**2. Data and methodology**

**1. Introduction**

The great success of shale gas exploration in North America has attracted increasing attention from petroleum geologists in the world. They are passionate about finding more commercial shale gas in other countries. Although the success of shale gas exploration in North America leads geologists to believe that shale gas exploration has a good prospect, how to find the sweet spot of shale gas in shale strata is still the main obstacle we need to face in the shale gas exploration and development. Sequence stratigraphy is a relatively new concept of stratigraphy and is a powerful tool to subdivide the stratigraphic intervals into geologically realistic and cyclic patterns and to predict the stratal patterns, the lithofacies variations, and the petroleum reservoirs within a chronostratigraphic unit. In the past 40 years, the sequence stratigraphic studies mainly focused on the coarse-grained siliciclastic and carbonate deposits and on the fine-grained turbiditic systems from lacustrine to deepwater settings, leading to a great success in the conventional petroleum exploration [1–5]. However, only limited studies on the high-frequency sequence and sedimentology have been conducted to study the sequence stratigraphy of shale strata deposited in deep water or restricted shallow water environment [6–8]. According to the traditional sequence stratigraphy, it is difficult to carry out a sequence stratigraphic analysis in relatively deep water areas where the shales are deposited [9]. This is due to the relatively homogenous and fine-grained nature of the shale interval [10] and to the conformity contact between upper and lower strata. Only newly acquired seismic data tied to geology can characterize the partial reservoir parameters, and it is very challenging to pick surfaces reflecting sealevel changes of fine-grained deposits on the seismic data if not tied by core and log data [11, 12]. Also, the GR log alone sometimes does not indicate high TOC of the maximum flooding surface within a sequence [13]. Nevertheless, some researchers have integrated the geochemistry, the geomechanics, the core and outcrop analysis, the seismic data, the well logs, the petrology, and the paleontology in order to perform a sequence stratigraphic analysis of the shales aimed at understanding the sequence boundaries, the major transgressive surface of erosion, the maximum flooding surface, the stacking patterns, the lithofacies variability, and the depositional models within the sequence [8–10, 14–25]. Some geologists have even tried to establish the criteria and models for sequence stratigraphy study in marine shale and predict the good reservoir potential shale gas vertically and laterally [8, 10, 12, 21, 22, 26]. So far, sequence stratigraphy of the shale is still poorly understood, and the traditional seismic stratigraphy approach to recognize the boundaries of sequences and system tracts does not work well for the fine-grained shale due to their relatively homogeneous fine-grained nature. Recent advances in property tests of the geochemistry, the mineralogy, the petrology, the petrophysics, and the geomechanics have provided the basis for the detailed characterization of the sequence stratigraphy of fine-grained shales. This chapter analyzes the spatial and temporal variations in the lithofacies, the well-logging responses, the geochemistry, the mineralogy, the hydrocarbon content and aims at developing the sequence stratigraphic frameworks for representative siliciclastic-dominated shales (marine Barnett, Woodford, Marcellus, and Mowry in the USA, Longmaxi and Chang7 shales in China) and carbonate-dominated shales (Niobrara, USA). The general variation trends of the shale properties have been analyzed within a sequence stratigraphic framework, so that they can be used to regionally correlate the shaly strata in a systematic manner and to identify, to predict, and to map the high-quality source rock and the productive fine-grained shale reservoirs.

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

The data for characterizing the sequence stratigraphy of fine-grained shales include the regional geologic data, the seismic data, the well logs, the lithofacies description in outcrops and cores, the organic geochemistry, the mineralogy, the thin sections, the SEMs, the QEMSCAN, and the geomechanics.

This chapter has employed a classic sequence stratigraphy model, consisting of the lowstand systems tract (LST), of the transgressive systems tract (TST), and of the highstand systems tract (HST) [2, 27]. The HST can be subdivided into early highstand systems tract (EHST) and late highstand systems tract (LHST) based on identifiable stacking patterns of parasequences and shale properties [24]. Slatt and Rodriguez [8] observed that many shales have cyclical patterns related to the eustatic sea-level fluctuations. They also stated that the general sequence stratigraphy of shales consists of a major transgressive surface of erosion or its correlative conformity and of an organic-rich TST bounded by a transgressive surface below and by a maximum flooding surface (MFS) or a condensed section (CS) above. The TST is then followed by upward decreasing gamma-ray trends within an interpreted HST (**Figure 1**, see appendix for the abbreviations used in this chapter). Typical time frames in which sea-level cycles (length of geologic time for a complete sea-level fluctuations) can be interpreted for shales (mainly based upon their fossil components) include second order, approximately 10–25 My; third order, approximately 1–3 My; fourth order, approximately 100,000–300,000 years [20].

For the methodology, this chapter adopts the following step-by-step workflow for the sequence stratigraphic analysis of fine-grained "shale" systems.


**Figure 1.** General sequence stratigraphic model of fine-grained shale. A conceptual gamma ray log is shown to indicate the log response of different fine-grained lithofacies (SS-shale rich in siltstone, DQ-shale rich in detrital quartz, O-CMorganic or clay rich mudstone, BQ-shale rich in biogenic quartz) in different stratigraphic interval within sequence stratigraphic framework. Modified from Slatt and Rodriguez (2012) and Slatt et al. (2014). See appendix for abbreviations of sequence stratigraphy from Van Wagoner et al. (1990).

3. Detailed lithological descriptions and preliminary interpretation of lithofacies from outcrop and cores;

4. Additional well logs and data collected from the cored wells were organized, including core-to-log depth corrections;

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 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.
