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ABSTRACT: New Role of Petroleum Geochemistry in Quantitative Prospect Evaluation and Basin Assessment

Jim Brooks

During the last few years, rapid progress has been made in petroleum geochemistry. Applications of petroleum geochemistry in exploration have dramatically changed from a postmortem science to a widely accepted predictive tool. Due mainly to progress in the development of new analytical techniques and interpretative concepts, applications of geochemistry have become more rapid and also capable of solving more specific petroleum exploration problems. Based upon a combination of petroleum geochemistry, geology, and geophysics, it is now possible to make quantitative predictions and assessment of hydrocarbon potential of prospects, basins, and regions during the initial stages of exploration. Such applications can also be used to reevaluate mature exploration areas.

Petroleum geochemical and geological methods and concepts, together with probability analysis, can be used as a relatively inexpensive, rapid, and successful tool in oil and gas exploration. Such integrated methods and concepts have been used to predict the location and amounts of oil and gas reserves in prospects, regions, or reservoir sequences.

The lecture will cover the essential steps in well, prospect, basin, and regional evaluation. The main focus is on the recognition and quantification of hydrocarbon source rocks and their oil and gas yields. Geological controls on oil and gas occurrences will be applied to understand the type of hydrocarbons generated, expulsion efficiencies, migration pathways and efficiencies, and accumulated hydrocarbons in prospects and basins. New concepts on hydrocarbon generation, expulsion, drainage, and migration will also be discussed and applied to give quantitative evaluation of prospects, basins, and hydrocarbon accumulations. Such applications are now essential in basin and prospect evaluations.

Case histories from northwestern European basins illustrate these new concepts and their applications to petroleum exploration. Mesozoic source rocks show good correlation with Mesozoic and Tertiary reservoired oils in the North Sea. Such correlations and mass balance studies have greatly assisted in our understanding of generation, expulsion, migration, and accumulation of hydrocarbons in sedimentary basins. Recent work has produced a new mechanism for the biodegradation of reservoired oils into heavy oil. It now seems possible to predict the API gravity of reservoired oil in some prospects.

Quantitative evaluation and risk analysis of generated, migrated, and accumulated hydrocarbons have been applied in exploration programs. It is possible to significantly improve exploration success ratios using quantitative prospect analysis and basin assessment. Plays, prospects, or reservoir sequences can be graded and their relative rankings used to influence exploration policy.

AAPG Search and Discovery Article #91001©1989-1990 AAPG Distinguished Lecture Tours 1989-1990