11.1 Introduction

Sequence stratigraphy is the study of sedimentary rock relationships within a chronostratigraphic or geologic-time framework. Its basis is identification of stratal surfaces, regional unconformities and their correlative conformities, and relationships among lithofacies and depositional environments, within this chronostratigraphic framework. Sequence stratigraphy differs fundamentally from lithostratigraphy. In sequence stratigraphy, stratal surfaces and rock bodies between the surfaces are defined on the basis of stratigraphic intervals that are time-synchronous, and laterally continuous and regionally correlative (e.g., bentonites or condensed sections), rather than on the basis of the lithologic character of the rocks and their stratigraphic relations (Fig. 11.1). Thus, chronostratigraphic horizons often crosscut lithostratigraphic horizons (Figs. 11.1 and 11.2), and well-log correlations differ substantially between these disciplines (Figs. 11.1 and 11.3). Before you initiate a well-log correlation project, it is wise to list the data available to you and to develop a correlation strategy, such as choosing lithostratigraphic versus chronostratigraphic correlation (Mulholland, 1994). Chronostratigraphic correlations are the recommended strategy.

Figure 11.1: Comparison of a lithostratigraphic and a chronostratigraphic correlation and stratigraphic interpretation of two pseudo-wells. After Bashore et al. (1994). (Reprinted with permission of AAPG, whose permission is required for further use.)

Figure 11.2: Comparison of a lithostratigraphic and a chronostratigraphic correlation for a series of progradational depositional complexes. Hiatal surfaces represent time intervals with no significant deposition. After Frasier (1974). (Reprinted with permission of AAPG, whose permission is required for further use.)

Figure 11.3: Comparison of well log cross-sections correlated on the basis of (A) chronostratigraphy and (B) lithostratigraphy.