Flat Spots in Carbonates: Rocks, Fluids, or Myth?

Questions are raised whether flat spots within carbonates are DHIs. High sensitivity of elastic properties to porosity and mineralogy, and the low sensitivity to fluids combined with high degree of heterogeneity all act to reduce the likelihood of strong and coherent fluid induced flat spots. In some regions of Sarawak Basin, offshore Malaysia apparent flat spots are observed within some carbonates. Where drilled the flat spots occur at or close to the GWC, but there is also a coincidence with changes in porosity and lithology. In some cases, the coincidence of changes in porosity coinciding with changes in fluid have been thought to be linked by diagenetic factors, where porosity is preserved or enhanced especially when CO2 level is high. Given not all flat spots are perfectly flat, question is raised whether these features can be predictive of gas. Recent well drilled into one of these carbonates and through the flat spot found gas but also found that the flat spot was caused by change in lithology and porosity. Further rock physics modeling showed that fluid induced flat spots might be present when the porosity and homogeneity were sufficiently high. Such examples were found on the flank of the carbonate.