Introduction
Seismic reflection is the main tool in the interpretation of offshore sedimentary sequences. The aim of these interpretations is to construct a geologic history of a region by correlating sequence geometries and the mechanisms that control this geometry. These mechanisms are eustacy, tectonics, subsidence, and sediment flux. We will explain the general rules used in defining seismic sequences and facies, followed by how these are related with respect to both geometry and time.
However, when interpreting seismic sequences, it is important to keep several things in mind. The reflectors on the seismic line are not necessarily a true depiction of lithologic features such as bed thickness, depositional dip, or the scale of sedimentary and tectonic structures. Seismic reflectors are a function of the velocity and time of wave propagation through rocks and sediment at depth and can only be converted to true thicknesses, etc., by constraint with geophysical well log data, usually supplied by industry, which can be used to quantify wave velocity through different geologic materials.

Seismic Stratigraphy: Rules and Definitions
The interpretation of seismic stratigraphy is based on the identification of seismic sequences and on seismic facies analysis.

Seismic Sequences:
A seismic sequence is the main element in the interpretation of seismic stratigraphy. It consists of a succession of reflections that are relatively concordant limited at the base and top by discontinuities shown by the lateral termination of reflectors. A seismic sequence has a chronostratigraphic significance becasue it was deposited during a time interval determined by the ages of the limits at the top and bottom of the sequence. However, these limits, which are often discontinuous, are not necessarily isochrons due to either a depositional hiatus or an erosional unconformity and could represent anywhere from thousands to millions of years.
The identification of seismic sequences is based on the geometry of the termination, at top and bottom, of a group of reflectors, and are interpreted as the lateral termination of strata. An angular unconformity on the seismic sections gives the limits by analyzing the termination of reflectors, e.g. truncation due to erosion, downlap, onlap, etc (Figure 1). The geometry of reflectors within an individual seismic sequence is described using seismic facies analysis.

Seismic Facies Analysis:
Seismic facies analysis consists of the parameters and reflection configuration studies, which determine a seismic sequence (Figure 2). The aim of the facies analysis is to interpret reflections with respect to lithology, stratification, and the characteristics of the depositional environment. These parameters are:
Amplitude (strong or weak)- amplitude explains the contrasts in density and the velocity of acoustic wave propagation through different lithologic material and helps to define the lateral variations within sequences.
Frequency (high to low)- The frequency, distance between reflectors, depends on the differences in distance between strata (bed thickness), as well as lateral variations in the velocity of acoustic waves due to changes in lithology.
Continuity (good to discontinuous)- The continuity of seismic reflectors is generally associated with the lateral extension of uniform strata.
External form and internal configuration of facies units- The form and internal configuration of facies units is generally the first parameter that can be described on a seismic profile. Traditionally certain forms and internal configurations (chaotic, laminated, draped, noisy zone, etc.) are associated with particular depositional environments (platform, gravity slides, filled channels, canyons, reefs, etc.).
In practice, the lithologic interpretation is done with preceding interpretation and correlated with lithologic and or geophysical well log data. It is important to remain cautious and prudent in seismic facies interpretaion without direct correlation with lithologic data.

Sequential Stratigraphy
The goal of sequential stratigraphy is to reconstruct in detail the modality of the deposits and the sedimentary geometry in a chronostratigraphic context. The underlying assumption of this method is that the sedimentary series is organized in a logical geometric succession of isochronous depositional units controlled by relative variations in basin scale sea level (Figure 3).
The sedimentary record in the basin is controlled by: eustacy (absolute variations in global sea level); tectonics over a range of scales (subsidence or uplift of the basin); the sediment flux.
In the sequential stratigraphy model, we consider depositional sequences as a whole, i.e., sedimentary sequence deposited in a complete cycle of relative sea level variation. The sequence itself can be divided into different depositional packages corresponding to different phases of relative sea level variation. For example, a regressive megasequence is a succession of progradational depositional sequences on a continental shelf and slope in the general context of regression.

Conclusions
The ability to carefully and accurately interpret seismic data is an important and useful tool for any geoscientist. However, the utility of this tool can only be maximized when it is carefully applied. Above, we have presented some general guidelines that should be followed in order to insure a robust sequential stratigraphic interpretation through seismic sequence and seismic facies analysis of 2-Dimensional seismic data. One method that we have not discussed but that is important to mention, is the use of a network of high-angle to orthogonal 2-D seismic lines. This type of network is the only way to precisely determine the lateral distribution of seismic, and therefore interpretation of lithologic, sequences and facies, and is a crucial tool in the reconstruction of something like paleogeographic maps. These are extremely useful in helping to determine sedimentary sources and sinks, whether or not sediment was bypassing the basin, and where, for industry, there are potential hydrocarbon source and reservoir rocks.