(SEISMIC) SEQUENCE STRATIGRAPHY

The stratigraphic interpretation that uses a chronostratigraphic framework of genetically related stratigraphic surfaces, and the genetically associated stratigraphic units between these surfaces, the so-called systems tracts that were deposited during specific phases of the relative sea level cycle. The bounding surfaces are assumed to represent time lines, such as Sequence Boundaries and Main Flooding Surfaces).

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Source: SEPM STRATA Encyclopedia.

Sequence Stratigraphy is a highly specialized part of petroleum geoscience; therefore OPS OES Thailand will provide industry experts to execute this type of work, which includes the following essential services:

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The essence of Sequence Stratigraphy is mapping of strata based on identification of surfaces, which are assumed to represent time lines (such as sequence boundaries (SB), unconformities and maximum flooding surfaces (MFS); see below), and therefore placing stratigraphy in chronostratigraphic framework. Sequence stratigraphy is a useful alternative to a lithostratigraphic approach, which emphasizes similarity of the lithology of rock units rather than time significance.

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Example of a type 1 sequence; i.e. where a relative fall in sealevel at the shoreline exists. As a result, rivers typically will incise in the shelf with channels filled up with fluvial sands forming petroleum reservoirs.

Sequence stratigraphy has become established as a primary interpretation methodology for petroleum geologists in the last 30 years for the following reasons:

 

  • It helps explain the true geometric relationship of sediment packages, or facies associations to one another; they are physically connected and genetically related. (This is useful for estimating reservoir volume, for example). This is more effective than simple, and potentially misleading, lithostratigraphic correlations, which clearly remain of key important.

  • It is an effective means of combining various data sets (e.g., biostratigraphy, sedimentology, wireline logs, seismic, etc.) within one integrated framework.

  • It is a powerful means of predicting away from known data points. The geometries of sequence stratigraphy predict what facies may be expected up and down systems tracts and laterally. Hence the occurrence of reservoirs, source rocks and seals may be predicted regionally from relatively sparse data sets.

 

Sequence boundaries (SB) are deemed the most significant surfaces. Sequence boundaries are defined as unconformities or their correlative conformities. Sequence boundaries are formed due to sea level fall.         

 

The concept of systems tracts evolved to link the contemporaneous depositional systems. They form subdivisions in a sequence, (see below).

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System tracts and surfaces in the four systems tract model

The Maximum Flooding Surface (MFS) is at its maximum landward position (i.e. the time of maximum transgression).

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  1. The lowstand systems tract (LST) is indicated by green section in the relative sealevel curve. It forms when the rate of sedimentation outpaces the rate of sea level rise during the early stage of the sea level curve. It is bounded by a sub aerial unconformity or its correlative conformity at the base and maximum regressive surface at the top.

  2. The transgressive systems tract (TST) is blue in the sealevel curve. It is bounded by maximum regressive surface at the base and maximum flooding surface at the top. This systems tract forms when the rate of sedimentation is outpaced by the rate of sea level rise in the sea level curves.

  3. The yellow highstand systems tract (HST) occurs during the late stage of base level rise when the rate of sea level rise drops below the sedimentation rate. In this period of sea level highstand is formed. It is bounded by maximum flooding surface at the base and composite surface at the top.

  4. The falling stage systems tract (FSST) in red or regressive systems tract includes all the regressional deposits that accumulated forms in the marine part of the basin after the onset of and during the base level fall and before the start of the next relative sea-level rise. Sub aerial unconformities form in the landward side of the basin at the same time.

 

These events have economic significance because these changes in sea level cause large lateral shifts in the depositional patterns of seafloor sediments. These lateral shifts in deposition create alternating layers of good reservoir quality rock (porous and permeable sands) and poorer-quality mudstones (capable of providing a reservoir "seal" to prevent the leakage of any accumulated hydrocarbons that may have migrated into the sandstones). This concept helps explain the spatial relationship between sediment packages.

 

A comprehensive seismic sequence stratigraphic project will only result from the integration of a multitude of geoscience tools. Proper processing and interpretation of seismic data is required to identify and map seismic facies, which eventually will lead to the mapping of the environment of deposition (EOD) from seismic data. Sedimentological core descriptions on wells, if available, will provide a real life calibration. Other well data that provide calibration points in the EOD mapping are the biostrat interpretation of specific stratigraphic levels in wells. These reports usually give information on the water depth, and are, as such, very important for the sequence stratigrapher. In case of a lack of seismic data, EOD mapping from well biostrat information, could be sufficient, but will have a much larger degree of uncertainty. As such, it may be sufficient for a regional understanding, but is not good enough at the prospect level. Any sequence stratigraphic correlation needs to be calibrated by a proper well correlation. If available, borehole imaging interpretation (BHI) is also a powerful tool utilized by our experts to calibrate the EOD mapping.

 

Sequence Stratigraphy is a highly specialized part of petroleum geoscience;

OPS OES Thailand aims to provide true industry experts. Due to their seniority, one of their main responsibilities is to closely interact with and train Client’s permanent G&G staff on the job.