A Quantitative Study of the Creation of Secondary Porosity in Carbonate Reservoirs by Dolomitization and Mesogenetic Dissolution

Abstract

Although most people believe that carbonate reservoir quality is largely controlled by diagenesis, one school of thinking claims this is supported by personal opinion, not by data (e.g., Ehrenberg et al., 2012). They argued that because carbonate minerals exhibit fast reaction kinetics and present ubiquitously, mesogenetic pore waters are always saturated. Even if pore water is initially under-saturated with respect to carbonates, it will become saturated during upward flow, and fail to create porosity. This on-going debate is important; as its outcome may have significant bearingon oil and gas exploration and production. Based on a reactive-transport model where a 1-D calcite column is subjected to flow of acidic water, Burk et al. (2015) demonstrated that due to calcite's retrograde solubility, porosity can be created by acidic water derived from kerogen maturation. However, that quantitative study is for a generic system where spatial and temporal attributes of some parameters did not have explicit physical meanings. For example, flow rate is not explicitly given. Therefore, the validity of the model and conclusions drawn from these simulations need to be further tested under realistic conditions. Moreover, the model only considers mesogenetic diagenesis due to retrograde solubility of calcite, its interactions with other diagenesis processes, such as dolomitization, are not included. In this study, we developed such a more realistic model to conduct a quantitative study, with each parameter assigned a value within the range believed to be reasonable and representative. This reactive-transport model generates direct answers. In addition, our model included dolomitization, and the interaction between dolomitization and mesogenetic dissolution. Conclusions: 1) mesogenetic dissolution indeed can create a significant amount of secondary porosity during upward flow, even if waters are initially saturated with calcite; 2) when dolomitization occurs, mesogenetic dissolution may be suppressed, as dolomitization releases a large amount of calcium. Based on our modeling result, we have reason to believe that dolomitization is a stronger agent for the creation of secondary porosity than mesogenetic dissolution. Therefore, it should be given more weight in reservoir quality prediction for carbonates.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016