucial reservoir types, carbonate reservoirs constitute approximately half of the global hydrocarbon resources [1]. In recent years, they have attracted increasing attention from researchers in the geological field [2-4]. Analyzing sequence stratigraphy of carbonates is of great significance to hydrocarbon accumulation and to the prediction of oil and gas exploration [5, 6]. The study of microfacies is not only the basic content of carbonate sedimentary facies and paleo-environment analysis but also plays an important role in sedimentary and reservoir geology [7, 8]. The distribution range of favorable reservoirs can be controlled by the dominant sedimentary zones. Sedimentation controls the distribution of high-quality reservoirs and also constitutes the material basis of later weathering karst in the Sichuan and Ordos basins (China), and the Middle East and Santos Basin (Brazil) [9-11].

The high-stand system tract (HST) is in the late stage of sea level rise and the early stage of decline, and the energy of the water is still strong. High-energy shoals are mostly developed here, which is the potential environment for a favorable reservoir. The sediments near the sequence boundary are prone to leaching and forming dissolution pores [12]. Generally, the closer to the boundary, the stronger the dissolution effect. Furthermore, it increases the porosity of the reservoir, and diagenetic processes, such as dissolution, are distributed along the sequence boundary [12, 13].

Understanding the characteristics of sequence and sedimentary facies is of great help to the development of carbonate platform and the distribution of reservoir [7, 14]. The third-order sequence has been uniformly divided by seismic reflection patterns, petrology, well logging, and isotope data [15-17], and many scholars have also done a lot of work on fourth-order sequences, and it is pointed out that the sequence boundary has a controlling effect on the development of Ordovician carbonate reservoirs. For sedimentary microfacies, the division is made by means of core, well logging, and thin-section observation [18, 19].

The Yijianfang Formation represents a crucial reservoir interval in the Tarim Basin [20-22]. In previous sequence stratigraphic studies of the northern Tarim and Shuntuoguole areas, the Yijianfang Formation was uniformly classified into one third-order sequence [20, 23-25]. However, the division schemes for fourth-order sequences remain inconsistent, mainly manifested in the number of sequences and the division criteria. Discrepancies exist among different scholars regarding the number of fourth-order sequences [20, 22, 23, 26, 27].

During the deposition period of the Yijianfang Formation in the Tahe area, the open platform facies were predominantly developed, with a diverse range of microfacies. Considerable research efforts have been made by predecessors, mainly focusing on the division of sedimentary microfacies and the distribution of shoals [23, 28], and these studies mainly utilized core data. Nevertheless, further exploration is still required on how to identify the sedimentary microfacies of the Yijianfang Formation using logging data.

Well logging and full-bore microscan imaging (FMI) have good response characteristics to sequence and sedimentary microfacies. Therefore, through logging and imaging logging combined with lithology, different sedimentary microfacies can be identified. The aims of this work are to: (1) identify the third-order and fourth-order sequence boundary, establish a high-precision sequence stratigraphic framework, and analyze the distribution of fourth-order sequences; (2) divide lithofacies and sedimentary microfacies within the fourth-order sequence framework and summarize their response patterns; (3) determine the distribution of each sedimentary microfacies under sequence framework.

As the largest petroliferous superposed basin in China, the Tarim Basin has a covering area of 56 × 10⁴ km² in the south of Xinjiang Uygur Autonomous Region. This basin is bordered by the Tianshan Mountains in the north and the Karakoram and Aerjin Mountains in the south (Figure 1(a)). The Tahe Oilfield is located in the south of the Akekule arch of the Tabei Uplift, Tarim Basin (Figure 1(b)). It is adjacent to Kuqa Depression in the north, Manjiaer Depression in the southeast, and Shuntuoguole Uplift in the south. Stratigraphically, the Ordovician strata are subdivided into six formations: Lower Ordovician Penglaiba Formation (O₁p), Lower-Middle Ordovician Yingshan Formation (O_1-2y), Middle Ordovician Yijianfang Formation (O₂yj), Upper Ordovician Qiaerbake Formation (O₃q), Lianglitage Formation (O₃l), and Sangtamu Formation (O₃s). The sedimentary environment changes from a restricted platform to an open platform. The sedimentary environment of Yijianfang Formation (O₂yj) is an open platform (Figure 2).

The Tarim Basin underwent multiple tectonic evolutions from Caledonian, Hercynian, Indosinian, Yanshanian to Himalaya orogenies, and finally formed the present tectonic form [29]. The late karstification was greatly influenced by the middle Caledonian and Hercynian orogenies, and the middle Caledonian orogeny was divided into three stages, including episodes I, II, and III. Due to the influence of Caledonian orogeny, the Yijianfang formation in the north of Tahe Oilfield was completely eroded [29], which resulted in the Yingshan Formation (O_1-2y) was directly covered by the Carboniferous Bachu Formation (C₁b). The lithology in the lower part of the Bachu Formation is sandstone, and the corresponding lithology in the upper part is limestone [30]. According to the statistics of a large number of drilling data in the study area, the Yijianfang formation is well preserved in the south part with an overall thickness ranging from 70 to 100 m. In addition, the Upper Silurian, Devonian, and Upper-Middle Ordovician strata were successively eroded [31, 32].

In this study, nine wells were selected for analysis of individual and well connection profile, and three wells (S76, S91, and T705) were used to analyze the sequence and sedimentary microfacies. Cores from more than 12 wells were observed and described. The data from 87 thin sections were carefully observed under a polarizing microscope at the China University of Geosciences (Beijing). These thin sections were stained with Alizarin Red A to distinguish calcite and dolomite. Based on photomicrographs of each thin section, the mineral composition, textural component, and grain content were intuitively estimated. The sedimentary structure, grain types, biological fossil fragments, and sedimentary microfacies division were analyzed.

A field outcrop section (named as Nanyigou profile) was selected for sequence boundary analysis and observation. The Nanyigou section contains complete Ordovician strata, with a primary focus on the stratigraphic development of the Yingshan and Yijianfang Formations.

Sequence boundary can be identified using outcrop characteristics, lithofacies assemblage, well log data analysis, and seismic reflection characteristics. The high-precision sequence stratigraphic framework was established through identifying the third- and fourth-order sequences in the study area.

1. Outcrop section: It always provides important formation to identify the sequence boundary [25]. Sequence boundaries exhibit relatively distinct phenomena in outcrop. First, sequence boundaries are generally unconformity surfaces. One can focus on observing the attitude and angle of strata, as well as the presence of sedimentary breaks. Subsequently, sequence boundaries typically manifest as lithologic mutation surfaces, where changes in lithology and color can be directly observed. For example, transitions between mudstone and grainstone, or between carbonate rocks and clastic rocks. Finally, sequence boundaries can also be identified based on changes in graded bedding. Generally, changes in graded bedding or the thickness of graded bedding occur at the boundaries.

2. Conventional log: Conventional log curves at the boundary change in morphology and data due to the influence of lithology change. Natural gamma ray (GR), deep and shallow dual lateral resistivity (RD and RS), acoustic (AC), compensated neutron (CNL), and density (DEN) were used to identify the third- and fourth-order sequence boundaries.

The process of identifying sequence boundaries involves the following three steps. First, changes in the shape of log curves can be visually observed, such as box shape, dentate shape, bell shape, and so on. Second, log curves can exhibit different trends. They may show an upward or downward trend, and abrupt changes may occur at the sequence boundaries. Finally, pay attention to the changes in the values of log curves. Notice the abrupt changes at the boundaries and the different value distribution ranges between different sequences.

1. FMI: FMI has the advantages of high accuracy and intuition, which is of great significance to identify sequence boundaries, to establish sequence frameworks, and to identify reservoir spaces. There is a complete color difference with two kinds of variation patterns on FMI logs above and below the boundary, abrupt change, and gradual change [26]. First, at the sequence boundaries, two different change patterns occur. One is the abrupt change from bright color to dark color, and the other is the gradual transition from bright color to dark color. Second, FMI can identify reservoir spaces such as pores, fractures, and caves, presented in forms like patchy and striped patterns.

2. Core and thin sections: The performance on the core is more intuitive. Lithological changes and combinations can be directly determined by the observation of core and thin sections. When identifying sequence boundaries using cores and thin sections, the following aspects are mainly involved: (1) The differences in lithology can be identified through cores and thin sections. (2) Second, the color differences can be directly observed. There are significant color distinctions above and below the sequence boundary. (3) The grain size of the lithology can be determined from the cores, including mudstone and grainstone. Moreover, in thin sections, different particle components such as ooids and bioclasts can be observed. (4) Regarding the differences in reservoir spaces, the development degrees of pores and fractures at different scales can be observed.

FMI shows bright or dark characteristics through different resistivity characteristics and can recognize lithology and sedimentary environment [33-36]. Hence, FMI and conventional logs were combined to correlate microfacies and well log response. Log data can be used to further analyze microfacies identification, development of sedimentary environment, and the distribution of favorable areas.

The Yijianfang Formation is identified as a third-order sequence, called SQ1, and three fourth-order sequences. From bottom to top, these fourth-order sequences are, respectively, denoted as Sq1, Sq2, and Sq3 (Table 1). Third-order sequence boundary SB2 is defined as the top boundary of the Middle Ordovician. For the study area, it is the top boundary of the Yijianfang Formation. There is an extensive range of regional unconformity at the top boundary of the Yijianfang Formation, which is caused by tectonic action or large-scale regression of the regional stratum exposure and leaching surface. There is an obvious change in lithology, FMI, and conventional logging response characteristics above and below the boundary. The third-order sequence boundary SB1 serves as the bottom boundary. FSB1 is the fourth-order boundaries between the first and second member, and FSB2 is the fourth-order boundaries between the second and third member of the Yijianfang Formation. The specific division scheme is as follows (Table 2).

1. Lithological difference at outcrop: The lithological characteristic of the Yijianfang Formation was observed in the outcrop. Lithological types change above and below the boundary, and there are intraclastic grainstone with sparry cements and mudstone, respectively (Figure 3). The main lithology of Yingshan Formation is intraclastic grainstone, intraclastic packstone, and mudstone. There are bioclastic packstone, intraclastic grainstone with bioclastic, and mudstone in the Yijianfang Formation. The lithology of the Yijianfang Formation changed with the overlying Qiaerbake Formation, and it was conformable contact with the underlying Yingshan Formation.

2. FMI features: The color changes in the above and lower parts of the boundary [37-39]. Overall, the FMI color of the Yijianfang Formation is brighter than the underlying Yingshan Formation. Specifically, the color of this formation is dark interbedded with dark spots (Figures 4(a) and 4(b)). For SB1, the color shows a gradual change from light to dark (Figures 4(d) and 4(e)), showing the integrating contact and lithological change caused by the stable transition of sedimentary environment.

3. Conventional logs: Yijianfang and Yingshan Formations have different response characteristics in logging curves. At the boundary between Yijianfang and Yingshan Formations, the curves of GR, RD, RS, DEN, AC, and CNL change in morphology and amplitude because of changes in lithology (Figure 5(a)).

For the GR curve, the swing amplitude of the curve changes obviously, and there is a mutation at the boundary. In the Yijianfang Formation, the curve oscillates widely in the range of 12–20 API, while in the Yingshan Formation below the boundary, it is concentrated around 10 API and shows a weak dentate shape. For the resistivity curve, the shape of the Yijianfang Formation above the boundary shows a weak box shape. The curve of the Yingshan Formation shows a very flat shape when the boundary becomes larger, and the value decreases continuously. The DEN curve of the Yijianfang Formation shows a box shape, while the Yingshan Formation below the boundary is dentate. The log values of Yingshan Formation and Yijianfang Formation are distributed on both sides of 2.6 g/cm³, but the Yingshan Formation is larger and the Yijianfang Formation is smaller. AC curve is dentate in the Yijianfang Formation, and the curvilinear morphology of the Yingshan Formation below the boundary is dentate and box shape (Figures 5(b) and 5(c)).

Lithological difference at core: The coring above and underlying the boundary is described to identify the boundary features. The Yijianfang Formation is dominated by intraclastic wackestone and bioclastic wackestone, with small segments of intraclastic packstone and mudstone, and oil spots can be observed on the core. The grain size is generally in the range of 200–300 μm.

Based on the lithology description of core from the Yijianfang Formation, the upper lithology is dominated by intraclastic packstone and thin-layered mudstone. The lower part is dominated by intraclastic wackestone and bioclastic wackestone. The grain size becomes coarser. Relatively rich fractures and dissolution pores can be observed intuitively, especially in the lower core of the Yijianfang Formation. Fractures and pores are well developed and accompanied by oil spots and oil stains. Underlying the boundary, the lithology of Yingshan Formation is finer than that of Yijianfang Formation, with large segments of mudstone.

1. Lithological difference: The SB2 boundary is the regional lithology conversion surface, the lithology changes from carbonate rock to clastic rock. Below the boundary, there are mudstone and packstone of the Yijianfang Formation. Above the boundary, it is directly connected with sandstone, breccia, and mudstone of the Carboniferous Bachu Formation.

2. FMI features: Overall, the color of FMI changes significantly at the boundary. The FMI color of the Yijianfang Formation is brighter than the overlying Qiaerbake Formation, but the characteristics of the top and the bottom boundary are different (Figures 4(a)–4(c)). The top boundary of the Yijianfang Formation shows color mutation from dark to light. The bright color reflects the possible tight sediments with a high resistivity.

3. Conventional logs: There is an obviously different response on the boundary of conventional logs curves. The response characteristics of values and amplitude are completely different (Figure 5(b)).

For GR curves, the morphology and value changes above and below the boundary are obvious. Above the boundary, the swing amplitude is large, and the value is greater than 30 API. Below the boundary, the value of the curve becomes smaller (around 10 API), and the shape becomes weak dentate. The resistivity curves (RD and RS) show opposite characteristics, with small value above the boundary, large amplitude, and poor coincidence of curves. The curve amplitude below the boundary is small, high coincidence degree, and weak dentate shape. The variation of the SP curve is similar to that of the resistivity curve. For the CNL curve, the curve shape changes obviously. The curve above the boundary is concentrated near 10%. The abrupt change of curve shape toward small direction at the boundary is located near 1%.

The boundary FSB1 is the boundary between the first and second members of the Yijianfang Formation. First, according to the amplitude, shape, and vertical distribution of the GR curve, it is divided into two sections; at the FSB1 boundary, the amplitude of the GR curve above and below is small and relatively smooth, and the value above the boundary is slightly smaller than the value below (Figure 6(a)). Below the boundary, the amplitude of cave is more obvious, and the value is more than 3 API. But above the boundary, the value is concentrated on 3 API, and the swing of the curve becomes smaller.

The boundary FSB2 is the boundary between the second and third members of the Yijianfang Formation. First, at the boundary FSB2, the GR curve changes from a considerable value above the boundary to a small value below the boundary. Above the boundary, the value of the curve is basically greater than 5 API, up to 50 or 60 API, with noticeable changes. Below the boundary, the curve is only concentrated around 3 API. The value above the boundary varies greatly, and the value below the boundary is small on the whole with a slight curve amplitude. The DEN curve has a larger value above the boundary, in the vicinity of 2.70 g/cm³ but a smaller value below the boundary, at about 2.68 g/cm³ with a larger amplitude. The AC and CNL curves have a smaller value above the boundary with a smaller amplitude, while the value below the boundary is significantly larger, showing a change from bell shape to funnel shape (Figure 6(b)). As for FMI, the color changes above and below the boundary, corresponding to the shift of lithology. The color at the boundary is bright.

Based on the observation of the sedimentary structures and lithology of the core and thin sections, eight lithofacies types could be distinguished. The facies are illustrated in Figure 7. These were divided into four groups: inter-shoal sea, low-energy shoal, medium-energy shoal, and high-energy shoal.

The grain content is more than 80%. The well-sorted grains are uneven in size, and the length of the major axis is usually between 200 and 300 μm. Most of the morphology is normal, and occasionally hollow and compound ooids can be seen (Figure 7(a)).

The grain size varies greatly, the length of the larger grain can be more than 500 μm, and the length of the smaller grain is about 200 μm. It is dominated by intraclasts and contains a small number of ooids. The poor-sorted intraclast is angular in shape, with different forms and poor roundness (Figure 7(b)).

The grain composition is dominated by bioclastic, with a few intraclasts. From the thin section, the grain content is more than 50%, which is supported by grains. There is sparry cementation between grains, and the sorting and roundness of grains are poor (Figure 7(c)).

The bioclasts include foraminifera, ostracoda, gastropods, and arthropods. They are abundant in volume (more than 75%). The micrite calcite often fills between the grains. Some biological fossils are relatively broken (Figure 7(d)).

Grains are mainly intraclasts, occasionally a small amount of bioclasts can be seen, and the sorting and roundness are good. The grain size is small on the whole, about 200 μm, and the grain concentration is more than 50%, micritic calcite fills between the grains (Figure 7(e)).

The grain content is generally between 10% and 30%, with a small grain size, moderate sorting, and good roundness, and the content of bioclast is minimal. The matrix is micrite calcite, and the grains are suspended on the micrite (Figure 7(f)).

The bioclast content is generally about 25%, and the bioclasts include gastropod, ostracoda, and foraminifera. The main manifestation is skeletal debris. But relatively complete biological grains can be observed locally. The matrix is micrite calcite (Figure 7(g)).

The lithofacies is widely distributed in the study area. The texture components are single, and there is a pure micritic matrix and very little particle content with strong homogeneity. The particle content is less than 5% (Figure 7(h) ).

Based on hydrodynamic conditions during the original deposition, eight lithofacies can be roughly classified into four types as mudstone, wackestone, packstone, and grainstone. These four types have good correspondence with four microfacies: inter-shoal sea, low-energy shoal, medium-energy shoal, and high-energy shoal. Each of these four sedimentary microfacies has a significant difference response on FMI (Figures 8 and 9).

The inter-shoal sea mainly consists of mudstone (Mf8) and intraclastic and bioclastic wackestone. The lithology is tight with very low porosity. Therefore, the response on FMI is highlighted block pattern. The blocky pattern shows a highlight reflection, with a tight and high-resistance characteristic. This reflects the low-energy sedimentary environment and quiet hydrodynamic force, and there is no disturbance from external debris, which shows the inter-shoal sea deposition, conventional logs shown as a wiggly dentate shape (Figure 9(a)).

The low-energy shoal mainly consists of intraclastic wackestone (Mf6) and bioclastic wackestone (Mf7). These grains have low abundance, and most bioclasts are well preserved with good shape. The rocks deposited in the low-energy shoal are less tight than those in the inter-shoal sea, and they show bright color with dark linear pattern on FMI. The linear pattern is characterized by bright color with alternate dark linear, which reflects the low resistivity layer within the overall tight background, and conventional logs are significantly different from those of the highlighted blocks (Figure 9(b)).

The medium-energy shoal is dominated by bioclastic grainstone, bioclastic packstone, and intraclastic packstone (Mf3, Mf4, and Mf5). The lithology compactness under the medium-energy shoal decreases further, and the lithology becomes coarser. The grains include intraclasts and bioclasts. The porphyritic pattern on FMI is characterized by a dark yellow background with dark porphyry, indicating many pores developed in the lithology. Some of pores are filled with argillaceous material. Unfilled dissolution cavities are also observed in the cores. The cores show oil spots and a few cracks. The variation is more reflected in the amplitude of the curve, with enormous swing and more obvious dentate (Figure 9(c)).

The high-energy shoal is mainly composed of oolitic grainstone, intraclastic grainstone, and bioclastic grainstone (Mf1, Mf2, and Mf3). The intra-platform shoal facies can be divided into bioclastic shoal and intraclastic shoal according to the different grain types. The sedimentary environment is a high-energy water environment with strong hydrodynamics. The lithology is mainly packstone or grainstone, including bioclastic packstone, intraclastic packstone, and oolitic grainstone. The lithology of this microfacies is coarser, and FMI is characterized by compound type.

This pattern is the darkest among the four patterns, with a dark yellow color. Besides, there are dark porphyritic and linear of different degrees. Pores and fractures can be observed from the core and thin sections, indicating that the lithology is the least tight. The amplitude of the curve changes significantly and increases compared with the first two types (Figure 9(d)).

Two sections in the north-south direction and the east-west direction were selected to establish the fourth-order sequence framework (Figures 10 and 11). The thickness distribution of the three fourth-order sequences is relatively uniform. There is a small difference in the strata thickness in the north-south direction, but the thickness in the south is larger than that in the north. The sequence thickness of Sq3 is the largest, and the thickness of the southern strata varies evenly. The stratigraphic thickness in the east-west direction is relatively stable and has little difference. Generally, it presents a thinning trend from west to east. In terms of the thickness comparison among three sequences, Sq3 is the largest one, followed by Sq2 and Sq1 (Figure 11).

The development of sequences is comprehensively influenced by tectonic and sea-level changes. The tectonic activities in the middle Caledonian period led to the early uplift of the northern Tarim area, resulting in severe stratigraphic erosion [29, 30, 33, 40-42]. Some drillings reveal that the Bachu Formation directly overlies the Yingshan Formation in the northern part of the Tahe area, with the Yijianfang Formation being absent. In the Shunbei area, which is adjacent to the study area in the south, the Yijianfang Formation is relatively completely developed, and the thickness of the strata is greater than that in the study area. The study area is located between the denudation area and the covered area, where there is a pinch-out boundary of the Yijianfang Formation. Most parts of the study area are situated south of the pinch-out boundary, and the strata in this area have undergone varying degrees of denudation. Therefore, compared with the first and second members of the Yijianfang Formation, the stratigraphic thickness of the third member shows relatively greater variations, and the strata of the second member are more stable. The influence of tectonic activities on the strata in the study area gradually decreases from north to south. Therefore, the strata exhibit the distribution characteristics described above, and this area belongs to a shallowly covered region. The drift characteristics of carbon isotope values are related to major geological environmental events and have a wide range of isochronism [31, 32]. According to the investigation of carbon isotope data in the adjacent areas, there are abrupt changes in carbon isotopes at the upper and lower interfaces of the Yijianfang Formation, showing a positive drift characteristic. The variation ranges of the two positive drifts of carbon isotopes are both above 1‰, making the Yijianfang Formation obviously different from the overlying and underlying strata [43, 44]. The change in isotopes reflects the rise of sea level, and the sedimentary environment has changed from a restricted to an open platform.

The control of sequences over sedimentary facies is mainly reflected in sedimentary environments, facies, and scale. Using high-precision sequences to understand the patterns and constraints of microfacies is the key to predict favorable reservoirs [7, 14]. The accommodation space increases slowly, the hydrodynamic force becomes stronger, and the water circulation is unobstructed, which is suitable for the formation of granular shoal and the rapid construction of shoal [45, 46]. Because in the HST, the sedimentary environment is mostly shallow water and high energy. The grainstone formed in high-energy environment has the material basis of forming reservoir [10].

Overall, the shoals in the first member (Sq1) of the Yijianfang Formation are relatively poorly developed. They are only thick in the northeastern part and relatively isolated, while the shoals in the southern and western areas are thinner. This reflects a process of gradual sea-level rise and deepening of seawater from the Yingshan Formation to the Yijianfang Formation.

The shoals in the second member (Sq2) and the third member (Sq3) are well developed. In the second member, the shoals in the southern and western areas are well developed, and the thickness of the shoals increases in the central and northern areas, gradually dominated by medium- to high-energy shoals. The third member is characterized by medium- to high-energy shoal facies. The shoals are continuously developed in two regions: the southern and central areas, and the eastern and northern areas, respectively. Compared with the second member, the shoals in the third member are thicker, cover a wider area, and show inheritance.

Vertically, the shoals in the second (Sq2) and third members (Sq3) are better developed than those in the first member (Sq1). According to individual well data, the shoals in the third member are thicker, and more high-energy shoals may be developed in this member.

Microfacies analysis shows that the top part of each fourth-order sequence, in the early stage of the slow rise and fall of relative sea level in HST, the hydrodynamic environment is high and turbulent. The lithofacies types reflecting the medium and high-energy shoal deposits are well developed (Mf1-Mf5). During this period, the scale of the shoal increased, and its thickness is relatively large in the eastern and southern parts as well as some local areas of the central region. During the transgressive system tract, the relative sea-level rise and the water depth are large. The lithofacies of low-energy shoal (Mf7, Mf8) or the thin shoal facies sedimentary was developed in the part far from the sequence boundary. During the rapid short-term sea-level changes of the fourth-order sequence, a cyclic process can be observed where the lithology changes from the grainstone of the HST to the mudstone during the transgressive period and then back to grainstone. This cyclic process also provides a foundation for the development of favorable reservoirs. Therefore, in the study area, the development locations of shoals are controlled by high-precision sequences.

With the deepening of research, the controlling effects of the sequence framework and sedimentary facies distribution on reservoirs have gradually become well known to researchers. Previous studies have explored the favorable reservoir development areas controlled by sedimentary facies. It has been proposed that sedimentary facies can control the distribution of large-scale carbonate reservoirs, and it is believed that the grainstones formed in high-energy facies serve as the lithofacies basis for high-quality reservoirs [13, 17, 47-49]. Previous researchers have studied the relationship between sequence boundaries and diagenetic characteristics [49, 50]. The results show that the third-order sequence boundaries are characterized by the regional exposure, leaching, or erosion of strata caused by long-period sea-level decline, they are strongly affected by the leaching of meteoric fresh water, and it has been pointed out that the sequence framework has a good controlling effect on karstification.

The Middle Ordovician Yijianfang Formation is a major reservoir developed interval in the Tahe area, featuring karst fracture-cavity reservoirs. The distribution of sedimentary microfacies within the fourth-order sequence framework of the Yijianfang Formation was analyzed. It was found that the shoal evolution in the third member of this formation exhibits inheritance and migration (Figure 12). Shoals mostly develop in the HST. The thickness of shoals is relatively large in the east, south, and central region. Judging from the distribution of shoals in the three sequences, the favorable development zones are mainly distributed in these three areas as well. Favorable reservoirs are mainly controlled by the distribution of shoals and mainly develop in high-energy shoal facies. In a high-energy environment, high primary porosity can be formed [45, 51, 52]. High-energy shoal reflects relatively strong hydrodynamic forces during the sedimentary period, with an open and turbulent sedimentary environment. The formed enclosing rocks have relatively large pores, providing ideal sedimentary conditions for the enrichment of intraclasts and bioclasts.

Due to the periodic changes in sea level, the sequence boundary is characterized by regional exposure of strata. Subjected to leaching and erosion by meteoric fresh water, reservoirs with well-developed large pores and cavities can be formed. It shows the characteristics of an unconformity surface, accompanied by beaded reflections terminating along the bedding plane on seismic (Figure 13(a)). Within a certain depth below the sequence boundary, dissolution is likely to occur. Good dissolution cavities are formed under the effects of exposure, leaching, and erosion [48]. Based on the data from adjacent single wells located in the shallowly covered region, the reservoir spaces within the Yijianfang Formation are interpreted through FMI (Figure 13(b)). As observed from the FMI, distinct dark-colored linear or porphyritic features can be seen at the top of the third member (Sq3) and the second member (Sq2) of the Yijianfang Formation, indicating well-developed fractures and solution pores. Overall, well-developed reservoir spaces are present at the top of the fourth-order sequence, with a high fracture DEN and good connectivity.

This dissolution increases the reservoir space, which is conducive to the formation of favorable reservoirs. Therefore, in the covered area, the high-energy shoal lithology with relatively well-developed primary pores, superimposed with the influence of dissolution, makes the high-energy shoal the main area of favorable distribution. The grainstone and packstone below the sequence boundary are the main lithologies for the development of favorable reservoirs. Similarly, below the sequence boundary of a typical single well, the FMI shows dark porphyry and linear features. According to the previous description, this reflects the occurrence state of pores and fractures below the sequence boundary. On the one hand, it is caused by the syndepositional karstification when the high-energy lithofacies develop in the HST. On the other hand, it is a large-scale modification controlled by the sequence boundary, revealing the fracture-cavity structure further formed by lithology dissolution [9, 18, 53].

Based on the reservoir thickness data provided by production wells, in the shallow covered area, dissolution is widespread. The areas with larger reservoir thickness basically coincide with those with greater shoal thickness (Figure 12). In summary, favorable reservoirs are mainly controlled by both sequences and sedimentary facies. The shoals of the Yijianfang Formation are widely distributed. The development of effective reservoirs is controlled by the distribution of dominant lithofacies, which also lays the foundation for diagenesis. High-precision sequence boundaries control the development of dissolution pores and cavities. Favorable reservoirs develop near the sequence boundaries and in the HST of the sequence.

1. The Yijianfang Formation in Tahe Oilfield can be divided into a third-order sequence and three fourth-order sequences. The thickness of the three fourth-order sequences is relatively uniform. The sequence Sq3 (third member of Yijianfang Formation) has the most considerable thickness, and the sequence Sq1 (first member of Yijianfang Formation) is generally thin. Eight sedimentary lithofacies can be identified, which can be divided into four microfacies based on hydrodynamic condition: inter-shoal sea, low-energy shoal, medium-energy shoal, and high-energy shoal, and each microfacies has a good correspondence with FMI.

2. The vertical evolution of sedimentary microfacies is mainly manifested in the development scale and migration of the intra-platform shoal. In the study area, the shoals were mainly punctate with poor connectivity in general. The high-energy shoal with good connectivity is concentrated in the third member of the Yijianfang Formation, and the medium-energy shoal has better connectivity in the second member. Favorable shoal facies reservoirs are distributed in the southern, eastern, and central parts of the study area. The environment is given priority with high-energy shoal.

3. Sequences control the distribution of high-energy sedimentary facies. The high-energy shoal developed relatively well in the HST. Moreover, due to prolonged or short-term exposure and leaching at the sequence boundaries, the intensity and distribution of karstification are controlled. According to production data, reservoirs are also more developed in areas where shoals develop better. Therefore, sequences and sedimentary facies have a favorable controlling and modifying effect on reservoirs.

All relevant data are within the paper.

The authors declare that there is no conflict of interest regarding the publication of this article.

This work was supported by the National Natural Science Foundation of China (42102151 and U24B6001) and the Fundamental Research Funds for the Central Universities (2-9-2022-034).

We are grateful to the SINOPEC Research Institute of Exploration and Development, Northwest Oilfield Branch Company for providing detailed data and support. Thanks to Man Xiang and Yaxin Shang for their help in core observations and preparations. Thanks Yu Li for providing assistance with the materials. We also thank the Editor and anonymous reviewers for their constructive suggestions and critical comments that significantly improved the manuscript.

AC

acoustic log

CNL

compensated neutron

DEN

density log

FMI

Full-bore microscan imaging

GR

Natural gamma ray log

HST

high-stand system tract

RD

deep dual lateral resistivity

RS

shallow dual lateral resistivity

SB

sequence boundary

TST

Transgressive System Tract