J. Cent. South Univ. (2012) 19: 1388-1398 

DOI: 10.1007/s11771-012-1155-6

Application of sequence stratigraphy to Triassic terrestrial strata in Tahe area of Tarim Basin

LIU Chen-sheng(刘辰生)^1,2, ZHANG Lin-ting(张琳婷)^1,2, GUO Jian-hua(郭建华)^1,2, WANG Ming-yan(王明艳)^1,2

1. School of Geosciences and Info-Physics, Central South University, Changsha 410083, China;

2. Department of Mineral Resources and Mine Management, Hunan Non-ferrous Metals Co. Ltd., Changsha 410015, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: Based on a synthetic geological study of drilling, well logging and seismic data, core observations and geochemical analysis, it is recognized that Triassic sedimentary facies in the Tahe area of Tarim Basin include braided river deposits filling erosional valleys, and sublacustrine fan, canyon and delta facies. Braided river deposits filling erosional valleys are dominated by coarse-grained lithic quartz sandstone with oblique bedding, and represent the most important sedimentation type of sandstone in the study area. Sublacustrine fan and canyon facies are mainly distributed in the Middle Oil Member. Most delta sediments are deposited in highstand system tract (HST). Because of frequent changes in base level, delta sediments are commonly eroded and rarely preserved. Sedimentary cycles are clearly reflected by lithology, sedimentary structures and well logging data, and are closely related to the changes in lacustrine level. In accordance with the basic principle of sequence subdivision, seven type-I boundaries can be recognized in Triassic strata and six type-I sequences are subdivided correspondingly. In general, lowstand system tract (LST) is well developed within stratigraphic sequences and forms the main body of reservoir sandstone in this area; highstand system tract (HST) and transgressive system tract (TST) are often eroded by upper sequences or missed. Although various factors have different influences on terrestrial sequence stratigraphy, the classical sequence stratigraphy theory proposed by VAIL can be applied to terrestrial strata.

Key words: sequence stratigraphy; erosional valley; sublacustrine fan; terrestrial sequence stratigraphy; Tahe area; Tarim Basin

1 Introduction

Sequence stratigraphy is applied to the interpretation of strata from seismic, well log, core, and outcrop data, and the identification of a hierarchy of sedimentary units, including the depositional sequence, and the correlation of their bounding chronostratigraphically significant surfaces. During the mid-1960s, VAIL’s research group in Exxon Production Research Co. worked on the new, greatly improved generation of multifold seismic reflection data for the continental margins of the world. Their work brought the stratigraphy and seismic interpretation together in developing the original concepts of seismic stratigraphy.

Later, these concepts were applied by VAIL and his co-workers to well logs, cores, and outcrops, broadening seismic stratigraphy into what is known today as sequence stratigraphy. Using these data, large-scale, basin-wide depositional patterns, stratal configurations, and unconformities in basins are interpreted around the world. A chronology of global sea-level fluctuation was proposed as a framework for global correlation, resulting in a world sea-level curve. This further led to a new eustatic sea-level model. The results of these studies impacted on many scientific disciplines, but its implications for the petroleum industry are extensive.

Classical sequence stratigraphy model of VAIL is based on study of passive continental margins [1-7], and whether or not this model can be applied to terrestrial stratigraphy has always been a focus of controversy [8-10]. Unlike passive continental margin systems, lake basins have limited accommodation space [11], and there are many provenances from which terrestrial sediments can be transported to lake basins [12-16]. Sediments and facies can vary significantly in response to change of lake level in lake basins [17-18]. Application of terrestrial sequence stratigraphy has recently attracted great attention of geoscientists because studies of terrestrial sequence stratigraphy can guide exploration and development of terrestrial oilfields [19-22]. Therefore, refined sequence stratigraphy models are needed for terrestrial basins. Furthermore, a lot of studies on terrestrial sequences have been conducted [23-28]. The results of these studies indicate that the VAIL model can be used in terrestrial sequences [29-31].

In the study area, terrestrial sediments were deposited in the Triassic, and the main rock types are sandstone and mudstone. According to the analysis of sporopollen, the paleoclimate in this area was semiarid to subhumid during deposition of Triassic sediments [32]. In recent years, numerous studies on Triassic sedimentary types have been carried out. There are two main viewpoints about the Triassic sedimentary facies, which suggest that they are dominated by braided delta and lacustrine facies [33] or meandering stream and lacustrine facies [34].

2 Regional tectonic and basin framework

The Tahe area refers to second-order uplift in the Shaya uplift zone that is situated in the northeastern depression of the Tarim Basin [35]. Studies on tectonic evolution history demonstrate that the southern Tianshan Ocean was closed while it was underthrust to the Tarim plate in the early Triassic. As a result, the southern Tianshan was rapidly uplifted, and the Kuche depression to the south rapidly subsided due to tectonic load [36-37]. The tectonic framework of the northern Tarim Basin was formed in this stage. The Kuche depression is a wedge-shaped depression with deposition center in its northern part; the southern Tianshan fold belt is located to the north of the Kuche depression and is the major source area of sediments in the depression; the Shaya uplift zone and the post-uplifting basin are located to the south of the Kuche depression; the Shaya foreland uplift is a major source area for sediments in both the study area and the Kuche depression. The Triassic sedimentary basin in the study area is located in the Shaya uplift zone and its southern slope, and belongs to the Kuche foreland basin system (Fig. 1).

The Triassic strata can be subdivided into three thick sandstone units: the upper, middle and lower sandstones that have good physical property and are named as the Upper Oil Member, Middle Oil Member and Lower Oil Member [27, 34]. The Lower and Middle Oil Member form the Akekule Formation and the Upper Oil Member forms the lower part of the Halahatang Formation (Fig. 2).

The Lower Oil Member is mainly composed of gray, coarse-grained pebbly sandstone that is the thickest and coarsest-grained sandstone in the study area. The Middle Oil Member contains middle- to coarse-grained sandstone, conglomerate, and miner fine-grained sandstone. The Upper Oil Member contains medium- to fine-grained sandstone, and minor mudstone. In general, Triassic rocks in the Tahe area are dominated by lithic quartz sandstone.

3 Types of sedimentary facies

Based on analysis of lithology, sedimentology and sedimentary mechanism of sandstone units, the sedimentary facies in the study area are subdivided into braided river, sublacustrine fan, delta, lacustrine, and canyon facies.

3.1 Braided river facies

Braided river facies is the dominant sedimentary facies of thick sandstone units of all oil members in the study area, and represents the infilling deposition of erosion valley that is related to rising lake level. Petrological and mineralogical analysis indicates that sandstones deposited in braided rivers are dominated by feldspathic litharenites, lithic quartz sandstones, lithic arkoses, lithic sandstones, and feldspathic quartz sandstones, with rare arkoses. Most sandstone is middle- and coarse-grained, with some pebbly sandstone.

Fig. 1 Location and tectonic setting of study area

Fig. 2 Sequence subdivision and facies interpretation of Triassic strata

Abundant sedimentary structures are observed in braded river facies, including large-scale oblique beddings, tabular cross beddings, parallel beddings, and wedge-shaped cross beddings (Fig. 3). Fossils are rare although there are some fragments of plants in inter-channel mudstones.

Braided river facies deposits are commonly composed of several coarsening-up, 1–3 m thick cyclothems. Clear erosional contacts are observed among cyclothems. Poorly sorted fine conglomerates or pebbly sandstones are present at the bottom of some cyclothems. Such coarse beds may be lag deposits at the braided river bottoms or debris flow deposits triggered by storms. One cyclothem represents a depositional event related to lateral migration of the river. Some cyclothems contain thin intervals of yellow mudstones with mud cracks.

The thickness of braided river deposits filling erosional valleys is variable in different areas. Well logging correlation and seismic inversion results show that braided river deposits vary significantly in thickness, and in some places, they have eroded and incised underlying strata, with partial incision depth of more than 100 m. The sedimentary facies is discontinuous across the unconformity surface formed by incision and erosion. Similar features are observed in Mesozoic–Tertiary sedimentary rocks in Louisiana, USA [38], where sandstones and conglomerates deposited in braided rivers have eroded and incised lower strata, resulting in loss of delta deposits forming during marine regression.

3.2 Sublacustrine fan facies

So far, sublacustrine fans in the study area have only been found in the Middle Oil Member and are distributed in the southern Akeyasu area (wells 3, 4, 5, 6 and 7) (Fig. 4). Based on core observation and well logging interpretation, sublacustrine fans can be subdivided into the central fan and distal fan subfacies. The central fan subfacies can be further subdivided into the braided channel and inter-channel microfacies.

Braided channel microfacies forms the major part of sublacustrine fan facies, and is mainly composed of coarse deposits, such as fine conglomerates, coarse pebbly sandstones and medium-grained sandstones. Fine conglomerates in the lower part of braded channel deposits are matrix-supported, with normal fining-up graded beddings. Trough cross-beddings and tabular beddings are present in the middle part of braded channel deposits (Fig. 5), and Bouma sequences are present in the upper part of braded channel deposits. Well logging curves and core data analysis indicate that the central fan subfacies are commonly composed of two sequences with normal fining-up graded beddings.

Stratigraphic correlation across different wells indicates that braided river deposits in the north are gradually changed into shore-shallow lacustrine deposits on slopes, and then into semi-deep lacustrine and sublacustrine fan deposits in the south. Several down-cutting channels are developed in the slope areas and transport sediments to sublacustrine fans. According to the plane graphics of seismic amplitudes and well logging data, five sublacustrine fans have been identified in the southern part of the study area, all of which are connected with down-cutting channels in the sloping areas.

Fig. 3 Infilling features of incised valley in well 1

Fig. 4 Facies trend map of Middle Oil Member in Triassic strata

Fig. 5 Lacustrine fan depositional features in Well 3

3.3 Delta facies

Delta facies is mainly made up of delta front subfacies and prodelta subfacies. The former is composed of distal bars, sheet-like sandstones and channel-mouth sand bar microfacies that become coarser upwards. Compared with thick sandstones in braided river facies, sandstones in delta deposits are relatively thin, commonly 2–8 m thick, and are dominated by siltstones and fine-grained sandstones.

3.4 Lacustrine facies

Lacustrine facies can be subdivided into shore-shallow lacustrine, and semi-deep to deep lacustrine subfacies. Shore-shallow lacustrine deposits can be subdivided into two types: shore-shallow lacustrine deposits inter-grown with braided river and sand spit and sandbar deposits inter-grown with delta frontal edge.

Slow rise of lake level causes the infilling of erosional valleys, and the middle and lower parts of the valleys are mainly filled by braided river deposits. The valleys are drowned with further rise of the lake level when extensive shore-shallow lacustrine sedimentation occurs. As a result, thick fine-grained sandstones and siltstones are present in the upper or top part of the fluvial facies deposits. These sandstones are well sorted, with highly matured minerals and strong homogeneity [19]. Horizontal beddings, small-scale cross beddings and wavy beddings are the dominant sedimentary structures in these sandstones.

Semi-deep and deep lacustrine subfacies deposits are mainly composed of gray mudstones, locally interbedded with thin siltstones.

3.5 Canyon fancies

Canyon facies is present only in the Middle Oil Mmember, and is inter-grown with sublacustrine fans and distributed on the southern slope of the study area. Comparison of well-correlating profiles shows that canyon deposits can be found in many wells. According to core data and well logging response characteristics, canyon deposits are dominated by sandstones, pebbly sandstones and mudstones, mostly with fining-up filling sequence. Canyons on the slopes are quite obvious in the attribute maps of seismic amplitude and three- dimensional seismic profiles (Fig. 6). Canyon deposits in wells 8, 9, 10 and 11 are composed of interbedding sandstones and mudstones, and the sandstones are thicker than mudstones. They contain two normal fining-up cyclothems, indicating that the lake level was changed twice during the depositional process. This is consistent with the lake level changes recorded by the depositional process of sublacustrine fans.

The average thickness of sandstones deposited in canyons is approximately 10 m in the study area, with a maximum thickness of 18 m in Well 10 and a  minimum thickness of 7.5 m in Well 8. In general, canyon sandstones gradually become thicker from north to south. Because the canyons are located closely adjacent to the oil-generating center of the basin and there are passage ways to transport oil and gas upwards, these sandstones are usually favorable reservoirs of oil and gas. For example, both Well 9 and Well 11 along a canyon are oil and gas wells.

Fig. 6 Canyons and lacustrine fans of seismic amplitude of Middle Oil Member

4 Subdivision of Triassic sequences and their characteristics

The Triassic deposits in the study area are subdivided into six sequences based on observations of drill cores, interpretation of seismic data and identification of well logging facies. These six sequences are named as SQ1, SQ2, SQ3, SQ4, SQ5 and SQ6   (Fig. 2), respectively, from bottom to top. The boundaries of these sequences are characterized by erosion, truncation, and abrupt change of depositional facies and lithofacies [39], and are easy to identify from drill cores, well logging response and seismic profiles [13].

Sequence stratigraphic correlation across different wells is based on sequence subdivision of individual wells, and is adjusted by using seismic profiles. Individual sequences are generally well correlated in the study area.

SQ1 is composed of the Ketuer Formation. It is underlain unconformably by Carboniferous strata, and overlain by the lower sandstones of the Lower Oil Member of the Akekule Formation, with an erosional contact (Fig. 7). TST and HST can be identified in the whole study area. The lowstand system tract (LST) is poorly developed and can only be observed in a few wells. LST is mainly composed of coarse-grained and pebbly sandstones that were deposited in erosional valleys and local depressions when lake level rose after the top Carboniferous strata was eroded. TST is a semi-deep to deep lacustrine facies that is composed of dark grey and black mudstones, shale and silty mudstones. Self-potential (SP) curves show that the grain size of sediments becomes fining-upwards; the water depth gradually becomes deeper and eventually reaches the maximum flooding surface. HST includes semi-deep to deep lacustrine, shore-shallow lacustrine and delta facies deposits that are dominated by coarsening-up dark grey mudstones and silty mudstones, with shallowing-up water depth. HST and part of TST in SQ1 are often eroded by LST in the overlying SQ2.

SQ2 consists of the lower sandstone section of the Lower Oil Member. LST in SQ2 is distributed throughout the study area whereas HST has been eroded and TST is rarely preserved. LST is dominated by medium-grained sandstones, pebbly sandstones and coarse-grained sandstones, among which medium- grained snadstones account for 70% of the LST deposits. The sandstones are interbedded with thinly bedded mudstones with an average thickness of 2–3 m. Such lithology association represents filling deposition of eroded vallys during the late stage of LST. The thickness of LST varies between 50 m and 80 m, with an average thickness of 59.7 m. TST and HST in SQ2 are partly eroded by LST of the overlying SQ3.

SQ3 consists of the upper sandstone section of the Lower Oil Member, and its bottom incises SQ2. This sequence has well-preserved LST and TST but lacks HST due to erosion by the above sequence. It spreads all over the study area. LST in SQ3 is composed of pebbly coarse sandstones, medium-grained sandstones and fine-grained sandstones deposited in erosional valleys during the late stage, with a thickness of 20-60 m and an average thickness of 47.2 m. During transgressive period, lake level rose rapidly to submerge the study area and semi-deep to deep lacustrine sediments of dark coloured mudstones and muddy siltstones were deposited. Accommodation space was decreased during deposition of HST, but the sediment supply was sufficient. Abundant delta sediments were deposited in the study area. The rock types of HST include fine-grained sandstones, mudy siltstones and dark coloured mudstones.

Fig. 7 Sequence boundaries shown by dash lines in outcrops and depositional features of incised valley infilling: (a) Sequence boundary between SQ2 (above dash line) and SQ1 (bellow dash line) with thick beds of sandstone above boundary representing incised valley infilling deposits of LST in SQ2; (b) Sequence boundary between SQ4 (above dash line) and SQ3 (bellow dash line) with dark mudstone and gray sandstone bellow dash line representing delta facies deposits in HST of SQ3; (c) Sequence boundary between SQ5 (above dash line) and SQ4 (bellow dash line); (d) Large scale cross-bedding in LST deposits of SQ2

SQ4 consists of the Middle Oil Member and represents the best preserved sequence in which LST, TST and HST can all be identified. LST at the bottom of SQ4 incises SQ3 and HST at the top is clearly eroded (Fig. 7). According to the characteristics of sedimentation and seismic reflection, three types of sedimentary systems can be identified in LST: early sublacustrine fan system, middle depositional system filling erosional channels on slopes, and late depositional system filling erosional valleys. The sublacustrine fan system is composed of coarse-grained and poorly sorted sandstones, conglomerates and pebbly medium-grained sandstones. The filling system of erosional valleys is mainly composed of medium- to fine-grained sandstones.

LST in SQ4 has a thickness of 0–50 m, and is much thinner than that of SQ2 and SQ3. The thickness of LST varies significantly from north to south. It is the thickest in the northern part of the study area, with no deposition in the middle part, and the sublacustrine fan deposits in the southern part has an average thickness of 40 m   (Fig. 4). TST with an average thickness of 45 m is mainly composed of dark-grey to balck mudstones and silty mudstones deposited in semi-deep to deep lakes. HST consists of dark coloured mudstones deposited in semi-deep to deep lacustrine subfacies, and grayish white fine-grained sandstones deposited in shore-shallow lacustrine subfacies, with local delta sedimentation. HST is 30–40 m thick, and the variation of thickness across the study area is relatively small.

SQ5 consists of the lower sandstone section of the Upper Oil Member and is bounded by clear erosional contacts. This sequence is the most unstable one in the study area. LST and TST are quite well developed in SQ5, but most of HST has been eroded as a result of falling base level. Because of rugged topography, the thickness of younger deposits filling erosional valleys becomes uneven. LST is mainly composed of medium- to coarse-grained sandstones and fine-grained sandstones, with a variable thickness of 0-40 m and an average thickness of 25 m. TST is composed of dark gray mudstones deposited in semi-deep to deep lakes, with a thickness of 5-30 m.

SQ6 is composed of the upper sandstone section of the Upper Oil Member, and contains LST, TST and HST. After deposition of SQ5, the study area was flat, and as a result, the distribution of SQ6 is relatively even. LST that is also formed by the filling sedimentation system of erosional valleys is made up of medium- to coarse- grained sandstones and fine-grained sandstones, with an average thickness of 70 m. TST with an average thickness of 5 m is mainly composed of semi-deep to deep lacustrine deposits, and the maximum flooding surface can be used as a stratigraphic correlation mark across the whole area. HST is composed of semi-deep to deep lacustrine, delta, and shore-shallow lacustrine deposits, with a maximum thickness of 80 m.

In summary, SQ2 and SQ3 were deposited when the lake level rose continuously, representing a transgressive stage of a second-order sequence. The lake level was highest during deposition of SQ4, representing a maximum flooding stage. The lake level began to fall during deposition of SQ5, representing a highstand stage of a second-order sequence. Therefore, SQ2-SQ5 correspond to a complete second-order sequence. The lake level rose again during deposition of SQ6, marking the development of a new second-order sequence    (Fig. 8).

5 Formation mechanism of slope break zones in lake basins

Based on a study of canyons in the Mississippi River, KOLLA et al (1993) proposed the relationship between canyons and slope sediments (Fig. 9). As shown in Fig. 9(a), when there are no canyons or if the canyons are small, thick wedge-shaped deposits or slope fans are deposited. In contrast, when the canyons are well developed (Fig. 9(b)), there are no wedge-shaped deposits. A non-depositional area on slope is also developed in LST of SQ4 (Middle Oil Member) in the central and southern parts of the Tahe area because several canyons on the slope transport deposits to the depositional area in southern sublacustrine fans. These canyons are clearly reflected in both seismic attribute map and plane facies map (Figs. 4 and 6).

Appearance of slope break zones of lake basins co-developed with the non-depositional area is a prerequisite for the formation of canyons and sublacustrine fans because the slope breaks zones provide sedimentary sources and indispensable gradients [33, 40]. Geological and geophysical analysis indicates that the gradient of the slope break zone in the study area is up to 35°. Its formation is related to the plastic flow of salt rocks in underlying Carboniferous strata, and the pinching-out surface of salt rocks coincides well with the slope break zone. Core observation results indicate that the average thickness of the salt rocks is 150 m. As the overlying sediments become thicker, the salt rocks are compacted and plastic flow starts in all directions. Adjacent to the pinching-out surface, the salt rocks are uplifted due to the resistance of clastic rocks, and the overlying strata close to the salt rocks are compressed and deformed (Fig. 10), forming a slope break zone in the southern part of the study area. Moreover, northeast-southwest-oriented shortening during the Indosinian Movement in the Triassic is also related to such a plastic flow of salt rocks.

Fig. 8 Sequence pattern of Triassic strata in Tahe area

Fig. 9 Slope deposit pattern of no-canyon (a) and with canyon (b)

6 Discussion

Similar to the sequence stratigraphy developed on passive continental margins, terrestrial sequence stratigraphy is also affected by various factors such as structures, climate and provenance [41] although these factors influence the development of the two types of sequences to different degrees. Structures have a greater influence on the development of terrestrial sequence stratigraphy than that of passive margins [42]. Lacustrine basins are relatively small, so that a regional tectonic uplift can form erosional valleys on the edge of the lacustrine basins. During the Indosinian Movement, compression and relaxation derived from the southern Tianshan plate and the Kunlun plate that are located to the north and south of the Tarim Basin, respectively, have resulted in frequent uplift and subsidence in the study area to form seven third-order sequence boundaries and six sequences within Triassic strata.

Sediment supply is an important factor affecting the third-order sequences [28]. As terrestrial sequences are deposited close to provenance, they are mainly composed of feldspar-quartz sandstones and lithic quartz sandstones with low stability, and pure quartz sandstones are rare. Terrestrial sequence boundaries are difficult to identify due to closeness to provenance, sufficient supply of sediments and rapid changes of depositional base level. In contrast, marine shore and tidal flat environments are relatively far away from provenance, with thinner sedimentary sequences but clear sequence boundaries that are easier to be identified [43].

Because lacustrine basins are much smaller than marine basins, climate has more impact on deposition of lacustrine sequences. The influence of climate is mainly reflected by the change of lake (sea) level. When it is moist, the lacustrine level is high, and TST and HST are developed in lacustrine basins; shore-shallow lacustrine, semi-deep to deep lacustrine, and delta deposits form the major sedimentary systems. When it is dry, the lacustrine level is low, and LST, TST and HST are developed; fluvial facies, shore-shallow lacustrine, semi-deep lacustrine, delta and lagoon deposits form the main sedimentary systems.

In summary, although terrestrial sequence stratigraphy is different from that of passive continental margins, this does not mean that the classical sequence stratigraphy model proposed by VAIL cannot be applied to the terrestrial environments. It is believed that the VAIL model can be well applied to terrestrial depositional environments according to our study of terrestrial sequence stratigraphy in the Tahe area. However, as there are several factors affecting terrestrial sequence stratigraphy, the structural patterns, basin evolution stages, and environmental factors during basin development must be taken into full consideration when the model is applied.

Fig. 10 Developing process of salt rock structure in Carboniferous and Triassic rocks

7 Conclusions

1) In the Triassic, the study area is situated in a special basin tectonic background, and the sedimentary response cycles that indicate changes in the lake level are quite obvious. As falling lake level has resulted in extensive exposure and erosion in the area, the sedimentary sequences are identified as sequences boundary, with erosional disconformity boundaries.

2) Because the lower strata are eroded while the lake level falls, three sedimentary system tracts within the sequence framework are not well developed. HST is severely denuded and even eroded, and TST in some sequences is also eroded. As a result, sand bodies of the overlying LST are directly contacted with those of the underlying LST.

3) Sand bodies in LST of SQ4 (i.e. Middle Oil Member) are deposited continuously at different stages of the lowstand period. Sublacustrine fans are deposited at the early stage, sediments filling canyons are deposited at the middle stage, and sediments in subaerial erosional valleys are deposited at the late stage.

4) Although tectonic movement, climate, and supply of sediments exert different influences upon the formation of terrestrial sequence stratigraphy and sequence stratigraphy of passive continental margins, the classical sequence stratigraphy model proposed by VAIL can still be applied to the analysis of terrestrial sequence stratigraphy.

5) Sequence framework plays an important role in controlling the sandstone reservoirs. Given the particularity of the basin tectonic framework in the study area, LST is the most important factor of controlling the development and distribution of sand bodies.

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(Edited by YANG Bing)

Foundation item: Project(2008ZX05002-005) supported by the State Major Special Science and Technology Foundation of China

Received date: 2011-08-22; Accepted date: 2011-12-27

Corresponding author: LIU Chen-sheng; Tel: +86-18773188632; E-mail: Lcsjed@163.com