An introduction—Carbonate geophysics

Abstract

Carbonates are strange rocks to most exploration geophysicists although they hold more than half the world's petroleum reserves. The geophysicist's exposure to geology in colleges and graduate shools was concentrated on basin analysis, structure, stratigraphy of clastic sedimentary rocks. This estrangement to carbonate sedimentary rocks was further perpetuated in the past two decades, primarily because of the industry's focus on deeper water exploration and development of turbidite sands. Owing to the advancement of geophysical technology, favorable rock physics and superior seismic data quality, geophysics played a critical role in finding many elusive oil and gas traps deep in the ocean. Geophysics was indispensable in aiding the development of the discovered deepwater fields by reducing the risk and cost of expensive deepwater wells. Geophysical applications in carbonate reservoirs are less mature and abundant than those associated with clastic reservoir. This lack of maturity is primarily the result of lower business priorities of carbonate reservoirs in the last two decades. Additionally, carbonate reservoirs are notoriously more difficult to characterize than siliciclastic reservoirs. Compared to siliciclastic reservoirs, carbonate reservoirs offer unique geophysical challenges with respect to reservoir characterization. These include: (1) tight rock fabric resulting in problematic and not widely accepted rock physics models; (2) greater heterogeneity due to rapid vertical and lateral facies variation; (3) lower seismic resolution due to higher velocities; (4) physical and chemical alterations causing fracturing and diagenesis; and (5) mostly land and shallow water seismic data, meaning relatively lower data quality over most carbonate fields. Recent business opportunities in the former Soviet Union and Middle East have triggered renewed interest in carbonate reservoirs and carbonate specific exploration, development, and production …