**6. Sedimentary model of Wenliu Area**

The reservoir properties depend on tectonism, eustasy, sediment flux, biological process, climate, and other allogenic and authigenic factors. At the basin scale, these factors mainly determine the formation of systems tracts [48], while at smaller scales, they influence colors, shapes, structures, and internal architectures of the sedimentary bodies. It is at these smaller scales that reservoir characterization (especially lithological and lithofacies analysis) becomes significant.

According to core observation and description as well as well logging analysis, this chapter concludes that the lithology of the target area is characterized by mudstones, sandy mudstones, muddy siltstones, shaley siltstones, and sandstones. The main color of the sandstones is light brown, whereas the main color of the mudstones is red to purple. Statistics shows that mudstone is the dominant lithology of the target interval (more than 63%), with the other components showing relatively smaller percentages (siltstone 14.5%, muddy siltstone 5%, limy siltstone 5%, and sandstone 12.5%). Shapes of grain size cumulative curves plotted on log-probability paper could reveal environmental characteristics. The plots of grain size as cumulative curves of the target interval show that suspension population occupies a large proportion (generally larger than 30%). Saltation population is dominant and its segment is relatively steep. The truncation points between these two populations are usually bigger than 3 (φ value). Traction population is usually missing within the target interval (**Figure 7**).

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

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

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

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

to 3000 m) of this block, originally described as a shallow water delta system deposit, is the

ful 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 strati-

Block based on abundant well logging data, 3D seismic data, and core data under the principles

The reservoir properties depend on tectonism, eustasy, sediment flux, biological process, climate, and other allogenic and authigenic factors. At the basin scale, these factors mainly determine the formation of systems tracts [48], while at smaller scales, they influence colors, shapes, structures, and internal architectures of the sedimentary bodies. It is at these smaller scales that reservoir characterization (especially lithological and lithofacies analysis) becomes

According to core observation and description as well as well logging analysis, this chapter concludes that the lithology of the target area is characterized by mudstones, sandy mudstones, muddy siltstones, shaley siltstones, and sandstones. The main color of the sandstones is light brown, whereas the main color of the mudstones is red to purple. Statistics shows that mudstone is the dominant lithology of the target interval (more than 63%), with the other components showing relatively smaller percentages (siltstone 14.5%, muddy siltstone 5%, limy siltstone 5%, and sandstone 12.5%). Shapes of grain size cumulative curves plotted on log-probability paper could reveal environmental characteristics. The plots of grain size as

main oil-bearing interval of this area (the oil-bearing area is nearly 13.4 km<sup>2</sup>

of high-resolution sequence stratigraphy and sedimentology (**Figure 5c**).

(**Figure 5b**).

L, lowermost Oligocene succession, ranging from approximately 2700 m

) and has plenti-

L, Paleogene Shahejie Formation of the W79

the Dongpu Depression [44].

Shahejie Formation (Es<sup>2</sup>

significant.

wide, covering an area of more than 2000 km<sup>2</sup>

graphic and sedimentary characteristics of the Es<sup>2</sup>

**6. Sedimentary model of Wenliu Area**

The ratio of stable transparent heavy mineral assemblage (mainly zircon and tourmaline) to the entire transparent heavy mineral amount (zircon, tourmaline, garnet, barite, epidote, etc.) indicates that the sourcelands are on NNW direction.

**Figure 7.** Grain size cumulative probability plots of target interval in Wenliu Oilfield (after [19]).

#### **6.1. Lithofacies characteristics**

The lithofacies characteristic analysis is also very important for depositional environment analysis [49–52]. The lithofacies recognition and categorization is based on lithology, grain size, physical and biogenic sedimentary structures, and stratification from core observation and description. Seventeen lithofacies have been recognized within the study interval, including from fine-grained sandstones of parallel bedding to mudstones of

**Figure 8.** Typical lithofacies of the W79 block for the target interval: (a) parallel bedding sandstone; (b) trough crossbedding sandstone; (c) scouring structured sandstone with mud gravels; (d) ripple cross-bedding siltstone; (e) wavy cross-bedding siltstone; (f) parallel bedding siltstone; (g) ripple cross-bedding muddy siltstone; (h) flasher structured mudstone (after [19]).

lenticular structures (**Figure 8**). The codes, features and interpretation are summarized in **Table 1**.

**6.1. Lithofacies characteristics**

mudstone (after [19]).

The lithofacies characteristic analysis is also very important for depositional environment analysis [49–52]. The lithofacies recognition and categorization is based on lithology, grain size, physical and biogenic sedimentary structures, and stratification from core observation and description. Seventeen lithofacies have been recognized within the study interval, including from fine-grained sandstones of parallel bedding to mudstones of

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

**Figure 8.** Typical lithofacies of the W79 block for the target interval: (a) parallel bedding sandstone; (b) trough crossbedding sandstone; (c) scouring structured sandstone with mud gravels; (d) ripple cross-bedding siltstone; (e) wavy cross-bedding siltstone; (f) parallel bedding siltstone; (g) ripple cross-bedding muddy siltstone; (h) flasher structured According to the reddish-brown mud of subaerial oxidizing features and the grain size cumulative plots revealing typical channel deposit features (**Figure 7**), along with the specific


supply of clay and silt


**Table 1.** Lithofacies of the target interval succession (after [19]).

 associations of the lithofacies as reconstructed from the core descriptions, a meandering river fan is interpreted to be responsible for the sedimentation.

#### **6.2. Meandering river fan**

The target interval of the Dongpu Depression was formed under a rift contraction basin environment far from the source and with an abundant sediment supply. Single well core section shows that channel and overbank deposits are two main sequence types of the sedimentary strata. The channel deposits generally have complete sequences and are relatively isolated. The overbank deposits are mainly composed of reddish-brown shale that could indicate the exposed and oxidic environment (together with the lack of subaqueous paleontological fossils, it may be revealed as subaerial sedimentary environment) (**Figure 9**). Lithofacies paleogeographic mapping shows many meandering channels distributed along the basin margin like a large fan-shaped river system, which shows distributive characteristics quite different of those of a fan delta system (**Figure 10**).

**Figure 9.** Single well facies (W133-12 Well) (after [19]).

associations of the lithofacies as reconstructed from the core descriptions, a meandering river

**ID Lithology Structure and texture Interpretation Schematic** 

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

**MSSp** Muddy siltstone Parallel bedding

**MSSla** Muddy siltstone Low-angle cross

**SMf** Silty mudstone Flaser bedding

SMw Silty mudstone Wavy bedding

Mw Mudstone Wavy bedding

Mm Mudstone Massive bedding

Ml Mudstone Lenticular bedding

**Table 1.** Lithofacies of the target interval succession (after [19]).

bedding

**representation**

Parallel and aclinic, result of suspension settling of fine-size sediment osr precipitation from solution, low energy

Low-angle bounding surfaces, originates by migration of planar bedforms, low energy

Fluctuating depositional conditions marked by periods of current activity,

Wavy lamina, parallel to the bounding surface as a whole, abundant sediment supply of clay and silt

Wavy lamina, parallel to the bounding surface as a whole, abundant sediment

Contain few or no visible internal lamina, overbank or abandoned channel

Fluctuating depositional conditions marked by periods of current activity,

mud < sand

supply of clay

deposits

mud > sand

The target interval of the Dongpu Depression was formed under a rift contraction basin environment far from the source and with an abundant sediment supply. Single well core section shows that channel and overbank deposits are two main sequence types of the sedimentary strata. The channel deposits generally have complete sequences and are relatively isolated. The overbank deposits are mainly composed of reddish-brown shale that could indicate the exposed and oxidic environment (together with the lack of subaqueous paleontological fossils, it may be revealed as subaerial sedimentary environment) (**Figure 9**). Lithofacies paleogeographic mapping shows many meandering channels distributed along

fan is interpreted to be responsible for the sedimentation.

**6.2. Meandering river fan**

This special depositional system is interpreted to be a meandering river fan, mainly composed of three subfacies (i.e., channel fill subfacies, overbank subfacies, and river flood lake subfacies) (**Figure 10**). In the study area, back-stepping and forward-stepping types both exist, revealing A/S > 1 and A/S < 1, respectively, and their logging curve (gamma ray) reflections are generally of toothed boxes or bell shapes (see **Table 2**).

**Figure 10.** Sedimentary facies model of the target interval in Dongpu Depression (after [19]).


**Table 2.** Comparison of meandering river fan and shallow water delta (after [19]).

The channel fill subfacies are mainly composed of sand-rich channel deposition. From bottom to top, Ss, St, Sla, and Sr constitute the main lithofacies and show a fining-upward cycle. The grain size is relatively coarse, and the sandbody connectivity is therefore high. The overbank subfacies contain two parts. The proximal overbank is next to the channel and is one of the most important reservoir types due to its good physical properties (i.e., after channel fill). The common lithofacies of the proximal overbank include Sla, SSla, and SSr. The distal overbank is distributed between channels and is mainly composed of purple to red mudstone. The common lithofacies include MSSw, SMw, and Mm. The river flood lake subfacies form near the distal overbank, recording stand water, and are mainly composed of grayish-green mudstone. The common lithofacies of this subfacies contain Mw, Mm, and Ml.
