First Steps Toward Focused ExplorationBy W. E. Schollnberger** Wolfgang E. Schollnberger, Vice President Exploration and Production Technology, Amoco Corp., Houston, Texas.Thisarticle was first published in OIL GAS European Magazine in 1996.SCHOLLNBERGER, W.E. (1996): First steps Toward Focused Exploration: OIL GAS European Magazine, vol. 22, (1/1996), pp 10 - 13, Hamburg - Wien (Urban Verlag).ABSTRACTFocus is an important ingredient of an exploration effort that aims for creation of financial wealth. The simple statistical methods compiled and discussed in this article are useful tools in targeting exploration at the right basins and plays.1.INTRODUCTIONExploration builds the future for the petroleum industry. Exploration also is a risky business that requires the application of complex and costly technologies. Over the last decade, return on investment has been disappointing for exploration and production companies, and it has become a big challenge to create value growth through exploration[1].The capability to build a diversified portfolio of exploration options that generate high financial value is based on recognizing the resource potential of basins or plays and the skill to focus on the best ones. Of course, fiscal aspects, operational/cost aspects and marketing/price aspects are as important as the resource aspect. In this article, however, we shall present a few simple methods which have proven useful to predict the remaining petroleum resources of basins and plays. All the discussed methods are based on the results of past drilling and cannot directly be applied to undrilled basins/plays.2.PREDICTING FUTURE RESOURCES FROM PAST DRILLINGWe found a few simple statistical cross-plots very helpful in focusing exploration early, before the appropriate high-tech explorations methods are applied.All cross-plots discussed in this article can he applied to basins or to plays. If applied to basins they require only three input parameters:
- location (where each exploration was drilled)
- time (when each exploration well was drilled)
- the petroleum resources (liquid or gas discovered by each exploration well, including zero for a dry hole).
Location and time are straightforward; the resources discovered are not because the estimated resources of a petroleum field may not be fully known from one exploration well only; later drilling and production history often leads to upward or downward revisions of resources. In this article, we always use the most current resource estimate for a field and attribute it back to the first exploration well drilled into that field.If the cross plots are applied to plays, a fourth input parameter is required:
- the rock formation(s) in which the petroleum resources have been found in a well.
All four inputs are generally openly available from a number of sources, such as national and state geological surveys, national and state petroleum companies, and from commercial enterprises such as Petroconsultants S.A.(Geneva, Switzerland) for fields outside the United States and from Petroleum Information Corporation (Littleton, Colorado) for fields within the United States.Let us now discuss how these simple inputs can help focus an exploration effort in the right areas before any time-consuming geological field work or extensive stratigraphic, geochemical, seismic and drilling activities is started.2.1.RESOURCES PER SUCCESSFUL EXPLORATION WELLOn the abscissa we plot the successful exploration wells in a basin or play in time sequence as they were drilled with the first well positioned nearest to the origin and equal spacing between each well. On the ordinate we scale the resources found per exploration well, e.g., in barrels of oil equivalent (BOE). In most petroliferous basins the resources found per successful exploration well increase rapidly after some initial dry holes, then the growth flattens, and finally, the resources added per well decline (Fig. 1). This cross plot is useful in roughly estimating at any time, resources expected to be found by future successful exploration wells in a basin or play; its usefulness can he further increased if it is combined with success ratios (Fig. 6).For many petroliferous basins, the curve has the general shape exhibited in Figure 1, and it can be subdivided into distinct portions which characterize the exploration maturity of a basin/play (Fig. 2). The portion before any commercial quantities of oil or gas have been found is called the frontier stage; as the resources found in each exploratory well increase we say the basin is in the emerging stage. When the resources found per well crest and taper off, the basin is in the established stage, and once the findings per well decline overall, the basin has reached the mature stage. Exploration in emerging and established basins has generally a higher chance to create financial growth than exploring in mature basins, and is much less risky than exploring in frontier basins. New technologies such as new 3D-seismic acquisition, 3D-processing and 3D-visualization, migration before stacking (especially in pre-salt and overthrust plays), amplitude versus offset (AVO) analysis, etc. can boost finding rates and flatten or even temporarily reverse declining finding rate trends. Empirically, we find that petroliferous basins with one exploration well per 500 km2 to 5000 km2 are in the emerging stage, basins with one exploration well per 50 km2 to 500 km2 are in the established stage, and basins with more than one exploration well per 50 km2 are in the mature stage. This empirically determined broad relationship between exploration well density and basin maturity stages can help in estimating how many wells it will take to fully explore a basin, although it is not valid for all basins.In many publications graphs similar to Figure 1 are plotted with time (e.g. years, equal spacing between each year) on the abscissa instead of successful exploration wells. We recommend against that practice because the resulting cross-plot can be very irregular