J. Cent. South Univ. (2019) 26: 1840-1855

DOI: https://doi.org/10.1007/s11771-019-4138-z

Discovery and prediction of high natural gamma sandstones in Chang 7₃ Submember of Triassic Yanchang Formation in Ordos Basin, China

ZHENG Qing-hua(郑庆华)¹, LIU Xing-jun(刘行军)², YOU Ji-yuan(尤继元)^{1, 3},

BAI Yun-yun(白云云)¹, WANG Jing-hui(王镜惠)¹, CHEN Xiao-liang(陈小亮)¹

1. School of Chemistry & Chemical Engineering, Yulin University, Yulin 719000, China;

2. Changqing Division, China Petroleum Well Logging Limited Company, Xi’an 710201, China;

3. Department of Geology, Northwest University, Xi’an 710069, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: The large scale development of high natural gamma sandstones has been discovered in the Chang 7₃ Submember of the Triassic Yanchang Formation in the Ordos Basin, China. High natural gamma sandstones consist of terrigenous detrital rocks with particle sizes ranging from silt to sand. They represent turbidite deposits characterized by high gamma ray values that are more than 180 American Petroleum Institute (API) units on a natural gamma ray log profile. For a long time, very high natural gamma sandstones had been identified as high-quality source rocks, such as oil shales, from conventional well log profiles, such as natural gamma ray well logs. Therefore, predicting the distribution of high natural gamma sandstones was studied. The sedimentary, lithological, and well log characteristics, as well as the genesis of the high radioactivity of high natural gamma sandstones were analyzed in the Chang 7₃ Submember. Thorium (Th), uranium (U) and other radioactive elements were found, carried by deep hydrothermal activity, and probably resulted in the formation of a relatively high radioactive zone in the cross-section, where high natural gamma sandstones usually develop in large quantities. This caused many turbidite sand bodies, which should have a continuous distribution in the cross-section, to appear to have a discontinuous distribution, when using conventional well log profiles, such as natural gamma ray well logs. From the above mentioned apparent discontinuous distribution of turbidite sand bodies in the cross-section, a continuous distribution can be predicted. It is obvious that the prediction of areas of continuous turbidite sand bodies in the cross-section usually corresponds with areas where high natural gamma sandstones are developed in large quantities. Exploration and development practice demonstrated that the developed method is fast and effective in predicting high natural gamma sandstones in the Chang 7₃ Submember.

Key words: Ordos Basin; Chang 7₃ Submember; high-quality source rock; high natural gamma sandstone; prediction

Cite this article as: ZHENG Qing-hua, LIU Xing-jun, YOU Ji-yuan, BAI Yun-yun, WANG Jing-hui, CHEN Xiao-liang. Discovery and prediction of high natural gamma sandstones in Chang 7₃ Submember of Triassic Yanchang Formation in Ordos Basin, China [J]. Journal of Central South University, 2019, 26(7): 1840-1855. DOI: https://doi.org/10.1007/s11771-019-4138-z.

1 Introduction

The Chang 7 Member of the Triassic Yanchang Formation of the Ordos Basin, China can be divided in descending order into the Chang 7₁ Submember, Chang 7₂ Submember, and Chang 7₃ Submember [1]. The depositional stage of the Chang 7₃ Submember represents the peak of the Ordos Basin development. During the deposition of the Chang 7₃ Submember, the areas of semi-deep and deep lake sedimentary environments were at a maximum, and source rocks, such as mudstone and oil shale, were mostly developed, while the turbidite fan was relatively poorly developed [1-10]. The lithology of the Chang 7₃ Submember formed in the semi-deep to deep lake environments in the central Ordos Basin is mainly oil shale, followed by dark grey to black mudstone, grey to ash black silty mudstone, pelitic siltstone, grey brown siltstone, and fine siltstone. Among these, the oil shale corresponds to a high-quality source rock, and is the primary source rock, followed by black mudstone, while the silty mudstone corresponds to a poor source rock, and pelitic siltstone, siltstone, and fine siltstone are not source rocks. The fine siltstone lithology is the main reservoir, and mainly consists of lithic feldspar sandstone and feldspar lithic sandstone, with dissolution pores accounting for an average of 66.7% of the developed porosity. The average porosity is 9.8% and the average permeability is 0.20×10^-3 μm², which corresponds to an ultra-low permeability tight sandstone reservoir.

Drilling of the Chang 7 Member seldom involves the source rocks, such as oil shale and black mudstone, but typically involves the fine siltstone of the Chang 7₁ and Chang 7₂ Submembers. For a long time, conventional well log profiles theoretically identified the lithology, whose natural gamma ray value is more than 180 American Petroleum Institute (API) units and whose density is less than 2.4 g/cm³, as oil shale corresponding to high-quality source rock [11]. But based on the above specifications, the thickness of the oil shale corresponding to high-quality source rock is too small for production; therefore it explained the lithology, whose natural gamma value is more than 180 American Petroleum Institute units, as oil shale corresponding to high-quality source rock. On the basis of core observations and analysis of well logs and geological logs of ten whole Chang 7₃ Submember wells, for the first time it was found that most of conventional well logs identified lithology as oil shale corresponding to high-quality source rock if its natural gamma ray value was greater than 180 American Petroleum Institute units. The lithology could be divided mainly into two types: 1) terrigenous detrital rocks with particle sizes ranging from silt to sand and corresponding to turbidite deposits, such as pelitic siltstone, siltstone and fine siltstone; and 2) a mixture of terrigenous detrital rocks with particle sizes ranging from silt to sand and corresponding to turbidite deposits, black mudstone, oil shale, etc., which formed interbedded layers, slump layers, seismicity layers, and muddy gravel-bearing layers. The above lithologies are collectively referred to as high natural gamma sandstones. For a long time this led to the underestimation of the development scale of turbidite sand bodies, especially the relatively high radioactive zone in the cross-section, such as in the Heshui Block (Figure 1). The result of this practice showed that oil shale corresponding to high-quality source rock, which was identified by conventional well logging, is actually high natural gamma sandstone, and it often has a high test oil yield, such as in Well N148 and Well G295, while the oil enrichment area of the Chang 8₁ Submember, which is close to the Chang 7₃ Submember, is usually undeveloped. Therefore, the prediction of high natural gamma sandstones has great significance in both theory and practice for tight oil exploration and development in the Yanchang Formation in the Ordos Basin.

On the basis of a study of the sedimentary characteristics, lithological characteristics, well log characteristics and analysis of the genesis of the high radioactivity of high natural gamma sandstones in the Chang 7₃ Submember of the Yanchang Formation in the Heshui Block, this study advances the prediction of the distribution of high natural gamma sandstones for the first time.

2 Definition of high natural gamma sand-stone

The study of the genesis of the high natural gamma sandstones within the sandstone reservoir was not systematic and related to tuff material [13-25], before LIU et al [26] performed comparatively systematic research in the Yanchang Formation.

LIU et al [26] discussed the definition, genesis, and log identification methods of high natural gamma sandstone in sandstone reservoirs, such as the Chang 4+5 and Chang 6 Members. High natural gamma sandstone is define as sandstone whose value of ΔGR_h/ΔGR_n is more than 66% in natural gamma ray log profiles, and whose lithology is sandstone with particle sizes coarser than or equal to fine sandstone, but not including siltstone, such as the high natural gamma sandstone of the Chang 6 Member in the Jiyun area. During identification with natural gamma ray logs, ΔGR_n is the difference between the normal value of mudstone and the normal value of sandstone, while ΔGR_h is the difference between the value of high natural gamma sandstone and the normal value of sandstone. According to thickness, the high natural gamma sandstone within a sandstone reservoir can be roughly divided into type Ⅰ and type Ⅱ, which represent relatively thin and relatively thick layers, respectively. The above particle size, the lower limit value of ΔGR_h/ΔGR_n in natural gamma ray log profiles, and the specifications of relatively thin layers and relatively thick layers reveal differences, while the distribution is not only within sandstone in different areas and layers of the Ordos Basin.

Figure 1 Comprehensive map of Chang 7₃ Submember of Yanchang Formation in Longdong (modified from Ref. [12])

The natural gamma ray values of lithologies of the Chang 7₃ Submember are significantly influenced by deep hydrothermal activity [26-31], and show a wide variety in different areas (Figure 1), as well as having the characteristics of high contents of uranium (U) or thorium (Th), as determined by natural gamma ray spectrometric logs (Figure 2).

Considering the lithology of the Chang 7 Member with relatively finer grain-size in the Triassic Yanchang Formation of the Ordos Basin, and to recognize high-quality source rocks, such as oil shale, the high natural gamma sandstone, whose natural gamma ray value is more than 180 API units on a natural gamma ray log profile, can be mainly divided into two types in the Chang 7₃ Submember. One is terrigenous detrital rock with particle sizes ranging from silt to sand and corresponding to turbidite deposit, such as pelitic siltstone, siltstone, and fine siltstone (Figure 2). The other is a mixture of terrigenous detrital rocks with particle sizes

ranging from silt to sand and corresponding to turbidite deposits, black mudstone, oil shale, etc., which form interbedded layers, slump layers, seismicity layers and muddy gravel-bearing layer (Figures 3 and 4). The former lithology is in a narrow sense, a high natural gamma sandstone, while the latter lithology is in a broad sense, a high natural gamma sandstone, and they are collectively called the target source rock’s high natural gamma sandstone (high natural gamma sandstone for short). The lithology which is in, a broad sense, a high natural gamma sandstone, is characterized by a sequence of terrigenous detrital rocks which is relatively thick, with particle sizes ranging from silt to sand, while the thickness of the source rock is relatively thin and has a lower hydrocarbon generation potential. The former lithology generally defines the lower limit of the natural gamma ray value of source rock of 180 API units in the Chang 7 Member, so the value of ΔGR_h/ΔGR_n of the target source rock’ high natural gamma sandstone is more than 100% in natural gamma ray logs. According to thickness, the relevant source rock’s high natural gamma sandstone can also be roughly divided into two types. Type Ⅰ is characterized by relatively thin layers, where the general range of thickness is 0.1-2.0 m, with an average thickness of 1.5 m. Type Ⅱ is characterized by relatively thick layers, where the general range of thickness is 2.0-5.0 m, with an average thickness of 2.5 m (Figures 3 and 4).

Figure 2 Well log interpretation profile of Chang 7₃ Submember in representative Well B36

Figure 3 Well log interpretation profile of Chang 7₃ Submember in representative Well N138

Figure 4 Well log interpretation profile of Chang 7₃ Submember in representative Well Z233

3 Sedimentary and lithological features

Deposition of the Chang 7₃ Submember occurred in the central part of the Ordos Basin and mainly represented semi-deep to deep lake environments, which developed deep water turbidite fan facies. The latter facies can be further divided into three subfacies (inner, middle, and outer fans), and four microfacies, such as the turbidite channel, interturbidite, turbidite front, and deep lacustrine mudstone [32-39]. The marginal parts of the Ordos Basin mainly represent a shallow lake environment, in which deposition of a river delta front sub-facies was developed in the Chang 7₃ Submember (Figure 1). The characteristics of the sedimentary environments, lithologies, oil-bearing properties, palaeontology, hydrocarbon generation potential, etc., of the Chang 7₃ Submember were studied, and as a result, it was divided in descending order into the Chang 7₃¹ Sublayer and Chang 7₃² Sublayer, where their average thicknesses are 15.0 and 20.0 m, respectively.

3.1 Chang 7₃¹ Sublayer

Deposition of the Chang 7₃¹ Sublayer occurred in the central parts of the Ordos Basin and mainly represented semi-deep to deep lake environments. The sublayer developed largely as the middle fan subfaces of the deep water turbidite fan facies, which was further divided into a turbidite front, interturbidite, and turbidite channel microfacies, and was followed by deep lacustrine mudstone of the outer fan subfacies of the deep water turbidite fan facies (Figures 4 and 5). The lithologies of the Chang 7₃¹ Sublayer are mainly terrigenous detrital rocks with particle sizes ranging from silt to sand that consist of mostly oily lithologies, and are followed by mudstone that contains only a minor amount of oily lithologies. The lithologies of the Chang 7₃¹ Sublayer are mainly terrigenous detrital rocks with particle sizes ranging from silt to sand, mainly consisting of pelitic siltstone, siltstone, and fine siltstone, whose thicknesses are generally between 2.0 and 13.0 m. These rocks developed mainly massive bedding, banded bedding or wave bedding, and slump structure, with muddy gravel observed at the base. The mudstones are mainly dark grey to black silty mudstone, and are followed by black mudstone and oil shale, and contain cyanobacteria, botryococcus, phytoclasts, and pyrite aggregates [40, 41], while fractures are well developed. Generally, the total organic carbon (TOC) values of the above mudstones are approximately 3.65%, and the content of chloroform asphalt “A” is approximately 0.25%. The value of potential hydrocarbon generation (S₁+S₂) is approximately 6.50 mg/g, and thus, the hydrocarbon potential is relatively poor, such as in Well H317 (Figure 5). The turbidite channel sand bodies are generally developed in the Chang 7₃¹ Sublayer, and relatively thick oil shale corresponding to high quality source rock is usually undeveloped.

Figure 5 Well log interpretation profile of Chang 7₃ Submember in representative Well H317

3.2 Chang 7₃² Sublayer

The deposition of the Chang 7₃² Sublayer occurred in the central parts of the Ordos Basin, and mainly represented a deep lake environment. The sublayer developed as the deep lacustrine mudstone microfacies, which developed in the outer fan subfacies of the deep water turbidite fan facies, and those are the most developed microfacies, followed by a turbidite channel, interturbidite, and turbidite front microfacies, which developed in the middle fan subfacies (Figures 4 and 5). The lithologies of the Chang 7₃² Sublayer are mainly mudstone, followed by terrigenous detrital rocks with particle sizes ranging from silt to sand, and followed by mudstone, which are mostly oily lithologies. The lithologies of the Chang 7₃² Sublayer are mainly terrigenous detrital rocks with particle sizes ranging from silt to sand, and are mainly fine siltstone, siltstone, and pelitic siltstone, whose thicknesses are generally between 0.1 and 6.0 m, which mainly developed massive bedding and slump structures. The mudstones mainly mostly consist of black oil shale, followed by black mudstone, with only minor amounts of black and grey silty mudstone, and contain cyanobacteria, botryococcus, phytoclasts, ostracods, and pyrite aggregates [40, 41]. Generally, the total organic carbon (TOC) values of the above mudstones are approximately 15.67%, and the content of chloroform asphalt “A” is approximately 0.61%. The value of potential hydrocarbon generation (S₁+S₂) is approximately 30.97 mg/g, and the hydro-carbon potential is very good, such as in Well H317 (Figure 5). The turbidite channel sand bodies are developed in the Chang 7₃² Sublayer, and generally have thicknesses between 0.5 and 2.0 m. The lithologies are mainly terrigenous detrital rocks with particle sizes ranging from silt to sand, and correspond to turbidite deposits, such as pelitic siltstone, siltstone, and fine siltstone. These usually consist of mixed black mudstone, oil shale, etc., and form interbedded layers, slump layers, seismicity layers, muddy gravel-bearing layers, and others. They are well developed on the margins and central parts of the semi-deep to deep lakes (especially on the margins), especially on the margins, and are unfavourable for the development of relatively thick oil shale corresponding to high quality source rock (Figures 1-4).

4 Conventional well log responses

The Chang 7₃² Sublayer rather than Chang 7₃¹ Sublayer is the primary target for this study because almost all of the relatively thick oil shales corresponding to high quality source rock were developed in the Chang 7₃² Sublayer.

4.1 Chang 7₃¹ sublayer

The lithologies of the Chang 7₃¹ Sublayer are generally not high-quality source rocks, such as fine siltstone, siltstone, pelitic siltstone, silty mudstone, and black mudstone, which can typically be identified by conventional well log analysis. They are generally characterized by a natural gamma ray of less than 180 American Petroleum Institute (API) units, a density between 2.2 and 2.6 g/cm³, an interval transit time between 160 and 350 μs/m, a neutron porosity between 15% and 50%, an induction between 20 and 100 Ω·m, and a shale volume (VSH) equal to or lower than 100% (Figures 4 and 5).

4.2 Chang 7₃² sublayer

The lithologies of the Chang 7₃² sublayer are generally oil shales corresponding to high-quality source rocks. In conventional well log analysis, these are generally characterized by a natural gamma ray of more than 180 American Petroleum Institute units, a density of smaller than 2.4 g/cm³, an interval transit time of greater than 250 μs/m, a neutron porosity of more than 30%, an induction of more than 100 Ω·m, and a shale volume (VSH) of equal to 100% (Figure 5).

A large amount of conventional well log data indicated that in a well log interpretation profile it is difficult to distinguish between oil shales corresponding to high quality source rocks and non-high quality source rocks in relatively high radioactive zones of the Chang 7₃ Submember, whether in the Chang 7₃¹ Sublayer or the Chang 7₃² Sublayer. The above non-high quality source rocks include fine siltstone, siltstone, pelitic siltstone, silty mudstone, and black mudstone, and can be generally characterized by a natural gamma ray of more than 180 American Petroleum Institute units, a density between 2.2 and 2.6 g/cm³, an interval transit time between 200 and 350 μs/m, a neutron porosity between 15% and 50%, an induction between 20 and 200 Ω·m, and a shale volume (VSH) of equal to 100% (Figures 2-4). Although the above non-high quality source rocks are characterized by a low density, a high interval transit time, a high neutron porosity, a high induced induction, etc., compared with oil shales corresponding to high quality source rocks, it is difficult to identify them effectively in practice.

According to the comprehensive analysis above, it is difficult for conventional well logging to effectively distinguish oil shales corresponding to high quality source rocks from high natural gamma sandstones, mainly for four reasons. Firstly, they all have high natural gamma ray values, all of which can be greater than 180 American Petroleum Institute units. Secondly, they are all oil-rich lithologies [42-46], which are characterized by high resistivity and there is no obvious difference in the characteristic of the induction resistivity log profiles. Thirdly, the dissolution pores are well developed in terrigenous detrital rocks with particle sizes ranging from silt to sand [47-51], and therefore log profiles of density, interval transit time, and neutron porosity show no obvious differences between them and oil shales corresponding to high quality source rocks. Fourthly, the thickness of terrigenous detrital rocks with particle sizes ranging from silt to sand are generally less than 40 cm, or occur usually mixed with oil shales, forming interbedded layers, slump layers, seismicity layers, and muddy gravel-bearing layers. Therefore, their characteristics in well logs are greatly influenced by oil shales corresponding to high quality source rocks.

5 Genetic and distribution characteristics of high natural gamma sandstones

5.1 Genetic characteristics of high natural gamma sandstones

The research objective of the genetic characteristics of the target source rocks’ high natural gamma sandstone is high natural gamma sandstone in a narrow, rather than in a broad sense, with characteristics in well logs that are greatly influenced by the relatively high radioactive signature of the surrounding rocks, such as oil shales corresponding to high quality source rocks. According to their mineralogical characteristics, the genetic characteristics of high natural gamma sandstone in the narrow sense are probably related to deep hydrothermal activity in the Chang 7₃ Submember, which is characterized by the development of a hydrothermal mineral assemblage, including minerals such as monazite, and rutile, which are higher in radioactive thorium (Th).

The genetic characteristics of high natural gamma sandstones are similar between high natural gamma sandstones in the narrow sense in the Chang 7₃ Submember and high natural gamma sandstones within sandstone reservoirs in other members of the Yanchang Formation, such as the Chang 6, Chang 4+5, Chang 2, and Chang 3. Both may have been affected by deep hydrothermal activity, which indicates that the content of radioactive thorium (Th), uranium (U), and potassium (K) are higher than in conventional sandstones, especially thorium (Th) (Figure 2). Thorium (Th) mainly exists in xenomorphic hydrothermal mineral assemblages, including minerals such as monazite, and rutile (Figure 6).

Taking two high natural gamma sandstone samples in the narrow sense from the Chang 7₃ Submember of the Yanchang Formation in the Heshui Block, as examples, such as the high-U, high natural gamma, fine siltstone, and the high-Th, high natural gamma, fine siltstone (Figures 6(a) and (d)), their contents of radioactive Th and K are comparable, but there are larger differences in the contents of radioactive uranium (U), and its high radioactivity is mainly caused by thorium (Th) (Figure 2). Both sandstone samples are rich in black oil and other organic matter, while dissolution pores are well developed in the fine siltstones (Figures 6(b) and (e)). Th-rich minerals, such as xenomorphic monazite, and rutile, are developed in intracrystalline pores or interstitial, intercrystalline pores (Figures 6(c) and (f)). Independent-U minerals, which may be of hydrothermal origin are not observed in the high-U high natural gamma, fine siltstone (Figures 6(a) and (c)) [29]. It is speculated that U may exist in homogeneous form or adsorbed form [27, 29]. Taking a high natural gamma, fine siltstone sample in the narrow sense from the Chang 7₃ Submember of the Yanchang Formation in the Pengyang Block, as an example, it developed syn-depositional deformation structures and was intercalated with a bedded thin layer of xenomorphic pyrite aggregates, which may correspond to syndepositional hydrothermal sediments (Figure 6(g)) [28]. The above high natural gamma, fine siltstone sample in the narrow sense is rich in black oil and other organic matter as well as dissolution pores (Figure 6(h)), and developed xenomorphic pyrite occurring together with xenomorphic monazite in intergranular pores, which may be related to syndepositional hydrothermal sediments (Figure 6 (i)).

Figure 6 Photographs showing sedimentary, textural, and mineralogical characteristics in macro (in core), petrographic and backscattered electron (BSE) images of high gamma sandstones in the Yanchang Formation:

High natural gamma sandstone within a sandstone reservoir of the Chang 6₂ Submember of the Yanchang Formation in the northern Shaanxi Province, contains rutile that developed: 1) a colloform structure, which may belong to a relatively typical hydrothermal sedimentary structure in intracrystalline pores or interstitial, intercrystalline pores (Figure 6(j)); 2) xenomorphic rutile occurring together with xenomorphic monazite, and enclosing monazite or quartz particles, but without forming chlorite films between them (Figure 6(k)); or 3) xenomorphic rutile and monazite filling intercrystalline pores and partially enclosing albite clastic particles, but without forming chlorite films (Figure 6(l)). Therefore, monazite likely formed before rutile, and both were likely formed before early diagenesis stage A, which may be deep hydrothermal origin [26].

Overall, the type and textural characteristics of the minerals that are rich in radioactive elements, are likely in the narrow sense between high natural gamma sandstone in the Chang 7₃ Submember and high natural gamma sandstone within sandstone reservoirs in other members of the Yanchang Formation, such as the Chang 6, Chang 4+5, Chang 2, and Chang 3. The above minerals are probably of deep hydrothermal origin and formed in the syndepositional period or before early diagenesis stage A.

In-situ microanalytical dating and tracing were not carried out because the diameters of the above individual radioactive minerals are smaller than 10 μm, such as for monazite and rutile. Therefore, the genesis of the high natural gamma sandstone needs further study.

5.2 Distribution characteristics of high natural gamma sandstone

A large amount of conventional well logging data, when viewed in the cross-section, indicates that there are many relatively high radioactive zones in the Chang 7₃ Submember of the Yanchang Formation of the Ordos Basin, such as in the Heshui Block, Baima Block, Tiebiancheng Block, and Zhengning Block, whose natural gamma ray values are generally between 300 and 700 American Petroleum Institute units (Figure 1). In the above blocks, both oil shales corresponding to high quality source rocks and high natural gamma sandstone of the target source rocks are developed, and their degree of development has a positive correlation with the radioactivity of the Chang 7₃ Submember. Taking the highest radioactive zone of the Chang 7₃ Submember in the cross-sections of the Ordos Basin as an example, such as in the Heshui Block, whose natural gamma ray value can reach 1000 American Petroleum Institute units, the high natural gamma sandstones of the target source rocks are developed on a relatively large scale and oil shales corresponding to high quality source rocks are also developed, such as in Well B36, Well Z233, and Well N138 (Figures 2-4).

6 Prediction of high natural gamma sand-stone

6.1 Basic principle

On the basis of the above sedimentary, lithological, and well logging characteristics of the Chang 7₃ Submember and the study of genetic characteristics of the high natural gamma sandstones, a prediction method for the high natural gamma sandstones of the Chang 7₃ Submember is proposed.

The Chang 7₃² Sublayer, where oil shales corresponding to high quality source rocks are the most developed, is taken as an example. The basic principle of the prediction method of high natural gamma sandstone of the Chang 7₃ Submember has five main points. Firstly, the lithologies of the Chang 7₃² Sublayer are mainly oil shales corresponding to high quality source rocks that mostly formed in a deep lake environment and correspond to terrigenous detrital rock with particle sizes ranging from silt to sand, belonging to turbidite deposits. Secondly, there are many relatively high and low radioactive zones in the Chang 7₃² Sublayer when viewed in cross-sections of the Ordos Basin. Thirdly, the high natural gamma sandstone of the target source rocks of the Chang 7₃² Sublayer are only developed in the relatively high radioactive zones of Chang 7₃² Sublayer. Fourthly, many turbidite sand bodies are developed, which should have a continuous distribution in the cross-section, but appear to have a discontinuous distribution when using conventional well log analyses, such as natural gamma ray well logging, in the relatively high radioactive zones of the Chang 7₃² Sublayer. Finally, on the basis of the above amentioned apparent discontinuous distribution of turbidite sand bodies in the cross- section, their continuous distribution can be predicted, and it is obvious that the prediction of areas of continuous turbidite sand bodies corresponds to areas where high natural gamma sandstones are usually developed in large quantities.

6.2 A forecast instance

The Heshui Block, which has the highest radioactive zone in the Chang 7₃ Submember in cross-sections of the Ordos Basin, was used as an example (Figures 1, 7, 8) for the prediction method on a high natural gamma sandstone of the Chang 7₃² Sublayer. The results show that a large quantity of high natural gamma sandstones of the target source rocks in the Chang 7₃² Sublayer are developed in Xuanma to Qingcheng (such as the area of Well B36), the west side of Chengguan (such as the area of Well Z80), the south side of Banqiao (such as the area of Well N138), Panke (such as the area of Well Z233), and that this is basically reflected in the core samples, such as from Well B36, Well Z80, Well N138, and Well Z233.

Figure 7 Natural gamma ray value isopleth map of Chang 7₃² Sublayer in Heshui Block

Figure 8 High natural gamma sandstones prediction map of Chang 7₃² Sublayer in Heshui Block

The Chang 8₁ Submember, which belongs to the deposition of the southwestern provenance system, closely underlies the bottom of the Chang 7₃² Sublayer, and its reservoir is mainly developed in underwater distributary channels in braided river delta front subfacies, whereas the non-reservoirs are mainly developed in underwater distributary bays [10, 52]. A large amount of work practice data indicates that the degree of development of oil shales corresponding to high quality source rocks has a positive correlation with the degree of oil enrichment in the main sand body of the Chang 8₁ Submember. Therefore, the oil enrichment area of the main sand body of the Chang 8₁ Submember is often not developed in areas of development of high natural gamma sandstone target source rocks in the Chang 7₃ Submember. They often develop low producing wells (e.g. Well Z80), water producing wells (e.g. Well B36, Well B107, Well N139 and Well X251), and non-oil showing wells (e.g. Well N32, Well N138 and Well Z233) (Figures 7, 8). However, the oil enrichment area of the main sand body of the Chang 8₁ Submember often develops in undeveloped areas of high natural gamma sandstone of the target source rocks in the Chang 7₃ Submember, and they often develop commercial oil flow wells, such as Well Z51, Well Z43, and Well B16 (Figures 7 and 8).

7 Conclusions

In this paper, the genetic characteristics and the prediction of the distribution of high natural gamma sandstones in the Chang 7₃ Submember of the Triassic Yanchang Formation in the Ordos Basin were studied. A novel method for predicting a continuous distribution of turbidite sand bodies in a cross-section, based on apparent discontinuous distri-butions, was proposed and tested. Based on the results obtained in the study, the following conclusions can be drawn:

1) Many so-called oil shales corresponding to high-quality source rocks of the Chang 7 Member, which are identified by conventional well log analysis and whose natural gamma ray value is greater than 180 American Petroleum Institute units, are actual terrigenous detrital rocks with particle sizes ranging from silt to sand, or a mixture of terrigenous detrital rocks with particle sizes ranging from silt to sand, black mudstones, and oil shales, which formed interbedded layers, slump layers, seismicity layers, and muddy gravel-bearing layers.

2) The formation of relatively high and low radioactive zones in the Chang 7 Member of the Yanchang Formation in cross-sections of the Ordos Basin is probably related to deep hydrothermal activity, which carried thorium (Th), uranium (U), and other radioactive elements. In relatively high radioactive zones in the cross-section, high natural gamma sandstones of the target source rocks and oil shales corresponding to high quality source rocks are developed. On the basis of the apparent discontinuous distribution of turbidite sand bodies in the cross-section, which is identified by conventional well log analysis, a continuous distribution of turbidite sand bodies and high natural gamma sandstones in the cross-section can be predicted.

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(Edited by YANG Hua)

中文导读

鄂尔多斯盆地三叠系延长组长7₃砂层组高自然伽马砂岩的发现及其预测

摘要：鄂尔多斯盆地上三叠统延长组长7₃砂层组大规模发育自然伽马测井值大于180 API、浊流沉积的粉砂级-细砂级高自然伽马砂岩，由于长期以来自然伽马等常规测井系列将其识别为“油页岩等优质烃源岩”，为此开展了其分布预测研究。通过盆地长7₃砂层组沉积特征、岩性特征、测井特征以及高自然伽马砂岩高放射性成因的分析，认为深部热水活动携带的Th、U等放射性元素可能形成了长7₃砂层组平面上放射性相对高值区且相对高值区内高自然伽马砂岩常大量发育，导致长7₃砂层组本应在平面上具有连续形态的浊流沉积砂体，而在根据自然伽马等常规测井系列识别上呈现出“平面上不连续的形态”，但可根据“平面上不连续的浊流沉积砂体形态”预测出浊流沉积砂体平面上可能具有的连续形态，显然平面上预测出的连续砂体形态部位为高自然伽马砂岩大量发育区。勘探开发实践表明，上述方法对长7₃砂层组高自然伽马砂岩的预测快速有效。

关键词：鄂尔多斯盆地；长7₃砂层组；优质烃源岩；高伽马砂岩；分布预测

Foundation item: Project(18GK28) supported by the Doctoral Scientific Research Starting Foundation for the Yulin University, China; Project(20106101110020) supported by the University Research Fund of Science and Technology Development Center of Ministry of Education, China; Project(BJ08133-3) supported by the Key Fund Project of Continental Dynamics National Key Laboratory of Northwest University, China

Received date: 2018-09-09; Accepted date: 2019-01-07

Corresponding author: ZHENG Qing-hua, PhD, Lecturer; Tel: +86-15388656689; E-mail: 272594012@qq.com; ORCID: 0000-0002- 1803-2989