**7. Sequence stratigraphy**

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**).

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

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

Paleo-climate Humid; oxidizing environment

Subfacies Channel fill subfacies, overbank

subfacies

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

Deposits between distributive/ distributary channels

Distance between distributive/ distributary channels

Wave or tide forces No Yes Sediment supply Sufficient Sufficient

Settings Mainly subaerial Subaqueous and subaerial

Subaqueous paleontological fossils Rare Relatively abundant

(major part) to slightly reductive environment (some small parts)

Sequence and cyclicity Mainly positive cycles Both positive and negative cycles

subfacies and river flood lake

**Meandering river fan Shallow water delta**

Mud Mud or sand sheet

Relatively large Small to medium

Relatively humid; reductive environment to slightly oxidizing

Delta plain subfacies, delta front subfacies, and pro-delta subfacies

environment

#### **7.1. Sequence stratigraphic surfaces**

The identification of sequence stratigraphic surfaces and classification of depositional trends is the core component of high-resolution stratigraphic correlation and division. Integrated seismic and well analyses show that two major sequence stratigraphic surfaces collaboratively limit the target interval.

SB1, the top surface of the target interval, has strong and continuous reflection in the seismic section (**Figure 11a**). It is a regional parallel unconformity and is characterized by overlying thick and continuous shale of Es<sup>2</sup> U. Its features on the well logging curves are high GR (gamma ray), AC/DT (acoustic) and SP (spontaneous potential) values and a low R25 (2.5 m resistivity) value. Below this surface, tooth-shaped AC/DT and GR curves become the main characteristics, revealing a frequent sand mud inter-bedding (**Figure 11b1**).

SB2, the bottom surface of the target interval, also has strong and continuous reflection in the seismic section (**Figure 11a**). It is also a parallel unconformity characterized by a stable mudstone layer of the underlying Es3 U. Its features on well logging curves are high GR, AC, SP, and R25 values (**Figure 6**). The most obvious characteristic is the in the SP curve, approximately 10 m above the surface, in which the curve has a v-shaped low value zone (**Figure 11b2**).

#### **7.2. Facies associations under sequence stratigraphic frame**

Based on core observations as well as integrated seismic and well analyses, more than 58 short-term base level cycles could be recognized in the target interval of the W79 Block. Genetic sequence stacking patterns can be used to define genetic sequence sets, showing base level rises and falls [1]. Thus, these short-term patterns could be combined into six middle-term base level cycles (from bottom to top—MSC1 and MSC6). In a similar way, given the regional setting, these six middle-term base level cycles could be further

**Figure 11.** Identification of main sequence stratigraphic surfaces and stratigraphic division framework (after [19]).

combined into two long-term base level cycles (from bottom to top—LSC1 and LSC2) (**Figure 11c**). Under this high-resolution sequence stratigraphic frame, according to the practice of oilfield development, a 58-small-layer plan was accepted for paleogeographic mapping (**Figure 12**) and for other subsequent studies. Precise paleogeographic mapping shows that the vertical facies distribution and association have strong regulations on the long-term base level cyclic scale.

LSC1, the first long-term base level cycle, composed of MSC1 to MSC4, approximately 90 m to 120 m long, is the lower part of the target interval and is limited by the SB2 at the bottom. At the lower part of this long-term base level cycle, proximal overbank deposition and distal overbank deposition are most commonly found, whereas channel fill deposition could sporadically be found. In contrast, at the upper part of this long-term base level cycle, apart from proximal and distal overbank, channel fill deposition is also commonly found and the connectivity of the sandbody is relatively higher than that of the lower part of LSC1, which has no or isolated channel fill deposition. The ratio of channel fill to overbank increases from bottom to top, and the sedimentary stratigraphy shows an Sequence Stratigraphy of Fluvial Facies: A New Type Representative from Wenliu Area, Bohai… http://dx.doi.org/10.5772/intechopen.71149 55

**Figure 12.** Sedimentary facies maps of the middle-term base level cycles of the target interval of W79 Block (after [19]).

combined into two long-term base level cycles (from bottom to top—LSC1 and LSC2) (**Figure 11c**). Under this high-resolution sequence stratigraphic frame, according to the practice of oilfield development, a 58-small-layer plan was accepted for paleogeographic mapping (**Figure 12**) and for other subsequent studies. Precise paleogeographic mapping shows that the vertical facies distribution and association have strong regulations on the

**Figure 11.** Identification of main sequence stratigraphic surfaces and stratigraphic division framework (after [19]).

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

LSC1, the first long-term base level cycle, composed of MSC1 to MSC4, approximately 90 m to 120 m long, is the lower part of the target interval and is limited by the SB2 at the bottom. At the lower part of this long-term base level cycle, proximal overbank deposition and distal overbank deposition are most commonly found, whereas channel fill deposition could sporadically be found. In contrast, at the upper part of this long-term base level cycle, apart from proximal and distal overbank, channel fill deposition is also commonly found and the connectivity of the sandbody is relatively higher than that of the lower part of LSC1, which has no or isolated channel fill deposition. The ratio of channel fill to overbank increases from bottom to top, and the sedimentary stratigraphy shows an

long-term base level cyclic scale.

overall coarsening-upward trend, revealing that the environment varied from a relatively higher accommodation condition to a relatively lower accommodation condition. This vertical change indicates that the entire LSC1 was formed during base level falling or an A/S decreasing period and is, in fact, a semi-cycle of the long-term scale. The vertical characteristic of LSC1 might reveal a relatively stable tectonic setting where subsidence tends to be slow and the depositional process is stable with abundant sediment supply.

LSC2, the second long-term base level cycle, composed of MSC5 to MSC6, approximately 150 to 190 m long, is the upper part of the target interval and is limited by SB1 at the top. In the lower part of this long-term base level cycle, similar to that of the upper part of LSC1, channel fill deposition is commonly found apart from the proximal and distal overbank deposition, and the connectivity of the sandbody is relatively high. In contrast, in the upper part of this long-term base level cycle, proximal overbank deposition, and distal overbank deposition are most commonly found, whereas channel fill deposition is sporadically found. The sedimentary stratigraphy shows an overall upward-fining trend, revealing that the environment varied from a relatively lower accommodation condition to a relatively higher accommodation condition. Similar to LSC1, the entire LSC2 was also a semi-cycle of the long-term scale, whereas it was formed during a base level rising or A/S increasing period. The vertical characteristics of LSC2 might reveal an active tectonic setting where subsidence tends to be fast and the base level rises rapidly, which makes the ratio of the channel to overbank change from high to low.
