**5. Geological background of the Wenliu Area**

The Bohai Bay Basin is located in the eastern China continental margin. It is a composite sedimentary basin formed in the Mesozoic and Cenozoic periods and the Proterozoic to the Paleozoic period, covering an area of 195,000 square kilometers in the Bohai Sea and the coastal areas. Various reservoir systems and reservoir rock types are well developed in the basin, and the Eocene Shahejie Formation is the major hydrocarbon bearing layer series (**Figure 5a**) [40]. Some new studies show that the formation and evolution of the Bohai Bay Basin was induced by both the thermal power of bottom of the lithosphere and plate boundary activity due to regional stress field. The dynamic process of the Cenozoic basin in the Bohai Bay Basin is related to the thermal activity of the lithosphere in eastern China. It is also related to the regional stress field controlled by the tectonic activity at the plate boundaries and belongs to the superimposition of many regional geological processes [41, 42]. The Bohai Bay Basin is a Meso-Cenozoic sedimentary basin superimposed on the Paleozoic basement. During previous geological times the basin was part of the North China craton. The upper Proterozoic and the lower Paleozoic sequences are constituted of marine sediments, including the upper Proterozoic strata of the Jixian system, the Changcheng system and Qingbaikou system, which are mainly composed of micritic limestones, quartz sandstones, and shales. The sedimentary cover is not only distributed in the lower part of the basin, but widely distributed around the basin. The lower Paleozoic sequence is made up of Cambrian and Ordovician carbonates, with shales, siltstones, and sandstones. The upper Paleozoic sequence is made up of Carboniferous and Permian strata, with paralic and continental clastic rocks and some coal seams. The Mesozoic strata range in age from the Triassic to the Cretaceous. They are composed of tuffaceous sandstones, conglomerates, mudstones, and coals (**Figure 6**).

The Dongpu Depression, located in the south margin of Bohai Bay Basin, is an oil-rich sedimentary depression. It is a 16-km long and 70-km wide NNE fault depression that covers an area of approximately 5300 km<sup>2</sup> (**Figure 5a**). The Dongpu Depression formed from the Huabei Movement in the late Paleogene. The evolution of the depression reveals an evident tectonic component and shows periodic features [43]. It is bounded by the

**Figure 5.** (a) Location of the Bohai Bay Basin (after [40]). (b) Location of the Dongpu Sag. (c) Location of Wenliu Area showing the main structures (after [19]).


Paleozoic period, covering an area of 195,000 square kilometers in the Bohai Sea and the coastal areas. Various reservoir systems and reservoir rock types are well developed in the basin, and the Eocene Shahejie Formation is the major hydrocarbon bearing layer series (**Figure 5a**) [40]. Some new studies show that the formation and evolution of the Bohai Bay Basin was induced by both the thermal power of bottom of the lithosphere and plate boundary activity due to regional stress field. The dynamic process of the Cenozoic basin in the Bohai Bay Basin is related to the thermal activity of the lithosphere in eastern China. It is also related to the regional stress field controlled by the tectonic activity at the plate boundaries and belongs to the superimposition of many regional geological processes [41, 42]. The Bohai Bay Basin is a Meso-Cenozoic sedimentary basin superimposed on the Paleozoic basement. During previous geological times the basin was part of the North China craton. The upper Proterozoic and the lower Paleozoic sequences are constituted of marine sediments, including the upper Proterozoic strata of the Jixian system, the Changcheng system and Qingbaikou system, which are mainly composed of micritic limestones, quartz sandstones, and shales. The sedimentary cover is not only distributed in the lower part of the basin, but widely distributed around the basin. The lower Paleozoic sequence is made up of Cambrian and Ordovician carbonates, with shales, siltstones, and sandstones. The upper Paleozoic sequence is made up of Carboniferous and Permian strata, with paralic and continental clastic rocks and some coal seams. The Mesozoic strata range in age from the Triassic to the Cretaceous. They are composed of tuffaceous sandstones, conglomerates, mudstones,

44 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

The Dongpu Depression, located in the south margin of Bohai Bay Basin, is an oil-rich sedimentary depression. It is a 16-km long and 70-km wide NNE fault depression that

from the Huabei Movement in the late Paleogene. The evolution of the depression reveals an evident tectonic component and shows periodic features [43]. It is bounded by the

**Figure 5.** (a) Location of the Bohai Bay Basin (after [40]). (b) Location of the Dongpu Sag. (c) Location of Wenliu Area

(**Figure 5a**). The Dongpu Depression formed

and coals (**Figure 6**).

covers an area of approximately 5300 km<sup>2</sup>

showing the main structures (after [19]).

**Figure 6.** Stratigraphy in Dongpu Depression, Bohai Bay Basin (left), enhancing the lower second member of Shahejie FM. In Well W167 of Wenliu Area (right). St, system; Sr, series; Fm, formation; Mb, member (after [19]).

Luxi Uplift on the east, the Lankao Uplift on the south, and the Neihuang Uplift on the west. Its north margin is the Maling Fault, which is also the south margin of the Linqing Depression. The Lanling, Changyuan, and Huanghe basement rift faults controlled the early evolution of the Dongpu Depression and changed it from a broom fault depression to a double-break fault depression. During the early Paleogene, the Tan-Lu Faults changed from transpression to dextral lateral strike-slip faults, altering the regional stress field from a shear-extrusion field to a shear-tensile field. Differential subsidence also occurred during this period. The regional shear-tensile effects caused the Central Uplift rise, then induced gravitational sliding and salt rock intrusion, and finally molded the present appearance of the Dongpu Depression [44].

The Wenliu Oilfield, located in Henan Province, is on the northern part of the central uplift of the Dongpu Depression. Bounded by two large sag belts (i.e., the West and the East Sub-sag Belt), the Wenliu Oilfield is a complicated fault zone that is characterized by horst and graben structures [44]. It is also of the NNE direction and is approximately 20 km long and 16 km wide, covering an area of more than 2000 km<sup>2</sup> (**Figure 5b**).

The W79 Block, one of the most important blocks of the Wenliu Area, has already entered the middle to late stage of the developing period. The Paleogene lower second member of the Shahejie Formation (Es<sup>2</sup> L, lowermost Oligocene succession, ranging from approximately 2700 m to 3000 m) of this block, originally described as a shallow water delta system deposit, is the main oil-bearing interval of this area (the oil-bearing area is nearly 13.4 km<sup>2</sup> ) and has plentiful remaining petroleum resources. However, because of its complicated sedimentary features, many studies [41, 44–47] concerning sequence stratigraphy and sedimentary facies models are still under scientific debate. Hence, this paper aims to provide an analysis of the sequence stratigraphic and sedimentary characteristics of the Es<sup>2</sup> L, Paleogene Shahejie Formation of the W79 Block based on abundant well logging data, 3D seismic data, and core data under the principles of high-resolution sequence stratigraphy and sedimentology (**Figure 5c**).
