Formation evaluation of carbonate rocks

Abstract

The unique pore characteristics and variation in mineral composition of carbonate rocks (dolomite, dolomitic limestone, calcitic dolomite, limestone, etc.) can significantly affect the e response of different types of wireline logs. For example, complications may occur because carbonate reservoir rocks can exhibit different types of porosity, e.g., intergranular, intercrystalline, vuggy, moldic, and fracture. Consequently, erroneous log-derived conclusions and recommendations as to whether or not to test a well are frequently made. An apparently “good or prolific” pay zone may test only marginal or even “dry”. Sometimes, therefore, economically attractive intervals are simply overlooked. Notwithstanding the significant recent advances in digital wellsite log evaluation capabilities, the present paper reviews and illustrates several basic manual log evaluation techniques. These techniques can assist the wellsite geologist as an interpretive quality check and provide guidelines for future action and decision making. Several field examples in carbonate rocks are presented which illustrate the utility of the techniques discussed.