Abstract
In recent years, the Ch6–Ch7 deep-water gravity flow deposits of the Yanchang Formation in the Southern Ordos Basin have been characterised as sandy debris flow sediments, a modification from previous sedimentary facies characterisations of braided river delta fronts, deep lacustrine turbidite fans and seismite deposits. This leads to the necessity of a detailed interpretation of the origin and reservoir properties of the deep-water deposits of the Jinghe Oil Field in the Ordos Basin. A large number of core images were analysed, identifying 15 lithofacies and 3 main sedimentary facies, including the sandy debris flow microfacies, turbidite microfacies and seismite-slump microfacies. Sedimentary facies determination was proved by particle size analysis and vertical and horizontal microfacies distributions. The sedimentary process can be described by earthquake and gravity deformation inducing a slide of the large deposit of delta front sediments on the slope break down the slope. Simultaneously, ambient lake water penetrated the sediments, forming seismite-slump microfacies with load structures, liquefied structures and slump deformation structures. With continuous sediment liquefaction, sandy debris flow microfacies, which were massive bedding sandstones, were formed during transportation. Leading the sediments, turbidite flows resulted from flow transformation which were possibly remolded by weak bottom currents in intermittent periods. Studies of reservoir properties and oil shows indicate that sandy debris flow sandstones have the best reservoir properties and oil shows, followed by turbidite sandstones, with seismite-slump sandstones being the poorest. The sandy debris flow and part of the turbidite sandstones have good oil production potential.
Keywords
Introduction
The deep-water gravity flow sediments are new fields for oil and gas exploration and have attracted great attention around the world over the past decades (Li et al., 2010b,c, 2011, 2013; Lowe, 1979, 1982; Shanmugam and Moiola, 1995; Shanmugam, 1997, 2000; Zou et al., 2009). They are widely developed in the centre of the large lacustrine basins in China. It is generally believed that these sediments are mainly turbidite or seismite deposits (Chen et al., 2012; Li et al., 2010a; Xia and Tian, 2007; Yang, 2005; Zhao et al., 2008), which are thin in sediment thickness and small in dimension and have low oil and gas potential. However, in recent years, thick massive bedding sandstones have frequently been found in deep water areas of the Ordos Basin. Many of these bedding sandstones have formed several huge oil fields which contain millions of tonnes of oil reserve. One example is the occurrence of the Ch6–Ch7 Member sandstones of the Late Triassic Yanchang Formation in the Jinghe Oil Field of the Southern Ordos Basin. Understanding the sedimentary facies of the thick massive bedding sandstones in the Southern Ordos Basin has undergone three stages.
In the early times, many researchers thought that the thick massive bedding sandstones were braided river delta front deposition (Chen, 2007; Chen et al., 2009; Liao et al., 2013; Tan and Chen, 2006; Tang et al., 2001; Zhang et al., 2000; Zhao et al., 2008). Later, they were considered to be deep lacustrine turbidite fans and seismite deposits (Chen et al., 2006, 2012; Ding et al., 2011; Li et al., 2010a; Xia et al., 2007; Yang et al., 2005; Zhao et al., 2008). Recently, they are reconsidered to be the deposition of the sandy debris flow (Chen et al., 2012; Li et al., 2010b,c, 2011, 2012; Zou et al., 2009). The researchers who believe that the study area is braided river delta front lack sufficient data of the study area and concluded that the deep lake is in the northern part of the study area. Considering the data provided by other researchers (Chen et al., 2012; Ding et al., 2011) and our research, the study area is semi-deep to deep water at the time of the Ch6–Ch7 Member sedimentation. Therefore, the main argument is the origin of the thick massive sandstones as turbidites or sandy debris flow sediments.
The study of deep water depositional systems began with an understanding of the turbidity current and the establishment of related theories (Bouma, 1962; Kuenan and Migliorini, 1950). In the 1960s, Bouma (1962) established the famous Bouma Sequence based on observations of the vertical combination of lithofacies and sedimentary structures of the turbidite sediments in the field outcrops. Following that, several scholars established submarine fan models in which the Bouma Sequence was the most important content (Mutti, 1977; Normark, 1970, 1978; Walker, 1978). Turbidity current was believed to be the fluid flow regime of the Bouma Sequence. However, this is only one type of the many mechanisms of deep water gravity flow sediment formation. Deep water gravity flow sediments were divided into four types according to support mechanisms: grain flow, liquidized sediment flow, debris flow and turbidity current, collectively referred to as turbidities (Lowe, 1979, 1982; Middleton and Hampton, 1973). The turbidity current is further divided into high-density turbidity current and low-density turbidity current. Mulder and Alexander (2001) proposed a new gravity flow classification based on the physical properties of the fluids and the dominant grain support mechanism. This classification categorised sediment gravity flow as cohesive flow and frictional flow by the cohesivity of particles. According to the sediment concentration in the fluid and the particle support mechanism, the frictional flow is subdivided into three types: hyper-concentrated density flow, concentrated density flow and turbidity flow. Therefore, the thick massive sandstones were interpreted as high density turbidity current (Chen et al., 2012; Ding et al., 2011).
Shanmugam (1997, 2002) proposed the concept of sandy debris flow based on the combination of field outcrop observation, core analysis and laboratory experiments. Shanmugam (2000) believed that the term ‘debris flow’ in a sandy debris flow refers to both depositional products of gravity flow and plastic rheology that forms this product. It is accepted that rheology is more important than particle size distribution in controlling the development of the sandy debris flow. Sandy debris flows can develop in slurries of any grain size (very fine sand to gravel), any sorting (poor to good), any clay content (low to high) and any modality (unimodal and bimodal). Theoretically, the grain flow (non-cohesive debris flow) and the muddy debris flow (cohesive debris flow) can be considered as two end members of the plastic flow, with the sandy debris flow as an intermediate product between the grain flow and muddy debris flow. The difference between the sandy debris flow and the turbidity current is mainly in the following aspects: First, the sandy debris flow is rheologically plastic (with cohesive strength), with a laminar flow regime exhibited as a mass flow with large deformation in the flow process. Therefore, shear structures, such as floating mud clasts in the top or bottom of the sandstones and floating quartz gravels, can be seen. The turbidity current displays Newtonian behaviour (no cohesive strength) with the turbulent flow regime with suspended particles transportation. This allows the graded bedding (A division of the Bouma Sequence) to be seen. Second, the sediment concentration in the turbidity current is relatively low (between 1 and 28%), whereas the sediment concentration is relatively high of 50–90% for the sandy debris flow. Third, the support mechanism of the turbidity current is mainly the turbulence, while the sandy debris flow has multiple sediment-support mechanisms (cohesive strength, frictional strength and buoyancy). Fourth, the clay content, which can be as low as about 5% of specific gravity, can be very low for the sandy debris flow.
In recent years, the characteristics of sandy debris flow sediments have also been found in the Southern Ordos Basin (Chen et al., 2012; Li et al., 2010b,c, 2011, 2012; Zou et al., 2009). These include (1) shear structures in the bottom of the sandstones; (2) floating mud clasts in the top or bottom of the sandstones with inversely graded bedding; (3) the mud clasts are elongated; (4) sharp top and bottom contacts. A sedimentary facies model for the sandy debris flow has been constructed. The development of the sandy debris flow is mainly dependent on the shear strength of sediments of the basin in the constructive delta front near the slope break. Slipping or slumping deformation occurs when the shear strength is equal to or less than the shear force generated by gravity or the earthquake. Sandy debris flow is then formed in the slump deformation period with turbiditic sediments deposited in front of the sandy debris flow (Cao et al., 2017a, 2017b; Li et al., 2011; Pyles and Jennette, 2009).
Although several sedimentary models have been constructed, a lack of sufficient data of the study area led to many interpretations. This paper uses a substantial amount of core observation data, grain size analysis data and sand body thickness data from the well-logging interpretation to understand the deep water sediments of the Ch6–Ch7 Members in the Jinghe Oilfield. The origin, microfacies, facies assembly and sedimentary model have been reanalysed, with an aim to reanalyse potential ‘sweet spots’ of the tight sandstone reservoirs and further provide a geological basis for tight oil exploration and development.
Geological setting
The Ordos Basin is one of the main hydrocarbon basins located in North-central China (Figure 1). The north of the Ordos Basin is adjacent to the Hetao Basin, the south to the Weibei Uplift and the east to the Jinxi Flexure Zone. The west of the basin is proximal to the Western Thrusted Belt. The main body of the basin is the Yishan Slope with an average dip of less than 1°. The Jinghe Oilfield in the Binchang Block is located on the border between the Weibei Uplift and the Yishan Slope, with an exploration area of 3012 km2.
Location, geological map, study area (a) and stratigraphy (b) of the Yanchang Formation, the Jinghe Oilfield, the Ordos Basin.
The Yanchang Formation was mainly developed during the late Triassic period in the Ordos Basin and can be divided into five lithologic intervals and 10 members. The Chang (Ch) 6 and Chang (Ch) 7 Members are deposited in the lacustrine expansion stage of the Ordos Basin, and the deep to semi-deep lacustrine facies are generally deposited in the south of the basin. The Ch6–Ch7 massive sandstones are 20–60 m in thickness with good oil shows and higher productivity discovered by the drilling the thicker massive tight sandstones.
Palaeogeomorphology was steep in the southern and western zones and gentle in the northern and eastern zones in late Triassic of the Ordos Basin (Chen et al., 2012; Ding et al., 2011; Liu et al., 2011; Pang et al., 2012). Liu et al. (2011) gave south-to-north seismic profiles and found that the Ch7 Member is thick in the south with decreasing thickness from south to north, indicating the existence of the slope and slope break. Pang et al. (2012) found that the oil shale thickness contour line is dense in the south and concluded that the Southern Ordos Basin is steep and sloped. At the time of the Ch7 Member sedimentation, the south basin boundary was uplifted and eroded because of the intensified Indo-China movement. These movements led to earthquakes and volcanic activities, which can be proved by the carbonate and tuff debris in the sandstone reservoir of the Ch6–Ch7 Members (Liu et al., 2011).
The sedimentary facies for the Ch6–Ch7 Members are variable. However, the gravity flow sediments and oil shale developed in the Ch6–Ch7 Members of the Jinghe Oil Field indicate the deep to semi-deep lacustrine sedimentary environments (Chen et al., 2006; Fu et al., 2009; Li et al., 2010a; Liu et al., 2011; Lv et al., 2008; Pang et al., 2012; Yang, 2005). The palaeogeomorphology and the deep and semi-deep sedimentary settings provided a favourable background in developing various types of deep-water gravity flows (seismite/slump, sandy debris flow and turbidite) in the Southern Ordos Basin.
Lithofacies and gravity flow sedimentary microfacies in the Ch6–Ch7 Members of the Jinghe Oilfield
Lithofacies of the gravity flow sediments
We carried out detailed observations and analyses for the cores of 34 drilling wells (about 1024 m in length) in the Jinghe Oilfield. Three major lithofacies were identified as the fine-grained sandstone, siltstone and argillaceous facies in the Ch6–Ch7 Members. These can be subdivided into 15 lithofacies (Table 1).
Characteristics of the lithofacies in Ch6–Ch7 Members in the Jinghe Oilfield.
According to the types of sedimentary structures in the cores, the fine-grained sandstone lithofacies set is further divided into: (1) massive fine-grained sandstone (S fm ), (2) massive fine-grained sandstone with floated mud clasts (S fmfc ), (3) inverse graded bedding fine-grained sandstone (S figb ), (4) quasi-parallel bedding fine-grained sandstone (S fpl ), (5) parallel bedding fine-grained sandstone (S fp ), (6) fine-grained sandstone with torn mud clasts (S ftmc ), (7) fine-grained sandstone with graded bedding (S fng ), (8) load structure fine-grained sandstone (S fls ), (9) fine-grained sandstone with the liquefied structure (S fliq ) and (10) fine-grained sandstone with slump structures (S fd ). The siltstone lithofacies set is categorised as the wavy and lenticular bedding siltstone (S sw ), parallel laminae siltstone (S spl ) and siltstone with slump deformable structure (S sd ). The argillaceous lithofacies set is divided into the oil shale (M osh ) and dark grey mudstone (M).
These lithofacies are the products of the sandy debris flow, turbidity current, traction current and earthquake-slump of the deep lacustrine background under the depositional mechanisms of the deep water gravity flow. They are typical gravity flow sediments in the deep-water environment.
Sedimentary microfacies of the deep water gravity flow sediments
Depositional characteristics of sandy debris flow microfacies
The lithofacies types of sandy debris flow sediments are mainly composed of S fm , S fmfc , S figb and S fpl . They are the fine lithic feldspar sandstone and feldspar lithic sandstone (Figure 2).
Photos of the main lithofacies of sandy debris flow microfacies. (a, b) Massive fine-grained sandstone (Sfm) from the Ch7 Member of Well JH8 and the Ch6 Member of Well JH9, respectively. Sections A and B represent the massive bedding sandstone and sliding shear zone. Sharp contact can be seen at the bottom of Section A. (c–g) Massive fine-grained sandstone with floating mudstone clasts (Sfmfc). The torn mudstone clasts are distributed along the bedding in the sandstone. (h) Inversed graded bedding fine-grained sandstone (Sfigb). (i) Quasi-parallel bedding fine-grained sandstone lithofacies (Sfpl). They are products of the plastic current for the sandy debris flow.
Massive bedding fine-grained sandstone lithofacies (Sfm)
Grey and brown in colour, it is fine grained, homogenous and structureless with low clay content and a sharp contact with no erosions at the top or bottom (Figure 2(a)). Thin interbeds of sandstone and mudstone are interpreted as the turbidite and can be found at the top or bottom (Figure 2(b)). Shanmugam (2000) pointed out that despite a very gentle slope, the sandstone can also be transported to the deep lake via hydroplaning. Low angle fractures at the bottom of the sandstone reflect shearing produced from the sliding of the sandstone body. S fm is considered to be a typical lithofacies in the sandy debris flow sediments, which is transported with multiple support mechanisms, such as the cohesive strength, frictional strength and buoyancy.
Massive fine-grained sandstone lithofacies with floating mud clasts (Sfmfc)
Grey, grey and brown in colour, homogeneous and structureless with good oil shows. There are scattered and elongated mudstone clasts in the middle and upper parts of the massive sandstone (Figure 2(c) to (f)). Mudstone clasts are up to 2–6 cm in diameter, exhibit floating, elongated and curled shapes from tears. They have a planar fabric. Floating mudstone clasts in the massive sandstone reflect the properties of the plastic fluid for the sandy debris flow. In other words, a large number of the torn mudstone clasts were made by the erosion of the turbidity current, inserted into the sandstone and were developed in the front or top of the sandy debris flow sediments. The internal strength of the plastic fluid in the transportation process led to the planar fabric of the mudstone clast.
Inverse-graded bedding fine-grained sandstone lithofacies (Sfigb)
It is grey and white in colour, tight and has poor oil shows. Massive fine-grained sandstone and muddy silt-fine-grained sandstones are observed from top to bottom (Figure 2(h)). The formation mechanism of the inverse-graded bedding can be explained by the fact that the subsiding of particles was hindered by the internal strength of the sandy debris flow.
Quasi-parallel bedding fine-grained sandstone lithofacies (Sfpl)
This occurs in grey–white and grey–brown colours. It is interpreted as a product of the laminar flow of the sandy debris flow, which is reflected by the intermittent parallel fabric of oil shows (Figure 2(i)).
Rheologically, the sandy debris flow is a plastic fluid which shows both gravity flow and laminar flow characteristics. Therefore, the C–M diagram of the sand debris flow sediments shows the gravity flow characteristics with a few instances of inconspicuous traction flow (Figure 3). In the study area, the thickness of the single sandy debris flow sediment is generally greater than 0.5 m with a maximum of up to tens of metres and an average of about 8 m. In the Ch6–Ch7 Members in the Jinghe Oilfield, the interstitial material in the sandstone is mainly clay, with an average content of 5.15%
Sandstone grain size analysis diagrams of the Ch6–Ch7 Members of the Jinghe Oilfield. Above: Probability cumulative curves; below: C–M diagram.
Turbidite microfacies
Turbidites are deposited in the Ch6–Ch7 Members, and lithofacies are mainly S ftmc , S fgb , S fp , S sw , S spl and M.
Specifically, the real turbidite lithofacies are composed of both the graded bedding fine-grained sandstone (S fgb ) and the fine-grained sandstone with torn mud clasts lithofacies (S ftmc ). The other lithofacies, such as the parallel bedding fine sand lithofacies (S fp ), wavy-lenticular bedding siltstone lithofacies (S sw ), parallel laminae siltstone lithofacies (S spl ) and deep grey mudstone lithofacies M, belong to products of the traction current. The traction flow can be transformed from the sandy debris flow liquefaction, the bottom current or the contour current (Shanmugam, 2000) in the deep water of the basin. These lithofacies, together with S fgb or S ftmc , constitute the sedimentary sequence of turbidite – the Bouma Sequence, which was formed during the deposition of a single turbidity current event. It can be either complete or incomplete. Therefore, we classify these lithofacies as the turbidite sequence. The fine-grained sandstone facies with torn mud clasts lithofacies (S ftmc ) is a relatively special lithofacies which reflects turbidite deposits in this area (Figure 4). It is greyish white in colour, with elongated torn mud clasts 2–5 cm in size, widely developed in the sandstone. The ends of the torn mud clasts are curly and turbulent. The origin of these mud clasts can be described by the fact that the high energy turbidity current formed at the bottom of the sand debris flow by the flow transformation eroded the lower mud. The mud was then rolled by the turbulence and subsequently formed the turbulent fabrics.
Turbidity channel at the bottom of the sand debris flow. (a) Turbidity sand sheet, multiple uncompleted Bouma Sequences (ABC–BC–AE) overlaid, Ch632 small layer of Well JH4; (b) turbidity channel at the bottom of the sandy debris flow, Ch721 small layer of Well JH4.
Turbidite sandstone sheet with the graded bedding. Ch622 small layer of Well JH21.
Superimposition of BCDE and BE division combinations in the uncompleted Bouma sequence, turbidite sand sheet with bottom erosion, groove and chute. Ch711 small layer of Well JH11.
Groove and chute structure at the bottom of turbidite. Ch711 small layer of Well JH11.
In general, the following characteristics for the turbidite are mainly:
The upward-fining grain size sequence can be seen in the thin sandstone. The graded bedding is formed as particles are supported by the turbulent current and deposited when the velocity of the turbulent current slows down or when there is a decrease in the disturbance intensity of the turbulent current. The bigger and heavier particles will be settled first followed by the finer or lighter particles. This results in the graded bedding in the sediments of the turbulent current. The sandstone with the graded bedding often appears in the form of thin interbeds of sandstone and mudstone and constitutes the multiple rhythmic layering, stable lateral extension and small thickness variation. The thickness of the single sandstone layer ranges from several centimetres to dozens of centimetres, with a maximum of no more than 0.5 m (Figure 5); The sandstone with the graded bedding can be combined with the fine-grained sandstone, siltstone and mudstone with variable sedimentary structures, such as the parallel bedding, wavy bedding and horizontal laminations, to form complete or incomplete Bouma sequences (Figure 6);
From the above analysis, the graded bedding in the sandstone indicates turbidite sedimentation. However, Lowe (1982) believed that B, C and D divisions in the Bouma Sequence were formed by the reduction of the density of turbidity current as a result of the dilution of the turbidity current concentration in the transportation process. This means that there was a transformation of turbidity flow into traction flow. In this process, the parallel bedding (B division), wavy bedding silty fine-grained sandstone (C division) and horizontal lamination siltstone (D division) were generated. Shanmugam (2000) believed that these divisions were formed as sandy debris flow sediments were reconstructed by bottom currents (traction flow). Nevertheless, these non-turbidity current products and graded bedding fine-grained sandstones (A Division) constitute a complete turbidite sequence, a milestone for the turbidite identification.
4. The turbidite sandstone bottom is uneven and sharp-contacted and often possesses a clear groove and chute structure at the bottom of the sandstone (Figure 7). This is co-genetic with some penecontemporaneous deformation structures, such as ball structure, pillow structure, flame-like structure and so on. 5. Torn mud clasts show a rolling state in the sandstone (Figure 4). 6. From the grain size curve, the classical turbidite shows that all of the samples are distributed by following a straight line of C = M on the C–M diagram, reflecting the characteristics of gravity flow (Figure 3).
Seismite-slump microfacies
Slumps are deformable bodies caused by sliding or slumping in deep water settings. There are many kinds of gravity-driven mechanisms which transport sediments from the edge of the shelf down the slope to the foot of the slope or the distal of the basin-plain in deep water environments. These include sliding, slump, debris flow and turbidity flow. The main characteristic of the slumps in the Ch6–Ch7 Members is that sand and mud are mixed with a variety of liquefied and deformed structures. In addition, characteristics of the seismites such as the microfold, microfault, boudinage, ring layer structure, sandy volcano (ejected structure), water escape vein structure, liquefied hydraulic pressure structure, load structure, etc. are often seen in the slumps mixed with the sand and mud. These characteristics indicate that the formation of the slumps was not only caused by earthquakes but transformed by earthquakes with the processes of slump and deposition. Therefore, we call the deformable bodies with features of slumps and vibration as the seismite-slump microfacies. This is the enabling mechanism of the development of the sandy debris flow and turbidity flow.
The main lithofacies of seismite-slump microfacies are the load structure fine-grained sandstone facies (S fls ), liquefied structure fine-grained sandstone (S fliq ) and slump deformation structure fine sand rock facies (S fd ).
The load structure fine-grained sandstone (S fls ) is grey–white in colour. Several kinds of load structures such as the ball structure and pillow structure (Figure 8(a) and (b)), flame-like structure (Figure 8(c)), squeezed structure (Figure 8(d)), boudinage (Figure 8(e)) and ring layer structure (Figure 8(f)) are developed at the bottom of the sandstone. S fls is formed by the differential settlement and vertical compressive shear in the process of rapid sliding and deposition. There are many characteristics of seismite in the S fls lithofacies, for example, the microfold, microfault and seismic mixed rock.
Photographs of typical sedimentary load structures in seismite-slump microfacies of the Ch6–Ch7 Members. (a) Ball and pillow structure. Ch711 small layer of Well JH9. (b) Pillow structure. Ch721 small layer of Well JH17. (c) Flame-like structure. Ch721 small layer of Well JH22. (d) Squeezed structure. Ch622 small layer of Well JH18. (e) Boudinage. Ch632 small layer of Well JH4. (f) Ring layer structure. Ch712 small layer of Well JH9.
The boudinage and ring layer structure are developed in the sandstone and mud interbeds which are compressed by the vertical compressive stress. The plastic rock (mudstone) is vertically compressed towards the rigid sandstone layers in the sandstone and mudstone interbeds. The process results in the extension in the horizontal direction and the vertical shear fracture, which breaks the sandstone layer to form the boudinage. If the interbeds of sandstone and mudstone are very thin in thickness, the ring layer structure is formed. The vertical stress that forms the boudinage or the ring layer structure mainly comes from the vertical vibration of the earthquake. Therefore, this kind of structure reflects the slump by earthquake events.
The liquefied structure fine-grained sandstone (S fliq ) is grey and white in colour. Various types of sedimentary structures have been observed. These include the dish structure (Figure 9(a)), ejected structure (Figure 9(b), (c), (f) and (g)), liquefied convolute bedding structure (Figure 9(d), (h) and (i)) and liquefied breccia structure (Figure 9(e)), all related to the earthquake, slide, slump and liquefaction.
Photographs of typical sedimentary liquefied structures in seismite-slump microfacies (S fliq ) in Ch6–Ch7 Members in the Jinghe Oilfield. (a) Dish structure. Ch611 small layer of Well CW1. (b) Sandy volcano (ejected structure). Ch631 small layer of Well JH10. (c) Liquefied hydraulic pressure structure. Ch711 small layer of Well JH9. (d) Liquefied contorted and diapir structure. Ch711 small layer of Well JH9. (e) Liquefied breccia structure. Ch632 small layer of Well JH22. (f) Water escape vein structure. Well JH13, Ch712 small layer. (g) Diapir structure. Ch631 small layer of Well JH3. (h) Liquefied contorted structure. Ch712 small layer of Well JH12. (i) Liquefied and squeezed structure. Ch631 small layer of Well JH4.
The slump deformation structure fine sand rock facies (S fd ) is grey–black in colour. Shaly siltstone, thin interbeds of sandstone and mudstone and the convolute bedding structure formed by the slump can be identified in the S fd . The marks of the seismites are usually seen in the S fd as earthquake influences. Examples include the earthquake hybrid rock (Figure 10(a)), microfolds (Figure 10(b), (d) and (e)) and microfaults (Figure 10(c)). Slump deformation structures indicate that the earthquake is the enabling mechanism for the sandy debris flows. The load structures and liquefied structures also reflect earthquake activities.
Photographs of typical sedimentary deformation structures in seismite-slump microfacies (S fd ) in the Ch6–Ch7 Members in the Jinghe Oilfield. (a) Earthquake hybrid rock. Ch712 small layer of Well JH13. (b) Earthquake micro folded rock. Ch712 small layer of Well JH9. (c) Earthquake micro faults rock. Ch611 small layer of Well JH10. (d) Squeezing mound structure. Ch721 small layer of Well JH9. (e) Slump deformation structure. Ch712 small layer of Well JH3.
Sedimentary assemblies and model of deep water gravity flow in the Ch6–Ch7 Member in the Jinghe Oilfield
Sedimentary assemblies and distribution of the deep water gravity flow sediments in the Ch6–Ch7 Member in the Jinghe Oilfield
Three main microfacies are identified from the core observation and particle size distribution. Exploration of the microfacies assembly and distribution is critical in understanding the whole sedimentary process.
There are three common vertical sequences of microfacies assemblies: (1) the assembly of the seismite-slump and sandy debris flow microfacies (Figure 11(a)), (2) the assembly of sandy debris flow and turbidite microfacies (including the turbidite sand sheet and turbidite channel) (Figure 11(b)) and (3) the assembly of the turbidite and deep water basin plain microfacies. Generally, the first type of the microfacies assembly is usually developed proximal to the basin slope. The second type is commonly developed in the middle or distal slope, and the third type is developed on the basin plain.
Common types of sandy debris flow, turbidite and seismite-slump microfacies in the Ch6–Ch7 Members of the Jinghe Oilfield. (a) Sandy debris flows and slumps sedimentary assembly, Ch711 small layer of Well JH2; (b): sandy debris flow-turbidite sedimentary assembly, Ch632 small layer of Well JH7.
Figure 12 gives an example of the microfacies distribution of the Ch711 small layer of the Jinghe Oil Field. The provenance is in the south of the study area. It is obvious that the assembly of the seismite-slump and sandy debris flow microfacies occurs in the south of the study area. The assembly of the sandy debris flow and turbidite microfacies is mainly distributed to the north. A complementary trend can be found on the vertical profile (Figure 13).
Deep water slope fan sedimentary microfacies of Ch711 small layer in the Jinghe Oilfield.
Deep water slope fan sedimentary microfacies section of Ch711–Ch73 small layers of the Jinghe Oilfield (the location is in Figure 12).
The sand body distribution also exhibits a different shape when compared with the delta model. The sand body in the thickness contour map is usually shaped like a lobe and an elongated lobe (Figure 12). The seismic facies are typically lump-like in shape and rarely channel-like, confirming that these features do not support the traditional views of braided distributary channel sand bodies.
Depositional model of deepwater gravity flow sediments
Shanmugam (2000) proposed his own deep-water slope fan model, dominated by the sandy debris flow, different from the conventional turbidite model and further divided into non-channelized and channelized systems. Zou et al. (2009) divided the deep water gravity flow sedimentary model of Ch6 Member in the Baibao area in the Ordos Basin into three microfacies according to the position from the slope break to the basin plain. Seismite-slump microfacies are developed mainly near the slope break, sandy debris flow microfacies are developed mainly on the slope and turbidite and bottom current transformation microfacies are developed on the basin plain. These models would be helpful for establishing the sedimentary facies model in the study area. The depositional processes can be described as follows (Figure 14):
Depositional system of the Ch6–Ch7 Members in the Jinghe Oilfield. (a) Sedimentary pattern map (solid map and section sediment gravity flow evolution profile); (b) Ch7 member sedimentary column section of Well JH7.
Following the deposition of the Ch8 sandstone reservoir in the Jinghe oilfield, the basement of the Ordos Basin was asymmetrically deformed as it was strongly affected by the Indosinian orogenic movement in the Western Qinling area, adjacent to the southwestern margin of the basin. This, in turn, led to the paleogeomorphology the basin, steep in the south and gentle in the north. At the start of the Ch7–Ch6 Member deposition, the basin slope break had been developed, determinable by the contour map of the oil shale thickness (Liu et al., 2011; Pang et al., 2012). Liu et al. (2011) estimated that the paleo slope has a dip of about 1°–3°, with an average ranging between 1.5° and 2°. Studies of the paleo slope where sandy debris flows were developed (including experiments, field outcrops observations and modern landslide descriptions) show that large-scale sandy debris flow sediments can slide for a long distance on slopes with a less than 5° dip. The same is the case for silty sandy debris flow sediments at a dip of even less than 1°. From this point of view, it is beneficial for sandy debris flows to develop in Ch6 and Ch7 Members which mainly consist of fine sand and silt. Simultaneously, the paleo slope can extend to dips of 3.5°–5° in the southern margin of the Jinghe Oilfield, encouraging deposition of the slump microfacies of the slope fans on the slope (Liu et al., 2011; Pang et al., 2012; Zhao et al., 2008).
Earthquake and gravity would trigger a slide of a large deposition of delta front sediments on the slope break down the slope. In this process, the ambient lake water invaded the sediments leading to a combination of each other and continuous liquefaction of the sediments. Following this, a large number of penecontemporaneous liquefaction structures, such as the liquefied contorted structure, convolute bedding structure, dish structure, escape structure, ejected structure and various seismite structures (microfold, microfault, sand volcano, boudinage, ring layer structure, liquefied water pressure structure, etc.), were formed. With the continuous liquefaction of the seismite and slump microfacies, sandy debris flow microfacies were formed during the transportation and deposition based on multiple sediment-support mechanisms. These included fluid overpressure, frictional strength (sandstone grains collided with each other) and cohesive force (the cohesion of water and clay). The turbidity current was formed at the upper part or the top of the sandy debris flow sediments, as a result of the flow transformation. With the decrease of the fluid concentration, the fluid became a low-density turbidity current when it entered the foot of the slope and the distal basin plain.
In summary, the deep water slope fan can be divided into upper fan subfacies dominated by the seismite-slump microfacies assembly (pleaogeographic location is in the transition between the slope break and the upper part of the slope), the mid-fan subfacies with the sand debris flow-turbidity flow miscrofacies assembly on the lower part of the slope and the slope foot and the lower fan subfacies of the turbidite flow-basin plain microfacies assembly on the slope foot and basin plain (Table 2).
Sedimentary facies, subfacies and microfacies in the Ch6–Ch7 Members in the Jinghe Oilfield.
BP: basin plain; SDF: sandy debris flow; SSLM: seismite-slump; TBF: turbidite.
The analysis of reservoir characteristics and oil bearing of deep water gravity flow sediments in the Ch6–Ch7 Members in the Jinghe Oilfield
Reservoir characteristics in the Ch6–Ch7 Members in the Jinghe Oilfield
By statistical analysis of 177 thin sections of core samples from 16 wells (Figure 15), the feldspar lithic fine-grained sandstone and lithic arkose fine-grained sandstone are seen to be dominant in the tight sandstone of the Ch6–Ch7 Members.
Lithology classification of the Ch6–Ch7 Members sandstone.
The quartz content ranges from 38 to 62%, the average being 48.02%. The feldspar content ranges from 9 to 47%, with an average of 28.05%. The debris content is between 12 and 56%, and the average is about 23.43%. Overall, the sandstone compositional maturity is low. In addition, the sandstones contain the carbonate debris with an average content of 0.5%.
According to the study of Liu et al. (2011), when the content of carbonate debris increases, it indicates that the structure of the surrounding basin is active and uplifting. Since the carbonate basement of the basin is exposed and eroded, the carbonate debris will be transported into the basin.
The sandstone grains are in line and/or in convex–concave contact, indicating strong compaction. Rounded and secondary angular grains of the sandstone show that the sorting is medium. Therefore, the texture maturity of the sandstone in the Ch6–Ch7 Members is poor. The content of the clay matrix in the sandstone is about 0.5–18%, with an average of 5.27%, indicating that the sandstone mostly belongs to the clean sandstone.
The average cement content of the Ch6–Ch7 sandstone is about 7.8%. The types of cement include the carbonate, authigenic clay and small amounts of siliceous cement, pyrite and siderite. The iron-calcite and iron-dolomite are the two main carbonate cements with an average content of 4.9%. Authigenic clay cement content was an average of 2.2%. The high content of cements indicates strong diagenesis and poor reservoir properties (low porosity and permeability).
Statistical analysis of porosity and permeability data from 1028 cores show that the porosity of the Ch6–Ch7 sandstone is between 0.2 and 17.68%, with an average of 9.36%. The permeability is between 0.04 and 61 mD with an average of 0.22 mD (Figure 16). The low porosity and permeability of the reservoir indicate that it is a typical tight sandstone reservoir. The crossplot between the porosity and permeability shows that the relationship is good though obviously affected by fractures (Figure 17).
Porosity and permeability distribution histograms of Ch6–Ch7 Members tight sandstone reservoir.
Porosity and permeability cross plots of the Ch6–Ch7 Members tight sandstone reservoir.
There are very complex relationships between the sandstone composition and porosity and/or permeability, demonstrating the large influence of diagenesis on the reservoir properties. In addition to the diagenesis, the grain size and microfacies of the sandstones also have certain effects on the reservoir properties.
Statistics of 343 fine-grained sandstone samples and 125 siltstone samples show that the average porosity is 10.1 and 8.8%, and the average permeability is 0.45 and 0.31 mD, respectively.
Further statistics show most favourable reservoir properties in the massive bedding and the parallel bedding sandstones with the sandy debris flow sediments. The average porosity is 10.3%, and the average permeability is 0.64 mD. The turbidite microfacies sandstones are usually poorly sorted thin interbeds of sandstone and mudstone. Their average porosity and permeability are 6.7% and 0.21 mD, respectively. The reservoir properties of the seismite-slump microfacies are relatively poor with an average porosity of 5.6% and average permeability of 0.16 mD because of the mixture of the sandstone and mudstone with deformed, contorted and convoluted beddings. The data are listed in Table 3.
Reservoir properties of gravity flow sedimentary microfacies in Ch6–Ch7 members.
Oil-bearing properties analysis
According to the relationship between reservoir properties and oil shows, it is clear that reservoir properties control the degree of oil bearing in the reservoir (Figure 18). Improved reservoir properties result in increased oil bearing.
Porosity and permeability cross plot for the Ch6–Ch7 sandstones with different oil-bearing properties.
Core logging results also show that different microfacies affect the oil show (Figure 19). All three microfacies have oil shows; however, massive sandstone of sandy debris flow sediments has the best oil bearing, with 11.9% samples of oil immersed, 55.3% samples of oil spot, 31.7% samples of oil stains and a small number of samples of oil immersed and fluorescence. The oil bearing of turbidite sandstone is poorer than that of sandy debris flow sediments. However, with mainly oil spots and oil stains samples, it is slightly better than that of seismite-slump. The oil bearing of the seismite-slump microfacies is poor, which may be related to the mixing of sand and mud as well as poor reservoir properties.
Oil-bearing properties frequency distribution of the Ch6–Ch7 sandstones for different microfacies.
Conclusions
This paper studied the characteristics of deep-water gravity flow sediments and their oil bearing in the Ch6–Ch7 Members of the Jinghe Oilfield and following conclusions were made:
The Jinghe Oilfield located in the southern margin of the Ordos Basin has the characteristics of the foreland basin influenced by the collision between the northern Yangtze Plate and the southern North-China Plate. The morphology of the basin is characterised by the steep south and gentle north trend. The slope breaks are developed at the Ch6–Ch7 Members deposition, providing good paleogeomorphic conditions for the development of sandy debris flow sediments. The deep-water sediments in the study area can be divided into 13 types of lithofacies dominated by fine-grained sandstone and siltstone. There are three types of sedimentary microfacies: the sandy debris flow microfacies, turbidite microfacies and seismite-slump microfacies, and three main types of sedimentary assemblies: the seismite-slump-sandy debris flow, sandy debris flow-turbidite and turbidite-basin plain. The deep water gravity flow deposition model in the Ch6–Ch7 Members can be summarised as the slope fan model and can be divided into three subfacies or assemblies: (1) the upper fan sub-facies dominated by the assembly of seismite-slump and sandy debris flow (pleaogeographic location is in the transition zone between the slope break and the upper part of the slope), (2) the mid-fan sub-facies dominated by the assembly of the sand debris flow-turbidite microfacies (pleaogeographic location is in the transition zone between the lower part of the slope and the slope foot) and (3) the lower-fan sub-facies dominated by turbidite flow-basin plain microfacies assembly (in the slope foot and basin plain). The sedimentary process of the deep water gravity flow in the slope fan can be described as the occurrence of seismic slide and slump deformation on the slope when a large amount of delta front sediments deposited in the slope break are induced by the earthquake. Seismites and slumps continuously liquefy to form the sandy debris flow sediments. The turbidity flow will be formed through the flow transformation and may be partially remoulded by the weak bottom current in the intermittent periods. Microfacies control both reservoir properties and oil shows. Sandy debris flow sandstones exhibit the best reservoir properties and oil shows, followed by turbidite sandstones while seismite-slump sandstones are the poorest. The sandy debris flow and part of the turbidite sandstones have good oil production potential.
Footnotes
Acknowledgements
The authors thank the two anonymous reviewers for the constructive suggestions and the associate editor Cao Jian for the editing work.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors received financial support from the National Natural Science Foundation of China (No. 41402086), China National Science and Technology Major Project under Contract 2016ZX05048-001–01-CS and SINOPEC Huabei Company for the research, authorship, and/or publication of this article.