Porosity Prediction in Shallow vs. Deepwater Limestones

Abstract

Summary Limestones can be divided arbitrarily into shallow- and deepwater types. Shallow-water limestones characteristically have complex depositional facies patterns and originally are composed primarily of unstable minerals (aragonite and high-Mg calcite), which make them highly susceptible to post-depositional alteration. Small differences in burial-uplift-water chemistry histories can produce radically different trends of porosity preservation, destruction, or creation as a consequence of the great susceptibility of unstable minerals to alteration by fresh (nonmarine) pore fluids. These factors make porosity prediction of shallow-water limestones very difficult. Only through careful and detailed mapping of both primary facies and mineralogical trends, coupled with equally painstaking determinations of diagenetic histories and patterns, can porosity predictions be made for shallow-water limestones. Deepwater, pelagic limestones, on the other hand, commonly are composed of a much more stable primary mineral (low-Mg calcite) and have more uniform and predictable facies patterns. Studies performed on Mesozoic and Cenozoic chalks throughout the U.S. and Europe have shown that porosity is lost as a direct function of burial depth. The rates of porosity loss vary with the pore-water chemistry and, more importantly, as a function of pore-fluid pressure. If these factors are known with reasonable certainty, then reasonable predictions can be made of average subsurface porosity of deepwater limestones. Only where such limestones have never been buried deeply, where they have had an early input of oil, or where they have elevated pore-fluid pressure can high porosities and high petroleum-reservoir potential be expected. Scanning electron microscopy (SEM), petrography, and isotopic analysis have proved useful techniques for deciphering the depositional and diagenetic histories of both shallow- and deepwater limestones. Introduction Prediction of porosity in carbonate rocks is at best a difficult undertaking. Most carbonate hydrocarbon reservoirs drilled to date have been in shallow-water limestones. In such rocks, depositional patterns are complex and variable and diagenetic alteration of original sediment textures and compositions may be extreme. Few, if any, general rules can be used to predict the porosity of such carbonate rocks in the absence of drilling data. Even where a significant number of wells have been drilled, useful porosity trends that enable accurate porosity prognostication over even relatively short distances may be difficult to discern. Porosity prediction in deepwater limestones, on the other hand, generally is easier. Deepwater strata have more uniform depositional facies, more predictable primary chemical compositions, and simpler diagenetic alteration patterns than their shallow-water equivalents. Alteration of deepwater limestones is controlled mainly by their maximum burial depth. Average rates for such diagenetic porosity loss can be determined as a function of burial.