ABSTRACT

In frontier basins where subsurface data are limited, or absent altogether, the study of reservoir rocks exposed in surface outcrops may be the dominant (or only available) means of predicting subsurface reservoir quality. This chapter provides a systematic, decision-tree-based procedure for using existing tools and techniques to evaluate potential subsurface reservoir quality when only surface outcrops are available. This approach is applicable to both carbonate and terrigenous clastic reservoirs. With this system, outcrop samples are classified into one of ten lithofacies types whose reservoir properties are codependent on common diagenetic or burial processes. The classification subdivides outcrop samples into either "tight" or "porous" lithofacies, depending on the measured porosity relative to economic minimum. "Tight" rocks include six end-member lithofacies that were either cemented or compacted during burial, or were originally tight at the time of deposition. "Porous" rocks include four lithofacies types that are categorized by original depositional fabric and the degree of alteration by recent surface weathering. Risk assessment for each of the ten lithofacies types using existing geological tools and techniques is discussed, along with guidelines for estimating potential subsurface porosity and permeability. Case histories that illustrate the recommended process for assessing risk are described from China, Myanmar (Burma), and Turkey.

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