Reservoir Implications of Modern Karst Topography

Abstract

Tropical karst landscapes exhibit a predominance of positive relief features relative to the negative relief features (i.e., sink holes, dolines, etc.) of temperate karst areas and yet, paradoxically, have never been observed in the subsurface on seismic sections. To understand why this is the case, topographic profiles were made over the positive karst relief features of China, Java, and Jamaica and were used to construct synthetic seismic sections. The results indicate that in most instances the magnitude of the topographic relief is sufficient to be seen on seismic sections. A possibility of misinterpreting karst highs as reefs seems clear, but more important in explaining the paradox is the fact that the magnitude of modern karst relief expression is largely attributable to uplift over an extended period. The probability of preservation of these positive relief solution residuals is dependent on the karstification process being aborted by the deposition of overlying sediment, an event not likely to occur in an uplift area. Uplift of well-lithified carbonate minimally results in an areally extensive runoff surface that controls the development of positive karst landforms. Uplift also commonly induces faulting and fracturing that further control the distribution of karst land forms. Hydrocarbon production from the resulting karst is geared to matrix porosity: the higher the matrix porosity, the greater the rate of deliverability of matrix oil to produced fracture and karst conduit oil. In contrast, the lowering of the base level of erosion due to a fall in sea level is generally less than that induced by uplift and, consequently, minimizes positive karst relief development relative to that of uplift. Subaerial exposure induced by passive sea level fall also minimizes the potential for the related occurrence of fracturing and faulting, which of itself mitigates against the general development of positive karst relief. The subaerial consequences of passive sea level fall are commonly preserved in the geologic record and result in karst reservoirs dominated by extensive moldic and vuggy porosity interconnected by solution channels. These reservoirs lack the production problems commonly attending fractured karst reservoirs with little matrix porosity.