Abstract

The key ‘flow drivers’ of an Early Cretaceous reservoir are described and correlated region-wide in Abu Dhabi – these are identified as being the same features in a series of fields and being layer-controlled. Three geological drivers control water-cut evolution, showing consistent stratigraphic relationships and, hence, will they provide guidance in both modelling and developing other fields, and in predicting water movement paths elsewhere. This is the first time that assessment and correlation of the key flow drivers has been made country-wide: in different basin settings with different hydrocarbons but with a comparatively consistent reservoir framework. Fractures organised in mechanical layers, connected and diagenetically modified burrows and preferential dissolution comprise the factors. These flow drivers are not randomly oriented within the reservoir but display distinct vertical arrangement that reflects an initial depositional motif. A fining- and deepening-upwards motif results in the algal floatstones concentrated in the lowermost 1/3rd, whilst the burrowed pack- and wackestones dominate the middle ~1/3rd. In the uppermost part nodular stylolitised mudstones prevail, where microporosity dominates and where the strength contrasts increasing cementation result in increasing fracture-prone thin layers that amalgamate towards top reservoir. Despite showing broad stratigraphic discretisation, in detail, some of these features overlap vertically. For example, burrows and fractures may be co-located near their interface; similarly, algal textures may be intertwined with minor fractures in pseudo-stratigraphic layers near base reservoir. This occurs consistently in all fields, irrespective of variation in overall reservoir thickness. Depositional heterogeneity is the key control on the basic reservoir framework, however, the flow driver development (K contrast severity) is further suppressed / accentuated by local to regional diagenetic modification. As all three heterogeneity systems are observed at the same relative vertical position, this reflects a high degree of depositional consistency on the platform passive margin.

Intensity of factors vary from field to field, also systematically between crest and flank areas of a given field. Example wells illustrate the development of all key drivers, to a greater or lesser extent in all fields: thereby, providing a relative ‘flow driver’ potential as a static model input and a dynamic model H-M sensitivity. Factors controlling the basic heterogeneity framework are stratigraphic / depositional, subsequently modified by diagenetic effects. Diagenesis can be represented by field-wide (depth on structure) or local (fault-proximal) controls, which can be compared with other factors, including hydrocarbon type (gas/oil), trap style and deformation intensity, water cut evolution, development scenario, amongst others. Implications of understanding these consistencies in relationships are applicable regionally, and can be used in learnings from one field to another, investigating the reservoir development impact of these drivers under different development schemes, and in assessing simulation history- matching within brownfields and for upcoming greenfield development.

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