Modelling Carbonate Diagenesis of Icehouse Platforms Using CARB3D+: Fundamental Controls on Secondary Porosity Distribution

Richard J. Paterson¹, Peter L. Smart², Fiona F. Whitaker¹, Gareth D. Jones², and Graham Felce^1
1 Bristol University, Bristol, United Kingdom
² ExxonMobil Production Company, Houston

A fundamental challenge in carbonate reservoir characterization is predicting the spatial distribution of early diagenesis, which in turn can have a critical control on porosity heterogeneity. During icehouse times, high-amplitude sea level oscillations cause repeated vertical migration of the water table and underlying hydrological zones, and large variations in vadose thickness. The consequent diagenetic overprinting generates a complex pattern of cementation and secondary porosity in the reservoir, which is difficult to predict from seismic data or interpolation between cored wells.

CARB3D+ is an innovative 3D forward modelling program that predicts the co-evolution of sedimentary facies and early diagenesis in a sequence stratigraphic context. This can improve the accuracy of predictions and yield multiple scenarios of reservoir quality for input to geological models.

In a diagenetic system that is both chemically- and hydrologically-controlled, CARB3D+ simulations show predictability in the distribution of porosity due to dissolution and cementation. Changes in relative sea level driven by eustasy, tectonics and sediment production affect porosity via residence time within hydrologically-controlled diagenetic zones. The exposure of sediments to meteoric diagenesis is also affected by climate via changing freshwater lens thickness. For a given residence time, both climate and depositional mineralogy (particularly the abundance of aragonite) determine rates of diagenesis and dissolutional lowering of the exposure surface. CARB3D+ enables assessment of the relative importance of individual controls in a quantitative manner. Simulations suggest that 5th order eustatic cycles have only a minor influence on secondary porosity. The relationship between 4th order 100 ka and 400 ka cycles is critical in controlling the nature and spatial distribution of secondary porosity. However rapid subsidence reduces overprinting and consequently the degree of secondary porosity development.