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Sequence Stratigraphy

Competency Statement:

In recent years, a better understanding of the geologic and sedimentary processes associated with sequence stratigraphy revived interest in its application to basin analysis. Sequence stratigraphy provides a means of interpreting the depositional history of a basin in terms of base level movements driven by eustatic sea level changes and by tectonic activity. These allogenic (external) drivers of base level movement interact with sediment supply to determine the creation (BL rise) or destruction (BL fall) of accommodation space for sediments in a basin.

Given that base level is continually changing, the key point for sequence stratigraphy is that this results in a number of distinct sedimentary surfaces that reflect depositional breaks and/or changes of depositional trend which bound stratigraphic sequences.

These surfaces include Material Based surfaces such as a Subaerial Unconformity (SU), a Regressive Surface of Marine Erosion (RSME), a Shoreline Ravinement (SR), a Maximum Regressive Surface (MRS), a Maximum Flooding Surface (MF), or a Slope Online Surface (SOS) and Time based surfaces termed the Basal Surface of Forced Regression (BSFR) and the Correlative Conformity (CC). Recognition and use of these surfaces in sequence stratigraphy are explored.

Upon completion of this module the learner will be able to place a stratigraphic succession into a chronostratigraphic framework to determine depositional environments and related facies by analyzing well logs, core samples, outcrops, seismic lines and other information. You will use a predictive model to evaluate the basin-wide distribution of petroleum source, reservoir and cap rocks.

Learning Objective:

  • Recognize and explain the principle material based surfaces used in sequence stratigraphy (e.g., SU, RSME, SR, MRS, MFS, and SOS) and the chronostratigraphic surfaces (BSFR and CC).
  • Construct stratigraphic cross-sections based on sequence stratigraphy correlation methods.
  • Interpret the stratigraphic evolution of a basin.
  • Predict petroleum reservoir occurrences utilizing the relationships between accommodation space, sediment supply, stacking patterns and unconformities in a sequence stratigraphic framework.

Assignment Instruction:

As a member of the Exploration team for Beta Petroleum Co., you have been selected for a sequence stratigraphy study in the Eastern Venezuelan Basin, one of the largest oil producing basins of the world. As part of your responsibilities, you will have to provide information on the continuity of reservoirs and regional seals that might be useful for the rest of your team.

The section comprises a fluvial deltaic system deposited during the Early Miocene (18.5 Ma) and Middle Miocene (13.8 Ma). The units deposited since the Early Miocene unconformably overlie Cretaceous shales with high organic matter content, which constitutes most of the source rock which for the hydrocarbons of the basin.

The geological interpretation of well logs was integrated with the biostratigraphic and sedimentological information obtained from core and sidewall samples. This integration allowed for the sedimentary section to be subdivided into geometrical packages bounded by unconformities. Based in this interpretation, third order sequences were identified.

To carry out this exercise GR and resistivity logs of 5 wells were used, as well as information from two cores taken from Wells 1 and 4.

The figure below shows the location of the wells studied.

Wells location map.

Well location map