Transtensional tectonism and its effects on the distribution of sandbodies in the Paleogene Baiyun Sag, Pearl River Mouth Basin, China
Jiahao Wang
0
1
2
Xiong Pang
0
1
2
Daqing Tang
0
1
2
Baojun Liu
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1
2
Donghao Xu
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1
2
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X. Pang B. Liu Institute CNOOC Ltd.
, Shenzhen, Guangzhou 510240,
China
1
J. Wang D. Tang D. Xu Faculty of Earth Resources, China University of Geosciences
, Wuhan 430074, Hubei,
China
2
J. Wang (&) Key Laboratory of Tectonics and Petroleum Resources
, Ministry of Education,
China University of Geosciences
, Wuhan 430074,
China
The Baiyun Sag, situated at the north continental slope of the South China Sea, is a main sub-unit in the Southern Depression Belt of the Pearl River Mouth Basin. In this Sag, the middle Eocene Wenchang and upper Eocenelower Oligocene Enping Formations had developed in the evolution stage of continental faulted basin. Seismic stratigraphic sequences and fault structures revealed that the Baiyun Sag was short of long-reaching boundary faults, and that it was a rifted basin greatly influenced by basement faults rather than a typical half-graben. Different from the sags in Northern Depression Belt of the Pearl River Mouth Basin which controlled by large-scale NEE-strike faults, the Baiyun Sag had been controlled by two groups of NWWstrike en echelon fault belts with approximate opposite dips, which developed in the southwest and northeast of this Sag respectively and had played the roles of boundary faults. These en echelon faults, together with narrow synclines, partial flower structures and fluid diapirs, indicated the leftlateral transtensional activities, which had resulted in subsidence center departing to main faults and stretching S-shaped. Moreover, the en echelon faults had constructed many composite transfer zones of relay ramps, and controlled the distribution of sandbodies. The en echelon fault belts are located in accordance with Nw-striking Mesozoic basement faults. Hence the left-lateral transtensional activities were responsible for the Western Pacific Plate subducting and strike slip reactivation of the basement faults. Significantly, NW-striking basement faults had forcefully determined the development of not only the Baiyun Sag but also the Xingning Sag.
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The Baiyun Sag is situated at the northern continental slope
of the South China Sea with water-depth from 300 to
4,000 m, and area of approximately 20,000 km2. With the
discoveries of large-scale Neogene submarine fans at the
beginning of twenty-first century, the Baiyun Sag became a
hotspot of deep-water oil and gas exploration (Pang et al.
2004, 2005; Peng et al. 2004, 2005). The Baiyun Sag
evolved from continental rift to passive continental
marginal basin since the Late Cretaceous (Liu et al. 2011). The
deep-seated middle Eocene Wenchang Formation and
upper Eocene-lower Oligocene Enping Formation was
deposited in the rifting stage. Both of them possessed good
hydrocarbon source rocks with sedimentary facies of deep
lacustrine, and had supplied abundant resources for the oil
and gas accumulation of the Neogene submarine fan, which
have been verified by recent petroleum discoveries (Chen
et al. 2006; Pang et al. 2006).
In recent years, benefited from the improved 3D seismic
data, many large-scale deposits with progradational
reflection configuration on seismic profiles were identified
as sedimentary facies of delta and fan delta in the
Fig. 1 The Paleogene tectonic
units of the Pearl river Mouth
basin (adapted after Pang et al.
2005). I Panyu Low Uplift, II
Yunkai Low Uplift, III Dongsha
Uplift, IV Baiyun Sag,
V Huizhou Sag, VI Xingning
Sag, 1 city, 2 boundary of Pearl
River Mouth Basin, 3 boundary
of first-order tectonic unit, 4
boundary of second-order
tectonic unit, 5 NW-strike fault
Wenchang and Enping Formations. Some of them broadly
distributed with buried-depth \3,000 m which were
regarded as potential exploration targets.
Therefore, the intensified researches on tectonic activities
and sedimentary filling in the development of Wenchang and
Enping Formations are very useful for revealing the primary
evolution of the passive continental slope of the South China
Sea and for finding the favorable deep-seated targets.
Geological background
Controlled by two groups of faults with NEE and NW
strike respectively, the Pearl River Mouth Basin presents a
segmental and zonal structural framework in the NEE
direction and in the NW direction respectively. This basin
is divided into five structural units of the Northern
FaultLadder Belt, Northern Depression Zone, Central Uplift
Belt, Southern Depression Zone and Southern Uplift Belt.
The Baiyun Sag located near the Panyu Low Uplift, the
Dongsha Uplift and the Yunkai Low Uplift is a sub-unit of
the Southern Depression Zone (Fig. 1).
The maximum thickness of the Wenchang and Enping
Formations in the Baiyun Sag is up to 4,700 m. Wenchang
and Enping formations corresponded to reflection units
between major unconformities Tg (49 Ma)T80 (36 Ma),
T80T70 (30 Ma) respectively on seismic profiles. Six
third-order sequence interfaces were further identified as
T84T81, T72 and T71, whic (...truncated)