Characterization and Genesis of Porosity below the Aptian-Albian Unconformity, Shuaiba Formation, Northern Oman

The recovery of hydrocarbon from carbonate reservoirs is influenced significantly by the inherent complexity of carbonate pore systems, yet the diagenetic processes that control porosity modification and the resultant reservoir complexity are often poorly described. Furthermore, little quantitative data exists on the size, shape and connectivity of the pore network. This study focuses on the uppermost strata of the Early Aptian Shuaiba Formation of north Oman, a few meters below the Albian-Aptian unconformity, where a zone of enhanced reservoir quality has been observed. In particular, total porosities of up to 40% and permeabilities of >1D have been measured in some wells. The study has three principal objectives:

1)to determine the timing of porosity modification, 2)to quantitatively describe the geometry of the pore network

, 3)to map the distribution of key pore-modifying events within the context of the tectono-stratigraphic and fluid evolution of the basin.

Petrographical observations demonstrate that heterogeneity in rock properties at the top of the Shuaiba Formation is controlled by several pore-destructive events (eg. neomorphism, cementation, compaction) coupled with multiple phases of allochem and matrix dissolution. Petrographically-determined cross-cutting relationships and textural observations under BSEM indicate that significant porosity generation took place in the burial realm. In particular, corrosion of micrite has enhanced micropore volumes adjacent to pressure solution seams and microfractures. X-Ray CT images reveal that these pores form an interconnected network at the micron- and sub-micron scale. The challenge now is to determine the source, composition and availability of corrosive fluids for this paragenetically late-stage porosity modification in order to build conceptual predictive models to explain the observed patterns of porosity and permeability. The results will have applications to other microporous Cretaceous reservoirs of the Arabian Plate and to the many reservoirs worldwide where burial corrosion has been identified as a key control on reservoir quality.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California