Fig. 1 Sketch map showing geographical position of tectonic units and
oil and gas fields in the Xihu Sag, East China Sea Shelf basin
The
Xihu Sag is located northeast of the ECSSB and belongs to a secondary
sag of the western geotectogene. The Xihu Sag is approximately 400 km
long from south to north and 100 km wide from east to west; it has a
total area of approximately 5.18 × 104km2. The sedimentary thickness in the sag is
approximately 9–15 km, making it the largest sedimentary sag in the
ECSSB; in particular, the sedimentary characteristics indicate a great
potential for oil and gas exploration and development as observed in
previous studies.
The sedimentary strata drilled in the Xihu Sag were primarily the
Paleogene and Neogene strata, during which there were many depositional
hiatuses and regional unconformities. The Paleogene Pinghu Formation in
the Xihu Sag underwent multiple tectonic movements and was deposited at
the stage of tectonic thermal subsidence in the post-rift period.
Consequently, the primary sedimentary facies are clastic rocks of the
transitional phase. The Pinghu Formation is divided into three members
from bottom to top, and each member has obvious sedimentary cycles of
fine sediment in the bottom and coarse sediment on the upside. The
lithology primarily comprises of yellow fine sandstone, gray siltstone,
gray mudstone, and coal, with a relatively fine grain size. The distinct
lithostratigraphic characteristics of the Pinghu Formation in the ECSSB
were affected by tectonic and sedimentary patterns, and the overall
lithology is finer from west to east (Zhang et al., 2006; Zhu et al.,
2012; Lin et al., 2017; Cheng et al., 2019; Wang et al., 2021).