Online Journal for E&P Geoscientists

Al-Neyadi, Abdulla¹; Al Suwaidi, Majid H.^*1; Deshpande, Anil ¹; Ottinger, Gary ¹; Edwards, H. Ewart ¹; Akatsuka, Kazuhito ²
(1) UZFD, ZADCO, Abu Dhabi, United Arab Emirates. (2) INPEX, Tokyo, Japan.

Lower Cretaceous carbonate sediments in a supergiant Middle Eastern oil field are characterised by extensive networks of dolomitized burrows which occur over areas of several kilometers. Due to their permeability contrast to the surrounding fine-grained matrix, the dolomitized burrows are believed to play a key role in reservoir fluid flow, particularly as a major contributor to early water breakthrough observed in the field. This paper will discuss the results of a recent investigation of burrows in core and their impact on reservoir flow properties. Additionally, a small-scale, effective property modeling approach is proposed to account for the potential impact of these burrows on fluid flow.

The burrows appear in core as apparently randomly oriented and distributed features on a scale of cm to 10’s of cm. They are characterized by grainier fill (packstone to grainstone), often dolomitized, within a background matrix of mud-dominated sediment. The surrounding matrix is not dolomitized, which implies only localised intra-burrow, diagenetic alteration. In many instances the grainy burrow centers have the highest permeability, several orders of magnitude greater than the matrix, with a third intermediate permeability zone present at the burrow walls.

The differences in texture and associated pore characteristics lead to heterogeneous poro-perm regimes that can have significant impact on recovery in oil fields that are subjected to water flood. A high concentration of connected burrows could provide permeability pathways leading to early water breakthrough. Moreover, the burrows can interact with additional heterogeneities such as fractures, vugs and stylolite zones to further enhance permeability pathways. Multiple detailed mini-permeameter measurements taken a few inches apart in several wells returned permeability ranges in the 10's of mD for individual dolomitized burrows. Where dolomitized burrows were found proximal to fractures or vugs, much higher measurements (100’s of mD) were observed compared to background matrix measurements (<1 mD). Consistently higher conventional core analyses from this stratigraphic zone indicate the influence of dolomitized burrow networks throughout the core.

The reservoir in this field has experienced earlier than expected water break through and steady increases in water cut have been observed over time. Recent evidence suggests water movement occurs in some reservoir layers that contain dolomitized burrows.