Case made for regional geology in hydrocarbon exploration

Henry W. DeJong
Consulting Geologist
Littleton, Colo.
Sunit K. Addy
CGG Petrosystems
Houston

The goal of a regional geologic evaluation is to determine those areas of the earth that offer the greatest economic opportunities for drilling successful wells in the most prospective area as defined by that study.

Without this step the explorationist may find himself searching for drillable prospects in a portion of a basin that has relatively low merit. Regional studies, usually done on a basinwide scale, or for contiguous multi-basins, show us what to look for and should include recommendations for the exploration methods and the sequence to follow in prospect hunting.

What and why

A regional study is a predictive tool, and one cannot properly evaluate a prospect without knowing its position in relation to the structure and stratigraphy of a basin together with its relationship to any shows or production.

The principal differences between looking for prospects and the study of regional geological parameters that make up prospective basins are shown in Fig. 1 [46,064 bytes].

On the other hand, the search for drillable prospects requires a detailed examination and synthesis of all geological and geophysical data in relatively small areas with success through the drilling stage more assured if the prospect is located in the more favorable areas of the basin. The difference is largely one of scale since basin analysis uses the broad-brush approach; prospect-finding is an intensely detailed analysis.

Regional studies are usually out of favor for the reasons shown in Fig. 2 [36,464 bytes]. The fallacy in those lines of thinking is that one good prospect developed in a favorable area possesses much lower risk than five located in an area of poor lithologies, antagonistic structures, and an adverse geological history. Low risk improves the chance of success and a favorable return on investment.

In addition, regional studies enable explorers to look at both the positive and negative aspects of large areas. This in turn permits establishment of priorities for evaluation. Regional work can be used to condemn areas (although no area can ever be completely condemned) as well as to explain productive trends. This requires knowledge of geologic history. Some regional problems worrisome to exploration activities will be highlighted.

Other minerals of interest occasionally make an appearance in a regional study: carbon dioxide (in the Colorado Front Range), coal (in the Powder River basin), helium (in east-central Arizona), and trona (in Southwest Wyoming). To these may be added oil shale in the Piceance basin of Northwest Colorado and interesting shows of carbon dioxide in the Mist area of Northwest Oregon and in Trans-Pecos Texas.

A bank of regional studies in one's files can give some insight into a discovery of oil or gas by others in a remote area presently not of interest to you. The discovery usually serves as a trigger for the frantic acquisition of an acreage position since as the play spreads any tract available appears desirable. This results in a haphazard assemblage of acreage that may or may not have any exploratory merit. Anyone who possesses some kind of evaluation of these areas in files is in a better position to obtain worthwhile acreage representation in the most favorable areas on short notice.

Preliminary to beginning a regional evaluation, current knowledge of all the area within one's jurisdiction will help the explorer decide those areas worthy of concentrated study.

Regional studies assist us in defining what we should look for and what to expect in any given area; it is a form of reconnaissance that outlines reservoirs, seals, timely migration, source, and the presence of trapping mechanisms. Regional thinking is primarily concerned with facies, environments, and the oil occurrences that are a function of both. It is an exercise in forecasting and hence becomes a management and budgeting tool.

Predicatability comes with understanding, and understanding the exploration environment enables us to predict the location of the most favorable prospective areas. That is, our knowledge and understanding of regional relationships permit us to selectively pursue economic opportunities through comprehensive knowledge by defining basin architecture, depositional patterns, and tectonic history. The geological regions, basins, trends, and plays that offer these opportunities ultimately put the exploration dollar to its best use in budgeting time, money, and personnel. Someone who knows where to concentrate the search for drillable prospects will need fewer technical people to reach that goal.

The reasons for embarking on regional work are many and varied (Fig. 3 [94,869 bytes]).

When to go regional

The timing for a regional evaluation varies from the first bit of interest in a basin to the need to understand what you have been working with for some time. That is, what is the goal and why are you making the evaluation? Is it a matter of money, time, or talent? Is there an urgent need, or is it to satisfy a long term goal?

Know the territory

Certain factors, which can only be determined from regional studies, are necessary to outline favorable prospective areas and to show where the concentration of effort will be most fruitful (Fig. 4 [71,254 bytes]).

More specifically, we should map and compare any parameter we believe may assist in our knowledge of units in the basin. How do we go about this? These factors (Fig. 5 [229,636 bytes]), not shown in any particular order, will help establish a work program. The game plan is to select work pertinent to the area to be evaluated, organize the sequence of investigation, and begin.

How one study worked

The computer is a powerful tool in both regional and local studies.

One regional study involved Trans-Pecos Texas (OGJ, Jan. 20, 1992, p. 59, and OGJ, Jan. 27, 1992, p. 97). The study involved work performed in 1983-84 but not previously published.

The data were fed into a computer, which permitted the authors to watch the develoment of each mapped horizon with time beginning with the lowermost portion of the section. Color enhanced the presentation. The maps and cross-sections fed into the computer by horizons developed a growing picture of sedimentation and structure.

We should point out that in any regional project, any existing maps, whether in-house or published, could be digitized and used in the analysis. PetroSystems of Houston performed the Trans-Pecos Texas computer work, which will be presented elsewhere.

Although our findings were generally negative we did not fully condemn the area. We did suggest that sizable accumulations are probably present. To the present time, these findings appear valid.

No exploration enjoys reaching negative conclusions about an area, but dollars not spent in poorly-prospective areas are dollars available for exploration in worthwhile areas.

Source beds are a problem. Is the unit still a source? Was it a source but the hydrocarbons have migrated? It may no longer look like a source rock. Lack of a readily determined source rock is not sufficient reason to condemn a horizon. The bulk of the world's oil is related to unconformities if you accept the concept that even minor unconformities assist greatly in the accumulation of hydrocarbons. Unconformities serve as the conduit along which fluids or gases move from a source area until a trap prevents further migration.

Unconformities can also serve as a trapping mechanism as well as a seal. Gentle unconformities offer the greatest opportunity for the occurrence of major trapping mechanisms and therefore major accumulations.

While given near the end of the article, more and more emphasis is and will be placed on the use and interpretation of geophysical data in the future, primarily seismic. Processing is all-important. Today we have techniques involving computers that have removed much of the guesswork formerly present in not only prospect-size areas gbut in the regional aspects as well. Geophysical data are vitally important today. For example, prediction of porosity and its lateral continuity are major exploration questions, and seismic facies maps provide information on fracture intensity and localized porosity development.

Three dimensional seismic data provide the explorationist with a high resolution picture of the subsurface and make it possible to delineate large and subtle variations in geometrical and physical properties at considerable depths. The finding and identification of flood plains, intricate channel systems, stratigraphic plays based on point bars, channel fill, crevasse splays, and fan delta systems are possible. Four dimensional seismic, or 3D time-lapse monitoring, brings in the time factor and permits observation of the movement of fluids during the development of a field and gives geologists and engineers additional control for exploration planning.

The use of seismic facies mapping is becoming more important. Where we have limited well information, we have the tendency to "create" sedimentary facies based on those few wells. We overlook the fact that we are not seeing a representative distribution of rocks in the subsurface. However, by using a seismic facies analysis approach at least we can recognize the variability in facies, which in turn can be used to predict the geology for a particular seismic signature. Seismic facies analysis differentiates areas of good porosity vs. bad porosity, development of reef and related features, etc. The technique is based on the fact that through "neural net" seismic analysis a map can be generated based on the distribution of similar wiggle trace shapes.

Shaping a report

Conclusions and recommendations should be presented on the first sheet of your report. If managers are interested at that point, they will read the balance of the report and examine your maps.

Not all companies want to spend the time necessary to produce a report, particularly for work that consisted of a multitude of maps, cross-sections, etc. We take a negative view of this outlook for a number of reasons:

A report forces the explorationists to organize their thoughts and to put them in understandable language.
It permits distribution of one's work to all who should have access to it.
It provides a foundation for future updating or as a reference if further work is required.
Since people do leave organizations for one reason or another, a report prevents the loss of valuable conclusions.

Your report should contain your selection of those areas that have the most exploratory merit in order of importance together with brief statements to justify that selection. Assuming that your study indicates the merit of detailed work in prime areas, what are your recommendations for exploration and refinement of those areas? What sequence should be followed, and what costs would be incurred? What are the manpower and time requirements to refine prospects? Keep your readers in mind: they control the purse strings.

If your conclusions are negative, management is entitled to know why as are workers who follow you.

Where and who

Regional evaluations are best done in offices or areas where the maximum geological and geophysical data are in-house or readily available and where the various disciplines needed are at hand.

Selection of personnel is important. Curiosity, imagination, and interest are necessary together with the ability to visualize rock sequences and events in three dimensions. The ability to coordinate information and to work with people representing other disciplines is vital. Everyone must be as close to being an expert in his field as possible.

Your regional explorationists should have experience in looking for drillable prospects so they can recognize potential areas outlined by the regional work.

Ideally, an explorationist needs a balance between training and exposure to several geological regimes. The geologist, for example, should have worked in several geological provinces, which will provide him with the knowledge of various parts of the geologic column and the structural and stratigraphic styles prevalent for several areas. He will have been exposed to different problems in these areas and their solutions. The exposure enhances an explorationist's "recognition factor."

On the other hand, management must do its part: show interest, need, and appreciation. When available, provide money both for the investigative program and to capitalize on the findings and recommendations.