Testing Regional Depositional Models and Capturing Stratigraphic and Diagenetic Heterogeneities in Carbonate Reservoirs Using Forward Numerical Modeling: A Case Study From the Lower Cretaceous of Oman

Abstract

Carbonate reservoirs are the product of complex interplays between mostly biologic in-situ carbonate production, sediment transport, and diagenesis. Despite the availability of seismic data and a large and growing number of outcrop analogue studies, questions remain about the 3D variability of carbonate attributes in ramps and platforms, and their exact controls through geological time (4D). Here, we demonstrate that diffusion-based forward numerical models (DionisosFlow) can be coupled with surface and outcrop data to test depositional scenarios, and to explore the variability of sedimentary and diagenetic attributes of carbonates at the regional to reservoir scale. We selected the Lower Cretaceous carbonate ramp of Oman as the target for our modeling study because of the abundance of regional data, the fact that giant reservoirs are hosted in the Shaiba and Kharaib Formations of the Middle East, and that well-documented geometries exist (prograding to aggrading clinoforms). The three main questions we aimed to address are: 1) what controls clinoform geometry, 2) can the prograding to aggrading regional trend be explained by changes in eustasy alone, and 3) are sedimentologic and diagenetic heterogeneities apparent in the model, and could they potentially impact hydrocarbon production? Our modeling results demonstrate that basement topography has the greatest influence on clinoform geometries, thus confirming existing conceptual models for the region. We also show conclusively that the Lower Cretaceous ramp geometry in Oman can be modeled using constant sedimentation rates and published eustatic curves, but that the aggradation pattern observed in the latter part of the Early Cretaceous requires markedly increased tectonic subsidence. Finally, the models show that sedimentologic heterogeneities in the extensive back-reef (where most reservoirs are located) will be controlled by carbonate production and wave energy dispersal, and that diagenetic heterogeneities will exist at the regional scale due to differential exposure times during sea-level lowstands. This creates potential zones of variable reservoir properties, which could impact production. In conclusion, our study suggests that diffusion-based models can be used to incorporate sedimentological heterogeneities in carbonate reservoir models both for prospection and production, but also that this approach has the potential to be powerful and time effective to estimate diagenesis in carbonate ramps.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018