Genesis of Miocene litho-stratigraphic trap and hydrocarbon accumulation in the Qiongdongnan Basin, northern South China Sea
Fan et al. Geosci. Lett.
Genesis of Miocene litho-stratigraphic trap and hydrocarbon accumulation in the Qiongdongnan Basin, northern South China Sea
Caiwei Fan 1
Tao Jiang 0
Kun Liu 1
Jiancai Tan 1
Hu Li 1
Anqi Li 1
0 Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences , Wuhan 430074 , China
1 Zhanjiang Branch of CNOOC Ltd. , Zhanjiang 524057, Guangdong , China
In recent years, several large gas fields have been discovered in western Qiongdongnan Basin. It is important and necessary to illustrate their sedimentary characteristics and hydrocarbon migration so that more gas fields could be discovered in the future. Previous regional tectonic-sedimentary researchers show that large-scale source rock of the Yacheng Formation developed in the Ledong and Lingshui sags due to the Red River Fault pull-apart strike slip in early Oligocene. The main targets for hydrocarbon exploration in this area are the Miocene deep water reservoirs. In late Miocene, the Huangliu Formation reservoirs are composed of the early channels which were sourced by river systems in Hainan uplift and the consequent channels were sourced by Qiupen River in Kunsong uplift. Both axial channels exhibit unique spatial distribution patterns and geometries. The other kind of reservoir developed in the middle Miocene Meishan Formation, which compose of slope break-controlled submarine fan. They can be further classified into three types-slope channelized fan, basin floor fan, and bottom current reworked fan. The various fans have different reservoir quality. These two kinds of reservoirs contribute to four types of litho-stratigraphic traps under the actions of sedimentation and subsidence. The overpressure caused by hydrocarbon generation can fracture deeper strata and result in regional fractured network for hydrocarbon migration. Therefore, free gas driven by overpressure and buoyancy force can be migrated into Miocene litho-stratigraphic traps to accumulate. The revealed genesis of Miocene lithologic trap and hydrocarbon accumulation in the Qiongdongnan Basin would greatly contribute to the further hydrocarbon exploration in northern South China Sea and can be helpful for other deep water areas around the world.
Qiongdongnan Basin; Ledong sag; Lingshui sag; Axial channel; Submarine fan; Litho-stratigraphic trap; Overpressure; Hydrocarbon accumulation
Introduction
Qiongdongnan basin is a Cenozoic petroliferous
extensional basin which is located in the continental margin
of the northern South China Sea and southeast of the
Hainan Island. It stretches northeastward with
alternating depressions and uplifts framework and can be
subdivided into northern depression, middle uplift zone,
middle depression, and south uplift zone, including 10
sub-depressions, such as Yannan sag, Yabei sag, Ledong
sag, Lingshui sag, and so on (Fig. 1). The Ledong and
Lingshui Sags are the two largest sags in the
Qiongdongnan Basin, characterized by deep burial strata, thick
deposition, high temperature, and overpressure
(Xie et al.
2006; Pettingill and Weimer 2002)
. However, the
hydrocarbon exploration potential in these two sags was not
considered to be promising for a long time due to three
reasons. Firstly, the resource potential was quite
uncertain. Limited by poor seismic data in the past, the scale
and distribution of Oligocene source rock (the Yacheng
Formation) could not be clearly identified. Secondly, fault
activity in most regions was absent or resting since
Neocene. Possibility and efficiency of hydrocarbon vertical
migration were dubious. Thirdly, owing to a lack of well
and 3D seismic data, there were enormous controversies
on provenance, formation mechanism, and reservoir
spatial distribution of axial channels
(Antbreh and
Krastel 2006; Su et al. 2013; Xu et al. 2012; He et al. 2011)
.
The shortage of basic geological information restricted
the understanding of the petroleum system in western
Qiongdongnan Basin. Moreover, exploration confidence
in this field was severely defeated by all failures in well
drilling in the past.
In recent several years, large-scale high-quality 3D
seismic acquisition and application, including the
widespread application of pre-stack depth migration in slope
break belt, make it possible to identify and evaluate
Paleogene the Yacheng Formation as source rock, which is
also favorable for Miocene seismic sedimentology,
paleotopography, and sedimentation researches. With the
help of high-resolution electric image logging
technology, sedimentary textures associated with reservoir
formation environment can be clearly identified. Moreover,
this study conducted the pressure–accumulation
relation research and description of fracture net which can
be served as vertical migration paths. Research results
illustrate reservoir-forming mechanism using new 3D
data and advanced imaging logging technology, which is
helpful for gas exploration and leads to the recent
breakt (...truncated)