Sedimentary and geochemical characteristics of the Triassic Chang 7 Member shale in the Southeastern Ordos Basin, Central China
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The Ordos Basin is the largest petroliferous basin in China, where the Chang 7 Member shale serves as the major source rock in the basin, with an area of more than 100,000 km2. So far, sedimentary and geochemical characterizations have rarely been conducted on the shale in shallow (< 1000 m) areas in the southeastern part of the basin, but such characterizations can help identify the genesis of organic-rich shale and promote the prediction and recovery of shale oil. In this paper, several outcrop sections of the Chang 7 Member in the Tongchuan area were observed and sampled, and sedimentary and geochemical characterizations were conducted for the well-outcropped YSC section. The study results show that the Chang 7 Member shale is widely distributed laterally with variable thickness. The organic-rich shale is 7–25 m thick in total and exhibits obvious horizontal variation in mineral composition. In the eastern sections, the shale contains organic matter of Type II2–III and is low in thermal maturity, with high clay mineral content, low K-feldspar content, and no pyrite. In the western sections, the shale contains Type II1 organic matter and is low in thermal maturity, with high clay mineral, K-feldspar, and pyrite contents. The YSC section reveals three obvious intervals in vertical mineral composition and organic abundance. The Chang 7 Member organic-rich shale (TOC > 10%) contains mainly sapropelite and liptinite, with Type II kerogen. It is generally characterized by a hydrocarbon potential of more than 70 mg/g, low maturity, and shallow–semideep lacustrine facies. In the western sections, the shale, still in a low maturity stage, has a higher hydrocarbon potential and is optional for shale oil recovery. However, the Chang 7 Member shale in the study area is highly heterogeneous and its shale oil recovery is practical only in the organic-rich intervals.
KeywordsOrdos Basin Chang 7 Member oil Organic-rich shale Sedimentary characteristics Geochemical characteristics
The Ordos Basin is the largest petroleum production basin in China, where the PetroChina Changqing Oilfield Company has produced more than 50 million tonnes of oil and gas each year for the past four consecutive years. The Mesozoic Triassic Chang 7 Member shale serves as the major source rock in the basin (Zhang and Li 2001; Yang et al. 2016). It is distributed extensively and has a high expulsion efficiency and supplies oil for a number of tight play zones (e.g., Chang 4 + 5, Chang 6, Chang 8, and Chang 9). The recently estimated oil resources exceed 12.8 billion tonnes (Zhang et al. 2006; Deng et al. 2009). Driven by the shale gas revolution in North America, the Chang 7 Member organic-rich shale is coming into the spotlight for exploration for, and research into, tight and shale oils (Miller et al. 2008; Jarvie 2012; Clarkson et al. 2012; Donovan et al. 2012; Yang et al. 2013; Zou et al. 2013; Zhang et al. 2015). Currently, the Chang 7 Member unconventional oil is mainly recovered by horizontal well volume fracturing techniques. The Xin’anbian oilfield, the first large tight oil reservoir in China, was discovered in the central part of the basin.
Previous studies of Chang 7 Member unconventional petroleum often focused on the genesis and structure of lacustrine gravity flow sandstone, the formation mechanism of the organic-rich shale, subtle reservoir evaluation, accumulation mechanisms of oil, evaluation of mobile oil, and identification of sweet spots (Zou et al. 2010; Zhang et al. 2009, 2010; Cui et al. 2016; Yang et al. 2015). In a State Key Program for Basic Research of China (937 Program) project, we reviewed the literature on unconventional petroleum geology in China and abroad, and found that fine-grained sediments were deemed as a key object (Schieber et al. 2007; Zou et al. 2013; Lazar et al. 2015). Unfortunately, there are only few continuous shale cores and little data on meter-scale macroscopic variations. Outcrops are the most practical choice for studying fine-grained sediments. For the southeastern Ordos Basin, especially the Tongchuan area, a lot of studies have been conducted on shale with respect to sedimentary environment, genesis, distribution, characteristics, and resources (Lu et al. 2006; Zhang et al. 2006; Li et al. 2009; Ren 2008), but no detailed sedimentary and geochemical characterizations have been made of outcrops. The horizontal and vertical sedimentary and geochemical characterizations of organic-rich shale in Chang 7 Member are insufficiently detailed. This has made it difficult to get a better understanding on the formation mechanisms and sedimentary patterns of fine-grained sediments and to select intervals for shale oil production. Even worse, the decline of global oil prices and limited knowledge of shale geology have forced many shale oil production plants to shut down.
In this paper, several outcrop sections of Chang 7 Member in the Tongchuan area were observed and sampled. Based on available logs and drilling data, the sedimentary and geochemical characterizations were conducted for the well-outcropped YSC section to describe the longitudinal variation of organic-rich shale. Finally, the sedimentary pattern of the Chang 7 Member shale was established.
2 Geological setting and experiments
2.1 Geological setting
In the southeastern Ordos Basin, the Quaternary overburden is dominant. Outcrops are found in the Triassic Yanchang Formation in valleys and hillsides, which lies disconformably with the Zhifang Formation with varying denudation at the top and in parallel unconformity with the Jurassic Yan’an or Fuxian Formation at the bottom. Overall, the Yanchang Formation developed in a period when the lacustrine basin was quite large. This formation contains ten oil formation (Chang 1–10) members from top to bottom (Fig. 1c). The Chang 7 Member represents the period that the lacustrine basin suffered the maximum flooding, when a semideep–deep lacustrine sedimentary system was deposited in the southern part of the Ordos Basin (Yang et al. 2010). A black shale sequence at the bottom of Chang 7 Member, also known as “Zhangjiatan shale,” is a regional marker bed. In the previous studies, the oil shale in the Yanchang Formation was believed to be controlled by sedimentary facies. In the Wuqing-Qingyang area at the center of the lacustrine basin, the deep lacustrine oil shale is generally 20–30 m thick, and even up to 40 m. In the northern Shaanxi area, the shallow lacustrine oil shale in the eastern part is generally less than 10 m thick, and the semideep–deep lacustrine oil shale in the southeastern part is generally 10–20 m thick. The Chang 7 Member oil shale in this part of the basin is characterized by wide distribution, miscellaneous occurrences, rich foliation, high asphaltene content, high resource potential, and high geochemical grade. The Chang 7 Member oil shale outcrops in the Yijunnan-Tongchuan-Xunyi area in the southern part of the basin. Typical outcrop sections include the Zaoyuan (ZY), Niejiahe (NJ), Beiyingou (BY), Yishicun (YSC), Jiaquhe (JQH), Qianlieqiao (QLQ), Liushutai (LST), Hejiafang (HJF), Bawangzhuang (BWZ), Tangnihe (TN), Maquan (MQ), and Mazhuangcun (MZ), as shown in Fig. 1d.
Samples were taken from eight outcrop sections, i.e., ZY, NJ, BY, YSC, BWZ, TN, MQ, and MZ, as shown in Fig. 1d. The shale samples were analyzed for properties and minerals. The geochemical characteristics of 294 samples over a thickness of 40 m in the YSC section were obtained.
Total organic carbon (TOC) and total sulfur (TS) were determined with a Leco CS230 carbon–sulfur detector according to GB/T 19145-2003. First, about 10 mg of powdered sample was weighed on an electronic balance and then placed into a porous porcelain crucible that had been heated in a 1000 °C Digital Muffle Furnace for 2 h. Second, a sufficient volume of 12.5% HCl was added to the crucible and heated on a hot plate for 2 h until it was fully reacted with sample. Third, the sample was washed with distilled water in crucible once every 30 min for 3 days. Finally, crucible and sample were dried in a 60 °C oven, and cooled before TOC and TS analysis.
Rock–Eval pyrolysis was conducted at the Key Laboratory of Petroleum Geochemistry, CNPC, following the procedure recommended in GB/T 18602-2012. In the experiment, 30–50 mg of powdered sample was weighed on an electronic balance and put into the Rock–Eval 6 crucible for pyrolysis. The measurement parameters included free hydrocarbon (S1 mg HC/g rock), hydrocarbon potential (S2 mg HC/g rock), maturity, and maximum pyrolysis temperature (Tmax, °C). Source rock parameters included hydrogen index (HI), oxygen index (OI), and productivity index (PI).
Whole-rock mineral and clay mineral compositions of the shales were analyzed by using a Rigaku TTR X-ray diffractometer under the conditions recommended in SY/T 5163-2010. During the whole-rock mineral composition analysis, 300-mesh powdered samples were selected to calculate the mass ratios of the minerals using the analysis software and the K value given in the international standard. The clay mineral composition was obtained by five steps. First, the samples were suspended to extract clay minerals. Second, the clay extracts were naturally dried at normal temperature and tested. Third, the sections were saturated with ethylene glycol at 60 °C for 8 h and tested again. Fourth, the sections were taken out of the diffractometer and heated at 550 °C for 2.5 h. Finally, the contents of clay minerals were calculated by using analysis software.
3 Results and discussion
3.1 Horizontal sedimentary and geochemical characteristics
The Tongchuan area is located in the Weibei Uplift and is a NW-dipping landform. The geological map of Binxian county (I4907, 1: 200000) shows that the oil shale intervals to the south of the oil shale outcrop line are denuded and the area to the northwest contains oil shale. According to the measurement of dip angles, among the eight outcrop sections, the ZY, BY, YSC, NJ, and MQ sections have a fairly flat or even horizontal attitude; the BWZ section shows a NNW inclination of 8°–10° and the MQ section shows a NNW inclination of 5°–8°; the TN section, near a fault belt, is much steeper,, with an NNE inclination of more than 40°, and up to 70° in oil shale intervals. Previous studies detected some liberal small-sized folds and minor fault structures locally in the southeastern part of the Ordos Basin, where the burial depth of oil shale generally increases from the southeast to the northwest; however, constrained by local landforms, the maximum valley-hillside height difference can be more than 200 m (Ren 2008; Li et al. 2014).
3.2 Vertical sedimentary and geochemical characteristics
The shale intervals observed from the outcrop sections show strong heterogeneity. Typically, the samples from the YSC section (Fig. 1) were selected for analyzing the vertical sedimentary and geochemical characteristics of shale. As the Chang 7 Member high-quality source rocks are rich in thin-layered tuffs, 156 layers of tuffs were sampled from the YSC section at a centimeter-scale. Zhang et al. (2009) identified more than 180 layers of tuffs at millimeter- to centimeter-scale in the Chang 7 Member source rocks from Well Zheng-8 in the southern part of the Ordos Basin and concluded after observation and calculation that the tuffs gradually thin from the southwest to the northeast (with a thickness of more than 1.0 m in the Zhengning-Huangling area) and the tuffs are either airborne or waterborne (Deng et al. 2008; Zhang et al. 2009). Most scholars believed that the volcanic crater was derived from the Qingling orogeny (Deng et al. 2008; Wang et al. 2014).
Obviously, the whole-rock mineral and clay mineral compositions are affected by deposition and diagenesis (Sonnenberg et al. 2011; Sruoga et al. 2004; Huang et al. 2009). In this study, the Chang 7 Member shale was investigated for vertical variation of the whole-rock and clay mineral compositions. It is found that the three sub-members (with a total thickness of 28 m) vary in an ascending order of whole-rock and clay mineral compositions vertically. When F1 (8 m) was deposited, the water began to deepen, and the shale was interrupted by thick tuffs (> 2 cm) and contained a quite high content of feldspar, followed by kaolinite; at the end of this deposition period, thick siltstone bodies (gravity flow) were developed. When F2 (13 m) was deposited, the water deepened further and tuffs with different thicknesses were deposited; the shale in this period contained a fairly low and stable content of quartz, and an alternating low–high–low content of feldspar, clay minerals. When F3 (7 m) was deposited, several layers of thin tuffs (< 2 cm) were developed, and the shale contained a relatively high content of quartz and an alternating low–high–low content of clay minerals.
Based on the geochemical characterization results, together with the shale thickness, hydrocarbon potential, and organic matter type, we suggest that the most favorable shale oil target should be the uppermost Chang 73, different from the medium-thick areas determined by Ren. The recovery priority should therefore be F3 > F2 > F1. Overall, the shale in the Tongchuan area is not comparable to that in the USA in view of distribution, thickness, organic matter type, or hydrocarbon potential (Smith 1960; Cashion 1967). As previously estimated, the Tongchuan area has shale oil reserves of 953 million tonnes, technically recoverable shale oil resources of 357 million tons, shale oil resources of 58.8 million tonnes, shale oil in-place of 23.2 million tonnes, and recoverable shale oil resources of 16.6 million tonnes (Li et al. 2014; Ren 2008). It is concluded that selecting the highest-quality horizon through heterogeneity study is critical for shale oil recovery.
3.3 Sedimentary environment and pattern of the Chang 7 Member organic-rich shale
The sedimentary environment and pattern of the Chang 7 Member shale in the southeastern part of the Ordos Basin is highly controversial. Some scholars believe that the shale in the Tongchuan area is semideep–deep lacustrine facies (Bai et al. 2009; Yang et al. 2010), while other scholars argue that it comprises shallow lacustrine facies (Bai et al. 2010; Ren 2008). The terms “(shore-) shallow lacustrine facies” and “semideep–deep lacustrine facies” are often confusing, because a lacustrine shore is hardly distinguishable from a shallow lacustrine and a semideep lacustrine is hardly distinguishable from a deep lacustrine. Shallow lacustrine facies can be distinguished from deep lacustrine facies easily; the shoreline is a very important divider, but the shoreline itself is not always easy to define as it is highly variable (Bai et al. 2010). In practice, geologists usually divide oil shale into three types of sedimentary facies, i.e., marine, lacustrine, and marine-continental, and assume marine oil shale to be mainly present in a deep lacustrine environment. Some scholars suggest that this misunderstanding might have been caused by the inclusion of mudstone as oil shale. Bai et al. (2010) investigated wells with typically different sedimentary facies in the Ordos Basin and concluded that the shallow lacustrine facies corresponded to a limited number of oil shale layers, with small single-layer thickness (0.5–2.0 m) and small total thickness, shallow–semideep lacustrine facies corresponded to more oil shale layers, with large single-layer thickness (2–11 m) and large total thickness, and deep lacustrine facies corresponded to multiple oil shale layers, with small single-layer thickness (0.5–2.0 m) and large total thickness. In this study, the development period and the sedimentary environment and pattern of the Chang 7 Member shale were determined based on the lithology, geochemical characterization results and oil shale formation environment.
In this study, eight outcrop sections in the Tongchuan area were observed and sampled to investigate the Chang 7 Member, and sedimentary and geochemical characterizations were conducted for the well-outcropped YSC section. The study results show that the Chang 7 Member shale is widely distributed laterally with variable thickness. The organic-rich interval is 7–25 m thick in total and exhibits obvious horizontal variation in mineral composition. In the eastern sections, the shale contains organic matter of Type II2–III and is low in thermal maturity, with high clay mineral content, low K-feldspar content, and no pyrite. In the western sections, the shale contains Type II1 organic matter and is low in thermal maturity, with high clay mineral, K-feldspar, and pyrite contents. Apparently, the immature shale in the eastern sections implies no shale oil is available for recovery. In contrast, the shale in the western sections has high hydrocarbon potential and is thus a potential for shale oil recovery.
We collected samples from 156 layers of tuffs from the YSC section at a centimeter-scale. The results of mineral composition and geochemical study suggest that the shale presents three obvious intervals in mineral composition and organic abundance vertically. Based on the TOC and hydrocarbon potential evaluation, we conclude that the recovery priority should be F3 > F2 > F1. Given the high heterogeneity of the oil shale in the Tongchuan area, selecting the highest-quality horizon through heterogeneity study is critical for shale oil recovery.
Depending on the regional position and the oil shale thickness map, it is believed that the western sections should be shallow/semideep lacustrine facies rather than deep lacustrine facies. The shale intervals in the eastern and western sections are generally shallow lacustrine facies, but only three shale intervals in the western sections reflect semideep lacustrine facies. It is concluded that the paleo-climate constrained the sources of organic matter, the paleo-lake water bodies controlled the deposition and storage conditions of organic matter, the sedimentary facies determined the type and thickness of the oil shale deposits, and the paleo-structures dominated the organic enrichment through volcanic and turbidity current events and controlled the elevation and exposure of oil shale in the study area.
This study was supported by the National Basic Research Program of China (973 Program, No. 2014CB239001). We are deeply grateful for Li C.C. of Geological Survey Center, Shanxi Province, for his help in the field survey and outcrop.
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