Introduction

Most of the terrigenous clastic rocks in China’s petroliferous basins are continental sediments, while marine sandstones and mudstones are found only in the Silurian–Devonian in the Tarim Basin, the Pinghu to Baoshi Formations in the East China Sea and the Benxi Formation in the Ordos Basin. Due to the small proportion of marine facies and limited outcrops, they are not intensively researched overall. The Benxi Fm in the Yanchang gas field is an important gas-producing horizon. Although the gas field has been known for many years, there are many differing opinions on the sandstone microfacies and its distribution and relationship with reservoir formation in the Benxi Formation due to the sparsity of wells and decreased correlation in the early stage. Previously, some people believed that the sandstones of the Benxi Fm were fan deltas (Chen et al. 2010) or shallow water deltas (Li 2011), and later, they were believed to have formed tidal channels (Duan et al. 2015). With an increasing amount of in well data and in-depth research, more researchers are beginning to believe that these sandstones mostly consist of barrier island sandbodies (Wang et al. 2016; Yu et al. 2017; Su et al. 2017; Hou ZS et al. 2018; Gao 2019; Wang et al. 2021), while others believe that they include tide-dominated deltas (Hou et al. 2018). The clastic sediments of the Benxi Formation in the central basin mostly originated from the Yinshan orogenic belt in the northeast, the eastern Qilian orogenic belt in the southwest, and the Qinling orogenic belt in the southeast (Lin et al. 2013; Jia et al. 2019). However, barrier sandbars often form in coastal areas with small to medium tidal ranges and weak wave environments (Galloway and Hobday 2012). When wave action intensifies or paleotopography changes, the barrier is destroyed or reworked into shoals or shoreface sandbodies (Short 1992). Due to the deposition of the Benxi Fm on an unconformity with a hiatus of 140 million years, the ancient topography was uneven, and along the grooves of the ancient topography, tidal inlets or channel sandbodies were typically thickened (Fitzgerald et al. 2012). When the wave energy is strong, either small, isolated, dispersed shoal or large scale, contiguous shoreface sandbodies can form. Therefore, there is a close relationship between the microfacies types of sandbodies and their scale and planar distribution (Reynolds 1999). Developing a method to distinguish different microfacies of sandbodies and determine their dimensions, morphology and extent is important in the geological evaluation and development deployment of gas fields.

In the absence of 3D seismic data, we lack a geological basis for whether a layer of sandstone in the Benxi Fm belongs to a tidal channel, barrier bar or shoal. In recent years, while accelerating the process of producing 10 million tons of oil equivalent gas in the Yanchang gas field, more than 1446 wells have been drilled in the study area, and the average well spacing in some areas has reached 2–3 km. Therefore, these drilling data can be used to determine the sandbody microfacies type and distribution tendency. Based on core, thin section, logging and gas testing data, we studied the paleogeomorphology, sedimentary microfacies and reservoir distribution of the Benxi Fm in this area. We believe that there are some similarities in the sandstones of the Benxi Fm according to the survey and that there are also some differences in the microfacies and distribution between the northern and southern regions of the shoreline.

The main progress made in this research is that with the transition from weak waves to strong wave coasts, barrier sandbars are transformed into scattered shoal and tidal channel microfacies due to the influence of ancient landforms, resulting in smaller sandbodies and poorer directionality. Perhaps because the wave action had not yet reached a very strong level or due to the limited supply of sandy sediments, no contiguous shoreface sandbodies were formed in the study area.

Geologic background

The Ordos Basin belongs to the western part of the North China Block in the Paleozoic and was transformed into a sedimentary basin surrounded by mountains in the Meso–Cenozoic. The basin covers an area of 3.7 × 105 km2 and can be divided into 6 secondary structural units, namely the Yimeng Uplift, Western Margin Thrust Belt, Tianhuan Depression, Yishan Slope, Jinxi Flexion Fold Belt and Weibei Uplift. The study area is in the mid-eastern Yishan slope of the Ordos Basin (Fig. 1).

Fig. 1
figure 1

a Location map of the Ordos Basin; b Ordos Basin structural unit map and location of the study area

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The Ordos Basin is a large, stacked basin that has undergone multiple phases of tectonic evolution and is presently a west-dipping slope with a dip of less than 1 degree. The basin has undergone four evolutionary stages: the lower Paleozoic carbonate rock platform, upper Paleozoic marine-continental transitional basin, Mesozoic continental depression basin and Cenozoic marginal rift basin. Except for the absence of Silurian–Devonian and Paleogene and Neogene strata in the basin interior, the basin is well developed stratigraphically (Fig. 2).

Fig. 2
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Comprehensive stratigraphic profile of the Upper Paleozoic strata in the Ordos Basin

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In the early Paleozoic, the Central Paleo Uplift formed in the southwestern part of the basin due to early regional extension and late extrusion. The study area was located east of the Central Paleo Uplift, and the end of the Caledonian movement caused the uplift of the Ordos Basin to receive up to 140 million years of denudation, resulting in the hiatus of the Silurian and early Carboniferous on the whole basin and intense erosion of the Central Paleo Uplift until the Permian Central Uplift gradually died (Yang et al 2013; He et al. 2020).

Oil and gas are distributed throughout most of the basin, mainly in the central part of the basin on the Yishan slope, and are mostly found in the Triassic, Jurassic, Carboniferous–Permian and Ordovician formations (Yang et al. 2005). The oil source rocks are mainly the lacustrine shales of the Late Triassic Yanchang Formation, and the gas source rocks are mostly the coal seams and dark mudstones of the Benxi–Shanxi Formation (Fig. 2).

In the past two years, the focus of exploration and research has shifted to Triassic shale oil and Permian coalbed methane development. Based on the wide azimuth 3D seismic technology of the Loess Plateau and the fine evaluation and fracturing technology of horizontal reservoir segmentation and hierarchy, the first intact shale oilfield in China, with proven reserves exceeding 1 billion tons, was discovered, and a shale oil development and production base with an annual output of 2 million tons was established in Longdong, Gansu Province. In 2022, shale oil production reached 2.21 million tons, accounting for more than two-thirds of the total shale oil production in China.

The first 100 billion cubic meter deep coalbed methane field in China, the Shenfu Deep Coalbed Methane Field in the 2000 m buried Permian, was discovered at the eastern edge of the Ordos Basin, with a proven geological reserve of more than 110 billion cubic meters. The thickness of a single layer of coal ranges from 6 to 23 m, with an average gas content of 15 m3/ton coal. The maximum daily gas production of a single well is 26,000 m3.

The exploitation of low-permeability and tight sandstone gas in the Ordos Basin is in the stage of reducing costs and increasing efficiency. The Benxi Fm gas reservoir studied in this article is a low-permeability and tight sandstone gas reservoir.

The Benxi Fm is overlain by the Taiyuan Fm limestone and underlain by Ordovician limestone that is widely distributed in the eastern part of the Ordos Basin. Its thickness is generally 20–70 m, thinning and pinching out toward the central ancient uplift in the western part of the basin. The Benxi Fm is divided into two members, namely Ben 1 and Ben 2, which are slightly equal in thickness or slightly thicker than Ben 1 Mem. The lithologies of Ben 1 and Ben 2 mainly consist of dark gray mudstone, carbonaceous mudstone, muddy siltstone and siltstone interbedded with sandstone. Most wells in the lower Ben1 region contain aluminiferous rocks (bauxite, mud bauxite and bauxitic mudstone), as well as 1–5 m of coal. Some wells in the Ben1 Mem deposit contain 1–2 m of limestone (Fig. 2). The top of the Benxi Formation is often bounded by approximately 1–5 m of coal from the Taiyuan Formation, and the bottom boundary is in unconformable contact with the lower Paleozoic carbonate rocks.

Data and methods

This paper is based on the results of two research projects entrusted by the Yanchang Oil and Gas Exploration Company. One project involved “Dedicate gas reservoir characterization of the Benxi Fm in the Ganquan–Fuxian area” in the southeastern basin; the other project involved “Sedimentary characteristics and favorable area prediction of the Benxi Fm in the central area of the Yan’an gas field” in the middle basin. The two adjacent research areas cover a total area of 8498 km2, with 1446 wells drilled. The distribution of development wells is uneven, and the average well spacing in the dense well area is approximately 2–3 km. There are four gas-producing formations in the study area, i.e., He8, Shan23, the Benxi Fm. and the Ordovician system. There are 296 test gas wells, 78 of which produced industrial gas in the Benxi Fm. All the wells underwent comprehensive logging, including natural gamma GR, acoustic AC, density, neutron, resistivity, etc., and the logging sampling rate was 0.125 m. Thirty-four wells were cored in the Benxi Fm., and two wells were cored for the whole Benxi Fm. Rock petrophysical samples, such as porosity, permeability, cast thin section, electron microscope and mercury injection samples, were collected and tested at most core intervals.

To analyze the microfacies types and plane extension trends of the sandbodies in the Benxi Fm., we carefully observed all the cores of the Benxi Fm. We mostly carried out rock and mineral type determination, sedimentary structure analysis, thin section examination, logging facies analysis, etc. By logging lithologic identification and statistics, a map of the percentages of Ben1 and Ben 2 Mem sandstone content in the study area was generated, and the distributional morphology and dimensions of the sandbodies in the small well spacing area were used as important bases for determining the microfacies type and paleocurrent direction of the sandbody (Reynolds 1999).

Three methods, including the impression method, paleogeological map method and first lithologic type and distribution on the unconformity surface, were used to analyze the karst paleogeomorphology and paleoenvironment of the earliest depositional stage of the Benxi Fm on the unconformity between the top Ordovician carbonate rocks and their control on the distribution of sedimentary microfacies and sandbodies.

Overall sedimentary characteristics of the Benxi Fm

The Yanchang Gas Field in the study area starts from southern Jingbian County in the north and ends at Huangling County in the south, with a length of approximately 150 km. It spans from west Zhidan County to east Yanchuan County, with a length of approximately 120 km (Fig. 1). The Benxi Formation is generally composed of terrigenous clastic coastal deposits. Spindle worm marine fossils are seldom observed in the core, and sandstone mainly occurs as microfacies such as barriers, tidal channels and shoals; its planar morphology, scale and extension trend all share some commonalities with terrestrial clastic coastal sedimentation.

The Benxi Fm is a sedimentary system of barrier coastal terrigenous clastic rocks developed on the Ordovician karst unconformity. The sedimentary structures in the cores mainly reflect the coexistence of tidal and wave sedimentation (Tavener 1982). The major facies indicators related to tides we observed in the cores included the erosion surface and its lag gravels, graded beddings, herringbone cross bedding sandstones, double or single clay laminations, wave beddings, lens beddings and flaser bedding. The sedimentary structures related to the coast are mostly low-angle cross-bedded and planar-bedded sandstones (Bridges 1976; Hobday 1978). Based on the differences in sandstone composition and textural maturity observed in thin sections, as well as in the logging facies, the sandstone microfacies of the Benxi Fm. These zones are considered to consist mostly of tidal channels, barriers and shoals, followed by a small number of washover fans and tidal deltas.

Petrology of the Benxi Fm

From south to north, bottom to top, there are four main rock types in the Benxi Fm., namely bauxite, coal seams and carbonaceous mudstone; sandstone to pebbly sandstone; limestone; and dark mudstone, reflecting various environments, such as epigenetic, exposed, strongly hydrodynamic and shallow to deep water sedimentation.

The first type of lithology is mostly composed of bauxite and bauxite mudstone at the bottom of the Benxi Formation and manifests as a sedimentary environment associated with a shallow lagoon. Bauxite is a kind of sedimentary rock composed of aluminum hydroxide. Its major mineral composition includes boehmite, clay, iron oxide and other trace minerals. The main element analysis of the whole rocks of Well Yan 1003, Well Yan 1227, Well Test 8, Well Yan 650, Well Yan 816 and Well Yan 818 reveals that the Al2O3 content is between 6 and 38%, indicating that these rocks are bauxite mudstones (Fig. 3a).

Fig. 3
figure 3

Partial core photos of the Benxi Fm. A The gray bauxite of the Y818 Well as the marker of the bottom Benxi Fm.; b and c the root clay of Well S8 indicating the exposed environment at the top barrier islands; d an abrupt change from mudstone to coarse sandstone at the tidal inlet; e graded bedding in fine gravel sandstone at the tidal inlet. See Fig. 10a and Fig. 15 for well sites

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The second type of lithology is mostly coal, carbonaceous mudstone and root soil rock; these lithologies are mostly found in delta plain or supralittoral zone swamps or mudflat of exposed environments (Fig. 3b, c).

The third type is sandstone, mostly fine- to coarse-grained and locally containing pebbles, with a light gray color and medium to thick layers. Lag gravels are mostly subrounded, accumulate near scours (Fig. 3d), form graded bedding (Fig. 3e) and change upward to tabular cross-bedded, trough cross-bedded, low-angle wedge cross-bedded, parallel bedded or massive sandstones.

The fourth type of lithology is mostly black and gray shale, which represents relatively deep water weakly reducing environments such as deep lagoons or mud shelves. Plant debris sometimes exits in mudstone, and thin to medium layers or massive mudstone, sandstone and siltstone are interbedded.

Sandstones have good compositions and moderate to poor textural maturity

Thin section observations under a microscope show that the compositional maturity of the Benxi Fm sandstone is generally high, with quartz and siliceous rock debris contents generally reaching approximately 95%. However, the textural maturity ranges from medium to low, with most sandstones being moderately sorted and subangular to subcircular (Fig. 4a, b, c). Some sandstones have low textural maturity with muddy gravels or a 5%-10% muddy or volcanic dust matrix (Fig. 4d). This moderate textural maturity might result from the hydrodynamic characteristics of tidal currents, which lack the washing effect of high-energy waves. A small amount of low textural maturity reflects rapid accumulation of storm or tidal wash masses or is related to the mixing of volcanic debris.

Fig. 4
figure 4

Microscopic photos of high-compositional maturity and medium to low textal maturity Benxi Fm sandstones. a Light gray medium sandstone with moderate sorting and roundness in Well Y1050; blue indicates pores; red indicates calcite; b same as (a), orthogonal; c light gray medium sandstone, moderately sorted and rounded in Well Y1126, single polarization; d deep gray fine sandstone in Well Y1059, containing a volcanic dust matrix, poor sorting and roundness, single polarized. See Figs. 10a and 11b for well sites

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Both tidal and coastal sedimentary structures exist

Tabular cross-bedded and trough cross-bedded sandstones are common in the cores of the Benxi Fm. This type of bedding can exist in various environments, such as shorefaces and tidal channels. In the study area, bidirectional cross bedding sandstones also occasionally occur, which reflects the reverse flow of rising and falling tides with the same intensity and should be typical tidal channel deposits (Fig. 5 a, b). Double or single clay laminae (Fig. 5c), wavy (Fig. 5d), lenticular (Fig. 5e) and flaser bedding can also be observed, which are sedimentary characteristics of tidal channels and tidal flat environments.

Fig. 5
figure 5

Sedimentary structures observed in the core of the Benxi Fm. a Herringbone cross-bedded fine sandstone in the Yan 600 Well; b herringbone cross-bedded fine sandstone in Yan 887; c double or single clay laminae in sandstone; d wavy cross bedding; e lenticular bedding; f low-angle wedge-shaped cross-bedded sandstone; g planar-bedded sandstones. See Fig. 11b for the wellsite

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Sandstones possibly related to coastal waves were observed in the cores. As earlier researchers noted (Bridges 1976; Hobday and Tankard 1978; Tavener 1982; Fitzgerald et al. 2012), these sandstones contain low-angle cross bedding (Fig. 5f) and planar bedding (Fig. 5g).

Sandstone bottoms undergo abrupt or gradual changes from mudstone

According to research on barrier islands in Europe and North America, the scours at sandstone bottoms and lag gravels represent tidal inlet depositions (Fitzgerald et al. 2012). Underlying the scours are often mid to deep lagoons or tidal flat mudstones, siltstones and swamp peat. On the natural gamma curve, the sandstone is mostly box shaped, with a small amount of bell and high-amplitude serrated shapes.

Similar depositional phenomena appear in the cores of the Benxi Fm in the study area. Most of the sandstones or fine conglomerates of the Benxi Fm have sudden contact with the underlying mudstone, and scours and lag pebbles frequently occur on the sandstone bases (Fig. 6a, b). These differences were related to tidal inlets at the bottom of the barrier islands. Underlying the erosion surface are often sedimentary microfacies such as mid-deep lagoons or low-energy mudstones, tidal flat siltstones and mudstones and swamp peat. The top boundary of the sandstone mostly experiences abrupt contact with the mudstone, after which the boundary area partially changes gradually. On the natural gamma curve, the sandstone is mostly box shaped, with a small amount of bell-shaped and high-amplitude serrations.

Fig. 6
figure 6

Core photos of the Benxi Fm in the study area indicating the abrupt change relationship between the bottom of the tidal inlet sandstone and the mudstone. a The scour of fine conglomerate with graded bedding inside in the Y864 Well; b the bottom of sandstone abruptly changes from mudstone in the Y1003 Well. See Figs. 10a and 11b for well sites

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Approximately one-third of the bottom boundary of sandstone gradually changes with respect to mudstone. The natural gamma logging curve is funnel-shaped or combined with a funnel shape. For example, the Y1045 well is located on the northeast boundary of the First Barrier. The logging and core data comprehensively indicate that the Ben 2 Mem block has a reverse coarsening cycle upwards, and it is comprehensively inferred that this event was an ebb tide delta (Fig. 7). In general, scours are common sedimentary structures at the bottoms of tidal channels and barrier bars.

Fig. 7
figure 7

The core and logging section from the ebb tidal delta to the barrier, shoal and lagoon in Well Y1045. The well site is shown in Fig. 10a

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Localized storms and tuff and limestone deposition in two cycles from transgression to regression

The fully cored Well S8 has relatively deep mudstone sedimentary backgrounds in both Ben 1 and Ben 2. In addition, storm-related sediments, tuffaceous sandstone and volcanic breccia tuff can seldom be observed. The mudstones in the Benxi Fm of Well Yan377 contain 20–30 cm volcanic breccia tuff locally (Fig. 8a), and the lagoon mudstones in Ben 2 of Well Yan1227 and Ben 1 of Well Yan887 contain dilacerated argillaceous debris scattered in sandstone, which may have formed by the scouring, agitation and redeposition of deeper mudstones by storms or may belong to the washover fan environment near barrier bars (Fig. 8 b, c, d).

Fig. 8
figure 8

a Sedimentary volcanic breccia tuff in Well Y377; b dilacerated argillaceous debris scattered in sandstone from Well Y1227; c dilacerated and short-distance staggered argillaceous debris scattered in calcareous mudstone; d storm-scouring related sandy and muddy debris. See Figs. 11b and 15 for well sites

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Locally, 1–2-m-thick layers of limestone occur within the Benxi Fm. These features can be recognized according to logging curves with acoustic time differences (ACs) < 190 μs/m and resistivities (Rd) > 2000 Ω m. The thin limestone of the Benxi Fm in the study area is typically less than 3 m thick for a single layer and is developed in both the Ben 1 and Ben 2 Mem layers. On the plane, they are distributed sporadically between barrier bars, slightly thicken within shoals, and vertically alternate with sandstones or mudstones (Fig. 9).

Fig. 9
figure 9

Cross-sections of wells linked through different microfacies and lithologies in the Benxi Fm. (see Fig. 11b for the section location). See Fig. 11b for the well site

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Based on a comprehensive interpretation of the core, logging lithology and sedimentary facies, two transgression to regression cycles occurred in the Benxi Fm. Ben 2 and Ben 1 experienced depositional processes of water deepening and shallowing, respectively (Fig. 9).

Differences in sandstone microfacies and distribution between the southern and northern fields

According to the analysis of the sedimentary characteristics of the Benxi Fm in the Ordos Basin, the Ganquan–Fuxian area in the southern part of the gas field is a typical weak wave barrier lagoon system, while the Ansai and Jingbian areas in the northern part of the gas field are part of a stronger wave barrier coast system.

Under the condition of a dense well network with an average well spacing of 2–3 km, through statistical analysis of the sandstone thickness and percentage of Ben 1 and Ben 2 Mem in nearly 1500 wells drilled in the Yanchang gas field, the sandstone percentage contour and sedimentary facies maps of the Ganquan to Fuxian area in the southern part (Fig. 10) and the Jingbian to Yan’an area in the northern part (Fig. 11) of the study area were constructed. The size, morphology, and distribution of the sand bodies in the Benxi Fm. differ significantly between the southern and northern fields.

Fig. 10
figure 10

Plans of the Sand-to-Ground Ratio and Sedimentary Phase Planes of Ben1 (a) and Ben2 (b) in the Ganquan–Fuxian Area

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Fig. 11
figure 11

Sandstone content percentages and sedimentary facies map of Ben1 (a) and Ben2 (b) in the northern part of the Yanchang gas field; the map location is shown in Fig. 15

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The Benxi Fm in the southern part of the Yanchang gas field features a medium–low tidal range and weak wave environment, forming a typical barrier sandbar and backbarrier sedimentation. The depositional pattern is shown in Fig. 12. Two large barrier sandbars extending northwest parallel to the shoreline occurred in Ben 1 and Ben 2 Mem, with each barrier being 5–15 km wide and 40–50 km long. The barrier sandbar is composed of bead-shaped sandbodies, with a general thickness of 4–10 m and a maximum thickness of nearly 20 m. The landward area from the barrier and the area between the two barriers are lagoon and tidal flat deposits (Fig. 10).

Fig. 12
figure 12

(Modified from Hudock et al. 2014)

Composition Model of Barrier Lagoon Coastal Sedimentary Units

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The northern part of the Yanchang Gas Field, namely the Jingbian South Yan’an area, features a medium–low tidal range and a strong wave environment. The Benxi Fm sandbody mostly features three microfacies, namely reworked barrier sandbars, shoals and tidal channels (Fig. 11). Due to wave modification and rework, the barrier sandbar is relatively small and is mainly distributed close to the western shoreline, extending northward to the northwest. The width of the barrier sandbars of Ben 1 Mem is 2–3 km, the length is 10–20 km, and the general thickness is 3–6 m. The width of the barrier sandbars in Ben 2 Mem is 3–5 km, with a length of 10–15 km, a general thickness of 10–20 m and a sandstone content percentage of 30–50% (Fig. 11).

The individual size of the shoal sandbody is small, with a diameter of generally 1–3 km and a thickness of approximately 5–15 m. The sandstone content is 10–30%, and it is isolated and scattered between barrier sandbars and tidal channels (Fig. 11).

According to their plane shape, shoals can be divided into three types. We call the narrow-banded sandbody the shoal ridge, which has a length-to-width ratio greater than 3:1. The short-axis shape is called the shoal bar, with a length-to-width ratio of 3:1–2:1. The nearly circular shape is called an oval shoal, and the length-to-width ratio is less than 2:1. The northern part of the Yanchang field includes numerous dispersed oval shoal sand bodies, some of which form shoal ridges or shoal bars and barrier bars. The different shoals are related to similar ancient landforms that exist on the unconformity between the top Ordovician carbonate rocks. Scattered oval-shaped shoals are also found on the modern Great Barrier Reef in the northern Australia Sea. Similarly, on the plane, small shoals in the northern Yanchang Gas Field appear nearly oval shaped, with a diameter of 1–3 km, weak directionality or irregular distribution. There are more than 20 dispersed shoals in all.

Tidal channel sandstone is mainly distributed at the northernmost end of the study area, filling nearly east‒west-extended incised valleys approximately perpendicular to the ancient coastline. The sandbody is 3–10 km wide, 20–30 km long, and 10–20 m thick and has a sandstone content of 30–50%. This tidal channel corresponds to an east‒west downward incised valley on the pre-Carboniferous paleogeological map (Fig. 13). Due to strong erosion and downward cutting, the Ma53 to Ma541 Sub Mem are exposed along the incised valley, while the Ma51 Sub Mem to M6 Mem are exposed on both sides of the valley (Fig. 13). The thickness of the Benxi Fm filled inside the incised valley is also significantly greater than that on both sides, with increases in sandstone content and thickness. The phenomenon of thick tidal channel sandstones deposited along unconformity valleys has also been reported in Quaternary barrier coastal sediments (Johannesen et al. 2010; Fitzgerald et al. 2012).

Fig. 13
figure 13

Pre-Carboniferous paleogeological map of Jingbian southern Yan’an area. The map location is shown in Fig. 15

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Previous studies have shown that barriers mainly occur in clastic coastal areas with small to medium tidal ranges and weak wave environments. Sandbodies in tidal environments with large tidal ranges are likely to be transported and accumulate into tidal bars arranged perpendicular to shorelines, but barriers are not present (Galloway and Hobday 2012). In a strong wave environment, the barrier will be reworked by waves to form beaches or shoals or will be completely transformed into a coastal sedimentary system. The modern west clastic coast of the Netherlands is of this type, where barrier sandbars and beach sand are distributed together. Multiple barrier sandbars are parallel to or at low angles to the coastline, and beach sandbodies occur on the landward side of the barrier (Short 1992). The sand bodies in the northern Yanchang field are mainly distributed on the basinward side of the barrier, which is slightly different from the situation in the Netherlands Peninsula. This difference might be related to the control of unconformity landforms on the seabed topography and sandbody distribution.

Discussion

Comparison between previous and present studies on barrier changes along the coast

We have seen only two cases of distribution and morphological changes in modern barrier sandbars along coasts and no reports on barrier changes along coasts during geological history, except for three cases of longshore microfacies changes in marine and lacustrine environments.

On the west coast of the central Netherlands, the barrier is modified by waves and coexists with beaches. Approximately ten barrier sandbars parallel to or at low angles intersect with the shoreline and are located at water depths of 10–20 m (Short 1992). Beach sands are present in water bodies at depths less than 10 m between barriers and shorelines.

Another example is the relationship between the modern Red River Delta front and barrier sandbars in Vietnam. Under the action of strong winds in the northeast direction, waves are formed to transport some delta sand to the southwest side of the delta front, forming two barrier sand bars that are close and parallel to the shoreline. They are approximately 7 and 10 km long and approximately 1 and 3 km wide, respectively, bordering a water depth line of 2 to 6 m; this indicates that delta and barrier sandbars can coexist along the coast and are adjacent to each other (Maren 2005).

The Middle Jurassic Fort Member Jaisalmer Formation in the western Rajasthan region of India consists of approximately 70 m of sandstone interbedded with mudstone. From bottom to top, there is a transformation from tidal-influenced fluvial to tidal flat to coastal environments, which may represent the transformation of the sedimentary environment of the Fort Member from west to east along an estuarine shoreline. Sandstones in three different environments commonly feature trough cross bedding, planar cross bedding and parallel bedding. In tidal flat sediments, there are thin interlayers of sand and mudstone, flaser bedding and Herringbone cross-bedded sandstones. In coastal sandstones, there are symmetrical ripple marks and hummocky cross beddings (Ahmad et al. 2020).

The Miocene carbonate rocks near the east coast of Saudi Arabia are interbedded with 5–10 m estuarine coastal shallow sea sandstone, which also has local characteristics caused by wind and wave modification and sea level decline. However, the sedimentary microfacies characteristics of this sandstone can be observed only on an outcrop profile approximately 5-km-long perpendicular to the shoreline, and no microfacies changes parallel to the shoreline have been observed (Ali et al. 2021).

There are three types of lacustrine beach bars in the Fourth Member of the Paleogene Shahejie Formation in the northern Jiangsu Sag, China, namely shallow littoral lacustrine sandy beach bars, semideep lacustrine storm-related sandy beach bars and shallow littoral lacustrine carbonate beach bars. The formation and distribution of these rocks are related to the supply of terrestrial clastics and the control of lake level rise and fall (Gao et al. 2017).

The subject of our study was the Carboniferous barrier coastal system. Due to the long ancient coastline involved, which was more than 150 km long, there was a longshore change from a barrier to other microfacies. In line with the findings of previous studies, both of these studies involve the reworking effect of waves on sand bodies. The difference is that the beaches on the modern coast of the Netherlands are developed on the shoreline side of the barrier, with a large scale and continuous distribution. The shoals of the Benxi Fm in the Ordos Basin developed on the seaward side of the barrier, with small single sand bodies and a scattered distribution. Compared to the second example of previous studies, we have not yet seen the phenomenon of deltas providing sandy debris for barrier sandbars.

Impact of ancient coastlines and provenance

During the sedimentation of the Benxi Fm, there was an ancient uplift erosion area in the central part of the Ordos Basin that became increasingly lower toward the north (Fig. 14). The Benxi Fm is missing from the uplift, and the strata deposited to the east and west of the uplift are called the Benxi Fm and Yanghugou Fm, respectively. The analysis of heavy minerals, clastic zircon ages, and trace elements in the rocks of the Benxi Fm indicates that the sand and mud in the Yanchang gas field mostly originated from the Yinshan Mountains to the northeast, the West Qinling Mountains to the southwest, and the East Qinling Mountains to the southeast (Lin et al. 2013; Jia et al. 2019). The boundary between the northeast and south mass supplies roughly extends in a northwest direction, from Well Y582 south of Jingbian to Well Y528. The boundary between the sediment from the southwest provenance and the sediment from the southeast provenance extends roughly north to northeast and is located along Well Y1051 to Well Y528 (Fig. 15).

Fig. 14
figure 14

Paleogeomorphic framework during sedimentation of the Benxi Fm in the Ordos Basin, with the Yanchang gas field shown in the red box

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Fig. 15
figure 15

Distribution of heavy minerals and provenance of the Benxi Fm sandstone in the Yanchang gas field

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As shown in Figs. 14 and 15, the sediments of the Benxi Fm in the Yanchang gas field mostly originated from the southeast and southwest and partially from the northeast. Ganquan to Fuxian County is adjacent to the northwest-oriented ancient coastline, which was the uplift boundary of the Central Ancient Uplift. Therefore, it is relatively close to the ancient coastline of the southwestern provenance, forming large barrier sandbars extending northwestward (Fig. 10 and Fig. 14). The sediments in the Benxi Fm from Yan’an to southern Jingbian mainly originated from the southwestern provenance and may have formed coastal barrier sand bodies in the nearshore area on the western side (Fig. 11 and Fig. 14). In Zichang and Yanchuan Counties, which are further from the western coastline and close to the interior of the epicontinental sea, the sediments may be more affected by northeasterly winds and wave action, forming many small wave-controlled shoal sandbodies arranged subtly northeast (Fig. 11 and Fig. 14).

Therefore, the changes in the microfacies and distribution of sand bodies in the Benxi Fm from south to north in the Yanchang gas field are also related to the ancient coastline and distance from the provenance. That is, coastal barrier sandstones developed better in areas close to provenances and shorelines, and shoal sandstones tended to form in areas far from provenances and shorelines.

Impact of ancient landforms

Karst paleogeomorphology on the unconformity of the top Ordovician carbonate rocks in the southern and northern parts of the Yanchang gas field significantly impacted the sandstone thickness and distribution, but the control patterns of sandbodies by paleogeomorphology in the southern and northern regions were slightly different. The ancient landform in the southern region is a northeast-dipping slope superimposed with two northwest-trending ancient grooves and protrusions (Fig. 16), forming two barrier sandbars in the Benxi Fm that also extend northwest (Fig. 10). The thickest barrier sandbar in the Benxi Fm is distributed neither within ancient protrusions nor within ancient grooves but within the transition from grooves to protrusions. An ancient protrusion occurred between the first and second barriers, which might have been supratidal and intertidal zones with shallow water depths. To the southwest of the first barrier and northeast of the second barrier were ancient geomorphic grooves belonging to the subtidal zone, lagoon and shelf mud environments, respectively (Figs. 10, 16).

Fig. 16
figure 16

Thickness map of the Benxi Fm in Ganquan to Fuxian County, indicating the paleolandform of the unconformity on top of the Ordovician carbonate rocks. The map location is shown in Fig. 15

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However, the sandstones of the Benxi Fm in the northern part of the Yanchang gas field are mostly thickened along ancient karst grooves. Whether the tidal channels filled the east‒west incised valley to the most north, the north to northwest extending barrier bars, or the scattered shoals, their sandstone thickening positions are consistent with the paleogeomorphic grooves (Figs. 9 and 11). This thickening of the Benxi Fm sandstone may have resulted from the existence of previous karst grooves (Fig. 17).

Fig. 17
figure 17

A schematic section of sandstone thickening in the Benxi Fm in the karst grooves in the northern Yanchang gas field. The section position is shown in Fig. 16

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Control of the sandbody on the gas reservoir

The Paleozoic natural gas in the Ordos Basin entered a stage of rapid increase in reserves and production. In 2023, gas production is expected to reach approximately 70 billion cubic meters, of which 60 billion cubic meters will be produced by the PetroChina Changqing Oilfield and 10 billion cubic meters will be produced by the Shaanxi Yanchang Gas Field. Approximately half of the gas produced by the latter comes from the Benxi Fm sandstone reservoir, and the Benxi Fm sandstone gas reservoir is the main potential contributor to future productivity increases. Research on microfacies changes, morphology, scale and extension in the Benxi Fm sand body has important guiding significance for the deployment of development wells and the enhancement of production capacity.

Compared to other gas reservoirs, such as the He8 Mem, Shanxi Fm, and Ordovician reservoirs, the Benxi Fm gas reservoirs have relatively greater single well production capacities and total gas outputs; the main reason is that the barrier and coastal sandstones are the largest sandstones in the paralic facies except for braided rivers (Reynolds 1999; Meng et al. 2023), which have large thicknesses, wide distributions and high component maturities, resulting in good reservoir petrophysical properties. In addition, the Benxi Fm generally contains a set of 4–16 m coal seams and carbonaceous mudstone source rocks; they are overlain by the 10–30 m tight micrite limestone of the Taiyuan Formation as well-qualified cap rocks, which causes the coalbed methane in the Benxi Fm to form a large source reservoir pressure difference and charge the sandstone reservoirs during the gas accumulation period, resulting in the pressure coefficient (over 1.0) of the gas reservoir in the Benxi Fm being slightly greater than that (approximately 0.9) of the other layers. These factors are important reasons for the high production of Benxi Fm gas reservoirs in the Yanchang gas field.

Overall, the southern part of the Yanchang gas field belongs to a weak wave environment with a medium to low tidal range, and the scale of barrier sandbars is large, causing it to be the main contributor to the reserves and to have a high production capacity in the Benxi Fm. The increased wind and wave forces in the northern part of the gas field, coupled with the influence of uneven ancient landforms, led to the reworking of barriers into smaller sizes or into small shoals and inferior production and reserves compared to those in the southern part.

Conclusions

  1. 1.

    Sandstone statistics and sedimentary microfacies analysis of 1446 wells within approximately 150 km parallel to the coastline indicate that the sedimentary environment of the Benxi Formation sandstones in the Yanchang gas field transitions from a typical terrestrial clastic barrier coast to a modified barrier coast environment from south to north. The former was a weak wave environment, mainly consisting of barrier sandbars, backbarrier tidal flats, lagoons and swamps. The latter is a strong wave environment that mainly features three types of microfacies: modified barriers, shoals and tidal channels.

  2. 2.

    Studies on the relationships between shoreline location and paleotopography and sedimentary microfacies have revealed that on the northern barrier coast reworked by strong waves, partial barrier sandstones are transformed into shoals with three planar shapes: elliptical shoals, short-axis shoals and long-axis ridges under the control of uneven ancient landforms. The scale of a single shoal sandbody is relatively small, is scattered far from the shoreline, and is arranged diagonally along the shoreline. The tidal sand body extends perpendicularly to the shoreline and mainly fills in unconformity incised valleys.

  3. 3.

    Longshore changes in sandstone microfacies can affect reservoir formation and production capacity. The barrier sandbars of the Benxi Fm in the southern part of the Yanchang gas field are large in scale, are distributed contiguously and have high reserves and production capacity. The scale of single sandbodies in the northern shoals and tidal channels is small and scattered, with relatively low reserves and production capacity.