Stratigraphy is a complex subject, dealing as it does with the occurrence of rocks in the earth’s crust and the problems associated with identifying both the characteristics and relationships of the various rock units encountered in the field. Thus, to develop a model to handle the many aspects of stratigraphy it is necessary to have an understanding as to its nature as described in this document.
The components that comprise the model are designed to accommodate the internationally accepted means of classifying and categorizing the stratigraphic rock units, the identification of these units as to how and where they occur, and the means of correlating their occurrence.
The data model allows the user to query the manifold attributes of stratigraphy as captured by the model. In addition the data can be used to support mapping and other geological modeling applications, as well as to supply stratigraphic interval information as a means of extracting other related data within the overall PPDM model.
How well the model meets the business needs of the user will be a critical measure of its success.
Business Process Overview
Data models provide a means of capturing and storing data so that it can be accessed for analysis, interpretation, reporting and manipulation. Good data models must manage data in a manner than takes into account the very nature of the data it has to capture. Consequently, an understanding of the elemental essence and make up of the subject is paramount. This is especially true of stratigraphic data, which has many attributes, is highly interpretive, and subject to qualification.
Secondly the model has to allow data to be accessed and queried as to its attributes, including presence, quality, source, etc., to satisfy the needs of the user.
Finally the model has to accommodate functions that relate to the actual use of the data as input material for analytical, manipulative, and modeling processes.
Stratigraphy is the science dealing with the description of all rock bodies forming the Earth's crust, sedimentary, igneous, and metamorphic and their organization into distinctive, useful, mappable units based on their inherent properties or attributes. Stratigraphic procedures include the description, classification, naming, and correlation of these units for the purpose of establishing their relationship in space and their succession in time.
In a number of respects stratigraphy is to the subsurface what geography is to the surface. It provides the geoscientist with a global, as well as local, means of reference and the necessary conventions for categorizing and identifying, the rocks that constitute the earth crust, interpreting their physical presence in the field, and correlating their occurrence.
Stratigraphy recognizes the uniqueness of the actual occurrence of rocks in a region that typically constitute the local “geologic record”, and which in turn represent the physical expression of its geologic history. This history may not only include deposition and intrusion, but also indicates the absence of strata by erosion or non-deposition, and the multiple occurrence of units, or the inversion of their normal sequence of occurrence as a result of tectonic events. The geologic record thus identified may represent the actual interpretation of the strata found in a wellbore, or at a surface exposure at a field station. It may indeed represent a conceptualized version of the stratigraphic section for a region used as a reference and displayed graphically as a “stratigraphic column”.
The model has been designed to capture all the above significant attributes and features of stratigraphic events, and to allow the model to be queried to answer questions related to the nature, occurrence, and capture of the data. However a major use is to map the structure and thickness of stratigraphic units in a defined area and to identify stratigraphic intervals of interest against which other down hole data types can be cross-referenced, queried, and extracted.
In essence, the data model has been constructed to accommodate all the important aspects of stratigraphic use and to meet the business needs identified in the business requirements document.
Significantly, rocks and the strata in which they occur have different properties. Therefore different categorization procedures have been adopted to match these properties. However, the changes in one set of properties may not match the changes in another set. Consequently, the boundaries between units will not necessarily coincide with that of a different set and may in fact transgress and cut across each other. Given this, it is not possible to assign these varying differences in properties to a single category. The various categories and their reference to the geological time scale are given below.
Inasmuch as space is the primary domain of the geographer, or more precisely the cartographer, for whom the latitude and longitude grid provides the spatial reference, the stratigrapher operates in an additional domain of geologic time or geochronology.
The geochronological time scale has been subdivided into units of geologic time that serve as a frame of reference for relating the age of occurrence of all rock strata in the earth’s history. Thus the geochronological time scale provides a unifying basis of reference for all categories of stratigraphy.
Note however, geochronological units are not stratigraphic units, as they represent time, a property that can neither be seen nor touched. The tangible, physical stratigraphic units that correspond to the time units are defined below as chronostratigraphy.
Chronostratigraphy is that component of stratigraphy that deals with the organization of strata into units based on their relative age of occurrence and duration of formation. Consequently, according to an internationally adopted convention, the sequence of rocks, dating from the very oldest to the most recent have been grouped by age relative to the geochronological time scale and identified accordingly.
In other words, rocks laid down during the time that correspond to the sequence and hierarchy of geochronological units are grouped into corresponding chronostratigraphic units.
Lithostratigraphy is that element of stratigraphy that applies to the actual lithological characteristics of strata; it identifies, groups, subdivides and defines associations and parent/child relationships accordingly.
Lithostratigraphic units are recognized on the basis of lithologic characteristics only. Consequently, while certain lithostratigraphic units may physically comprise a chronostratigraphic sequence or system or rocks, boundaries between units may be transgressive and overlap chronostratigraphic boundaries. Thus while it may be convenient to sequence stratigraphic units according to age, more realistically it may be more appropriate and accurate to assign a simple sequence number to designate the relative sequential association. Both these options are available within the data model.
Biostratigraphy is that aspect of stratigraphy that organizes strata into units based on their fossil content, on specific fossil assemblages, or the range of individual or collective fossil occurrence, etc.
The same comments apply to biostratigraphic units as to lithostratigraphic units re relationships and sequencing.
Sequence stratigraphy consists of the identification and correlation of unconformities and conformable surfaces that have low diachroneity. The main sequence stratigraphic units are a sequence and its two component system tracts, the transgressive system tract and the regressive system tract.
A sequence is a stratigraphic unit bounded by subaerial unconformities, or ravinements, which have eroded through the subaerial unconformities, and by correlative conformable transgressive surfaces. A maximum flooding surface within a sequence allows the sequence to be subdivided into a transgressive systems tract below, and a regressive systems tract above.
(After Ashton Embry, PhD. Geological Survey of Canada.)
There are other categories of stratigraphic classification identified in the International Stratigraphic Guide published by the IUGS and the GSA. These can be accommodated, provided the modeling rules are observed.
Identification covers nomenclature; that is, the naming of units, their membership of various suites (name sets) assembled by different sources (vendors, oil companies, etc.), lexicon references, and aliases. Particular stratigraphic information covers the stratigraphic position and range of occurrence, “equivalencies” to other units, hierarchical classification, topological juxtaposition with other units, association to specific regions, and constituent units of conceptualized stratigraphic columns. The model provides for the capture of this data to meet the client needs.
This covers the observation as to the actual physical occurrence (or absence) of a stratigraphic unit and the point or depth at which it was encountered and whether or not the unit was located in normal stratigraphic sequence, and if not, why not.
The model meets the need to store the stratigraphic interpretations according to the preferences of the user, and the interpretations can be assessed as to the degree of certainty or the quality of the interpretation, method of interpretation, sources and versions.
Correlation is a process of identifying the corresponding presence of analogous stratigraphic units within (usually) nearby wells and/or field sections. These correlations are therefore applied to actual specific physical occurrences of these units and are based on identity, or similarities of character or stratigraphic position.
However, correlation is used in a general sense when it is applied to “correlation charts”. These correlation charts contain idealized stratigraphic columns representing the theoretical sequence of units in a given basin. Placing these columns together in juxtaposition relative to a geochronological time scale implies certain correlations between the units in the different basins.
The four types of correlation accepted by the model are as follows:
Between Wells/Field sections
The most common form of correlation is done between well or field sections. The process identifies formation tops on a given well or field section, with respect to criteria supplied by a reference well or field section. The current table design recognizes and allows for this.
Inter- and Intra-Basin (sub-basin)
The use of the term in this instance refers to identifying specific stratigraphic units existing in different basins, or in different parts of the same basin (e.g., sub-basins). Here an explicit correlation may be warranted to indicate some definite similarity in stratigraphic position (not necessarily age), which goes beyond simply expressing “stratigraphic equivalence”. This is the sort of correlation shown on a stratigraphic (inter-basin) correlation chart (where correlations are implied but not specifically stated).
Specific Localized Zone
In this instance, a particular zone having some generic facies property (such as porosity) and occurring throughout a specific depth interval within a well section, is deemed to correlate with a similar interval within another well. One problem, however, is that a zone in one well may split into two or more zones in another well.
This relationship indicates a correlation between a unit of one stratigraphic type and a unit of another type, where the occurrence of one type is associated with the occurrence of the other. In certain circumstances this could be unidirectional, and this would have to be indicated. An example would be where a certain faunal zone is always associated with and occurs throughout a lithostratigraphic zone (but the occurrence of the lithostratigraphic zone does not necessarily indicate the presence of the faunal zone). Another example might represent the case where the presence of certain fauna gives rise to, and indicates the presence of a porous zone.
Stratigraphic Information Processes
The creation and use of stratigraphic information involves the selection or generation of a stratigraphic model and the application of this model to specific occurrences or interpretations. A stratigraphic model describes the relationships and characteristics of stratigraphic units in the area of interest, and is a conceptual version of the actual geologic record. The graphical expression of the model is referred to as a “stratigraphic column”. The relationship of the model to other stratigraphic models (or columns) can be represented by arranging all the columns alongside each other into a stratigraphic correlation chart. A chart of this type provides a frame of reference from which to apply interpretations to actual occurrences of stratigraphic units in wellbores or field sections within and between adjacent geologic regions (basins).
Building a Frame of Reference
The correlation chart places the stratigraphic columns for the area(s) of interest in juxtaposition within its own frame of reference, which consists of a geological time scale defined and subdivided by a recognized, and generally accepted, global chronostratigraphic convention. The stratigraphic columns can represent models created by different authors, or expected occurrences in different areas.
Construction of a correlation chart as a framework for stratigraphic interpretation is a complex process requiring detailed stratigraphic knowledge and analysis. The stratigrapher’s task is to assimilate a globally or regionally known stratigraphy into the uncertain stratigraphy of the area of interest. This process evolves stratigraphic understanding from an initial vague or indefinite preliminary perception towards a fuller awareness, and may involve a number of stratigraphic scenarios (different stratigraphic models) in the process. As stratigraphic knowledge evolves, stratigraphers create stratigraphic columns that tie the new knowledge into the framework, thus linking past knowledge to a new understanding.
The initial stage of populating the stratigraphy data model is to describe the reference framework of generally accepted stratigraphic nomenclature, characteristics, and relationships. This requires:
loading information on all the data types identified in Figure 2 that refer to the various stratigraphic categories.
loading information on the stratigraphic units that the user is expected to encounter, such as stratigraphic name sets, stratigraphic relationships (ages, sequencing, aliases, etc.), and stratigraphic columns.
Having built the framework, the user can now add data representing the interpretations as to the actual physical presence of the various stratigraphic units to the requisite tables (see Figure 2). That is, the various entities used by the geologist to describe and interpret the occurrence of stratigraphic units in the field or in the subsurface, and their position in the actual geologic record, can then be captured.
Usage of Stratigraphic Information
When the frame of reference has been built and the initial interpretations loaded, the user is in a position to perform tasks typical of manipulating and managing stratigraphic information; namely:
Queries may be related to specific attributes of the data itself, such as its origin, quality, source, etc., or more likely it will relate to questions as to its occurrence and association in a geological or stratigraphic sense. In this latter form, queries would identify wells penetrating a certain stratigraphic unit or horizon, or identify well or field sections where certain stratigraphic units occurred in conjunction with one another. The user may wish to query what units are likely to be encountered in specific regions or named areas, or examine the correlative nature between certain units. The results of a query may be a number, a statement, a list, a file for export, or a map.
Modify & Edit
The interpretative nature of stratigraphic information demands that revisions will take place from time to time. As such, the data will be subject to errors of commission as well as omission, resulting in inconsistencies with respect to sequence and actuality of occurrence.
This refers to extensions beyond the query stage that result in some manipulation being performed on the data, or a further selection process than that implied by the query stage. An example might include the creation of a mappable data set that represents the stratigraphic equivalent of a given horizon or the first occurring horizon subcropping at an erosional surface. Another could include the definition of a stratigraphic interval for which an isopach thickness is derived and/or against which a search is made for other down hole data (e.g., formation test, cores, etc.) occurring wholly or partially within that interval.
Sources of Stratigraphic Formation Lexicons
Many lexicons have been developed by various organisations. In recent years, some attempts have been launched that target better integration and correlation of these lists. Some useful links follow. More should be added by the reader as sources are published.
- http://www.onegeology.org/ OneGeology is an international initiative of the geological surveys of the world. This ground-breaking project was launched in 2007 and contributed to the 'International Year of Planet Earth', becoming one of their flagship projects.
- http://cgkn.net/weblex/weblex_e.pl This site provides information about over 16,000 published geological names used in Canada. The names are from several sources, now stored in a single database. The names are of two types: lithological units and chronological units. The lexicon is a "living" database; some names remain to be added, others are incompletely described, and many descriptions require updating.
- http://data.gns.cri.nz/stratlex/ Precise definitions of units of rock and periods of geological time are required, especially for mapping and study of geological history. The International Stratigraphic Guide (Hedberg 1976; revised Salvador 1994) was adopted for New Zealand use by the Geological Society of New Zealand soon after its initial formulation (Hornibrook et al. 1965), and its principles are used in the maps and publications of GNS.
- http://ngmdb.usgs.gov/Geolex/geolex_home.html The Geolex database contains 16,072 entries. 75% of the unit names from the USGS Geologic Names Committee (GNC) card catalog have been entered in the database. Several thousand unit names remain to be checked and entered.
- http://www.nhm.uio.no/norlex/ Norwegian Interactive Offshore Stratigraphic Lexicon