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Risk Reduction or Why Stratigraphy?

MARKS, EDWARD, Marks and Associates, Whittier, CA

Risks and uncertainties are involved in the exploration for resources. This paper identifies these risks in terms of oil and gas exploration and how an interdisciplinary stratigraphic approach to exploration can reduce these risks.

Risk and uncertainty are related. Risk is the opportunity for loss; uncertainty is the range of probabilities that certain conditions may exist that could cause this loss. Risk factors involved in the search for oil and gas include four geologic factors: structure, reservoir, trapping conditions, and hydrocarbon charge. Exploration decisions involve the consideration of these risks and uncertainties when determining budgets, geotechnical predictions and serial exploration.

To find oil and gas, three geologic factors must be determined--the source rock, the reservoir, and the trapping mechanism. The geologic branch known as stratigraphy is best equipped to deal with these unknowns. Using lithostratigraphy, biostratigraphy, and seismic stratigraphy, geologists can determine the nature and type of stratigraphic traps, as well as the risk factors involved with each. Once these factors are identified, ways to reduce them can be determined for development areas.

Traps are identified using various tools. Drilling logs, fossil distribution charts, clastic analysis, and carbonate analysis, stratigraphic summaries and columns can be used for individual well sections. Area stratigraphy uses stratigraphic columns, strand line sequences, lithostratigraphic units and facies, previously known oil occurrences, cross sections, and stratigraphic sections. Seismic stratigraphy yields important information about the paleoenvironment and subsurface structures. With all the information collected, stratigraphers can apply Walther's law of facies succession to reduce risks in development areas.

