**4. Sequence stratigraphy of siliciclastic-dominated shales**

Siliciclastic shales are mainly composed of silicate minerals and transported and deposited as fine-grained particles. The basic sequence stratigraphy of representative siliciclastic Barnett, Woodford, Marcellus, and Mowry shales is herein presented based on the variations in lithofacies, geochemistry, well log responses, and cyclicity of relative sea-level change and its effects upon stacking patterns and stratigraphy.

#### **1.** Barnett shale

3. Detailed lithological descriptions and preliminary interpretation of lithofacies from out-

4. Additional well logs and data collected from the cored wells were organized, including

5. Lithofacies were identified based on core description, QEMSCAN analysis, thin section description, well log responses of spectral gamma ray, resistivity, FMI, density, and XRD data; 6. Seismo-stratigraphic, paleontological, geochemical, and mechanical data were used to

In the last few decades, the most of organic-rich shale-related plays, for example Barnett, Niobrara, Eagle Ford, Haynesville, and Bakken, have been labeled as "shale plays," and the terms "shale oil," "tight oil," and "resource play" are often used interchangeably in a public discourse [28–32]. The Barnett shale is a typical organic-rich siliceous shale (**Figure 2a**). The Eagle Ford shale is an organic-rich shale interbedded with thin fine-grained carbonate beds (**Figure 2b**). The lacustrine Green River shale consists of organic-rich shale with fine-grained ostracod rich carbonate (**Figure 2c**). The Niobrara shale play is dominated by a low permeability fine-grained chalk sourced by its adjacent marl source rock with continuous hydrocarbon accumulation [33]. The chalk and marl cannot be differentiated by naked eyes and appear like

**Figure 2.** Lithofacies of typical fine-grained "shales" in U.S. The Green River Fm is of lacustrine environment and the

crop and cores;

core-to-log depth corrections;

**3. Lithofacies of shales**

rest are of marine environment.

identify the boundaries of different order sequences;

7. Vertical and lateral stacking patterns of the sequences were recognized; 8. High-frequency sequence stratigraphic framework was developed.

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

The Mississippian Barnett shale is divided into lower and upper Barnett shale intervals separated in the northern part of the basin by the Forestburg Limestone. The lower Barnett sits directly above an SB/TSE, which caps the underlying Viola/Ellenburger limestones. The lower Barnett is dominated by siliceous mudstones and the upper Barnett is dominated by calcareous mudstones [8]. These lithofacies form distinctive stacking patterns termed "gamma-ray parasequences" (GRP) [34] and termed "high frequency-sequences" by Abouelresh and Slatt [21]. We classify the Lower Barnett shale and Forsberg limestone as a third-order sequence consisting of organic-rich high gamma-ray TST above the Ellenburger limestone and HST of generally decreasing gamma-ray interval. The high gamma-ray lower section of Upper Barnett shale represents TST of another third-order sequence (**Figure 3a**).

#### **2.** Woodford shale

The Late Devonian-Early Mississippian Woodford shale comprises three members. They are represented by the Lower Woodford Black shale, unconformably (SB/TSE) overlying (SB/ TSE) the Hunton Group carbonates, by the Middle Woodford organic-rich black pyritic shale, and by the more quartzose-phosphatic Upper Woodford. The Woodford shale was deposited during a second-order sea-level cycle consisting of several third-order sequences [22]. Each third-order sequence comprises a TST with upward increasing in gamma ray and a HST with upward decreasing in gamma ray (**Figure 3b**).

**Figure 3.** Sequence stratigraphy of typical Barnett, Woodord, Marcellus and Mowry siliciclastic marine shales in U.S. Original data of Barnett shale, Woodford shale, and Marcellus shale are from Slatt and Rodriguez (2012), Slatt and Rodriguez (2014), and Lash and Engelder (2011), respectively.

#### **3.** Marcellus shale

The organic-rich lower Union Springs Member of Devonian Marcellus Shale in the northeastern USA. was deposited as a third-order sequence in the Appalachian foreland basin [35]. This sequence consists of a lower, upward increasing API gamma-ray TST and an upper, upward decreasing API gamma-ray HST. The MFS constituting the highest gamma-ray shale separates the TST and HST, and the HST can further be divided into EHST (early stage) and LHST (late stage) (**Figure 3c**). Generally, the TST and EHST are rich in TOC and quartz content.

#### **4.** Mowry shale

The Cretaceous Mowry shale in Wyoming consists of third-order sequences with each consisting of TST and HST. TST sits sharply above the SB/TSE and is indicated by a shale interval of upward increasing in gamma ray and resistivity (Rt ). The MFS has the highest gamma ray representing the highest sea level. HST generally consists of shale with upward decreasing in gamma ray and resistivity (Rt ).

#### **5.** Longmaxi shale

**3.** Marcellus shale

**4.** Mowry shale

gamma ray and resistivity (Rt

The organic-rich lower Union Springs Member of Devonian Marcellus Shale in the northeastern USA. was deposited as a third-order sequence in the Appalachian foreland basin [35]. This sequence consists of a lower, upward increasing API gamma-ray TST and an upper, upward decreasing API gamma-ray HST. The MFS constituting the highest gamma-ray shale separates the TST and HST, and the HST can further be divided into EHST (early stage) and LHST (late stage) (**Figure 3c**). Generally, the TST and EHST are rich in TOC and quartz content.

**Figure 3.** Sequence stratigraphy of typical Barnett, Woodord, Marcellus and Mowry siliciclastic marine shales in U.S. Original data of Barnett shale, Woodford shale, and Marcellus shale are from Slatt and Rodriguez (2012), Slatt and

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

The Cretaceous Mowry shale in Wyoming consists of third-order sequences with each consisting of TST and HST. TST sits sharply above the SB/TSE and is indicated by a shale interval

representing the highest sea level. HST generally consists of shale with upward decreasing in

). The MFS has the highest gamma ray

of upward increasing in gamma ray and resistivity (Rt

Rodriguez (2014), and Lash and Engelder (2011), respectively.

).

The Lower Silurian Longmaxi shale with the best marine shale gas potential is a typical example. This shale is characterized by organic-rich marine shale with abundant graptolite [24]. The thicker organic-rich portion of this Silurian shale was mainly deposited in intra-shelf lows (bathymetric lows on the shelf) since the foredeep area was subject to clastic sediments dilution from the orogenic belt. The Lower Silurian Longmaxi shale was interpreted as a third-order sequence consisting of transgressive systems tract (TST) and highstand systems tract (HST). Based on the lithofacies and geochemical characteristics, the HST can be further subdivided into early highstand systems tract (EHST) and late highstand systems tract (LHST) (**Figure 4**). During the deposition of the lowstand systems tract (LST), wide shelf areas have experienced erosion, and the sediments have deposited in proximal intra-shelf lows. During the deposition of the transgressive systems tract (TST), the shoreline backstepped toward sediment source areas. The shale is generally deposited in the wide shelf, intra-shelf low, and slope area. The organic-rich shale interval with abundant in-situ graptolite, for example *Diplograptus* [24], high gamma-ray value, and high quartz content, is deposited in anoxic settings. During the deposition of the early highstand systems tract (EHST), the total organic content (TOC) in the shale may be high since the redox condition is still anoxic to dysoxic. The graptolite in EHST is characterized by transported microoffsite graptolite, for example *Monograptus* [24]. During

**Figure 4.** Sequence stratigraphy of Silurian siliciclastic dominated Longmaxi marine shale in Jiaoye1 well in SE Sichuan Basin, Southwest China.

the deposition of the late highstand systems tract (LHST), a further fall of sea level resulted in an increased input of siltstone and sandstone within the shales (see silty layer in core images of the LHST shale in **Figure 4**). During this time interval, sedimentary environment changes to oxygenated shallow marine setting and the coarsening-upward sequence made up of carbonatic shale and silty shales progrades basinward. The shale in LHST is organic-lean and clay-rich shale due to the dilution of clastic input. Generally, the Paleozoic marine shale in TST and EHST has high TOC, high quartz content, and high gas content and is the main target for shale gas play in China. The intervals with high TOC and quartz content in TST and EHST have higher porosity. The porosity in LHST is the highest due to the contribution of silty shale. The sandstone or carbonate in the LHST may trap gas migrated from the TST and EHST shale source rock, and they can form tight sand/carbonate gas play (**Figure 4**).
