CHINA PETROLEUM EXPLORATION

Volume 21, Issue 4, July 2016

Received date: 28 Mar. 2016; Revised date: 16 Jun. 2016. Corresponding author. E-mail: sipc@sinopec.com Foundation item: Specialized Research Fund for the Doctoral Program of Higher Education of China (20050491001). Copyright © 2016, Petroleum Industry Press, PetroChina. All rights reserved.

The depositional pattern and dynamic mechanism of the rifting stage in the northwestern margin of Beibuwan Basin

Jiang Hui1, Yu Xinghe2

1. Sinopec International Petroleum Exploration and Production Corporation;

2. School of Energy Resources, China University of Geosciences (Beijing)

Abstract: The northwestern margin of the Beibuwan Basin, a significant area for hydrocarbon generation and accumulation, is

characterized by a major stage of rifting during the Paleogene and deposition dominated by terrigenous clastic rocks. With vertical

grain-sizes and color variation showing coarse-fine-coarse and red-dark-motley respectively, the sedimentary environment gradu-

ally changes, going from a continental alluvial fan to a lacustrine to a fluvial and then a delta environment, which forms a whole

sedimentary cycle. The sedimentary history was analyzed and Eocene Liushagang Formation depositional pattern established.

Moreover, the paper preliminarily considers that the dynamic mechanism determining sedimentary system formation and devel-

opment is jointly affected by tectonic dynamics, filling dynamics, and climatic power. From the Palaeocene to Oligocene, the tec-

tonic dynamics were continually decreasing and the climatic power was kept stable; when the climatic power was adjusted, the

deposition rates of the SW Weizhou sag and the Haizhong sag were more than 100m/Ma during the deposition of both the 2nd

member of the Eocene Liushagang Formation and the Oligocene Weizhou Formation, which led to the two most favorable sets of

the Paleogene reservoirs.

Key words: Beibuwan Basin, northwestern margin, rifting stage, depositional pattern, dynamic mechanics

The Beibuwan Basin, located in the northern South China Sea, is a Cenozoic rift basin developed in the southwest of the South China fold system. It has mainly experienced two stages: the rifting stage and the depression stage. The former is divided into several sub-stages, such as the initial rifting, the rift development (which can be subdivided into three periods, namely rift expansion early period, rift expansion peak period, and rift shrinking period), and the rifting-de-pression transition, which is when the Paleocene Changliu Formation, the Eocene Liushagang Formation, and the Oli-gocene Weizhou Formation were developed. Located in the northwestern margin of the basin are the SW Weizhou sag, the SW Weizhou low bulge, and the Haizhong sag, all three of which form the structural framework of NE-SW “one bulge between two sags”. These areas are considered im-portant hydrocarbon generation and accumulation areas. In the past 50 years or so, petroleum discoveries were mainly found in the SW Weizhou sag[12], with commercial oil and gas flows revealed successively in the Carboniferous lime-stone, the Liushagang Formation clastic rock, and the Weizhou Formation clastic rock. In other sags of the basin, there have been less or even no breakthroughs whatsoever in petroleum exploration.

Previous petroleum geology studies[310] were mostly fo-cused on the basin configuration, stratigraphic correlation, structural evolution, and on fault activity, but rarely on deposition. Currently, there is insufficient and limited log-

ging (coring) data. In this paper, based on rock debris, pa-leobios, and seismic data, the paleo-sedimentary environ-ment is reconstructed, and the sedimentary pattern during the rifting stage and the dynamic mechanism that controlled the filling of the deposit are proposed so as to provide some references for oil and gas exploration in the northwestern margin of the basin.

1. Palaeo-sedimentary environment evolution

1.1. Paleocene initial rifting sub-stage

During the Late Cretaceous-Paleocene, mantle uplifting resulted from a disturbance in the deep mantle due to the subduction of the Pacific plate triggered by a NW-SE ten-sion stress in the northern continental shelf of South China Sea, which then led to intensive rifting in the northern mar-gin of South China Sea. As the Paleocene deposition was controlled by rifting, several narrowly segmented half-gra-bens were formed, and the detrital materials eroded from the uplifted side of the fault filled depressions rapidly, which gave rise to a massive set of thick red sandy conglomerates interbedded with mudstone[11]. The spores there were too poor to form combinations, which indicated a continental alluvial environment (Fig.1).

1.2. Eocene rift development sub-stage

In the Early Eocene, with the further subduction of the

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Fig. 1 Paleogene sedimentary paleography of the Beibuwan Basin

Pacific plate to the South China continent, a series of NNE-SWW sag-controlling faults were developed, and the basin began to rift and expand; multiple half-grabens were uniformly expanded to the lake basin at the northwest mar-gin[12]. The lake level rose gradually where glutenite and dark grey shale, interbedded with aubergine mudstone, were predominantly deposited. Carbonaceous shale and thin coal seams were also deposited. In freshwater lakes, Candona and Saliaxipollenites belonging to swamp water plants were observed, which reflected the characteristics of a lakeshore sedimentary environment with a locally developed swamp.

In the Middle Eocene, the basin came into the rift expan-sion peak period. In this period, the climate was humid, the area covered by water expanded, and dark mud shale and oil shale intercalated by thin and fine siltstone were deposited. Spherulitic pyrite and siderite were commonly seen in for-mations. In addition to the terrigenous sporopollens, many aquatic algae, ostracoda, and gastropod fossils were ob-served, and the immense and deep water bodies formed a semi-deep lacustrine reducing environment. Moreover, the deposition rate was greater than the recharge rate in the open lake basin.

During the Late Eocene, rifting gradually receded, and the whole Beibuwan Basin began to shrink. As a result, the lake basin became shallower to form a shallow lake sedi-mentary environment. Off-lapping sedimentation could be observed locally. The deposits were composed of interbed-ded dark shale and sandstone in unequal layers of thickness, with characteristics including the presence of fine grained particles, a small single-layer thickness, and a small amount of pyrite and carbonaceous fragments.

1.3. Oligocene rifting-depression transition period

In the Oligocene, with the continuous subduction of the West Pacific plate to the Eurasian plate, the regional stress field in the Beibuwan Basin continued to rotate clockwise eastwards, and the stress field at the plate boundary gradu-ally transferred into the plate, which resulted in a dextral strike-slip extension and the deformation of basin-con-trolling and sag-controlling faults. Furthermore, the whole Beibuwan Basin was in a rifting-depression transition. Shallower lakes and deeper rivers created a generally flu-vial-lacustrine sedimentary environment, with a high level of delta and flooding plain development. The deposits were dominated by interbedded variegated mudstone, sandstone, and sandy conglomerates in unequal levels of thickness, with cross bedding and many sporopollen fossils of alpine plants (e.g. spruce, cedar and hemlock) present. This sug-gests that the relative height difference between the external provenance and the lake basin was increasing, and the warped blocks within the basin were gradually leveled. The underlying formations suffered local erosion when the deposition rate was smaller than recharge rate[13].

2. Sedimentary pattern during rifting stage

According to paleontological and rock-mineral analy-sis[14], the Paleogene formation in the Beibuwan Basin is dominated by terrigenous clastic rocks, with coarse and fine-coarse grain sizes and a red-dark-motley color present from bottom to top. The sedimentary construction is sig-nificantly controlled by rifting. Analysis based on a large

Jiang Hui et al., The depositional pattern and dynamic mechanism of the rifting stage in the northwestern margin of ... 3

amount of seismic data and well logging sedimentary facies templates (Fig.2, and Table 1) indicates that because the Paleocene (Changliu Formation), the northwestern margin of the basin, was in an initial rifting sub-stage, the basin basement began to subside rapidly, and near-source fan del-

tas and coarse clastic flood plains characterized by wedge- prograding seismic reflection wave groups were extensively developed. The southern ramp of the Haizhong sag turned into a shore-shallow lake locally.

Until the Eocene (Liushagang Formation) (Fig.3), the

Fig. 2 Sedimentary electrofacies template in the northwestern margin of the Beibuwan Basin

Table 1 Representative seismic facies-sedimentary facies in the northwestern margin of the Beibuwan Basin

Seismic facies Seismic subfacies Typical seismic section Main sedimentary environment

Sheet parallel facies

Line BWL20 Semi-deep lake, shore-shallow

lake

Sheet faices

Sheet waveform facies

Line BW 78 Shore-shallow lake, beach bar,

flood plain

Sphenoid waveform facies

Line BWL 42 Shore-shallow lake, flood plain

Sphenoid chaotic facies

Line BW 98 Underwater fan, turbidite fan Sphenoid facies

Sphenoid foreset facies

Line BWL 26 Fan delta, delta

Filling facies Filling foreset facies

Line BW 98 Channel and flood plain

Broom-like facies Broom-like divergent facies

Line BWL 20 Underwater fan, delta

Blank facies —

Line BW 74 Fan delta

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Fig. 3 Eocene Liushagang Formation depositional pattern in the northwestern margin of the Beibuwan Basin

faults (e.g. SW Weizhou fault, and No.3 fault) controlling the SW Weizhou sag and the Haizhong sag were growing in level of activity, the dustpan-like features of the two sags were increasingly obvious, and the gradual NNE-SSW de-flection of the local stress field led to the folding of the downthrown side and the finalization of rollover anticlines. During this period, the climate started to change from semi-arid to warm-moist, the area covered by water was continuously expanding, and shore-shallow lakes were widely distributed in the SW Weizhou sag and in the Ha-izhong sag. In the lower parts of the central sags, there were semi-deep lake facies represented by sheet-parallel seismic reflection wave groups; in the higher parts, coarse fan deltas were observed. With the continuous expansion in the rifting stage, the climate in the second member of Liushagang Formation (Liu 2 Member) deposition period became more humid, the area covered by water achieved its largest cov-erage, and shore-shallow and semi-deep lake facies occu-pied the most part of sages. Near the SW Weizhou fault, fan deltas and deltas were developed, and slump turbidite fans were observed in the front-end. In the south of the SW Weizhou low bulge, there were a series of submarine fans. From here to the Late Eocene (the first member of Liushagang Formation or Liu 1 Member), the basin suffered from the intensive South China Sea tectonic movement. As a result, it was uplifted and eroded. The two sags in the northwestern margin of the basin began to shrink after ex-panding to their maximum size. The range of the semi-deep lake decreased, and the deposition rate and the recharge rate

increased alternatively. In the Oligocene (Weizhou Formation), the Beibuwan

Basin approached the end of the rifting stage, and then it gradually transitioned to a depression. Until the late Oligo-cene, a sharp inversion took place. The intensity of the sag-controlling faults in the northwestern margin clearly decreased, which gave rise to many local structural con-figurations. In this period, rivers and river-dominated deltas with overbank drainages and flood plain backgrounds ap-peared in succession; the seismic reflection features were represented by a filling progradation and broom-like diver-gency, and the deposits were characterized by a gradual transition from retrogradation to progradation in terms of vertical stacking.

3. Dynamic mechanism of the sedimentary system development

The formation and development of the sedimentary sys-tem in continental basins were jointly determined by many dynamic factors. Li Sitian et al[15] focused on structural dy-namics. Zheng Rongcai et al.[16] investigated sequence-fill-ing dynamics. Other scholars[1719] proposed various factors that could control the sedimentary system from a different perspective (e.g. climate and fluid). The paper indicates that the formation and alternation of sedimentary systems in continental basins are jointly and organically controlled by tectonic dynamics (e.g. extension, compression, and shearing), filling dynamics (e.g. physical action, chemical action, and

Jiang Hui et al., The depositional pattern and dynamic mechanism of the rifting stage in the northwestern margin of ... 5

biological action), and climate dynamics (e.g. water molecu-lar condensation and heat diffusion caused by climate chang-es). Specifically, tectonic dynamics create the accommoda-tion space for sedimentary systems, filling dynamics (includ-ing biological forces, although with a weak impact) provide various fabric elements for sedimentary systems, and climate dynamics form dynamic effects and environmental differ-ences for a sedimentary system. These dynamics are different in stages, with variable influences on deposition rate (Fig.4).

Generally, from the Paleocene to the Oligocene, the intensity of the tectonic dynamics decreased, while the filling dy-namics were essentially kept stable. Meanwhile, promoted by climate dynamics, the deposition rates in the deposition periods of the second member of the Eocene Liushagang Formation and the Oligocene Weizhou Formation in the SW Weizhou sag and the Haizhong sag reached high values successively and led to the formation of the two most fa-vorable Paleogene clastic reservoirs in the Beibuwan Basin.

Fig. 4 Paleogene dynamical mechanisms in the northwestern margin of the Beibuwan Basin

Firstly, the Paleocene tectonic dynamics refer to the ten-sion stress derived from the delamination-derooting between the continental margin of East Asia and the pre-tethys tec-tonic-domain subduction plate. It facilitated the rudiment of the Beibuwan Basin and started the process of rifting and expanding. It was considered as the main basin-forming driving force. Since the water body of the lake basin was small and shallow, with weak wave action, the filling dy-namics was mainly derived from physical weathering and erosion, gravitational collapse, paroxysmal strong water dynamics, and chemical evaporation crystallization. The deposition rates were 75 m/Ma and 102 m/Ma in the SW Weizhou sag and the Haizhong sag respectively. Climate dynamics belonged to the regional strong evaporation-weak rainfall effect, where the surface runoff was small with local and intermittently strong divergent flows and tubular flows.

Secondly, the Eocene tectonic dynamics were slightly weakened, which was derived from the enlarged accommo-dation space as a result of horizontal compression caused by the Eurasian plate collision extending and spreading to the

southeast and the clockwise tension-torsional stress of the Pacific plate on the South China Sea basin. The broad and deep lake basins strengthened the transposition capacity of the water filling dynamics. In addition to buoyancy and gravity subsidence, there were storm deposits under the storm, slump deposits under gravity imbalance, and turbid-ity deposits generated by underwater density flows. For example, underwater fan groups were formed on the SW Weizhou low bulge. Underbalanced zones were present in the deep depocenter of the Haizhong sag, with weak chemi-cal action. The deposition rates were 113 m/Ma and 107 m/Ma in the SW Weizhou sag and the Haizhong sag respec-tively. The climate gradually changed to become warm and humid, the environment continued to be humid and semi- humid with an abundant level of rainfall, and the low-lying areas were favorable for the development of delta-lake sedimentary systems rich in coal-bearing swamps.

Thirdly, the Oligocene tectonic dynamics were made up of the tension-torsional stresses generated by the NW-SE extension of South China Sea basin. The basin was in the

6 CHINA PETROLEUM EXPLORATION Vol. 21, No. 4, 2016

rifting-depression transition period when the basement at the northwestern margin was uplifted, causing the Weizhou Formation to be exposed to weathering and erosion. In this period, filling and leveling both occurred in the basin, and as the height difference gradually decreased, fault activities tended to stop, weathering and erosion became more prominent, and the sedimentary source supply was kept abundant. Filling dynamics were mainly derived from in-termittent tubular traction current power, constant wind power, and biological power. The deposition rates were 130 m/Ma and 142 m/Ma in the SW Weizhou sag and the Ha-izhong sag, making them the highest. Moreover, climate dynamics were characterized by the coexistence of strong evaporation and low rainfall, which indirectly affected the strength changes of the hydro-dynamic conditions.

4. Conclusions

(1) The northwestern margin of Beibuwan Basin experi-enced initial rifting, rift development, and rifting-depression transition sub-stages during the Paleogene. The sedimentary construction was significantly controlled by rifting, as noted by the predominance of terrigenous clastic rocks. Rhythmic sedimentary cycle features were reflected in terms of grain-size, color, and superposition mode. The strength changes of the sag-controlling faults and the shrinking and expanding of the lake basins decided 8 types of sedimentary facies and their distribution pattern in the northwestern margin of the basin.

(2) From the Paleocene to the Oligocene, the sedimentary system in the northwestern margin of the Beibuwan Basin was controlled by three major dynamic factors. In this pe-riod, regional tension-torsional stress continually decreased, filling dynamics generated by water transportation were kept stable, and the climate dynamics of evaporation and rainfall appeared alternatively. During the Middle Eocene and Middle-Late Oligocene, the accommodation space and the source supply were well-matched, and the depositional rates in the two sags successively reached the peak, which explains in terms of sedimentary dynamics why the best reservoirs were developed in the two sets of formations.

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