China's Yanchi area, western Ordos basin derived from mixed source

July 6, 2015
The Yanchi area lies in the west of China's Ordos basin and to the west of the giant Sulige gas field, site of recent breakthroughs in Lower Permian exploration.

Zhou Shichao
Wang Xingzhi

Southwest Petroleum University
Chengdu, China

Liu Xinshe
Han Peng

PetroChina Changqing
Oilfield Co.
Xi'an, China

The Yanchi area lies in the west of China's Ordos basin and to the west of the giant Sulige gas field, site of recent breakthroughs in Lower Permian exploration. Two tectonic units, the Tianhuan syncline and Yishan slope, cross the basin, which is located where Shanxi, Gansu, Ningxia, and Inner Mongolia converge to share a common border.

The area extends to Huanxian in the south, Otog Front Banner in the north, and Anbian and Majiatan in the east and west, respectively. Its 21,500 sq km (8,300 sq miles) include several counties and cities such as Otog Front Banner, Yanchi, Dingbian, and Huanxian (Fig.1).

For areas such as China's Ordos basin, source analysis plays an important role in understanding its sedimentary facies. Important geological studies that further oil and gas exploration include:

• The identification of ancient erosion zones.
• Reproduction of ancient river systems.
• The delineation of parent rock properties in the source area.1
• Identification of climate and tectonic setting for a particular sedimentary basin. The sedimentary system, paleocurrent, and source directions and the close relationship of these features to those of the source area have a direct influence on the oil and gas exploration process.

Yanchi area

At present, methods such as heavy mineral analysis, paleocurrent direction, geochemistry, clastic rock classification, lithic composition analysis, sandstone composition, and fission track dating are available for analyzing source data in a specific basin.2-4

Researchers have put forward many views concerning the development of circumbasin material, tectonic setting, and general source direction in adjacent formations and areas of Ordos basin, but few have carried out studies on the source of the Yanchi area to the west of Sulige gas field.5 6

On the basis of previous research, this article analyzes parent rock properties and tectonic setting with a discussion about the sedimentary source direction and the condition of mixed sources in Member 8 of the Shihezi formation and Member 1 of Shanxi formation, in the Yanchi area.

This work provides a theoretical basis for future oil and gas exploration in the Yanchi area by deploying a variety of source analysis methods, including sandstone mineral composition, quartz cathodoluminescence, heavy mineral assemblage, identifying rare earth elements (REE), trace elements, and measuring zircon ages and paleocurrent direction. In addition, the study of these data provides an inferred form of the mixed source area by employing uniformitarianism.

The sediment body in the river course found in the Lower Permian of the Yanchi area shows a regular pattern of spreading in a NW-SE direction. Therefore, the area is divided into three regions from West to East: Areas 1, 2, and 3 (Fig. 1).

Mineral composition, characteristics

In addition to climate, topography, transportation distance, and diagenesis, the properties of parent rocks and the tectonic setting of a source area are among the main factors affecting the composition of clastic rocks.

To obtain the plate tectonic information for the three sections of the Yanchi area, this study analyzed sandstone mineral composition of 429 samples from 65 wells in Member 8 of the Shihezi formation and 108 samples from 30 wells in Member 1 of the Shanxi formation. The components were point-counted for QFR (quartz, feldspar, and rock fragments), as well as QmFLt, QtFL, and QpLvLs (Table 1).7 Figs. 2-3 compare sandstone composition with that of the Sulige field.

Due to crystal lattice defects and differences in the trace element content of crystals, quartz grains of different origins show different cathodoluminescence characteristics under electron irradiation. The differences in luminescence of quartz grains can discriminate their genetic environments and determine their source direction.

In recent years, many researchers have used this source analysis method.8

The quartz grains in the Yanchi area mainly emit brown light with a small number emitting blue light under cathodoluminescence. This indicates the quartz is metamorphic, with a small amount of magmatic quartz emitting purple light (Tables 2-3).

Heavy minerals provide an important method for identifying source directions as they are corrosion resistant and stable, retaining many characteristics of their parent rock.4 9 10 Furthermore, heavy minerals are well documented in the Yanchi area, and their comparison results are reliable. Heavy mineral analyses include heavy mineral assemblage analysis and a heavy mineral characteristic index, or ZTR, analysis (Fig. 4). With these two analytic methods, the stability, spatial distribution, and other characteristics of heavy mineral assemblages can determine the types of parent rocks and tectonic settings of the source area, infer the sediment transport distance, and determine the source direction.

Rare earth elements (REE) and trace elements such as Th, Sc, Cr, and Co and other trace elements in sedimentary rocks have low solubility and are relatively stable (See Box). They are transported mainly in the form of particulate matter and quickly enter fine-grained sediments. They are highly resistant to transport in sedimentation and metamorphism processes. Later weathering, diagenesis, and alteration have a weak impact on these elements.

REE characteristics are controlled by the rock composition in a source area and can show the geological characteristics of source rocks.3 REE properties of a sedimentary rock provide information about its parent rock in a source area, which allows sedimentologists to corroborate traced source-area study results (Fig. 5-6).

Trace elements play important roles in researching the type of source area and the identification of tectonic settings.2 11 Source indicators for sediment typically include Zr, Th, Sc, and Y (Figs. 7-8).

The age distribution of zircon uranium-lead (U-Pb) in the clastic rock reflects the magma-metamorphism that occurred in the source area, further reflecting the structure of tectonic layers. The composition of sources during sedimentation of layers can be deduced by comparing the age information obtained from the zircon in the sediment and the age of rock mass outcrops on the adjacent mountains in the basin.12-14

Our study took 90 zircon U-Pb samples from the sandstone samples obtained at both the Li 4 well in the central study area and the Lian 54 well in the southeast of the research area.

These samples are mostly transparent and of a partially light brown color. The samples were mostly round in shape, supplemented by idiomorphism, meaning they have experienced weathering, transportation, and abrasion.

Most of the Th/U values for zircon are high, between those expected for magmatic origin and metamorphogenetic zircon, which may be caused by incomplete metamorphic recrystallization or a later geological event. Through comparative analysis of the 207Pb/206Pb age frequency distribution diagram at the corresponding sections, the two wells show different peak values, meaning these two wells had different source compositions during sedimentation (Fig. 9).

Parent rock properties

Point counting in the QFL diagram concludes that the main rock types in Member 8 of the Shihezi formation include lithic quartz sandstone, quartz sandstone, and lithic sandstone, and the types in Member 1 of the Shanxi formation include lithic quartz sandstone, lithic sandstone, and quartz sandstone in small amounts (Fig. 2).

When projected on an REE-La/Yb diagram, the 26 samples collected from the research area display sourcing from sedimentary or metamorphic rocks (Fig. 5). This conclusion is consistent with the characteristics of abundant sedimentary or metamorphic rocks and scarce magmatic rocks found through lithic fragment analysis (Table 1).

The major source areas of the Shehezi and Shanxi formations in the Yanchi are basically the same. Characteristics of matrix mineral compositions, REEs, zircon U-Pb dating in clastic rock, and chronologic isotope research of strata from Yinshan Mountain on the north edge of the Ordos basin combine to show that parent rocks in the source area include epimetamorphic rock, moderately deep metamorphic rock, sedimentary rock, and various kinds of intrusive and extrusive rocks in more limited quantities.15

The data from Li 4 indicate that the zircon at the strongest peak-1,800-2,000 million years (Ma.)-is predominantly metamorphogenetic, accounting for 56.7% of its content. Zircon aged 2,000-2,600 Ma. is metamorphogenetic with a certain amount of magmatic zircon. This result is consistent with the age of zircon (dominated by metamorphic zircon) in clastic rock aged 2,000-2,300 Ma. and 1,850-1,950 Ma. in the khondalite zone on the north and northwest edge of Ordos basin; the age of TTG gneiss and granulite in the western Yinshan Mountain region (2,500-2,600 Ma.); the formation age of gneissic granite (2,400-2,500 Ma.); the emplacement age of magma in the Daqing Mountain region in North Ordos basin; and the thermal tectonic event from Neoarchean to Paleoproterozoic.

The ages of all zircon samples in clastic rock at the strongest peak (300-400 Ma., Devonian-Carboniferous) indicate that the sedimentation at Member 8 of Shihezi formation and Member 1 of Shanxi were supplied by the continental crust substances in the same period.

The rocks at these sections in the Yanchi are quartz sandstone characterized by high contents of quartz scrap and quartzite debris, where the source is controlled by a khondalite zone, rich in quartz and quartzite formed in the Paleoproterozoic in the Yin Mountains region (or source area where the khondalite zone is located) and the TTG gneiss. Lithic sandstone is characterized by many metamorphic rocks, sedimentary rock debris, and some magmatic debris, where the source is affected by the khondalite zone and the TTG gneiss, as well as the Archean Eonothem old metamorphic rock, Devonian-Carboniferous granite, and the continental crust volcanic substances in the Yin Mountains region. The source of the lithic quartz sandstone is jointly controlled by the same source of quartz sandstone and lithic sandstone.5

Tectonic settings

The distribution patterns of the sedimentary basin and the source area are controlled by tectonic structure to a considerable extent. Therefore, the composition and structural characteristics of clastic sediment in the sedimentary basin are closely related to the tectonic properties in the source area.

The discriminant analysis of tectonic settings in the source area obtained with the QmFLt triangular model for continental detrital sandstone shows the sediment as primarily from the internal craton region, or that a considerable quantity of detrital materials were transported when a sand-carrying water system passed through the interior of the craton (Fig. 3).

The QtFL model shows all samples in the recycling orogenic belt source area. Point counting of lithic contents on QpLvLs diagrams proves that the tectonic settings in the source area are of the orogenic belt with a relatively complex structure. A few points within the volcanic arc orogenic belt source area imply that volcanic materials related to the Xingmeng trough on the northern side may have migrated into the basin.

La/Th-Hf and La/Sc-Co/Th data obtained from 83 samples from 33 wells in the research area show the source rocks as dominated by a felsic source of the average upper crust, an active continental margin source with orogenic belts, and a passive continental margin source. The rocks are partially subject to the intrusions of older sediments such as Phanerozoic craton sandstone and light amounts of andesite (Fig. 7).

Analysis of Th-Co-Zr/10, Th-Sc-Zr/10, and La-Th-Sc shows all samples as within active and passive continental margin areas and continental island arc areas, which feature complex tectonic settings (Fig. 8).2

Source directions

Comparison of sandstone composition in the Yanchi area with that in the Sulige area shows that the Yanchi features abundant quartz, lithic fragments (28.8%), and relatively scarce feldspar (part of the feldspar underwent alteration to kaolinite); (Table 1). In particular, there is a much lower component of magmatic rock and a much higher proportion of metamorphic and sedimentary rock components among the lithic components.

Previous research has demonstrated that the source north of the basin (including the Sulige area) is mainly from the direction of the Yin Mountains on the north margin of the basin.15

Quartz cathodoluminescence test results show that the main source of the quartz in the research area is sedimentary rock, followed by metamorphic rock, with the lowest composition from magmatic rock (Table 2). The comparison of the composition of quartz grains in Yanchi and that in the northern margin of the basin indicates that the quartz differ in origin, a fact demonstrated by the lower magmatic rock component in the Yanchi, and its higher metamorphic and sedimentary rock components. This conclusion is consistent with the lithic component analysis in the matrix mineral analysis (Table 1).

Based on the rock matrix mineral analysis, the source of these formations in Yanchi is not only from the north of the basin, but also from other directions.

Two main paleocurrent directional values are based on the measurement and calibration of the field outcrop paleocurrent direction in Member 8 of Shihezi formation and Member 1 of Shanxi formation in the northwest of the research area (Hulusitai profile: 144°-189°; and Qianlishan profile: 92°-165°)(Fig. 10). This demonstrates that the main paleocurrent direction at the northwestern field profile is NW-SE, showing a source from the northwest direction. When taken together with the data from Zaozhuang at the northeastern field profile of the research area, the measured data indicate that the main paleocurrent direction is NE-SW.15 According to the measurement results of the mineral composition, cathode luminescence, and Permian paleocurrent, the source of the Yanchi area commonly consists from the Alxa direction in the northwest and from the Yin Mountains direction in the north.

Mixed source

The comparison of the cathodoluminescence characteristics of quartz in Areas 1, 2, and 3 indicate that the degree of metamorphism of the metamorphic rocks changes from shallow to deep from west to east (Table 3). The quartz grains in the origin of the western portion of Area 1 are consistent with a source feed from the Alxa area in the northwest. Quartz grains from the eastern portion of Area 3 are affected by the western section of the Yin Mountains archicontinent in the north. The central portion of Area 2 is a mixed source region, combining source characteristics from both directions, with shallow-medium metamorphic quartz grains, quartz of magmatic rock origin, and a predominant sedimentary quartz. The heavy mineral assemblages in 15 wells in the Yanchi area show a broad variation of characteristics across all three study areas (Fig. 4).

The heavy mineral assemblage in Areas 1 and 3 is zircon + leucosphenite + garnet, while that in the mixed-source Area 2 is mainly dominated by tourmaline + zircon + leucosphenite. Its abundant tourmaline content is different from other regions. A higher ZTR index indicates higher maturity.9 Comparison of Area 2 with Areas 1 and 3 in terms of their ZTR indices for 10 northern wells, shows Area 2's index as lower than Area 1 or 3. A comparison of the ZTR indices of nine wells in Area 2 reveals that the ZTR index in the northern portion is lower than that in the south. The maturity of the heavy minerals increases from north to south. More than 10 sample wells in the research area were selected for comparison with REEs in the field profile and the basin periphery. The researchers discovered that REE distribution in Area 1 shows characteristics of serious depletion of Eu, Ho, and other heavy REEs.

The main source for Area 1 is the Alxa in the northwest, and Area 3 is highly affected by the source in the north of the basin, which is demonstrated by the depletion in Eu, enrichment in light REEs, and slight depletion in heavy REEs.15 The curve for Area 2 is smooth, with no depletion in Eu. The mixed source area is wedge-shaped and formed under the influences of the Alxa and Yinshan from the NW and north, respectively (Fig. 6).

Age determination

On the basis of uniformitarianism, the Yanchi area is sourced from two directions. The source runoff and the amounts of material from Alxa and Yin Mountains form the mixed source Yanchi in a fusiform "S," with two pointed ends and a wider middle portion (Fig. 10).

The original data this study employed for dating orogenic belts in the research area and zircon U-Pb isotopes in the basin had already been collected to provide evidence of ages for comparison between orogenic belts and the basin and the approximate location of the source area.13 16 Comparison between the zircon U-Pb data in the adjacent areas and the positions and age measurement for zircon obtained from the Li 4 and Lian 54 show two sources dominate the research area: The Alxa to the northwest is represented by 2,500-2,600 Ma. TTG gneiss and the Yin Mountains archicontinent to the north consists of 1,800-2,300 Ma. khondalite (Fig. 10).

The measurements for zircon in the Li 4 well and the Uxin Banner area are similar, sharing the characteristics of the measured values of zircon obtained in the northern and northwestern regions of the basin.5

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The authors
Zhou Shichao ([email protected]) is a PhD student at the College of Earth Science and Technology, Southwest Petroleum University, Chengdu. She holds a BE from Southwest Petroleum University (2009), and achieved her ME from Southwest Petroleum University in 2012.
Wang Xingzhi ([email protected]) is a professor and doctoral supervisor at the College of Earth Science and Technology, Southwest Petroleum University, Chengdu. He serves as the deputy dean of the College of Earth Science and Technology, Southwest Petroleum University. He also serves as the leader of Sichuan province's Academic and Technical Council, as the deputy director of CNPC's carbonate key laboratory, and deputy director of Ministry of Land and Resources sedimentary basin and oil and gas resources key laboratory. He received a PhD from the China University of Petroleum, Beijing, in 1996. He is a member of the China Mineral Rock Geochemical Lithofacies Paleogeographic professional committee and of the China Unconventional Oil and Gas professional committee.
Liu Xinshe ([email protected]) is a senior engineer of exploration geology and deputy chief geologist at the Institute of Exploration, Development, and Research of PetroChina Co. Ltd.'s Changqing Oilfield Branch, Xi'an, China. He holds a PhD from the Northwest University, Xi'an.
Han Peng ([email protected]) is an engineer of exploration geology at the Institute of Exploration, Development, and Research of PetroChina Co. Ltd.'s Changqing Oilfield Branch, Xi'an. He holds an MS from Northwest University, Xi'an.