High frequency cycle and parasequence Interpretation of carbonates- well logs & core
Well logs and cores of carbonates, as with clastics, are used to establish the lithology, porosity and permeability of the rocks penetrated by a well or wells. Data gathered from this medium can be used to predict the geometry and extent of carbonate lithofacies, particularly within oil and gas fields. The process used to identify and predict carbonate lithofacies distribution from this well data requires the identification and mapping of carbonate cycles and/or parasequences from the cores and logs.
It is important that you recognize that the use of well logs alone to determine the lithology and fabric of carbonate sediments and so determine depositional setting and predict facies geometries is not at straightforward as it is with logs for clastic facies interpretation and prediction. The reason that the character of carbonate well logs usually cannot be used for direct lithofacies interpretation is that as carbonates accumulate they tend to become cemented and/or leached. This diagenetic alteration changes the carbonate petrological fabrics and so obscures the log signals that might indicate grain size change (a process that conversely can be achieved in clastic rocks). The solution to the identification of lithofacies and reservoir fabric of carbonates is to calibrate well log character with cores and/or with seismic and outcrops when the latter is available. Together these data sources can establish the lithology, porosity and permeability of the carbonate rocks penetrated by a well or wells. Information gathered this way can then be subjected to sequences stratigraphic analysis, and will lead to the identification and mapping of carbonate cycles and/or parasequences and making it possible to predict the geometry and extent of carbonate lithofacies, particularly within oil and gas fields.Ancient carbonate parasequences - rational for their studyThe exercises in this section are designed to introduce geologists to the creation of carbonate from the logs of wells that penetrate locations across carbonate dominated margins, shelves, and basins. The well-log signals are often repeated vertically (cycles and/or parasequences) but have "unique" local characteristic responses to depositional setting. These cycles and "unique" signals are used to predict the distribution and geometry of the carbonate sedimentary facies.
Using the above rational this set of exercises examines the high frequency sequence stratigraphy of shallow water carbonate sedimentary rocks from the perspective of the analysis of cores and well logs. The exercises are based on the concept that a better understanding of ancient carbonate sedimentary rocks and their depositional systems is helped by initially subdividing the rock section into the synchronous units of high frequency depositional cycles and/or parasequence. The four-dimensional relationship of these units of sedimentary rocks (time, depth and area) can be established by combining the constraints of the relative age of each cycle and/or parasequence to its neighbors, and their associated facies and geometries. The subdivision of shallow water carbonate sections into these units using cores and well logs is explained below.
The subdivision of carbonates into cycles or parasequences is aided by the fact that:
1) The accumulation of shallow water carbonates is particularly dependent on the depth of the sea and so their vertical position across a shallow water shelf.
2) Each cycle or parasequence matches the extent that the sea surface onlapped over the accommodation that is now filled by the sediments of that cycle or parasequence (See simulation of Neuquen basin).
The subdivision of carbonates into cycles or parasequences, just as is done with clastics, provides a "relative" time control on the component units of the section being studied and may also contain the fundaformmental units of a hydrocarbon reservoir (reservoir rock, seal and source rock). cycles and/or parasequences of carbonates are bounded by synchronous surfaces that envelope the layered wedges of sediment. Though these sediments were deposited at different times within the accommodation envelope, in terms of the boundaries of this envelope, these sediments can be considered to have accumulated synchronously. Not surprisingly these sediments vary in character within their time, or "chronostratigraphic", envelope in response to their depositional setting. Using the exercises provided in the sections that follow it is possible to identify these surfaces and see how the effects of changes in sea level (base level) cause both vertical and lateral changes in the facies of the cycle and parasequence.High-frequency "cycle" or parasequenceThe exercises further demonstrate that the high-frequency "cycle" or "parasequence" is the smallest set of genetically related facies deposited during a single base-level cycle. The cycle boundaries mark the turnaround from base-level fall to base-level rise (a period of time during which sea level rises from a highstand position, through a lowstand, and returns to a highstand). cycles can be mapped across multiple facies tracts and include multiple vertical facies successions (VFS) and are therefore chronostratigraphic units (Kerans & Tinker, 1997 and Mitchum & Van Wagoner, 1991). The commonest manifestation of a cycle or parasequence is the shoaling upward cycle, with finer deeper water facies at their base and coarser better-sorted facies towards their top. Less common but equally important is the deepening upward cycle, with coarser shallow water facies at their base and finer less well-sorted facies towards their top.
The next section provides links to exercises working with and interpreting high-frequency "cycles":
Then follows a section that contains the references that may help with the Exercises. For more detailed discussion of high frequency sequence analysis and the Hanifa sequence interpretation on which this exercise is based, please refer to Nassir's Alnaji's thesis sections: