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GIS Below the Earth's Surface
Where traditional GIS concentrates on the surface of the Earth, a Geodigital Framework allows the petroleum geologist to analyze what lies under the Earth's surface.
By John R. Forster and John C. Horne Introduction
Traditional Geographic Information Systems relate diverse phenomena that occur on the Earth's surface. These phenomena acquired from airborne, satellite photography, radar, surface facilities such as telephone poles, buildings and other resources and infrastructure, are incorporated into the GIS mapping and analysis process. The goal of the GIS analysis is to represent the spatial relationships of the natural and man made features with the intent of providing an answer to a spatial and temporal problem. The oil and gas industry extends this technology from the surface of the Earth into the subsurface, thus, back millions of years in time. Integrating numerous tools and methodologies, the essence of oil and gas exploration is to describe the topology and environmental attributes of the Earth's surface at some time in the past. The exploration geologist reconstructs how these GIS attributes or phenomena relate to the distribution of the hydrocarbon reservoirs. Integration of a reconstructed Earth surface, at some time in the past, with the present day structural configuration will normally define the location of oil and gas accumulations. Successful integration of data relating to the Earth's surface millions of years in the past requires construction of a comprehensive stratigraphic framework to restrict the spatial analysis to a unique time or stratigraphic unit. In digital form, the stratigraphic framework becomes a Geodigital Framework or GDF and software can integrate data both spatially and temporally. It is through this restriction of the stratigraphic interval that successful GIS type analyses of the Earth's history are accomplished.
With the introduction of computers in nearly every phase of the petroleum industry, the stratigraphic framework defined by formation tops is a required digital data set of nearly every software program, from economics to hydrocarbon generation and migration (basin modeling) and subsurface mapping to reservoir simulation. This is apparent to any geologist, geophysicist, or engineer using new software applications. The first major task, upon installation of new software, is to gather a data set including a set of formation picks for the software to manipulate. From seismic modeling and interpretation software to hydrocarbon generation and migration packages, formation picks are a required data set for the application. To successfully utilize the software, the digital stratigraphic formation tops must be consistent.
A Geodigital Framework (GDF) is the compilation of consistent stratigraphic picks in a digital format for various software applications. The Geodigital Framework assures that every application of computer technology consistently employs the correct stratigraphic intervals in the processing sequence. Correct application of the GDF essentially integrates all subsurface data into a three-dimensional, stratigraphically-controlled, matrix or framework. The GDF is not a software application nor is it a database management system. It is simply a digital file of the primary geological framework used to integrate diverse industry data sets. Definition of a GDF
A Geodigital Framework is a digital working model of internally consistent and complete stratigraphic boundaries in the subsurface. This model can be constructed from surface outcrop geology, subsurface wells, seismic surveys, paleontology, and other related data. In many areas of the world, seismic is the dominant media for subsurface definition, and seismic data are recorded in time. With the exception of seismic, the other data sets are recorded as elevations in feet or meters. Unless the associated data are to be converted to time to match the seismic data, the GDF should be integrated as a series of depth converted data sets.
The GDF can vary in size from a single pay zone in one field, to the entire stratigraphic column across a basin or series of basins. The vertical detail or resolution of a GDF can vary from a coarse sequence stratigraphic model down to detailed lithologic subdivisions of flow units in a reservoir. The internal consistency and completeness differentiate a true geodigital framework from traditionally reported formation tops and seismic time picks. Every pick or horizon must be described across the entire region, if only to note that an interval was never deposited in portions of the area. For a Geodigital Framework to be considered useful and valid in multiple software applications, a few simple rules must be strictly employed. The rules of a GDF are:
1. All wellbores, seismic surveys, and other incorporated data sets must be uniquely identified and correctly located on and below the Earth's surface.
2. The GDF must be consistently picked to a valid stratigraphic model, and every horizon within the model must be described at every well and/or shot point.
3. The Geodigital Framework should normally be generated in depth, not time or velocity, to offer easy integration with other subsurface data sets.
Within these rules, the stratigraphic horizons comprising a GDF can be derived from any or all available data such as well logs, cores, outcrops, paleontologic data, and seismic data. Requirements to Build a GDF
The data required to construct a Geodigital Framework is generally as simple as a set of well logs. Regional cross section grids assure that complete, consistent, stratigraphic picks can be made. The constructed stratigraphic framework explains the observed vertical sequences and establishes definitions where tops are not correlated or are absent. Stratigraphic complexities may require the utilization of cores, paleontology, seismic surveys, and other data to refine the stratigraphic model. Similarly, where seismic data are used in the construction of a GDF, looping and tying of seismic lines must occur to develop a seismic-stratigraphic model. In addition, velocity data are needed to construct velocity gradient maps for the conversion of seismic time values to depth values for integration with wellbore data sets.
While the data requirements to construct a Geodigital Framework are fairly simple, the data must strictly adhere to the rules of a GDF. Prior to the utilization of computers, traditional exploration methods required extensive hours of work by drafting departments to update and maintain basemaps of well locations and seismic surveys. The location on a map, at times with a well name or line name, described the unique identification of individual wells and/or seismic lines. These basemaps were the backbone of nearly every oil company and generally regarded as confidential information. Most of the major oil companies considered this information so valuable that the maps were locked in a vault nightly to protect them from theft or damage.
Today, computerization has changed this approach. Digital databases have replaced these basemaps and often are considered valuable confidential information by oil companies. The unique identification of a wellbore and/or seismic survey now requires a much more sophisticated method of identification.
Commonly, wellbores are uniquely categorized with an identification number such as an "API Number." Theoretically, this number is unique. Seismic lines and shot points are commonly classified with a cryptic identifier of numbers and letters. Every control point or surface location must also include an X and Y Coordinate (commonly Latitude and Longitude) for location on the surface of the Earth. Requirements of other data sets to be integrated
A properly constructed Geodigital Framework assures the digital integrity and geological reliability of all stratigraphic surfaces. Similarly, other subsurface data should be comprehensively compiled. Primarily, wellbore and seismic survey identifiers must relate to the constructed GDF. In addition, every attribute or associated subsurface data element must be unique.
The petroleum industry has developed numerous data models to assure the uniqueness of data elements as well as establish a relationship between various data. Some of these data models are well known such as The Mercury Data Model, The Petrotechnical Open Software Corporation Data Model (POSC), and The Public Petroleum Data Model (PPDM). Numerous other data models exist and are utilized daily by the petroleum industry. Individual oil companies such as Exxon, Amoco, and Aramco have adapted internal data models, often within single divisions of a company. In fact, nearly every software application which stores data has an internal data model to establish relationships between data sets. While a data model is extremely important in the storage and manipulation of data, a single data model is not a requirement of a Geodigital Framework. Geodigital Frameworks can be compiled on any software from a simple spreadsheet program to a relational database manager. Obviously, a comprehensive data model implemented in a relational database management system is preferred from an operational standpoint. The construction of geodigital framework databases should not be delayed by the absence of such a data model or database management system. Most software applications provide functionality to import data such as ASCII flat files. Utility of a GDF
The utility of a well constructed Geodigital Framework is in the application of a consistent geological model to the integration of all subsurface data types. All wellbore data are defined in relationship to a depth within the wellbore. Data reported by depth include drill stem tests, cores, perforations, stimulations, plugging methods, casing programs, mud programs, drill bit programs, and many others. Subdividing these disparate data sources with the depth-related stratigraphy from the Geodigital Framework permits the integration of the data for analytical and operational purposes. There is a general perception that most oil and gas entities already have a Geodigital Framework. Formation tops acquired over years of activity in a given area are generally considered to be sufficiently accurate data. This is a common mistake which ultimately leads to erroneous results. The most obvious distinction between a set of formation tops developed over a period of years and a Geodigital Framework is the consistency and completeness of the file. Traditional stratigraphic tops files may have a few of the formation picks well populated. However, the consistency of individual picks in every well never occurs.
Figure 3 shows three typical reported tops files in the same field of the Powder River Basin of Wyoming. While the operator for all three wells is the same, and the same geologist picked the tops, not all the same tops are reported. This example of inconsistent reporting becomes a problem if the zone of interest is the Spearfish Formation. It is only reported in Wells #1 and #2. The question facing a mapper using mapping software is whether the Spearfish was present in Well #3. Should the thickness of this formation be zero in that well? A Geodigital Framework provides an explanation to the mapping software in the alpha qualifier field. In this case, the formation is present and would have been picked in the Geodigital Framework. If the formation had been absent due to non-deposition, the qualifier field would be populated with "NPN" for not picked, not deposited.
The utilization of good alpha qualifiers for explanations as to why a formation pick is not made is one of the strengths of a GDF. Often formations cannot be picked in a given wellbore, and alpha qualifier explanations could include: omitted by faulting (NPF), never deposited (NPN), removed by erosion (NPE), and log not available (NL), or poor quality (ND). Mapping software can interpret these qualifier explanations resulting in the ability to map such things as a zero edge of a formation isopach. In the integration of other subsurface data, the qualifier provides logic in the assignment of formation names. The qualifier also removes questions by the user about the status of the formation tops. If a formation top is not present in a well, the qualifier explains the reason why it is not present. Generally, most organizations and individuals perceive regional Geodigital Frameworks to be nearly impossible to generate. For most geologists, it is overwhelming to consider the correlation of numerous formation tops for tens of thousands of wells. However, RPI International Inc. successfully completed the correlation of 32,000 wells with 64 formation picks in the Powder River Basin of Wyoming. This Geodigital Framework study took 18 months with 43 geologists and geologic technicians to complete. It is currently the finest Geodigital Framework in North America. Conclusions
Stratigraphy has always been the primary method of communication in the subsurface. It is through the stratigraphy that GIS type analyses are utilized to define hydrocarbon accumulations. Geochemists communicate with engineers about the geochemical make up of oils by formation name. Geologists convey anticipated pay zones and potential problem areas in a proposed well to the drilling engineers by formation name. The geology of oil and gas requires that stratigraphic boundaries define the rock sequences in the subsurface containing hydrocarbons. The introduction of computer technologies in the industry will not alter this basic method of communication. In fact, computers will enhance the utilization of formation names as the tool of subsurface communication and integration. Stratigraphy is the tool utilized in the petroleum industry to "strip off" the overlying rocks and analyze surface of the Earth millions of years ago with conventional GIS methodologies. The transformation of the industry into the age of computers requires that formation tops become digitally accessible. Digital tops files define the stratigraphic framework in the subsurface. The requirements of this Geodigital Framework is that it be a valid digital file that can be accepted by various software applications. Most importantly, the stratigraphy must be acceptable to the multitude of specialists that will employ these digital files.
The operational benefits to Geodigital Framework construction come from the ease of data integration. All facets of the industry will utilize the GDF for subsurface definition of various entities. This results in higher quality and reliability of products being produced by software applications. This will ultimately transform into lower costs of finding and producing oil and gas. In addition, the cost of conducting additional studies is reduced. For example, up to 50 percent of the cost in constructing a simulation model is in the geological characterization phase of the project. Prior construction of a GDF can greatly reduce the cost of this phase of the project and also decrease the time required to complete the simulation model. Additionally, the GDF assures that all geologic characterizations for simulation models, geochemical studies, petrophysical studies, and many more, can be integrated easily in the subsurface.
Ultimately, with the addition of new software applications, all companies and government entities that utilize this software, will need to construct a geodigital framework. Those entities that construct the Geodigital Framework early will have a tremendous advantage over those that fail to recognize the need of such a geologic model. About the Authors:
John Forster is an exploration/exploitation geologist with over 18 years of experience in the petroleum industry. This includes more than 10 years as a manager of exploration programs in the Rocky Mountains Region for Dekalb and Texaco. More recently, he has over five years experience as a consulting geologist on projects in North America and South America. He may be reached at 303-771-9640. John Horne is a sedimentologist/stratigrapher who has more than 30 years technical and management experience in the petroleum industry. This includes 19 years of energy consulting in Rocky Mountain and Gulf Coast basins. He has also participated in hydrocarbon exploration and exploitation projects in North America, South America, Europe, and Africa. He may be reached at 303-771-9640. The authors wish to express their appreciation to Geoquest - Reservoir Technologies and RPI International for their expertise and contributions.
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