Structural styles and hydrocarbon accumulation of the northern piedmont belt in the Taibei Sag, Turpan-Hami Basin

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 38, Issue 2, April 2011 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2011, 38(2): 152–158.

RESEARCH PAPER

Structural styles and hydrocarbon accumulation of the northern piedmont belt in the Taibei Sag, Turpan-Hami Basin Liu Bo1,*, Huang Zhilong1, Tu Xiaoxian2, Zhang Jinxue3, Mu Kexun3 1. State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing 102249, China; 2. PetroChina Turpan Hami Oilfield Company, CNPC, Hami 839009, China; 3. BGP Urumqi Branch, CNPC, Urumqi 830016, China

Abstract: The northern piedmont belt of the Taibei Sag is one of the oil rich structural belts in the Turpan-Hami Basin. Based on the interpretation of seismic cross sections and the analyses of discovered hydrocarbon accumulations, the geologic structure can be divided into six structural styles, thrust-imbricate structure, imbricate fan structure, thrust-opposite structure, pop-up structure, forward thrust structure and duplex structure. According to the study of the distribution regularity of the structural styles, the fold thrust belt controlled by three levels tectonic transform zones is characterized by zonation from the south to the north and segmentation from the west to the east, and it can be classified into thrust-nappe belt, breaking belt and frontal belt by mechanism of tectonic deformation. The leading edge with two accumulation forming models which are the leading edge pop-up model and the leading edge imbricate fan model is the most favorable hydrocarbon enriched belt in the northern margin of the Taibei Sag. The thrust-opposite structures in the rear edge of the deformable zone have good preservation, potential source kitchen on the footwall, higher fault dip, and good communication of sandbodies. They are the next favorable exploration structures. Key words: structural style; thrust faults system; structural deformation belt; tectonic transform zone; accumulation forming model

The Tuha Basin is an intermontane basin formed by the northward extrusion of the Indian plate[1]. The current geological structure is mainly controlled by the intensive late Himalayan age uplift reformation of the Bogota Mountain in the north to the basin. Foreland fold-thrust belt, controlled by this orogenic stress system, is developed in the north of the Taibei Sag, i.e. the northern piedmont zone[2]. Recently, thanks to the extensive development of foreland basin hydrocarbon exploration[39], the study of foreland thrust belt structure and hydrocarbon distribution by using fold theories of fold-thrust made great progress. However, the study on the geological structure of the northern piedmont zone in the Tuha Basin is still insufficient[10]; the understanding of the structural framework of its fold-thrust belt is not detailed enough; and study on structural styles and distribution, structure segmentation and cause is inadequate. Based on a large amount of interpretation results of seismic profile, emphasizing on the structural style differences of the northern piedmont zones in the Taibei Sag in different locations, starting from the deformation mechanism, the foreland fold belt is fragmented in terms of structure deformation, the effect of structure transfer zone is analyzed, hydrocarbon accumulation

condition of deformable zone front is contrasted, and hydrocarbon accumulation patterns are concluded, so as further to understand the structure development mechanism, hydrocarbon enrichment rule and differences in the area.

1

Regional tectonic setting

The north area of Xinjiang is an important part of the central Asian orogenic belt, and can be divided into three plates in regions, which are Siberia Plate, Kazakhstan Plate and Tarim-North China Plate[11]. The Tuha Basin is located in the northeast part of the Kazakhstan Plate, in the junction part of the three plates and between the Bogota uplift belt and the Jueluotag uplift belt (Fig. 1). The structural framework of the Taibei Sag is a large thrust-sheet thrusting and slipping southwards due to constant plate activities. The south boundary is the Huoyanshan- Qiketai structural belt, and the northern piedmont zone is foothill fold-thrust system formed due to the uplift of the Bogota Mountain[12]. The uplift of Bogota area started in late Permian[13], and the area experienced differential uplifting of Indo-China movement, re-lifting of Yanshan movement and intensive uplifting orogenic stage of Himalayan movement. The structural framework today is a

Received date: 29 Apr. 2010; Revised date: 19 Jan. 2011. * Corresponding author. E-mail: [email protected] Foundation item: Support by the State Major Basic Research Development Planning (973) Program (2006CB202300). Copyright © 2011, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

Liu Bo et al. / Petroleum Exploration and Development, 2011, 38(2): 152–158

complex superposition structure of the Yanshanian structure intensively reformed during Himalayan age[14-15]. Five series of detachment layers are developed in the area: Upper-Permian mudstone and coal bed, Badaowan Formation coal bed and high carbon shale, Lower Xishanyao Formation shale and sandy shale, Upper Qiketai Formation and Qigu Formation shale and silty shale[16], which have great effect on structural deformation style.

Fig. 1

2 2.1

Regional geological location of the Tuha Basin

Structural types and geologic characteristics Classification of structural styles

Under the effect of the NS-trending compressive stress caused by the uplift and thrusting of the Bogota Mountain, the formation of the northern piedmont zone was deformed intensively from north to south, and its structural type mainly belongs to compressional structure[17], which is controlled by geometric characteristics and kinematics of the main faults[18,19]. According to the relationship between the fault and basement, arrangement and combination of abnormal fault and the related fold styles, the structural styles of the northern piedmont zone are divided into six types, namely, thrust-imbricate structure, imbricate fan structure, thrustopposite structure, pop-up structure, forward-thrust structure and duplex structure[2023] (Fig. 2).

Fig. 2 Structure styles of the northern piedmont zone of the Taibei Sag

The imbricate thrust structure and the imbricate fan structure are a series of listric thrust faults with similar occurrence which successively developed and formed under the thrusting action. The thrust faults with imbricate thrust structure are separate from each other, without the common basal thrust

fault, and mainly developed in the Qiabei Structural Belt in the trailing edge of the deformation zone. The imbricate fan converged into a thrust fault with low angle in the deep formation, and its development location is influenced by the distribution of the underlying detachment surfaces. Both the Qiquanhu structural belt in the western section of front edge of the deformation zone and Qialekan structural belt belong to the imbricate fan structure (Fig. 3). The thrust fault is formed while the forward displacement of the upper wall is arrested. According to the location of thrust faults, the thrust-opposite structures usually developed in the Hetaogou structural belt and Aktashi-Shanle structural belt in the trailing edge of the deformation zone, and the pop-up structure developed in the Taoergou structural belt, Kekeya structural belt and Qiuling structural belt in the front stage of the deformation zone. In the zone with strong structural deformation, such as the Guobei structural belt, the thrusting and decoupling system slip from bedding fault to the earth surface by the way of cutting-bedding, and developed into forward-thrust structure after denudation. The Zhaobishan-Hongqikan structural belt and the Hongqikanbei structural belt have many sets of detachment surfaces; therefore they are the development zone of duplex structure. The distribution of structural styles is related to the deformation process. The most possible situation is that the imbricate fan structure and pop-up structure or forward-thrust structure formed at the early period of deformation; the structural types depend on the distribution of detachment surfaces and the deformation stress. And then, with progressive deformation, these structures developed into duplex structure, thrust-opposite structure or thrust-imbricate structure in the trailing edge. The distribution of detachment surfaces and location combination of thrust fault determines the structural styles. 2.2

Distribution of faults

The compressive stress with mainly NS-trending made an approximate WE-trending thrust fault system begin to develop in the northern piedmont zone at the early Yanshanian period, and the en echelon thrust fault controlled the trend of the structural belt (Fig. 3). There are also some approximate NS-trending tearing faults in the thrust fault system, and these faults often generate the relatively steep fault planes on the section. But no obvious structural deformation is observed, and there exists strike slip displacement, which has the function of adjusting the displacement, direction, intensity and type of deformation between main thrust faults. These faults are caused by the sharp uplifting and uneven tectonic stress of the Bogota Mountain at the late Himalayan[24]. 2.3 Difference in structural styles between the east and the west The formation location of thrust fold structures in the northern piedmont zone has the regular pattern of migrating from north to south and from east to west. This regular pattern

Liu Bo et al. / Petroleum Exploration and Development, 2011, 38(2): 152–158

Fig. 3

Distribution of fold-thrust belt of the northern piedmont zone of the Taibei Sag, Tuha Basin

Fig. 4

Seismic profile of the northern piedmont zone of the Taibei sag, Tuha Basin

is in accordance with the migration of the depocenters of the foreland basin at different stages[25]. The characteristic analysis of structural types show that (Figs. 3 and 4), to the east of the Kekeya structural belt, the distribution of structural line is mainly form north to west, and the Qigu Formation of Jurassic and Cretaceous are only locally developed. Due to the low deformation degree and the gentle structure of the Tertiary System, the period having strong tectonic compression activ-

ity is the late Yanshanian. To west of the Qialekan structural belt, the distribution of structural line is mainly from north to east, and the period with strong tectonic compression is the Himalayan, and the Tertiary System has different degrees of denudation due to the structural uplifting and forward thrust. It can be inferred that the major stress direction at the Yanshanian is NWW, mainly had effect on the Shanle-Qiuling section; and the major stress direction at the Himalayan is

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NEE, mainly had effect on the Qialekan section, which formed the arc structure protruding southwards.

3 Deformation segmentation of piedmont zone structure The thrust activity of the piedmont zone in the north of the Tuha Basin has the obvious characteristic of piggy-back sequence[25]. The difference between thrust direction and displacement direction horizontally, regulating action of tearing fault and the existence of structure transform zone, lead to the tectonic framework of longitudinal zonation and lateral segmentation (Fig. 5), which results in the difference of tectonic deformation type, development scale and hydrocarbon distribution rules[26]. 3.1

Structural features of longitudinal zonation

The mountain fold of the Bogota Mountain in the north uplifted continuously after the late Indo-China movement, the extrusion stress resulted from thrust released gradually from north to south, the structural deformation layers were gradually shallow, and tectonic type also changed which is shown as three belts: thrust-nappe belt, fault step belt and thrust frontal belt in turn from north to south. The fracture of the north border formed by the thrust-nappe belt is the bordering fault in the north of the piedmont zone, which develops the nappe structure. Fault step belt is mainly shown as thrust imbricate structure, and the base of the Guobei-GuobeibeiQiabei structure is located below the thrust-nappe block, which makes the areas be lack of this belt in the plain[27]. The thrust frontal belt is the main body of the northern piedmont zone, often forms pop-up structure and breaking fold, which are the most favorable hydrocarbon accumulation belt. 3.2

Structural features of lateral segmentation

The northern piedmont zone is divided into different tectonic deformable zones in the light of deformation mechanism. The same deformable zone has the similar tectonic deformation mechanism, but can be shown as different structural

Fig. 5

styles, which are controlled by the distribution of slip surface, deformation degree, thrust distance and fault combination. The structural belts in NEE and NWW trends which develop in two periods on the whole constitute the arc distribution of piedmont tectonic zone. The main body of piedmont zone forms two rows of fold belts, and distributes in the diagonal form. Three rows of fold belts developed in the Yuguo section are actually overlapped by two rows of NWW-trending fold belts in the N-S direction. 3.3

Features of tectonic transform zone

The piedmont zone in the north is divided into several deformable zones by structure transform zones with different structural levels[28,29]. The structure transform zones can be classified into three levels (Fig. 5)[30] by terms of the influence of structure transform zones on regional structure segmentation: the level I structure transform zone is located in the middle part of the piedmont zone, the structural evolution, stratigraphic distribution, deformation type, deformation degree and tectonic line distribution of blocks in both sides of the structure transform zone are very different. Level II structure transform zone mainly develops to the west of the Qiquanhu structural belt, to the east of the Qialekan structural belt and to the west of the Hongqikan structural belt. The deformation mechanism of adjacent deformation blocks is similar, but the structural style and deformation degree are different, which are the longitudinal marks of structural deformation segmentation. Level III structure transform zone is distributed in the tectonic deformable zone to adjust the difference of local structural deformation.

4

Hydrocarbon accumulation modes

In the northern piedmont zone from east to west, there are differences in deformation structure styles, source rock development horizons, faulting feature and structure forming time, which lead to the differences in hydrocarbon accumulation time, control factors of hydrocarbon accumulation and the rule of hydrocarbon enrichment. The diversity in structure

Deformation segmentation and distribution of structure transform zones in northern piedmont zone, Tuha Basin

Liu Bo et al. / Petroleum Exploration and Development, 2011, 38(2): 152–158

Fig. 6

Hydrocarbon accumulation patterns of the front edge of northern piedmont zone in Tuha Basin

pattern, hydrocarbon supplying condition and the efficiency of the conduction pathway made the front of deformable zone be divided into two hydrocarbon accumulation patterns: pop-up type (Qiuling-Kekeya deformable zone) and imbricate fan type (Youzi-Qiquanhu deformable zone, Qialekan-Guobei deformable zone) (Fig. 6, Table 1). 4.1

Structure styles

The front of the deformable zone, near the basin center and Table 1 Hydrocarbon accumulation modes

close to the Shengbei hydrocarbon generation depression, is the most favorable hydrocarbon enrichment area. It is possible for the structures in the trailing edge of the deformable zone to form favorable hydrocarbon traps, but it is far away from the hydrocarbon generation center, so hydrocarbon generally accumulated and became reservoirs in the front of the deformable zone. The imbricate fan structure and pop-up structure are the most favorable structure styles. Forward thrust structure is unfavorable for hydrocarbon preservation because

Comparison of two kinds of hydrocarbon accumulation modes at the front of the deformable zone Structure modes

Development zones

Composition of Tectonic stress

deformable materials

Hydrocarbon supply conditions Deformation characteristics

Maturation and evolution Position of hyof hydrocarbon source drocarbon source rocks

The included angle Both flanks of the antiThe formation between the strucclinal core are on the Pop-up of Qiuling- tural belt trend and Wide spread thick shortens due to hanging wall with deep the front of Kekeya regional main Mid-Jurassic sand- recoil faults, and it buried depth, and the hydrocarbon source the defordeformable tectonic stress is body with brittleis easy to form small and the rocks of Mid-Lower mable zones zone ness structural traps on stress transmitted Jurassic Shuixigou the hanging wall is less Group is at mature stage The formation curls QialekanGuobei deformable Imbricate fan at the front of the deformable zones

zone

Mainly lacastrine Stress is relatively facies Mid-Jurassic concentrated

deposition with strong plasticity

over, and the formation both on the hanging wall and the foot wall uplifts in high range with the arch-like shape

kitchen The trap in the core of the structure is relatively close to hydrocarbon generation area

The foot wall is doming

It is far away

up. The source rocks of

from the hydro-

Shuixigou Group are

carbon generation

shallow buried and no a

area both hori-

large quantity of hydro-

zontally and

carbon generated

vertically

The thrusting distance is long on the hanging wall, Medium-Lower The main fracture is It is far away the fault throw is large, YouziJurassic delta the detachment fault and the hydrocarbon from the effective Qiquanhu Stress is relatively sandbody pinching with relative small source rocks are immahydrocarbon deformable concentrated near the main fault dip angle, and ture. The foot wall is generation area zone with relatively thrusting distance is deeply buried, and the vertically strong plasticity large hydrocarbon source rocks are mature

Efficiency of the migration pathway Closure prop-

Development

erty of frac-

degree of the

tures

sand body In general, it is

The dip angle thick sandstone of the fault is on both sides of steep, and the

the fractured

vertical sealing surface, and the ability is poor

lateral sealing property is poor

Mudstone and The fractured

coal bed are

surface is gen- dominant on the tle, the pressure

hanging wall

of the overlying and the foot wall formation is

of the fracture,

high, and the and the conducvertical sealing tion system is ability is good

relatively not effective

Liu Bo et al. / Petroleum Exploration and Development, 2011, 38(2): 152–158

of the high amplitude of pop-up, even though it developed in the front of the deformable zone. 4.2

Hydrocarbon supplying conditions

It is unidirectional hydrocarbon supply in the Qialekan and Qiquanhu zones, and the hydrocarbon is mainly from the hydrocarbon source rock in southern foot wall. The Qiuling zone is close to the Shengbei hydrocarbon generation center in the west, and the Qiudong hydrocarbon generation center in the east, with bilateral hydrocarbon supply from these two great hydrocarbon generation centers. Moreover, the middle-lower Jurassic Shuixigou Group hydrocarbon source rocks in two flanks of the anticline of the Qiuling zone also supply hydrocarbon for this area. Thus, the Qiuling zone has multilateral hydrocarbon supply, with abundant hydrocarbon supply. 4.3

Migration

For both normal faults and reverse faults, after the faulting activities, the more gentle the dip of fault section, the higher the pressure from overlying formation to fault section, the better the fault tight closure degree, and the easier it is to form good vertical seal, otherwise, the vertical seal of faults is poor[31]. The relatively poorer conduction systems in the Qialekan zone and Qiquanhu zone is one of the main reasons for insufficient hydrocarbon supply in these two zones. The vertical faulting and lateral sand bodies in the Qiuling area form much effective network hydrocarbon fluid conduction system, which enables abundant hydrocarbon to migrate and accumulate into reservoirs. Comparison indicates that, whether the structure style is favorable for hydrocarbon generation depends on the following two factors: the tectonic stress and the component of the deformation materials. The tectonic stress decides the intensity degree of structure deformation, which is related to the reservoir preservation conditions; the component of deformation materials, on the one hand, controls the tectonic deformation feature and has influence on the buried depth of the formation developed in the foot wall hydrocarbon source rock and the fault sealing ability. On the other hand, it controls the distribution of effective hydrocarbon kitchen and the connectivity of lateral sand bodies. Therefore, it is inferred that, the thrust-hedge structure in the trailing edge of the deformable zone has great preservation conditions; the foot wall is potential hydrocarbon kitchen development area[32], with steep fault dip and good sand body connectivity, being the favorable structural belt for next hydrocarbon exploration.

5

Conclusions

The structural styles of the northern piedmont zone of the Taibei Sag of the Tuha Basin can be divided into six styles, controlled by nearly east-west trending thrust faulting system and a series of nearly south-north trending tearing faults. The generation mechanism is related to the change of tectonic stress direction, the transfer of the concentrated major stress area, as well as the superposition of the Yanshan tectonic

movement and Himalaya tectonic movement. The structure transfer belts mainly presented as tearing fault divided the piedmont thrusting belt in the north of the Taibei Sag of the Tuha Basin into several structural deformable zones, with tectonic framework featured by longitudinal zonation and latitudinal segmentation. Hydrocarbon accumulations in the northern piedmont deformable zone can be basically divided into two types, respectively with different structure styles, hydrocarbon supplying conditions and migration pathways, as well as different hydrocarbon enrichment degrees.

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