**2. Seismic stratigraphy**

rifting and the formation of oceanic lithosphere. In a well-developed Atlantic-type continental

Sequence stratigraphic interpretations of the alluvial systems and of the fine-grained shales, studied in this book, may be considered as both a counterpart and an integration of sequence stratigraphic analyses of marine deposits in continental shelf, slope and basin environments. The sequence stratigraphic setting of the fluvial depositional systems has been studied by several authors in different geological frameworks [29–35]. The distributive fluvial systems (DFS) [35], which have been investigated in this book, are a particular type of fluvial system, which is characterized by a downstream whose size decreases, is not bounded from valleys and shows a pattern of different rays coming from an apex. The sequence stratigraphy of the fine-grained shales is an interesting research topic of this book, and theoretical aspects applied to several geological settings have been pointed out by several papers [36–40]. In particular, the sequence stratigraphy of the Barnett Shale and subordinately of the Woodford Shale is among the most studied research topics regarding the shales and has been coupled with other geological methodologies, including the geochemistry and the evaluation of the

In this book, different case studies located in China have been presented. To this aim, it should be useful to clarify the type of geological structure of the Chinese-type basins. The present-day geological setting of the Asia continent and, in particular, of China has been controlled by the amalgamation of several Paleozoic continental blocks and many insular arcs. Ziegler et al. [46] have attempted to follow the traces of the migration of some of these blocks up to their unification in the Laurasia continent of the Pangea. The paleogeographic reconstructions of the Chinese region at the end of the Paleozoic have allowed to distinguish three Precambrian platforms, which have been captured during the growth processes in the Paleozoic (i.e., the Tarim platforms, Northern China, and Southern China) [47]. During the formation of the Meso-Cenozoic megasuture belt, a new set of plates was produced, with the capture of the blocks of the Lut, Iran, Tibetan block, and Indochinese platform, merging with the initial Paleozoic nucleus. In the time interval ranging from the Upper Cretaceous to the Pliocene, the collision of the Asia with the Arabian block occurred, while during the Cenozoic, the collision of the Asia with the Indian block occurred [48]. The Chinese basins include the Ordos, Pre-Nan Shan, Tsaidam, Tarim, Turfan, and Dzungarian and are characterized by the occurrence of Mesozoic-Tertiary continental successions deformed by both strike-slip and reverse faults involving the Paleozoic basement [49]. Their individuation appears to be related to compressional stresses [50]. Three types of basins, hosting important oil and gas resources, have been distinguished in China, namely the extensional basins, the compressional basins, and the transitional basins [50]. The extensional basins prevail in the Eastern China, including the Songliao and Bohai Gulf basins (the second one has been investigated in this book). The compressional basins are mainly located in the Western China, including the Tarim and Junggar basins, while the transitional ones are mainly located in the Central China, including the Sichuan and Ordos

In this book, another important research topic is represented by the Andean foreland basin, whose age is Cenozoic and located in northern Argentina. Its formation has been modeled

margin, a continental shelf, continental slope and rise and basin occur [14–28].

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

gas content for petroleum studies [41–45].

basins [50].

In this book, the seismo-stratigraphic setting of the Northern Taiwan offshore has been reconstructed based on the geologic interpretation of seismic sections (see Chapter 3). Moreover, in Chapter 5, significant results on the stratigraphic unconformities have been shown, focusing on examples of Paleozoic successions. Perhaps, it should be useful to clarify some seismostratigraphic concepts and methods.

While the stratigraphic analysis was previously based on the field geological survey, on the measurement of stratigraphic sections and on the lithologic and paleontologic descriptions, aimed at reconstructing the depositional environments and at correlating the stratigraphic sequences among them, this work methodology has been deeply changed after the onset of the seismic stratigraphy, which has allowed for obtaining detailed seismic records of the stratigraphic successions.

The approach to the seismic stratigraphy is based on the key concept that the seismic reflectors may be compared with the strata plans and, perhaps, the geometry of the seismic reflectors corresponds to the depositional geometry [55]. In this sense, the seismic stratigraphy represents a geological and geophysical approach to the stratigraphic analysis and interpretation.

The seismic reflectors occur in correspondence with significant contrasts of the acoustic impedance, which is a significant parameter in seismic stratigraphy. When an acoustic wave meets the interface separating two media having a different acoustic impedance, a part of the wave is transmitted to the other medium, while another part is reflected on the interface among the two media. The concept of the acoustic impedance allows for the calculation of the quantity of transmitted and reflected acoustic energy.

If we consider U as the energy of the wave crossing the media M1 and M2 and we suppose that Z1 is the acoustic impedance of the medium M1 and Z2 is the acoustic impedance of the medium M2 , the transmitted energy U<sup>t</sup> can be calculated through the following equation:

$$\mathbf{U}\_{i} = \frac{2\,\mathrm{Z}\_{i}}{\mathrm{Z}\_{1} + \mathrm{Z}\_{2}}.\,\mathrm{U}\tag{1}$$

while the reflected energy U<sup>r</sup> can be calculated through the following equation:

$$\mathbf{U}\_r = \frac{\mathbf{Z}\_2 - \mathbf{Z}\_1}{\mathbf{Z}\_1 + \mathbf{Z}\_2} \mathbf{J} \mathbf{U} \tag{2}$$

The contrasts of acoustic impedance controlling the individuation of the seismic reflectors are located along surfaces corresponding to strata surfaces or to other discontinuities having a chronostratigraphic meaning. The strata surfaces represent the old surfaces of deposition, and then, they are coeval in the depositional area. The discontinuities are old erosional or nondepositional surfaces corresponding to significant stratigraphic gaps. Also if they represent events varying during the geological time, the discontinuities are considered as chronostratigraphic surfaces, since all the strata overlying the discontinuity are younger than the underlying strata [2, 9–12]. When identified on a seismic section, the discontinuities let to identify the most important lateral variations in the deposition of a stratigraphic succession. Moreover, they offer a geological basis in order to subdivide the stratigraphic successions in depositional sequences, which are the basic stratigraphic units of seismic stratigraphy [2, 9–12].

The main steps of the seismo-stratigraphic analysis are represented by the identification of the discontinuities and consequently of the depositional sequences, by the reconstruction of the original geometry of the sedimentary bodies and related sedimentary environments and by the chronostratigraphic correlation [2, 9–12].

The seismic sequence analysis allows for the identification of the depositional sequences. The geometric relationships between the lateral terminations of the strata and the discontinuities or the correlative conformities define the boundaries of the depositional sequences [2]. The lateral terminations of the strata with respect to the sequence boundaries individuate the configurations of onlap, downlap, continuity (lower boundaries) and of erosional truncation, toplap and continuity (upper boundaries) [1, 2, 7–12].

The seismic facies analysis deals with both the individuation and the geologic interpretation of the geometry, continuity, amplitude, frequency and velocity of the seismic reflectors, more than the outer shape of the sedimentary bodies and the seismic facies associations in a depositional sequence [2, 56–61]. In the modern development of this methodology, one aim is represented by the recognition of clusters or groups, representative of significant variations in the properties of the rocks, in the lithology and in the content of fluids. The cluster analysis offers a significant instrument in order to perform the classification of the shapes of the seismic traces grouping them into clusters, often using an unsupervised process without a previous definition of the clusters [57, 60, 61].

The analysis of relative sea-level fluctuations is based on the construction of chronostratigraphic diagrams and of curves of relative sea-level cycles [1, 6–10, 62]. In a chronostratigraphic section, reporting the chronological units in the ordinates of the graph, each layer has an equal time duration. Both erosional and non-depositional hiatuses may occur among the time surfaces corresponding to the layers of the depositional sequences. Three-dimensional Wheeler chronostratigraphic diagrams represent a useful tool in the geological interpretation of the seismic sections [63–65]. While the conventional Wheeler diagrams, which are usually made by hand, include sketch diagrams showing the extent of chronostratigraphic sequences, new methods have been recently developed in order to construct a Wheeler diagram for a seismic three-dimensional volume [63–65].
